Chongwu Zhou

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

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Publications (240)1506.05 Total impact

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    ABSTRACT: Monolithic integration of III-V with Si has been pursued for some time in the semiconductor industry. However, the mismatch of lattice constants and thermal expansion coefficients represents a large technological challenge for the heteroepitaxial growth. Nanowires, due to their small lateral dimension, can relieve strain and mitigate dislocation formation to allow single crystal III-V material to be grown on Si. Here, we report a facile five-step heteroepitaxial growth of GaAs nanowires on Si using selective area growth (SAG) in metalorganic chemical vapor deposition (MOCVD), and we further report in-depth study on the twin formation mechanism. Rotational twin defects were observed in the nanowire structures and showed strong dependence on the growth condition and nanowire size. We adopt a model of faceted growth to demonstrate the formation of twins during growth, which is well supported by both transmission electron microscopy (TEM) study and simulation based on nucleation energetics. Our study has led to twin-free segments in the length up to 80 nm, a significant improvement compared to previous work using SAG. The achievements may open up opportunities for future functional III-V-on-Si heterostructure devices.
    No preview · Article · Feb 2016 · ACS Nano
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    ABSTRACT: Semiconducting single-wall carbon nanotubes (SWCNTs) with long lengths are highly desirable for many applications such as thin-film transistors and circuits. Previously reported length sorting techniques usually require sophisticated instrumentation and are hard to scale up. In this paper, we report for the first time a general phenomenon of a length-dependent precipitation of surfactant-dispersed carbon nanotubes by polymers, salts, and their combinations. Polyelectrolytes such as polymethacrylate (PMAA) and polystyrene sulfonate (PSS) are found to be especially effective on cholate and deoxycholate dispersed SWCNTs. By adding PMAA to these nanotube dispersions in a stepwise fashion, we have achieved nanotube precipitation in a length-dependent order: first nanotubes with an average length of 650 nm, and then successively of 450 nm, 350 nm, and 250 nm. A similar effect of nanotube length sorting has also been observed for PSS. To demonstrate the utility of the length fractionation, the 650 nm-long nanotube fraction was subjected to an aqueous two-phase separation to obtain semiconducting enriched nanotubes. Thin-film transistors fabricated with the resulting semiconducting SWCNTs showed a carrier mobility up to 18 cm(2) (V s)(-1) and an on/off ratio up to 10(7). Our result sheds new light on the phase behavior of aqueous nanotube dispersions under high concentrations of polymers and salts, and offers a facile, low-cost, and scalable method to produce length sorted semiconducting nanotubes for macroelectronics applications.
    No preview · Article · Jan 2016 · Nanoscale
  • Xuan Cao · Yu Cao · Chongwu Zhou
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    ABSTRACT: Flexible thin-film transistors based on semiconducting single-wall carbon nanotubes are promising for flexible digital circuits, artificial skins, radio frequency devices, active-matrix-based displays, and sensors due to the outstanding electrical properties and intrinsic mechanical strength of carbon nanotubes. Nevertheless, previous research effort only led to nanotube thin-film transistors with the smallest bending radius down to 1 mm. In this paper, we have realized the full potential of carbon nanotubes by making ultraflexible and imperceptible p-type transistors and circuits with a bending radius down to 40 μm. In addition, the resulted transistors show mobility up to 12.04 cm(2) V(-1) S(-1), high on-off ratio (∼10(6)), ultralight weight (<3 g/m(2)), and good mechanical robustness (accommodating severe crumpling and 67% compressive strain). Furthermore, the nanotube circuits can operate properly with 33% compressive strain. On the basis of the aforementioned features, our ultraflexible p-type nanotube transistors and circuits have great potential to work as indispensable components for ultraflexible complementary electronics.
    No preview · Article · Dec 2015 · ACS Nano
  • Yu Cao · Yuchi Che · Hui Gui · Xuan Cao · Chongwu Zhou
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    ABSTRACT: In this paper, we report polyfluorene-separated ultra-high purity semiconducting carbon nanotube radio frequency transistors with a self-aligned T-shape gate structure. Because of the ultra-high semiconducting tube purity and self-aligned T-shape gate structure, these transistors showed an excellent direct current and radio frequency performance. In regard to the direct current characteristics, these transistors showed a transconductance up to 40 μS/μm and an excellent current saturation behavior with an output resistance greater than 200 kΩ·μm. In terms of the radio frequency characteristics, an extrinsic maximum oscillation frequency (f max) of 19 GHz was achieved, which is a record among all kinds of carbon nanotube transistors, and an extrinsic current gain cut-off frequency (f T) of 22 GHz was achieved, which is the highest among transistors based on carbon nanotube networks. Our results take the radio frequency performance of carbon nanotube transistors to a new level and can further accelerate the application of carbon nanotubes for future radio frequency electronics.
    No preview · Article · Nov 2015 · Nano Research
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    ABSTRACT: Lithium-ion batteries have attracted great attention as one of the most versatile electrochemical energy storage devices. However, to meet the ever-growing energy needs for wide applications, further improvements on energy density of batteries are expected, which requires the development of innovative high-energy electrode materials. Silicon (Si) and sulfur (S) are two promising candidates and have been studied intensively as anode and cathode materials in lithium-ion batteries. Nevertheless, the excellent performance achieved with Li-Si and Li-S half cells usually does not easily translate to high-performance Si-S full cell. Here, we will discuss the challenges in the Si-S full cell integration, and a failure mechanism of Si-S full cell is proposed, which is due to the spontaneous reaction between Si (and lithiated Si) and polysulfides. On this basis, we report one prototype of Si-S full cells using lithiated Nafion-coated porous Si as anode and sulfur as cathode, and our study on the functionality of Nafion in shielding Si from reaction with polysulfides. With optimized mass ratio between sulfur and silicon, the full cell yields specific capacity of 330. mA. h/g and energy density of 590. W. h/kg after 100 cycles based on the total mass of sulfur and silicon. The achieved energy density is more than 2 times higher than commercially available lithium-ion batteries. The investigation of issues in Si-S full cell research and the proposed full cell prototype will shed light on the development of next-generation lithium-ion batteries.
    No preview · Article · Nov 2015 · Nano Energy
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    ABSTRACT: Multijunction solar cells provide us a viable approach to achieve efficiencies higher than the Shockley-Queisser limit. Semiconductor nanowires, due to their unique optical, electrical, and crystallographic features, are good candidates to achieve monolithic integration of solar cell materials that are not lattice-matched. Here, we report the first realization of nanowire-on-Si tandem cells with the observation of voltage addition of the GaAs nanowire top cell and the Si bottom cell with an open circuit voltage of 0.956 V and an efficiency of 11.4%. Our simulation showed that the current-matching condition plays an important role in the overall efficiency. Furthermore, we characterized GaAs nanowire arrays grown on lattice-mismatched Si substrates and estimated the carrier density using photoluminescence. A low-resistance connecting junction was obtained using n+-GaAs/p+-Si heterojunction. Finally, we demonstrated tandem solar cells based on top GaAs nanowire array solar cells grown on bottom planar Si solar cells. The reported nanowire-on-Si tandem cell opens up great opportunities for high-efficiency, low-cost multijunction solar cells.
    No preview · Article · Oct 2015 · Nano Letters
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    ABSTRACT: One-step simultaneous diameter and metallicity enrichment of semiconducting (9,8) single-walled carbon nanotubes is achieved by a new aqueous two-phase separation method. The enriched nanotubes have a band gap of 0.88 eV and a narrow emission window at 1400 nm, showing excellent potential for application in electronic and optoelectronic devices.
    No preview · Article · Oct 2015
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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
    No preview · Article · Aug 2015 · Carbon
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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.
    No preview · Article · Jul 2015 · ACS Nano
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    ABSTRACT: Sodium (Na)-ion batteries offer an attractive option for low cost large scale energy storage due to the earth abundance of Na. SnO2 is considered as a high capacity anode for Na-ion batteries with a theoretical capacity of 1378 mA h/g. However, several limitations, such as large volume expansion with cycling, slow kinetics and low electrical conductance, have severely limited its performance. In this article, we demonstrate an anode consisting of a SnO2 nanocrystal layer grown on hierarchical microfibers of carbon cloth (CC) with extra surface coating to addresses the above challenges associated with SnO2 anodes. The soft nature of CC and the nanocrystal structure of SnO2 layers can effectively accommodate the volume change associated with the sodiation process. In addition, the effect from an extra coating layer of carbon (C/SnO2/CC) and Al2O3 (Al2O3/SnO2/CC) have been explored and the results showed that the extra coating layer can further enhance the performance of SnO2 anode. The C/SnO2/CC core–shell structure anode achieved a 501 mA h/g and a 144 mA h/g capacity at 0.1 C and 30 C charge/discharge rate, respectively. Meanwhile, a 375 mA h/g specific capacity after 100 deep cycles with an 80% retention is achieved by Al2O3/SnO2/CC anode. The designed surface-coating/nanocrystal-active-material-layer/conductive-soft-platform core–shell system paves the way to high performance Na-ion batteries.
    No preview · Article · Jul 2015 · Nano Energy
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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.
    No preview · Article · Jun 2015 · ACS Nano
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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).
    Full-text · Article · May 2015 · Nanotechnology
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    Full-text · Dataset · May 2015
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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.
    Full-text · Article · May 2015 · Advanced Materials
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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.
    No preview · Article · May 2015 · ACS Nano
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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.
    Full-text · Article · May 2015 · ACS Nano
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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.
    No preview · Article · Apr 2015 · Nano Research
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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.
    Full-text · Article · Mar 2015 · Nano Energy
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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.
    No preview · Article · Mar 2015 · Journal of the American Chemical Society
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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.
    Full-text · Article · Feb 2015 · Nano Letters

Publication Stats

12k Citations
1,506.05 Total Impact Points

Institutions

  • 2001-2016
    • University of Southern California
      • • Department of Electrical Engineering
      • • Department of Chemical Engineering and Materials Science
      • • Department of Chemistry
      • • Division of Cardiovascular Medicine
      Los Ángeles, California, United States
  • 2004-2015
    • 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