Co-reporter:Ethan B. Secor, Theodore Z. Gao, Ahmad E. Islam, Rahul Rao, Shay G. Wallace, Jian Zhu, Karl W. Putz, Benji Maruyama, and Mark C. Hersam
Chemistry of Materials March 14, 2017 Volume 29(Issue 5) pp:2332-2332
Publication Date(Web):February 27, 2017
DOI:10.1021/acs.chemmater.7b00029
Recent developments in liquid-phase processing of carbon nanomaterials have established graphene as a promising candidate for printed electronics. Of great importance in the ink formulation is the stabilizer, which has to provide excellent dispersion stability and tunability in the liquid state, and also decompose into chemical moieties that promote high electrical conductivity and robust mechanical and environmental stability. Here we demonstrate the promise of nitrocellulose as a synergistic polymer stabilizer for graphene inks. Graphene processed with nitrocellulose is formulated into inks with viscosities ranging over 4 orders of magnitude for compatibility with a wide range of deposition methods. Following thermal treatment, the graphene/nitrocellulose films offer high electrical conductivity of ∼40 000 S/m, along with mechanical flexibility. Moreover, in contrast to state-of-the-art graphene inks based on ethyl cellulose, the nitrocellulose residue offers superior mechanical and environmental stability as assessed by a suite of stress tests, including the Scotch tape test, a water sonication test, and an 85/85 damp heat test. By exploring the fundamental chemistry underlying these macroscopic benefits, we provide insight into binder selection for functional nanomaterial inks while producing a high-performance graphene ink with strong potential for printed and flexible electronics.
Co-reporter:Stephanie Bettis Homan;Emily A. Weiss;Itamar Balla;Vinod K. Sangwan;Hadallia Bergeron
Nano Letters January 11, 2017 Volume 17(Issue 1) pp:164-169
Publication Date(Web):December 5, 2016
DOI:10.1021/acs.nanolett.6b03704
van der Waals heterojunctions between two-dimensional (2D) layered materials and nanomaterials of different dimensions present unique opportunities for gate-tunable optoelectronic devices. Mixed-dimensional p–n heterojunction diodes, such as p-type pentacene (0D) and n-type monolayer MoS2 (2D), are especially interesting for photovoltaic applications where the absorption cross-section and charge transfer processes can be tailored by rational selection from the vast library of organic molecules and 2D materials. Here, we study the kinetics of excited carriers in pentacene–MoS2 p–n type-II heterojunctions by transient absorption spectroscopy. These measurements show that the dissociation of MoS2 excitons occurs by hole transfer to pentacene on the time scale of 6.7 ps. In addition, the charge-separated state lives for 5.1 ns, up to an order of magnitude longer than the recombination lifetimes from previously reported 2D material heterojunctions. By studying the fractional amplitudes of the MoS2 decay processes, the hole transfer yield from MoS2 to pentacene is found to be ∼50%, with the remaining holes undergoing trapping due to surface defects. Overall, the ultrafast charge transfer and long-lived charge-separated state in pentacene–MoS2 p–n heterojunctions suggest significant promise for mixed-dimensional van der Waals heterostructures in photovoltaics, photodetectors, and related optoelectronic technologies.Keywords: charge transfer; Organic; transient absorption spectroscopy; transition metal dichalcogenide; ultrafast;
Co-reporter:William A. Gaviria Rojas, Julian J. McMorrow, Michael L. Geier, Qianying Tang, Chris H. Kim, Tobin J. Marks, and Mark C. Hersam
Nano Letters August 9, 2017 Volume 17(Issue 8) pp:4976-4976
Publication Date(Web):July 3, 2017
DOI:10.1021/acs.nanolett.7b02118
With the growing adoption of interconnected electronic devices in consumer and industrial applications, there is an increasing demand for robust security protocols when transmitting and receiving sensitive data. Toward this end, hardware true random number generators (TRNGs), commonly used to create encryption keys, offer significant advantages over software pseudorandom number generators. However, the vast network of devices and sensors envisioned for the “Internet of Things” will require small, low-cost, and mechanically flexible TRNGs with low computational complexity. These rigorous constraints position solution-processed semiconducting single-walled carbon nanotubes (SWCNTs) as leading candidates for next-generation security devices. Here, we demonstrate the first TRNG using static random access memory (SRAM) cells based on solution-processed SWCNTs that digitize thermal noise to generate random bits. This bit generation strategy can be readily implemented in hardware with minimal transistor and computational overhead, resulting in an output stream that passes standardized statistical tests for randomness. By using solution-processed semiconducting SWCNTs in a low-power, complementary architecture to achieve TRNG, we demonstrate a promising approach for improving the security of printable and flexible electronics.Keywords: cybersecurity; encryption; Internet of Things; printed electronics; Thin-film transistor;
Co-reporter:Joohoon Kang, Vinod K. Sangwan, Joshua D. Wood, and Mark C. Hersam
Accounts of Chemical Research April 18, 2017 Volume 50(Issue 4) pp:943-943
Publication Date(Web):February 27, 2017
DOI:10.1021/acs.accounts.6b00643
ConspectusExfoliation of single-layer graphene from bulk graphite and the subsequent discovery of exotic physics and emergent phenomena in the atomically thin limit has motivated the isolation of other two-dimensional (2D) layered nanomaterials. Early work on isolated 2D nanomaterial flakes has revealed a broad range of unique physical and chemical properties with potential utility in diverse applications. For example, the electronic and optical properties of 2D nanomaterials depend strongly on atomic-scale variations in thickness, enabling enhanced performance in optoelectronic technologies such as light emitters, photodetectors, and photovoltaics. Much of the initial research on 2D nanomaterials has relied on micromechanical exfoliation, which yields high-quality 2D nanomaterial flakes that are suitable for fundamental studies but possesses limited scalability for real-world applications. In an effort to overcome this limitation, solution-processing methods for isolating large quantities of 2D nanomaterials have emerged. Importantly, solution processing results in 2D nanomaterial dispersions that are amenable to roll-to-roll fabrication methods that underlie lost-cost manufacturing of thin-film transistors, transparent conductors, energy storage devices, and solar cells. Despite these advantages, solution-based exfoliation methods typically lack control over the lateral size and thickness of the resulting 2D nanomaterial flakes, resulting in polydisperse dispersions with heterogeneous properties. Therefore, post-exfoliation separation techniques are needed to achieve 2D nanomaterial dispersions with monodispersity in lateral size, thickness, and properties.In this Account, we survey the latest developments in solution-based separation methods that aim to produce monodisperse dispersions and thin films of emerging 2D nanomaterials such as graphene, boron nitride, transition metal dichalcogenides, and black phosphorus. First, we motivate the need for precise thickness control in 2D nanomaterials by reviewing thickness-dependent physical properties. Then we present a succinct survey of solution-based exfoliation methods that yield 2D nanomaterial dispersions in organic solvents and aqueous media. The Account subsequently focuses on separation methods, including a critical analysis of their relative strengths and weaknesses for 2D nanomaterials with different buoyant densities, van der Waals interactions, and chemical reactivities. Specifically, we evaluate sedimentation-based density gradient ultracentrifugation (sDGU) and isopycnic DGU (iDGU) for post-exfoliation 2D nanomaterial dispersion separation. The comparative advantages of sedimentation and isopycnic methods are presented in both aqueous and nonaqueous media for 2D nanomaterials with varying degrees of chemical reactivity. Finally, we survey methods for forming homogeneous thin films from 2D nanomaterial dispersions and emerging technologies that are likely to benefit from these structures. Overall, this Account provides not only an overview of the present state-of-the-art but also a forward-looking vision for the field of solution-processed monodisperse 2D nanomaterials.
Co-reporter:Kan-Sheng Chen, Rui Xu, Norman S. Luu, Ethan B. Secor, Koichi Hamamoto, Qianqian Li, Soo Kim, Vinod K. Sangwan, Itamar Balla, Linda M. Guiney, Jung-Woo T. Seo, Xiankai Yu, Weiwei Liu, Jinsong Wu, Chris Wolverton, Vinayak P. Dravid, Scott A. Barnett, Jun Lu, Khalil Amine, and Mark C. Hersam
Nano Letters April 12, 2017 Volume 17(Issue 4) pp:2539-2539
Publication Date(Web):February 27, 2017
DOI:10.1021/acs.nanolett.7b00274
Efficient energy storage systems based on lithium-ion batteries represent a critical technology across many sectors including consumer electronics, electrified transportation, and a smart grid accommodating intermittent renewable energy sources. Nanostructured electrode materials present compelling opportunities for high-performance lithium-ion batteries, but inherent problems related to the high surface area to volume ratios at the nanometer-scale have impeded their adoption for commercial applications. Here, we demonstrate a materials and processing platform that realizes high-performance nanostructured lithium manganese oxide (nano-LMO) spinel cathodes with conformal graphene coatings as a conductive additive. The resulting nanostructured composite cathodes concurrently resolve multiple problems that have plagued nanoparticle-based lithium-ion battery electrodes including low packing density, high additive content, and poor cycling stability. Moreover, this strategy enhances the intrinsic advantages of nano-LMO, resulting in extraordinary rate capability and low temperature performance. With 75% capacity retention at a 20C cycling rate at room temperature and nearly full capacity retention at −20 °C, this work advances lithium-ion battery technology into unprecedented regimes of operation.Keywords: high packing density; high rate capability; Lithium manganese oxide; low temperature; nanoparticle; spinel;
Co-reporter:Itamar Balla;Erik Luijten;Zonghui Wei;Xiaolong Liu;Andrew J. Mannix;Nathan P. Guisinger
Science Advances 2017 Volume 3(Issue 2) pp:
Publication Date(Web):
DOI:10.1126/sciadv.1602356
Integrating borophene with organic molecules results in electronically abrupt self-assembled lateral heterostructures.
Co-reporter:Menghua Zhu;Weiwei Liu;Weijun Ke;Sarah Clark;Ethan B. Secor;Tze-Bin Song;Mercouri G. Kanatzidis;Xin Li
Journal of Materials Chemistry A 2017 vol. 5(Issue 46) pp:24110-24115
Publication Date(Web):2017/11/28
DOI:10.1039/C7TA07969A
In recent years, hybrid organic–inorganic halide perovskite solar cells (PVSCs) have emerged as leading candidates for next-generation photovoltaic technologies. The architecture of PVSCs rely on charge transport layers to enable optimal efficiency and stability. In particular, tin oxide (SnO2) has been shown to be an effective electron transport layer (ETL) due to favorable energy level alignment, relatively wide band gap, and high environmental and thermal stability. However, traditional processing of SnO2 necessitates a high-temperature and/or long-duration sintering step that limits substrate choice and introduces manufacturing challenges. To overcome this limitation, we present here a rapid, low-temperature, solution-based method for SnO2 film fabrication based on intense pulsed photonic annealing. Following a comprehensive survey of the precursor solution and photonic annealing conditions, light exposure as short as 20 milliseconds is shown to provide a high-quality nanocrystalline SnO2 film at room temperature, enabling PVSCs with low hysteresis and high power conversion efficiencies exceeding 15%. Overall, this work establishes a processing pathway for SnO2-based PVSCs that is compatible with flexible substrates and high-throughput, roll-to-roll manufacturing.
Co-reporter:Kyle A. Luck;Vinod K. Sangwan;Patrick E. Hartnett;Heather N. Arnold;Michael R. Wasielewski;Tobin J. Marks
Advanced Functional Materials 2017 Volume 27(Issue 48) pp:
Publication Date(Web):2017/12/01
DOI:10.1002/adfm.201703805
AbstractNon-fullerene acceptors based on perylenediimides (PDIs) have garnered significant interest as an alternative to fullerene acceptors in organic photovoltaics (OPVs), but their charge transport phenomena are not well understood, especially in bulk heterojunctions (BHJs). Here, charge transport and current fluctuations are investigated by performing correlated low-frequency noise and impedance spectroscopy measurements on two BHJ OPV systems, one employing a fullerene acceptor and the other employing a dimeric PDI acceptor. In the dark, these measurements reveal that PDI-based OPVs have a greater degree of recombination in comparison to fullerene-based OPVs. Furthermore, for the first time in organic solar cells, 1/f noise data are fit to the Kleinpenning model to reveal underlying current fluctuations in different transport regimes. Under illumination, 1/f noise increases by approximately four orders of magnitude for the fullerene-based OPVs and three orders of magnitude for the PDI-based OPVs. An inverse correlation is also observed between noise spectral density and power conversion efficiency. Overall, these results show that low-frequency noise spectroscopy is an effective in situ diagnostic tool to assess charge transport in emerging photovoltaic materials, thereby providing quantitative guidance for the design of next-generation solar cell materials and technologies.
Co-reporter:Yongsuk Choi;Junmo Kang;Deep Jariwala;Moon Sung Kang;Tobin J. Marks;Jeong Ho Cho
Advanced Materials 2016 Volume 28( Issue 19) pp:3742-3748
Publication Date(Web):
DOI:10.1002/adma.201506450
Co-reporter:Jian Zhu;Xiaolong Liu;Michael L. Geier;Julian J. McMorrow;Deep Jariwala;Megan E. Beck;Wei Huang;Tobin J. Marks
Advanced Materials 2016 Volume 28( Issue 1) pp:63-68
Publication Date(Web):
DOI:10.1002/adma.201504501
Co-reporter:Tejas A. Shastry;Sarah C. Clark;Andrew J. E. Rowberg;Kyle A. Luck;Kan-Sheng Chen;Tobin J. Marks
Advanced Energy Materials 2016 Volume 6( Issue 2) pp:
Publication Date(Web):
DOI:10.1002/aenm.201501466
Single-walled carbon nanotube (SWCNT) fullerene solar cells have recently attracted attention due to their low-cost processing, high environmental stability, and near-infrared absorption. While SWCNT–fullerene bulk-heterojunction photovoltaics employing an inverted architecture and polychiral SWCNTs have achieved efficiencies exceeding 3% over device areas of ≈1 mm2, large-area SWCNT solar cells have not yet been demonstrated. In particular, with increasing device area, spatial inhomogeneities in the SWCNT film have limited overall device performance. Here, 1,8-diiodooctane (DIO) is utilized as a solvent additive to reduce fullerene domain size and to improve SWCNT–fullerene bulk-heterojunction morphology. Under optimized conditions, DIO elucidates the influence of SWCNT chiral distribution on overall device performance, revealing a tradeoff between short-circuit current density and fill factor as a function of the chirality distribution present. The combination of SWCNT chirality distribution engineering and improved spatial homogeneity via solvent additives enables area-scaling of SWCNT–fullerene solar cells with performance comparable to small-area cells.
Co-reporter:Michael L. Geier, Karttikay Moudgil, Stephen Barlow, Seth R. Marder, and Mark C. Hersam
Nano Letters 2016 Volume 16(Issue 7) pp:4329-4334
Publication Date(Web):June 2, 2016
DOI:10.1021/acs.nanolett.6b01393
Single-walled carbon nanotube (SWCNT) transistors are among the most developed nanoelectronic devices for high-performance computing applications. While p-type SWCNT transistors are easily achieved through adventitious adsorption of atmospheric oxygen, n-type SWCNT transistors require extrinsic doping schemes. Existing n-type doping strategies for SWCNT transistors suffer from one or more issues including environmental instability, limited carrier concentration modulation, undesirable threshold voltage control, and/or poor morphology. In particular, commonly employed benzyl viologen n-type doping layers possess large thicknesses, which preclude top-gate transistor designs that underlie high-density integrated circuit layouts. To overcome these limitations, we report here the controlled n-type doping of SWCNT thin-film transistors with a solution-processed pentamethylrhodocene dimer. The charge transport properties of organorhodium-treated SWCNT thin films show consistent n-type behavior when characterized in both Hall effect and thin-film transistor geometries. Due to the molecular-scale thickness of the organorhodium adlayer, large-area arrays of top-gated, n-type SWCNT transistors are fabricated with high yield. This work will thus facilitate ongoing efforts to realize high-density SWCNT integrated circuits.
Co-reporter:Junmo Kang, Deep Jariwala, Christopher R. Ryder, Spencer A. Wells, Yongsuk Choi, Euyheon Hwang, Jeong Ho Cho, Tobin J. Marks, and Mark C. Hersam
Nano Letters 2016 Volume 16(Issue 4) pp:2580-2585
Publication Date(Web):March 7, 2016
DOI:10.1021/acs.nanolett.6b00144
Black phosphorus (BP) has recently emerged as a promising narrow band gap layered semiconductor with optoelectronic properties that bridge the gap between semimetallic graphene and wide band gap transition metal dichalcogenides such as MoS2. To date, BP field-effect transistors have utilized a lateral geometry with in-plane transport dominating device characteristics. In contrast, we present here a vertical field-effect transistor geometry based on a graphene/BP van der Waals heterostructure. The resulting device characteristics include high on-state current densities (>1600 A/cm2) and current on/off ratios exceeding 800 at low temperature. Two distinct charge transport mechanisms are identified, which are dominant for different regimes of temperature and gate voltage. In particular, the Schottky barrier between graphene and BP determines charge transport at high temperatures and positive gate voltages, whereas tunneling dominates at low temperatures and negative gate voltages. These results elucidate out-of-plane electronic transport in BP and thus have implications for the design and operation of BP-based van der Waals heterostructures.
Co-reporter:Deep Jariwala, Sarah L. Howell, Kan-Sheng Chen, Junmo Kang, Vinod K. Sangwan, Stephen A. Filippone, Riccardo Turrisi, Tobin J. Marks, Lincoln J. Lauhon, and Mark C. Hersam
Nano Letters 2016 Volume 16(Issue 1) pp:497-503
Publication Date(Web):December 10, 2015
DOI:10.1021/acs.nanolett.5b04141
The recent emergence of a wide variety of two-dimensional (2D) materials has created new opportunities for device concepts and applications. In particular, the availability of semiconducting transition metal dichalcogenides, in addition to semimetallic graphene and insulating boron nitride, has enabled the fabrication of “all 2D” van der Waals heterostructure devices. Furthermore, the concept of van der Waals heterostructures has the potential to be significantly broadened beyond layered solids. For example, molecular and polymeric organic solids, whose surface atoms possess saturated bonds, are also known to interact via van der Waals forces and thus offer an alternative for scalable integration with 2D materials. Here, we demonstrate the integration of an organic small molecule p-type semiconductor, pentacene, with a 2D n-type semiconductor, MoS2. The resulting p–n heterojunction is gate-tunable and shows asymmetric control over the antiambipolar transfer characteristic. In addition, the pentacene/MoS2 heterojunction exhibits a photovoltaic effect attributable to type II band alignment, which suggests that MoS2 can function as an acceptor in hybrid solar cells.
Co-reporter:KunHo Yoon, Jae-Hyeok Lee, Joohoon Kang, Junmo Kang, Michael J. Moody, Mark C. Hersam, and Lincoln J. Lauhon
Nano Letters 2016 Volume 16(Issue 12) pp:7370-7375
Publication Date(Web):November 10, 2016
DOI:10.1021/acs.nanolett.6b02691
The decreasing cost of silicon-based photovoltaics has enabled significant increases in solar electricity generation worldwide. Silicon photoanodes could also play an important role in the cost-effective generation of solar fuels, but the most successful methods of photoelectrode passivation and performance enhancement rely on a combination of precious metals and sophisticated processing methods that offset the economic arguments for silicon. Here we show that metal-free carbon-based nanomaterial coatings deposited from solution can protect silicon photoanodes carrying out the oxygen evolution reaction in a range of working environments. Purified semiconducting carbon nanotubes (CNTs) act as a hole extraction layer, and a graphene (Gr) capping layer both protects the CNT film and acts as a hole exchange layer with the electrolyte. The performance of semiconducting CNTs is found to be superior to that of metallic or unsorted CNTs in this context. Furthermore, the insertion of graphene oxide (GO) between the n-Si and CNTs reduces the overpotential relative to photoanodes with CNTs deposited on hydrogen-passivated silicon. The composite photoanode structure of n-Si/GO/CNT/Gr shows promising performance for oxygen evolution and excellent potential for improvement by optimizing the catalytic properties and stability of the graphene protective layer.Keywords: Carbon nanotube; graphene; graphene oxide; photoanode; solar fuel; water-splitting;
Co-reporter:Zhuhua Zhang, Andrew J. Mannix, Zhili Hu, Brian Kiraly, Nathan P. Guisinger, Mark C. Hersam, and Boris I. Yakobson
Nano Letters 2016 Volume 16(Issue 10) pp:6622-6627
Publication Date(Web):September 22, 2016
DOI:10.1021/acs.nanolett.6b03349
Two-dimensional (2D) materials tend to be mechanically flexible yet planar, especially when adhered on metal substrates. Here, we show by first-principles calculations that periodic nanoscale one-dimensional undulations can be preferred in borophenes on concertedly reconstructed Ag(111). This “wavy” configuration is more stable than its planar form on flat Ag(111) due to anisotropic high bending flexibility of borophene that is also well described by a continuum model. Atomic-scale ultrahigh vacuum scanning tunneling microscopy characterization of borophene grown on Ag(111) reveals such undulations, which agree with theory in terms of topography, wavelength, Moiré pattern, and prevalence of vacancy defects. Although the lattice is coherent within a borophene island, the undulations nucleated from different sides of the island form a distinctive domain boundary when they are laterally misaligned. This structural model suggests that the transfer of undulated borophene onto an elastomeric substrate would allow for high levels of stretchability and compressibility with potential applications to emerging stretchable and foldable devices.Keywords: atomic structure; Boron nanostructure; defect; density functional theory calculation; substrate; two-dimensional material;
Co-reporter:Joohoon Kang, Vinod K. Sangwan, Joshua D. Wood, Xiaolong Liu, Itamar Balla, David Lam, and Mark C. Hersam
Nano Letters 2016 Volume 16(Issue 11) pp:7216-7223
Publication Date(Web):October 4, 2016
DOI:10.1021/acs.nanolett.6b03584
Isopycnic density gradient ultracentrifugation (iDGU) has been widely applied to sort nanomaterials by their physical and electronic structure. However, the commonly used density-gradient medium iodixanol has a finite maximum buoyant density that prevents the use of iDGU for high-density nanomaterials. Here, we overcome this limit by adding cesium chloride (CsCl) to iodixanol, thus increasing its maximum buoyant density to the point where the high-density two-dimensional nanomaterial rhenium disulfide (ReS2) can be sorted in a layer-by-layer manner with iDGU. The resulting aqueous ReS2 dispersions show photoluminescence at ∼1.5 eV, which is consistent with its direct bandgap semiconductor electronic structure. Furthermore, photocurrent measurements on thin films formed from solution-processed ReS2 show a spectral response that is consistent with optical absorbance and photoluminescence data. In addition to providing a pathway for effective solution processing of ReS2, this work establishes a general methodology for sorting high-density nanomaterials via iDGU.Keywords: cesium chloride; iodixanol; liquid-phase exfoliation; photocurrent; photoluminescence; Two-dimensional nanomaterials;
Co-reporter:Won-Kyu Lee, Junmo Kang, Kan-Sheng Chen, Clifford J. Engel, Woo-Bin Jung, Dongjoon Rhee, Mark C. Hersam, and Teri W. Odom
Nano Letters 2016 Volume 16(Issue 11) pp:7121-7127
Publication Date(Web):October 11, 2016
DOI:10.1021/acs.nanolett.6b03415
This paper describes how delamination-free, hierarchical patterning of graphene can be achieved on prestrained thermoplastic sheets by surface wrinkling. Conformal contact between graphene and the substrate during strain relief was maintained by the presence of a soft skin layer, resulting in the uniform patterning of three-dimensional wrinkles over large areas (>cm2). The graphene wrinkle wavelength was tuned from the microscale to the nanoscale by controlling the thickness of the skin layer with 1 nm accuracy to realize a degree of control not possible by crumpling, which relies on delamination. Hierarchical patterning of the skin layers with varying thicknesses enabled multiscale graphene wrinkles with predetermined orientations to be formed. Significantly, hierarchical graphene wrinkles exhibited tunable mechanical stiffness at the nanoscale without compromising the macroscale electrical conductivity.Keywords: conductive atomic force microscopy; Graphene; hierarchical patterning; polystyrene; texturing; wrinkles;
Co-reporter:Ethan B. Secor, Jeremy Smith, Tobin J. Marks, and Mark C. Hersam
ACS Applied Materials & Interfaces 2016 Volume 8(Issue 27) pp:17428-17434
Publication Date(Web):June 21, 2016
DOI:10.1021/acsami.6b02730
Recent developments in solution-processed amorphous oxide semiconductors have established indium-gallium-zinc-oxide (IGZO) as a promising candidate for printed electronics. A key challenge for this vision is the integration of IGZO thin-film transistor (TFT) channels with compatible source/drain electrodes using low-temperature, solution-phase patterning methods. Here we demonstrate the suitability of inkjet-printed graphene electrodes for this purpose. In contrast to common inkjet-printed silver-based conductive inks, graphene provides a chemically stable electrode-channel interface. Furthermore, by embedding the graphene electrode between two consecutive IGZO printing passes, high-performance IGZO TFTs are achieved with an electron mobility of ∼6 cm2/V·s and current on/off ratio of ∼105. The resulting printed devices exhibit robust stability to aging in ambient as well as excellent resilience to thermal stress, thereby offering a promising platform for future printed electronics applications.
Co-reporter:Heather N. Arnold, Cory D. Cress, Julian J. McMorrow, Scott W. Schmucker, Vinod K. Sangwan, Laila Jaber-Ansari, Rajan Kumar, Kanan P. Puntambekar, Kyle A. Luck, Tobin J. Marks, and Mark C. Hersam
ACS Applied Materials & Interfaces 2016 Volume 8(Issue 8) pp:5058
Publication Date(Web):February 16, 2016
DOI:10.1021/acsami.5b12259
Solution-processed semiconductor and dielectric materials are attractive for future lightweight, low-voltage, flexible electronics, but their response to ionizing radiation environments is not well understood. Here, we investigate the radiation response of graphene field-effect transistors employing multilayer, solution-processed zirconia self-assembled nanodielectrics (Zr-SANDs) with ZrOx as a control. Total ionizing dose (TID) testing is carried out in situ using a vacuum ultraviolet source to a total radiant exposure (RE) of 23.1 μJ/cm2. The data reveal competing charge density accumulation within and between the individual dielectric layers. Additional measurements of a modified Zr-SAND show that varying individual layer thicknesses within the gate dielectric tuned the TID response. This study thus establishes that the radiation response of graphene electronics can be tailored to achieve a desired radiation sensitivity by incorporating hybrid organic–inorganic gate dielectrics.Keywords: chemical vapor deposition graphene; field-effect transistor; hybrid dielectrics; low-voltage electronics; radiation effects; total ionizing dose
Co-reporter:Tejas A. Shastry, Patrick E. Hartnett, Michael R. Wasielewski, Tobin J. Marks, and Mark C. Hersam
ACS Energy Letters 2016 Volume 1(Issue 3) pp:548
Publication Date(Web):August 17, 2016
DOI:10.1021/acsenergylett.6b00291
Polymer solar cells (PSCs) have achieved power conversion efficiencies exceeding 10%, but their performance has been limited under concentrated sunlight because of poor stability and recombination processes despite their potential for low-cost concentrated solar power. Recently, ternary polymer solar cell blends have been explored as a strategy to improve PSC performance; however, this approach has been demonstrated only for polymer–fullerene solar cells with organic ternary additives and has not addressed stability issues under supersolar irradiation. Here, we present the first polymer solar cells comprising ternary blends of high efficiency polymers, nonfullerene perylenediimide acceptors, and semiconducting single-walled carbon nanotube additives. We find that the addition of carbon nanotubes reduces efficiency-degrading recombination and improves performance and photostability, most notably under concentrated sunlight exceeding 10 suns. The utilization of carbon nanomaterials as ternary additives in organic photovoltaics creates new opportunities for integrating solution-processed solar cells with energy conversion systems based on solar concentrators.
Co-reporter:Tejas A. Shastry, Itamar Balla, Hadallia Bergeron, Samuel H. Amsterdam, Tobin J. Marks, and Mark C. Hersam
ACS Nano 2016 Volume 10(Issue 11) pp:10573
Publication Date(Web):October 26, 2016
DOI:10.1021/acsnano.6b06592
Two-dimensional transition metal dichalcogenides (TMDCs) have recently attracted attention due to their superlative optical and electronic properties. In particular, their extraordinary optical absorption and semiconducting band gap have enabled demonstrations of photovoltaic response from heterostructures composed of TMDCs and other organic or inorganic materials. However, these early studies were limited to devices at the micrometer scale and/or failed to exploit the unique optical absorption properties of single-layer TMDCs. Here we present an experimental realization of a large-area type-II photovoltaic heterojunction using single-layer molybdenum disulfide (MoS2) as the primary absorber, by coupling it to the organic π-donor polymer PTB7. This TMDC–polymer heterojunction exhibits photoluminescence intensity that is tunable as a function of the thickness of the polymer layer, ultimately enabling complete quenching of the TMDC photoluminescence. The strong optical absorption in the TMDC–polymer heterojunction produces an internal quantum efficiency exceeding 40% for an overall cell thickness of less than 20 nm, resulting in exceptional current density per absorbing thickness in comparison to other organic and inorganic solar cells. Furthermore, this work provides insight into the recombination processes in type-II TMDC–polymer heterojunctions and thus provides quantitative guidance to ongoing efforts to realize efficient TMDC-based solar cells.Keywords: heterojunction solar cell; photoluminescence; photovoltaic effect; transition metal dichalcogenide; two-dimensional materials
Co-reporter:Christopher R. Ryder, Joshua D. Wood, Spencer A. Wells, and Mark C. Hersam
ACS Nano 2016 Volume 10(Issue 4) pp:3900
Publication Date(Web):March 28, 2016
DOI:10.1021/acsnano.6b01091
Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) and black phosphorus (BP) have beneficial electronic, optical, and physical properties at the few-layer limit. As atomically thin materials, 2D TMDCs and BP are highly sensitive to their environment and chemical modification, resulting in a strong dependence of their properties on substrate effects, intrinsic defects, and extrinsic adsorbates. Furthermore, the integration of 2D semiconductors into electronic and optoelectronic devices introduces unique challenges at metal–semiconductor and dielectric–semiconductor interfaces. Here, we review emerging efforts to understand and exploit chemical effects to influence the properties of 2D TMDCs and BP. In some cases, surface chemistry leads to significant degradation, thus necessitating the development of robust passivation schemes. On the other hand, appropriately designed chemical modification can be used to beneficially tailor electronic properties, such as controlling doping levels and charge carrier concentrations. Overall, chemical methods allow substantial tunability of the properties of 2D TMDCs and BP, thereby enabling significant future opportunities to optimize performance for device applications.Keywords: anisotropy; chemistry; contacts; covalent; electronics; excitons; noncovalent; optoelectronics
Co-reporter:Xiaolong Liu, Itamar Balla, Hadallia Bergeron, Gavin P. Campbell, Michael J. Bedzyk, and Mark C. Hersam
ACS Nano 2016 Volume 10(Issue 1) pp:1067
Publication Date(Web):November 13, 2015
DOI:10.1021/acsnano.5b06398
Atomically thin MoS2/graphene heterostructures are promising candidates for nanoelectronic and optoelectronic technologies. Among different graphene substrates, epitaxial graphene (EG) on SiC provides several potential advantages for such heterostructures, including high electronic quality, tunable substrate coupling, wafer-scale processability, and crystalline ordering that can template commensurate growth. Exploiting these attributes, we demonstrate here the thickness-controlled van der Waals epitaxial growth of MoS2 on EG via chemical vapor deposition, giving rise to transfer-free synthesis of a two-dimensional heterostructure with registry between its constituent materials. The rotational commensurability observed between the MoS2 and EG is driven by the energetically favorable alignment of their respective lattices and results in nearly strain-free MoS2, as evidenced by synchrotron X-ray scattering and atomic-resolution scanning tunneling microscopy (STM). The electronic nature of the MoS2/EG heterostructure is elucidated with STM and scanning tunneling spectroscopy, which reveals bias-dependent apparent thickness, band bending, and a reduced band gap of ∼0.4 eV at the monolayer MoS2 edges.Keywords: chemical vapor deposition; scanning tunneling microscopy; silicon carbide; synchrotron X-ray scattering; transition metal dichalcogenide; van der Waals heterostructure;
Co-reporter:Xiaolong Liu, Itamar Balla, Hadallia Bergeron, and Mark C. Hersam
The Journal of Physical Chemistry C 2016 Volume 120(Issue 37) pp:20798-20805
Publication Date(Web):March 22, 2016
DOI:10.1021/acs.jpcc.6b02073
With reduced degrees of freedom, structural defects are expected to play a greater role in two-dimensional materials in comparison to their bulk counterparts. In particular, mechanical strength, electronic properties, and chemical reactivity are strongly affected by crystal imperfections in the atomically thin limit. Here, ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) and spectroscopy (STS) are employed to interrogate point and line defects in monolayer MoS2 grown on epitaxial graphene (EG) at the atomic scale. Five types of point defects are observed with the majority species showing apparent structures that are consistent with vacancy and interstitial models. The total defect density is observed to be lower than MoS2 grown on other substrates and is likely attributed to the van der Waals epitaxy of MoS2 on EG. Grain boundaries (GBs) with 30° and 60° tilt angles resulting from the rotational commensurability of MoS2 on EG are more easily resolved by STM than atomic force microscopy at similar scales due to the enhanced contrast from their distinct electronic states. For example, band gap reduction to ∼0.8 and ∼0.5 eV is observed with STS for 30° and 60° GBs, respectively. In addition, atomic resolution STM images of these GBs are found to agree well with proposed structure models. This work offers quantitative insight into the structure and properties of common defects in MoS2 and suggests pathways for tailoring the performance of MoS2/graphene heterostructures via defect engineering.
Co-reporter:Kayla S. B. Culver, Yu Jin Shin, Matthew W. Rotz, Thomas J. Meade, Mark C. Hersam, and Teri W. Odom
The Journal of Physical Chemistry C 2016 Volume 120(Issue 38) pp:22103-22109
Publication Date(Web):September 13, 2016
DOI:10.1021/acs.jpcc.6b08362
Gold nanostars functionalized with Gd(III) have shown significant promise as contrast agents for magnetic resonance imaging (MRI) because of their anisotropic, branched shape. However, the size and shape polydispersity of as-synthesized gold nanostars have precluded efforts to develop a rigorous relationship between the gold nanostar structure (e.g., number of branches) and relaxivity of surface-bound Gd(III). This paper describes the use of a centrifugal separation method that can produce structurally refined populations of gold nanostars and is compatible with Gd(III) functionalization. Combined transmission electron microscopy and relaxivity analyses revealed that the increased number of nanostar branches was correlated with enhanced relaxivity. By identifying the underlying relaxivity mechanisms for Gd(III)-functionalized gold nanostars, we can inform the design of high-performance MRI contrast agents.
Co-reporter:Kyle A. Luck, Heather N. Arnold, Tejas A. Shastry, Tobin J. Marks, and Mark C. Hersam
The Journal of Physical Chemistry Letters 2016 Volume 7(Issue 20) pp:4223-4229
Publication Date(Web):October 10, 2016
DOI:10.1021/acs.jpclett.6b02079
Polyfluorenes have achieved noteworthy performance in organic electronic devices but exhibit undesired green band emission under photo-oxidative conditions that have limited their broad utility in optoelectronic applications. In addition, polyfluorenes are well-known dispersants of single-walled carbon nanotubes (SWCNTs), although the influence of SWCNTs on polyfluorene photo-oxidative stability has not yet been defined. Here we quantitatively explore the photophysical properties of poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) under photo-oxidative conditions when it is in van der Waals contact with SWCNTs. Photoluminescence spectroscopy tracks the spectral evolution of the polymer emission following ambient ultraviolet (UV) exposure, confirming that PFN exhibits green band emission. In marked contrast, PFN-wrapped SWCNTs possess high spectral stability without green band emission under the same ambient UV exposure conditions. By investigating a series of PFN thin films as a function of SWCNT content, it is shown that SWCNT loadings as low as ∼23 wt % suppress photo-oxidative degradation. These findings suggest that PFN–SWCNT composites provide an effective pathway toward utilizing polyfluorenes in organic optoelectronics.
Co-reporter:Joohoon Kang;Spencer A. Wells;Joshua D. Wood;Jae-Hyeok Lee;Xiaolong Liu;Christopher R. Ryder;Jian Zhu;Jeffrey R. Guest;Chad A. Husko
PNAS 2016 Volume 113 (Issue 42 ) pp:11688-11693
Publication Date(Web):2016-10-18
DOI:10.1073/pnas.1602215113
Understanding and exploiting the remarkable optical and electronic properties of phosphorene require mass production methods
that avoid chemical degradation. Although solution-based strategies have been developed for scalable exfoliation of black
phosphorus, these techniques have thus far used anhydrous organic solvents in an effort to minimize exposure to known oxidants,
but at the cost of limited exfoliation yield and flake size distribution. Here, we present an alternative phosphorene production
method based on surfactant-assisted exfoliation and postprocessing of black phosphorus in deoxygenated water. From comprehensive
microscopic and spectroscopic analysis, this approach is shown to yield phosphorene dispersions that are stable, highly concentrated,
and comparable to micromechanically exfoliated phosphorene in structure and chemistry. Due to the high exfoliation efficiency
of this process, the resulting phosphorene flakes are thinner than anhydrous organic solvent dispersions, thus allowing the
observation of layer-dependent photoluminescence down to the monolayer limit. Furthermore, to demonstrate preservation of
electronic properties following solution processing, the aqueous-exfoliated phosphorene flakes are used in field-effect transistors
with high drive currents and current modulation ratios. Overall, this method enables the isolation and mass production of
few-layer phosphorene, which will accelerate ongoing efforts to realize a diverse range of phosphorene-based applications.
Co-reporter:Ethan B. Secor;Bok Y. Ahn;Theodore Z. Gao;Jennifer A. Lewis
Advanced Materials 2015 Volume 27( Issue 42) pp:6683-6688
Publication Date(Web):
DOI:10.1002/adma.201502866
Co-reporter:Laila Jaber-Ansari;Kanan P. Puntambekar;Soo Kim;Muratahan Aykol;Langli Luo;Jinsong Wu;Benjamin D. Myers;Hakim Iddir;John T. Russell;Spencer J. Saldaña;Rajan Kumar;Michael M. Thackeray;Larry A. Curtiss;Vinayak P. Dravid;Chris Wolverton
Advanced Energy Materials 2015 Volume 5( Issue 17) pp:
Publication Date(Web):
DOI:10.1002/aenm.201500646
Spinel-structured LiMn2O4 (LMO) is a desirable cathode material for Li-ion batteries due to its low cost, abundance, and high power capability. However, LMO suffers from limited cycle life that is triggered by manganese dissolution into the electrolyte during electrochemical cycling. Here, it is shown that single-layer graphene coatings suppress manganese dissolution, thus enhancing the performance and lifetime of LMO cathodes. Relative to lithium cells with uncoated LMO cathodes, cells with graphene-coated LMO cathodes provide improved capacity retention with enhanced cycling stability. X-ray photoelectron spectroscopy reveals that graphene coatings inhibit manganese depletion from the LMO surface. Additionally, transmission electron microscopy demonstrates that a stable solid electrolyte interphase is formed on graphene, which screens the LMO from direct contact with the electrolyte. Density functional theory calculations provide two mechanisms for the role of graphene in the suppression of manganese dissolution. First, common defects in single-layer graphene are found to allow the transport of lithium while concurrently acting as barriers for manganese diffusion. Second, graphene can chemically interact with Mn3+ at the LMO electrode surface, promoting an oxidation state change to Mn4+, which suppresses dissolution.
Co-reporter:Deep Jariwala, Vinod K. Sangwan, Jung-Woo Ted Seo, Weichao Xu, Jeremy Smith, Chris H. Kim, Lincoln J. Lauhon, Tobin J. Marks, and Mark C. Hersam
Nano Letters 2015 Volume 15(Issue 1) pp:416-421
Publication Date(Web):December 1, 2014
DOI:10.1021/nl5037484
The emergence of semiconducting materials with inert or dangling bond-free surfaces has created opportunities to form van der Waals heterostructures without the constraints of traditional epitaxial growth. For example, layered two-dimensional (2D) semiconductors have been incorporated into heterostructure devices with gate-tunable electronic and optical functionalities. However, 2D materials present processing challenges that have prevented these heterostructures from being produced with sufficient scalability and/or homogeneity to enable their incorporation into large-area integrated circuits. Here, we extend the concept of van der Waals heterojunctions to semiconducting p-type single-walled carbon nanotube (s-SWCNT) and n-type amorphous indium gallium zinc oxide (a-IGZO) thin films that can be solution-processed or sputtered with high spatial uniformity at the wafer scale. The resulting large-area, low-voltage p–n heterojunctions exhibit antiambipolar transfer characteristics with high on/off ratios that are well-suited for electronic, optoelectronic, and telecommunication technologies.
Co-reporter:Jian Zhu, Joohoon Kang, Junmo Kang, Deep Jariwala, Joshua D. Wood, Jung-Woo T. Seo, Kan-Sheng Chen, Tobin J. Marks, and Mark C. Hersam
Nano Letters 2015 Volume 15(Issue 10) pp:7029-7036
Publication Date(Web):September 8, 2015
DOI:10.1021/acs.nanolett.5b03075
Gate dielectrics directly affect the mobility, hysteresis, power consumption, and other critical device metrics in high-performance nanoelectronics. With atomically flat and dangling bond-free surfaces, hexagonal boron nitride (h-BN) has emerged as an ideal dielectric for graphene and related two-dimensional semiconductors. While high-quality, atomically thin h-BN has been realized via micromechanical cleavage and chemical vapor deposition, existing liquid exfoliation methods lack sufficient control over h-BN thickness and large-area film quality, thus limiting its use in solution-processed electronics. Here, we employ isopycnic density gradient ultracentrifugation for the preparation of monodisperse, thickness-sorted h-BN inks, which are subsequently layer-by-layer assembled into ultrathin dielectrics with low leakage currents of 3 × 10–9 A/cm2 at 2 MV/cm and high capacitances of 245 nF/cm2. The resulting solution-processed h-BN dielectric films enable the fabrication of graphene field-effect transistors with negligible hysteresis and high mobilities up to 7100 cm2 V–1 s–1 at room temperature. These h-BN inks can also be used as coatings on conventional dielectrics to minimize the effects of underlying traps, resulting in improvements in overall device performance. Overall, this approach for producing and assembling h-BN dielectric inks holds significant promise for translating the superlative performance of two-dimensional heterostructure devices to large-area, solution-processed nanoelectronics.
Co-reporter:Brian Kiraly, Robert M. Jacobberger, Andrew J. Mannix, Gavin P. Campbell, Michael J. Bedzyk, Michael S. Arnold, Mark C. Hersam, and Nathan P. Guisinger
Nano Letters 2015 Volume 15(Issue 11) pp:7414-7420
Publication Date(Web):October 27, 2015
DOI:10.1021/acs.nanolett.5b02833
Epitaxially oriented wafer-scale graphene grown directly on semiconducting Ge substrates is of high interest for both fundamental science and electronic device applications. To date, however, this material system remains relatively unexplored structurally and electronically, particularly at the atomic scale. To further understand the nature of the interface between graphene and Ge, we utilize ultrahigh vacuum scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) along with Raman and X-ray photoelectron spectroscopy to probe interfacial atomic structure and chemistry. STS reveals significant differences in electronic interactions between graphene and Ge(110)/Ge(111), which is consistent with a model of stronger interaction on Ge(110) leading to epitaxial growth. Raman spectra indicate that the graphene is considerably strained after growth, with more point-to-point variation on Ge(111). Furthermore, this native strain influences the atomic structure of the interface by inducing metastable and previously unobserved Ge surface reconstructions following annealing. These nonequilibrium reconstructions cover >90% of the surface and, in turn, modify both the electronic and mechanical properties of the graphene overlayer. Finally, graphene on Ge(001) represents the extreme strain case, where graphene drives the reorganization of the Ge surface into [107] facets. From this work, it is clear that the interaction between graphene and the underlying Ge is not only dependent on the substrate crystallographic orientation, but is also tunable and strongly related to the atomic reconfiguration of the graphene–Ge interface.
Co-reporter:Brian Kiraly, Andrew J. Mannix, Mark C. Hersam, and Nathan P. Guisinger
Chemistry of Materials 2015 Volume 27(Issue 17) pp:6085
Publication Date(Web):August 27, 2015
DOI:10.1021/acs.chemmater.5b02602
The integration of heterogeneous two-dimensional materials has the potential to yield electronic behavior approaching theoretical limits and facilitate the exploration of new fundamental physical phenomena. Here, we report the integration of graphene with two-dimensional, semiconducting crystalline silicon. Sequential deposition of carbon and silicon on Ag(111) in ultrahigh vacuum results in the synthesis of both lateral and vertical graphene–silicon heterostructures. The one-dimensional in-plane interfaces demonstrate atomically precise material transitions both structurally and electronically. The vertical heterostructures show noninteracting van der Waals behavior as shown by energetically resolved scanning tunneling microscopy coupled with ex-situ Raman analysis. The pristine and direct integration of graphene with two-dimensional, semiconducting crystalline silicon couples two of the most studied electronic materials into a hybrid structure with high potential for next-generation nanoelectronics.
Co-reporter:Julian J. McMorrow, Amanda R. Walker, Vinod K. Sangwan, Deep Jariwala, Emily Hoffman, Ken Everaerts, Antonio Facchetti, Mark C. Hersam, and Tobin J. Marks
ACS Applied Materials & Interfaces 2015 Volume 7(Issue 48) pp:26360
Publication Date(Web):October 19, 2015
DOI:10.1021/acsami.5b07744
The coupling of hybrid organic–inorganic gate dielectrics with emergent unconventional semiconductors has yielded transistor devices exhibiting record-setting transport properties. However, extensive electronic transport measurements on these high-capacitance systems are often convoluted with the electronic response of the semiconducting silicon substrate. In this report, we demonstrate the growth of solution-processed zirconia self-assembled nanodielectrics (Zr-SAND) on template-stripped aluminum substrates. The resulting Zr-SAND on Al structures leverage the ultrasmooth (r.m.s. roughness <0.4 nm), chemically uniform nature of template-stripped metal substrates to demonstrate the same exceptional electronic uniformity (capacitance ∼700 nF cm–2, leakage current <1 μA cm–2 at −2 MV cm–1) and multilayer growth of Zr-SAND on Si, while exhibiting superior temperature and voltage capacitance responses. These results are important to conduct detailed transport measurements in emergent transistor technologies featuring SAND as well as for future applications in integrated circuits or flexible electronics.Keywords: capacitor; nanodielectric; self-assembly; template strip; unconventional electronics
Co-reporter:Maogang Gong, Tejas A. Shastry, Qiannan Cui, Ryan R. Kohlmeyer, Kyle A. Luck, Andrew Rowberg, Tobin J. Marks, Michael F. Durstock, Hui Zhao, Mark C. Hersam, and Shenqiang Ren
ACS Applied Materials & Interfaces 2015 Volume 7(Issue 13) pp:7428
Publication Date(Web):March 23, 2015
DOI:10.1021/acsami.5b01536
Semiconducting single-walled carbon nanotube/fullerene bulk heterojunctions exhibit unique optoelectronic properties highly suitable for flexible, efficient, and robust photovoltaics and photodetectors. We investigate charge-transfer dynamics in inverted devices featuring a polyethylenimine-coated ZnO nanowire array infiltrated with these blends and find that trap-assisted recombination dominates transport within the blend and at the active layer/nanowire interface. We find that electrode modifiers suppress this recombination, leading to high performance.Keywords: bulk heterojunction; charge transfer; fullerenes; mechanism; nanocarbon
Co-reporter:Mark C. Hersam
The Journal of Physical Chemistry Letters 2015 Volume 6(Issue 14) pp:2738-2739
Publication Date(Web):July 16, 2015
DOI:10.1021/acs.jpclett.5b01218
Co-reporter:Xiaolong Liu; Joshua D. Wood; Kan-Sheng Chen; EunKyung Cho
The Journal of Physical Chemistry Letters 2015 Volume 6(Issue 5) pp:773-778
Publication Date(Web):February 9, 2015
DOI:10.1021/acs.jpclett.5b00043
With a semiconducting band gap and high charge carrier mobility, two-dimensional (2D) black phosphorus (BP)—often referred to as phosphorene—holds significant promise for next generation electronics and optoelectronics. However, as a 2D material, it possesses a higher surface area to volume ratio than bulk BP, suggesting that its chemical and thermal stability will be modified. Herein, an atomic-scale microscopic and spectroscopic study is performed to characterize the thermal degradation of mechanically exfoliated 2D BP. From in situ scanning/transmission electron microscopy, decomposition of 2D BP is observed to occur at ∼400 °C in vacuum, in contrast to the 550 °C bulk BP sublimation temperature. This decomposition initiates via eye-shaped cracks along the [001] direction and then continues until only a thin, amorphous red phosphorus like skeleton remains. In situ electron energy loss spectroscopy, energy-dispersive X-ray spectroscopy, and energy-loss near-edge structure changes provide quantitative insight into this chemical transformation process.
Co-reporter:Ethan B. Secor
The Journal of Physical Chemistry Letters 2015 Volume 6(Issue 4) pp:620-626
Publication Date(Web):January 28, 2015
DOI:10.1021/jz502431r
Carbon and post-carbon nanomaterials present desirable electrical, optical, chemical, and mechanical attributes for printed electronics, offering low-cost, large-area functionality on flexible substrates. In this Perspective, recent developments in carbon nanomaterial inks are highlighted. Monodisperse semiconducting single-walled carbon nanotubes compatible with inkjet and aerosol jet printing are ideal channels for thin-film transistors, while inkjet, gravure, and screen-printable graphene-based inks are better-suited for electrodes and interconnects. Despite the high performance achieved in prototype devices, additional effort is required to address materials integration issues encountered in more complex systems. In this regard, post-carbon nanomaterial inks (e.g., electrically insulating boron nitride and optically active transition-metal dichalcogenides) present promising opportunities. Finally, emerging work to extend these nanomaterial inks to three-dimensional printing provides a path toward nonplanar devices. Overall, the superlative properties of these materials, coupled with versatile assembly by printing techniques, offer a powerful platform for next-generation printed electronics.
Co-reporter:Satyaprasad P. Senanayak;Vinod K. Sangwan;Julian J. McMorrow;Ken Everaerts;Zhihua Chen;Antonio Facchetti;Tobin J. Marks;K. S. Narayan
Advanced Electronic Materials 2015 Volume 1( Issue 12) pp:
Publication Date(Web):
DOI:10.1002/aelm.201500226
Solution-processed polymer-based logic circuits are typically associated with high operating voltage and slow switching speeds. Here, polymer field-effect transistors (PFETs) fabricated on hybrid self-assembled nanodielectric (SAND) structures are reported, the latter consisting of alternating organic–inorganic layers exhibiting low leakage current (≈10−9 A cm−2) and high capacitance (≈0.8 μF cm−2). Suitable device engineering, controllable dielectric parameters, and interface energetics enable PFET operation at ±1 V, field-effect mobility (μ FET) > 2.0 cm2 V−1 s−1, subthreshold swing ≈100 mV dec−1, and switching response ≈150 ns. These performance parameters are orders of magnitude higher than similar devices fabricated from other polymer dielectrics. Inverter and NAND logic circuits fabricated from these SAND-based PFETs possess voltage gain up to 38 and maximum-frequency bandwidth of 2 MHz. A systematic study comparing different classes of dielectric and semiconducting material attributes the enhanced performance to improved relaxation dynamics of the SAND layer and tunable chemically functionalized interfaces.
Co-reporter:Joohoon Kang, Joshua D. Wood, Spencer A. Wells, Jae-Hyeok Lee, Xiaolong Liu, Kan-Sheng Chen, and Mark C. Hersam
ACS Nano 2015 Volume 9(Issue 4) pp:3596
Publication Date(Web):March 18, 2015
DOI:10.1021/acsnano.5b01143
Solution dispersions of two-dimensional (2D) black phosphorus (BP)—often referred to as phosphorene—are achieved by solvent exfoliation. These pristine, electronic-grade BP dispersions are produced with anhydrous organic solvents in a sealed-tip ultrasonication system, which circumvents BP degradation that would otherwise occur via solvated O2 or H2O. Among conventional solvents, N-methylpyrrolidone (NMP) is found to provide stable, highly concentrated (∼0.4 mg/mL) BP dispersions. Atomic force microscopy, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy show that the structure and chemistry of solvent-exfoliated BP nanosheets are comparable to mechanically exfoliated BP flakes. Additionally, residual NMP from the liquid-phase processing suppresses the rate of BP oxidation in ambient conditions. Solvent-exfoliated BP nanosheet field-effect transistors exhibit ambipolar behavior with current on/off ratios and mobilities up to ∼104 and ∼50 cm2 V–1 s–1, respectively. Overall, this study shows that stable, highly concentrated, electronic-grade 2D BP dispersions can be realized by scalable solvent exfoliation, thereby presenting opportunities for large-area, high-performance BP device applications.Keywords: anhydrous; centrifugation; degradation; field-effect transistor; liquid phase; organic solvent; phosphorene;
Co-reporter:Adam E. Jakus, Ethan B. Secor, Alexandra L. Rutz, Sumanas W. Jordan, Mark C. Hersam, and Ramille N. Shah
ACS Nano 2015 Volume 9(Issue 4) pp:4636
Publication Date(Web):April 10, 2015
DOI:10.1021/acsnano.5b01179
The exceptional properties of graphene enable applications in electronics, optoelectronics, energy storage, and structural composites. Here we demonstrate a 3D printable graphene (3DG) composite consisting of majority graphene and minority polylactide-co-glycolide, a biocompatible elastomer, 3D-printed from a liquid ink. This ink can be utilized under ambient conditions via extrusion-based 3D printing to create graphene structures with features as small as 100 μm composed of as few as two layers (<300 μm thick object) or many hundreds of layers (>10 cm thick object). The resulting 3DG material is mechanically robust and flexible while retaining electrical conductivities greater than 800 S/m, an order of magnitude increase over previously reported 3D-printed carbon materials. In vitro experiments in simple growth medium, in the absence of neurogenic stimuli, reveal that 3DG supports human mesenchymal stem cell (hMSC) adhesion, viability, proliferation, and neurogenic differentiation with significant upregulation of glial and neuronal genes. This coincides with hMSCs adopting highly elongated morphologies with features similar to axons and presynaptic terminals. In vivo experiments indicate that 3DG has promising biocompatibility over the course of at least 30 days. Surgical tests using a human cadaver nerve model also illustrate that 3DG has exceptional handling characteristics and can be intraoperatively manipulated and applied to fine surgical procedures. With this unique set of properties, combined with ease of fabrication, 3DG could be applied toward the design and fabrication of a wide range of functional electronic, biological, and bioelectronic medical and nonmedical devices.Keywords: 3D printing; graphene; neurogenesis; tissue engineering;
Co-reporter:Andrew J. Mannix;Xiang-Feng Zhou;Joshua D. Wood;Brian Kiraly;Diego Alducin;Benjamin D. Myers;Xiaolong Liu;Brandon L. Fisher;Ulises Santiago;Jeffrey R. Guest;Miguel Jose Yacaman;Arturo Ponce;Artem R. Oganov;Nathan P. Guisinger
Science 2015 Volume 350(Issue 6267) pp:1513-1516
Publication Date(Web):18 Dec 2015
DOI:10.1126/science.aad1080
Borophene: Boron in two dimensions
Although bulk allotropes of carbon and boron differ greatly, small clusters of these elements show remarkable similarities. Boron analogs of two-dimensional carbon allotropes such as graphene have been predicted. Now Mannix et al. report the formation of two-dimensional boron by depositing the elemental boron onto a silver surface under ultrahigh-vacuum conditions (see the Perspective by Sachdev). The graphene-like structure was buckled, weakly bonded to the substrate, and metallic.
Science, this issue p. 1513; see also p. 1468
Co-reporter:Young-Geun Ha, Ken Everaerts, Mark C. Hersam, and Tobin J. Marks
Accounts of Chemical Research 2014 Volume 47(Issue 4) pp:1019
Publication Date(Web):January 15, 2014
DOI:10.1021/ar4002262
Recent advances in semiconductor performance made possible by organic π-electron molecules, carbon-based nanomaterials, and metal oxides have been a central scientific and technological research focus over the past decade in the quest for flexible and transparent electronic products. However, advances in semiconductor materials require corresponding advances in compatible gate dielectric materials, which must exhibit excellent electrical properties such as large capacitance, high breakdown strength, low leakage current density, and mechanical flexibility on arbitrary substrates. Historically, conventional silicon dioxide (SiO2) has dominated electronics as the preferred gate dielectric material in complementary metal oxide semiconductor (CMOS) integrated transistor circuitry. However, it does not satisfy many of the performance requirements for the aforementioned semiconductors due to its relatively low dielectric constant and intransigent processability. High-k inorganics such as hafnium dioxide (HfO2) or zirconium dioxide (ZrO2) offer some increases in performance, but scientists have great difficulty depositing these materials as smooth films at temperatures compatible with flexible plastic substrates. While various organic polymers are accessible via chemical synthesis and readily form films from solution, they typically exhibit low capacitances, and the corresponding transistors operate at unacceptably high voltages. More recently, researchers have combined the favorable properties of high-k metal oxides and π-electron organics to form processable, structurally well-defined, and robust self-assembled multilayer nanodielectrics, which enable high-performance transistors with a wide variety of unconventional semiconductors.In this Account, we review recent advances in organic–inorganic hybrid gate dielectrics, fabricated by multilayer self-assembly, and their remarkable synergy with unconventional semiconductors. We first discuss the principals and functional importance of gate dielectric materials in thin-film transistor (TFT) operation. Next, we describe the design, fabrication, properties, and applications of solution-deposited multilayer organic–inorganic hybrid gate dielectrics, using self-assembly techniques, which provide bonding between the organic and inorganic layers. Finally, we discuss approaches for preparing analogous hybrid multilayers by vapor-phase growth and discuss the properties of these materials.
Co-reporter:Ethan B. Secor;Sooman Lim;Heng Zhang;C. Daniel Frisbie;Lorraine F. Francis
Advanced Materials 2014 Volume 26( Issue 26) pp:4533-4538
Publication Date(Web):
DOI:10.1002/adma.201401052
Co-reporter:Joshua D. Wood, Spencer A. Wells, Deep Jariwala, Kan-Sheng Chen, EunKyung Cho, Vinod K. Sangwan, Xiaolong Liu, Lincoln J. Lauhon, Tobin J. Marks, and Mark C. Hersam
Nano Letters 2014 Volume 14(Issue 12) pp:6964-6970
Publication Date(Web):November 7, 2014
DOI:10.1021/nl5032293
Unencapsulated, exfoliated black phosphorus (BP) flakes are found to chemically degrade upon exposure to ambient conditions. Atomic force microscopy, electrostatic force microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy are employed to characterize the structure and chemistry of the degradation process, suggesting that O2 saturated H2O irreversibly reacts with BP to form oxidized phosphorus species. This interpretation is further supported by the observation that BP degradation occurs more rapidly on hydrophobic octadecyltrichlorosilane self-assembled monolayers and on H-Si(111) versus hydrophilic SiO2. For unencapsulated BP field-effect transistors, the ambient degradation causes large increases in threshold voltage after 6 h in ambient, followed by a ∼103 decrease in FET current on/off ratio and mobility after 48 h. Atomic layer deposited AlOx overlayers effectively suppress ambient degradation, allowing encapsulated BP FETs to maintain high on/off ratios of ∼103 and mobilities of ∼100 cm2 V–1 s–1 for over 2 weeks in ambient conditions. This work shows that the ambient degradation of BP can be managed effectively when the flakes are sufficiently passivated. In turn, our strategy for enhancing BP environmental stability will accelerate efforts to implement BP in electronic and optoelectronic applications.
Co-reporter:Maogang Gong, Tejas A. Shastry, Yu Xie, Marco Bernardi, Daniel Jasion, Kyle A. Luck, Tobin J. Marks, Jeffrey C. Grossman, Shenqiang Ren, and Mark C. Hersam
Nano Letters 2014 Volume 14(Issue 9) pp:5308-5314
Publication Date(Web):August 7, 2014
DOI:10.1021/nl5027452
Single-walled carbon nanotubes (SWCNTs) have highly desirable attributes for solution-processable thin-film photovoltaics (TFPVs), such as broadband absorption, high carrier mobility, and environmental stability. However, previous TFPVs incorporating photoactive SWCNTs have utilized architectures that have limited current, voltage, and ultimately power conversion efficiency (PCE). Here, we report a solar cell geometry that maximizes photocurrent using polychiral SWCNTs while retaining high photovoltage, leading to record-high efficiency SWCNT–fullerene solar cells with average NREL certified and champion PCEs of 2.5% and 3.1%, respectively. Moreover, these cells show significant absorption in the near-infrared portion of the solar spectrum that is currently inaccessible by many leading TFPV technologies.
Co-reporter:Bongjun Kim, Seonpil Jang, Michael L. Geier, Pradyumna L. Prabhumirashi, Mark C. Hersam, and Ananth Dodabalapur
Nano Letters 2014 Volume 14(Issue 6) pp:3683-3687
Publication Date(Web):May 21, 2014
DOI:10.1021/nl5016014
The materials combination of inkjet-printed single-walled carbon nanotubes (SWCNTs) and zinc tin oxide (ZTO) is very promising for large-area thin-film electronics. We compare the characteristics of conventional complementary inverters and ring oscillators measured in air (with SWCNT p-channel field effect transistors (FETs) and ZTO n-channel FETs) with those of ambipolar inverters and ring oscillators comprised of bilayer SWCNT/ZTO FETs. This is the first such comparison between the performance characteristics of ambipolar and conventional inverters and ring oscillators. The measured signal delay per stage of 140 ns for complementary ring oscillators is the fastest for any ring oscillator circuit with printed semiconductors to date.
Co-reporter:Jordan M. Klingsporn ; Nan Jiang ; Eric A. Pozzi ; Matthew D. Sonntag ; Dhabih Chulhai ; Tamar Seideman ; Lasse Jensen ; Mark C. Hersam ;Richard P. Van Duyne
Journal of the American Chemical Society 2014 Volume 136(Issue 10) pp:3881-3887
Publication Date(Web):February 18, 2014
DOI:10.1021/ja411899k
Tip-enhanced Raman spectroscopy (TERS) provides chemical information for adsorbates with nanoscale spatial resolution, single-molecule sensitivity, and, when combined with scanning tunneling microscopy (STM), Ångstrom-scale topographic resolution. Performing TERS under ultrahigh-vacuum conditions allows pristine and atomically smooth surfaces to be maintained, while liquid He cooling minimizes surface diffusion of adsorbates across the solid surface, allowing direct STM imaging. Low-temperature TER (LT-TER) spectra differ from room-temperature TER (RT-TER), RT surface-enhanced Raman (SER), and LT-SER spectra because the vibrational lines are narrowed and shifted, revealing additional chemical information about adsorbate–substrate interactions. As an example, we present LT-TER spectra for the rhodamine 6G (R6G)/Ag(111) system that exhibit such unique spectral shifts. The high spectral resolution of LT-TERS provides intramolecular insight in that the shifted modes are associated with the ethylamine moiety of R6G. LT-TERS is a promising approach for unraveling the intricacies of adsorbate–substrate interactions that are inaccessible by other means.
Co-reporter:Charles Kiseok Song ; Kyle A. Luck ; Nanjia Zhou ; Li Zeng ; Henry M. Heitzer ; Eric F. Manley ; Samuel Goldman ; Lin X. Chen ; Mark A. Ratner ; Michael J. Bedzyk ; Robert P. H. Chang ; Mark C. Hersam ;Tobin J. Marks
Journal of the American Chemical Society 2014 Volume 136(Issue 51) pp:17762-17773
Publication Date(Web):November 24, 2014
DOI:10.1021/ja508453n
To achieve densely packed charge-selective organosilane-based interfacial layers (IFLs) on the tin-doped indium oxide (ITO) anodes of organic photovoltaic (OPV) cells, a series of Ar2N-(CH2)n-SiCl3 precursors with Ar = 3,4-difluorophenyl, n = 3, 6, 10, and 18, was synthesized, characterized, and chemisorbed on OPV anodes to serve as IFLs. To minimize lateral nonbonded -NAr2···Ar2N- repulsions which likely limit IFL packing densities in the resulting self-assembled monolayers (SAMs), precursor mixtures having both small and large n values are simultaneously deposited. These “heterogeneous” SAMs are characterized by a battery of techniques: contact angle measurements, X-ray reflectivity, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy (UPS), cyclic voltammetry, and DFT computation. It is found that the headgroup densities of these “supersaturated” heterogeneous SAMs (SHSAMs) are enhanced by as much as 17% versus their homogeneous counterparts. Supersaturation significantly modifies the IFL properties including the work function (as much as 16%) and areal dipole moment (as much as 49%). Bulk-heterojunction OPV devices are fabricated with these SHSAMs: ITO/IFL/poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][2-[[(2-ethylhexyl)oxy]carbonyl]-3-fluorothieno[3,4-b]thiophenediyl]]:phenyl-C71-butyric acid methyl ester (PTB7:PC71BM)/LiF/Al. OPVs having SHSAM IFLs exhibit significantly enhanced performance (PCE by 54%; Voc by 35%) due to enhanced charge selectivity and collection, with the PCE rivaling or exceeding that of PEDOT:PSS IFL devices −7.62%. The mechanism underlying the enhanced performance involves modified hole collection and selectivity efficiency inferred from the UPS data. The ITO/SAM/SHSAM surface potential imposed by the dipolar SAMs causes band bending and favorably alters the Schottky barrier height. Thus, interfacial charge selectivity and collection are enhanced as evident in the greater OPV Voc.
Co-reporter:Patrick E. Hartnett ; Amod Timalsina ; H. S. S. Ramakrishna Matte ; Nanjia Zhou ; Xugang Guo ; Wei Zhao ; Antonio Facchetti ; Robert P. H. Chang ; Mark C. Hersam ; Michael R. Wasielewski ;Tobin J. Marks
Journal of the American Chemical Society 2014 Volume 136(Issue 46) pp:16345-16356
Publication Date(Web):October 28, 2014
DOI:10.1021/ja508814z
Perylenediimide (PDI)-based acceptors offer a potential replacement for fullerenes in bulk-heterojunction (BHJ) organic photovoltaic cells (OPVs). The most promising efforts have focused on creating twisted PDI dimers to disrupt aggregation and thereby suppress excimer formation. Here, we present an alternative strategy for developing high-performance OPVs based on PDI acceptors that promote slip-stacking in the solid state, thus preventing the coupling necessary for rapid excimer formation. This packing structure is accomplished by substitution at the PDI 2,5,8,11-positions (“headland positions”). Using this design principle, three PDI acceptors, N,N-bis(n-octyl)-2,5,8,11-tetra(n-hexyl)-PDI (Hexyl-PDI), N,N-bis(n-octyl)-2,5,8,11-tetraphenethyl-PDI (Phenethyl-PDI), and N,N-bis(n-octyl)-2,5,8,11-tetraphenyl-PDI (Phenyl-PDI), were synthesized, and their molecular and electronic structures were characterized. They were then blended with the donor polymer PBTI3T, and inverted OPVs of the structure ITO/ZnO/Active Layer/MoO3/Ag were fabricated and characterized. Of these, 1:1 PBTI3T:Phenyl-PDI proved to have the best performance with Jsc = 6.56 mA/cm2, Voc = 1.024 V, FF = 54.59%, and power conversion efficiency (PCE) = 3.67%. Devices fabricated with Phenethyl-PDI and Hexyl-PDI have significantly lower performance. The thin film morphology and the electronic and photophysical properties of the three materials are examined, and although all three materials undergo efficient charge separation, PBTI3T:Phenyl-PDI is found to have the deepest LUMO, intermediate crystallinity, and the most well-mixed domains. This minimizes geminate recombination in Phenyl-PDI OPVs and affords the highest PCE. Thus, slip-stacked PDI strategies represent a promising approach to fullerene replacements in BHJ OPVs.
Co-reporter:Albert L. Lipson, Kanan Puntambekar, David J. Comstock, Xiangbo Meng, Michael L. Geier, Jeffrey W. Elam, and Mark C. Hersam
Chemistry of Materials 2014 Volume 26(Issue 2) pp:935
Publication Date(Web):January 7, 2014
DOI:10.1021/cm402451h
Application of a functional surface coating on Li-ion battery electrodes can potentially result in a significant enhancement of the lifespan of the battery cell. In particular, atomic layer deposition (ALD), which can create highly conformal ultrathin oxide films on many different electrodes has been shown to increase the cyclability in these systems. In this study, we explore the impact of such films on the formation of the solid electrolyte interphase (SEI), which may explain why these films show improvements in the cycling performance. Specifically, we characterize, using in situ scanning ion conductance microscopy and other ex situ surface characterization techniques, the SEI formed on ALD Al2O3 coated and uncoated MnO electrodes. We ascertain that ∼9 Å is the minimum thickness of ALD Al2O3 that will inhibit thick SEI formation. Furthermore, we show that the ALD surface coating is robust and prevents SEI formation for at least 100 cycles. Lastly, we investigated the differences between our in situ and ex situ measurements to help determine what artifacts can result that are due to post-processing for ex situ studies.
Co-reporter:Hunter J. Karmel, John J. Garramone, Jonathan D. Emery, Sumit Kewalramani, Michael J. Bedzyk and Mark C. Hersam
Chemical Communications 2014 vol. 50(Issue 64) pp:8852-8855
Publication Date(Web):13 Jun 2014
DOI:10.1039/C4CC02761B
Scanning tunnelling microscopy and X-ray reflectivity are used to characterize adlayers of perylenetetracarboxylic diimide (PTCDI) deposited on epitaxial graphene (EG) on SiC(0001). PTCDI adopts a herringbone structural phase on EG/SiC that can accommodate sub-5 nm voids with molecularly defined boundaries and isolated molecular vacancies at room temperature. The PTCDI monolayer remains intact up to substrate temperatures of ∼260 °C, thus demonstrating enhanced thermal stability compared to previously studied perylene derivatives on EG/SiC.
Co-reporter:Laila Jaber-Ansari, Kanan P. Puntambekar, Hadi Tavassol, Handan Yildirim, Alper Kinaci, Rajan Kumar, Spencer J. Saldaña, Andrew A. Gewirth, Jeffrey P. Greeley, Maria K. Y. Chan, and Mark C. Hersam
ACS Applied Materials & Interfaces 2014 Volume 6(Issue 20) pp:17626
Publication Date(Web):September 29, 2014
DOI:10.1021/am503715g
Despite rapidly growing interest in the application of graphene in lithium ion batteries, the interaction of the graphene with lithium ions and electrolyte species during electrochemical cycling is not fully understood. In this work, we use Raman spectroscopy in a model system of monolayer graphene transferred on a Si(111) substrate and density functional theory (DFT) to investigate defect formation as a function of lithiation. This model system enables the early stages of defect formation to be probed in a manner previously not possible with commonly used reduced graphene oxide or multilayer graphene substrates. Using ex situ and Ar-atmosphere Raman spectroscopy, we detected a rapid increase in graphene defect level for small increments in the number of lithiation/delithiation cycles until the I(D)/I(G) ratio reaches ∼1.5–2.0 and the 2D peak intensity drops by ∼50%, after which the Raman spectra show minimal changes upon further cycling. Using DFT, the interplay between graphene topological defects and chemical functionalization is explored, thus providing insight into the experimental results. In particular, the DFT results show that defects can act as active sites for species that are present in the electrochemical environment such as Li, O, and F. Furthermore, chemical functionalization with these species lowers subsequent defect formation energies, thus accelerating graphene degradation upon cycling. This positive feedback loop continues until the defect concentration reaches a level where lithium diffusion through the graphene can occur in a relatively unimpeded manner, with minimal further degradation upon extended cycling. Overall, this study provides mechanistic insight into graphene defect formation during lithiation, thus informing ongoing efforts to employ graphene in lithium ion battery technology.Keywords: defects; density functional theory; graphene; lithium ion battery; Raman spectroscopy; silicon
Co-reporter:Andrew J. Mannix, Brian Kiraly, Brandon L. Fisher, Mark C. Hersam, and Nathan P. Guisinger
ACS Nano 2014 Volume 8(Issue 7) pp:7538
Publication Date(Web):July 5, 2014
DOI:10.1021/nn503000w
Having fueled the microelectronics industry for over 50 years, silicon is arguably the most studied and influential semiconductor. With the recent emergence of two-dimensional (2D) materials (e.g., graphene, MoS2, phosphorene, etc.), it is natural to contemplate the behavior of Si in the 2D limit. Guided by atomic-scale studies utilizing ultrahigh vacuum (UHV), scanning tunneling microscopy (STM), and spectroscopy (STS), we have investigated the 2D limits of Si growth on Ag(111). In contrast to previous reports of a distinct sp2-bonded silicene allotrope, we observe the evolution of apparent surface alloys (ordered 2D silicon–Ag surface phases), which culminate in the precipitation of crystalline, sp3-bonded Si(111) nanosheets. These nanosheets are capped with a √3 honeycomb phase that is isostructural to a √3 honeycomb-chained-trimer (HCT) reconstruction of Ag on Si(111). Further investigations reveal evidence for silicon intermixing with the Ag(111) substrate followed by surface precipitation of crystalline, sp3-bonded silicon nanosheets. These conclusions are corroborated by ex situ atomic force microscopy (AFM), transmission electron microscopy (TEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Even at the 2D limit, scanning tunneling spectroscopy shows that the sp3-bonded silicon nanosheets exhibit semiconducting electronic properties.Keywords: molecular beam epitaxy; scanning tunneling microscopy; silicene; surface reconstruction; two-dimensional materials
Co-reporter:Deep Jariwala, Vinod K. Sangwan, Lincoln J. Lauhon, Tobin J. Marks, and Mark C. Hersam
ACS Nano 2014 Volume 8(Issue 2) pp:1102
Publication Date(Web):January 29, 2014
DOI:10.1021/nn500064s
With advances in exfoliation and synthetic techniques, atomically thin films of semiconducting transition metal dichalcogenides have recently been isolated and characterized. Their two-dimensional structure, coupled with a direct band gap in the visible portion of the electromagnetic spectrum, suggests suitability for digital electronics and optoelectronics. Toward that end, several classes of high-performance devices have been reported along with significant progress in understanding their physical properties. Here, we present a review of the architecture, operating principles, and physics of electronic and optoelectronic devices based on ultrathin transition metal dichalcogenide semiconductors. By critically assessing and comparing the performance of these devices with competing technologies, the merits and shortcomings of this emerging class of electronic materials are identified, thereby providing a roadmap for future development.Keywords: digital electronics; field-effect transistor; flexible electronics; light-emitting diode; molybdenum disulfide; nanoelectronics; optoelectronics; photodetector; photovoltaic; sensor; solar cell; valleytronics; van der Waals heterostructure
Co-reporter:Laila Jaber-Ansari, Hakim Iddir, Larry A. Curtiss, and Mark C. Hersam
ACS Nano 2014 Volume 8(Issue 3) pp:2399-2409
Publication Date(Web):February 8, 2014
DOI:10.1021/nn405921t
Single-walled carbon nanotubes (SWCNTs) have emerged as one of the leading additives for high-capacity nanocomposite lithium ion battery electrodes due to their ability to improve electrode conductivity, current collection efficiency, and charge/discharge rate for high power applications. However, since as-grown SWCNTs possess a distribution of physical and electronic structures, it is of high interest to determine which subpopulations of SWCNTs possess the highest lithiation capacity and to develop processing methods that can enhance the lithiation capacity of underperforming SWCNT species. Toward this end, SWCNT electronic type purity is controlled via density gradient ultracentrifugation, enabling a systematic study of the lithiation of SWCNTs as a function of metal versus semiconducting content. Experimentally, vacuum-filtered freestanding films of metallic SWCNTs are found to accommodate lithium with an order of magnitude higher capacity than their semiconducting counterparts, which is consistent with ab initio molecular dynamics and density functional theory calculations in the limit of isolated SWCNTs. In contrast, SWCNT film densification leads to the enhancement of the lithiation capacity of semiconducting SWCNTs to levels comparable to metallic SWCNTs, which is corroborated by theoretical calculations that show increased lithiation of semiconducting SWCNTs in the limit of small SWCNT–SWCNT spacing. Overall, these results will inform ongoing efforts to utilize SWCNTs as conductive additives in nanocomposite lithium ion battery electrodes.Keywords: ab initio molecular dynamics; density functional theory; density gradient ultracentrifugation; lithium ion battery; metallic; semiconducting;
Co-reporter:Andy H. Hung, Robert J. Holbrook, Matthew W. Rotz, Cameron J. Glasscock, Nikhita D. Mansukhani, Keith W. MacRenaris, Lisa M. Manus, Matthew C. Duch, Kevin T. Dam, Mark C. Hersam, and Thomas J. Meade
ACS Nano 2014 Volume 8(Issue 10) pp:10168
Publication Date(Web):September 16, 2014
DOI:10.1021/nn502986e
The delivery of bioactive molecules into cells has broad applications in biology and medicine. Polymer-modified graphene oxide (GO) has recently emerged as a de facto noncovalent vehicle for hydrophobic drugs. Here, we investigate a different approach using native GO to deliver hydrophilic molecules by co-incubation in culture. GO adsorption and delivery were systematically studied with a library of 15 molecules synthesized with Gd(III) labels to enable quantitation. Amines were revealed to be a key chemical group for adsorption, while delivery was shown to be quantitatively predictable by molecular adsorption, GO sedimentation, and GO size. GO co-incubation was shown to enhance delivery by up to 13-fold and allowed for a 100-fold increase in molecular incubation concentration compared to the alternative of nanoconjugation. When tested in the application of Gd(III) cellular MRI, these advantages led to a nearly 10-fold improvement in sensitivity over the state-of-the-art. GO co-incubation is an effective method of cellular delivery that is easily adoptable by researchers across all fields.Keywords: adsorption; cell culture; delivery vehicle; gadolinium; graphene oxide; sedimentation; surface interaction;
Co-reporter:In Soo Kim, Vinod K. Sangwan, Deep Jariwala, Joshua D. Wood, Spencer Park, Kan-Sheng Chen, Fengyuan Shi, Francisco Ruiz-Zepeda, Arturo Ponce, Miguel Jose-Yacaman, Vinayak P. Dravid, Tobin J. Marks, Mark C. Hersam, and Lincoln J. Lauhon
ACS Nano 2014 Volume 8(Issue 10) pp:10551
Publication Date(Web):September 15, 2014
DOI:10.1021/nn503988x
Ultrathin transition metal dichalcogenides (TMDCs) of Mo and W show great potential for digital electronics and optoelectronic applications. Whereas early studies were limited to mechanically exfoliated flakes, the large-area synthesis of 2D TMDCs has now been realized by chemical vapor deposition (CVD) based on a sulfurization reaction. The optoelectronic properties of CVD grown monolayer MoS2 have been intensively investigated, but the influence of stoichiometry on the electrical and optical properties has been largely overlooked. Here we systematically vary the stoichiometry of monolayer MoS2 during CVD via controlled sulfurization and investigate the associated changes in photoluminescence and electrical properties. X-ray photoelectron spectroscopy is employed to measure relative variations in stoichiometry and the persistence of MoOx species. As MoS2−δ is reduced (increasing δ), the field-effect mobility of monolayer transistors increases while the photoluminescence yield becomes nonuniform. Devices fabricated from monolayers with the lowest sulfur content have negligible hysteresis and a threshold voltage of ∼0 V. We conclude that the electrical and optical properties of monolayer MoS2 crystals can be tuned via stoichiometry engineering to meet the requirements of various applications.Keywords: chemical vapor deposition; field-effect mobility; molybdenum disulfide; photoluminescence; stoichiometry; transition metal dichalcogenides; X-ray photoelectron spectroscopy;
Co-reporter:Hunter J. Karmel, TeYu Chien, Vincent Demers-Carpentier, John J. Garramone, and Mark C. Hersam
The Journal of Physical Chemistry Letters 2014 Volume 5(Issue 2) pp:270-274
Publication Date(Web):December 24, 2013
DOI:10.1021/jz4025518
The development of graphene functionalization strategies that simultaneously achieve two-dimensional (2D) spatial periodicity and substrate registry is of critical importance for graphene-based nanoelectronics and related technologies. Here, we demonstrate the generation of a hydrogen-bonded molecularly thin organic heteromolecular nanoporous network on epitaxial graphene on SiC(0001) using room-temperature ultrahigh vacuum scanning tunneling microscopy. In particular, perylenetetracarboxylic diimide (PTCDI) and melamine are intermixed to form a spatially periodic 2D nanoporous network architecture with hexagonal symmetry and a lattice parameter of 3.45 ± 0.10 nm. The resulting adlayer is in registry with the underlying graphene substrate and possesses a characteristic domain size of 40–50 nm. This molecularly defined nanoporous network holds promise as a template for 2D ordered chemical modification of graphene at lengths scales relevant for graphene band structure engineering.Keywords: melamine; nanostructures; PTCDI; scanning tunneling microscopy; self-assembly; surface chemistry;
Co-reporter:Deep Jariwala, Vinod K. Sangwan, Lincoln J. Lauhon, Tobin J. Marks and Mark C. Hersam
Chemical Society Reviews 2013 vol. 42(Issue 7) pp:2824-2860
Publication Date(Web):05 Nov 2012
DOI:10.1039/C2CS35335K
In the last three decades, zero-dimensional, one-dimensional, and two-dimensional carbon nanomaterials (i.e., fullerenes, carbon nanotubes, and graphene, respectively) have attracted significant attention from the scientific community due to their unique electronic, optical, thermal, mechanical, and chemical properties. While early work showed that these properties could enable high performance in selected applications, issues surrounding structural inhomogeneity and imprecise assembly have impeded robust and reliable implementation of carbon nanomaterials in widespread technologies. However, with recent advances in synthesis, sorting, and assembly techniques, carbon nanomaterials are experiencing renewed interest as the basis of numerous scalable technologies. Here, we present an extensive review of carbon nanomaterials in electronic, optoelectronic, photovoltaic, and sensing devices with a particular focus on the latest examples based on the highest purity samples. Specific attention is devoted to each class of carbon nanomaterial, thereby allowing comparative analysis of the suitability of fullerenes, carbon nanotubes, and graphene for each application area. In this manner, this article will provide guidance to future application developers and also articulate the remaining research challenges confronting this field.
Co-reporter:James E. Johns and Mark C. Hersam
Accounts of Chemical Research 2013 Volume 46(Issue 1) pp:77
Publication Date(Web):October 2, 2012
DOI:10.1021/ar300143e
Although graphene’s physical structure is a single atom thick, two-dimensional, hexagonal crystal of sp2 bonded carbon, this simple description belies the myriad interesting and complex physical properties attributed to this fascinating material. Because of its unusual electronic structure and superlative properties, graphene serves as a leading candidate for many next generation technologies including high frequency electronics, broadband photodetectors, biological and gas sensors, and transparent conductive coatings. Despite this promise, researchers could apply graphene more routinely in real-world technologies if they could chemically adjust graphene’s electronic properties. For example, the covalent modification of graphene to create a band gap comparable to silicon (∼1 eV) would enable its use in digital electronics, and larger band gaps would provide new opportunities for graphene-based photonics. Toward this end, researchers have focused considerable effort on the chemical functionalization of graphene. Due to its high thermodynamic stability and chemical inertness, new methods and techniques are required to create covalent bonds without promoting undesirable side reactions or irreversible damage to the underlying carbon lattice.In this Account, we review and discuss recent theoretical and experimental work studying covalent modifications to graphene using gas phase atomic radicals. Atomic radicals have sufficient energy to overcome the kinetic and thermodynamic barriers associated with covalent reactions on the basal plane of graphene but lack the energy required to break the C–C sigma bonds that would destroy the carbon lattice. Furthermore, because they are atomic species, radicals substantially reduce the likelihood of unwanted side reactions that confound other covalent chemistries. Overall, these methods based on atomic radicals show promise for the homogeneous functionalization of graphene and the production of new classes of two-dimensional materials with fundamentally different electronic and physical properties.Specifically, we focus on recent studies of the addition of atomic hydrogen, fluorine, and oxygen to the basal plane of graphene. In each of these reactions, a high energy, activating step initiates the process, breaking the local π structure and distorting the surrounding lattice. Scanning tunneling microscopy experiments reveal that substrate mediated interactions often dominate when the initial binding event occurs. We then compare these substrate effects with the results of theoretical studies that typically assume a vacuum environment. As the surface coverage increases, clusters often form around the initial distortion, and the stoichiometric composition of the saturated end product depends strongly on both the substrate and reactant species. In addition to these chemical and structural observations, we review how covalent modification can extend the range of physical properties that are achievable in two-dimensional materials.
Co-reporter:Yu Jin Shin;Emilie Ringe;Michelle L. Personick;M. Ferna Cardinal;Chad A. Mirkin;Laurence D. Marks;Richard P. Van Duyne
Advanced Materials 2013 Volume 25( Issue 29) pp:4023-4027
Publication Date(Web):
DOI:10.1002/adma.201301278
Co-reporter:Justice M. P. Alaboson, Chun-Hong Sham, Sumit Kewalramani, Jonathan D. Emery, James E. Johns, Aparna Deshpande, TeYu Chien, Michael J. Bedzyk, Jeffrey W. Elam, Michael J. Pellin, and Mark C. Hersam
Nano Letters 2013 Volume 13(Issue 12) pp:5763-5770
Publication Date(Web):March 6, 2013
DOI:10.1021/nl4000932
Molecular-scale control over the integration of disparate materials on graphene is a critical step in the development of graphene-based electronics and sensors. Here, we report that self-assembled monolayers of 10,12-pentacosadiynoic acid (PCDA) on epitaxial graphene can be used to template the reaction and directed growth of atomic layer deposited (ALD) oxide nanostructures with sub-10 nm lateral resolution. PCDA spontaneously assembles into well-ordered domains consisting of one-dimensional molecular chains that coat the entire graphene surface in a manner consistent with the symmetry of the underlying graphene lattice. Subsequently, zinc oxide and alumina ALD precursors are shown to preferentially react with the functional moieties of PCDA, resulting in templated oxide nanostructures. The retention of the original one-dimensional molecular ordering following ALD is dependent on the chemical reaction pathway and the stability of the monolayer, which can be enhanced via ultraviolet-induced molecular cross-linking.
Co-reporter:Michael L. Geier, Pradyumna L. Prabhumirashi, Julian J. McMorrow, Weichao Xu, Jung-Woo T. Seo, Ken Everaerts, Chris H. Kim, Tobin J. Marks, and Mark C. Hersam
Nano Letters 2013 Volume 13(Issue 10) pp:4810-4814
Publication Date(Web):September 10, 2013
DOI:10.1021/nl402478p
In this Letter, we demonstrate thin-film single-walled carbon nanotube (SWCNT) complementary metal-oxide-semiconductor (CMOS) logic devices with subnanowatt static power consumption and full rail-to-rail voltage transfer characteristics as is required for logic gate cascading. These results are enabled by a local metal gate structure that achieves enhancement-mode p-type and n-type SWCNT thin-film transistors (TFTs) with widely separated and symmetric threshold voltages. These complementary SWCNT TFTs are integrated to demonstrate CMOS inverter, NAND, and NOR logic gates at supply voltages as low as 0.8 V with ideal rail-to-rail operation, subnanowatt static power consumption, high gain, and excellent noise immunity. This work provides a direct pathway for solution processable, large area, power efficient SWCNT advanced logic circuits and systems.
Co-reporter:Vinod K. Sangwan, Heather N. Arnold, Deep Jariwala, Tobin J. Marks, Lincoln J. Lauhon, and Mark C. Hersam
Nano Letters 2013 Volume 13(Issue 9) pp:4351-4355
Publication Date(Web):August 14, 2013
DOI:10.1021/nl402150r
Ubiquitous low-frequency 1/f noise can be a limiting factor in the performance and application of nanoscale devices. Here, we quantitatively investigate low-frequency electronic noise in single-layer transition metal dichalcogenide MoS2 field-effect transistors. The measured 1/f noise can be explained by an empirical formulation of mobility fluctuations with the Hooge parameter ranging between 0.005 and 2.0 in vacuum (<10–5 Torr). The field-effect mobility decreased, and the noise amplitude increased by an order of magnitude in ambient conditions, revealing the significant influence of atmospheric adsorbates on charge transport. In addition, single Lorentzian generation-recombination noise was observed to increase by an order of magnitude as the devices were cooled from 300 to 6.5 K.
Co-reporter:Vinod K. Sangwan, Deep Jariwala, Stephen A. Filippone, Hunter J. Karmel, James E. Johns, Justice M. P. Alaboson, Tobin J. Marks, Lincoln J. Lauhon, and Mark C. Hersam
Nano Letters 2013 Volume 13(Issue 3) pp:1162-1167
Publication Date(Web):February 7, 2013
DOI:10.1021/nl3045553
The full potential of graphene in integrated circuits can only be realized with a reliable ultrathin high-κ top-gate dielectric. Here, we report the first statistical analysis of the breakdown characteristics of dielectrics on graphene, which allows the simultaneous optimization of gate capacitance and the key parameters that describe large-area uniformity and dielectric strength. In particular, vertically heterogeneous and laterally homogeneous Al2O3 and HfO2 stacks grown via atomic-layer deposition and seeded by a molecularly thin perylene-3,4,9,10-tetracarboxylic dianhydride organic monolayer exhibit high uniformities (Weibull shape parameter β > 25) and large breakdown strengths (Weibull scale parameter, EBD > 7 MV/cm) that are comparable to control dielectrics grown on Si substrates.
Co-reporter:James E. Johns ; Justice M. P. Alaboson ; Sameer Patwardhan ; Christopher R. Ryder ; George C. Schatz
Journal of the American Chemical Society 2013 Volume 135(Issue 48) pp:18121-18125
Publication Date(Web):November 8, 2013
DOI:10.1021/ja408248z
Chemically interfacing the inert basal plane of graphene with other materials has limited the development of graphene-based catalysts, composite materials, and devices. Here, we overcome this limitation by chemically activating epitaxial graphene on SiC(0001) using atomic oxygen. Atomic oxygen produces epoxide groups on graphene, which act as reactive nucleation sites for zinc oxide nanoparticle growth using the atomic layer deposition precursor diethyl zinc. In particular, exposure of epoxidized graphene to diethyl zinc abstracts oxygen, creating mobile species that diffuse on the surface to form metal oxide clusters. This mechanism is corroborated with a combination of scanning probe microscopy, Raman spectroscopy, and density functional theory and can likely be generalized to a wide variety of related surface reactions on graphene.
Co-reporter:Ken Everaerts ; Jonathan D. Emery ; Deep Jariwala ; Hunter J. Karmel ; Vinod K. Sangwan ; Pradyumna L. Prabhumirashi ; Michael L. Geier ; Julian J. McMorrow ; Michael J. Bedzyk ; Antonio Facchetti ; Mark C. Hersam ;Tobin J. Marks
Journal of the American Chemical Society 2013 Volume 135(Issue 24) pp:8926-8939
Publication Date(Web):May 21, 2013
DOI:10.1021/ja4019429
Ambient and solution-processable, low-leakage, high capacitance gate dielectrics are of great interest for advances in low-cost, flexible, thin-film transistor circuitry. Here we report a new hafnium oxide-organic self-assembled nanodielectric (Hf-SAND) material consisting of regular, alternating π-electron layers of 4-[[4-[bis(2-hydroxyethyl)amino]phenyl]diazenyl]-1-[4-(diethoxyphosphoryl) benzyl]pyridinium bromide) (PAE) and HfO2 nanolayers. These Hf-SAND multilayers are grown from solution in ambient with processing temperatures ≤150 °C and are characterized by AFM, XPS, X-ray reflectivity (2.3 nm repeat spacing), X-ray fluorescence, cross-sectional TEM, and capacitance measurements. The latter yield the largest capacitance to date (1.1 μF/cm2) for a solid-state solution-processed hybrid inorganic–organic gate dielectric, with effective oxide thickness values as low as 3.1 nm and have gate leakage <10–7 A/cm2 at ±2 MV/cm using photolithographically patterned contacts (0.04 mm2). The sizable Hf-SAND capacitances are attributed to relatively large PAE coverages on the HfO2 layers, confirmed by X-ray reflectivity and X-ray fluorescence. Random network semiconductor-enriched single-walled carbon nanotube transistors were used to test Hf-SAND utility in electronics and afforded record on-state transconductances (5.5 mS) at large on:off current ratios (ION:IOFF) of ∼105 with steep 150 mV/dec subthreshold swings and intrinsic field-effect mobilities up to 137 cm2/(V s). Large-area devices (>0.2 mm2) on Hf-SAND (6.5 nm thick) achieve mA on currents at ultralow gate voltages (<1 V) with low gate leakage (<2 nA), highlighting the defect-free and conformal nature of this nanodielectric. High-temperature annealing in ambient (400 °C) has limited impact on Hf-SAND leakage densities (<10–6 A/cm2 at ±2 V) and enhances Hf-SAND multilayer capacitance densities to nearly 1 μF/cm2, demonstrating excellent compatibility with device postprocessing methodologies. These results represent a significant advance in hybrid organic–inorganic dielectric materials and suggest synthetic routes to even higher capacitance materials useful for unconventional electronics.
Co-reporter:Tejas A. Shastry ; Adam J. Morris-Cohen ; Emily A. Weiss
Journal of the American Chemical Society 2013 Volume 135(Issue 18) pp:6750-6753
Publication Date(Web):February 1, 2013
DOI:10.1021/ja312235n
Two-dimensional diffusion ordered spectroscopy (2D DOSY) NMR was used to probe the micellar structure of sodium dodecyl sulfate (SDS) and sodium cholate (SC) in aqueous solutions with and without semiconducting and metallic single-walled carbon nanotubes (SWCNTs). The solutions contain SDS and SC at weight ratios of 1:4 and 3:2, the ratios commonly used to isolate semiconducting and metallic SWCNTs through density gradient ultracentrifugation (DGU). These results show that the coverage of surfactant on the semiconducting and metallic SWCNTs is nearly identical in the 1:4 surfactant mixture, and a lower degree of bundling is responsible for the greater buoyancy of semiconducting SWCNTs. In the 3:2 surfactant mixture, the metallic SWCNTs are only encapsulated in SC while the semiconducting SWCNTs remain encapsulated in a poorly packed two-surfactant micelle, leading to a large buoyant density difference between the electronic species. This work provides insight into future directions to increase the purity of semiconducting and metallic SWCNTs sorted through DGU and demonstrates the utility of 2D DOSY NMR in probing SWCNT–surfactant complexes.
Co-reporter:Ken Everaerts, Li Zeng, Jonathan W. Hennek, Diana I. Camacho, Deep Jariwala, Michael J. Bedzyk, Mark C. Hersam, and Tobin J. Marks
ACS Applied Materials & Interfaces 2013 Volume 5(Issue 22) pp:11884
Publication Date(Web):November 4, 2013
DOI:10.1021/am403585n
Solution-processed amorphous oxide semiconductors (AOSs) are emerging as important electronic materials for displays and transparent electronics. We report here on the fabrication, microstructure, and performance characteristics of inkjet-printed, low-temperature combustion-processed, amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) grown on solution-processed hafnia self-assembled nanodielectrics (Hf-SANDs). TFT performance for devices processed below 300 °C includes >4× enhancement in electron mobility (μFE) on Hf-SAND versus SiO2 or ALD-HfO2 gate dielectrics, while other metrics such as subthreshold swing (SS), current on:off ratio (ION:IOFF), threshold voltage (Vth), and gate leakage current (Ig) are unchanged or enhanced. Thus, low voltage IGZO/SAND TFT operation (<2 V) is possible with ION:IOFF = 107, SS = 125 mV/dec, near-zero Vth, and large electron mobility, μFE(avg) = 20.6 ± 4.3 cm2 V–1 s–1, μFE(max) = 50 cm2 V–1 s–1. Furthermore, X-ray diffraction analysis indicates that the 300 °C IGZO combustion processing leaves the underlying Hf-SAND microstructure and capacitance intact. This work establishes the compatibility and advantages of all-solution, low-temperature fabrication of inkjet-printed, combustion-derived high-mobility IGZO TFTs integrated with self-assembled hybrid organic–inorganic nanodielectrics.Keywords: amorphous oxide field-effect transistor; electron mobility; hybrid dielectric; inkjet-printing; low-voltage electronics; thin-film transistor;
Co-reporter:Kyle A. Luck, Tejas A. Shastry, Stephen Loser, Gabriel Ogien, Tobin J. Marks and Mark C. Hersam
Physical Chemistry Chemical Physics 2013 vol. 15(Issue 48) pp:20966-20972
Publication Date(Web):08 Nov 2013
DOI:10.1039/C3CP54623C
Organic photovoltaics have the potential to serve as lightweight, low-cost, mechanically flexible solar cells. However, losses in efficiency as laboratory cells are scaled up to the module level have to date impeded large scale deployment. Here, we report that a 3-aminopropyltriethoxysilane (APTES) cathode interfacial treatment significantly enhances performance reproducibility in inverted high-efficiency PTB7:PC71BM organic photovoltaic cells, as demonstrated by the fabrication of 100 APTES-treated devices versus 100 untreated controls. The APTES-treated devices achieve a power conversion efficiency of 8.08 ± 0.12% with histogram skewness of −0.291, whereas the untreated controls achieve 7.80 ± 0.26% with histogram skewness of −1.86. By substantially suppressing the interfacial origins of underperforming cells, the APTES treatment offers a pathway for fabricating large-area modules with high spatial performance uniformity.
Co-reporter:Albert L. Lipson and Mark C. Hersam
The Journal of Physical Chemistry C 2013 Volume 117(Issue 16) pp:7953-7963
Publication Date(Web):February 26, 2013
DOI:10.1021/jp312594s
Recent years have seen a proliferation of scanning probe microscopy (SPM) techniques that can probe and manipulate a diverse range of materials and devices. In particular, SPM methods that employ a conductive tip are well suited for probing electronic and electrochemical phenomena of direct relevance to electronic and energy technologies. Conductive SPM is also a versatile nanofabrication tool, which can create nearly arbitrary nanopatterns of oxide, metals, and organics on solid substrates. In this Feature Article, we provide an overview of recent conductive SPM work from our laboratory regarding the characterization and nanopatterning of electronic and energy materials. The discussion begins by describing the methodologies used to characterize organic photovoltaics and transparent conducting oxides. We then illustrate how different SPM techniques are applied to the more complex electrochemical environments presented by Li-ion batteries and other electrochemical systems. Lastly, the use of conductive atomic force microscopy to probe and nanopattern electronically inhomogeneous substrates, such as epitaxial graphene layers on silicon carbide, is presented.
Co-reporter:Andy H. Hung, Matthew C. Duch, Giacomo Parigi, Matthew W. Rotz, Lisa M. Manus, Daniel J. Mastarone, Kevin T. Dam, Colton C. Gits, Keith W. MacRenaris, Claudio Luchinat, Mark C. Hersam, and Thomas J. Meade
The Journal of Physical Chemistry C 2013 Volume 117(Issue 31) pp:16263-16273
Publication Date(Web):July 17, 2013
DOI:10.1021/jp406909b
Gd(III) associated with carbon nanomaterials relaxes water proton spins at an effectiveness that approaches or exceeds the theoretical limit for a single bound water molecule. These Gd(III)-labeled materials represent a potential breakthrough in sensitivity for Gd(III)-based contrast agents used for magnetic resonance imaging (MRI). However, their mechanism of action remains unclear. A gadographene library encompassing GdCl3, two different Gd(III) complexes, graphene oxide (GO), and graphene suspended by two different surfactants and subjected to varying degrees of sonication was prepared and characterized for their relaxometric properties. Gadographene was found to perform comparably to other Gd(III)–carbon nanomaterials; its longitudinal (r1) and transverse (r2) relaxivity are modulated between 12–85 mM–1 s–1 and 24–115 mM–1 s–1, respectively, depending on the Gd(III)–carbon backbone combination. The unusually large relaxivity and its variance can be understood under the modified Florence model incorporating the Lipari–Szabo approach. Changes in hydration number (q), water residence time (τM), molecular tumbling rate (τR), and local motion (τfast) sufficiently explain most of the measured relaxivities. Furthermore, results implicated the coupling between graphene and Gd(III) as a minor contributor to proton spin relaxation.
Co-reporter:Vinod K. Sangwan;Deep Jariwala;Pradyumna L. Prabhumirashi;Michael L. Geier;Chung-Chiang Wu;Lincoln J. Lauhon;Tobin J. Marks
PNAS 2013 Volume 110 (Issue 45 ) pp:18076-18080
Publication Date(Web):2013-11-05
DOI:10.1073/pnas.1317226110
The p-n junction diode and field-effect transistor are the two most ubiquitous building blocks of modern electronics and optoelectronics.
In recent years, the emergence of reduced dimensionality materials has suggested that these components can be scaled down
to atomic thicknesses. Although high-performance field-effect devices have been achieved from monolayered materials and their
heterostructures, a p-n heterojunction diode derived from ultrathin materials is notably absent and constrains the fabrication
of complex electronic and optoelectronic circuits. Here we demonstrate a gate-tunable p-n heterojunction diode using semiconducting
single-walled carbon nanotubes (SWCNTs) and single-layer molybdenum disulfide as p-type and n-type semiconductors, respectively.
The vertical stacking of these two direct band gap semiconductors forms a heterojunction with electrical characteristics that
can be tuned with an applied gate bias to achieve a wide range of charge transport behavior ranging from insulating to rectifying
with forward-to-reverse bias current ratios exceeding 104. This heterojunction diode also responds strongly to optical irradiation with an external quantum efficiency of 25% and fast
photoresponse <15 μs. Because SWCNTs have a diverse range of electrical properties as a function of chirality and an increasing
number of atomically thin 2D nanomaterials are being isolated, the gate-tunable p-n heterojunction concept presented here
should be widely generalizable to realize diverse ultrathin, high-performance electronics and optoelectronics.
Co-reporter:Ethan B. Secor, Pradyumna L. Prabhumirashi, Kanan Puntambekar, Michael L. Geier, and Mark C. Hersam
The Journal of Physical Chemistry Letters 2013 Volume 4(Issue 8) pp:1347-1351
Publication Date(Web):April 8, 2013
DOI:10.1021/jz400644c
The ability to print high conductivity, conformal, and flexible electrodes is an important technological challenge in printed electronics, especially for large-area formats with low cost considerations. In this Letter, we demonstrate inkjet-printed, high conductivity graphene patterns that are suitable for flexible electronics. The ink is prepared by solution-phase exfoliation of graphene using an environmentally benign solvent, ethanol, and a stabilizing polymer, ethyl cellulose. The inkjet-printed graphene features attain low resistivity of 4 mΩ·cm after a thermal anneal at 250 °C for 30 min while showing uniform morphology, compatibility with flexible substrates, and excellent tolerance to bending stresses.Keywords: carbon; electrode; electronics; exfoliation; nanomaterial; solution-phase;
Co-reporter:Jung-Woo T. Seo, Nathan L. Yoder, Tejas A. Shastry, Jefford J. Humes, James E. Johns, Alexander A. Green, and Mark C. Hersam
The Journal of Physical Chemistry Letters 2013 Volume 4(Issue 17) pp:2805-2810
Publication Date(Web):August 7, 2013
DOI:10.1021/jz4013596
Arc discharge single-walled carbon nanotubes (SWCNTs) possess superlative optical and electronic properties that are of high interest for technologically important applications including fiber optic communications, biomedical imaging, and field-effect transistors. However, as-grown arc discharge SWCNTs possess a mixture of metallic and semiconducting species in addition to a wide diameter distribution (1.2 to 1.7 nm) that limit their performance in devices. While previous postsynthetic sorting efforts have achieved separation by electronic type and diameter refinement for metallic arc discharge SWCNTs, tight diameter distributions of semiconducting arc discharge SWCNTs have not yet been realized. Herein, we present two advances in density gradient ultracentrifugation that enable the isolation of high purity (>99%) semiconducting arc discharge SWCNTs with narrow diameter distributions centered at ∼1.6 and ∼1.4 nm. The resulting diameter-refined populations of semiconducting arc discharge SWCNTs possess monodisperse characteristics that are well-suited for high-performance optical and electronic technologies.Keywords: DGU; electronic; near-infrared; optical; separation; sorting;
Co-reporter:Timothy P. Tyler;Tejas A. Shastry;Benjamin J. Leever
Advanced Materials 2012 Volume 24( Issue 35) pp:4765-4768
Publication Date(Web):
DOI:10.1002/adma.201201728
Co-reporter:N. Jiang, E. T. Foley, J. M. Klingsporn, M. D. Sonntag, N. A. Valley, J. A. Dieringer, T. Seideman, G. C. Schatz, M. C. Hersam, and R. P. Van Duyne
Nano Letters 2012 Volume 12(Issue 10) pp:5061-5067
Publication Date(Web):December 26, 2011
DOI:10.1021/nl2039925
Multiple vibrational modes have been observed for copper phthalocyanine (CuPc) adlayers on Ag(111) using ultrahigh vacuum (UHV) tip-enhanced Raman spectroscopy (TERS). Several important new experimental features are introduced in this work that significantly advance the state-of-the-art in UHV-TERS. These include (1) concurrent sub-nm molecular resolution STM imaging using Ag tips with laser illumination of the tip–sample junction, (2) laser focusing and Raman collection optics that are external to the UHV-STM that has two cryoshrouds for future low temperature experiments, and (3) all sample preparation steps are carried out in UHV to minimize contamination and maximize spatial resolution. Using this apparatus we have been able to demonstrate a TERS enhancement factor of 7.1 × 105. Further, density-functional theory calculations have been carried out that allow quantitative identification of eight different vibrational modes in the TER spectra. The combination of molecular-resolution UHV-STM imaging with the detailed chemical information content of UHV-TERS allows the interactions between large polyatomic molecular adsorbates and specific binding sites on solid surfaces to be probed with unprecedented spatial and spectroscopic resolution.
Co-reporter:Aparna Deshpande ; Chun-Hong Sham ; Justice M. P. Alaboson ; Jonathan M. Mullin ; George C. Schatz
Journal of the American Chemical Society 2012 Volume 134(Issue 40) pp:16759-16764
Publication Date(Web):August 28, 2012
DOI:10.1021/ja307061e
While graphene has attracted significant attention from the research community due to its high charge carrier mobility, important issues remain unresolved that prevent its widespread use in technologically significant applications such as digital electronics. For example, the chemical inertness of graphene hinders integration with other materials, and the lack of a bandgap implies poor switching characteristics in transistors. The formation of ordered organic monolayers on graphene has the potential to address each of these challenges. In particular, functional groups incorporated into the constituent molecules enable tailored chemical reactivity, while molecular-scale ordering within the monolayer provides sub-2 nm templates with the potential to tune the electronic band structure of graphene via quantum confinement effects. Toward these ends, we report here the formation of well-defined one-dimensional organic nanostructures on epitaxial graphene via the self-assembly of 10,12-pentacosadiynoic acid (PCDA) in ultrahigh vacuum (UHV). Molecular resolution UHV scanning tunneling microscopy (STM) images confirm the one-dimensional ordering of the as-deposited PCDA monolayer and show domain boundaries with symmetry consistent with the underlying graphene lattice. In an effort to further stabilize the monolayer, in situ ultraviolet photopolymerization induces covalent bonding between neighboring PCDA molecules in a manner that maintains one-dimensional ordering as verified by UHV STM and ambient atomic force microscopy (AFM). Further quantitative insights into these experimental observations are provided by semiempirical quantum chemistry calculations that compare the molecular structure before and after photopolymerization.
Co-reporter:Sudeshna Chattopadhyay, Albert L. Lipson, Hunter J. Karmel, Jonathan D. Emery, Timothy T. Fister, Paul A. Fenter, Mark C. Hersam, and Michael J. Bedzyk
Chemistry of Materials 2012 Volume 24(Issue 15) pp:3038
Publication Date(Web):July 23, 2012
DOI:10.1021/cm301584r
The solid electrolyte interphase (SEI) plays a critical role in the performance and safety of Li-ion batteries, but the crystal structure of the materials formed have not been previously studied. We employ the model system of epitaxial graphene on SiC to provide a well-defined graphitic surface to study the crystallinity and texture formation in the SEI. We observe, via in situ synchrotron X-ray scattering, the formation and growth of LiF crystallites at the graphene surface, which increase in size with lithiation dose and are textured such that the LiF (002) planes are approximately parallel to the graphene sheets. Furthermore, X-ray photoelectron spectroscopy (XPS) reveals the composition of the SEI formed in this system to consist of LiF and organic compounds similar to those found previously on graphite. SEI components, other than LiF, do not produce X-ray diffraction peaks and are categorized as amorphous. From high-resolution transmission electron microscopy, the LiF crystallites are seen in near proximity to the graphene surface along with additional apparently amorphous material, which is likely to be other SEI components detected by XPS and/or misoriented LiF. This new understanding that LiF crystallites grow on the graphene surface with strong texturing will assist future efforts to model and engineer the SEI formed on graphitic materials.Keywords: graphene; graphite; Li-ion batteries; solid electrolyte interphase; X-ray scattering;
Co-reporter:James E. Johns, Hunter J. Karmel, Justice M. P. Alaboson, and Mark C. Hersam
The Journal of Physical Chemistry Letters 2012 Volume 3(Issue 15) pp:1974-1979
Publication Date(Web):July 11, 2012
DOI:10.1021/jz300802k
The superlative electronic properties of graphene suggest its use as the foundation of next-generation integrated circuits. However, this application requires precise control of the interface between graphene and other materials, especially the metal oxides that are commonly used as gate dielectrics. Toward that end, organic seeding layers have been empirically shown to seed ultrathin dielectric growth on graphene via atomic layer deposition (ALD), although the underlying chemical mechanisms and structural details of the molecule/dielectric interface remain unknown. Here, confocal resonance Raman spectroscopy is employed to quantify the structure and chemistry of monolayers of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on graphene before and after deposition of alumina with the ALD precursors trimethyl aluminum (TMA) and water. Photoluminescence measurements provide further insight into the details of the growth mechanism, including the transition between layer-by-layer growth and island formation. Overall, these results reveal that PTCDA is not consumed during ALD, thereby preserving a well-defined and passivating organic interface between graphene and deposited dielectric thin films.Keywords: atomic layer deposition; graphene; PTCDA; Raman spectroscopy; scanning tunneling microscopy; surface chemistry;
Co-reporter:Yu Teng Liang, Baiju K. Vijayan, Olga Lyandres, Kimberly A. Gray, and Mark C. Hersam
The Journal of Physical Chemistry Letters 2012 Volume 3(Issue 13) pp:1760-1765
Publication Date(Web):June 19, 2012
DOI:10.1021/jz300491s
Due to their unique optoelectronic structure and large specific surface area, carbon nanomaterials have been integrated with titania to enhance photocatalysis. In particular, recent work has shown that nanocomposite photocatalytic performance can be improved by minimizing the covalent defect density of the carbon component. Herein, carbon nanotube–titania nanosheet and graphene–titania nanosheet composites with low carbon defect densities are compared to investigate the role of carbon nanomaterial dimensionality on photocatalytic response. The resulting 2D–2D graphene–titania nanosheet composites yield superior electronic coupling compared to 1D–2D carbon nanotube–titania nanosheet composites, leading to greater enhancement factors for CO2 photoreduction under ultraviolet irradiation. On the other hand, 1D carbon nanotubes are shown to be more effective titania photosensitizers, leading to greater photoactivity enhancement factors under visible illumination. Overall, this work suggests that carbon nanomaterial dimensionality is a key factor in determining the spectral response and reaction specificity of carbon–titania nanosheet composite photocatalysts.Keywords: carbon nanotubes; graphene; interfaces; photocatalysis; titanium dioxide;
Co-reporter:Timothy P. Tyler, Pin Ann Lin, Yuan Tian, Hong-Jun Gao, Xuan P. A. Gao, R. Mohan Sankaran, and Mark C. Hersam
The Journal of Physical Chemistry Letters 2012 Volume 3(Issue 11) pp:1484-1487
Publication Date(Web):May 9, 2012
DOI:10.1021/jz3005283
Highly refined shape populations of gold nanoparticles (AuNPs) are important for emerging applications in catalysis, plasmonics, and nanomaterials growth. To date, research efforts have focused on achieving monodisperse shape by synthetic control or postsynthetic processing that relies on centrifugal sedimentation-based sorting schemes where differences in the particle mass and aspect ratios (e.g., rods and spheres) provide a driving force for separation. Here, we present a technique to reversibly modify the sedimentation coefficients of AuNPs possessing different shapes that would otherwise be virtually indistinguishable during centrifugal sedimentation due to their similar densities, masses, and aspect ratios by exploiting the preferential affinity of the surfactant cetyltrimethylammonium bromide (CTAB) for the Au(100) facet. The resulting tailored sedimentation coefficients enable AuNP shape sorting via density gradient centrifugation (DGC). DGC-refined populations of faceted AuNPs are shown to significantly enhance the growth rate of InAs nanowires when used as seed particles, emphasizing the importance of shape control for nanomaterials growth applications.Keywords: density gradient centrifugation; gold nanoparticles; nanowire growth; separation; transmission electron microscopy;
Co-reporter:Albert L. Lipson, Sudeshna Chattopadhyay, Hunter J. Karmel, Timothy T. Fister, Jonathan D. Emery, Vinayak P. Dravid, Michael M. Thackeray, Paul A. Fenter, Michael J. Bedzyk, and Mark C. Hersam
The Journal of Physical Chemistry C 2012 Volume 116(Issue 39) pp:20949-20957
Publication Date(Web):September 11, 2012
DOI:10.1021/jp307220y
The electrochemical lithiation capacity of 6H silicon carbide (0001) is found to increase by over 1 order of magnitude following graphitization at 1350 °C in ultrahigh vacuum. Through several control experiments, this Li-ion capacity enhancement is correlated with SiC substrate doping and removal of the native oxide surface layer by thermal annealing, which renders both the bulk and surface electrically conductive. Characterization via multiple depth-resolved spectroscopies shows that lithium penetrates the activated SiC upon lithiation, the bulk lattice spacing does not appreciably change, and the surface structure remains largely intact. The electron energy-loss spectroscopy (EELS) extracted compositional ratio of Li to Si is approximately 1:1, which indicates an intrinsic bulk Li capacity in activated SiC of 670 mAh g–1. In addition, inelastic X-ray scattering spectra show changes in the Si chemical bonding configuration due to lithiation. X-ray scattering data show a decrease in the SiC Bragg peak intensity during lithiation, suggesting changes to the bulk crystallinity, whereas the emergence of a diffuse scattering feature suggests that lithiation is associated with the development of substrate defects. Overall, these results illustrate that the electrochemical capacity of a traditionally inert refractory material can be increased substantially via surface modification, thus suggesting a new strategy for improving the performance of next generation Li-ion battery electrodes.
Co-reporter:Alexander L. Antaris, Jung-Woo T. Seo, Ryan E. Brock, Jane E. Herriman, Michael J. Born, Alexander A. Green, and Mark C. Hersam
The Journal of Physical Chemistry C 2012 Volume 116(Issue 37) pp:20103-20108
Publication Date(Web):August 28, 2012
DOI:10.1021/jp3063564
The method by which surfactants selectively interact with particular electronic types of single-walled carbon nanotubes (SWCNTs) and thereby enable the isolation of metallic and semiconducting species is not well understood. While density gradient ultracentrifugation (DGU) has demonstrated its potential as a powerful nanomaterial separation technique, this study utilizes DGU as an analytic tool to probe the interactions between amphiphilic block copolymers, surfactants capable of electronic type extraction, and the SWCNT surface. By modulating the pH during DGU, we find that the linear shaped Pluronic copolymers can extract either metallic or semiconducting SWCNTs at purities in excess of 99%. Furthermore, the first electronic type sorting mechanism is given by which oxygen absorption and subsequent protonation of the SWCNT surface acts to template copolymer adhesion. Detailed characterization reveals the underlying mechanism for pH-shifted DGU and is thus likely to enable future development of more efficient and facile SWCNT electronic type sorting methods.
Co-reporter:Vinod K. Sangwan, Rocio Ponce Ortiz, Justice M. P. Alaboson, Jonathan D. Emery, Michael J. Bedzyk, Lincoln J. Lauhon, Tobin J. Marks, and Mark C. Hersam
ACS Nano 2012 Volume 6(Issue 8) pp:7480
Publication Date(Web):July 11, 2012
DOI:10.1021/nn302768h
In the past decade, semiconducting carbon nanotube thin films have been recognized as contending materials for wide-ranging applications in electronics, energy, and sensing. In particular, improvements in large-area flexible electronics have been achieved through independent advances in postgrowth processing to resolve metallic versus semiconducting carbon nanotube heterogeneity, in improved gate dielectrics, and in self-assembly processes. Moreover, controlled tuning of specific device components has afforded fundamental probes of the trade-offs between materials properties and device performance metrics. Nevertheless, carbon nanotube transistor performance suitable for real-world applications awaits understanding-based progress in the integration of independently pioneered device components. We achieve this here by integrating high-purity semiconducting carbon nanotube films with a custom-designed hybrid inorganic–organic gate dielectric. This synergistic combination of materials circumvents conventional design trade-offs, resulting in concurrent advances in several transistor performance metrics such as transconductance (6.5 μS/μm), intrinsic field-effect mobility (147 cm2/(V s)), subthreshold swing (150 mV/decade), and on/off ratio (5 × 105), while also achieving hysteresis-free operation in ambient conditions.Keywords: density gradient ultracentrifugation; mobility; nanoelectronics; self-assembled nanodielectrics; subthreshold swing; transconductance
Co-reporter:Albert L. Lipson;Ryan S. Ginder
Advanced Materials 2011 Volume 23( Issue 47) pp:
Publication Date(Web):
DOI:10.1002/adma.201190192
Co-reporter:Albert L. Lipson;Ryan S. Ginder
Advanced Materials 2011 Volume 23( Issue 47) pp:5613-5617
Publication Date(Web):
DOI:10.1002/adma.201103094
Co-reporter:Alexer A. Green
Advanced Materials 2011 Volume 23( Issue 19) pp:
Publication Date(Web):
DOI:10.1002/adma.201190067
Co-reporter:Justice M. P. Alaboson;Qing Hua Wang;Joshua A. Kellar;Joohee Park;Jeffrey W. Elam;Michael J. Pellin
Advanced Materials 2011 Volume 23( Issue 19) pp:2181-2184
Publication Date(Web):
DOI:10.1002/adma.201100367
Co-reporter:Alexer A. Green
Advanced Materials 2011 Volume 23( Issue 19) pp:2185-2190
Publication Date(Web):
DOI:10.1002/adma.201100034
Co-reporter:Timothy P. Tyler;Ryan E. Brock;Hunter J. Karmel;Tobin J. Marks
Advanced Energy Materials 2011 Volume 1( Issue 5) pp:
Publication Date(Web):
DOI:10.1002/aenm.201190021
Co-reporter:Timothy P. Tyler;Ryan E. Brock;Hunter J. Karmel;Tobin J. Marks
Advanced Energy Materials 2011 Volume 1( Issue 5) pp:785-791
Publication Date(Web):
DOI:10.1002/aenm.201100274
Co-reporter:Matthew C. Duch, G. R. Scott Budinger, Yu Teng Liang, Saul Soberanes, Daniela Urich, Sergio E. Chiarella, Laura A. Campochiaro, Angel Gonzalez, Navdeep S. Chandel, Mark C. Hersam, and Gökhan M. Mutlu
Nano Letters 2011 Volume 11(Issue 12) pp:5201-5207
Publication Date(Web):October 24, 2011
DOI:10.1021/nl202515a
To facilitate the proposed use of graphene and its derivative graphene oxide (GO) in widespread applications, we explored strategies that improve the biocompatibility of graphene nanomaterials in the lung. In particular, solutions of aggregated graphene, Pluronic dispersed graphene, and GO were administered directly into the lungs of mice. The introduction of GO resulted in severe and persistent lung injury. Furthermore, in cells GO increased the rate of mitochondrial respiration and the generation of reactive oxygen species, activating inflammatory and apoptotic pathways. In contrast, this toxicity was significantly reduced in the case of pristine graphene after liquid phase exfoliation and was further minimized when the unoxidized graphene was well-dispersed with the block copolymer Pluronic. Our results demonstrate that the covalent oxidation of graphene is a major contributor to its pulmonary toxicity and suggest that dispersion of pristine graphene in Pluronic provides a pathway for the safe handling and potential biomedical application of two-dimensional carbon nanomaterials.
Co-reporter:Yu Teng Liang, Baiju K. Vijayan, Kimberly A. Gray, and Mark C. Hersam
Nano Letters 2011 Volume 11(Issue 7) pp:2865-2870
Publication Date(Web):June 20, 2011
DOI:10.1021/nl2012906
With its unique electronic and optical properties, graphene is proposed to functionalize and tailor titania photocatalysts for improved reactivity. The two major solution-based pathways for producing graphene, oxidation–reduction and solvent exfoliation, result in nanoplatelets with different defect densities. Herein, we show that nanocomposites based on the less defective solvent-exfoliated graphene exhibit a significantly larger enhancement in CO2 photoreduction, especially under visible light. This counterintuitive result is attributed to their superior electrical mobility, which facilitates the diffusion of photoexcited electrons to reactive sites.
Co-reporter:Qing Hua Wang and Mark C. Hersam
Nano Letters 2011 Volume 11(Issue 2) pp:589-593
Publication Date(Web):December 17, 2010
DOI:10.1021/nl103590j
Nanoscale control of surface chemistry holds promise for tailoring the electronic, optical, and chemical properties of graphene. Toward this end, the nanofabrication of sub-5-nm heteromolecular organic nanostructures is demonstrated on epitaxial graphene using room temperature ultrahigh vacuum scanning tunneling microscopy. In particular, monolayers of the organic semiconductor 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) are nanopatterned on epitaxial graphene using feedback-controlled lithography (FCL) and then used as chemical resists to template the deposition of N,N′-dioctyl-3,4,9,10-perylene-tetracarboxylic diimide (PTCDI-C8). The generality of this FCL-based nanofabrication procedure suggests its applicability to a wide range of fundamental studies and prototype device fabrication on chemically functionalized graphene.
Co-reporter:Michael D. Irwin, Jonathan D. Servaites, D. Bruce Buchholz, Benjamin J. Leever, Jun Liu, Jonathan D. Emery, Ming Zhang, Jung-Hwan Song, Michael F. Durstock, Arthur J. Freeman, Michael J. Bedzyk, Mark C. Hersam, Robert P. H. Chang, Mark A. Ratner, and Tobin J. Marks
Chemistry of Materials 2011 Volume 23(Issue 8) pp:2218
Publication Date(Web):April 1, 2011
DOI:10.1021/cm200229e
The functionality of NiO interfacial layers in enhancing bulk heterojunction (BHJ) organic photovoltaic (OPV) cell performance is investigated by integrated characterization of the electrical properties, microstructure, electronic structure, and optical properties of thin NiO films grown on glass/ITO electrodes. These NiO layers are found to be advantageous in BHJ OPV applications due to favorable energy band levels, interface passivation, p-type character, crystallinity, smooth surfaces, and optical transparency. The NiO overlayers are fabricated via pulsed-laser deposition and found to have a work function of ∼5.3 eV. They are investigated by both topographic and conductive atomic force microscopy and shown to passivate interfacial charge traps. The films also have an average optical transparency of >80% in the visible range, crucial for efficient OPV function, and have a near-stoichiometric Ni:O surface composition. By grazing-incidence X-ray diffraction, the NiO thin films are shown to grow preferentially in the (111) direction and to have the fcc NaCl crystal structure. Diodes of p−n structure and first-principles electronic structure calculations indicate that the NiO interlayer is preferentially conductive to holes, with a lower hole charge carrier effective mass versus that of electrons. Finally, the implications of these attributes in advancing efficiencies for state-of-the-art OPV systems—in particular, improving the open circuit voltage (VOC)—are discussed.Keywords: electron blocking layer; hole transport layer; interfacial layer; nickel oxide; NiO; organic photovoltaics; organic solar cells;
Co-reporter:Jonathan D. Emery, Qing Hua Wang, Marie Zarrouati, Paul Fenter, Mark C. Hersam, Michael J. Bedzyk
Surface Science 2011 Volume 605(17–18) pp:1685-1693
Publication Date(Web):September 2011
DOI:10.1016/j.susc.2010.11.008
Epitaxial graphene, grown by thermal decomposition of the SiC (0001) surface, is a promising material for future applications due to its unique and superlative electronic properties. However, the innate chemical passivity of graphene presents challenges for integration with other materials for device applications. Here, we present structural characterization of epitaxial graphene functionalized by the organic semiconductor perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA). A combination of ultra-high vacuum scanning tunneling microscopy (STM) and high-resolution X-ray reflectivity (XRR) is used to extract lateral and vertical structures of 0, 1, and 2 monolayer (ML) PTCDA on epitaxial graphene. Both Fienup-based phase-retrieval algorithms and model-based least-squares analyses of the XRR data are used to extract an electron density profile that is interpreted in terms of a stacking sequence of molecular layers with specific interlayer spacings. Features in the STM and XRR analysis indicate long-range molecular ordering and weak π–π* interactions binding PTCDA molecules to the graphene surface. The high degree of both lateral and vertical ordering of the self-assembled film demonstrates PTCDA functionalization as a viable route for templating graphene for the growth and deposition of additional materials required for next-generation electronics and sensors.Research Highlights► PTCDA adlayers on graphene are investigated with X-ray and scanning probe techniques. ► PTCDA adlayers are found to be ordered in both lateral and vertical dimensions. ► Results suggest that PTCDA binds to graphene via a π- π* interaction. ► PTCDA may be used for non-covalent functionalization of graphene.
Co-reporter:Jung-Woo T. Seo, Alexander A. Green, Alexander L. Antaris, and Mark C. Hersam
The Journal of Physical Chemistry Letters 2011 Volume 2(Issue 9) pp:1004-1008
Publication Date(Web):April 12, 2011
DOI:10.1021/jz2003556
The ability to disperse pristine graphene at high concentrations in aqueous solutions is an enabling step for large-scale processing and emerging biomedical applications. Herein we demonstrate that nonionic, biocompatible block copolymers are able to produce graphene dispersions with concentrations exceeding 0.07 mg mL–1 via sonication and centrifugation, resulting in optical densities above 4 OD cm–1 in the visible and near-infrared regions of the electromagnetic spectrum. The dispersion efficiency of graphene using Pluronic and Tetronic block copolymers varies substantially depending on the lengths of their hydrophilic and hydrophobic domains, with the best of these copolymers sharing similar domain molecular weight ratios and comparable overall molecular weights. This study presents a new class of biocompatible dispersing agents for graphene in aqueous solution, thus suggesting a facile route to employ graphene in biomedical sensing, imaging, and therapeutic applications.Keywords: block copolymer; dispersion; graphene; pluronic; suspension; tetronic;
Co-reporter:Timothy P. Tyler, Anne-Isabelle Henry, Richard P. Van Duyne, and Mark C. Hersam
The Journal of Physical Chemistry Letters 2011 Volume 2(Issue 3) pp:218-222
Publication Date(Web):January 18, 2011
DOI:10.1021/jz101690f
Noble metal nanoparticle clusters underlie a variety of plasmonic devices and measurements including surface-enhanced Raman spectroscopy (SERS). Because of the strong dependence of plasmonic properties on nanoparticle cluster aggregation state, the elimination of non-SERS-active structures and the refinement of the nanoparticle cluster population are critical to realizing uniform and reproducible structures for plasmonic nanoantenna applications such as SERS-based sensors. In this Letter, we report a centrifugal sorting technique for gold core/silica shell nanoparticles that host SERS reporter molecules at the gold/silica interface. The relatively massive nanoparticle clusters are sorted by sedimentation coefficient via centrifugation in a high-viscosity density gradient medium, iodixanol, which yields solutions that contain a preponderance of one aggregation state and a diminished monomer population, as determined by transmission electron microscopy, extinction spectroscopy, and SERS. A quantitative analysis of the nanoparticle sedimentation coefficients is presented, thus allowing this approach to be predictably generalized to other nanoparticle systems.Keywords (keywords): centrifugation; density gradient; gold nanoparticle; sedimentation coefficient; separation; SERS; sorting; surface-enhanced Raman scattering; TEM; transmission electron microscopy;
Co-reporter:Ian P. Murray, Sylvia J. Lou, Laura J. Cote, Stephen Loser, Cameron J. Kadleck, Tao Xu, Jodi M. Szarko, Brian S. Rolczynski, James E. Johns, Jiaxing Huang, Luping Yu, Lin X. Chen, Tobin J. Marks, and Mark C. Hersam
The Journal of Physical Chemistry Letters 2011 Volume 2(Issue 24) pp:3006-3012
Publication Date(Web):November 12, 2011
DOI:10.1021/jz201493d
Organic photovoltaic (OPV) materials have recently garnered significant attention as enablers of high power conversion efficiency (PCE), low-cost, mechanically flexible solar cells. Nevertheless, further understanding-based materials developments will be required to achieve full commercial viability. In particular, the performance and durability of many current generation OPVs are limited by poorly understood interfacial phenomena. Careful analysis of typical OPV architectures reveals that the standard electron-blocking layer, poly-3,4-ethylenedioxy-thiophene:poly(styrene sulfonate) (PEDOT:PSS), is likely a major factor limiting the device durability and possibly performance. Here we report that a single layer of electronically tuned graphene oxide is an effective replacement for PEDOT:PSS and that it significantly enhances device durability while concurrently templating a performance-optimal active layer π-stacked face-on microstructure. Such OPVs based on graphene oxide exhibit PCEs as high as 7.5% while providing a 5× enhancement in thermal aging lifetime and a 20× enhancement in humid ambient lifetime versus analogous PEDOT:PSS-based devices.Keywords: bulk heterojunction; GO; interfacial layer; lifetime; PTB7; reliability; solar cell;
Co-reporter:Justice M. P. Alaboson, Qing Hua Wang, Jonathan D. Emery, Albert L. Lipson, Michael J. Bedzyk, Jeffrey W. Elam, Michael J. Pellin, and Mark C. Hersam
ACS Nano 2011 Volume 5(Issue 6) pp:5223
Publication Date(Web):May 9, 2011
DOI:10.1021/nn201414d
The development of high-performance graphene-based nanoelectronics requires the integration of ultrathin and pinhole-free high-k dielectric films with graphene at the wafer scale. Here, we demonstrate that self-assembled monolayers of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) act as effective organic seeding layers for atomic layer deposition (ALD) of HfO2 and Al2O3 on epitaxial graphene on SiC(0001). The PTCDA is deposited via sublimation in ultrahigh vacuum and shown to be highly ordered with low defect density by molecular-resolution scanning tunneling microscopy. Whereas identical ALD conditions lead to incomplete and rough dielectric deposition on bare graphene, the chemical functionality provided by the PTCDA seeding layer yields highly uniform and conformal films. The morphology and chemistry of the dielectric films are characterized by atomic force microscopy, ellipsometry, cross-sectional scanning electron microscopy, and X-ray photoelectron spectroscopy, while high-resolution X-ray reflectivity measurements indicate that the underlying graphene remains intact following ALD. Using the PTCDA seeding layer, metal-oxide-graphene capacitors fabricated with a 3 nm Al2O3 and 10 nm HfO2 dielectric stack show high capacitance values of ∼700 nF/cm2 and low leakage currents of ∼5 × 10–9 A/cm2 at 1 V applied bias. These results demonstrate the viability of sublimated organic self-assembled monolayers as seeding layers for high-k dielectric films in graphene-based nanoelectronics.Keywords: alumina; gate insulator; graphene; hafnia; organic seeding layer; PTCDA; transistor
Co-reporter:Alexander A. Green and Mark C. Hersam
ACS Nano 2011 Volume 5(Issue 2) pp:1459
Publication Date(Web):January 28, 2011
DOI:10.1021/nn103263b
Double-walled carbon nanotubes (DWNTs) can adopt four distinct permutations arising from the electronic type (metallic or semiconducting) of their inner and outer walls. This polydispersity limits the utility of DWNTs in applications such as thin film electronics. We demonstrate that density gradient ultracentrifugation can be employed to address this source of heterogeneity by producing DWNTs with well-defined outer-wall electronic types. Optical absorption measurements of sorted DWNTs reveal outer-wall purities of 96% and 98% for sorted semiconducting and metallic samples, respectively. Electrical characterization of semiconducting and metallic outer-wall DWNTs in thin film transistors directly confirms the efficacy of these separations, with semiconducting DWNT devices yielding on/off ratios 2 orders of magnitude higher than comparable metallic DWNT devices.Keywords (keywords): carbon nanotube; density gradient ultracentrifugation; double; purification; separation; sorting; transistor
Co-reporter:Benjamin J. Leever ; Ian P. Murray ; Michael F. Durstock ; Tobin J. Marks
The Journal of Physical Chemistry C 2011 Volume 115(Issue 45) pp:22688-22694
Publication Date(Web):October 10, 2011
DOI:10.1021/jp209570h
A correlation between anode surface treatment and spatially localized photocurrent variations has been found in bulk heterojunction poly(3-hexylthiophene):[6,6]-phenyl-C-61-butyric acid methyl ester (P3HT:PCBM) organic photovoltaic (OPV) devices. Atomic force photovoltaic microscopy was used to scan arrays of 2 μm diameter OPV devices with varied indium tin oxide (ITO) surface treatments. The standard deviation of the average photocurrent was found to be 11.4% for devices fabricated on untreated ITO, 8.6% for devices with a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) interlayer, and 6.7% for devices with a HCl-treated ITO surface. These results suggest that spatial variations in the structure and electronic properties of the anode surface degrade the overall performance of OPVs, including reductions in short-circuit current by up to 20%, thus highlighting the importance of surface treatments that improve the homogeneity of ITO.
Co-reporter:Gökhan M. Mutlu, G. R. Scott Budinger, Alexander A. Green, Daniela Urich, Saul Soberanes, Sergio E. Chiarella, George F. Alheid, Donald R. McCrimmon, Igal Szleifer and Mark C. Hersam
Nano Letters 2010 Volume 10(Issue 5) pp:1664-1670
Publication Date(Web):April 8, 2010
DOI:10.1021/nl9042483
Excitement surrounding the attractive physical and chemical characteristics of single walled carbon nanotubes (SWCNTs) has been tempered by concerns regarding their potential health risks. Here we consider the lung toxicity of nanoscale dispersed SWCNTs (mean diameter ∼1 nm). Because dispersion of the SWCNTs increases their aspect ratio relative to as-produced aggregates, we directly test the prevailing hypothesis that lung toxicity associated with SWCNTs compared with other carbon structures is attributable to the large aspect ratio of the individual particles. Thirty days after their intratracheal administration to mice, the granuloma-like structures with mild fibrosis in the large airways observed in mice treated with aggregated SWCNTs were absent in mice treated with nanoscale dispersed SWCNTs. Examination of lung sections from mice treated with nanoscale dispersed SWCNTs revealed uptake of the SWCNTs by macrophages and gradual clearance over time. We conclude that the toxicity of SWCNTs in vivo is attributable to aggregation of the nanomaterial rather than the large aspect ratio of the individual nanotubes. Biocompatible nanoscale dispersion provides a scalable method to generate purified preparations of SWCNTs with minimal toxicity, thus allowing them to be used safely in commercial and biomedical applications.
Co-reporter:Md. Zakir Hossain ; Michael A. Walsh
Journal of the American Chemical Society 2010 Volume 132(Issue 43) pp:15399-15403
Publication Date(Web):October 8, 2010
DOI:10.1021/ja107085n
The reduction of diazonium salts has recently been proposed as a robust covalent modification scheme for graphene surfaces. While preliminary studies have provided indirect evidence that this strategy decorates graphene with aryl moieties, the molecular ordering and conformation of the resulting adlayer have not been directly measured. In this Article, we report molecular-resolution characterization of the adlayer formed via the spontaneous reduction of 4-nitrophenyl diazonium (4-NPD) tetrafluoroborate on epitaxial graphene on SiC(0001) using ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) and spectroscopy (STS). An atomically flat inhomogeneous layer of covalently bonded organic molecules is observed after annealing the chemically treated surface at ∼500 °C in UHV. STM and STS results indicate that the adlayer consists predominantly of aryl oligomers that sterically prevent uniform and complete covalent modification of the graphene surface. The adsorbed species can be selectively desorbed by the STM tip above a threshold sample bias of −5 V and tunneling current of 1 nA, thus enabling the fabrication of a diverse range of graphene nanopatterns at the sub-5 nm length scale.
Co-reporter:Yu Teng Liang
Journal of the American Chemical Society 2010 Volume 132(Issue 50) pp:17661-17663
Publication Date(Web):November 29, 2010
DOI:10.1021/ja107661g
Efficient graphene exfoliation in a nontraditional solvent, ethanol, is achieved through the addition of a stabilizing polymer, ethyl cellulose. Iterative solvent exchange is further demonstrated as a rapid, room-temperature, ultracentrifugation-free approach to concentrate the graphene solution to a level exceeding 1 mg/mL. The outstanding processability and electrical properties of these graphene inks are verified through the realization of aligned graphene−polymer nanocomposites and transparent conductive graphene thin films.
Co-reporter:Norma E. Sosa ; Christopher Chen ; Jun Liu ; Sujing Xie ; Tobin J. Marks
Journal of the American Chemical Society 2010 Volume 132(Issue 21) pp:7347-7354
Publication Date(Web):May 11, 2010
DOI:10.1021/ja9092242
Realizing optically transparent functional circuitry continues to fuel scientific and technological interest in transparent conducting oxides (TCOs). However, precise means for creating transparent interconnects for device-to-device integration has remained elusive. Here we report on the chemical, microstructural, and electronic properties of transparent conducting oxide nanowires (Ga-doped In2O3) created by direct-write focused ion beam (Ga+) implantation within an insulating oxide substrate (In2O3). First, methodology for preparing TEM-ready samples is presented that enables detailed TEM-based analysis of individual nanowires. Differences in diffraction features between doped and undoped oxide regions, accompanied by RTA results, support a model in which oxygen vacancies and amorphization comprise the predominant doping/carrier creation mechanism. The same isolated nanowires are then subjected to chemical profiling, providing quantitative information on the lateral Ga doping dimensions, which are in good agreement with conductive AFM images. Furthermore, spatially selective nanoscale EELS spectroscopy provides additional evidence for changes in the oxygen site chemical environment in the FIB-processed/doped In2O3, and for negligible changes in the surrounding non-FIB-processed/undoped oxide. The nanowires exhibit ohmic electrical behavior and with an average estimated conductivity of 1600−3600 S cm−1, similar to macroscale Ga-doped In2O3 films grown by conventional processes.
Co-reporter:Michael A. Walsh ; Stephanie R. Walter ; Kirk H. Bevan ; Franz M. Geiger
Journal of the American Chemical Society 2010 Volume 132(Issue 9) pp:3013-3019
Publication Date(Web):February 12, 2010
DOI:10.1021/ja909139n
Using ultrahigh vacuum (UHV) scanning tunneling microscopy (STM), many olefins have been shown to self-assemble on the hydrogen-passivated Si(100)-2 × 1 surface into one-dimensional nanostructures. This paper demonstrates that similar one-dimensional nanostructures can also be realized using alkynes. In particular, UHV STM, sum frequency generation (SFG), and density functional theory (DFT) are employed to study the growth mechanism and binding configuration of phenylacetylene (PA) one-dimensional nanostructures on the Si(100)-2 × 1:H surface. Molecular-resolution UHV STM images reveal the binding position and spacing of PA with respect to the underlying silicon dimer rows. Furthermore, UHV STM characterization of heteromolecular one-dimensional nanostructures of styrene and PA shows distinct electronic contrast between the two molecules, which is confirmed using simulated STM images derived from DFT and provides insight into the nature of PA binding to silicon. Additional evidence from SFG measurements corroborates the conclusion that the terminal carbon atoms of PA retain π-conjugation following reaction to the Si(100)-2 × 1:H surface.
Co-reporter:David J. Comstock;Steven T. Christensen;Jeffrey W. Elam;Michael J. Pellin
Advanced Functional Materials 2010 Volume 20( Issue 18) pp:
Publication Date(Web):
DOI:10.1002/adfm.201090082
Abstract
Nanostructured metal films have been widely studied for their roles in sensing, catalysis, and energy storage. In this work, the synthesis of compositionally controlled and nanostructured Pt/Ir films by atomic layer deposition (ALD) into porous anodized aluminum oxide templates is demonstrated. Templated ALD provides advantages over alternative synthesis techniques, including improved film uniformity and conformality as well as atomic-scale control over morphology and composition. Nanostructured Pt ALD films are demonstrated with morphological control provided by the Pt precursor exposure time and the number of ALD cycles. With these approaches, Pt films with enhanced surface areas, as characterized by roughness factors as large as 310, are reproducibly synthesized. Additionally, nanostructured PtIr alloy films of controlled composition and morphology are demonstrated by templated ALD, with compositions varying systematically from pure Pt to pure Ir. Lastly, the application of nanostructured Pt films to electrochemical sensing applications is demonstrated by the non-enzymatic sensing of glucose.
Co-reporter:David J. Comstock;Steven T. Christensen;Jeffrey W. Elam;Michael J. Pellin
Advanced Functional Materials 2010 Volume 20( Issue 18) pp:3099-3105
Publication Date(Web):
DOI:10.1002/adfm.201000389
Abstract
Nanostructured metal films have been widely studied for their roles in sensing, catalysis, and energy storage. In this work, the synthesis of compositionally controlled and nanostructured Pt/Ir films by atomic layer deposition (ALD) into porous anodized aluminum oxide templates is demonstrated. Templated ALD provides advantages over alternative synthesis techniques, including improved film uniformity and conformality as well as atomic-scale control over morphology and composition. Nanostructured Pt ALD films are demonstrated with morphological control provided by the Pt precursor exposure time and the number of ALD cycles. With these approaches, Pt films with enhanced surface areas, as characterized by roughness factors as large as 310, are reproducibly synthesized. Additionally, nanostructured PtIr alloy films of controlled composition and morphology are demonstrated by templated ALD, with compositions varying systematically from pure Pt to pure Ir. Lastly, the application of nanostructured Pt films to electrochemical sensing applications is demonstrated by the non-enzymatic sensing of glucose.
Co-reporter:Michael A. Walsh and Mark C. Hersam
Chemical Communications 2010 vol. 46(Issue 7) pp:1153-1155
Publication Date(Web):23 Dec 2009
DOI:10.1039/B917502D
Scanning tunneling microscopy reveals multiple binding configurations for o-phthalaldehyde (OP) molecules on the Si(100)-2 × 1:H surface, including evidence for a chain reaction in which OP molecules bind to the surface via two covalent bonds.
Co-reporter:David J. Comstock, Jeffrey W. Elam, Michael J. Pellin and Mark C. Hersam
Analytical Chemistry 2010 Volume 82(Issue 4) pp:1270
Publication Date(Web):January 14, 2010
DOI:10.1021/ac902224q
Scanning ion conductance microscopy (SICM) has developed into a powerful tool for imaging a range of biophysical systems. In addition, SICM has been integrated with a range of other techniques, allowing for the simultaneous collection of complementary information including near-field optical and electrophysiological properties. However, SICM imaging remains insensitive to electrochemical properties, which play an important role in both biological and nonbiological systems. In this work, we demonstrate the fabrication and application of a nanopipet probe with an integrated ultramicroelectrode (UME) for concurrent SICM and scanning electrochemical microscopy (SECM). The fabrication process utilizes atomic layer deposition (ALD) of aluminum oxide to conformally insulate a gold-coated nanopipet and focused ion beam (FIB) milling to precisely expose a UME at the pipet tip. Fabricated probes are characterized by both scanning electron microscopy and cyclic voltammetry and exhibit a 100 nm diameter nanopipet tip and a UME with an effective radius of 294 nm. The probes exhibit positive and negative feedback responses on approach to conducting and insulating surfaces, respectively. The suitability of the probes for SECM−SICM imaging is demonstrated by both feedback-mode and substrate generation/tip collection-mode imaging on patterned surfaces. This probe geometry enables successful SECM−SICM imaging on features as small as 180 nm in size.
Co-reporter:David J. Comstock, Steven T. Christensen, Jeffrey W. Elam, Michael J. Pellin, Mark C. Hersam
Electrochemistry Communications 2010 Volume 12(Issue 11) pp:1543-1546
Publication Date(Web):November 2010
DOI:10.1016/j.elecom.2010.08.029
Iridium oxide (IrOx) has been widely studied due to its applications in electrochromic devices, pH sensing, and neural stimulation. Previous work has demonstrated that both Ir and IrOx films with porous morphologies prepared by sputtering exhibit significantly enhanced charge storage capacities. However, sputtering provides only limited control over film porosity. In this work, we demonstrate an alternative scheme for synthesizing nanoporous Ir and activated IrOx films (AIROFs). This scheme utilizes atomic layer deposition to deposit a thin conformal Ir film within a nanoporous anodized aluminum oxide template. The Ir film is then activated by potential cycling in 0.1 M H2SO4 to form a nanoporous AIROF. The morphologies and electrochemical properties of the films are characterized by scanning electron microscopy and cyclic voltammetry, respectively. The resulting nanoporous AIROFs exhibit a nanoporous morphology and enhanced cathodal charge storage capacities as large as 311 mC/cm2.
Co-reporter:Alexander A. Green and Mark C. Hersam
The Journal of Physical Chemistry Letters 2010 Volume 1(Issue 2) pp:544-549
Publication Date(Web):December 29, 2009
DOI:10.1021/jz900235f
With the recent burst of activity surrounding the solution-phase production of graphene, comparatively little progress has been made toward the generation of graphene dispersions with tailored thickness, lateral area, and shape. The polydispersity of graphene dispersions, however, can lead to unpredictable or nonideal behavior once they are incorporated into devices since the properties of graphene vary as a function of its structural parameters. In this brief perspective, we overview the problem of graphene polydispersity, the production of graphene dispersions, and the methods under development to produce dispersions of monodisperse graphene.
Co-reporter:Alexander L. Antaris, Jung-Woo T. Seo, Alexander A. Green and Mark C. Hersam
ACS Nano 2010 Volume 4(Issue 8) pp:4725
Publication Date(Web):July 29, 2010
DOI:10.1021/nn101363m
As-synthesized single-walled carbon nanotubes (SWNTs) typically possess a range of diameters and electronic properties. This polydispersity has hindered the development of many SWNT-based technologies and encouraged the development of postsynthetic methods for sorting SWNTs by their physical and electronic structure. Herein, we demonstrate that nonionic, biocompatible block copolymers can be used to isolate semiconducting and metallic SWNTs using density gradient ultracentrifugation. Separations conducted with different Pluronic block copolymers reveal that Pluronics with shorter hydrophobic chain lengths lead to higher purity semiconducting SWNTs, resulting in semiconducting purity levels in excess of 99% obtained for Pluronic F68. In contrast, X-shaped Tetronic block copolymers display an affinity for metallic SWNTs, yielding metallic purity levels of 74% for Tetronic 1107. These results suggest that high fidelity and high yield density gradient separations can be achieved using nonionic block copolymers with rationally designed homopolymer segments, thus generating biocompatible monodisperse SWNTs for a range of applications.Keywords: biotechnology; carbon nanotube; density gradient ultracentrifugation; Pluronic; separation; sorting; Tetronic
Co-reporter:Alexander A. Green and Mark C. Hersam
Nano Letters 2009 Volume 9(Issue 12) pp:4031-4036
Publication Date(Web):September 25, 2009
DOI:10.1021/nl902200b
Graphene flakes with controlled thicknesses are isolated in solution using density gradient ultracentrifugation. These stable graphene dispersions are produced using the bile salt sodium cholate, which promotes graphite exfoliation and results in graphene−surfactant complexes having buoyant densities that vary with graphene thickness. The sorted graphene flakes are characterized using atomic force microscopy and Raman spectroscopy. Graphene dispersions produced using density differentiation offer superior performance in transparent conductors than those produced using conventional sedimentation-based centrifugation techniques.
Co-reporter:Nathan L. Yoder ; James S. Fakonas
Journal of the American Chemical Society 2009 Volume 131(Issue 29) pp:10059-10065
Publication Date(Web):July 2, 2009
DOI:10.1021/ja9010546
Dissociation of individual cyclopentene molecules on the Si(100) surface is induced and investigated using cryogenic ultrahigh vacuum scanning tunneling microscopy (STM). Using a secondary feedback loop during elevated tunneling current and sample biasing conditions, the cyclopentene dissociation products are isolated and then characterized with atomic-scale spatial resolution. Using multibias STM and density functional theory, the cyclopentene dissociation products are shown to consist of a C5H7 fragment and an individual H atom. The C5H7 fragment contains a C═C double bond and is bound to the Si(100) surface via a single Si−C covalent bond, while the individual H atom can be induced to hop between two sites on a single silicon dimer with the STM tip. This study shows that the use of feedback control during STM-induced single molecule reactions allows transient reaction products to be captured and thus more thoroughly studied. While demonstrated here for cyclopentene on Si(100), this feedback-controlled approach can likely be applied to a wide array of chemical reactions on semiconductor surfaces.
Co-reporter:Steven T. Christensen, Jeffrey W. Elam, Byeongdu Lee, Zhenxing Feng, Michael J. Bedzyk and Mark C. Hersam
Chemistry of Materials 2009 Volume 21(Issue 3) pp:516
Publication Date(Web):January 2, 2009
DOI:10.1021/cm8026863
The early stages of nucleation and growth of atomic layer deposition (ALD) platinum on SrTiO3 (001) have been studied. Scanning electron microscopy reveals the ALD Pt deposits as discrete nanoparticles that grow and coalesce with increasing number of ALD cycles, ultimately resulting in a continuous film after ∼40 cycles. Atomic force microscopy shows the films to be fine-grained and highly conformal such that the 0.4 nm atomic steps of the underlying SrTiO3 (001) surface remain visible even after 80 Pt ALD cycles. Grazing-incidence small-angle X-ray scattering (GISAXS) studies demonstrate that the early stages of Pt ALD yields nanoparticles that are well approximated as cylinders with a height to radius ratio that is nearly unity. Consistent with nanoparticle coalescence, GISAXS also reveals an interparticle spacing that increases with the number of ALD cycles. X-ray fluorescence measurements of the Pt coverage reveal growth dynamics in which the Pt deposition is initially faster than the steady-state growth rate that emerges after 40−70 ALD cycles. These experimental results are understood through the application of a model that suggests that the SrTiO3 surface is more reactive than the Pt species and that Pt diffusion is operative in nanoparticle formation. Overall, this study delineates ALD growth conditions for forming either Pt nanoparticles or continuous Pt thin films on SrTiO3 (001), thus presenting potentially useful substrates for catalysis and microelectronics, respectively.
Co-reporter:Alexander A. Green;Matthew C. Duch
Nano Research 2009 Volume 2( Issue 1) pp:69-77
Publication Date(Web):2009 January
DOI:10.1007/s12274-009-9006-y
Current methods of synthesizing single-walled carbon nanotubes (SWNTs) result in racemic mixtures that have impeded the study of left- and right-handed SWNTs. Here we present a method of isolating different SWNT enantiomers using density gradient ultracentrifugation. Enantiomer separation is enabled by the chiral surfactant sodium cholate, which discriminates between left- and right-handed SWNTs and thus induces subtle differences in their buoyant densities. This sorting strategy can be employed for simultaneous enrichment by handedness and roll-up vector of SWNTs having diameters ranging from 0.7 to 1.5 nm. In addition, circular dichroism of enantiomer refined samples enables identification of high-energy optical transitions in SWNTs.
Co-reporter:Joshua A. Kellar, Jui-Ching Lin, Jun-Hyun Kim, Nathan L. Yoder, Kirk H. Bevan, Grace Y. Stokes, Franz M. Geiger, SonBinh T. Nguyen, Michael J. Bedzyk and Mark C. Hersam
The Journal of Physical Chemistry C 2009 Volume 113(Issue 7) pp:2919-2927
Publication Date(Web):2017-2-22
DOI:10.1021/jp8100249
Highly conjugated molecules bound to silicon are promising candidates for organosilicon electronic devices and sensors. In this study, 1-bromo-4-ethynylbenzene was synthesized and reacted with a hydrogen-passivated Si(111) surface via ultraviolet irradiation. Through an array of characterization and modeling tools, the binding configuration and morphology of the reacted molecule were thoroughly analyzed. Atomic force microscopy confirmed an atomically flat surface morphology following reaction, while X-ray photoelectron spectroscopy verified reaction to the surface via the terminal alkyne moiety. In addition, synchrotron X-ray characterization, including X-ray reflectivity, X-ray fluorescence, and X-ray standing wave measurements, enabled sub-angstrom determination of the position of the bromine atom with respect to the silicon lattice. This structural characterization was quantitatively compared with density functional theory (DFT) calculations, thus enabling the π-conjugation of the terminal carbon atoms to be deduced. The X-ray and DFT results were additionally corroborated with the vibrational spectrum of the organic adlayer, which was measured with sum frequency generation. Overall, these results illustrate that the terminal carbon atoms in 1-bromo-4-ethynylbenzene adlayers on Si(111) retain π-conjugation, thus revealing alkyne molecules as promising candidates for organosilicon electronics and sensing.
Co-reporter:Alexander A. Green and Mark C. Hersam
Nano Letters 2008 Volume 8(Issue 5) pp:1417-1422
Publication Date(Web):April 8, 2008
DOI:10.1021/nl080302f
Single-walled carbon nanotubes (SWNTs) are promising materials for transparent conduction as a result of their exceptional electrical, optical, mechanical, and chemical properties. However, since current synthetic methods yield polydisperse mixtures of SWNTs, the performance of SWNT transparent conductive films has previously been hindered by semiconducting species. Here, we describe the performance of transparent conductors produced using predominantly metallic SWNTs. Compared with unsorted material, films enriched in metallic SWNTs can enhance conductivity by factors of over 5.6 in the visible and 10 in the infrared. Moreover, by using monodisperse metallic SWNTs sorted with angstrom-level resolution in diameter, semitransparent conductive coatings with tunable optical transmittance can be produced.
Co-reporter:Gordana N. Ostojic, John R. Ireland and Mark C. Hersam
Langmuir 2008 Volume 24(Issue 17) pp:9784-9789
Publication Date(Web):July 23, 2008
DOI:10.1021/la801311j
A method for noncovalent functionalization of DNA-wrapped single-walled carbon nanotubes (SWNTs) using platinum-based DNA cross-linkers is investigated. In particular, cisplatin and potassium tetrachloroplatinate are shown to bind to DNA that encapsulates SWNTs in aqueous solution. The bound platinum salt can then be reduced to decorate the DNA-encapsulated SWNTs with platinum nanoparticles. The resulting SWNT/DNA/Pt hybrids are investigated by optical absorption spectroscopy, circular dichroism spectroscopy, Raman spectroscopy, X-ray diffraction, transmission electron microscopy, and atomic force microscopy. The unique combination of catalytic activity of nanoscale platinum, biological functionality of DNA, and optoelectronic properties of SWNTs suggests a myriad of applications including fuel cells, catalysts, biosensors, and electrochemical devices.
Co-reporter:Michael S. Arnold, Jin Suntivich, Samuel I. Stupp and Mark C. Hersam
ACS Nano 2008 Volume 2(Issue 11) pp:2291
Publication Date(Web):October 3, 2008
DOI:10.1021/nn800512t
The hydrodynamic properties of surfactant encapsulated single-walled carbon nanotubes (SWNTs) have been characterized by optically measuring their spatial and temporal redistribution in situ in an analytical ultracentrifuge. The measured redistribution profiles are fit to the Lamm equation, thus determining the sedimentation, diffusion, and hydrodynamic frictional coefficients of the surfactant encapsulated SWNTs. For sodium cholate encapsulated SWNTs, we demonstrate that the technique of analytical ultracentrifugation can be utilized to determine the linear packing density of surfactant molecules along the length of the SWNTs, 3.6 ± 0.8 nm−1, and the anhydrous molar volume of the surfactant molecules on the SWNT surfaces, 270 ± 20 cm3 mol−1. Additionally, analytical ultracentrifugation is used to measure and compare the sedimentation rates of bundled and isolated carbon nanotubes. This study should serve as a guide for designing centrifuge-based processing procedures for preparing samples of SWNTs for a wide variety of applications and studies. Additionally, the results obtained here should aid in understanding the hydrodynamic properties of SWNTs and the interactions between SWNTs and surfactants in aqueous solution.Keywords: carbon nanotubes; density; diffusion; hydrodynamic; sedimentation; surfactant encapsulated; ultracentrifuge
Co-reporter:Liam S.C. Pingree, Matthew T. Russell, Brian J. Scott, Tobin J. Marks, Mark C. Hersam
Organic Electronics 2007 Volume 8(Issue 5) pp:465-479
Publication Date(Web):October 2007
DOI:10.1016/j.orgel.2007.02.008
Two recently developed atomic force microscopy (AFM) techniques are used to characterize the impedance and charge transport/emission characteristics of individually addressed micro- and nano-scale organic light-emitting diodes (OLEDs). To fabricate independent diodes at this length scale, a suspended silicon nitride membrane shadow mask scheme is employed with semiconductor processing and electron beam lithography. This approach enables the fabrication of individually addressable OLEDs ranging in size from microns down to hundreds of nanometers. Atomic force electroluminescence microscopy (AFEM) and bridge enhanced nanoscale impedance microscopy (BE-NIM) are used to characterize these devices. AFEM offers real-time nanometer-scale spatial resolution mapping of simultaneously acquired current, topography, and light emission data while BE-NIM enables real-time impedance spectroscopy studies of functioning OLEDs. These two AFM techniques are shown to be capable of analyzing device-to-device response variations across a broad range of length scales and to provide unique quantification of intra-array device variations.
Co-reporter:L.S.C. Pingree, M.T. Russell, T.J. Marks, M.C. Hersam
Thin Solid Films 2007 Volume 515(Issue 11) pp:4783-4787
Publication Date(Web):9 April 2007
DOI:10.1016/j.tsf.2006.11.186
Electronic trap densities at the indium tin oxide (ITO)/hole transport layer (HTL) interface in operating organic light-emitting diodes (OLEDs) are characterized in situ using impedance spectroscopy. For OLEDs with a high density of active trap states, negative values of the frequency derivative of resistance are clearly observable for frequencies on the order of 10 kHz, whereas positive values are observed when the trap density is low With this technique, it is revealed that the trap density is minimized via the introduction of a TPD-Si2 (4,4′-bis[(p-trichlorosilylpropylphenyl) phenylamino]-biphenyl) passivation layer at the ITO/HTL interface or by the application of large electric fields during device operation. Furthermore, impedance spectroscopy illustrates that the ITO/HTL interface is not a simple series resistance when traps are present since they are shown not to contribute to high frequency conduction. Overall, this paper demonstrates that the parasitic effects of interface traps can mask the underlying negative capacitive transport in OLEDs and presents a technique capable of monitoring the trap density of buried interfaces in organic electronic devices.
Co-reporter:
Nature Nanotechnology 2006 1(1) pp:
Publication Date(Web):
DOI:10.1038/nnano.2006.52
The heterogeneity of as-synthesized single-walled carbon nanotubes (SWNTs) precludes their widespread application in electronics, optics and sensing. We report on the sorting of carbon nanotubes by diameter, bandgap and electronic type using structure-discriminating surfactants to engineer subtle differences in their buoyant densities. Using the scalable technique of density-gradient ultracentrifugation, we have isolated narrow distributions of SWNTs in which >97% are within a 0.02-nm-diameter range. Furthermore, using competing mixtures of surfactants, we have produced bulk quantities of SWNTs of predominantly a single electronic type. These materials were used to fabricate thin-film electrical devices of networked SWNTs characterized by either metallic or semiconducting behaviour.
Co-reporter:C. R. Kinser;M. J. Schmitz;M. C. Hersam
Advanced Materials 2006 Volume 18(Issue 11) pp:1377-1380
Publication Date(Web):24 APR 2006
DOI:10.1002/adma.200501231
Conductive atomic force microscope (AFM) nanopatterning on hydrogen-terminated silicon in a hydrophobic organic solvent under ambient conditions produces features consistent with AFM field-induced oxidation. The growth rate of the oxide features (see figure) exhibits modulation consistent with a space-charge-limited growth mechanism.
Co-reporter:M. E. Greene;A. N. Chiaramonti;S. T. Christensen;L. X. Cao;M. J. Bedzyk;M. C. Hersam
Advanced Materials 2005 Volume 17(Issue 14) pp:
Publication Date(Web):8 JUL 2005
DOI:10.1002/adma.200401459
Stable α-Fe2O3 surfaces: A 3D rendering of the controlled nanoscale morphology observed on a hematite (0001) surface is shown in the Figure. This atomically flat surface is characterized by circular depressions with diameters on the order of hundreds of nanometers and depths of 2.2 ± 0.2 Å. At room temperature, the surface is exceptionally stable, with no measured change following storage in air over several months.
Co-reporter:Nathan P. Guisinger;Nathan L. Yoder
PNAS 2005 102 (25 ) pp:8838-8843
Publication Date(Web):2005-06-21
DOI:10.1073/pnas.0501214102
A cryogenic variable-temperature ultra-high vacuum scanning tunneling microscope is used for measuring the electrical properties
of isolated cyclopentene molecules adsorbed to the degenerately p-type Si(100)-2×1 surface at a temperature of 80 K. Current–voltage
curves taken under these conditions show negative differential resistance at positive sample bias, in agreement with previous
observations at room temperature. Because of the enhanced stability of the scanning tunneling microscope at cryogenic temperatures,
repeated measurements can be routinely taken over the same molecule. Taking advantage of this improved stability, we show
that current–voltage curves on isolated cyclopentene molecules are reproducible and possess negligible hysteresis for a given
tip–molecule distance. On the other hand, subsequent measurements with variable tip position show that the negative differential
resistance voltage increases with increasing tip–molecule distance. By using a one-dimensional capacitive equivalent circuit
and a resonant tunneling model, this behavior can be quantitatively explained, thus providing insight into the electrostatic
potential distribution across a semiconductor-molecule-vacuum-metal tunnel junction. This model also provides a quantitative
estimate for the alignment of the highest occupied molecular orbital of cyclopentene with respect to the Fermi level of the
silicon substrate, thus suggesting that this experimental approach can be used for performing chemical spectroscopy at the
single-molecule level on semiconductor surfaces. Overall, these results serve as the basis for a series of design rules that
can be applied to silicon-based molecular electronic devices.
Co-reporter:Mark E. Greene, Nathan P. Guisinger, Rajiv Basu, Andrew S. Baluch, Mark C. Hersam
Surface Science 2004 Volume 559(Issue 1) pp:16-28
Publication Date(Web):10 June 2004
DOI:10.1016/j.susc.2004.04.012
The ultra-high vacuum scanning tunneling microscope (UHV-STM) was used to investigate the addition of the 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) radical to the Si(1 0 0) surface. Room temperature studies performed on clean Si(1 0 0)-2 × 1 confirm the proposed binding of the unpaired valence electron associated with the singly occupied molecular orbital (SOMO) of the molecule with a Si dangling bond. A strong bias dependence in the topography of isolated molecules was observed in the range of −2.0 to +2.5 V. Semiempirical and density functional calculations of TEMPO bound to a three-dimer silicon cluster model yield occupied state density isosurfaces below the highest occupied (HOMO) and unoccupied state densities isosurfaces above the lowest unoccupied molecular orbital (LUMO) which trend in qualitative agreement with the bias dependent STM topography. Furthermore, the placement of TEMPO molecules on dangling bonds was controlled with atomic precision on the monohydride Si(1 0 0) surface via electron stimulated desorption of H, demonstrating the compatibility of nitroxyl free radical binding chemistries with nanopatterning techniques such as feedback controlled lithography.
Co-reporter:M.W. Such, D.E. Kramer, M.C. Hersam
Ultramicroscopy 2004 Volume 99(2–3) pp:189-196
Publication Date(Web):May 2004
DOI:10.1016/j.ultramic.2003.12.005
Atomic force microscopy (AFM) is a widely used technique for characterizing the topography and frictional properties of nanostructures. Inherent misalignments between the AFM cantilever and the feedback hardware lead to crosstalk between topography data and lateral force microscopy (LFM) data. Because the degree of crosstalk depends on the positioning of the cantilever, LFM and topography data of the same structure can vary from one experiment to the next. For nanostructures with large LFM contrast, errors as large as 50% in topography and LFM can be observed. This paper describes an empirical strategy for correcting this alignment error. The technique is used to characterize the frictional properties of scanning probe-induced oxide nanostructures and the hydrogen-terminated Si(1 1 1) surfaces on which they are patterned. Reproducible differences in the frictional properties of the oxide nanostructures patterned on HF-treated and NH4F-treated Si(1 1 1) surfaces are observed and attributed to the mixed-hydride versus monohydride termination of each surface. The observed frictional contrast is consistent with known differences in surface reactivity and demonstrates how LFM measurements can provide insight into the frictional and chemical properties of nanostructures
Co-reporter:Hunter J. Karmel, John J. Garramone, Jonathan D. Emery, Sumit Kewalramani, Michael J. Bedzyk and Mark C. Hersam
Chemical Communications 2014 - vol. 50(Issue 64) pp:NaN8855-8855
Publication Date(Web):2014/06/13
DOI:10.1039/C4CC02761B
Scanning tunnelling microscopy and X-ray reflectivity are used to characterize adlayers of perylenetetracarboxylic diimide (PTCDI) deposited on epitaxial graphene (EG) on SiC(0001). PTCDI adopts a herringbone structural phase on EG/SiC that can accommodate sub-5 nm voids with molecularly defined boundaries and isolated molecular vacancies at room temperature. The PTCDI monolayer remains intact up to substrate temperatures of ∼260 °C, thus demonstrating enhanced thermal stability compared to previously studied perylene derivatives on EG/SiC.
Co-reporter:Michael A. Walsh and Mark C. Hersam
Chemical Communications 2010 - vol. 46(Issue 7) pp:NaN1155-1155
Publication Date(Web):2009/12/23
DOI:10.1039/B917502D
Scanning tunneling microscopy reveals multiple binding configurations for o-phthalaldehyde (OP) molecules on the Si(100)-2 × 1:H surface, including evidence for a chain reaction in which OP molecules bind to the surface via two covalent bonds.
Co-reporter:Deep Jariwala, Vinod K. Sangwan, Lincoln J. Lauhon, Tobin J. Marks and Mark C. Hersam
Chemical Society Reviews 2013 - vol. 42(Issue 7) pp:NaN2860-2860
Publication Date(Web):2012/11/05
DOI:10.1039/C2CS35335K
In the last three decades, zero-dimensional, one-dimensional, and two-dimensional carbon nanomaterials (i.e., fullerenes, carbon nanotubes, and graphene, respectively) have attracted significant attention from the scientific community due to their unique electronic, optical, thermal, mechanical, and chemical properties. While early work showed that these properties could enable high performance in selected applications, issues surrounding structural inhomogeneity and imprecise assembly have impeded robust and reliable implementation of carbon nanomaterials in widespread technologies. However, with recent advances in synthesis, sorting, and assembly techniques, carbon nanomaterials are experiencing renewed interest as the basis of numerous scalable technologies. Here, we present an extensive review of carbon nanomaterials in electronic, optoelectronic, photovoltaic, and sensing devices with a particular focus on the latest examples based on the highest purity samples. Specific attention is devoted to each class of carbon nanomaterial, thereby allowing comparative analysis of the suitability of fullerenes, carbon nanotubes, and graphene for each application area. In this manner, this article will provide guidance to future application developers and also articulate the remaining research challenges confronting this field.
Co-reporter:Kyle A. Luck, Tejas A. Shastry, Stephen Loser, Gabriel Ogien, Tobin J. Marks and Mark C. Hersam
Physical Chemistry Chemical Physics 2013 - vol. 15(Issue 48) pp:NaN20972-20972
Publication Date(Web):2013/11/08
DOI:10.1039/C3CP54623C
Organic photovoltaics have the potential to serve as lightweight, low-cost, mechanically flexible solar cells. However, losses in efficiency as laboratory cells are scaled up to the module level have to date impeded large scale deployment. Here, we report that a 3-aminopropyltriethoxysilane (APTES) cathode interfacial treatment significantly enhances performance reproducibility in inverted high-efficiency PTB7:PC71BM organic photovoltaic cells, as demonstrated by the fabrication of 100 APTES-treated devices versus 100 untreated controls. The APTES-treated devices achieve a power conversion efficiency of 8.08 ± 0.12% with histogram skewness of −0.291, whereas the untreated controls achieve 7.80 ± 0.26% with histogram skewness of −1.86. By substantially suppressing the interfacial origins of underperforming cells, the APTES treatment offers a pathway for fabricating large-area modules with high spatial performance uniformity.
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