Chongwu Zhou

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

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Publications (193)1000.14 Total impact

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    ABSTRACT: Due to unique structural, optical and electrical properties, solar cells based on semiconductor nanowires are a rapidly-evolving scientific enterprise. Various approaches employing III-V nanowires have emerged, among which GaAs, especially, is under intense research and development. Most reported GaAs nanowire solar cells form p-n junctions in the radial direction; however, nanowires using axial junction may enable the attainment of high open circuit voltage (Voc) and integration into multi-junction solar cells. Here, we report GaAs nanowire solar cells with axial p-i-n junctions that achieve 7.58% efficiency. Simulations show that axial junctions are more tolerant to doping variation than radial junctions and lead to higher Voc under certain conditions. We further study the effect of wire diameter and junction depth using electrical characterization and cathodoluminescence. The results show that large diameter and shallow junctions are essential for high extraction efficiency. Our approach opens up great opportunity for future low-cost, high-efficiency photovoltaics.
    Nano letters. 05/2014;
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    ABSTRACT: Semiconducting SnO2 nanowires have been used to demonstrate high quality field-effect transistors, optically transparent devices, photodetectors, and gas sensors. However, controllable assembly of rutile SnO2 nanowires is necessary for scalable and practical device applications. Here we demonstrate aligned, planar SnO2 nanowires grown on A-plane, M-plane, and R-plane sapphire substrates. These parallel nanowires can reach 100 μm in length with sufficient density to be patterned photolithographically for field-effect transistors and sensor devices. As proof-of-concept, we show that transistors made this way can achieve on/off current ratios on the order of 10(6), mobilities around 71.68 cm(2)/V∙s, and sufficiently high currents to drive external organic light-emitting diode displays. Furthermore, the aligned SnO2 nanowire devices are shown to be photosensitive to UV light, with the capability to distinguish between 254 nm and 365 nm wavelengths. Their alignment is advantageous for polarized UV light detection; we have measured a polarization ratio of photoconductance (σ) of 0.3. Lastly, we show that the nanowires can detect NO2 at a concentration of 0.2 ppb, making them a scalable, ultra-sensitive gas sensing technology. Aligned SnO2 nanowires offer a straightforward method to fabricate scalable SnO2 nano-devices for a variety of future electronic applications.
    Nano letters. 05/2014;
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    ABSTRACT: Bottom-up synthesis of graphene nanoribbons (GNRs) is an essential step toward utilizing them in electronic and sensing applications due to their defined edge structure and high uniformity. Recently, structurally perfect GNRs with variable lengths and edge structures were created using various chemical synthesis techniques. Nonetheless, issues like GNR deposition, characterization, electronic properties, and applications are not fully explored. Here we report optimized conditions for deposition, characterization, and device fabrication of individual and thin films of ultra-long chemically synthesized GNRs. Moreover, we have demonstrated exceptional NO2 gas sensitivity of the GNR film devices down to parts per billion (ppb) levels. The results lay the foundation for using chemically synthesized GNRs for future electronic and sensing applications.
    Journal of the American Chemical Society 05/2014; 136(21):7555–7558. · 10.68 Impact Factor
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    ABSTRACT: Carbon nanotubes have the potential to spur future development in electronics due to their unequalled electrical properties. In this article, we present a review on carbon nanotube-based circuits in terms of their electrical performance in two major directions: nanoelectronics and macroelectronics. In the nanoelectronics direction, we direct our discussion to the performance of aligned carbon nanotubes for digital circuits and circuits designed for radio-frequency applications. In the macroelectronics direction, we focus our attention on the performance of thin films of carbon nanotube random networks in digital circuits, display applications, and printed electronics. In the last part, we discuss the existing challenges and future directions of nanotube-based nano- and microelectronics.
    Semiconductor Science and Technology 05/2014; 29(7):073001. · 1.92 Impact Factor
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    ABSTRACT: Lightweight and high-power LiNi0.5Mn1.5O4/carbon nanofiber (CNF) network electrodes are developed as a high-voltage cathode for lithium ion batteries. The LiNi0.5Mn1.5O4/CNF network electrodes are free-standing and can be used as a cathode without using any binder, carbon black, or metal current collector, and hence the total weight of the electrode is highly reduced while keeping the same areal loading of active materials. Compared with conventional electrodes, the LiNi0.5Mn1.5O4/CNF network electrodes can yield up to 55% reduction in total weight and 2.2 times enhancement in the weight percentage of active material in the whole electrode. Moreover, the LiNi0.5Mn1.5O4/carbon nanofiber (CNF) network electrodes showed excellent current rate capability in the large-current test up to 20C (1C = 140 mAh/g), when the conventional electrodes showed almost no capacity at the same condition. Further analysis of polarization resistance confirmed the favorable conductivity from the CNF network compared with the conventional electrode structure. By reducing the weight, increasing the working voltage, and improving the large-current rate capability simultaneously, the LiNi0.5Mn1.5O4/CNF electrode structure can highly enhance the energy/power density of lithium ion batteries and thus holds great potential to be used with ultrathin, ultralight electronic devices as well as electric vehicles and hybrid electric vehicles.
    ACS Nano 04/2014; · 12.03 Impact Factor
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    ABSTRACT: Trace chemical detection is important for a wide range of practical applications. Recently emerged two-dimensional (2D) crystals offer unique advantages as potential sensing materials with high sensitivity, owing to their very high surface-to-bulk atom ratios and semiconducting properties. Here, we report the first use of Schottky-contacted chemical vapor deposition grown monolayer MoS2 as high-performance room temperature chemical sensors. The Schottky-contacted MoS2 transistors show current changes by 2-3 orders of magnitude upon exposure to very low concentrations of NO2 and NH3. Specifically, the MoS2 sensors show clear detection of NO2 and NH3 down to 20 ppb and 1 ppm, respectively. We attribute the observed high sensitivity to both well-known charger transfer mechanism and, more importantly, the Schottky barrier modulation upon analyte molecule adsorption, the latter of which is made possible by the Schottky contacts in the transistors and is not reported previously for MoS2 sensors. This study shows the potential of 2D semiconductors as high-performance sensors and also benefits the fundamental studies of interfacial phenomena and interactions between chemical species and monolayer 2D semiconductors.
    ACS Nano 04/2014; · 12.03 Impact Factor
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    ABSTRACT: Bandgap engineering of graphene is an essential step toward employing graphene in electronic and sensing applications. Recently, graphene nanoribbons (GNRs) were used to create a bandgap in graphene and function as a semiconducting switch. Although GNRs with widths of <10 nm have been achieved, problems like GNR alignment, width control, uniformity, high aspect ratios, and edge roughness must be resolved in order to introduce GNRs as a robust alternative technology. Here we report patterning, characterization, and superior chemical sensing of ultranarrow aligned GNR arrays down to 5 nm width using helium ion beam lithography (HIBL) for the first time. The patterned GNR arrays possess narrow and adjustable widths, high aspect ratios, and relatively high quality. Field-effect transistors were fabricated on such GNR arrays and temperature-dependent transport measurements show the thermally activated carrier transport in the GNR array structure. Furthermore, we have demonstrated exceptional NO2 gas sensitivity of the 5 nm GNR array devices down to parts per billion (ppb) levels. The results show the potential of HIBL fabricated GNRs for the electronic and sensing applications.
    ACS Nano 01/2014; · 12.03 Impact Factor
  • J. Mater. Chem. C. 01/2014; 2(7).
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    ABSTRACT: The effects of near-interfacial trapping induced by ionizing radiation exposure of aligned single-walled carbon nanotube (SWCNT) arrays are investigated via measurements of gate hysteresis in the transfer characteristics of aligned SWCNT field-effect transistors. Gate hysteresis is attributed to charge injection (i.e., trapping) from the SWCNTs into radiation-induced traps in regions near the SWCNT/dielectric interface. Self-consistent calculations of surface-potential, carrier density, and trapped charge are used to describe hysteresis as a function of ionizing radiation exposure. Hysteresis width (h) and its dependence on gate sweep range are investigated analytically. The effects of non-uniform trap energy distributions on the relationship between hysteresis, gate sweep range, and total ionizing dose are demonstrated with simulations and verified experimentally.
    Journal of Applied Physics 01/2014; 115(5):054506-054506-8. · 2.21 Impact Factor
  • Haitian Chen, Yu Cao, Jialu Zhang, Chongwu Zhou
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    ABSTRACT: Carbon nanotubes and metal oxide semiconductors have emerged as important materials for p-type and n-type thin-film transistors, respectively; however, realizing sophisticated macroelectronics operating in complementary mode has been challenging due to the difficulty in making n-type carbon nanotube transistors and p-type metal oxide transistors. Here we report a hybrid integration of p-type carbon nanotube and n-type indium-gallium-zinc-oxide thin-film transistors to achieve large-scale (>1,000 transistors for 501-stage ring oscillators) complementary macroelectronic circuits on both rigid and flexible substrates. This approach of hybrid integration allows us to combine the strength of p-type carbon nanotube and n-type indium-gallium-zinc-oxide thin-film transistors, and offers high device yield and low device variation. Based on this approach, we report the successful demonstration of various logic gates (inverter, NAND and NOR gates), ring oscillators (from 51 stages to 501 stages) and dynamic logic circuits (dynamic inverter, NAND and NOR gates).
    Nature Communications 01/2014; 5:4097. · 10.02 Impact Factor
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    ABSTRACT: The objective was to use carbon nanotubes (CNT) coupled with near-infrared radiation (NIR) to induce hyperthermia as a novel non-ionizing radiation treatment for primary brain tumors, glioblastoma multiforme (GBM). In this study, we report the therapeutic potential of hyperthermia-induced thermal ablation using the sequential administration of carbon nanotubes (CNT) and NIR. In vitro studies were performed using glioma tumor cell lines (U251, U87, LN229, T98G). Glioma cells were incubated with CNTs for 24 h followed by exposure to NIR for 10 min. Glioma cells preferentially internalized CNTs, which upon NIR exposure, generated heat, causing necrotic cell death. There were minimal effects to normal cells, which correlate to their minimal uptake of CNTs. Furthermore, this protocol caused cell death to glioma cancer stem cells, and drug-resistant as well as drug-sensitive glioma cells. This sequential hyperthermia therapy was effective in vivo in the rodent tumor model resulting in tumor shrinkage and no recurrence after only one treatment. In conclusion, this sequence of selective CNT administration followed by NIR activation provides a new approach to the treatment of glioma, particularly drug-resistant gliomas.
    Frontiers in oncology. 01/2014; 4:180.
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    ABSTRACT: A generic and facile method of coating graphene oxide (GO) on particles is reported, with sulfur/GO core-shell particles demonstrated as an example for lithium-sulfur (Li-S) battery application with superior performance. Particles of different diameters (ranging from 100 nm to 10 μm), geometries, and compositions (sulfur, silicon, and carbon) are successfully wrapped up by GO, by engineering the ionic strength in solutions. Importantly, our method does not involve any chemical reaction between GO and the wrapped particles, and therefore, it can be extended to vast kinds of functional particles. The applications of sulfur/GO core-shell particles as Li-S battery cathode materials are further investigated, and the results show that sulfur/GO exhibit significant improvements over bare sulfur particles without coating. Galvanic charge-discharge test using GO/sulfur particles shows a specific capacity of 800 mAh/g is retained after 1000 cycles at 1 A/g current rate if only the mass of sulfur is taken into calculation, and 400 mAh/g if the total mass of sulfur/GO is considered. Most importantly, the capacity decay over 1000 cycles is less than 0.02% per cycle. The coating method developed in this study is facile, robust, and versatile and is expected to have wide range of applications in improving the properties of particle materials.
    Nano Letters 12/2013; · 13.03 Impact Factor
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    ABSTRACT: Atomic layer deposition (ALD) has been used to modify the surface of the high-voltage cathode LiNi0.5Mn1.5O4 by coating ultrathin Al2O3 layers on the electrodes. The ultrathin layer can suppress the undesirable reactions during cycling while retaining the electron and ion conductivity of the electrode. The Al2O3-coated LiNi0.5Mn1.5O4 showed remarkable improvement over bare LiNi0.5Mn1.5O4. After 200 cycles, the Al2O3-coated cathode showed 91 % capacity retention whereas the bare LiNi0.5Mn1.5O4 can only maintain 75 % under the same testing conditions. In addition, the Al2O3-coated LiNi0.5Mn1.5O4 retained 63 % of its capacity 900 cycles. At an elevated temperature of 55 °C, the Al2O3-coated LiNi0.5Mn1.5O4 still delivered 116 mAh g−1 at the 100th cycle; in comparison, the capacity for bare LiNi0.5Mn1.5O4 decreased to 98 mAh g−1. According to the results from charge/discharge and AC impedance experiments, the improvement is ascribed to the reduced overpotential and Li ion surface diffusion impedance. The promising results demonstrate the potential of developing high-energy lithium ion batteries with a long cycle life by using a highly scalable preparation method for LiNi0.5Mn1.5O4 and the broadly applicable ALD process.
    Energy Technology. 12/2013;
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    ABSTRACT: Recently, silicon-based lithium-ion battery anodes have shown encouraging results, as they can offer high capacities and long cyclic lifetimes. The applications of this technology are largely impeded by the complicated and expensive approaches in producing Si with desired nanostructures. We report a cost-efficient method to produce nanoporous Si particles from metallurgical Si through ball-milling and inexpensive stain-etching. The porosity of porous Si is derived from particle's three-dimensional reconstructions by scanning transmission electron microscopy (STEM) tomography, which shows the particles' highly porous structure when etched under proper conditions. Nanoporous Si anodes with reversible capacity of 2900 mAh/g was attained at a charging rate of 400 mA/g, and stable capacity above 1100 mAh/g was retained for extended 600 cycles tested at 2000 mA/g. The synthetic route is low-cost and scalable for mass production, promising Si as a potential anode material for the next-generation lithium-ion batteries with enhanced capacity and energy density.
    Nano Letters 11/2013; · 13.03 Impact Factor
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    ABSTRACT: Single-wall carbon nanotubes (SWNTs) possess superior geometrical, electronic, chemical, thermal, and mechanical properties and are very attractive for applications in electronic devices and circuits. To make this a reality, the nanotube orientation, density, diameter, electronic property, and even chirality should be well controlled. This Feature article focuses on recent achievements researchers have made on the controlled growth of horizontally aligned SWNTs and SWNT arrays on substrates and their electronic applications. Principles and strategies to control the morphology, structure, and properties of SWNTs are reviewed in detail. Furthermore, electrical properties of field-effect transistors fabricated on both individual SWNTs and aligned SWNT arrays are discussed. State-of-the-art electronic devices and circuits based on aligned SWNTs and SWNT arrays are also highlighted.
    Nanoscale 08/2013; · 6.23 Impact Factor
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    ABSTRACT: Structurally uniform and chirality-pure single-wall carbon nanotubes are highly desired for both fundamental study and many of their technological applications, such as electronics, optoelectronics, and biomedical imaging. Considerable efforts have been invested in the synthesis of nanotubes with defined chiralities by tuning the growth recipes, but the approach has only limited success. Recently, we have shown that chirality-pure short nanotubes can be used as seeds for vapour-phase epitaxial cloning growth, opening up a new route towards chirality-controlled carbon nanotube synthesis. Nevertheless, the yield of vapour-phase epitaxial growth is rather limited at the present stage, due in large part to the lack of mechanistic understanding of the process. Here we report chirality-dependent growth kinetics and termination mechanism for the vapour-phase epitaxial growth of seven single-chirality nanotubes of (9, 1), (6, 5), (8, 3), (7, 6), (10, 2), (6, 6), and (7, 7), covering near zigzag, medium chiral angle, and near armchair semiconductors, as well as armchair metallic nanotubes. Our results reveal that the growth rates of nanotubes increase with their chiral angles while the active lifetimes of the growth hold opposite trend. Consequently, the chirality distribution of a nanotube ensemble is jointly determined by both growth rates and lifetimes. These results correlate nanotube structures and properties with their growth behaviors and deepen our understanding of chirality-controlled growth of nanotubes.
    Nano Letters 08/2013; · 13.03 Impact Factor
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    ABSTRACT: Periodic arrays of n-GaAs nanowires have been grown by selective-area metal–organic chemical-vapor deposition on Si and GaAs substrates. The optical absorption characteristics of the nanowire-arrays were investigated experimentally and theoretically, and the photoelectrochemical energy-conversion properties of GaAs nanowire arrays were evaluated in contact with one-electron, reversible, redox species in non-aqueous solvents. The radial semiconductor/liquid junction in the nanowires produced near-unity external carrier-collection efficiencies for nanowire-array photoanodes in contact with non-aqueous electrolytes. These anodes exhibited overall inherent photoelectrode energy-conversion efficiencies of 8.1% under 100 mW cm−2 simulated Air Mass 1.5 illumination, with open-circuit photovoltages of 590 ± 15 mV and short-circuit current densities of 24.6 ± 2.0 mA cm−2. The high optical absorption, and minimal reflection, at both normal and off-normal incidence of the GaAs nanowire arrays that occupy <5% of the fractional area of the electrode can be attributed to efficient incoupling into radial nanowire guided and leaky waveguide modes.
    Energy & Environmental Science 05/2013; 6(6):1879-1890. · 11.65 Impact Factor
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    ABSTRACT: In this paper, we applied self-aligned T-gate design to aligned carbon nanotube array transistors and achieved an extrinsic current-gain cut-off frequency (ft) of 25 GHz, which is the best on-chip performance for nanotube radio frequency (RF) transistors reported to date. Meanwhile, an intrinsic current-gain cut-off frequency up to 102 GHz is obtained, comparable to the best value reported for nanotube RF transistors. Armed with the excellent extrinsic RF performance, we performed both single-tone and two-tone measurements for aligned nanotube transistors at a frequency up to 8 GHz. Furthermore, we utilized T-gate aligned nanotube transistors to construct mixing and frequency doubling analog circuits operated in gigahertz frequency regime. Our results confirm the great potential of nanotube-based circuit applications and indicate that nanotube transistors are promising building blocks in high-frequency electronics.
    ACS Nano 04/2013; · 12.03 Impact Factor
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    ABSTRACT: Toward the increasing demands of portable energy storage and electric vehicle applications, the widely used graphite anodes with significant drawbacks become more and more unsuitable. Herein, we report a novel scaffold of hierarchical silicon nanowires-carbon textiles anodes fabricated via a facile method. Further, complete lithium-ion batteries based on Si and commercial LiCoO2 materials were assembled to investigate their corresponding across-the-aboard performances, demonstrating their enhanced specific capacity (2950 mAh g(-1) at 0.2 C), good repeatability/rate capability (even >900 mAh g(-1) at high rate of 5 C), long cycling life, and excellent stability in various external conditions (curvature, temperature, and humidity). Above results light the way to principally replacing graphite anodes with silicon-based electrodes which was confirmed to have better comprehensive performances.
    Scientific Reports 04/2013; 3:1622. · 5.08 Impact Factor
  • Yi Zhang, Luyao Zhang, Chongwu Zhou
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    ABSTRACT: Since its debut in 2004, graphene has attracted enormous interest because of its unique properties. Chemical vapor deposition (CVD) has emerged as an important method for the preparation and production of graphene for various applications since the method was first reported in 2008/2009. In this Account, we review graphene CVD on various metal substrates with an emphasis on Ni and Cu. In addition, we discuss important and representative applications of graphene formed by CVD, including as flexible transparent conductors for organic photovoltaic cells and in field effect transistors. Growth on polycrystalline Ni films leads to both monolayer and few-layer graphene with multiple layers because of the grain boundaries on Ni films. We can greatly increase the percentage of monolayer graphene by using single-crystalline Ni(111) substrates, which have smooth surface and no grain boundaries. Due to the extremely low solubility of carbon in Cu, Cu has emerged as an even better catalyst for the growth of monolayer graphene with a high percentage of single layers. The growth of graphene on Cu is a surface reaction. As a result, only one layer of graphene can form on a Cu surface, in contrast with Ni, where more than one layer can form through carbon segregation and precipitation. We also describe a method for transferring graphene sheets from the metal using polymethyl methacrylate (PMMA). CVD graphene has electronic properties that are potentially valuable in a number of applications. For example, few-layer graphene grown on Ni can function as flexible transparent conductive electrodes for organic photovoltaic cells. In addition, because we can synthesize large-grain graphene on Cu foil, such large-grain graphene has electronic properties suitable for use in field effect transistors.
    Accounts of Chemical Research 03/2013; · 20.83 Impact Factor

Publication Stats

4k Citations
1,000.14 Total Impact Points

Institutions

  • 2001–2014
    • University of Southern California
      • • Department of Electrical Engineering
      • • Department of Chemistry
      Los Angeles, California, United States
  • 2009–2013
    • Huazhong University of Science and Technology
      • Wuhan National Laboratory for Optoelectronics
      Wuhan, Hubei, China
  • 2010–2012
    • Purdue University
      • Department of Electrical and Computer Engineering Technology (ECET)
      West Lafayette, IN, United States
    • Harvard University
      • Department of Chemistry and Chemical Biology
      Cambridge, MA, United States
  • 2002–2011
    • University of California, Los Angeles
      • Department of Electrical Engineering
      Los Angeles, CA, United States
  • 2008
    • Kyonggi University
      Sŏul, Seoul, South Korea