Chorng Haur Sow

National University of Singapore, Tumasik, Singapore

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Publications (179)797.51 Total impact

  • Wenhao Dong · Feng Pan · Leilei Xu · Minrui Zheng · Chorng Haur Sow · Kai Wu · Guo Qin Xu · Wei Chen
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    ABSTRACT: Amorphous TiO2 layers with a controllable thickness from 3.5 to 40 nm were coated on the one-dimensional CdS nanorods surface under mild conditions. Compared to the bare CdS nanorods, the as-prepared CdS@TiO2 nanorods exhibit enhanced photocatalytic activities for phenol photodecomposition under visible light irradiation. The improved photoactivity is ascribed to the efficient separation of photogenerated electron and hole charge carriers between CdS cores and TiO2 shells. This study promises a simple approach to fabricating CdS@TiO2 core–shell structure nanocomposites, and can be applied for other semiconductor cores with TiO2 shells.
    Applied Surface Science 05/2015; 349. DOI:10.1016/j.apsusc.2015.04.207 · 2.54 Impact Factor
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    ABSTRACT: As-grown transition metal dichalcogenides are usually chalcogen deficient, and therefore contain a high density of chalcogen vacancies, deep electron traps which can act as charged scattering centers, reducing the electron mobility. However, we show that chalcogen vacancies can be effectively passivated by oxygen, healing the electronic structure of the material. We proposed that this can be achieved by means of surface laser modification, and demonstrate the efficiency of this processing technique, which can enhance the conductivity of monolayer WSe2 by ~ 400 times, and its photoconductivity by ~ 150 times.
    Nano Letters 04/2015; 15(5). DOI:10.1021/acs.nanolett.5b00952 · 13.59 Impact Factor
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    Junpeng Lu · Sharon Xiaodai Lim · Chorng Haur Sow
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    ABSTRACT: Corresponding author. Prof.; Tel.: +65 6516 2603; Fax: +65 6777 6126.
    Journal of Materials Science and Technology -Shenyang- 03/2015; 31(6). DOI:10.1016/j.jmst.2014.12.006 · 1.61 Impact Factor
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    ABSTRACT: Vertically aligned, beaded zinc germinate (Zn2GeO4)/ zinc oxide (ZnO) hybrid nanowire arrays were successfully synthesized by a vapor-solid process via a catalyst-free approach. The as-synthesized products were characterized using X-ray diffraction, scanning electron microscopy and transmission electron microscopy equipped with an energy-dispersive X-ray spectrometer. TEM studies revealed the beaded microstructures of the Zn2GeO4/ZnO nanowire. Furthermore, Zn2GeO4 nanotubes were synthesized after wet etching treatments on Zn2GeO4/ZnO hybrid nanowires.
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    ABSTRACT: Using in situ field effect transistor (FET) characterization combined with the molecular beam epitaxy technique, we demonstrate a significant depletion of electron charge carriers in single zinc oxide (ZnO) nanowire through the surface modification with molybdenum trioxide (MoO3) and 1, 4, 5, 8, 9, 11-hexaazatriphenylene hexacarbonitrile (HATCN) layers. The electron mobility of ZnO nanowire was found to sharply decrease after the surface modification with MoO3; in contrast, the electron mobility significantly increased after functionalization with HATCN layers. Such depletion of n-type conduction originates from the interfacial charge transfer, corroborated by in situ photoelectron spectroscopy studies. The air exposure effect on MoO3- and HATCN-coated ZnO nanowire devices was also investigated.
    Nanotechnology 02/2015; 26(9):095202. DOI:10.1088/0957-4484/26/9/095202 · 3.67 Impact Factor
  • Y. C. Wan · H. F. Teoh · E. S. Tok · C. H. Sow
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    ABSTRACT: We report a facile, two-step method for the micro-landscaping of Au nanoparticles(NPs) on reduced graphene oxide (rGO) film en route to micro-patterned Au(NPs)-rGO hybrid functional materials. This method employs a focused laser beam to first locally convert GO to rGO before immersing the micro-patterned GO-rGO film into HAuCl4 solution. The rGO micro-pattern, shaped by the focused laser beam, serves as nucleation sites for the reduction of Au ions. The reduction mechanism that governs the decoration of Au NPs on rGO films is akin to electroless deposition process. In this instance, surface charges that are formed during laser reduction of GO to rGO provide active nucleation sites for Au 3+ ions to form Au NPs when HAuCl4 solution is introduced. The number density, the size, and size distribution of the Au NPs can thus be directly tuned and preferentially anchored onto the rGO micro-pattern by varying the incident laser power, the scanning speed of the laser, or the concentration of HAuCl4. The resulting hybrid materials can be used as a substrate for Surface Enhanced Raman Spectroscopy (SERS). Using Rhodamine 6G as the test subject, we found an improvement of SERS enhancement over bare rGO of up to four times, depending on the size of the Au NPs.
    Journal of Applied Physics 02/2015; 117(5):054304. DOI:10.1063/1.4906043 · 2.19 Impact Factor
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    ABSTRACT: Micro landscaping of gold nanoparticles on a thin MoS2 film is achieved via dipping a laser-activated MoS2 film into gold chloride. On page 1792, the activated micropatterns are first created on the MoS2 film by C. H. Sow and co-workers using a scanning focused laser beam. When the activated film is subsequently immersed in gold chloride for only 10 s, gold nanoparticles are found on the modified micropatterns. These decorated MoS2 micropatterns turn out to be sensitive detectors for aromatic molecules via the production of strong SERS signals. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Small 01/2015; 11(15). DOI:10.1002/smll.201402591 · 8.37 Impact Factor
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    ABSTRACT: Arrays of ZnO/CdSSe core/shell nanowires with shells of tunable band gaps represent a class of interesting hybrid nanomaterials with unique optical and photoelectrical properties due to their type II heterojunctions and chemical compositions. In this work, we demonstrate that direct focused laser beam irradiation is able to achieve localized modification of the hybrid structure and chemical composition of the nanowire arrays. As a result, the photoresponsivity of the laser modified hybrid is improved by a factor of ,3. A 3D photodetector with improved performance is demonstrated using laser modified nanowire arrays overlaid with monolayer graphene as the top electrode. Finally, by controlling the power of the scanning focused laser beam, micropatterns with different fluorescence emissions are created on a substrate covered with nanowire arrays. Such a pattern is not apparent when imaged under normal optical microscopy but the pattern becomes readily revealed under fluorescence microscopy i.e. a form of Micro-Steganography is achieved. B and gap engineering is an attractive strategy for the control of physical properties of semiconductors in photoelectronics 1–5 . Band gap engineering of multinary alloyed nanostructures has been achieved by adjusting their relative composition as these alloys show a strong dependence of electronic energy on the effective exciton mass 6–9 . In addition, another approach to achieve band gap engineering is the controlled synthesis of hetero/hybrid nanostructures. Among these structures, the band gap in type II core/shell hetero-structures forms a stepwise energy alignment at the interfaces where both the conduction and the valence bands of the shell are either higher or lower in energy than those in the core. Majority and minority carriers would preferably transfer across the junction in opposite directions to form an excitonic charge separation state 10,11
    Scientific Reports 09/2014; 4:6350. DOI:10.1038/srep06350 · 5.58 Impact Factor
  • Kian Keat Lee · Wee Shong Chin · Chorng Haur Sow
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    ABSTRACT: Transition metal compounds (oxides, hydroxides etc.) are emerging electrode materials for electrochemical capacitors (ECs) due to their rich redox properties involving multiple oxidation states and different ions. Pseudocapacitance derived from the reversible faradaic reactions can be ten times or even higher than the state-of-the-art carbon-based electric double layer capacitors (EDLCs). As one of the most well-known electroactive inorganic materials, extensive studies of cobalt-based compounds (Co3O4, Co(OH)2, CoOOH, CoS etc.) for ECs have mushroomed, and the relevant literatures have grown exponentially in the past ten years. This review consolidates and evaluates the recent progress, achievements, weaknesses and challenges in the research of cobalt-based compounds and nanocomposites for ECs. The triangular relationship between synthesis strategies, tailored material properties and the electrochemical performances are thoroughly assessed, unveiling the advanced electrode material design and development.
    08/2014; 2(41). DOI:10.1039/C4TA02074J
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    ABSTRACT: In the present study, we report the synthesis of a high-quality, single-crystal hexagonal β-Co(OH)2 nanosheet, exhibiting a thickness down to ten atomic layers and an aspect ratio exceeding 900, by using graphene oxide (GO) as an exfoliant of β-Co(OH)2 nanoflowers. Unlike conventional approaches using ionic precursors in which morphological control is realized by structure-directing molecules, the β-Co(OH)2 flower-like superstructures were first grown by a nanoparticle-mediated crystallization process, which results in large 3D superstructure consisting of ultrathin nanosheets interspaced by polydimethoxyaniline (PDMA). Thereafter, β-Co(OH)2 nanoflowers were chemically exfoliated by surface-active GO under hydrothermal conditions into unilamellar single-crystal nanosheets. In this reaction, GO acts as a two-dimensional (2D) amphiphile to facilitate the exfoliation process through tailored interactions between organic and inorganic molecules. Meanwhile, the on-site conjugation of GO and Co(OH)2 promotes the thermodynamic stability of freestanding ultrathin nanosheets and restrains further growth through Oswald ripening. The unique 2D structure combined with functionalities of the hybrid ultrathin Co(OH)2 nanosheets on rGO resulted in a remarkably enhanced lithium-ion storage performance as anode materials, maintaining a reversible capacity of 860 mA h g−1 for as many as 30 cycles. Since mesocrystals are ubiquitous and rich in morphological diversity, the strategy of the GO-assisted exfoliation of mesocrystals developed here provides an opportunity for the synthesis of new functional nanostructures that could bear importance in clean renewable energy, catalysis, photoelectronics, and photonics.
    Chemistry - A European Journal 08/2014; 20(39). DOI:10.1002/chem.201403068 · 5.70 Impact Factor
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    ABSTRACT: In this work, we report a novel and feasible strategy for the practical applications of one-dimensional ultrasensitive phototransistors made of tungsten-doped VO2 single nanowires. The photoconductive response of the single nanowire device was investigated under different visible light excitations (405 nm, 532 nm, and 660 nm). The phototransistor device exhibited ultrafast photoresponse, high responsivity, broad multispectral response, and rapid saturation characteristic curves. These promising results help to promote the applications of this material in nano-scale optoelectronic devices such as efficient multispectral phototransistors and optical switches.
    Nanoscale 06/2014; 6(13). DOI:10.1039/c4nr00898g · 7.39 Impact Factor
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    Pui Yee Loh · Kian Keat Lee · Yuting Ng · Chorng Haur Sow · Wee Shong Chin
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    ABSTRACT: We present here a study of Co/Al layered double hydroxides (LDH) nanoflakes supported on Co nanowires as a binderless electrode for electrochemical capacitor. This nanostructured array were prepared easily via electrodeposition of Co in anodic alumina (AAO) template followed by alkaline treatment. This is the first report AAO template is used as a source of Al3 + to form the LDH directly. Capacitance was found to increase with the amount of Co/Al-LDH, as indicated by the Al content from EDX analysis. This content is controllable by varying the duration of alkaline treatment. High capacitance of 0.510 F/cm2 was achieved for sample containing 12% Al measured at current density 2.5 mA/cm2 in 1 M KOH. The electrodes also exhibited good stability, maintaining more than 90% of their original capacitance after 3000 continuous charge–discharge cycles.
    Electrochemistry Communications 06/2014; 43. DOI:10.1016/j.elecom.2014.03.001 · 4.85 Impact Factor
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    ABSTRACT: Direct patterning of ultrathin MoS2 films with well-defined structures and controllable thickness is appealing since the properties of MoS2 sheets are sensitive to the number of layer and surface properties. In this work, we employed a facial, effective and well-controlled technique to achieve micropatterning of MoS2 films with a focused laser beam. We demonstrated that a direct focused laser beam irradiation was able to achieve localized modification and thinning of as-synthesized MoS2 films. With a scanning laser beam, micro-domains with well-defined structures and controllable thickness were created on the same film. We found that laser modification altered the photoelectrical property of the MoS2 films and subsequently photodetectors with improved performance have been fabricated and demonstrated using laser modified films.
    ACS Nano 05/2014; 8(6). DOI:10.1021/nn501821z · 12.88 Impact Factor
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    ABSTRACT: We developed a simple process to fabricate deeply buried micro- and nanoscale channels in glass and porous silicon from bulk silicon using a combination of ion beam irradiation, electrochemical anodization and high temperature oxidation. The depth, width and length of these structures can be controllably varied and we successfully fabricated an array of centimeter-long buried micro- and nanochannels. This process allows densely packed, arbitrary-shaped channel geometries with micro- to nanoscale dimensions to be produced in a three-dimensional multilevel architecture, providing a route to fabricate complex devices for use in nanofluidics and lab-on-a-chip systems. We demonstrate the integration of these channels with large reservoirs for DNA linearization in high aspect ratio nanochannels.
    Lab on a Chip 05/2014; DOI:10.1039/c4lc00062e · 5.75 Impact Factor
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    ABSTRACT: Large area self-standing nanosheets of mixed iron (III) oxyhydroxide/oxide (lepidocrocite, γ-FeOOH and maghemite, γ-Fe2O3) were grown directly from an iron foil based on a simple chemical oxidation route. The γ-FeOOH and γ-Fe2O3 nanosheets on a conductive substrate can be used directly as an electrode, eliminating additional electrode fabrication procedures. The electrode exhibited high areal capacitance (0.3–0.4 F/cm2) and good cycling stability in Na2SO3 electrolyte. Additionally, cycling studies in Na2SO4 disclosed that iron (III) oxyhydroxide/oxide are not stable in Na2SO4, undergoing reductive dissolution with repeated cycling in negative potential window.
    Materials Letters 03/2014; 118:150–153. DOI:10.1016/j.matlet.2013.12.059 · 2.27 Impact Factor
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    ABSTRACT: A direct and facile method for micro-landscaping of Ag nanoparticles on reduced graphene oxide (rGO) is presented. This method employs a focused laser beam to achieve local reduction of Ag(+) ions to Ag NPs by laser irradiation on a GO film that is submerged in AgNO3 solution. Using this method, the Ag nanoparticles can be directly anchored on a rGO film, creating a microlandscape of Ag nanoparticles on the rGO film. In addition, varying the intensity of the laser beam can control the shapes, sizes and distributions of Ag nanoparticles. The resulting hybrid materials exhibit surface enhanced Raman scattering of up to 16 times depending on the size and number density of silver nanoparticles. In addition, the hybrid Ag-rGO material shows superior photoresponse when compared to rGO.
    Nanoscale 02/2014; 6(6). DOI:10.1039/c3nr05373c · 7.39 Impact Factor
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    Hongwei Liu · Junpeng Lu · Sing Hai Tang · Chorng Haur Sow · Xinhai Zhang
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    ABSTRACT: We present a study on the composition-dependent electron transport in ternary CdS<sub>x</sub>Se<sub>1-x</sub> nanobelts at equilibrium and nonequilibrium conditions via THz spectroscopy. The measured spectra are analyzed using a Drude-Smith model combined with a harmonic oscillator. The physical origin of parameters in the Drude-Smith model is studied in detail. Under equilibrium conditions, the surface depletion region is the dominant factor to free-carrier backscattering. However, under nonequilibrium conditions, the influence of the surface depletion region is masked by the high bulk concentration and the free carriers are mainly localized by composition disorder. The contributions from different mechanisms to the carrier mobility are also explored. In equilibrium, alloy scattering is the most vital scattering mechanism for nanobelts with x=0.25→0.9 since composition disorder is significant in this range. On the other hand, the effect of electron-phonon interaction increases under photoexcitation.
    Optics Letters 02/2014; 39(3):567-70. DOI:10.1364/OL.39.000567 · 3.18 Impact Factor
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    ABSTRACT: There has been a growing interest in 1-D metal oxide semiconducting nanostructures due to their stable chemical properties and potential applications in large-area, low-cost and flexible substrates. In this current work, we used field effect transistors (FETs) built on sub-millimetre scale metal oxide nanonet to characterize the nanowire surface properties. Two variations of SnO2 nanowires, denoted as 0% O2 and 0.5% O2–Ar, were grown by changing the O2 concentration in the growth atmosphere. HR-TEM images exhibit two dissimilar surface morphologies which represent diverse surface-defect levels. While the devices showed very little semiconducting behaviour in humid air, the flow in the dry air decreased the density of free carriers dramatically. Both water vapour and oxygen were observed to contribute to the hysteresis of transfer curves. Under white light illumination, 0% O2 nanonet devices exhibited a significant photocurrent response in the controlled environment while almost no changes were observed for 0.5% O2 ones. These results confirmed the significant role of surface defects in metal-oxide nanowires and implied great potential for SnO2 nanonet FETs in the application of water gas sensors and photodetectors.
    01/2014; 2(5). DOI:10.1039/C3TC31713G
  • Pui Yee Loh · Chenmin Liu · Chorng Haur Sow · Wee Shong Chin
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    ABSTRACT: This report presents a versatile “Pore Widening” method that can readily lead to arrays of multi-layered one-dimensional (1D) hetero-nanostructures. Using this simple and facile method, we demonstrate that the thickness of the shell component is controllable by the degree of “Pore Widening”. With careful selection of materials and sequences of deposition steps, a variety of polymer-metal, metal-metal and polymer-metal-metal core-shell nanowires, as well as metal oxide nanotubes and metal oxide-metal double-walled nanotubes can be successfully achieved. This opens up a possibility to tailor new properties of 1D hetero-nanostructures through the judicious combination of different core and shell components in the nanometer size regime.
    RSC Advances 01/2014; 4(17):8735. DOI:10.1039/c4ra00045e · 3.84 Impact Factor
  • Xiaoli Liu · Minrui Zheng · Yunbo Lv · Jie Fang · Chorng Haur Sow · Haiming Fan · Jun Ding
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    ABSTRACT: High-content Fe hollow nanostructures from nanotubes (NTs) to nanorings (NRs) have been successfully fabricated by reduction of α-Fe2O3 hollow nanostructures at designated temperature. We investigated the influence of reduction condition on the structure and property of the products. With increasing reduction temperature, α-Fe2O3 with rhombohedral phase could be converted to cubic inverse spinel phase Fe3O4. Thereafter the phase was converted to the dominant cubic Fe preserving the same morphology. The highest Ms is 126 emu/g and 123 emu/g for 70 nm NRs and NTs reduced at 450 °C, respectively. Our results provide a general strategy of reducing single-crystalline α-Fe2O3 hollow nanostructures to get high magnetization which is required for many applications.
    Materials Research Bulletin 12/2013; 48(12):5003. DOI:10.1016/j.materresbull.2013.05.077 · 2.29 Impact Factor