Co-reporter:Yuan Liu, Jian Guo, Qiyuan He, Hao Wu, Hung-Chieh Cheng, Mengning Ding, Imran Shakir, Vincent Gambin, Yu Huang, and Xiangfeng Duan
Nano Letters September 13, 2017 Volume 17(Issue 9) pp:5495-5495
Publication Date(Web):August 20, 2017
DOI:10.1021/acs.nanolett.7b02161
Negative transconductance (NTC) devices have been heavily investigated for their potential in low power logical circuit, memory, oscillating, and high-speed switching applications. Previous NTC devices are largely attributed to two working mechanisms: quantum mechanical tunneling, and mobility degradation at high electrical field. Herein we report a systematic investigation of charge transport in multilayer two-dimensional semiconductors (2DSCs) with optimized van der Waals contact and for the first time demonstrate NTC and antibipolar characteristics in multilayer 2DSCs (such as MoS2, WSe2). By varying the measurement temperature, bias voltage, and body thickness, we found the NTC behavior can be attributed to a vertical potential barrier in the multilayer 2DSCs and the competing mechanisms between intralayer lateral transport and interlayer vertical transport, thus representing a new working mechanism for NTC operation. Importantly, this vertical potential barrier arises from inhomogeneous carrier distribution in 2DSC from the near-substrate region to the bulk region, which is in contrast to conventional semiconductors with homogeneous doping defined by bulk dopants. We further show that the unique NTC behavior can be explored for creating frequency doublers and phase shift keying circuits with only one transistor, greatly simplifying the circuit design compared to conventional technology.Keywords: antibipolar; graphene contact; multilayer MoS2 transistor; Negative transconductance;
Co-reporter:Yiliu Wang;Xun Guan;Dehui Li;Hung-Chieh Cheng;Xidong Duan
Nano Research 2017 Volume 10( Issue 4) pp:1223-1233
Publication Date(Web):2017 April
DOI:10.1007/s12274-016-1317-1
Organic–inorganic hybrid halide perovskites, such as CH3NH3PbI3, have emerged as an exciting class of materials for solar photovoltaic applications; however, they are currently plagued by insufficient environmental stability. To solve this issue, all-inorganic halide perovskites have been developed and shown to exhibit significantly improved stability. Here, we report a single-step chemical vapor deposition growth of cesium lead halide (CsPbX3) microcrystals. Optical microscopy studies show that the resulting perovskite crystals predominantly adopt a square-platelet morphology. Powder X-ray diffraction (PXRD) studies of the resulting crystals demonstrate a highly crystalline nature, with CsPbCl3, CsPbBr3, and CsPbI3 showing tetragonal, monoclinic, and orthorhombic phases, respectively. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) studies show that the resulting platelets exhibit well-faceted structures with lateral dimensions of the order of 10–50 μm, thickness around 1 μm, and ultra-smooth surface, suggesting the absence of obvious grain boundaries and the single-crystalline nature of the individual microplatelets. Photoluminescence (PL) images and spectroscopic studies show a uniform and intense emission consistent with the expected band edge transition. Additionally, PL images show brighter emission around the edge of the platelets, demonstrating a wave-guiding effect in high-quality crystals. With a well-defined geometry and ultra-smooth surface, the square platelet structure can function as a whispering gallery mode cavity with a quality factor up to 2,863 to support laser emission at room temperature. Finally, we demonstrate that such microplatelets can be readily grown on a variety of substrates, including silicon, graphene, and other two-dimensional materials such as molybdenum disulfide, which can readily allow the construction of heterostructure optoelectronic devices, including a graphene/perovskite/graphene vertically-stacked photodetector with photoresponsivity > 105 A/W. The extraordinary optical properties of CsPbX3 platelets, combined with their ability to be grown on diverse materials to form functional heterostructures, can lead to exciting opportunities for broad optoelectronic applications.
Co-reporter:Longlu Wang;Xia Liu;Jinming Luo;Xidong Duan; John Crittenden; Chengbin Liu;Shuqu Zhang; Yong Pei;Yunxiong Zeng; Xiangfeng Duan
Angewandte Chemie International Edition 2017 Volume 56(Issue 26) pp:7610-7614
Publication Date(Web):2017/06/19
DOI:10.1002/anie.201703066
AbstractThe metallic 1T-MoS2 has attracted considerable attention as an effective catalyst for hydrogen evolution reactions (HERs). However, the fundamental mechanism about the catalytic activity of 1T-MoS2 and the associated phase evolution remain elusive and controversial. Herein, we prepared the most stable 1T-MoS2 by hydrothermal exfoliation of MoS2 nanosheets vertically rooted into rigid one-dimensional TiO2 nanofibers. The 1T-MoS2 can keep highly stable over one year, presenting an ideal model system for investigating the HER catalytic activities as a function of the phase evolution. Both experimental studies and theoretical calculations suggest that 1T phase can be irreversibly transformed into a more active 1T′ phase as true active sites in photocatalytic HERs, resulting in a “catalytic site self-optimization”. Hydrogen atom adsorption is the major driving force for this phase transition.
Co-reporter:Xiangfeng Duan
Science China Materials 2017 Volume 60( Issue 7) pp:681-682
Publication Date(Web):16 May 2017
DOI:10.1007/s40843-017-9046-x
Co-reporter:Yanbing Yang;Xiangdong Yang;Xuming Zou;Shiting Wu;Da Wan;Anyuan Cao;Lei Liao;Quan Yuan
Advanced Functional Materials 2017 Volume 27(Issue 19) pp:
Publication Date(Web):2017/05/01
DOI:10.1002/adfm.201604096
Graphene is an attractive material for flexible electronics and biosensors, yet its zero bandgap nature has limited the on/off ratio of field-effect transistors (FETs) and the sensitivity of biosensors based on graphene. Graphene nanomesh (GNM), a continuous 2D graphene nanostructure with a high density of holes punched in the basal plane, has been created to introduce lateral confinement and enable improved on/off ratio. However, the GNMs produced to date typically have a relatively large dimension (constriction neck width >5 nm) and low on/off ratio (≈100) limited by the resolution of the lithography process used. Here, the exploration of a directly grown mesoporous silica template is reported for the preparation of ultrafine GNMs with considerably narrower neck width (<3 nm) and strong quantum confinement to enable flexible FETs with greatly improved on/off ratio (up to 1000). With excellent electronic properties and high surface area for the functionalization of specific receptors, it is further shown that the GNM FETs can be readily used to construct highly sensitive biosensors for selective detection of human epidermal growth factor receptor 2, which is further demonstrated for real-time detection of breast cancer cells overexpressed with receptor 2 down to single-cell level. The studies provide a simple and scalable method to GNMs with potential applications for flexible nanoelectronics and biosensors.
Co-reporter:Zhaoyang Lin;Courtney Hollar;Joon Sang Kang;Anxiang Yin;Yiliu Wang;Hui-Ying Shiu;Yu Huang;Yongjie Hu;Yanliang Zhang
Advanced Materials 2017 Volume 29(Issue 21) pp:
Publication Date(Web):2017/06/01
DOI:10.1002/adma.201606662
A solid-state thermoelectric device is attractive for diverse technological areas such as cooling, power generation and waste heat recovery with unique advantages of quiet operation, zero hazardous emissions, and long lifetime. With the rapid growth of flexible electronics and miniature sensors, the low-cost flexible thermoelectric energy harvester is highly desired as a potential power supply. Herein, a flexible thermoelectric copper selenide (Cu2Se) thin film, consisting of earth-abundant elements, is reported. The thin film is fabricated by a low-cost and scalable spin coating process using ink solution with a truly soluble precursor. The Cu2Se thin film exhibits a power factor of 0.62 mW/(m K2) at 684 K on rigid Al2O3 substrate and 0.46 mW/(m K2) at 664 K on flexible polyimide substrate, which is much higher than the values obtained from other solution processed Cu2Se thin films (<0.1 mW/(m K2)) and among the highest values reported in all flexible thermoelectric films to date (≈0.5 mW/(m K2)). Additionally, the fabricated thin film shows great promise to be integrated with the flexible electronic devices, with negligible performance change after 1000 bending cycles. Together, the study demonstrates a low-cost and scalable pathway to high-performance flexible thin film thermoelectric devices from relatively earth-abundant elements.
Co-reporter:Hongtao Sun;Zipeng Zhao;Lin Mei;Jonathan Lau;Junfei Liang;Huilong Fei;Benjamin Papandrea;Imran Shakir;Xu Xu;Bruce Dunn;Chain Lee;Yu Huang;Chen Wang;Mufan Li;Guolin Hao;Mengning Ding
Science 2017 Volume 356(Issue 6338) pp:
Publication Date(Web):
DOI:10.1126/science.aam5852
As with donuts, the holes matter
Improving the density of stored charge and increasing the speed at which it can move through a material are usually opposing objectives. Sun et al. developed a Nb2O5/holey graphene framework composite with tailored porosity. The three-dimensional, hierarchically porous holey graphene acted as a conductive scaffold to support Nb2O5. A high mass loading and improved power capability were reached by tailoring the porosity in the holey graphene backbone with higher charge transport in the composite architecture. The interconnected graphene network provided excellent electron transport, and the hierarchical porous structure in the graphene sheets facilitated rapid ion transport and mitigated diffusion limitations.
Science, this issue p. 599
Co-reporter:Fei Chen 陈菲;Shuzhen Yang 杨抒臻;Zhenjun Wu 吴振军;Wei Hu 胡伟
Science China Materials 2017 Volume 60( Issue 1) pp:39-48
Publication Date(Web):2017 January
DOI:10.1007/s40843-016-5136-6
Bottom-up assembly of nanostructured thin films could offer an alternative low-cost approach to electronic thin films. However, such solution-processed thin films are often plagued by excessive inter-particle resistance and only exhibit limited current delivering capability. Here, we report a novel approach to fabricate highly conductive free-standing metallic thin film, accomplished by combining interfacial self-assembly of nanoparticles (NPs) and a light welding process. We found that light from a xenon lamp can weld adjacent Ag and Au NPs assembled at the water-air interface, forming a highly interconnected, free-standing metallic thin film structure with excellent electrical transport properties. With such a unique structure, the resultant thin metallic films show not only high flexibility and robustness, but also high conductivity comparable to bulk metallic thin films. Our studies offer a low-cost, room-temperature, and solution-processable approach to highly conductive metallic films. It can significantly impact solution-processable electronic and optoelectronic devices.金属薄膜在透明导电极、化学传感器、催化和光电器件等方面具有广泛应用. 发展溶液加工技术可以大幅度降低金属薄膜的制作成本, 灵活地调控其性能, 从而促进其在多方面的应用. 我们发现氙灯光源可以高效地焊接在水-气界面上自组装的金和银纳米粒子薄膜;其焊接程度取决于光照时间和光强度. 最终, 自组装的纳米粒子膜形成一种自支撑、高度交联的网状结构, 并具有和同样厚度的体相金属薄膜相当的导电性. 这一发现将纳米粒子界面自组装技术与光焊接技术相结合, 可以将液相金属溶胶加工成高柔性、鲁棒性和导电性的金属粒子薄膜. 这不仅可以促进金属粒子薄膜自身的应用, 而且可以促进全液相加工的电子和光电器件的发展. 例如, 利用该金属粒子薄膜作为叉指电极的钙钛矿光电探测器展现出了良好的性能.
Co-reporter:Yunran Zhai;Lingyan Meng;Lijun Xu;Lin Yuan;Zhilin Yang;Jiawen Hu
Plasmonics 2017 Volume 12( Issue 2) pp:263-269
Publication Date(Web):2017 April
DOI:10.1007/s11468-016-0259-5
Fluorescence intensity is vital for fluorescence sensing and imaging because it determines the sensing sensitivity and imaging brightness. This study reports plasmon-enhanced fluorescence by engineering plasmonic nanostructures, that are SiO2-coated Au nanoshell dimers with a high yield exceeding 60 %. With this elaborately designed nanostructure, we show that the thin SiO2 shell can conveniently distance the fluorophore from the underneath metal, thereby effectively avoiding fluorescence quenching. Meanwhile, the inner Au nanoshell dimers create abundant hot spots at particle-particle junctions and enable near-infrared fluorescence enhancement. The largest fluorescence enhancement achieved is 69 times for the design with a 9 nm external SiO2 shell, as is also confirmed by three-dimensional finite-difference time-domain simulations. This dramatically increased fluorescence has great significance in fluorescence-based sensing and imaging.
Co-reporter:Dehui LiHung-Chieh Cheng, Hao Wu, Yiliu Wang, Jian Guo, Gongming Wang, Yu Huang, Xiangfeng Duan
The Journal of Physical Chemistry Letters 2017 Volume 8(Issue 2) pp:
Publication Date(Web):January 4, 2017
DOI:10.1021/acs.jpclett.6b02841
Understanding the intrinsic charge transport in organolead halide perovskites is essential for the development of high-efficiency photovoltaics and other optoelectronic devices. Despite the rapid advancement of the organolead halide perovskite in photovoltaic and optoelectronic applications, the intrinsic charge-carrier transport in these materials remains elusive partly due to the difficulty of fabricating electrical devices and obtaining good electrical contact. Here we report the fabrication of organolead halide perovskite microplates with mono- or bilayer graphene as low barrier electrical contact. Systematic charge-transport studies reveal an insulator to band-like transport transition. Our studies indicate that the insulator to band-like transport transition depends on the orthorhombic-to-tetragonal phase-transition temperature and defect densities of the organolead halide perovskite microplates. Our findings not only are important for the fundamental understanding of charge-transport behavior but also offer valuable practical implications for photovoltaics and optoelectronic applications based on the organolead halide perovskite.
Co-reporter:Yuan Liu, Guo Zhang, Hailong Zhou, Zheng Li, Rui Cheng, Yang XuVincent Gambin, Yu Huang, Xiangfeng Duan
Nano Letters 2017 Volume 17(Issue 3) pp:
Publication Date(Web):February 6, 2017
DOI:10.1021/acs.nanolett.6b04417
Vertical heterostructures based on graphene have emerged as a unique architecture for novel electronic devices with unusual characteristics. Here we report a new design of vertical ambipolar barristors based on metal–graphene–silicon–graphene sandwich structure, using the bottom graphene as a gate-tunable “active contact”, the top graphene as an adaptable Ohmic contact, and the low doping thin silicon layer as the switchable channel. Importantly, with finite density of states and weak screening effect of graphene, we demonstrate, for the first time, that both the carrier concentration and majority carrier type in the sandwiched silicon can be readily modulated by gate potential penetrating through graphene. It can thus enable a new type of ambipolar barristors with an ON-OFF ratio exceeding 103. Significantly, these ambipolar barristors can be flexibly configured into either p-type or n-type transistors and used to create integrated circuits with reconfigurable logic functions. This unconventional device structure and ambipolar reconfigurable characteristics can open up exciting opportunities in future electronics based on graphene or two-dimensional van der Waals heterostructures.Keywords: ambipolar; graphene weak screening; tunable diode; van der Waals heterostructure; Vertical transistor;
Co-reporter:Khurram Shehzad, Yang XuChao Gao, Hanying Li, Zhi-Min Dang, Tawfique HasanJack Luo, Xiangfeng Duan
ACS Applied Materials & Interfaces 2017 Volume 9(Issue 8) pp:
Publication Date(Web):February 3, 2017
DOI:10.1021/acsami.6b14984
Polymer dielectrics offer key advantages over their ceramic counterparts such as flexibility, scalability, low cost, and high breakdown voltages. However, a major drawback that limits more widespread application of polymer dielectrics is their temperature-dependent dielectric properties. Achieving dielectric constants with low/zero-temperature coefficient (L/0TC) over a broad temperature range is essential for applications in diverse technologies. Here, we report a hybrid filler strategy to produce polymer composites with an ultrawide L/0TC window of dielectric constant, as well as a significantly enhanced dielectric value, maximum energy storage density, thermal conductivity, and stability. By creating a series of percolative polymer composites, we demonstrated hybrid carbon filler based composites can exhibit a zero-temperature coefficient window of 200 °C (from −50 to 150 °C), the widest 0TC window for all polymer composite dielectrics reported to date. We further show the electric and dielectric temperature coefficient of the composites is highly stable against stretching and bending, even under AC electric field with frequency up to 1 MHz. We envision that our method will push the functional limits of polymer dielectrics for flexible electronics in extreme conditions such as in hybrid vehicles, aerospace, power electronics, and oil/gas exploration.Keywords: carbon nanotubes; dielectric constant; energy storage and conversion; polymer nanocomposites; zero temperature coefficient;
Co-reporter:Xing Zhong;Benjamin Papandrea;Yuxi Xu;Zhaoyang Lin;Hua Zhang;Yuan Liu
Nano Research 2017 Volume 10( Issue 2) pp:472-482
Publication Date(Web):2017 February
DOI:10.1007/s12274-016-1306-4
Lithium-air batteries have attracted significant interest for applications in high energy density mobile power supplies, yet there are considerable challenges to the development of rechargeable Li-air batteries with stable cycling performance under ambient conditions. Here we report a three-dimensional (3D) hydrophobic graphene membrane as a moisture-resistive cathode for high performance Li-air batteries. The 3D graphene membrane features a highly interconnected graphene network for efficient charge transport, a highly porous structure for efficient diffusion of oxygen and electrolyte ions, a large specific surface area for high capacity storage of the insulating discharge product, and a network of highly tortuous hydrophobic channels for O2/H2O selectivity. These channels facilitate O2 ingression while retarding moisture diffusion and ensure excellent charge/discharge cycling stability under ambient conditions. The membrane can thus enable robust Li-air batteries with exceptional performance, including a maximum cathode capacity that exceeds 5,700 mAh/g and excellent recharge cycling behavior (>2,000 cycles at 140 mAh/g, and >100 cycles at 1,400 mAh/g). The graphene membrane air cathode can deliver a lifetime capacity of 100,000–300,000 mAh/g, comparable to that of a typical lithium ion battery cathode. The stable operation of Li-air batteries with significantly improved single charge capacities and lifetime capacities comparable to those of Li-ion batteries may offer an attractive high energy density storage alternative for future mobile power supplies. These batteries may provide much longer battery lives and greatly reduced recharge frequency.
Co-reporter:Chin-Yi Chiu;Zhaoyang Lin;Lingyan Ruan;Yongjia Li;Teng Xue;Zipeng Zhao;Yu Huang;Gongming Wang;Chain Lee
Science Advances 2017 Volume 3(Issue 1) pp:
Publication Date(Web):
DOI:10.1126/sciadv.1600615
Molecular ligand modulation of Pd nanoparticle catalysts achieves exceptional activity and stability.
Co-reporter:Yuan Liu;Jiming Sheng;Hao Wu;Qiyuan He;Hung-Chieh Cheng;Muhammad Imran Shakir;Yu Huang
Advanced Materials 2016 Volume 28( Issue 21) pp:4120-4125
Publication Date(Web):
DOI:10.1002/adma.201506173
Co-reporter:Hung-Chieh Cheng, Gongming Wang, Dehui Li, Qiyuan He, Anxiang Yin, Yuan Liu, Hao Wu, Mengning Ding, Yu Huang, and Xiangfeng Duan
Nano Letters 2016 Volume 16(Issue 1) pp:367-373
Publication Date(Web):December 14, 2015
DOI:10.1021/acs.nanolett.5b03944
The recently emerged organohalide perovskites (e.g., CH3NH3PbI3) have drawn intense attention for high efficiency solar cells. However, with a considerable solubility in many solvents, these perovskites are not typically compatible with conventional lithography processes for more complicated device fabrications that are important for both fundamental studies and technological applications. Here, we report the creation of novel heterojunction devices based on perovskites and two-dimensional (2D) crystals by taking advantage of the layered characteristic of lead iodide (PbI2) and vapor-phase intercalation. We show that a graphene/perovskite/graphene vertical stack can deliver a highest photoresponsivity of ∼950 A/W and photoconductive gain of ∼2200, and a graphene/WSe2/perovskite/graphene heterojunction can display a high on/off ratio (∼106) transistor behavior with distinct gate-tunable diode characteristics and open-circuit voltages. Such unique perovskite–2D heterostructures have significant potential for future optoelectronic research and can enable broad possibilities with compositional tunability of organohalide perovskites and the versatility offered by diverse 2D materials.
Co-reporter:Xidong Duan, Chen Wang, Zheng Fan, Guolin Hao, Liangzhi Kou, Udayabagya Halim, Honglai Li, Xueping Wu, Yicheng Wang, Jianhui Jiang, Anlian Pan, Yu Huang, Ruqin Yu, and Xiangfeng Duan
Nano Letters 2016 Volume 16(Issue 1) pp:264-269
Publication Date(Web):December 3, 2015
DOI:10.1021/acs.nanolett.5b03662
Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have recently emerged as a new class of atomically thin semiconductors for diverse electronic, optoelectronic, and valleytronic applications. To explore the full potential of these 2D semiconductors requires a precise control of their band gap and electronic properties, which represents a significant challenge in 2D material systems. Here we demonstrate a systematic control of the electronic properties of 2D-TMDs by creating mixed alloys of the intrinsically p-type WSe2 and intrinsically n-type WS2 with variable alloy compositions. We show that a series of WS2xSe2–2x alloy nanosheets can be synthesized with fully tunable chemical compositions and optical properties. Electrical transport studies using back-gated field effect transistors demonstrate that charge carrier types and threshold voltages of the alloy nanosheet transistors can be systematically tuned by adjusting the alloy composition. A highly p-type behavior is observed in selenium-rich alloy, which gradually shifts to lightly p-type, and then switches to lightly n-type characteristics with the increasing sulfur atomic ratio, and eventually evolves into highly n-doped semiconductors in sulfur-rich alloys. The synthesis of WS2xSe2–2x nanosheets with tunable optical and electronic properties represents a critical step toward rational design of 2D electronics with tailored spectral responses and device characteristics.
Co-reporter:Yuan Liu, Jian Guo, Yecun Wu, Enbo Zhu, Nathan O. Weiss, Qiyuan He, Hao Wu, Hung-Chieh Cheng, Yang Xu, Imran Shakir, Yu Huang, and Xiangfeng Duan
Nano Letters 2016 Volume 16(Issue 10) pp:6337-6342
Publication Date(Web):August 31, 2016
DOI:10.1021/acs.nanolett.6b02713
Two-dimensional semiconductors (2DSCs) such as molybdenum disulfide (MoS2) have attracted intense interest as an alternative electronic material in the postsilicon era. However, the ON-current density achieved in 2DSC transistors to date is considerably lower than that of silicon devices, and it remains an open question whether 2DSC transistors can offer competitive performance. A high current device requires simultaneous minimization of the contact resistance and channel length, which is a nontrivial challenge for atomically thin 2DSCs, since the typical low contact resistance approaches for 2DSCs either degrade the electronic properties of the channel or are incompatible with the fabrication process for short channel devices. Here, we report a new approach toward high-performance MoS2 transistors by using a physically assembled nanowire as a lift-off mask to create ultrashort channel devices with pristine MoS2 channel and self-aligned low resistance metal/graphene hybrid contact. With the optimized contact in short channel devices, we demonstrate sub-100 nm MoS2 transistor delivering a record high ON-current of 0.83 mA/μm at 300 K and 1.48 mA/μm at 20 K, which compares well with that of silicon devices. Our study, for the first time, demonstrates that the 2DSC transistors can offer comparable performance to the 2017 target for silicon transistors in International Technology Roadmap for Semiconductors (ITRS), marking an important milestone in 2DSC electronics.Keywords: large current density; low contact resistance; MoS2 transistor; self-alignment; short channel;
Co-reporter:Boris Volosskiy, Huilong Fei, Zipeng Zhao, Stacy Lee, Mufan Li, Zhaoyang Lin, Benjamin Papandrea, Chen Wang, Yu Huang, and Xiangfeng Duan
ACS Applied Materials & Interfaces 2016 Volume 8(Issue 40) pp:26769
Publication Date(Web):September 16, 2016
DOI:10.1021/acsami.6b08320
An efficient non-noble metal catalyst for the oxygen reduction reaction (ORR) is of great importance for the fabrication of cost-effective fuel cells. Nitrogen-doped carbons with various transition metal co-dopants have emerged as attractive candidates to replace the expensive platinum catalysts. Here we report the preparation of various copper- and nitrogen-doped carbon materials as highly efficient ORR catalysts by pyrolyzing porphyrin based metal organic frameworks and investigate the effects of air impurities during the thermal carbonization process. Our results indicate that the introduction of air impurities can significantly improve ORR activity in nitrogen-doped carbon and the addition of copper co-dopant further enhances the ORR activity to exceed that of platinum. Systematic structural characterization and electrochemical studies demonstrate that the air-impurity-treated samples show considerably higher surface area and electron transfer numbers, suggesting that the partial etching of the carbon by air leads to increased porosity and accessibility to highly active ORR sites. Our study represents the first example of using air or oxygen impurities to tailor the ORR activity of metal and nitrogen co-doped carbon materials and open up a new avenue to engineer the catalytic activity of these materials.Keywords: electrocatalyst; fuel cell; metal−organic framework; oxygen reduction reaction
Co-reporter:Tao Cheng;Zipeng Zhao;Mufan Li;Alessandro Fortunelli;Chih-Yen Chen;Qinghua Zhang;Rong Yu;Enbo Zhu;Boris V. Merinov;Jinghua Guo;Qingying Jia;Yu Huang;William A. Goddard III;Lin Gu;Ted Yu;Zhaoyang Lin;Liang Zhang
Science 2016 Volume 354(Issue 6318) pp:
Publication Date(Web):
DOI:10.1126/science.aaf9050
An activity lift for platinum
Platinum is an excellent but expensive catalyst for the oxygen reduction reaction (ORR), which is critical for fuel cells. Alloying platinum with other metals can create shells of platinum on cores of less expensive metals, which increases its surface exposure, and compressive strain in the layer can also boost its activity (see the Perspective by Stephens et al.). Bu et al. produced nanoplates—platinum-lead cores covered with platinum shells—that were in tensile strain. These nanoplates had high and stable ORR activity, which theory suggests arises from the strain optimizing the platinum-oxygen bond strength. Li et al. optimized both the amount of surface-exposed platinum and the specific activity. They made nanowires with a nickel oxide core and a platinum shell, annealed them to the metal alloy, and then leached out the nickel to form a rough surface. The mass activity was about double the best reported values from previous studies.
Science, this issue p. 1410, p. 1414; see also p. 1378
Co-reporter:Kenan Zhang, Tianning Zhang, Guanghui Cheng, Tianxin Li, Shuxia Wang, Wei Wei, Xiaohao Zhou, Weiwei Yu, Yan Sun, Peng Wang, Dong Zhang, Changgan Zeng, Xingjun Wang, Weida Hu, Hong Jin Fan, Guozhen Shen, Xin Chen, Xiangfeng Duan, Kai Chang, and Ning Dai
ACS Nano 2016 Volume 10(Issue 3) pp:3852
Publication Date(Web):March 7, 2016
DOI:10.1021/acsnano.6b00980
We demonstrate the type-II staggered band alignment in MoTe2/MoS2 van der Waals (vdW) heterostructures and an interlayer optical transition at ∼1.55 μm. The photoinduced charge separation between the MoTe2/MoS2 vdW heterostructure is verified by Kelvin probe force microscopy (KPFM) under illumination, density function theory (DFT) simulations and photoluminescence (PL) spectroscopy. Photoelectrical measurements of MoTe2/MoS2 vdW heterostructures show a distinct photocurrent response in the infrared regime (1550 nm). The creation of type-II vdW heterostructures with strong interlayer coupling could improve our fundamental understanding of the essential physics behind vdW heterostructures and help the design of next-generation infrared optoelectronics.Keywords: interlayer transition; MoS2; MoTe2; type-II band alignment; van der Waals heterostructure
Co-reporter:Dehui Li, Hao Wu, Hung-Chieh Cheng, Gongming Wang, Yu Huang, and Xiangfeng Duan
ACS Nano 2016 Volume 10(Issue 7) pp:6933
Publication Date(Web):June 17, 2016
DOI:10.1021/acsnano.6b02795
Ion migration has been postulated as the underlying mechanism responsible for the hysteresis in organolead halide perovskite devices. However, the electronic and ionic transport dynamics and how they impact each other in organolead halide perovskites remain elusive to date. Here we report a systematic investigation of the electronic and ionic transport dynamics in organolead halide perovskite microplate crystals and thin films using temperature-dependent transient response measurements. Our study reveals that thermally activated ionic and electronic conduction coexist in perovskite devices. The extracted activation energies suggest that the electronic transport is easier, but ions migrate harder in microplates than in thin films, demonstrating that the crystalline quality and grain boundaries can fundamentally modify electronic and ionic transport in perovskites. These findings offer valuable insight on the electronic and ionic transport dynamics in organolead halide perovskites, which is critical for optimizing perovskite devices with reduced hysteresis and improved stability and efficiency.Keywords: activation energy; electronic transport; hysteresis; ionic transport; organolead halide perovskite
Co-reporter:Zhaoyang Lin;Jun Mao;Anxiang Yin;Yi Xia;Qiyuan He;Nicholas Kempf;Chih-Yen Chen;Yiliu Wang;Yanliang Zhang;Vidvuds Ozolins;Zhifeng Ren;Yu Huang
Science Advances 2016 Volume 2(Issue 10) pp:e1600993
Publication Date(Web):07 Oct 2016
DOI:10.1126/sciadv.1600993
Lin et al. report 2D layered materials as a soft template for symmetry-mismatched epitaxial growth of PbSe/Bi2Se3 heterostructures.
Co-reporter:Meng Wang, Xidong Duan, Yuxi Xu, and Xiangfeng Duan
ACS Nano 2016 Volume 10(Issue 8) pp:7231
Publication Date(Web):July 12, 2016
DOI:10.1021/acsnano.6b03349
Integration of graphene with polymers to construct three-dimensional porous graphene/polymer composites (3DGPCs) has attracted considerable attention in the past few years for both fundamental studies and diverse technological applications. With the broad diversity in molecular structures of graphene and polymers via rich chemical routes, a number of 3DGPCs have been developed with unique structural, electrical, and mechanical properties, chemical tenability, and attractive functions, which greatly expands the research horizon of graphene-based composites. In particular, the properties and functions of the 3DGPCs can be readily tuned by precisely controlling the hierarchical porosity in the 3D graphene architecture as well as the intricate synergistic interactions between graphene and polymers. In this paper, we review the recent progress in 3DGPCs, including their synthetic strategies and potential applications in environmental protection, energy storage, sensors, and conducting composites. Lastly, we will conclude with a brief perspective on the challenges and future opportunities.Keywords: composites; energy; environment; graphene; polymer; sensor; three-dimensional
Co-reporter:Mengning Ding, Hui-Ying Shiu, Shiue-Lin Li, Calvin K. Lee, Gongming Wang, Hao Wu, Nathan O. Weiss, Thomas D. Young, Paul S. Weiss, Gerard C. L. Wong, Kenneth H. Nealson, Yu Huang, and Xiangfeng Duan
ACS Nano 2016 Volume 10(Issue 11) pp:9919
Publication Date(Web):October 27, 2016
DOI:10.1021/acsnano.6b03655
The electrical conductivity measured in Shewanella and Geobacter spp. is an intriguing physical property that is the fundamental basis for possible extracellular electron transport (EET) pathways. There is considerable debate regarding the origins of the electrical conductivity reported in these microbial cellular structures, which is essential for deciphering the EET mechanism. Here, we report systematic on-chip nanoelectronic investigations of both Shewanella and Geobacter spp. under physiological conditions to elucidate the complex basis of electrical conductivity of both individual microbial cells and biofilms. Concurrent electrical and electrochemical measurements of living Shewanella at both few-cell and the biofilm levels indicate that the apparent electrical conductivity can be traced to electrochemical-based electron transfer at the cell/electrode interface. We further show that similar results and conclusions apply to the Geobacter spp. Taken together, our study offers important insights into previously proposed physical models regarding microbial conductivities as well as EET pathways for Shewanella and Geobacter spp.Keywords: bioelectrochemical system; bioelectronics; electrogenic bacteria; extracellular electron transfer; microbial electrochemistry; microbiome; nanoelectronics
Co-reporter:Dr. Anxiang Yin;Dr. Qiyuan He;Zhaoyang Lin;Dr. Liang Luo;Yuan Liu;Sen Yang;Hao Wu;Dr. Mengning Ding; Yu Huang; Xiangfeng Duan
Angewandte Chemie International Edition 2016 Volume 55( Issue 2) pp:583-587
Publication Date(Web):
DOI:10.1002/anie.201508586
Abstract
Herein, we report the design and synthesis of plasmonic/non-linear optical (NLO) material core/shell nanostructures that can allow dynamic manipulation of light signals using an external electrical field and enable a new generation of nanoscale optical voltage sensors. We show that gold nanorods (Au NRs) can be synthesized with tunable plasmonic properties and function as the nucleation seeds for continued growth of a shell of NLO materials (such as polyaniline, PANI) with variable thickness. The formation of a PANI nanoshell allows dynamic modulation of the dielectric environment of the plasmonic Au NRs, and therefore the plasmonic resonance characteristics, by an external electrical field. The finite element simulation confirms that such modulation is originated from the field-induced modulation of the dielectric constant of the NLO shell. This approach is general, and the coating of the Au NRs with other NLO materials (such as barium titanate, BTO) is found to produce a similar effect. These findings can not only open a new pathway to active modulation of plasmonic resonance at the sub-wavelength scale but also enable the creation of a new generation of nanoscale optical voltage sensors (NOVS).
Co-reporter:Dr. Anxiang Yin;Dr. Qiyuan He;Zhaoyang Lin;Dr. Liang Luo;Yuan Liu;Sen Yang;Hao Wu;Dr. Mengning Ding; Yu Huang; Xiangfeng Duan
Angewandte Chemie 2016 Volume 128( Issue 2) pp:593-597
Publication Date(Web):
DOI:10.1002/ange.201508586
Abstract
Herein, we report the design and synthesis of plasmonic/non-linear optical (NLO) material core/shell nanostructures that can allow dynamic manipulation of light signals using an external electrical field and enable a new generation of nanoscale optical voltage sensors. We show that gold nanorods (Au NRs) can be synthesized with tunable plasmonic properties and function as the nucleation seeds for continued growth of a shell of NLO materials (such as polyaniline, PANI) with variable thickness. The formation of a PANI nanoshell allows dynamic modulation of the dielectric environment of the plasmonic Au NRs, and therefore the plasmonic resonance characteristics, by an external electrical field. The finite element simulation confirms that such modulation is originated from the field-induced modulation of the dielectric constant of the NLO shell. This approach is general, and the coating of the Au NRs with other NLO materials (such as barium titanate, BTO) is found to produce a similar effect. These findings can not only open a new pathway to active modulation of plasmonic resonance at the sub-wavelength scale but also enable the creation of a new generation of nanoscale optical voltage sensors (NOVS).
Co-reporter:Yuxi Xu, Gaoquan Shi, and Xiangfeng Duan
Accounts of Chemical Research 2015 Volume 48(Issue 6) pp:1666
Publication Date(Web):June 4, 2015
DOI:10.1021/acs.accounts.5b00117
Graphene and its derivatives are versatile building blocks for bottom-up assembly of advanced functional materials. In particular, with exceptionally large specific surface area, excellent electrical conductivity, and superior chemical/electrochemical stability, graphene represents the ideal material for various electrochemical energy storage devices including supercapacitors. However, due to the strong π–π interaction between graphene sheets, the graphene flakes tend to restack to form graphite-like powders when they are processed into practical electrode materials, which can greatly reduce the specific surface area and lead to inefficient utilization of the graphene layers for electrochemical energy storage. The self-assembly of two-dimensional graphene sheets into three-dimensional (3D) framework structures can largely retain the unique properties of individual graphene sheets and has recently garnered intense interest for fundamental investigations and potential applications in diverse technologies.In this Account, we review the recent advances in preparing 3D graphene macrostructures and exploring them as a unique platform for supercapacitor applications. We first describe the synthetic strategies, in which reduction of a graphene oxide dispersion above a certain critical concentration can induce the reduced graphene oxide sheets to cross-link with each other via partial π–π stacking interactions to form a 3D interconnected porous macrostructure. Multiple reduction strategies, including hydrothermal/solvothermal reduction, chemical reduction, and electrochemical reduction, have been developed for the preparation of 3D graphene macrostructures. The versatile synthetic strategies allow for easy incorporation of heteroatoms, carbon nanomaterials, functional polymers, and inorganic nanostructures into the macrostructures to yield diverse composites with tailored structures and properties. We then summarize the applications of the 3D graphene macrostructures for high-performance supercapacitors. With a unique framework structure in which the graphene sheets are interlocked in 3D space to prevent their restacking, the graphene macrostructures feature very high specific surface areas, rapid electron and ion transport, and superior mechanical strength. They can thus be directly used as supercapacitor electrodes with excellent specific capacitances, rate capabilities, and cycling stabilities. We finally discuss the current challenges and future opportunities in this research field.By regarding the graphene as both a single-atom-thick carbon sheet and a conjugated macromolecule, our work opens a new avenue to bottom-up self-assembly of graphene macromolecule sheets into functional 3D graphene macrostructures with remarkable electrochemical performances. We hope that this Account will promote further efforts toward fundamental investigation of graphene self-assembly and the development of advanced 3D graphene materials for their real-world applications in electrochemical energy storage devices and beyond.
Co-reporter:Hailong Zhou, Chen Wang, Jonathan C. Shaw, Rui Cheng, Yu Chen, Xiaoqing Huang, Yuan Liu, Nathan O. Weiss, Zhaoyang Lin, Yu Huang, and Xiangfeng Duan
Nano Letters 2015 Volume 15(Issue 1) pp:709-713
Publication Date(Web):November 29, 2014
DOI:10.1021/nl504256y
Transition metal dichacogenides represent a unique class of two-dimensional layered materials that can be exfoliated into single or few atomic layers. Tungsten diselenide (WSe2) is one typical example with p-type semiconductor characteristics. Bulk WSe2 has an indirect band gap (∼1.2 eV), which transits into a direct band gap (∼1.65 eV) in monolayers. Monolayer WSe2, therefore, is of considerable interest as a new electronic material for functional electronics and optoelectronics. However, the controllable synthesis of large-area WSe2 atomic layers remains a challenge. The studies on WSe2 are largely limited by relatively small lateral size of exfoliated flakes and poor yield, which has significantly restricted the large-scale applications of the WSe2 atomic layers. Here, we report a systematic study of chemical vapor deposition approach for large area growth of atomically thin WSe2 film with the lateral dimensions up to ∼1 cm2. Microphotoluminescence mapping indicates distinct layer dependent efficiency. The monolayer area exhibits much stronger light emission than bilayer or multilayers, consistent with the expected transition to direct band gap in the monolayer limit. The transmission electron microscopy studies demonstrate excellent crystalline quality of the atomically thin WSe2. Electrical transport studies further show that the p-type WSe2 field-effect transistors exhibit excellent electronic characteristics with effective hole carrier mobility up to 100 cm2 V–1 s–1 for monolayer and up to 350 cm2 V–1 s–1 for few-layer materials at room temperature, comparable or well above that of previously reported mobility values for the synthetic WSe2 and comparable to the best exfoliated materials.
Co-reporter:Yuan Liu, Hao Wu, Hung-Chieh Cheng, Sen Yang, Enbo Zhu, Qiyuan He, Mengning Ding, Dehui Li, Jian Guo, Nathan O. Weiss, Yu Huang, and Xiangfeng Duan
Nano Letters 2015 Volume 15(Issue 5) pp:3030-3034
Publication Date(Web):April 16, 2015
DOI:10.1021/nl504957p
Two-dimensional layered semiconductors such as molybdenum disulfide (MoS2) have attracted tremendous interest as a new class of electronic materials. However, there are considerable challenges in making reliable contacts to these atomically thin materials. Here we present a new strategy by using graphene as the back electrodes to achieve ohmic contact to MoS2. With a finite density of states, the Fermi level of graphene can be readily tuned by a gate potential to enable a nearly perfect band alignment with MoS2. We demonstrate for the first time a transparent contact to MoS2 with zero contact barrier and linear output behavior at cryogenic temperatures (down to 1.9 K) for both monolayer and multilayer MoS2. Benefiting from the barrier-free transparent contacts, we show that a metal–insulator transition can be observed in a two-terminal MoS2 device, a phenomenon that could be easily masked by Schottky barriers found in conventional metal-contacted MoS2 devices. With further passivation by boron nitride (BN) encapsulation, we demonstrate a record-high extrinsic (two-terminal) field effect mobility up to 1300 cm2/(V s) in MoS2 at low temperature.
Co-reporter:Gongming Wang, Xiangheng Xiao, Wenqing Li, Zhaoyang Lin, Zipeng Zhao, Chi Chen, Chen Wang, Yongjia Li, Xiaoqing Huang, Ling Miao, Changzhong Jiang, Yu Huang, and Xiangfeng Duan
Nano Letters 2015 Volume 15(Issue 7) pp:4692-4698
Publication Date(Web):June 8, 2015
DOI:10.1021/acs.nanolett.5b01547
Titanium oxide (TiO2) represents one of most widely studied materials for photoelectrochemical (PEC) water splitting but is severely limited by its poor efficiency in the visible light range. Here, we report a significant enhancement of visible light photoactivity in nitrogen-implanted TiO2 (N-TiO2) nanowire arrays. Our systematic studies show that a post-implantation thermal annealing treatment can selectively enrich the substitutional nitrogen dopants, which is essential for activating the nitrogen implanted TiO2 to achieve greatly enhanced visible light photoactivity. An incident photon to electron conversion efficiency (IPCE) of ∼10% is achieved at 450 nm in N-TiO2 without any other cocatalyst, far exceeding that in pristine TiO2 nanowires (∼0.2%). The integration of oxygen evolution reaction (OER) cocatalyst with N-TiO2 can further increase the IPCE at 450 nm to ∼17% and deliver an unprecedented overall photocurrent density of 1.9 mA/cm2, by integrating the IPCE spectrum with standard AM 1.5G solar spectrum. Systematic photoelectrochemical and electrochemical studies demonstrated that the enhanced PEC performance can be attributed to the significantly improved visible light absorption and more efficient charge separation. Our studies demonstrate the implantation approach can be used to reliably dope TiO2 to achieve the best performed N-TiO2 photoelectrodes to date and may be extended to fundamentally modify other semiconductor materials for PEC water splitting.
Co-reporter:Yuxi Xu, Chih-Yen Chen, Zipeng Zhao, Zhaoyang Lin, Chain Lee, Xu Xu, Chen Wang, Yu Huang, Muhammad Imran Shakir, and Xiangfeng Duan
Nano Letters 2015 Volume 15(Issue 7) pp:4605-4610
Publication Date(Web):June 9, 2015
DOI:10.1021/acs.nanolett.5b01212
Scalable preparation of solution processable graphene and its bulk materials with high specific surface areas and designed porosities is essential for many practical applications. Herein, we report a scalable approach to produce aqueous dispersions of holey graphene oxide with abundant in-plane nanopores via a convenient mild defect-etching reaction and demonstrate that the holey graphene oxide can function as a versatile building block for the assembly of macrostructures including holey graphene hydrogels with a three-dimensional hierarchical porosity and holey graphene papers with a compact but porous layered structure. These holey graphene macrostructures exhibit significantly improved specific surface area and ion diffusion rate compared to the nonholey counterparts and can be directly used as binder-free supercapacitor electrodes with ultrahigh specific capacitances of 283 F/g and 234 F/cm3, excellent rate capabilities, and superior cycling stabilities. Our study defines a scalable pathway to solution processable holey graphene materials and will greatly impact the applications of graphene in diverse technological areas.
Co-reporter:Honglai Li; Qinglin Zhang; Xidong Duan; Xueping Wu; Xiaopeng Fan; Xiaoli Zhu; Xiujuan Zhuang; Wei Hu; Hong Zhou; Anlian Pan
Journal of the American Chemical Society 2015 Volume 137(Issue 16) pp:5284-5287
Publication Date(Web):April 14, 2015
DOI:10.1021/jacs.5b01594
Band gap engineering of transition-metal dichalcogenides is an important task for their applications in photonics, optoelectronics, and nanoelectronics. We report for the first time the continuous lateral growth of composition graded bilayer MoS2(1–x)Se2x alloys along single triangular nanosheets by an improved chemical vapor deposition approach. From the center to the edge of the nanosheet, the composition can be gradually tuned from x = 0 (pure MoS2) to x = 0.68, leading to the corresponding bandgap being continuously modulated from 1.82 eV (680 nm) to 1.64 eV (755 nm). Local photoluminescence scanning from the center to the edge gives single band edge emission peaks, indicating high crystalline quality for the achieved alloy nanosheets, which was further demonstrated by the microstructure characterizations. These novel 2D structures offer an interesting system for probing the physical properties of layered materials and exploring new applications in functional nanoelectronic and optoelectronic devices.
Co-reporter:Li Tao, Xidong Duan, Chen Wang, Xiangfeng Duan and Shuangyin Wang
Chemical Communications 2015 vol. 51(Issue 35) pp:7470-7473
Publication Date(Web):25 Mar 2015
DOI:10.1039/C5CC01981H
Herein, for the first time, we reported a general approach of plasma engineering to tune the surface properties and edge reactive sites for greatly improving HER (hydrogen evolution) activity of MoS2. We show that the Ar or O2 plasma can be used to generate a large number of physical and chemical defects in 2D crystals to modify the electronic properties and increase the number of active sites in MoS2. Electrocatalytic studies show that the plasma-treated MoS2 exhibits significantly enhanced electrocatalytic activity for HER.
Co-reporter:Boris Volosskiy, Kenta Niwa, Yu Chen, Zipeng Zhao, Nathan O. Weiss, Xing Zhong, Mengning Ding, Chain Lee, Yu Huang, and Xiangfeng Duan
ACS Nano 2015 Volume 9(Issue 3) pp:3044
Publication Date(Web):February 22, 2015
DOI:10.1021/nn5072446
With well-defined porous structures and dimensions, metal-organic frameworks (MOFs) can function as versatile templates for the growth of metallic nanostructures with precisely controlled shapes and sizes. Using MOFs as templates, metallic nanostructures can be grown without the need of bulky surfactants and thus preserve their intrinsic surface. Additionally, the high surface area of MOFs can also ensure that the surface of the template metallic nanostructures is readily accessible, which is critical for the proper function of catalysts or sensors. The hybrid metal@MOF structures have been demonstrated to exhibit useful properties not found in either component separately. Here we report the growth of ultrafine metallic nanowires inside one-dimensional MOF pores with well-controlled shape and size. Our study shows that solvent selection plays an important role in controlling precursor loading and the reduction rate inside the MOF pores for the formation of the nanowires. The growth of the well-aligned, ultrathin nanowires was monitored and characterized by transmission electron microscopy, X-ray diffraction, UV–vis spectroscopy, fluorescence studies, and Brunauer–Emmet–Teller surface area analysis.Keywords: host−guest systems; metal−organic frameworks; nanoparticles; nanowires;
Co-reporter:Zhaoyang Lin, Qiyuan He, Anxiang Yin, Yuxi Xu, Chen Wang, Mengning Ding, Hung-Chieh Cheng, Benjamin Papandrea, Yu Huang, and Xiangfeng Duan
ACS Nano 2015 Volume 9(Issue 4) pp:4398
Publication Date(Web):April 13, 2015
DOI:10.1021/acsnano.5b00886
Low-temperature solution-processable electronic materials are of considerable interest for large-area, low-cost electronics, thermoelectrics, and photovoltaics. Using a soluble precursor and suitable solvent to formulate a semiconductor ink is essential for large-area fabrication of semiconductor thin films. To date, it has been shown that hydrazine can be used as a versatile solvent to process a wide range of inorganic semiconductors. However, hydrazine is highly toxic and not suitable for large-scale manufacturing. Here we report a binary mixed solvent of amine and thiol for effective dispersion and dissolution of a large number of inorganic semiconductors including Cu2S, Cu2Se, In2S3, In2Se3, CdS, SnSe, and others. The mixed solvent is significantly less toxic and safer than hydrazine, while at the same time offering the comparable capability of formulating diverse semiconductor ink with a concentration as high as >200 mg/mL. We further show that such ink material can be readily processed into high-performance semiconducting thin films (Cu2S and Cu2Se) with the highest room-temperature conductivity among solution-based materials. Furthermore, we show that complex semiconductor alloys with tunable band gaps, such as CuIn(SxSe1–x)2 (0 ≤ x ≤ 1), can also be readily prepared by simply mixing Cu2S, Cu2Se, In2S3, and In2Se3 ink solutions in a proper ratio. Our study outlines a general strategy for the formulation of inorganic semiconductor ink for low-temperature processing of large-area electronic thin films on diverse substrates and can greatly impact diverse areas including flexible electronics, thermoelectrics, and photovoltaics.Keywords: flexible electronics; semiconductor; solution process; solvent; thin films;
Co-reporter:Yuan Liu, Hailong Zhou, Nathan O Weiss, Yu Huang, and Xiangfeng Duan
ACS Nano 2015 Volume 9(Issue 11) pp:11102
Publication Date(Web):October 15, 2015
DOI:10.1021/acsnano.5b04612
Here we present a general strategy for the fabrication of high-performance organic vertical thin film transistors (OVTFTs) based on the heterostructure of graphene and different organic semiconductor thin films. Utilizing the unique tunable work function of graphene, we show that the vertical carrier transport across the graphene–organic semiconductor junction can be effectively modulated to achieve an ON/OFF ratio greater than 103. Importantly, with the OVTFT design, the channel length is determined by the organic thin film thickness rather than by lithographic resolution. It can thus readily enable transistors with ultrashort channel lengths (<200 nm) to afford a delivering current greatly exceeding that of conventional planar TFTs, thus enabling a respectable operation frequency (up to 0.4 MHz) while using low-mobility organic semiconductors and low-resolution lithography. With this vertical device architecture, the entire organic channel is sandwiched and naturally protected between the source and drain electrodes, which function as the self-passivation layer to ensure stable operation of both p- and n-type OVTFTs in ambient conditions and enable complementary circuits with voltage gain. The creation of high-performance and highly robust OVTFTs can open up exciting opportunities in large-area organic macroelectronics.Keywords: air-stable; barrier modulation; cutoff frequency; graphene contact; high current density; macroelectronics; OTFT;
Co-reporter:Matthew L. Gethers, John C. Thomas, Shan Jiang, Nathan O. Weiss, Xiangfang Duan, William A. Goddard III, and Paul S. Weiss
ACS Nano 2015 Volume 9(Issue 11) pp:10909
Publication Date(Web):October 1, 2015
DOI:10.1021/acsnano.5b03936
We demonstrate the use of “holey” graphene as a mask against molecular adsorption. Prepared porous graphene is transferred onto a Au{111} substrate, annealed, and then exposed to dilute solutions of 1-adamantanethiol. In the pores of the graphene lattice, we find islands of organized, self-assembled molecules. The bare Au in the pores can be regenerated by postdeposition annealing, and new molecules can be self-assembled in the exposed Au region. Graphene can serve as a robust, patternable mask against the deposition of self-assembled monolayers.Keywords: chemical patterning; graphene; mask; nanoscience; scanning tunneling microscopy; self-assembly;
Co-reporter:Gongming Wang;Dehui Li;Anxiang Yin;Yongjia Li;Hung-Chieh Cheng;Chih-Yen Chen;Zipeng Zhao;Zhaoyang Lin;Hao Wu;Yuan Liu;Mengning Ding;Yu Huang;Qiyuan He
Science Advances 2015 Volume 1(Issue 9) pp:e1500613
Publication Date(Web):02 Oct 2015
DOI:10.1126/sciadv.1500613
Scalable growth of regular arrays of perovskite microplates opens a new platform for electronic and optoelectronic device arrays.
Co-reporter:Xing Zhong;Gongming Wang;Benjamin Papandrea;Mufan Li;Yuxi Xu;Yu Chen
Nano Research 2015 Volume 8( Issue 9) pp:2850-2858
Publication Date(Web):2015 September
DOI:10.1007/s12274-015-0790-2
Silicon nanowires (SiNWs) have been widely studied as light harvesting antennas in photocatalysts due to their ability to absorb broad-spectrum solar radiation, but they are typically limited by poor photoelectrochemical stability. Here, we report the synthesis of reduced graphene oxide-SiNW (rGO-SiNW) heterostructures to achieve greatly improved photocatalytic activity and stability. The SiNWs were synthesized through a metal-assisted electroless etching process and functionalized with reduced graphene oxide (rGO) flakes through a chemical absorption process. Here, the rGO not only functions as a physical protection layer to isolate the SiNWs from the harsh electrochemical environment but also serves as a charge mediator to facilitate the charge separation and transport processes. Furthermore, the rGO may also function as a redox catalyst to ensure efficient utilization of photo-carriers for the desired chemical reactions. Photocatalytic dye degradation studies show that the photoactivity of the heterostructures can be significantly enhanced with an initial activation process and maintained without apparent decay over repeated reaction cycles. Electrochemical and photoelectrochemical studies indicate that the enhanced photoactivity and photostability can be attributed to the more efficient separation of photoexcited charge carriers in SiNWs and the reduced self-oxidation of the surface of the SiNWs during the photocatalytic dye degradation process. The ability to significantly improve the photocatalytic activity and stability in rGO-SiNW heterostructures can not only lead to more opportunities for the application of silicon-based photocatalysts/photoelectrodes for solar energy harvesting but also provide new insights into the stabilization of other unstable photocatalytic systems.
Co-reporter:Dr. Yuxi Xu;Zhaoyang Lin;Dr. Xing Zhong;Ben Paprea; Yu Huang
Angewandte Chemie 2015 Volume 127( Issue 18) pp:5435-5440
Publication Date(Web):
DOI:10.1002/ange.201500677
Abstract
A solvent-exchange approach for the preparation of solvated graphene frameworks as high-performance anode materials for lithium-ion batteries is reported. The mechanically strong graphene frameworks exhibit unique hierarchical solvated porous networks and can be directly used as electrodes with a significantly improved electrochemical performance compared to unsolvated graphene frameworks, including very high reversible capacities, excellent rate capabilities, and superior cycling stabilities.
Co-reporter:Dr. Yuxi Xu;Zhaoyang Lin;Dr. Xing Zhong;Ben Paprea; Yu Huang
Angewandte Chemie International Edition 2015 Volume 54( Issue 18) pp:5345-5350
Publication Date(Web):
DOI:10.1002/anie.201500677
Abstract
A solvent-exchange approach for the preparation of solvated graphene frameworks as high-performance anode materials for lithium-ion batteries is reported. The mechanically strong graphene frameworks exhibit unique hierarchical solvated porous networks and can be directly used as electrodes with a significantly improved electrochemical performance compared to unsolvated graphene frameworks, including very high reversible capacities, excellent rate capabilities, and superior cycling stabilities.
Co-reporter:Rui Cheng, Dehui Li, Hailong Zhou, Chen Wang, Anxiang Yin, Shan Jiang, Yuan Liu, Yu Chen, Yu Huang, and Xiangfeng Duan
Nano Letters 2014 Volume 14(Issue 10) pp:5590-5597
Publication Date(Web):August 26, 2014
DOI:10.1021/nl502075n
The p–n diodes represent the most fundamental device building blocks for diverse optoelectronic functions, but are difficult to achieve in atomically thin transition metal dichalcogenides (TMDs) due to the challenges in selectively doping them into p- or n-type semiconductors. Here, we demonstrate that an atomically thin and sharp heterojunction p–n diode can be created by vertically stacking p-type monolayer tungsten diselenide (WSe2) and n-type few-layer molybdenum disulfide (MoS2). Electrical measurements of the vertically staked WSe2/MoS2 heterojunctions reveal excellent current rectification behavior with an ideality factor of 1.2. Photocurrent mapping shows rapid photoresponse over the entire overlapping region with a highest external quantum efficiency up to 12%. Electroluminescence studies show prominent band edge excitonic emission and strikingly enhanced hot-electron luminescence. A systematic investigation shows distinct layer-number dependent emission characteristics and reveals important insight about the origin of hot-electron luminescence and the nature of electron–orbital interaction in TMDs. We believe that these atomically thin heterojunction p–n diodes represent an interesting system for probing the fundamental electro-optical properties in TMDs and can open up a new pathway to novel optoelectronic devices such as atomically thin photodetectors, photovoltaics, as well as spin- and valley-polarized light emitting diodes, on-chip lasers.
Co-reporter:Yuan Liu, Hailong Zhou, Rui Cheng, Woojong Yu, Yu Huang, and Xiangfeng Duan
Nano Letters 2014 Volume 14(Issue 3) pp:1413-1418
Publication Date(Web):February 6, 2014
DOI:10.1021/nl404484s
Flexible thin-film transistors (TFTs) are of central importance for diverse electronic and particularly macroelectronic applications. The current TFTs using organic or inorganic thin film semiconductors are usually limited by either poor electrical performance or insufficient mechanical flexibility. Here, we report a new design of highly flexible vertical TFTs (VTFTs) with superior electrical performance and mechanical robustness. By using the graphene as a work-function tunable contact for amorphous indium gallium zinc oxide (IGZO) thin film, the vertical current flow across the graphene–IGZO junction can be effectively modulated by an external gate potential to enable VTFTs with a highest on–off ratio exceeding 105. The unique vertical transistor architecture can readily enable ultrashort channel devices with very high delivering current and exceptional mechanical flexibility. With large area graphene and IGZO thin film available, our strategy is intrinsically scalable for large scale integration of VTFT arrays and logic circuits, opening up a new pathway to highly flexible macroelectronics.
Co-reporter:Zhaoyang Lin, Yu Chen, Anxiang Yin, Qiyuan He, Xiaoqing Huang, Yuxi Xu, Yuan Liu, Xing Zhong, Yu Huang, and Xiangfeng Duan
Nano Letters 2014 Volume 14(Issue 11) pp:6547-6553
Publication Date(Web):October 24, 2014
DOI:10.1021/nl503140c
Low-temperature solution-processed electronic materials on plastic substrates are of considerable interest for flexible electronics. Solution dispersible inorganic nanostructures (e.g., zero-dimensional (0D) quantum dots or one-dimensional (1D) nanowires) have emerged as interesting ink materials for low-temperature solution processing of electronic thin films on flexible substrates, but usually with limited performance due to the large number of grain boundaries (0D) or incomplete surface coverage (1D). Here, we report two-dimensional (2D) colloidal nanoplates of layered materials as a new ink material for solution assembly of high-performance electronic thin films. The 2D colloidal nanoplates exhibit few dangling bonds and represent an ideal geometry for the assembly of highly uniform continuous thin films with greatly reduced grain boundaries dictated by large-area conformal plane–plane contact with atomically flat/clean interfaces. It can therefore promise efficient charge transport across neighboring nanoplates and throughout the entire thin film to enable unprecedented electronic performance. We show that Bi2Se3 and Bi2Te3 nanoplates can be synthesized with well-controlled thickness (6–15 nm) and lateral dimension (0.5–3 μm) and can be used for the assembly of highly uniform continuous thin films with a full surface coverage and an excellent room temperature carrier mobility >100 cm2·V–1·s–1, approaching that of chemical vapor deposition grown materials. Our study demonstrates a general strategy to using 2D nanoplates as a unique building block for the construction of high-performance electronic thin films on plastic substrates for future flexible electronics and optoelectronics.
Co-reporter:Jonathan C. Shaw;Hailong Zhou;Yu Chen;Nathan O. Weiss;Yuan Liu
Nano Research 2014 Volume 7( Issue 4) pp:511-517
Publication Date(Web):2014 April
DOI:10.1007/s12274-014-0417-z
The synthesis of two-dimensional (2D) layered materials with controllable thickness is of considerable interest for diverse applications. Here we report the first chemical vapor deposition growth of single- and few-layer MoSe2 nanosheets. By using Se and MoO3 as the chemical vapor supply, we demonstrate that highly crystalline MoSe2 can be directly grown on the 300 nm SiO2/Si substrates to form optically distinguishable single- and multi-layer nanosheets, typically in triangular shaped domains with edge lengths around 30 μm, which can merge into continuous thin films upon further growth. Micro-Raman spectroscopy and imaging was used to probe the thickness-dependent vibrational properties. Photoluminescence spectroscopy demonstrates that MoSe2 monolayers exhibit strong near band edge emission at 1.55 eV, while bilayers or multi-layers exhibit much weaker emission, indicating of the transition to a direct band gap semiconductor as the thickness is reduced to a monolayer.
Co-reporter:Yongquan Qu and Xiangfeng Duan
Chemical Society Reviews 2013 vol. 42(Issue 7) pp:2568-2580
Publication Date(Web):29 Nov 2012
DOI:10.1039/C2CS35355E
There is increasing interest in developing artificial systems that can mimic natural photosynthesis to directly harvest and convert solar energy into usable or storable energy resources. Photocatalysis, in which solar photons are used to drive redox reactions to produce chemical fuel, is the central process to achieve this goal. Despite significant efforts to date, a practically viable photocatalyst with sufficient efficiency, stability and low cost is yet to be demonstrated. It is often difficult to simultaneously achieve these different performance metrics with a single material component. The heterogeneous photocatalysts with multiple integrated functional components could combine the advantages of different components to overcome the drawbacks of single component photocatalysts. A wide range of heterostructures, including metal/semiconductor, semiconductor/semiconductor, molecule/semiconductor and multi-heteronanostructures, have been explored for improved photocatalysts by increasing the light absorption, promoting the charge separation and transportation, enhancing the redox catalytic activity and prolonging the functional life-time. The present review gives a concise overview of heterogeneous photocatalysts with a focus on the relationship between the structural architecture and the photocatalytic activity and stability.
Co-reporter:Hua Zhang, Xing Zhong, Jonathan C. Shaw, Lixin Liu, Yu Huang and Xiangfeng Duan
Energy & Environmental Science 2013 vol. 6(Issue 9) pp:2621-2625
Publication Date(Web):29 Jul 2013
DOI:10.1039/C3EE41157E
High density electrochemical energy storage is of central importance for mobile power applications. The relatively low energy density and high cost associated with the current approaches to electrochemical energy storage, including various battery and supercapacitor technologies, have been the main hurdles to its more widespread application. Herein we report a new family of silicide based anode materials for high energy density metal–air primary batteries. Several intrinsic features of silicide materials including high electron capacity, high conductivity and high operating voltage make them an excellent class of materials for very high density energy storage. We show that various silicide anodes (Mg2Si, TiSi2, CoSi2 and VSi2) can exhibit excellent electrochemical performance with unparalleled capacity in both thin films and bulk powder pellet forms. With further optimization, silicide materials can promise a new battery family with ultra-high anode capacity and can open up exciting opportunities for future mobile power applications.
Co-reporter:Yuxi Xu;Zhaoyang Lin;Xiaoqing Huang;Yang Wang;Yu Huang
Advanced Materials 2013 Volume 25( Issue 40) pp:5779-5784
Publication Date(Web):
DOI:10.1002/adma.201301928
Co-reporter:Yuxi Xu, Zhaoyang Lin, Xiaoqing Huang, Yuan Liu, Yu Huang, and Xiangfeng Duan
ACS Nano 2013 Volume 7(Issue 5) pp:4042
Publication Date(Web):April 4, 2013
DOI:10.1021/nn4000836
Flexible solid-state supercapacitors are of considerable interest as mobile power supply for future flexible electronics. Graphene or carbon nanotubes based thin films have been used to fabricate flexible solid-state supercapacitors with high gravimetric specific capacitances (80–200 F/g), but usually with a rather low overall or areal specific capacitance (3–50 mF/cm2) due to the ultrasmall electrode thickness (typically a few micrometers) and ultralow mass loading, which is not desirable for practical applications. Here we report the exploration of a three-dimensional (3D) graphene hydrogel for the fabrication of high-performance solid-state flexible supercapacitors. With a highly interconnected 3D network structure, graphene hydrogel exhibits exceptional electrical conductivity and mechanical robustness to make it an excellent material for flexible energy storage devices. Our studies demonstrate that flexible supercapacitors with a 120 μm thick graphene hydrogel thin film can exhibit excellent capacitive characteristics, including a high gravimetric specific capacitance of 186 F/g (up to 196 F/g for a 42 μm thick electrode), an unprecedented areal specific capacitance of 372 mF/cm2 (up to 402 mF/cm2 for a 185 μm thick electrode), low leakage current (10.6 μA), excellent cycling stability, and extraordinary mechanical flexibility. This study demonstrates the exciting potential of 3D graphene macrostructures for high-performance flexible energy storage devices.Keywords: 3D network; energy storage; flexible electronics; graphene; solid-state devices; supercapacitor
Co-reporter:Yuxi Xu;Xiaoqing Huang;Zhaoyang Lin;Xing Zhong;Yu Huang
Nano Research 2013 Volume 6( Issue 1) pp:65-76
Publication Date(Web):2013 January
DOI:10.1007/s12274-012-0284-4
Co-reporter:Chunlan Wang, Rui Cheng, Lei Liao, Xiangfeng Duan
Nano Today 2013 Volume 8(Issue 5) pp:514-530
Publication Date(Web):October 2013
DOI:10.1016/j.nantod.2013.08.001
Thin film transistors (TFTs) represent the central device building block for large area electronics. The integration of TFTs on flexible substrates is of considerable interest for diverse applications including electronic displays and radio frequency identification, and can enable a new generation of flexible, wearable and disposable electronics. The current approach to TFTs is largely based on amorphous Si or polysilicon on glass technology, which is limited by a number of intrinsic factors, including relatively poor electronic performance or indispensable high temperature deposition processes preventing the use of flexible plastic substrate. The organic semiconductors can be processed in solution and applied onto plastic substrate at room temperature, but are typically limited by rather poor electronic performance. To this end, inorganic nanostructures have emerged as an interesting alternative material system for thin film electronics that could promise the high electronic performance of crystalline inorganic materials, and at the same time offer a low temperature processibility of organic materials. These unique attributes have opened up completely new possibilities for future thin film electronics. Here we focus on the recent advancements in the fabrication of high-performance TFTs based inorganic nanostructures, specifically involving thin films of (1) carbon nanotubes, (2) semiconductor nanowires, (3) two-dimensional layered materials, (4) quantum dots, and (5) metal oxide/nanostructure composites. We will discuss the state-of-the-art, key merits and limitations of each of these material systems.
Co-reporter:Nathan O. Weiss
PNAS 2013 Volume 110 (Issue 38 ) pp:15171-15172
Publication Date(Web):2013-09-17
DOI:10.1073/pnas.1313743110
Co-reporter:Hailong Zhou, Yongquan Qu, Tahani Zeid and Xiangfeng Duan
Energy & Environmental Science 2012 vol. 5(Issue 5) pp:6732-6743
Publication Date(Web):21 Mar 2012
DOI:10.1039/C2EE03447F
The search for clean renewable energy sources is of central importance to address the ever-increasing challenges of diminishing fossil fuels and global warming. Photocatalytic processes can mimic natural photosynthesis to directly convert solar energy into chemical energy, and represent an attractive strategy for renewable energy generation and environmental remediation. Nanostructured semiconductors can play an important role in photocatalysis due to their unique structures, and chemical and physical properties. Here we present a brief overview of the recent progress in the development of semiconductor nanostructure based photocatalysts. In particular, we focus our discussions on four essential problems that dictate the performance of a photocatalyst material: visible light absorption for efficient solar energy harvesting, efficient charge separation and transportation, effective cocatalysts for efficient charge utilization, and photoelectrochemical stability for robust photocatalysis. Challenges, potential solutions, and recent efforts to address each one of these problems are discussed. Lastly, we finish the perspective with the discussion of a recent concept of using freestanding photoelectrochemical nanodevices as a potential solution to a new generation of highly efficient and stable photocatalysts.
Co-reporter:Nathan O. Weiss;Hailong Zhou;Lei Liao;Yuan Liu;Shan Jiang;Yu Huang
Advanced Materials 2012 Volume 24( Issue 43) pp:5782-5825
Publication Date(Web):
DOI:10.1002/adma.201201482
Abstract
Graphene, a single layer of carbon atoms in a honeycomb lattice, offers a number of fundamentally superior qualities that make it a promising material for a wide range of applications, particularly in electronic devices. Its unique form factor and exceptional physical properties have the potential to enable an entirely new generation of technologies beyond the limits of conventional materials. The extraordinarily high carrier mobility and saturation velocity can enable a fast switching speed for radio-frequency analog circuits. Unadulterated graphene is a semi-metal, incapable of a true off-state, which typically precludes its applications in digital logic electronics without bandgap engineering. The versatility of graphene-based devices goes beyond conventional transistor circuits and includes flexible and transparent electronics, optoelectronics, sensors, electromechanical systems, and energy technologies. Many challenges remain before this relatively new material becomes commercially viable, but laboratory prototypes have already shown the numerous advantages and novel functionality that graphene provides.
Co-reporter:Nathan O. Weiss;Hailong Zhou;Lei Liao;Yuan Liu;Shan Jiang;Yu Huang
Advanced Materials 2012 Volume 24( Issue 43) pp:
Publication Date(Web):
DOI:10.1002/adma.201290269
Co-reporter:Lei Liao, Xiangfeng Duan
Materials Today 2012 Volume 15(7–8) pp:328-338
Publication Date(Web):July–August 2012
DOI:10.1016/S1369-7021(12)70138-4
Graphene is emerging as an attractive electronic material for future electronics. With the highest carrier mobility, high saturation velocity, high critical current densities, and single atomic thickness, graphene has great potential for ultra-high speed transistors, with the highest projected cut-off frequency exceeding 1 THz. However, the fabrication of high speed graphene transistors is of significant challenge, since conventional electronic fabrication processes often introduce undesirable defects into graphene lattices. Significant efforts have made to mitigate these challenges. Here we review the opportunities, challenges, as well as the recent advances in the development of high speed graphene transistors and circuits.
Co-reporter:Gang Liu, Yung-Chen Lin, Lei Liao, Lixin Liu, Yu Chen, Yuan Liu, Nathan O. Weiss, Hailong Zhou, Yu Huang, and Xiangfeng Duan
Nano Letters 2012 Volume 12(Issue 4) pp:1972-1976
Publication Date(Web):April 2, 2012
DOI:10.1021/nl204510p
We report the synthesis of single crystalline Co2Si nanowires and the electrical transport studies of single Co2Si nanowire devices at low temperature. The butterfly shaped magnetoresistance shows interesting ferromagnetic features, including negative magnetoresistance, hysteretic switch fields, and stepwise drops in magnetoresistance. The nonsmooth stepwise magnetoresistance response is attributed to magnetic domain wall pinning and depinning motion in the Co2Si nanowires probably at crystal or morphology defects. The temperature dependence of the domain wall depinning field is observed and described by a model based on thermally assisted domain wall depinning over a single energy barrier.
Co-reporter:Lei Liao, Jingwei Bai, Rui Cheng, Hailong Zhou, Lixin Liu, Yuan Liu, Yu Huang, and Xiangfeng Duan
Nano Letters 2012 Volume 12(Issue 6) pp:2653-2657
Publication Date(Web):June 7, 2011
DOI:10.1021/nl201922c
Graphene transistors are of considerable interest for radio frequency (rf) applications. High-frequency graphene transistors with the intrinsic cutoff frequency up to 300 GHz have been demonstrated. However, the graphene transistors reported to date only exhibit a limited extrinsic cutoff frequency up to about 10 GHz, and functional graphene circuits demonstrated so far can merely operate in the tens of megahertz regime, far from the potential the graphene transistors could offer. Here we report a scalable approach to fabricate self-aligned graphene transistors with the extrinsic cutoff frequency exceeding 50 GHz and graphene circuits that can operate in the 1–10 GHz regime. The devices are fabricated on a glass substrate through a self-aligned process by using chemical vapor deposition (CVD) grown graphene and a dielectrophoretic assembled nanowire gate array. The self-aligned process allows the achievement of unprecedented performance in CVD graphene transistors with a highest transconductance of 0.36 mS/μm. The use of an insulating substrate minimizes the parasitic capacitance and has therefore enabled graphene transistors with a record-high extrinsic cutoff frequency (> 50 GHz) achieved to date. The excellent extrinsic cutoff frequency readily allows configuring the graphene transistors into frequency doubling or mixing circuits functioning in the 1–10 GHz regime, a significant advancement over previous reports (∼20 MHz). The studies open a pathway to scalable fabrication of high-speed graphene transistors and functional circuits and represent a significant step forward to graphene based radio frequency devices.
Co-reporter:Yongquan Qu and Xiangfeng Duan
Journal of Materials Chemistry A 2012 vol. 22(Issue 32) pp:16171-16181
Publication Date(Web):25 Jun 2012
DOI:10.1039/C2JM32267F
Nanostructures have been the focus of considerable interest for solar energy conversion in the areas of renewable green energy sources as well as environmental remediation due to their unique physicochemical properties. Here we highlight the recent efforts on developing new materials for solar energy conversion with a focus on one-dimensional homogeneous and heterogeneous nanowires. We first outline challenges and strategies to develop highly efficient and stable semiconductor materials for solar energy conversion, and then discuss the potential advantages and recent progress in exploring one-dimensional homogeneous and heterogeneous structures. We will particularly focus our discussion on the photovoltaic performance of various one-dimensional nanowire materials. Lastly, the perspectives for further improving the efficiency and stability of the solar energy conversion system using one-dimensional homogeneous and heterogeneous nanowires and their potential applications will be discussed.
Co-reporter:Lixin Liu, Hailong Zhou, Rui Cheng, Yu Chen, Yung-Chen Lin, Yongquan Qu, Jingwei Bai, Ivan A. Ivanov, Gang Liu, Yu Huang and Xiangfeng Duan
Journal of Materials Chemistry A 2012 vol. 22(Issue 4) pp:1498-1503
Publication Date(Web):24 Nov 2011
DOI:10.1039/C1JM14272K
Graphene has attracted considerable interest as a potential material for future electronics. Although mechanical peel is known to produce high quality graphene flakes, practical applications require continuous graphene layers over a large area. The catalyst-assisted chemical vapor deposition (CVD) is a promising synthetic method to deliver wafer-sized graphene. Here we present a systematic study on the nucleation and growth of crystallized graphene domains in an atmospheric pressure chemical vapor deposition (APCVD) process. Parametric studies show that the mean size of the graphene domains increases with increasing growth temperature and CH4 partial pressure, while the density of domains decreases with increasing growth temperature and is independent of the CH4 partial pressure. Our studies show that nucleation of graphene domains on copper substrate is highly dependent on the initial annealing temperature. A two-step synthetic process with higher initial annealing temperature but lower growth temperature is developed to reduce domain density and achieve high quality full-surface coverage of monolayer graphene films. Electrical transport measurements demonstrate that the resulting graphene exhibits a high carrier mobility of up to 3000 cm2 V−1 s−1 at room temperature.
Co-reporter:Xing Zhong;Hua Zhang;Yuan Liu;Dr. Jingwei Bai;Dr. Lei Liao; Yu Huang; Xiangfeng Duan
ChemSusChem 2012 Volume 5( Issue 1) pp:177-180
Publication Date(Web):
DOI:10.1002/cssc.201100426
Co-reporter:Lixin Liu, Hailong Zhou, Rui Cheng, Woo Jong Yu, Yuan Liu, Yu Chen, Jonathan Shaw, Xing Zhong, Yu Huang, and Xiangfeng Duan
ACS Nano 2012 Volume 6(Issue 9) pp:8241
Publication Date(Web):August 20, 2012
DOI:10.1021/nn302918x
Bernal-stacked (AB-stacked) bilayer graphene is of significant interest for functional electronic and photonic devices due to the feasibility to continuously tune its band gap with a vertical electric field. Mechanical exfoliation can be used to produce AB-stacked bilayer graphene flakes but typically with the sizes limited to a few micrometers. Chemical vapor deposition (CVD) has been recently explored for the synthesis of bilayer graphene but usually with limited coverage and a mixture of AB- and randomly stacked structures. Herein we report a rational approach to produce large-area high-quality AB-stacked bilayer graphene. We show that the self-limiting effect of graphene growth on Cu foil can be broken by using a high H2/CH4 ratio in a low-pressure CVD process to enable the continued growth of bilayer graphene. A high-temperature and low-pressure nucleation step is found to be critical for the formation of bilayer graphene nuclei with high AB stacking ratio. A rational design of a two-step CVD process is developed for the growth of bilayer graphene with high AB stacking ratio (up to 90%) and high coverage (up to 99%). The electrical transport studies demonstrate that devices made of the as-grown bilayer graphene exhibit typical characteristics of AB-stacked bilayer graphene with the highest carrier mobility exceeding 4000 cm2/V·s at room temperature, comparable to that of the exfoliated bilayer graphene.Keywords: AB stacking; band gap; bilayer graphene; chemical vapor deposition; copper foil
Co-reporter:Teng Xue;Shan Jiang;Dr. Yongquan Qu;Qiao Su;Rui Cheng;Sergey Dubin;Chin-Yi Chiu; Richard Kaner; Yu Huang; Xiangfeng Duan
Angewandte Chemie International Edition 2012 Volume 51( Issue 16) pp:3822-3825
Publication Date(Web):
DOI:10.1002/anie.201108400
Co-reporter:Teng Xue;Shan Jiang;Dr. Yongquan Qu;Qiao Su;Rui Cheng;Sergey Dubin;Chin-Yi Chiu; Richard Kaner; Yu Huang; Xiangfeng Duan
Angewandte Chemie International Edition 2012 Volume 51( Issue 16) pp:
Publication Date(Web):
DOI:10.1002/anie.201201328
Co-reporter:Jingwei Bai;Yung-Chen Lin;Rui Cheng;Lei Liao;Yu Chen;Hailong Zhou;Yu Huang;Shan Jiang;Lixin Liu
PNAS 2012 Volume 109 (Issue 29 ) pp:
Publication Date(Web):2012-07-17
DOI:10.1073/pnas.1205696109
Graphene has attracted enormous attention for radio-frequency transistor applications because of its exceptional high carrier
mobility, high carrier saturation velocity, and large critical current density. Herein we report a new approach for the scalable
fabrication of high-performance graphene transistors with transferred gate stacks. Specifically, arrays of gate stacks are
first patterned on a sacrificial substrate, and then transferred onto arbitrary substrates with graphene on top. A self-aligned
process, enabled by the unique structure of the transferred gate stacks, is then used to position precisely the source and
drain electrodes with minimized access resistance or parasitic capacitance. This process has therefore enabled scalable fabrication
of self-aligned graphene transistors with unprecedented performance including a record-high cutoff frequency up to 427 GHz.
Our study defines a unique pathway to large-scale fabrication of high-performance graphene transistors, and holds significant
potential for future application of graphene-based devices in ultra–high-frequency circuits.
Co-reporter:Jingwei Bai, Lei Liao, Hailong Zhou, Rui Cheng, Lixin Liu, Yu Huang, and Xiangfeng Duan
Nano Letters 2011 Volume 11(Issue 6) pp:2555-2559
Publication Date(Web):May 6, 2011
DOI:10.1021/nl201331x
Graphene transistors are of considerable interest for radio frequency (rf) applications. In general, transistors with large transconductance and drain current saturation are desirable for rf performance, which is however nontrivial to achieve in graphene transistors. Here we report high-performance top-gated graphene transistors based on chemical vapor deposition (CVD) grown graphene with large transconductance and drain current saturation. The graphene transistors were fabricated with evaporated high dielectric constant material (HfO2) as the top-gate dielectrics. Length scaling studies of the transistors with channel length from 5.6 μm to 100 nm show that complete current saturation can be achieved in 5.6 μm devices and the saturation characteristics degrade as the channel length shrinks down to the 100–300 nm regime. The drain current saturation was primarily attributed to drain bias induced shift of the Dirac points. With the selective deposition of HfO2 gate dielectrics, we have further demonstrated a simple scheme to realize a 300 nm channel length graphene transistors with self-aligned source–drain electrodes to achieve the highest transconductance of 250 μS/μm reported in CVD graphene to date.
Co-reporter:Woo Jong Yu, Lei Liao, Sang Hoon Chae, Young Hee Lee, and Xiangfeng Duan
Nano Letters 2011 Volume 11(Issue 11) pp:4759-4763
Publication Date(Web):October 10, 2011
DOI:10.1021/nl2025739
The bilayer graphene has attracted considerable attention for potential applications in future electronics and optoelectronics because of the feasibility to tune its band gap with a vertical displacement field to break the inversion symmetry. Surface chemical doping in bilayer graphene can induce an additional offset voltage to fundamentally affect the vertical displacement field and the band gap opening in bilayer graphene. In this study, we investigate the effect of chemical molecular doping on band gap opening in bilayer graphene devices with single or dual gate modulation. Chemical doping with benzyl viologen molecules modulates the displacement field to allow the opening of a transport band gap and the increase of the on/off ratio in the bilayer graphene transistors. Additionally, Fermi energy level in the opened gap can be rationally controlled by the amount of molecular doping to obtain bilayer graphene transistors with tunable Dirac points, which can be readily configured into functional devices, such as complementary inverters.
Co-reporter:Yongquan Qu ; Rui Cheng ; Qiao Su
Journal of the American Chemical Society 2011 Volume 133(Issue 42) pp:16730-16733
Publication Date(Web):October 3, 2011
DOI:10.1021/ja204383q
We report the plasmonic enhancement of the photocatalytic properties of Pt/n-Si/Ag photodiode photocatalysts using Au/Ag core/shell nanorods. We show that Au/Ag core/shell nanorods can be synthesized with tunable plasmon resonance frequencies and then conjugated onto Pt/n-Si/Ag photodiodes using well-defined chemistry. Photocatalytic studies showed that the conjugation with Au/Ag core/shell nanorods can significantly enhance the photocatalytic activity by more than a factor of 3. Spectral dependence studies further revealed that the photocatalytic enhancement is strongly correlated with the plasmonic absorption spectra of the Au/Ag core/shell nanorods, unambiguously demonstrating the plasmonic enhancement effect.
Co-reporter:Min Xue ; Xing Zhong ; Zory Shaposhnik ; Yongquan Qu ; Fuyuhiko Tamanoi ; Xiangfeng Duan ;Jeffrey I. Zink
Journal of the American Chemical Society 2011 Volume 133(Issue 23) pp:8798-8801
Publication Date(Web):May 19, 2011
DOI:10.1021/ja201252e
Porous silicon nanoparticles (PSiNPs) were synthesized by silver-assisted electroless chemical etching of silicon nanowires generated on a silicon wafer. The rod-shaped particles (200–400 nm long and 100–200 nm in diameter) were derivatized with a cyclodextrin-based nanovalve that was closed at the physiological pH of 7.4 but open at pH <6. Release profiles in water and tissue culture media showed that no cargo leaked when the valves were closed and that release occurred immediately after acidification. In vitro studies using human pancreatic carcinoma PANC-1 cells proved that these PSiNPs were endocytosed and carried cargo molecules into the cells and released them in response to lysosomal acidity. These studies show that PSiNPs can serve as an autonomously functioning delivery platform in biological systems and open new possibilities for drug delivery.
Co-reporter:Hua Zhang, Yujing Li, Yungchen Lin, Yu Huang and Xiangfeng Duan
Nanoscale 2011 vol. 3(Issue 3) pp:963-966
Publication Date(Web):25 Jan 2011
DOI:10.1039/C0NR00823K
Single crystal hexagonal NaYF4:Yb/Tm nanocrystals have been synthesized with uniform size, morphology and controlled chemical composition. Spectroscopic studies show that these nanocrystals exhibit strong energy upconversion emission when excited with a 980 nm diode laser, with two primary emission peaks centered around 452 nm and 476 nm. Importantly, the overall and relative emission intensity at these wavelengths can be readily tuned by controlling the concentration of the trivalent rare earth element dopants at the beginning of the synthesis which has been confirmed by EDX for the first time. Through systematic studies, the optimum rare earth ion doping concentration can be determined for the strongest emission intensity at the selected peak(s). Confocal microscopy studies show that the upconversion emission from individual NCs can be readily visualized. These studies demonstrate a rational approach for fine tuning the upconversion properties in rare-earth doped nanostructures and can broadly impact areas ranging from energy harvesting, energy conversion to biomedical imaging and therapeutics.
Co-reporter:Yongquan Qu, Hailong Zhou and Xiangfeng Duan
Nanoscale 2011 vol. 3(Issue 10) pp:4060-4068
Publication Date(Web):25 Aug 2011
DOI:10.1039/C1NR10668F
In this mini-review, we summarize recent progress in the synthesis, properties and applications of a new type of one-dimensional nanostructures—single crystalline porous silicon nanowires. The growth of porous silicon nanowires starting from both p- and n-type Si wafers with a variety of dopant concentrations can be achieved through either one-step or two-step reactions. The mechanistic studies indicate the dopant concentration of Si wafers, oxidizer concentration, etching time and temperature can affect the morphology of the as-etched silicon nanowires. The porous silicon nanowires are both optically and electronically active and have been explored for potential applications in diverse areas including photocatalysis, lithium ion batteries, gas sensors and drug delivery.
Co-reporter:Yongquan Qu, Jingwei Bai, Lei Liao, Rui Cheng, Yung-Chen Lin, Yu Huang, Ting Guo and Xiangfeng Duan
Chemical Communications 2011 vol. 47(Issue 4) pp:1255-1257
Publication Date(Web):22 Nov 2010
DOI:10.1039/C0CC03922E
Single crystalline Co2Si nanobelts were synthesized for the first time. Temperature-dependent electrical transport studies show the Co2Si nanobelts exhibit metallic behavior with a large negative magnetoresistance over 10% at low temperature, which may be attributed to alignment of the spins in surface cobalt atoms.
Co-reporter:Hua Zhang, Di Xu, Yu Huang and Xiangfeng Duan
Chemical Communications 2011 vol. 47(Issue 3) pp:979-981
Publication Date(Web):16 Nov 2010
DOI:10.1039/C0CC03566A
We report a five-fold overall enhancement of upconversion emission in NaYF4:Yb/Er nanocrystals when coupled with gold island films. Spectroscopic studies show that the enhancement factors are highly dependent on the exact spectral positions and excitation power density, with a largest enhancement factor of more than 12 observed at selected spectral positions, which may be attributed to different upconversion processes involved.
Co-reporter:Xing Zhong, Yongquan Qu, Yung-Chen Lin, Lei Liao, and Xiangfeng Duan
ACS Applied Materials & Interfaces 2011 Volume 3(Issue 2) pp:261
Publication Date(Web):January 18, 2011
DOI:10.1021/am1009056
Porous silicon nanowire is emerging as an interesting material system due to its unique combination of structural, chemical, electronic, and optical properties. To fully understand their formation mechanism is of great importance for controlling the fundamental physical properties and enabling potential applications. Here we present a systematic study to elucidate the mechanism responsible for the formation of porous silicon nanowires in a two-step silver-assisted electroless chemical etching method. It is shown that silicon nanowire arrays with various porosities can be prepared by varying multiple experimental parameters such as the resistivity of the starting silicon wafer, the concentration of oxidant (H2O2) and the amount of silver catalyst. Our study shows a consistent trend that the porosity increases with the increasing wafer conductivity (dopant concentration) and oxidant (H2O2) concentration. We further demonstrate that silver ions, formed by the oxidation of silver, can diffuse upwards and renucleate on the sidewalls of nanowires to initiate new etching pathways to produce a porous structure. The elucidation of this fundamental formation mechanism opens a rational pathway to the production of wafer-scale single crystalline porous silicon nanowires with tunable surface areas ranging from 370 to 30 m2 g−1 and can enable exciting opportunities in catalysis, energy harvesting, conversion, storage, as well as biomedical imaging and therapy.Keywords (keywords): catalysis; electroless chemical etching; porous silicon; silicon nanowires; surface area
Co-reporter:Yongquan Qu, Lei Liao, Rui Cheng, Yue Wang, Yung-Chen Lin, Yu Huang and Xiangfeng Duan
Nano Letters 2010 Volume 10(Issue 5) pp:1941-1949
Publication Date(Web):April 7, 2010
DOI:10.1021/nl101010m
Photocatalysts are of significant interest in solar energy harvesting and conversion into chemical energy. However, the photocatalysts available to date are limited by either poor efficiency in the visible light range or insufficient photoelectrochemical stability. Here we report the rational design of a new generation of freestanding photoelectric nanodevices as highly efficient and stable photocatalysts by integrating a nanoscale photodiode with two redox catalysts in a single nanowire heterostructure. We show that a platinum−silicon−silver nanowire heterostructure can be synthesized to integrate a nanoscale metal−semiconductor Schottky diode encased in a protective insulating shell with two exposed metal catalysts. We further demonstrated that the Schottky diodes exhibited a pronounced photovoltaic effect with nearly unity internal quantum efficiency and that the integrated nanowire heterostructures could be used as highly efficient photocatalysts for a wide range of thermodynamically downhill and uphill reactions including the photocatalytic degradation of organic dyes and the reduction of metal ions and carbon dioxide using visible light. Our studies for the first time demonstrated the integration of multiple distinct functional components into a single nanostructure to form a standalone active nanosystem and for the first time successfully realized a photoelectric nanodevice that is both highly efficient and highly stable throughout the entire solar spectrum. It thus opens a rational avenue to the design and synthesis of a new generation of photoelectric nanosystems with unprecedented efficiency and stability and will have a broad impact in areas including environmental remediation, artificial photosynthesis and solar fuel production.
Co-reporter:Lei Liao, Jingwei Bai, Rui Cheng, Yung-Chen Lin, Shan Jiang, Yu Huang and Xiangfeng Duan
Nano Letters 2010 Volume 10(Issue 5) pp:1917-1921
Publication Date(Web):April 9, 2010
DOI:10.1021/nl100840z
The integration ultrathin high dielectric constant (high-k) materials with graphene nanoribbons (GNRs) for top-gated transistors can push their performance limit for nanoscale electronics. Here we report the assembly of Si/HfO2 core/shell nanowires on top of individual GNRs as the top-gates for GNR field-effect transistors with ultrathin high-k dielectrics. The Si/HfO2 core/shell nanowires are synthesized by atomic layer deposition of the HfO2 shell on highly doped silicon nanowires with a precise control of the dielectric thickness down to 1−2 nm. Using the core/shell nanowires as the top-gates, high-performance GNR transistors have been achieved with transconductance reaching 3.2 mS μm−1, the highest value for GNR transistors reported to date. This method, for the first time, demonstrates the effective integration of ultrathin high-k dielectrics with graphene with precisely controlled thickness and quality, representing an important step toward high-performance graphene electronics.
Co-reporter:Lei Liao, Jingwei Bai, Rui Cheng, Yung-Chen Lin, Shan Jiang, Yongquan Qu, Yu Huang, and Xiangfeng Duan
Nano Letters 2010 Volume 10(Issue 10) pp:3952-3956
Publication Date(Web):September 3, 2010
DOI:10.1021/nl101724k
Here we report high-performance sub-100 nm channel length graphene transistors fabricated using a self-aligned approach. The graphene transistors are fabricated using a highly doped GaN nanowire as the local gate with the source and drain electrodes defined through a self-aligned process and the channel length defined by the nanowire size. This fabrication approach allows the preservation of the high carrier mobility in graphene and ensures nearly perfect alignment between source, drain, and gate electrodes. It therefore affords transistor performance not previously possible. Graphene transistors with 45−100 nm channel lengths have been fabricated with the scaled transconductance exceeding 2 mS/μm, comparable to the best performed high electron mobility transistors with similar channel lengths. Analysis of and the device characteristics gives a transit time of 120−220 fs and the projected intrinsic cutoff frequency (fT) reaching 700−1400 GHz. This study demonstrates the exciting potential of graphene based electronics in terahertz electronics.
Co-reporter:Yongquan Qu;Teng Xue;Xing Zhong;Yung-Chen Lin;Lei Liao;Jina Choi
Advanced Functional Materials 2010 Volume 20( Issue 18) pp:3005-3011
Publication Date(Web):
DOI:10.1002/adfm.201000857
Abstract
Photocatalyst mediated photoelectrochemical processes can make use of the photogenerated electrons and holes onsite for photocatalytic redox reactions, and enable the harness and conversion of solar energy into chemical energy, in analogy to natural photosynthesis. However, the photocatalysts available to date are limited by either poor efficiency in the visible light range or insufficient photoelectrochemical stability. Here, it is shown that a Pt/Si/Ag nanowire heterostructure can be rationally synthesized to integrate a nanoscale metal-semiconductor Schottky diode encased in a protective insulating shell with two exposed metal catalysts. The synthesis of Pt/Si/Ag nanowire diodes involves a scalable process including the formation of silicon nanowire array through wet chemical etching, electrodeposition of platinum and photoreduction of silver. The Pt/Si/Ag diodes exhibit highly efficient photocatalytic activity for a wide range of applications including environmental remediation and solar fuel production in the visible range. In this article, photodegradation of indigo carmine and 4-nitrophenol are used to evaluate the photoactivity of Pt/Si/Ag diodes. The Pt/Si/Ag diodes also show high activity for photoconversion of formic acid into carbon dioxide and hydrogen.
Co-reporter:Yongquan Qu;Teng Xue;Xing Zhong;Yung-Chen Lin;Lei Liao;Jina Choi
Advanced Functional Materials 2010 Volume 20( Issue 18) pp:
Publication Date(Web):
DOI:10.1002/adfm.201090081
Abstract
Photocatalyst mediated photoelectrochemical processes can make use of the photogenerated electrons and holes onsite for photocatalytic redox reactions, and enable the harness and conversion of solar energy into chemical energy, in analogy to natural photosynthesis. However, the photocatalysts available to date are limited by either poor efficiency in the visible light range or insufficient photoelectrochemical stability. Here, it is shown that a Pt/Si/Ag nanowire heterostructure can be rationally synthesized to integrate a nanoscale metal-semiconductor Schottky diode encased in a protective insulating shell with two exposed metal catalysts. The synthesis of Pt/Si/Ag nanowire diodes involves a scalable process including the formation of silicon nanowire array through wet chemical etching, electrodeposition of platinum and photoreduction of silver. The Pt/Si/Ag diodes exhibit highly efficient photocatalytic activity for a wide range of applications including environmental remediation and solar fuel production in the visible range. In this article, photodegradation of indigo carmine and 4-nitrophenol are used to evaluate the photoactivity of Pt/Si/Ag diodes. The Pt/Si/Ag diodes also show high activity for photoconversion of formic acid into carbon dioxide and hydrogen.
Co-reporter:Yongquan Qu, Xing Zhong, Yujing Li, Lei Liao, Yu Huang and Xiangfeng Duan
Journal of Materials Chemistry A 2010 vol. 20(Issue 18) pp:3590-3594
Publication Date(Web):09 Apr 2010
DOI:10.1039/C0JM00493F
Porous silicon nanowires are synthesized by metal assisted wet-chemical etching of highly-doped silicon wafers. The resulting porous silicon nanowires exhibit a large surface area of 337 m2 g−1 and a wide spectrum absorption across the entire ultraviolet, visible and near infrared regime. We further demonstrate that platinum nanoparticles can be loaded onto the surface of the porous silicon nanowires with controlled density. These combined advancements make the porous silicon nanowires an interesting material for photocatalytic applications. We show that the porous silicon nanowires and platinum nanoparticle loaded porous silicon nanowires can be used as effective photocatalysts for photocatalytic degradation of organic dyes and toxic pollutants under visible irradiation, and thus are of significant interest for organic waste treatment and environmental remediation.
Co-reporter:Lei Liao, Xiangfeng Duan
Materials Science and Engineering: R: Reports 2010 70(3–6) pp: 354-370
Publication Date(Web):22 November 2010
DOI:10.1016/j.mser.2010.07.003
Graphene is emerging as an interesting electronic material for future electronics due to its exceptionally high carrier mobility and single-atomic thickness. Graphene–dielectric integration is of critical importance for the development of graphene transistors and a new generation of graphene based electronics. Deposition of dielectric materials onto graphene is of significant challenge due to the intrinsic material incompatibility between pristine graphene and dielectric oxide materials. Here we review various strategies being researched for graphene–dielectric integration. Physical vapor deposition (PVD) can be used to directly deposit dielectric materials on graphene, but often introduces significant defects into the monolayer of carbon lattice; atomic layer deposition (ALD) process has also been explored to deposit high-κ dielectrics on graphene, which however requires functionalization of graphene surface with reactive groups, inevitably leading to a significant degradation in carrier mobilities. Using naturally oxidized thin aluminum or polymer as buffer layer for dielectric deposition can mitigate the damages to graphene lattice and improve the carrier mobility of the resulted top-gated transistors. Lastly, a physical assembly approach has recently been explored to integrate dielectric nanostructures with graphene without introducing any appreciable defects, and enabled top-gated graphene transistors with the highest carrier mobility reported to date. We will conclude with a brief summary and perspective on future opportunities.
Co-reporter:Hua Zhang;Yujing Li;IvanA. Ivanov;Yongquan Qu Dr.;Yu Huang
Angewandte Chemie International Edition 2010 Volume 49( Issue 16) pp:2865-2868
Publication Date(Web):
DOI:10.1002/anie.200905805
Co-reporter:Lei Liao;Jingwei Bai;Yongquan Qu;Yung-chen Lin;Yujing Li;Yu Huang
PNAS 2010 107 (15 ) pp:6711-6715
Publication Date(Web):2010-04-13
DOI:10.1073/pnas.0914117107
Deposition of high-κ dielectrics onto graphene is of significant challenge due to the difficulties of nucleating high quality oxide on pristine
graphene without introducing defects into the monolayer of carbon lattice. Previous efforts to deposit high-κ dielectrics on graphene often resulted in significant degradation in carrier mobility. Here we report an entirely new strategy
to integrate high quality high-κ dielectrics with graphene by first synthesizing freestanding high-κ oxide nanoribbons at high temperature and then transferring them onto graphene at room temperature. We show that single crystalline
Al2O3 nanoribbons can be synthesized with excellent dielectric properties. Using such nanoribbons as the gate dielectrics, we have
demonstrated top-gated graphene transistors with the highest carrier mobility (up to 23,600 cm2/V·s) reported to date, and a more than 10-fold increase in transconductance compared to the back-gated devices. This method
opens a new avenue to integrate high-κ dielectrics on graphene with the preservation of the pristine nature of graphene and high carrier mobility, representing
an important step forward to high-performance graphene electronics.
Co-reporter:Jingwei Bai, Xiangfeng Duan and Yu Huang
Nano Letters 2009 Volume 9(Issue 5) pp:2083-2087
Publication Date(Web):April 3, 2009
DOI:10.1021/nl900531n
We report a rational approach to fabricate graphene nanoribbons (GNRs) with sub-10 nm width by employing chemically synthesized nanowires as the physical protection mask in oxygen plasma etch. Atomic force microscopy study shows that the patterns of the resulted nanoribbons replicate exactly those of mask nanowires so that ribbons or branched or crossed graphene nanostructures can be produced. Our study shows a linear scaling relation between the resulted GNR widths and mask nanowire diameters with variable slopes for different etching times. GNRs with controllable widths down to 6 nm have been demonstrated. We have fabricated GNR field effect transistors (FETs) with nanoribbons directly connected to bulk graphene electrodes. Electrical measurements on an 8 nm GNR-FET show room temperature transistor behavior with an on/off ratio around 160, indicating appreciable band gaps arise due to lateral confinement. We find the on/off ratio in the log scale inversely scales with ribbon width. This approach opens a new avenue to graphene nanoribbons and other graphene nanostructures in the deep nanometer regime without sophisticated lithography. It thus opens exciting new opportunities for graphene nanodevice engineering.
Co-reporter:Yongquan Qu, Lei Liao, Yujing Li, Hua Zhang, Yu Huang and Xiangfeng Duan
Nano Letters 2009 Volume 9(Issue 12) pp:4539-4543
Publication Date(Web):2017-2-22
DOI:10.1021/nl903030h
We report the synthesis of vertical silicon nanowire array through a two-step metal-assisted chemical etching of highly doped n-type silicon (100) wafers in a solution of hydrofluoric acid and hydrogen peroxide. The morphology of the as-grown silicon nanowires is tunable from solid nonporous nanowires, nonporous/nanoporous core/shell nanowires, to entirely nanoporous nanowires by controlling the hydrogen peroxide concentration in the etching solution. The porous silicon nanowires retain the single crystalline structure and crystallographic orientation of the starting silicon wafer and are electrically conductive and optically active with visible photoluminescence. The combination of electronic and optical properties in the porous silicon nanowires may provide a platform for novel optoelectronic devices for energy harvesting, conversion, and biosensing.
Co-reporter:Longlu Wang, Xidong Duan, Gongming Wang, Chengbin Liu, Shenglian Luo, Shuqu Zhang, Yunxiong Zeng, Yuzi Xu, Yutang Liu, Xiangfeng Duan
Applied Catalysis B: Environmental (5 June 2016) Volume 186() pp:
Publication Date(Web):5 June 2016
DOI:10.1016/j.apcatb.2015.12.056
•Well-integrated “cauline leaf” architecture with multiple active materials.•Omnidirectional enhancement of photocatalytic hydrogen evolution.•High HER rate and apparent quantum efficiency.•Excellent long term stability of HER.The scrupulous design and integration of multiple active materials into hierarchical nanoarchitectures is essential for the creation of photocatalytic hydrogen evolution reaction (HER) system that can mimic natural photosynthesis. Here we report the design and preparation of a “cauline leaf”-like structure for highly efficient HER, by decorating TiO2 nanofibers with vertical arrays of atomically-thin MoS2 nanosheets and CdS nanocrystals. The unique integrated “cauline leaf” design can promote light trapping and absorption for highly efficient light harvesting and photocarrier generation, and offer unblocked electron transport pathway for rapid charge separation/transport to suppress charge recombination, as well as high surface area and high density of active sites for highly efficient utilization of photo-generated carriers for productive HER. Structural characterizations by transmission electron microscopy show well-integrated nanoarchitectures. Significantly, photocatalytic studies demonstrate rapid HER rates as high as 12.3 or 6.2 mmol h−1 g−1 under simulated solar light or visible light irradiation, with apparent quantum efficiencies of 70.5% at 365 nm or 57.6% at 420 nm, and excellent long term stability, representing one of the best reported MoS2 hybrid HER photocatalysts. The study could open new opportunities for the rational design of nanoscale architectures for HER or other application.Download full-size image
Co-reporter:Honglai Li ; Xidong Duan ; Xueping Wu ; Xiujuan Zhuang ; Hong Zhou ; Qinglin Zhang ; Xiaoli Zhu ; Wei Hu ; Pinyun Ren ; Pengfei Guo ; Liang Ma ; Xiaopeng Fan ; Xiaoxia Wang ; Jinyou Xu ; Anlian Pan
Journal of the American Chemical Society () pp:
Publication Date(Web):February 24, 2014
DOI:10.1021/ja500069b
Band gap engineering of atomically thin two-dimensional layered materials is critical for their applications in nanoelectronics, optoelectronics, and photonics. Here we report, for the first time, a simple one-step chemical vapor deposition approach for the simultaneous growth of alloy MoS2xSe2(1–x) triangular nanosheets with complete composition tunability. Both the Raman and the photoluminescence studies show tunable optical properties consistent with composition of the alloy nanosheets. Importantly, all samples show a single bandedge emission peak, with the spectral peak position shifting from 668 nm (for pure MoS2) to 795 nm (for pure MoSe2), indicating the high quality for these complete composition alloy nanosheets. These band gap engineered 2D structures could open up an exciting opportunity for probing their fundamental physical properties in 2D and may find diverse applications in functional electronic/optoelectronic devices.
Co-reporter:Yongquan Qu, Xing Zhong, Yujing Li, Lei Liao, Yu Huang and Xiangfeng Duan
Journal of Materials Chemistry A 2010 - vol. 20(Issue 18) pp:NaN3594-3594
Publication Date(Web):2010/04/09
DOI:10.1039/C0JM00493F
Porous silicon nanowires are synthesized by metal assisted wet-chemical etching of highly-doped silicon wafers. The resulting porous silicon nanowires exhibit a large surface area of 337 m2 g−1 and a wide spectrum absorption across the entire ultraviolet, visible and near infrared regime. We further demonstrate that platinum nanoparticles can be loaded onto the surface of the porous silicon nanowires with controlled density. These combined advancements make the porous silicon nanowires an interesting material for photocatalytic applications. We show that the porous silicon nanowires and platinum nanoparticle loaded porous silicon nanowires can be used as effective photocatalysts for photocatalytic degradation of organic dyes and toxic pollutants under visible irradiation, and thus are of significant interest for organic waste treatment and environmental remediation.
Co-reporter:Yongquan Qu and Xiangfeng Duan
Journal of Materials Chemistry A 2012 - vol. 22(Issue 32) pp:NaN16181-16181
Publication Date(Web):2012/06/25
DOI:10.1039/C2JM32267F
Nanostructures have been the focus of considerable interest for solar energy conversion in the areas of renewable green energy sources as well as environmental remediation due to their unique physicochemical properties. Here we highlight the recent efforts on developing new materials for solar energy conversion with a focus on one-dimensional homogeneous and heterogeneous nanowires. We first outline challenges and strategies to develop highly efficient and stable semiconductor materials for solar energy conversion, and then discuss the potential advantages and recent progress in exploring one-dimensional homogeneous and heterogeneous structures. We will particularly focus our discussion on the photovoltaic performance of various one-dimensional nanowire materials. Lastly, the perspectives for further improving the efficiency and stability of the solar energy conversion system using one-dimensional homogeneous and heterogeneous nanowires and their potential applications will be discussed.
Co-reporter:Lixin Liu, Hailong Zhou, Rui Cheng, Yu Chen, Yung-Chen Lin, Yongquan Qu, Jingwei Bai, Ivan A. Ivanov, Gang Liu, Yu Huang and Xiangfeng Duan
Journal of Materials Chemistry A 2012 - vol. 22(Issue 4) pp:NaN1503-1503
Publication Date(Web):2011/11/24
DOI:10.1039/C1JM14272K
Graphene has attracted considerable interest as a potential material for future electronics. Although mechanical peel is known to produce high quality graphene flakes, practical applications require continuous graphene layers over a large area. The catalyst-assisted chemical vapor deposition (CVD) is a promising synthetic method to deliver wafer-sized graphene. Here we present a systematic study on the nucleation and growth of crystallized graphene domains in an atmospheric pressure chemical vapor deposition (APCVD) process. Parametric studies show that the mean size of the graphene domains increases with increasing growth temperature and CH4 partial pressure, while the density of domains decreases with increasing growth temperature and is independent of the CH4 partial pressure. Our studies show that nucleation of graphene domains on copper substrate is highly dependent on the initial annealing temperature. A two-step synthetic process with higher initial annealing temperature but lower growth temperature is developed to reduce domain density and achieve high quality full-surface coverage of monolayer graphene films. Electrical transport measurements demonstrate that the resulting graphene exhibits a high carrier mobility of up to 3000 cm2 V−1 s−1 at room temperature.
Co-reporter:Yongquan Qu and Xiangfeng Duan
Chemical Society Reviews 2013 - vol. 42(Issue 7) pp:NaN2580-2580
Publication Date(Web):2012/11/29
DOI:10.1039/C2CS35355E
There is increasing interest in developing artificial systems that can mimic natural photosynthesis to directly harvest and convert solar energy into usable or storable energy resources. Photocatalysis, in which solar photons are used to drive redox reactions to produce chemical fuel, is the central process to achieve this goal. Despite significant efforts to date, a practically viable photocatalyst with sufficient efficiency, stability and low cost is yet to be demonstrated. It is often difficult to simultaneously achieve these different performance metrics with a single material component. The heterogeneous photocatalysts with multiple integrated functional components could combine the advantages of different components to overcome the drawbacks of single component photocatalysts. A wide range of heterostructures, including metal/semiconductor, semiconductor/semiconductor, molecule/semiconductor and multi-heteronanostructures, have been explored for improved photocatalysts by increasing the light absorption, promoting the charge separation and transportation, enhancing the redox catalytic activity and prolonging the functional life-time. The present review gives a concise overview of heterogeneous photocatalysts with a focus on the relationship between the structural architecture and the photocatalytic activity and stability.
Co-reporter:Yongquan Qu, Jingwei Bai, Lei Liao, Rui Cheng, Yung-Chen Lin, Yu Huang, Ting Guo and Xiangfeng Duan
Chemical Communications 2011 - vol. 47(Issue 4) pp:NaN1257-1257
Publication Date(Web):2010/11/22
DOI:10.1039/C0CC03922E
Single crystalline Co2Si nanobelts were synthesized for the first time. Temperature-dependent electrical transport studies show the Co2Si nanobelts exhibit metallic behavior with a large negative magnetoresistance over 10% at low temperature, which may be attributed to alignment of the spins in surface cobalt atoms.
Co-reporter:Li Tao, Xidong Duan, Chen Wang, Xiangfeng Duan and Shuangyin Wang
Chemical Communications 2015 - vol. 51(Issue 35) pp:NaN7473-7473
Publication Date(Web):2015/03/25
DOI:10.1039/C5CC01981H
Herein, for the first time, we reported a general approach of plasma engineering to tune the surface properties and edge reactive sites for greatly improving HER (hydrogen evolution) activity of MoS2. We show that the Ar or O2 plasma can be used to generate a large number of physical and chemical defects in 2D crystals to modify the electronic properties and increase the number of active sites in MoS2. Electrocatalytic studies show that the plasma-treated MoS2 exhibits significantly enhanced electrocatalytic activity for HER.
Co-reporter:Hua Zhang, Di Xu, Yu Huang and Xiangfeng Duan
Chemical Communications 2011 - vol. 47(Issue 3) pp:NaN981-981
Publication Date(Web):2010/11/16
DOI:10.1039/C0CC03566A
We report a five-fold overall enhancement of upconversion emission in NaYF4:Yb/Er nanocrystals when coupled with gold island films. Spectroscopic studies show that the enhancement factors are highly dependent on the exact spectral positions and excitation power density, with a largest enhancement factor of more than 12 observed at selected spectral positions, which may be attributed to different upconversion processes involved.
