Co-reporter:Bo Wang, Fatemeh Rezaeifar, Jihan Chen, Sisi Yang, Rehan Kapadia, and Stephen B. Cronin
ACS Photonics November 15, 2017 Volume 4(Issue 11) pp:2706-2706
Publication Date(Web):September 27, 2017
DOI:10.1021/acsphotonics.7b00455
We observe bright electroluminescence from suspended carbon nanotube (CNT) field effect transistors (FETs) under extremely low applied electrical powers (∼nW). Here, light emission occurs under positive applied gate voltages, with the FET in its “off” state. This enables us to apply high bias voltages (4 V) without heating the CNT. Under these conditions, we observe light emission at currents as small as 1 nA and corresponding electrical powers of 4nW, which is 3 orders of magnitude lower than previous studies. Thermal emission is ruled out by monitoring the G band Raman frequency, which shows no evidence of heating under these small electrical currents. The mechanism of light emission is understood on the basis of steep band bending that occurs in the conduction and valence band profiles at the contacts, which produces a peak electric field of 500 kV/cm, enabling the acceleration of carriers beyond the threshold of exciton emission. The exciton-generated electrons and holes are then accelerated in this field and emit excitons in an avalanche process. This is evidenced by an extremely sharp increase in the current with bias voltage (45 mV/dec). We also observe light emission at negative applied gate voltages when the FET is in its “on” state at comparable electrical powers to those reported previously (∼5 μW). However, substantial Joule heating (T > 1000 K) is also observed under these conditions, and it is difficult to separate the mechanisms of thermal emission from hot carrier photoemission in this regime.Keywords: avalanche; ballistic; band-to-band; high-field; photoemission;
Co-reporter:Zhen LiSage R. Bauers, Nirakar Poudel, Danielle Hamann, Xiaoming WangDavid S. Choi, Keivan Esfarjani, Li Shi, David C. Johnson, Stephen B. Cronin
Nano Letters 2017 Volume 17(Issue 3) pp:
Publication Date(Web):February 8, 2017
DOI:10.1021/acs.nanolett.6b05402
We report cross-plane thermoelectric measurements of misfit layered compounds (SnSe)n(TiSe2)n (n = 1,3,4,5), approximately 50 nm thick. Metal resistance thermometers are fabricated on the top and bottom of the (SnSe)n(TiSe2)n material to measure the temperature difference and heat transport through the material directly. By varying the number of layers in a supercell, n, we vary the interface density while maintaining a constant global stoichiometry. The Seebeck coefficient measured across the (SnSe)n(TiSe2)n samples was found to depend strongly on the number of layers in the supercell (n). When n decreases from 5 to 1, the cross-plane Seebeck coefficient decreases from −31 to −2.5 μV/K, while the cross-plane effective thermal conductivity decreases by a factor of 2, due to increased interfacial phonon scattering. The cross-plane Seebeck coefficients of the (SnSe)n(TiSe2)n are very different from the in-plane Seebeck coefficients, which are higher in magnitude and less sensitive to the number of layers in a supercell, n. We believe this difference is due to the different carrier types in the n-SnSe and p-TiSe2 layers and the effect of tunneling on the cross-plane transport.Keywords: 2D material stack; cross-plane; Intergrowth compounds; Seebeck coefficient;
Co-reporter:Ioannis Chatzakis;Zhen Li;Alexander V. Benderskii
Nanoscale (2009-Present) 2017 vol. 9(Issue 4) pp:1721-1726
Publication Date(Web):2017/01/26
DOI:10.1039/C6NR07054J
We report a terahertz optical modulator consisting of randomly stacked trilayer graphene (TLG) deposited on an oxidized silicon substrate by means of THz-Time Domain Spectroscopy (THz-TDS). Here, the gate tuning of the Fermi level of the TLG provides the fundamental basis for the modulation of THz transmission. We measured a 15% change in the THz transmission of this device over a broad frequency range (0.6–1.6 THz). We also observed a strong absorption >80% in the time-domain signals and a frequency independence of the conductivity. Furthermore, unlike previous studies, we find that the underlying silicon substrate, which serves as a gate electrode for the graphene, also exhibits substantial modulation of the transmitted THz radiation under applied voltage biases.
Co-reporter:Bingya Hou;Lang Shen;Haotian Shi;Rehan Kapadia
Physical Chemistry Chemical Physics 2017 vol. 19(Issue 4) pp:2877-2881
Publication Date(Web):2017/01/25
DOI:10.1039/C6CP07542H
We report measurements of photocatalytic water splitting using Au films with and without TiO2 coatings. In these structures, a thin (3–10 nm) film of TiO2 is deposited using atomic layer deposition (ALD) on top of a 100 nm thick Au film. We utilize an AC lock-in technique, which enables us to detect the relatively small photocurrents (∼μA) produced by the short-lived hot electrons that are photoexcited in the metal. Under illumination, the bare Au film produces a small AC photocurrent (<1 μA) for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) due to hot electrons and hot holes, respectively, that are photoexcited in the Au film. The samples with TiO2 produce a larger AC photocurrent indicating that hot electrons are being injected from the metal into the TiO2 semiconductor where they then reduce hydrogen ions in solution forming H2 (i.e., 2H+ + 2e− → H2). The AC photocurrent exhibits a narrow peak when plotted as a function of reference potential, which is a signature of hot electrons. Here, we photoexcite a monoenergetic source of hot electrons, which produces a peak in the photocurrent, as the electrode potential is swept through the resonance with the redox potential of the desired half-reaction. This stands in contrast to conventional bulk semiconductor photocatalysts, whose AC photocurrent saturates beyond a certain potential (i.e., light limited photocurrent). The photocurrents produced at the metal–liquid interface are smaller than those of the metal–semiconductor system, mainly because, in the metal–semiconductor system, there is a continuum of energy and momentum states that each hot electron can be injected into, while for an ion in solution, the number of energy and momentum states are very small.
Co-reporter:Zhen Li, Sisi Yang, Rohan Dhall, Ewa Kosmowska, Haotian Shi, Ioannis Chatzakis, and Stephen B. Cronin
ACS Nano 2016 Volume 10(Issue 7) pp:6836
Publication Date(Web):July 8, 2016
DOI:10.1021/acsnano.6b02488
We report Raman and photoluminescence spectra of mono- and few-layer WSe2 and MoSe2 taken before and after exposure to a remote oxygen plasma. For bilayer and trilayer WSe2, we observe an increase in the photoluminescence intensity and a blue shift of the photoluminescence peak positions after oxygen plasma treatment. The photoluminescence spectra of trilayer WSe2 exhibit features of a bilayer after oxygen plasma treatment. Bilayer WSe2 exhibits features of a monolayer, and the photoluminescence of monolayer WSe2 is completely absent after the oxygen plasma treatment. These changes are observed consistently in more than 20 flakes. The mechanism of the changes observed in the photoluminescence spectra of WSe2 is due to the selective oxidation of the topmost layer. As a result, N-layer WSe2 is reduced to N–1 layers. Raman spectra and AFM images taken from the WSe2 flakes before and after the oxygen treatment corroborate these findings. Because of the low kinetic energy of the oxygen radicals in the remote oxygen plasma, the oxidation is self-limiting. By varying the process duration from 1 to 10 min, we confirmed that the oxidation will only affect the topmost layer of the WSe2 flakes. X-ray photoelectron spectroscopy shows that the surface layer WOx of the sample can be removed by a quick dip in KOH solution. Therefore, this technique provides a promising way of controlling the thickness of WSe2 layer by layer.Keywords: dichalcogenide; MoSe2; oxidation; oxygen plasma; photoluminescence; WSe2
Co-reporter:Rohan Dhall, Kyle Seyler, Zhen Li, Darshana Wickramaratne, Mahesh R. Neupane, Ioannis Chatzakis, Ewa Kosmowska, Roger K. Lake, Xiaodong Xu, and Stephen B. Cronin
ACS Photonics 2016 Volume 3(Issue 3) pp:
Publication Date(Web):February 10, 2016
DOI:10.1021/acsphotonics.5b00593
We report circularly polarized photoluminescence spectra taken from few layer MoS2 after treatment with a remotely generated oxygen plasma. Here, the oxygen plasma decouples the individual layers in MoS2 by perturbing the weak interlayer van der Waals forces without damaging the lattice structure. This decoupling causes a transition from an indirect to a direct band gap material, which causes a strong enhancement of the photoluminescence intensity. Furthermore, up to 80% circularly polarized photoluminescence is observed after plasma treatment of few layer MoS2 flakes, consistent with high spin polarization of the optically excited carriers. A strong degree of polarization continues up to room temperature, further indicating that the quality of the crystal does not suffer degradation due to the oxygen plasma exposure. Our results show that the oxygen plasma treatment not only engineers the van der Waals separation in these TMDC multilayers for enhanced PL quantum yields, but also produces high quality multilayer samples for strong circularly polarized emission, which offers the benefit of layer index as an additional degree of freedom, absent in monolayer MoS2.
Co-reporter:Rohan Dhall;Mahesh R. Neupane;Darshana Wickramaratne;Matthew Mecklenburg;Zhen Li;Cameron Moore;Roger K. Lake;Stephen Cronin
Advanced Materials 2015 Volume 27( Issue 9) pp:1573-1578
Publication Date(Web):
DOI:10.1002/adma.201405259
We report a robust method for engineering the optoelectronic properties of many-layer MoS2 using low-energy oxygen plasma treatment. Gas phase treatment of MoS2 with oxygen radicals generated in an upstream N2–O2 plasma is shown to enhance the photoluminescence (PL) of many-layer, mechanically exfoliated MoS2 flakes by up to 20 times, without reducing the layer thickness of the material. A blueshift in the PL spectra and narrowing of linewidth are consistent with a transition of MoS2 from indirect to direct bandgap material. Atomic force microscopy and Raman spectra reveal that the flake thickness actually increases as a result of the plasma treatment, indicating an increase in the interlayer separation in MoS2. Ab initio calculations reveal that the increased interlayer separation is sufficient to decouple the electronic states in individual layers, leading to a transition from an indirect to direct gap semiconductor. With optimized plasma treatment parameters, we observed enhanced PL signals for 32 out of 35 many-layer MoS2 flakes (2–15 layers) tested, indicating that this method is robust and scalable. Monolayer MoS2, while direct bandgap, has a small optical density, which limits its potential use in practical devices. The results presented here provide a material with the direct bandgap of monolayer MoS2, without reducing sample thickness, and hence optical density.
Co-reporter:Zhen Li, Goutham Ezhilarasu, Ioannis Chatzakis, Rohan Dhall, Chun-Chung Chen, and Stephen B. Cronin
Nano Letters 2015 Volume 15(Issue 6) pp:3977-3982
Publication Date(Web):May 20, 2015
DOI:10.1021/acs.nanolett.5b00885
Transition metal dichalcogenides (TMDCs), such as MoS2 and WSe2, are free of dangling bonds and therefore make more “ideal” Schottky junctions than bulk semiconductors, which produce Fermi energy pinning and recombination centers at the interface with bulk metals, inhibiting charge transfer. Here, we observe a more than 10× enhancement in the indirect band gap photoluminescence of transition metal dichalcogenides (TMDCs) deposited on various metals (e.g., Cu, Au, Ag), while the direct band gap emission remains unchanged. We believe the main mechanism of light emission arises from photoexcited hot electrons in the metal that are injected into the conduction band of MoS2 and WSe2 and subsequently recombine radiatively with minority holes in the TMDC. Since the conduction band at the K-point is 0.5 eV higher than at the Σ-point, a lower Schottky barrier exists for the Σ-point band, making electron injection more favorable. Also, the Σ band consists of the sulfur pz orbital, which overlaps more significantly with the electron wave functions in the metal. This enhancement in the indirect emission only occurs for thick flakes of MoS2 and WSe2 (≥100 nm) and is completely absent in monolayer and few-layer (∼10 nm) flakes. Here, the flake thickness must exceed the depletion width of the Schottky junction, in order for efficient radiative recombination to occur in the TMDC. The intensity of this indirect peak decreases at low temperatures, which is consistent with the hot electron injection model.
Co-reporter:Jing Qiu, Guangtong Zeng, Mai-Anh Ha, Mingyuan Ge, Yongjing Lin, Mark Hettick, Bingya Hou, Anastassia N. Alexandrova, Ali Javey, and Stephen B. Cronin
Nano Letters 2015 Volume 15(Issue 9) pp:6177-6181
Publication Date(Web):August 12, 2015
DOI:10.1021/acs.nanolett.5b02511
Here, we report photocatalytic CO2 reduction with water to produce methanol using TiO2-passivated InP nanopillar photocathodes under 532 nm wavelength illumination. In addition to providing a stable photocatalytic surface, the TiO2-passivation layer provides substantial enhancement in the photoconversion efficiency through the introduction of O vacancies associated with the nonstoichiometric growth of TiO2 by atomic layer deposition. Plane wave-density functional theory (PW-DFT) calculations confirm the role of oxygen vacancies in the TiO2 surface, which serve as catalytically active sites in the CO2 reduction process. PW-DFT shows that CO2 binds stably to these oxygen vacancies and CO2 gains an electron (−0.897e) spontaneously from the TiO2 support. This calculation indicates that the O vacancies provide active sites for CO2 absorption, and no overpotential is required to form the CO2– intermediate. The TiO2 film increases the Faraday efficiency of methanol production by 5.7× to 4.79% under an applied potential of −0.6 V vs NHE, which is 1.3 V below the Eo(CO2/CO2–) = −1.9 eV standard redox potential. Copper nanoparticles deposited on the TiO2 act as a cocatalyst and further improve the selectivity and yield of methanol production by up to 8-fold with a Faraday efficiency of 8.7%.
Co-reporter:Guangtong Zeng;Jing Qiu;Bingya Hou;Haotian Shi;Dr. Yongjing Lin;Mark Hettick; Ali Javey; Stephen B. Cronin
Chemistry - A European Journal 2015 Volume 21( Issue 39) pp:13502-13507
Publication Date(Web):
DOI:10.1002/chem.201501671
Abstract
A robust and reliable method for improving the photocatalytic performance of InP, which is one of the best known materials for solar photoconversion (i.e., solar cells). In this article, we report substantial improvements (up to 18×) in the photocatalytic yields for CO2 reduction to CO through the surface passivation of InP with TiO2 deposited by atomic layer deposition (ALD). Here, the main mechanisms of enhancement are the introduction of catalytically active sites and the formation of a pn-junction. Photoelectrochemical reactions were carried out in a nonaqueous solution consisting of ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]BF4), dissolved in acetonitrile, which enables CO2 reduction with a Faradaic efficiency of 99 % at an underpotential of +0.78 V. While the photocatalytic yield increases with the addition of the TiO2 layer, a corresponding drop in the photoluminescence intensity indicates the presence of catalytically active sites, which cause an increase in the electron-hole pair recombination rate. NMR spectra show that the [EMIM]+ ions in solution form an intermediate complex with CO2−, thus lowering the energy barrier of this reaction.
Co-reporter:Shermin Arab, P. Duke Anderson, Chun-Yung Chi, P. Daniel Dapkus, Michelle L. Povinelli, and Stephen B. Cronin
ACS Photonics 2015 Volume 2(Issue 8) pp:
Publication Date(Web):July 13, 2015
DOI:10.1021/acsphotonics.5b00179
GaAs nanosheets with no twin defects, stacking faults, or dislocations are excellent candidates for optoelectrical applications. Their outstanding optical behavior and twin free structure make them superior to traditionally studied GaAs nanowires. While many research groups have reported optically resonant cavities (i.e., Fabry–Perot) in 1D nanowires, here, we report an optical cavity resonance in GaAs nanosheets consisting of complex 2D asymmetric modes, which are fundamentally different from one-dimensional cavities. These resonant modes are detected experimentally using photoluminescence (PL) spectroscopy, which exhibits a series of peaks or “fringes” superimposed on the bulk GaAs photoluminescence spectrum. Finite-difference time-domain (FDTD) simulations confirm these experimental findings and provide a detailed picture of these complex resonant modes. Here, the complex modes of this cavity are formed by the three nonparallel edges of the GaAs nanosheets. Due to the asymmetrical nature of the nanosheets, the mode profiles are largely unintuitive. We also find that by changing the substrate from Si/SiO2 to Au, we enhance the resonance fringes as well as the overall optical emission by 5× at room temperature. Our FDTD simulation results confirm that this enhancement is caused by the local field enhancement of the Au substrate and indicate that the thickness of the nanosheets plays an important role in the formation and enhancement of fringes.
Co-reporter:Guangtong Zeng;Jing Qiu;Bingya Hou;Haotian Shi;Dr. Yongjing Lin;Mark Hettick; Ali Javey; Stephen B. Cronin
Chemistry - A European Journal 2015 Volume 21( Issue 39) pp:
Publication Date(Web):
DOI:10.1002/chem.201583961
Co-reporter:Shermin Arab, Chun-Yung Chi, Teng Shi, Yuda Wang, Daniel P. Dapkus, Howard E. Jackson, Leigh M. Smith, and Stephen B. Cronin
ACS Nano 2015 Volume 9(Issue 2) pp:1336
Publication Date(Web):January 7, 2015
DOI:10.1021/nn505227q
Unlike nanowires, GaAs nanosheets exhibit no twin defects, stacking faults, or dislocations even when grown on lattice mismatched substrates. As such, they are excellent candidates for optoelectronic applications, including LEDs and solar cells. We report substantial enhancements in the photoluminescence efficiency and the lifetime of passivated GaAs nanosheets produced using the selected area growth (SAG) method with metal organic chemical vapor deposition (MOCVD). Measurements are performed on individual GaAs nanosheets with and without an AlGaAs passivation layer. Both steady-state photoluminescence and time-resolved photoluminescence spectroscopy are performed to study the optoelectronic performance of these nanostructures. Our results show that AlGaAs passivation of GaAs nanosheets leads to a 30- to 40-fold enhancement in the photoluminescence intensity. The photoluminescence lifetime increases from less than 30 to 300 ps with passivation, indicating an order of magnitude improvement in the minority carrier lifetime. We attribute these enhancements to the reduction of nonradiative recombination due to the compensation of surface states after passivation. The surface recombination velocity decreases from an initial value of 2.5 × 105 to 2.7 × 104 cm/s with passivation.Keywords: AlGaAs; GaAs; lifetime; MOCVD; nanosheet; photoluminescence;
Co-reporter:Shun-Wen Chang, Jubin Hazra, Moh Amer, Rehan Kapadia, and Stephen B. Cronin
ACS Nano 2015 Volume 9(Issue 12) pp:11551
Publication Date(Web):October 24, 2015
DOI:10.1021/acsnano.5b03873
We present a comparative study of quasi-metallic (Eg ∼ 100 meV) and semiconducting (Eg ∼ 1 eV) suspended carbon nanotube pn-junctions introduced by electrostatic gating. While the built-in fields of the quasi-metallic carbon nanotubes (CNTs) are 1–2 orders of magnitude smaller than those of the semiconducting CNTs, their photocurrent is 2 orders of magnitude higher than the corresponding semiconducting CNT devices under the same experimental conditions. Here, the large exciton binding energy in semiconducting nanotubes (∼400 meV) makes it difficult for excitons to dissociate into free carriers that can contribute to an externally measured photocurent. As such, semiconducting nanotubes require a phonon to assist in the exciton dissociation process, in order to produce a finite photocurrent, while quasi-metallic nanotubes do not. The quasi-metallic nanotubes have much lower exciton binding energies (∼50 meV) as well as a continuum of electronic states to decay into and, therefore, do not require the absorption of a phonon in order to dissociate, making it much easier for these excitons to produce a photocurrent. We performed detailed simulations of the band energies in quasi-metallic and semiconducting nanotube devices in order to obtain the electric field profiles along the lengths of the nanotubes. These simulations predict maximum built-in electric field strengths of 2.3 V/μm for semiconducting and 0.032–0.22 V/μm for quasi-metallic nanotubes under the applied gate voltages used in this study.Keywords: carbon nanotube; exciton; photocurrent; pn-junction; quasi-metallic;
Co-reporter:Guangtong Zeng, Jing Qiu, Zhen Li, Prathamesh Pavaskar, and Stephen B. Cronin
ACS Catalysis 2014 Volume 4(Issue 10) pp:3512
Publication Date(Web):September 3, 2014
DOI:10.1021/cs500697w
In the past, the electrochemical instability of III–V semiconductors has severely limited their applicability in photocatlaysis. As a result, a vast majority of the research on photocatalysis has been done on TiO2, which is chemically robust over a wide range of pH. However, TiO2 has a wide band gap (3.2 eV) and can only absorb ∼4% of the solar spectrum, and thus, it will never provide efficient solar energy conversion/storage on its own. Here, we report photocatalytic CO2 reduction with water to produce methanol using TiO2-passivated GaP photocathodes under 532 nm wavelength illumination. The TiO2 layer prevents corrosion of the GaP, as evidenced by atomic force microscopy and photoelectrochemical measurements. Here, the GaP surface is passivated using a thin film of TiO2 deposited by atomic layer deposition (ALD), which provides a viable, stable photocatalyst without sacrificing photocatalytic efficiency. In addition to providing a stable photocatalytic surface, the TiO2 passivation provides substantial enhancement in the photoconversion efficiency through passivation of surface states, which cause nonradiative carrier recombination. In addition to passivation effects, the TiO2 deposited by ALD is n-type due to oxygen vacancies and forms a pn-junction with the underlying p-type GaP photocathode. This creates a built-in field that assists in the separation of photogenerated electron–hole pairs, further reducing recombination. This reduction in the surface recombination velocity (SRV) corresponds to a shift in the overpotential of almost 0.5 V. No enhancement is observed for TiO2 thicknesses above 10 nm, due to the insulating nature of the TiO2, which eventually outweighs the benefits of passivation.Keywords: CO2 reduction; GaP; methanol; photoelectrochemical; TiO2 passivated
Co-reporter:Jing Qiu, Guangtong Zeng, Prathamesh Pavaskar, Zhen Li and Stephen B. Cronin
Physical Chemistry Chemical Physics 2014 vol. 16(Issue 7) pp:3115-3121
Publication Date(Web):20 Dec 2013
DOI:10.1039/C3CP54674H
Integrating plasmon resonant nanostructures with photocatalytic semiconductors shows great promise for high efficiency photocatalytic water splitting. However, the electrochemical instability of most III–V semiconductors severely limits their applicability in photocatalysis. In this work, we passivate p-type GaP with a thin layer of n-type TiO2 using atomic layer deposition. The TiO2 passivation layer prevents corrosion of the GaP, as evidenced by atomic force microscopy and photoelectrochemical measurements. In addition, the TiO2 passivation layer provides an enhancement in photoconversion efficiency through the formation of a charge separating pn-region. Plasmonic Au nanoparticles deposited on top of the TiO2-passivated GaP further increases the photoconversion efficiency through local field enhancement. These two enhancement mechanisms are separated by systematically varying the thickness of the TiO2 layer. Because of the tradeoff between the quickly decaying plasmonic fields and the formation of the pn-charge separation region, an optimum performance is achieved for a TiO2 thickness of 0.5 nm. Finite difference time domain (FDTD) simulations of the electric field profiles in this photocatalytic heterostructure corroborate these results. The effects of plasmonic enhancement are distinguished from the natural catalytic properties of Au by evaluating similar photocatalytic TiO2/GaP structures with catalytic, non-plasmonic metals (i.e., Pt) instead of Au. This general approach of passivating narrower band gap semiconductors enables a wider range of materials to be considered for plasmon-enhanced photocatalysis for high efficiency water splitting.
Co-reporter:Jing Qiu, Guangtong Zeng, Prathamesh Pavaskar, Zhen Li and Stephen B. Cronin
Physical Chemistry Chemical Physics 2014 vol. 16(Issue 48) pp:27134-27134
Publication Date(Web):11 Nov 2014
DOI:10.1039/C4CP90165G
Correction for ‘Plasmon-enhanced water splitting on TiO2-passivated GaP photocatalysts’ by Jing Qiu et al., Phys. Chem. Chem. Phys., 2014, 16, 3115–3121.
Co-reporter:Leyre Gomez, Victor Sebastian, Manuel Arruebo, Jesus Santamaria and Stephen B. Cronin
Physical Chemistry Chemical Physics 2014 vol. 16(Issue 29) pp:15111-15116
Publication Date(Web):10 Feb 2014
DOI:10.1039/C4CP00229F
Increasing water demand and water scarcity around the world requires the development of robust and efficient methods for water purification in the coming decades. Here, we report a photocatalytic water purification method using visible light (532 nm) utilizing 5 nm gold nanoparticles and their enhancement when attached on the surface of silica nanospheres as an inactive support to prevent nanoparticle coalescence or sintering. This is a non-toxic, low-cost, and easy photocatalytic process which provides high decomposition rates. Decomposition of the methyl orange dye is tested as a reaction model and trichloroethylene is selected as an example of a real water pollutant. When irradiated at their plasmon resonant frequency, the gold nanoparticles generate hydroxyl radicals that degradate organic pollutants into non-toxic molecules representing a basic mechanism of photocatalytic water purification.
Co-reporter:Chia-Chi Chang;Chun-Yung Chi;Chun-Chung Chen;Ningfeng Huang
Nano Research 2014 Volume 7( Issue 2) pp:163-170
Publication Date(Web):2014 February
DOI:10.1007/s12274-013-0383-x
We have recently demonstrated that GaAs nanosheets can be grown by metal-organic chemical vapor deposition (MOCVD). Here, we investigate these nanosheets by secondary electron scanning electron microscopy (SE-SEM) and electron beam induced current (EBIC) imaging. An abrupt boundary is observed between an initial growth region and an overgrowth region in the nanosheets. The SE-SEM contrast between these two regions is attributed to the inversion of doping at the boundary. EBIC mapping reveals a p-n junction formed along the boundary between these two regions. Rectifying I–V behavior is observed across the boundary further indicating the formation of a p-n junction. The electron concentration (ND) of the initial growth region is around 1 × 1018 cm−3, as determined by both Hall effect measurements and low temperature photoluminescence (PL) spectroscopy. Based on the EBIC data, the minority carrier diffusion length of the nanosheets is 177 nm, which is substantially longer than the corresponding length in unpassivated GaAs nanowires measured previously.
Co-reporter:Shermin Arab;P. Duke Anderson;Maoqing Yao;Chongwu Zhou
Nano Research 2014 Volume 7( Issue 8) pp:1146-1153
Publication Date(Web):2014 August
DOI:10.1007/s12274-014-0477-0
Co-reporter:Zhen Li;Shun-Wen Chang;Chun-Chung Chen
Nano Research 2014 Volume 7( Issue 7) pp:973-980
Publication Date(Web):2014 July
DOI:10.1007/s12274-014-0459-2
We report substantial improvements and modulation in the photocurrent (PC) and photoluminescence (PL) spectra of monolayer MoS2 recorded under electrostatic and ionic liquid gating conditions. The photocurrent and photoluminescence spectra show good agreement with a dominant peak at 1.85 eV. The magnitude of the photoluminescence can be increased 300% by ionic liquid gating due to the passivation of surface states and trapped charges that act as recombination centers. The photocurrent also doubles when passivated by the ionic liquid. Interestingly, a significant shift of the PL peak position is observed under electrostatic (14 meV) and ionic liquid (30 meV) gating, as a result of passivation. The ionic liquid provides significant screening without any externally applied voltage, indicating that these surface recombination centers have net charge. The acute sensitivity of monolayer MoS2 to ionic liquid gating and passivation arises because of its high surface-to-volume ratio, which makes it especially sensitive to trapped charge and surface states. These results reveal that, in order for efficient optoelectronic devices to be made from monolayer MoS2, some passivation strategy must be employed to mitigate the issues associated with surface recombination.
Co-reporter:Jesse Theiss;Mehmet Aykol;Prathamesh Pavaskar
Nano Research 2014 Volume 7( Issue 9) pp:1344-1354
Publication Date(Web):2014 September
DOI:10.1007/s12274-014-0499-7
We fabricate arrays of metallic nanoparticle dimers with nanometer separation using electron beam lithography and angle evaporation. These “nanogap” dimers are fabricated on thin silicon nitride membranes to enable high resolution transmission electron microscope imaging of the specific nanoparticle geometries. Plasmonic resonances of the pairs are characterized by dark-field scattering micro-spectroscopy, which enables the optical scattering from individual nanostructures to be measured by using a spatially-filtered light source to illuminate a small area. Scattering spectra from individual dimers are correlated with transmission electron microscope images and finite-difference time-domain simulations of their electromagnetic response, with excellent agreement between simulation and experiment. We observe a strong polarization dependence with two dominant scattering peaks in spectra taken with the polarization aligned along the dimer axis. This response arises from a unique Fano interference, in which the bright hybridized modes of an asymmetric dimer are able to couple to the dark higherorder hybridized modes through substrate-mediated coupling. The presence of this interference is strongly dependent on the nanoparticle geometry that defines the plasmon energy profile but also on the intense localization of charge at the dielectric surface in the nanogap region for separations smaller than 6 nm.
Co-reporter:Moh. R. Amer, Shun-Wen Chang, Rohan Dhall, Jing Qiu, and Stephen B. Cronin
Nano Letters 2013 Volume 13(Issue 11) pp:5129-5134
Publication Date(Web):October 15, 2013
DOI:10.1021/nl402334e
We investigate the electronic and optoelectronic properties of quasi-metallic nanotube pn-devices, which have smaller band gaps than most known bulk semiconductors. These carbon nanotube-based devices deviate from conventional bulk semiconductor device behavior due to their low-dimensional nature. We observe rectifying behavior based on Zener tunneling of ballistic carriers instead of ideal diode behavior, as limited by the diffusive transport of carriers. We observe substantial photocurrents at room temperature, suggesting that these quasi-metallic pn-devices may have a broader impact in optoelectronic devices. A new technique based on photocurrent spectroscopy is presented to identify the unique chirality of nanotubes in a functional device. This chirality information is crucial in obtaining a theoretical understanding of the underlying device physics that depends sensitively on nanotube chirality, as is the case for quasi-metallic nanotube devices. A detailed model is developed to fit the observed I–V characteristics, which enables us to verify the band gap from these measurements as well as the dimensions of the insulating tunneling barrier region.
Co-reporter:Wenbo Hou
Advanced Functional Materials 2013 Volume 23( Issue 13) pp:1612-1619
Publication Date(Web):
DOI:10.1002/adfm.201202148
Abstract
In the past decade, the surface plasmon resonance of Ag and Au nanoparticles has been investigated to improve the efficiency of photocatalytic processes. The photocatalytic production of fuels is particularly interesting for its ability to store the sun's energy in chemical bonds that can be released later without producing harmful byproducts. This Feature Article reviews recent work demonstrating plasmon-enhanced photocatalytic water splitting, reduction of CO2 with H2O to form hydrocarbon fuels, and degradation of organic molecules. Focus is placed on several possible mechanisms that have been previously discussed in the literature. A particular emphasis is given to several aspects of these mechanisms that are not fully understood and will require further investigation.
Co-reporter:Chia-Chi Chang, Chun-Yung Chi, Maoqing Yao, Ningfeng Huang, Chun-Chung Chen, Jesse Theiss, Adam W. Bushmaker, Stephen LaLumondiere, Ting-Wei Yeh, Michelle L. Povinelli, Chongwu Zhou, P. Daniel Dapkus, and Stephen B. Cronin
Nano Letters 2012 Volume 12(Issue 9) pp:4484-4489
Publication Date(Web):August 13, 2012
DOI:10.1021/nl301391h
We report a systematic study of carrier dynamics in AlxGa1–xAs-passivated GaAs nanowires. With passivation, the minority carrier diffusion length (Ldiff) increases from 30 to 180 nm, as measured by electron beam induced current (EBIC) mapping, and the photoluminescence (PL) lifetime increases from sub-60 ps to 1.3 ns. A 48-fold enhancement in the continuous-wave PL intensity is observed on the same individual nanowire with and without the AlxGa1–xAs passivation layer, indicating a significant reduction in surface recombination. These results indicate that, in passivated nanowires, the minority carrier lifetime is not limited by twin stacking faults. From the PL lifetime and minority carrier diffusion length, we estimate the surface recombination velocity (SRV) to range from 1.7 × 103 to 1.1 × 104 cm·s–1, and the minority carrier mobility μ is estimated to lie in the range from 10.3 to 67.5 cm2 V–1 s–1 for the passivated nanowires.
Co-reporter:Moh. R. Amer, Adam Bushmaker, and Stephen B. Cronin
Nano Letters 2012 Volume 12(Issue 9) pp:4843-4847
Publication Date(Web):August 29, 2012
DOI:10.1021/nl302321k
We report a detailed comparison of ultraclean suspended and on-substrate carbon nanotubes (CNTs) in order to quantify the effect of the substrate interaction on the effective band gap of metallic nanotubes. Here, individual CNTs are grown across two sets of electrodes, resulting in one segment of the nanotube that is suspended across a trench and the other segment supported on the substrate. The suspended segment shows a significant change in the conductance (ΔG/G = 0.84) with applied gate voltage, which is attributed to a small band gap. The on-substrate segment, however, only shows a change in the measured conductance of ΔG/G = 0.11. A Landauer model is used to fit the low bias conductance of these devices. From these fits, the band gaps in the suspended region range from 75 to 100 meV but are only 5–14.3 meV when the nanotube is in contact with the substrate. The decreased band gap is attributed to localized doping caused by trapped charges in the substrate that result in inhomogeneous broadening of the Fermi energy, which in turn limits the ability to modulate the conductance.
Co-reporter:Moh Amer;Adam Bushmaker;Steve Cronin
Nano Research 2012 Volume 5( Issue 3) pp:172-180
Publication Date(Web):2012 March
DOI:10.1007/s12274-012-0197-2
Electrically-heated suspended, nearly defect-free, carbon nanotubes (CNTs) exhibiting negative differential conductance in the high bias regime experience a sudden drop in current (or “kink”). The bias voltage at the kink (Vkink) is found to depend strongly on gate voltage, substrate temperature, and gas environment. After subtracting the voltage drop across the contacts, however, the kink bias voltages converge around 0.2 V, independent of gate voltage and gas environment. This bias voltage of 0.2 V corresponds to the threshold energy of optical phonon emission. This phenomenon is corroborated by simultaneously monitoring the Raman spectra of these nanotubes as a function of bias voltage. At the kink bias voltage, the G band Raman modes experience a sudden downshift, further indicating threshold optical phonon emission. A Landauer model is used to fit these kinks in various gas environments where the kink is modeled as a change in the optical phonon lifetime, which corresponds to a change in the non-equilibrium factor that describes the existence of hot phonons in the system.
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Co-reporter:Chia-Chi Chang;Chun-Chung Chen;Wei-Hsuan Hung;I. -Kai Hsu
Nano Research 2012 Volume 5( Issue 12) pp:854-862
Publication Date(Web):2012 December
DOI:10.1007/s12274-012-0269-3
Co-reporter:Wenbo Hou, Prathamesh Pavaskar, Zuwei Liu, Jesse Theiss, Mehmet Aykol and Stephen B. Cronin
Energy & Environmental Science 2011 vol. 4(Issue 11) pp:4650-4655
Publication Date(Web):20 Sep 2011
DOI:10.1039/C1EE02120F
We report an improvement in the efficiency of dye sensitized solar cells (DSSCs) by exploiting the plasmonic resonance of Au nanoparticles. By comparing the performance of DSSCs with and without Au nanoparticles, we demonstrate a 2.4-fold enhancement in the photoconversion efficiency. Enhancement in the photocurrent extends over the wavelength range from 460 nm to 730 nm. The underlying mechanism of enhancement is investigated by comparing samples with different geometries, including nanoparticles deposited on top of and embedded in the TiO2 electrode, as well as samples with the light absorbing dye molecule deposited on top of and underneath the Au nanoparticles. The mechanism of enhancement is attributed to the local electromagnetic response of the plasmonic nanoparticles, which couples light very effectively from the far field to the near field at the absorbing dye molecule monolayer, thereby increasing the local electron–hole pair (or exciton) generation rate significantly. The UV-vis absorption spectra and photocurrent spectra provide further information regarding the energy transfer between the plasmonic nanoparticles and the light absorbing dye molecules. Based on scanning electron microscope images, we perform electromagnetic simulations of these different Au nanoparticle/dye/TiO2 configurations, which corroborate the enhancement observed experimentally.
Co-reporter:Wenbo Hou, Wei Hsuan Hung, Prathamesh Pavaskar, Alain Goeppert, Mehmet Aykol, and Stephen B. Cronin
ACS Catalysis 2011 Volume 1(Issue 8) pp:929
Publication Date(Web):June 29, 2011
DOI:10.1021/cs2001434
A systematic study of the mechanisms of Au nanoparticle/TiO2-catalyzed photoreduction of CO2 and water vapor is carried out over a wide range of wavelengths. When the photon energy matches the plasmon resonance of the Au nanoparticles (free carrier absorption), which is in the visible range (532 nm), we observe a 24-fold enhancement in the photocatalytic activity because of the intense local electromagnetic fields created by the surface plasmons of the Au nanoparticles. These intense electromagnetic fields enhance sub-bandgap absorption in the TiO2, thereby enhancing the photocatalytic activity in the visible range. When the photon energy is high enough to excite d band electronic transitions in the Au, in the UV range (254 nm), a different mechanism occurs resulting in the production of additional reaction products, including C2H6, CH3OH, and HCHO. This occurs because the energy of the d band excited electrons lies above the redox potentials of the additional reaction products CO2/C2H6, CO2/CH3OH, and CO2/HCHO. We model the plasmon excitation at the Au nanoparticle-TiO2 interface using finite difference time domain (FDTD) simulations, which provides a rigorous analysis of the electric fields and charge at the Au nanoparticle-TiO2 interface.Keywords: CO2; hydrocarbon fuels; interband transition; photocatalysis; photocatalytic; plasmon; plasmonic
Co-reporter:Zuwei Liu, Adam Bushmaker, Mehmet Aykol, and Stephen B. Cronin
ACS Nano 2011 Volume 5(Issue 6) pp:4634
Publication Date(Web):May 5, 2011
DOI:10.1021/nn200444x
We study the thermal emission spectra of individual suspended carbon nanotubes induced by electrical heating. Semiconducting and metallic devices exhibit different spectra, based on their distinctive band structures. These spectra are compared with the ideal blackbody emission spectrum. In the visible wavelength range, the thermal emission spectra of semiconducting devices agree well with Planck’s law, while the spectra of metallic devices show an additional peak between 1.5 and 1.9 eV. In the near-infrared wavelength range, the semiconducting nanotubes exhibit a peak around 1 eV. These additional peaks are attributed to the E11M and E22SC transitions that are thermally driven under these high applied bias voltages. These peaks show a strong polarization dependence, while the blackbody tail is unpolarized, which provides further evidence for electron–hole recombination in thermal emission. For semiconducting devices, the temperature of the nanotube is fit to Planck’s law and compared with the temperatures obtained from the G band and 2D band Raman downshifts, as well as the anti-Stokes/Stokes intensity ratio. For devices showing thermal non-equilibrium, the electron temperature agrees well with G+ downshift but deviates from G– downshift.Keywords: carbon nanotube; phonon; photon; Raman spectrum; thermal emission; transition
Co-reporter:Wei Hsuan Hung, Mehmet Aykol, David Valley, Wenbo Hou and Stephen B. Cronin
Nano Letters 2010 Volume 10(Issue 4) pp:1314-1318
Publication Date(Web):March 30, 2010
DOI:10.1021/nl9041214
Irradiating gold nanoparticles at their plasmon resonance frequency creates immense plasmonic charge and high temperatures, which can be used to drive catalytic reactions. By integrating strongly plasmonic nanoparticles with strongly catalytic metal oxides, significant enhancements in the catalytic activity can be achieved. Here, we study the plasmonically driven catalytic conversion of CO to CO2 by irradiating Au nanoparticle/Fe2O3 composites. The reaction rate of this composite greatly exceeds that of the Au nanoparticles or Fe2O3 alone, indicating that this reaction is not driven solely by the thermal (plasmonic) heating of the gold nanoparticles but relies intimately on the interaction of these two materials. A comparison of the plasmonically driven catalytic reaction rate with that obtained under uniform heating shows an enhancement of at least 2 orders of magnitude.
Co-reporter:Chia-Chi Chang, I-Kai Hsu, Mehmet Aykol, Wei-Hsuan Hung, Chun-Chung Chen, and Stephen B. Cronin
ACS Nano 2010 Volume 4(Issue 9) pp:5095
Publication Date(Web):August 10, 2010
DOI:10.1021/nn100946q
We apply immense strain to ultralong, suspended, single-walled carbon nanotubes while monitoring their Raman spectra. We can achieve strains up to 13.7 ± 0.3% without slippage, breakage, or defect formation based on the observation of reversible change in Raman spectra. This is more than twice that of previous observations. The rate of G band downshift with strain is found to span a wide range from −6.2 to −23.6 cm−1/% strain. Under these immense strains, the G band is observed to downshift by up to 157 cm−1 (from 1592 to 1435 cm−1). Interestingly, under these significant lattice distortions, we observe no detectable D band Raman intensity. Also, we do not observe any broadening of the G band line width until a threshold downshift of ΔωG > 75 cm−1 is achieved at high strains, beyond which the fwhm of the G band increases sharply and reversibly. On the basis of a theoretical nonlinear stress−strain response, we estimate the maximum applied stress of the nanotubes in this study to be 99 GPa with a strength-to-weight ratio of almost 74 000 kN·m/kg, which is 30 times that of Kevlar and 117 times that of steel.Keywords: CNTs; CVD; individual; Raman; strain; strength; stress
Co-reporter:Adam W. Bushmaker, Vikram V. Deshpande, Scott Hsieh, Marc W. Bockrath and Stephen B. Cronin
Nano Letters 2009 Volume 9(Issue 2) pp:607-611
Publication Date(Web):January 22, 2009
DOI:10.1021/nl802854x
Raman spectra and electrical conductance of individual, pristine, suspended, metallic single-walled carbon nanotubes are measured under applied gate potentials. The G- band is observed to downshift with small applied gate voltages, with the minima occurring at EF = ±1/2Ephonon, contrary to adiabatic predictions. A subsequent upshift in the Raman frequency at higher gate voltages results in a “W”-shaped Raman shift profile that agrees well with a nonadiabatic phonon renormalization model. This behavior constitutes the first experimental confirmation of the theoretically predicted breakdown of the Born−Oppenheimer approximation in individual single-walled carbon nanotubes.
Co-reporter:Adam W. Bushmaker, Vikram V. Deshpande, Scott Hsieh, Marc W. Bockrath and Stephen B. Cronin
Nano Letters 2009 Volume 9(Issue 8) pp:2862-2866
Publication Date(Web):July 10, 2009
DOI:10.1021/nl901042w
In this work, we measure the electrical conductance and temperature of individual, suspended quasi-metallic single-walled carbon nanotubes under high voltage biases using Raman spectroscopy, while varying the doping conditions with an applied gate voltage. By applying a gate voltage, the high-bias conductance can be switched dramatically between linear (Ohmic) behavior and nonlinear behavior exhibiting negative differential conductance (NDC). Phonon populations are observed to be in thermal equilibrium under Ohmic conditions but switch to nonequilibrium under NDC conditions. A typical Landauer transport model assuming zero bandgap is found to be inadequate to describe the experimental data. A more detailed model is presented, which incorporates the doping dependence in order to fit this data.
Co-reporter:I-Kai Hsu, Michael T. Pettes, Adam Bushmaker, Mehmet Aykol, Li Shi and Stephen B. Cronin
Nano Letters 2009 Volume 9(Issue 2) pp:590-594
Publication Date(Web):January 13, 2009
DOI:10.1021/nl802737q
A focused laser beam is used to heat individual single-walled carbon nanotube bundles bridging two suspended microthermometers. By measurement of the temperature rise of the two thermometers, the optical absorption of 7.4−10.3 nm diameter bundles is found to be between 0.03 and 0.44% of the incident photons in the 0.4 μm diameter laser spot. The thermal conductance of the bundle is obtained with the additional measurement of the temperature rise of the nanotubes in the laser spot from shifts in the Raman G band frequency. According to the nanotube bundle diameter determined by transmission electron microscopy, the thermal conductivity is obtained.
Co-reporter:Bardia Zandian, Rajay Kumar, Jesse Theiss, Adam Bushmaker, Stephen B. Cronin
Carbon 2009 Volume 47(Issue 5) pp:1292-1296
Publication Date(Web):April 2009
DOI:10.1016/j.carbon.2009.01.012
We demonstrate selective burnout of individual carbon nanotubes that are electronically resonant with the incident laser energy. Raman spectroscopy and atomic force microscopy are used to quantify the burnout of nanotubes. The threshold laser power for rapid burnout is found to occur between 0.4 and 0.9 W/μm2. At lower laser powers of 80 mW/μm2, the burnout depends linearly on time, over tens of minutes. Non-resonant nanotubes could not be burned out even with high laser power or long exposure times. This preferential burnout of resonant nanotubes demonstrates the possibility of selective removal of metallic nanotubes from an inhomogeneous sample.
Co-reporter:Wenbo Hou, Zuwei Liu, Prathamesh Pavaskar, Wei Hsuan Hung, Stephen B. Cronin
Journal of Catalysis (24 January 2011) Volume 277(Issue 2) pp:149-153
Publication Date(Web):24 January 2011
DOI:10.1016/j.jcat.2010.11.001
By integrating strongly plasmonic Au nanoparticles with strongly catalytic TiO2, we observe enhanced photocatalytic decomposition of methyl orange under visible illumination. Irradiating Au nanoparticles at their plasmon resonance frequency creates intense electric fields, which can be used to increase electron–hole pair generation rate in semiconductors. As a result, the photocatalytic activity of large bandgap semiconductors, like TiO2, can be extended into the visible region of the electromagnetic spectrum. Here, we report a 9-fold improvement in the photocatalytic decomposition rate of methyl orange driven by a photocatalyst consisting of strongly plasmonic Au nanoparticles deposited on top of strongly catalytic TiO2. Finite-difference time-domain (FDTD) simulations indicate that the improvement in photocatalytic activity in the visible range can be attributed to the electric field enhancement near the metal nanoparticles. The intense local fields produced by the surface plasmons couple light efficiently to the surface of the TiO2. This enhancement mechanism is particularly effective because of TiO2’s short exciton diffusion length, which would otherwise limit its photocatalytic efficiency. Our electromagnetic simulations of this process suggest that enhancement factors many times larger than this are possible if this mechanism can be optimized.Graphical abstract9-fold plasmonic enhancement in the photocatalytic decomposition rate of methyl orange was observed under visible illumination by integrating strongly plasmonic Au nanoparticles with strongly catalytic TiO2. Finite-difference time-domain (FDTD) simulations indicate that the improvement in photocatalytic activity in the visible range can be attributed to the electric field enhancement in the interface of Au nanoparticles and TiO2, rather than charge transfer.Download high-res image (136KB)Download full-size imageResearch highlights► Plasmonic improvement of methyl orange photocatalytic decomposition under visible illumination. ► New mechanism for increasing the photocatalytic activity of TiO2 under visible illumination. ► The intense local fields produced by the surface plasmons couple light efficiently to the surface of the TiO2.
Co-reporter:Leyre Gomez, Victor Sebastian, Manuel Arruebo, Jesus Santamaria and Stephen B. Cronin
Physical Chemistry Chemical Physics 2014 - vol. 16(Issue 29) pp:NaN15116-15116
Publication Date(Web):2014/02/10
DOI:10.1039/C4CP00229F
Increasing water demand and water scarcity around the world requires the development of robust and efficient methods for water purification in the coming decades. Here, we report a photocatalytic water purification method using visible light (532 nm) utilizing 5 nm gold nanoparticles and their enhancement when attached on the surface of silica nanospheres as an inactive support to prevent nanoparticle coalescence or sintering. This is a non-toxic, low-cost, and easy photocatalytic process which provides high decomposition rates. Decomposition of the methyl orange dye is tested as a reaction model and trichloroethylene is selected as an example of a real water pollutant. When irradiated at their plasmon resonant frequency, the gold nanoparticles generate hydroxyl radicals that degradate organic pollutants into non-toxic molecules representing a basic mechanism of photocatalytic water purification.
Co-reporter:Bingya Hou, Lang Shen, Haotian Shi, Rehan Kapadia and Stephen B. Cronin
Physical Chemistry Chemical Physics 2017 - vol. 19(Issue 4) pp:NaN2881-2881
Publication Date(Web):2017/01/05
DOI:10.1039/C6CP07542H
We report measurements of photocatalytic water splitting using Au films with and without TiO2 coatings. In these structures, a thin (3–10 nm) film of TiO2 is deposited using atomic layer deposition (ALD) on top of a 100 nm thick Au film. We utilize an AC lock-in technique, which enables us to detect the relatively small photocurrents (∼μA) produced by the short-lived hot electrons that are photoexcited in the metal. Under illumination, the bare Au film produces a small AC photocurrent (<1 μA) for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) due to hot electrons and hot holes, respectively, that are photoexcited in the Au film. The samples with TiO2 produce a larger AC photocurrent indicating that hot electrons are being injected from the metal into the TiO2 semiconductor where they then reduce hydrogen ions in solution forming H2 (i.e., 2H+ + 2e− → H2). The AC photocurrent exhibits a narrow peak when plotted as a function of reference potential, which is a signature of hot electrons. Here, we photoexcite a monoenergetic source of hot electrons, which produces a peak in the photocurrent, as the electrode potential is swept through the resonance with the redox potential of the desired half-reaction. This stands in contrast to conventional bulk semiconductor photocatalysts, whose AC photocurrent saturates beyond a certain potential (i.e., light limited photocurrent). The photocurrents produced at the metal–liquid interface are smaller than those of the metal–semiconductor system, mainly because, in the metal–semiconductor system, there is a continuum of energy and momentum states that each hot electron can be injected into, while for an ion in solution, the number of energy and momentum states are very small.
Co-reporter:Jing Qiu, Guangtong Zeng, Prathamesh Pavaskar, Zhen Li and Stephen B. Cronin
Physical Chemistry Chemical Physics 2014 - vol. 16(Issue 48) pp:
Publication Date(Web):
DOI:10.1039/C4CP90165G
Co-reporter:Jing Qiu, Guangtong Zeng, Prathamesh Pavaskar, Zhen Li and Stephen B. Cronin
Physical Chemistry Chemical Physics 2014 - vol. 16(Issue 7) pp:NaN3121-3121
Publication Date(Web):2013/12/20
DOI:10.1039/C3CP54674H
Integrating plasmon resonant nanostructures with photocatalytic semiconductors shows great promise for high efficiency photocatalytic water splitting. However, the electrochemical instability of most III–V semiconductors severely limits their applicability in photocatalysis. In this work, we passivate p-type GaP with a thin layer of n-type TiO2 using atomic layer deposition. The TiO2 passivation layer prevents corrosion of the GaP, as evidenced by atomic force microscopy and photoelectrochemical measurements. In addition, the TiO2 passivation layer provides an enhancement in photoconversion efficiency through the formation of a charge separating pn-region. Plasmonic Au nanoparticles deposited on top of the TiO2-passivated GaP further increases the photoconversion efficiency through local field enhancement. These two enhancement mechanisms are separated by systematically varying the thickness of the TiO2 layer. Because of the tradeoff between the quickly decaying plasmonic fields and the formation of the pn-charge separation region, an optimum performance is achieved for a TiO2 thickness of 0.5 nm. Finite difference time domain (FDTD) simulations of the electric field profiles in this photocatalytic heterostructure corroborate these results. The effects of plasmonic enhancement are distinguished from the natural catalytic properties of Au by evaluating similar photocatalytic TiO2/GaP structures with catalytic, non-plasmonic metals (i.e., Pt) instead of Au. This general approach of passivating narrower band gap semiconductors enables a wider range of materials to be considered for plasmon-enhanced photocatalysis for high efficiency water splitting.
