Chongwu Zhou

University of Southern California, Los Ángeles, California, United States

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Publications (230)1421.61 Total impact

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    ABSTRACT: Single-walled carbon nanotube (SWCNT) was synthesized from short nanotubes using chemical vapor deposition (CVD) and the associated factors affecting the re-growth of the SWCNT were both investigated and optimized. Long, dense nanotubes were prepared from a mixture of acetylene and ethanol on air-annealed ST-cut quartz substrates by hot-wall CVD. Raman and photoluminescence analyses of the resulting material demonstrated that SWCNT was generated from the initial seeds since the chiralities of the seeds were maintained in the re-grown SWCNT. The re-growth of SWCNT was also achieved by cold-wall CVD. In both CVD systems, the efficiency of SWCNT re-growth was largely determined by the pretreatment conditions and growth parameters. By varying these factors, the growth of SWCNT from seeds was controlled. The re-growth mechanism is discussed based on experimental observations. Full paper link: http://authors.elsevier.com/a/1Rdgz1zUA2ch5
    Carbon 08/2015; 95:497-502. DOI:10.1016/j.carbon.2015.08.039 · 6.16 Impact Factor
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    ABSTRACT: Two-dimensional (2D) materials beyond graphene have drawn a lot of attention recently. Among the large family of 2D materials, transitional metal dichalcogenides (TMDCs), for example, molybdenum disulfides (MoS2) and tungsten diselenides (WSe2), have been demonstrated to be good candidates for advanced electronics, optoelectronics, and other applications. Growth of large single-crystalline domains and continuous films of monolayer TMDCs has been achieved recently. Usually, these TMDC flakes nucleate randomly on substrates, and their orientation cannot be controlled. Nucleation control and orientation control are important steps in 2D material growth, because randomly nucleated and orientated flakes will form grain boundaries when adjacent flakes merge together, and the formation of grain boundaries may degrade mechanical and electrical properties of as-grown materials. The use of single crystalline substrates enables the alignment of as-grown TMDC flakes via a substrate-flake epitaxial interaction, as demonstrated recently. Here we report a step-edge-guided nucleation and growth approach for the aligned growth of 2D WSe2 by a chemical vapor deposition method using C-plane sapphire as substrates. We found that at temperatures above 950 °C the growth is strongly guided by the atomic steps on the sapphire surface, which leads to the aligned growth of WSe2 along the step edges on the sapphire substrate. In addition, such atomic steps facilitate a layer-over-layer overlapping process to form few-layer WSe2 structures, which is different from the classical layer-by-layer mode for thin-film growth. This work introduces an efficient way to achieve oriented growth of 2D WSe2 and adds fresh knowledge on the growth mechanism of WSe2 and potentially other 2D materials.
    ACS Nano 07/2015; DOI:10.1021/acsnano.5b03043 · 12.88 Impact Factor
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    ABSTRACT: Two-dimensional (2D) semiconducting monolayer transition metal dichalcogenides (TMDCs) have stimulated lots of interest because they are direct bandgap materials that have reasonably good mobility values. However, contact between most metals and semiconducting TMDCs like 2H phase WSe2 are highly resistive, thus degrading the performance of field effect transistors (FETs) fabricated with WSe2 as active channel materials. Recently, a phase engineering concept of 2D MoS2 materials was developed, with improved device performance. Here, we applied this method to chemical vapor deposition (CVD) grown monolayer 2H-WSe2 and demonstrated semiconducting-to-metallic phase transition in atomically thin WSe2. We have also shown that metallic phase WSe2 can be converted back to semiconducting phase, demonstrating the reversibility of this phase transition. In addition, we fabricated FETs based on these CVD-grown WSe2 flakes with phase-engineered metallic 1T-WSe2 as contact regions and intact semiconducting 2H-WSe2 as active channel materials. The device performance is substantially improved with metallic phase source/drain electrodes, showing on/off current ratios of 10(7) and mobilities up to 66 cm(2)/V·s for monolayer WSe2. These results further suggest that phase engineering can be a generic strategy to improve device performance for many kinds of 2D TMDC materials.
    ACS Nano 06/2015; DOI:10.1021/acsnano.5b02399 · 12.88 Impact Factor
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    ABSTRACT: Engineering silicon into nanostructures has been a well-adopted strategy to improve the cyclic performance of silicon as a lithium-ion battery anode. Here, we show that the electrode performance can be further improved by alloying silicon with germanium. We have evaluated the electrode performance of SixGe1-x nanoparticles (NPs) with different compositions. Experimentally, SixGe1-x NPs with compositions approaching Si50Ge50 are found to have better cyclic retention than both Si-rich and Ge-rich NPs. During the charge/discharge process, NP merging and Si-Ge homogenization are observed. In addition, a distinct morphology difference is observed after 100 cycles, which is believed to be responsible for the different capacity retention behavior. The present study on SixGe1-x alloy NPs sheds light on the development of Si-based electrode materials for stable operation in lithium-ion batteries (e.g., through a comprehensive design of material structure and chemical composition). The investigation of composition-dependent morphology evolution in the delithiated Li-SiGe ternary alloy also significantly broadens our understanding of dealloying in complex systems, and it is complementary to the well-established understanding of dealloying behavior in binary systems (e.g., Au-Ag alloys).
    Nanotechnology 05/2015; 26(25):255702. DOI:10.1088/0957-4484/26/25/255702 · 3.67 Impact Factor
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    ABSTRACT: New layered anisotropic infrared semiconductors, black arsenic-phosphorus (b-AsP), with highly tunable chemical compositions and electronic and optical properties are introduced. Transport and infrared absorption studies demonstrate the semiconducting nature of b-AsP with tunable band gaps, ranging from 0.3 to 0.15 eV. These band gaps fall into long-wavelength infrared regime and cannot be readily reached by other layered materials. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Advanced Materials 05/2015; DOI:10.1002/adma.201501758 · 17.49 Impact Factor
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    ABSTRACT: Semiconducting transition metal dichalcogenides (TMDCs) have attracted a lot of attention recently, because of their interesting electronic, optical, and mechanical properties. Among large numbers of TMDCs, monolayer of tungsten diselenides (WSe2) is of particular interest since it possesses a direct band gap and tunable charge transport behaviors, which make it suitable for a variety of electronic and optoelectronic applications. Direct synthesis of large domains of monolayer WSe2 and their growth mechanism studies are important steps toward applications of WSe2. Here, we report systematical studies on ambient pressure chemical vapor deposition (CVD) growth of monolayer and few layer WSe2 flakes directly on silica substrates. The WSe2 flakes were characterized using optical microscopy, atomic force microscopy, Raman spectroscopy, and photoluminescence spectroscopy. We investigated how growth parameters, with emphases on growth temperatures and durations, affect the sizes, layer numbers, and shapes of as-grown WSe2 flakes. We also demonstrated that transport properties of CVD-grown monolayer WSe2, similar to mechanically-exfoliated samples, can be tuned into either p-type or ambipolar electrical behavior, depending on the types of metal contacts. These results deepen our understandings on the vapor phase growth mechanism of WSe2, and may benefit the uses of these CVD-grown monolayer materials in electronic and optoelectronics.
    ACS Nano 05/2015; DOI:10.1021/acsnano.5b01301 · 12.88 Impact Factor
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    ABSTRACT: The utilization of black phosphorus and its monolayer (phosphorene) and few-layers in field-effect transistors has attracted a lot of attention to this elemental two-dimensional material. Various studies on optimization of black phosphorus field-effect transistors, PN junctions, photodetectors, and other applications have been demonstrated. Although chemical sensing based on black phosphorus devices was theoretically predicted, there is still no experimental verification of such an important study of this material. In this article, we report on chemical sensing of nitrogen dioxide (NO2) using field-effect transistors based on multilayer black phosphorus. Black phosphorus sensors exhibited increased conduction upon NO2 exposure and excellent sensitivity for detection of NO2 down to 5 ppb. Moreover, when the multilayer black phosphorus field-effect transistor was exposed to NO2 concentrations of 5, 10, 20, and 40 ppb, its relative conduction change followed the Langmuir isotherm for molecules adsorbed on a surface. Additionally, on the basis of an exponential conductance change, the rate constants for adsorption and desorption of NO2 on black phosphorus were extracted for different NO2 concentrations, and they were in the range of 130-840 s. These results shed light on important electronic and sensing characteristics of black phosphorus, which can be utilized in future studies and applications.
    ACS Nano 05/2015; 9(5). DOI:10.1021/acsnano.5b01961 · 12.88 Impact Factor
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    ABSTRACT: Carbon nanotubes (CNTs) have emerged as an important material for printed macroelectronics. However, achieving printed complementary macroelectronics solely based on CNTs is difficult because it is still challenging to make reliable n-type CNT transistors. In this study, we report threshold voltage (V th) tuning and printing of complementary transistors and inverters composed of thin films of CNTs and indium zinc oxide (IZO) as p-type and n-type transistors, respectively. We have optimized the V th of p-type transistors by comparing Ti/Au and Ti/Pd as source/drain electrodes, and observed that CNT transistors with Ti/Au electrodes exhibited enhancement mode operation (V thI on) and enhancement mode operation (V th > 0). For example, an In:Zn ratio of 2:1 yielded an enhancement mode n-type transistor with V th ∼ 1 V and I on of 5.2 μA. Furthermore, by printing a CNT thin film and an IZO thin film on the same substrate, we have fabricated a complementary inverter with an output swing of 99.6% of the supply voltage and a voltage gain of 16.9. This work shows the promise of the hybrid integration of p-type CNT and n-type IZO for complementary transistors and circuits.
    Nano Research 04/2015; 8(4):1159-1168. DOI:10.1007/s12274-014-0596-7 · 6.96 Impact Factor
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    ABSTRACT: We report the integration of high voltage cathode material LiNi0.5Mn1.5O4 and multiwall carbon nanotube network for the first time, which has led to charge/discharge rate as high as 20C and has also been demonstrated to work as cathode for flexible batteries. The LiNi0.5Mn1.5O4/ multiwall carbon nanotube network is flexible and light-weight since the use of binder, conductive additive and metal current collector is eliminated. Due to the high voltage provided by LiNi0.5Mn1.5O4 particles, the total energy of the battery is significantly enhanced. In addition, with the high conductivity from multiwall carbon nanotube network, the LiNi0.5Mn1.5O4/multiwall carbon nanotube network electrodes can deliver 80% of the 1C capacity even when the charge/discharge current density increased to 20C (1C=140 mA/g). During the high current rate cycling test, no obvious capacity decay is observed after 100 cycles at 10C. Calculation of the polarization resistance Rp reveals that the Rp of the LiNi0.5Mn1.5O4/multiwall carbon nanotube electrodes is less than 25% of that of the conventional electrodes fabricated through slurry-casting on metal current collector. In addition, the power density calculated from LiNi0.5Mn1.5O4/multiwall carbon nanotube electrodes is over two times larger than that provided by the conventional electrodes. The combined effect from high voltage, high current rate performance and reduced weight has led our LiNi0.5Mn1.5O4/multiwall carbon nanotube electrodes to be a promising candidate for high-power lithium ion batteries. Moreover, the features of flexibility and light-weight also demonstrate the potential of applying the LiNi0.5Mn1.5O4 / multiwall carbon nanotube electrodes in new-generation flexible or ultrathin/ultralight electronic devices in the future.
    Nano Energy 03/2015; 12. DOI:10.1016/j.nanoen.2014.11.052 · 10.21 Impact Factor
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    ABSTRACT: Recently, chemical synthesis of a range of large nanographene molecules with various shapes and sizes opened a new path to utilize them in various applications and devices. However, due to their extended aromatic cores and high molecular weight, film formation of large nanographene molecules, bearing more than 90 sp(2) carbon atoms in aromatic cores, is very challenging, which has prevented their applications such as in thin-film transistors. Here, we developed an effective approach to prepare films of such large nanographene molecules using a vapor-phase transport (VPT) technique based on molecule sublimation. The VPT of these molecules was made possible by combining the molecules and the target substrate in a small confinement of vacuum-sealed glass tube, so that a small amount of sublimation can be utilized to create films. Surprisingly, such heavy and large molecules can be deposited on any substrate by this method to create films of assembled large nanographene molecules while maintaining their aromatic cores intact, which was confirmed using mass spectrometry measurements. Moreover, field-effect transistors based on these films are depleted and show significantly improved current on/off ratio compared to previous large nanographene-based transistors fabricated using liquid-phase-based process. Our work shows that VPT deposition can be a viable technique to prepare films based on large nanographene molecules and potentially other high molecular weight compounds, which may find exciting applications in electronics and optoelectronics.
    Journal of the American Chemical Society 03/2015; 137(13). DOI:10.1021/ja513207e · 11.44 Impact Factor
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    ABSTRACT: This work expands the redox chemistry of single-wall carbon nanotubes (SWCNTs) by investigating its role in a number of SWCNT sorting processes. Using a polyethylene glycol (PEG)/dextran (DX) aqueous two-phase system, we show that electron-transfer between redox molecules and SWCNTs triggers reorganization of the surfactant coating layer, leading to strong modulation of nanotube partition in the two phases. While the DX phase is thermodynamically more favored by an oxidized SWCNT mixture, the mildly reducing PEG phase is able to recover SWCNTs from oxidation and extract them successively from the DX phase. Remarkably, the extraction order follows SWCNT bandgap: semiconducting nanotubes of larger bandgap first, followed by semiconducting nanotubes of smaller bandgap, then non-armchair metallic tubes of small but nonvanishing bandgap, and finally armchair metallic nanotubes of zero bandgap. Furthermore, we show that redox-induced surfactant reorganization is a common phenomenon, affecting nanotube buoyancy in a density gradient field, affinity to polymer matrices, and solubility in organic solvents. These findings establish redox modulation of surfactant coating structures as a general mechanism for tuning a diverse range of SWCNT sorting processes, and demonstrate for the first time that armchair and non-armchair metallic SWCNTs can be separated by their differential response to redox.
    Nano Letters 02/2015; 15(3). DOI:10.1021/nl504189p · 13.59 Impact Factor
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    ABSTRACT: Nanostructure field-effect transistor (FET) biosensors have shown great promise for ultra sensitive biomolecular detection. Top-down assembly of these sensors increases scalability and device uniformity but faces fabrication challenges in achieving the small dimensions needed for sensitivity. We report top-down fabricated indium oxide (In2O3) nanoribbon FET biosensors using highly scalable radio frequency (RF) sputtering to create uniform channel thicknesses ranging from 50 nm to 10 nm. We combine this scalable sensing platform with amplification from electronic enzyme-linked immunosorbent assay (ELISA) to achieve high sensitivity to target analytes such as streptavidin and human immunodeficiency virus type 1 (HIV-1) p24 proteins. Our approach circumvents Debye screening in ionic solutions and detects p24 protein at 20 fg/ml (about 250 viruses/ml or about three orders of magnitude lower than commercial ELISA) with a 35% conduction change in human serum. The In2O3 nanoribbon biosensors have 100% device yield and use a simple 2 mask photolithography process. The electrical properties of 50 In2O3 nanoribbon FETs showed good uniformity in on-state current, on/off current ratio, mobility, and threshold voltage. In addition, the sensors show excellent pH sensitivity over a broad range (pH 4 to 9) as well as over the physiological-related pH range (pH 6.8 to 8.2). With the demonstrated sensitivity, scalability and uniformity, the In2O3 nanoribbon sensor platform makes great progress toward clinical testing, such as for early diagnosis of acquired immunodeficiency syndrome (AIDS).
    Nano Letters 01/2015; 15(3). DOI:10.1021/nl5047889 · 13.59 Impact Factor
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    ABSTRACT: The inability to synthesize single-wall carbon nanotubes (SWCNTs) possessing uniform electronic properties and chirality represents the major impediment to their widespread applications. Recently, there is growing interest to explore and synthesize well-defined carbon nanostructures, including fullerenes, short nanotubes, and sidewalls of nanotubes, aiming for controlled synthesis of SWCNTs. One noticeable advantage of such processes is that no metal catalysts are used, and the produced nanotubes will be free of metal contamination. Many of these methods, however, suffer shortcomings of either low yield or poor controllability of nanotube uniformity. Here, we report a brand new approach to achieve high efficiency metal-free growth of nearly pure SWCNT semiconductors, as supported by extensive spectroscopic characterization, electrical transport measurements, and density functional theory calculations. Our strategy combines bottom-up organic chemistry synthesis with vapour phase epitaxy elongation. We identify a strong correlation between the electronic properties of SWCNTs and their diameters in nanotube growth. This study not only provides material platforms for electronic applications of semiconducting SWCNTs, but also contributes to fundamental understanding of the growth mechanism and controlled synthesis of SWCNTs.
    Nano Letters 12/2014; 15(1). DOI:10.1021/nl504066f · 13.59 Impact Factor
  • Xuan Cao · Haitian Chen · Xiaofei Gu · Bilu Liu · Wenli Wang · Yu Cao · Fanqi Wu · Chongwu Zhou
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    ABSTRACT: Semiconducting single-wall carbon nanotubes are very promising materials in printed electronics due to their excellent mechanical and electrical property, outstanding printability, and great potential for flexible electronics. Nonetheless, developing scalable and low-cost approaches for manufacturing fully printed high-performance single-wall carbon nanotube thin-film transistors remains a major challenge. Here we report that screen printing, which is a simple, scalable, and cost-effective technique, can be used to produce both rigid and flexible thin-film transistors using separated single-wall carbon nanotubes. Our fully printed top-gated nanotube thin-film transistors on rigid and flexible substrates exhibit decent performance, with mobility up to 7.67 cm(2) V(-1) s(-1), on/off ratio of 10(4) ∼ 10(5), minimal hysteresis, and low operation voltage (<10 V). In addition, outstanding mechanical flexibility of printed nanotube thin-film transistors (bent with radius of curvature down to 3 mm) and driving capability for organic light-emitting diode have been demonstrated. Given the high performance of the fully screen-printed single-wall carbon nanotube thin-film transistors, we believe screen printing stands as a low-cost, scalable, and reliable approach to manufacture high-performance nanotube thin-film transistors for application in display electronics. Moreover, this technique may be used to fabricate thin-film transistors based on other materials for large-area flexible macroelectronics, and low-cost display electronics.
    ACS Nano 12/2014; 8(12). DOI:10.1021/nn505979j · 12.88 Impact Factor
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    ABSTRACT: Two-dimensional (2D) layered tungsten diselenides (WSe2) material has recently drawn a lot of attention due to its unique optoelectronic properties and ambipolar transport behavior. However, direct chemical vapor deposition (CVD) synthesis of 2D WSe2 is not as straightforward as other 2D materials due to the low reactivity between reactants in WSe2 synthesis. In addition, the growth mechanism of WSe2 in such CVD process remains unclear. Here we report the observation of a screw-dislocation-driven (SDD) spiral growth of 2D WSe2 flakes and pyramid-like structures using a sulfur-assisted CVD method. Few-layer and pyramid-like WSe2 flakes instead of monolayer were synthesized by introducing a small amount of sulfur as a reducer to help the selenization of WO3, which is the precursor of tungsten. Clear observations of steps, helical fringes, and herring-bone contours under atomic force microscope characterization reveal the existence of screw dislocations in the as-grown WSe2. The generation and propagation mechanisms of screw dislocations during the growth of WSe2 were discussed. Back-gated field-effect transistors were made on these 2D WSe2 materials, which show on/off current ratios of 106 and mobility up to 44 cm2/Vs.
    ACS Nano 10/2014; 8. DOI:10.1021/nn504775f · 12.88 Impact Factor
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    ABSTRACT: We report formation of an optical cavity and observation of Fabry-Perot resonance in GaAs nanowires and nanosheets grown by metal organic chemical vapor deposition (MOCVD) with selective area growth (SAG). These nanostructures are grown along the (111) B direction. The formation of an optical cavity in the nanowires and nanosheets are fundamentally different from each other. In nanowires the optical cavity is formed along the length of the nanowire with ends of the nanowire behaving as two parallel mirrors. In nanosheets, however, the three non-parallel edges of the GaAs nanosheets are involved in trapping of the light through total internal reflection, thus forming a 2D cavity. We show that through surface passivation and local field enhancement, both the photoluminescence intensity and hence Fabry-Perot peak intensity increases significantly. Transferring the GaAs nanowires and nanosheets to the gold substrate (instead of Si/SiO2 substrate) leads to substantial enhancement in the photoluminescence intensity by 5X (for nanowires) and 3.7X (for nanosheets) to infinite enhancement of the FP peaks intensities. In order to reduce the non-radiative recombination in these nanowires the surface states in the nanowires can be passivated by either an ionic liquid (EMIM-TFSI) or an AlGaAs surface layer. Both passivations methods lead to an enhancement of the optical response by up to 12X.
    SPIE NanoScience + Engineering; 09/2014

Publication Stats

9k Citations
1,421.61 Total Impact Points

Institutions

  • 2001–2015
    • University of Southern California
      • • Department of Chemical Engineering and Materials Science
      • • Department of Electrical Engineering
      • • Department of Chemistry
      • • Division of Cardiovascular Medicine
      Los Ángeles, California, United States
  • 2004–2014
    • University of California, Los Angeles
      • Department of Electrical Engineering
      Los Ángeles, California, United States
  • 2013
    • Michigan State University
      • Department of Electrical and Computer Engineering
      Ист-Лансинг, Michigan, United States
  • 2010
    • Harvard University
      • Department of Chemistry and Chemical Biology
      Cambridge, MA, United States