Chorng Haur Sow

National University of Singapore, Tumasik, Singapore

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Publications (187)832.38 Total impact

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    ABSTRACT: We report electrical and optoelectrical properties of a cross-junction of two semiconducting nanowires. Semiconducting nanowires and their junction play an important role in nanonetwork device. By mechanically manipulating the nanowires, cross-junction nanodevices are fabricated on SiO2/Si substrate using VO2 and ZnO nanowires. These junctions are formed across prepatterned two-probe Au electrodes and contacted through Pt metal deposition. The cross-junction devices were studied using global and focused laser beam irradiation with a wavelength of 532 nm at sweeping bias and fixed external bias. Furthermore multi-junction in nanonetwork between VO2 and ZnO nanowires device is demonstrated as a viable photodetector for potential application.
    No preview · Article · Jan 2016 · Nanoscience and Nanotechnology Letters
  • Lili Gong · Liangjun Wang · Junpeng Lu · Cheng Han · Wei Chen · Chorng Haur Sow
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    ABSTRACT: In this report, few layered molybdenum disulfide (MoS2) shell was coated on core multiwalled carbon nanotube (CNT) by a facile solvothermal method. The morphology and high crystallinity of this structure were demonstrated and verified by transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). After integrated into planar device, CNT-MoS2 core-shell structure exhibits clear photoresponse and a wide response range upon laser illumination. In addition, the device shows a bias dependent and position sensitive photocurrent effect. Further experiments show that larger photocurrent was obtained under laser illumination with longer wavelength. Both the photocurrent and response speed are enhanced when the device is placed under vacuum condition. The simple material synthesis and device fabrication method used in this work may provide a practical strategy for low cost and large scale optical applications.
    No preview · Article · Oct 2015 · The Journal of Physical Chemistry C
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    ABSTRACT: We demonstrate a straightforward and effective laser pruning approach to reduce multilayer black phosphorus (BP) to few-layer BP under ambient condition. Phosphorene oxides and suboxides are formed and the degree of laser induced oxidation is controlled by the laser power. Since the band gaps of the phosphorene suboxide depend on the oxygen concentration, this simple technique is able to realize localized band gap engineering of the thin BP. Micropatterns of few-layer phosphorene suboxide flakes with unique optical and fluorescence properties are created. Remarkably some of these suboxide flakes display long term (up to 2 weeks) stability in ambient condition. Comparing against the optical properties predicted by first-principle calculations, we develop a "calibration" map in using focused laser power as a handle to tune the band gap of the BP suboxide flake. Moreover, the surface of the laser patterned region is altered to be sensitive to toxic gas by way of fluorescence contrast. Therefore, the multicolored display is further demonstrated as a toxic gas monitor. In addition, the BP suboxide flake is demonstrated to exhibit higher drain current modulation and mobility comparable to that of the pristine BP in the electronic application.
    Full-text · Article · Sep 2015 · ACS Nano
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    ABSTRACT: The successful design and synthesis of earth-abundant and efficient catalysts for the oxygen evolution reaction (OER) will be a major step forward towards the use of electrochemical water splitting as an environmentally-friendly process for producing H2 fuel. Due to their poor activity, copper-based materials have not been considered apt for catalysing OER. In this work, we demonstrate that unique copper(II) oxide nanostructures obtained via hydrothermal synthesis and subsequent hydrogen peroxide treatment exhibit unusually high and sustainable OER activity. In 0.1 M KOH electrolyte, the CuO nanostructures catalyse OER with current densities of 2.6–3.4 mA cm−2 at 1.75 V (vs. RHE). The calculated turnover frequency (per Cu site) of ~2 × 10−3 s−1 for O2 production is markedly higher than that of high-surface area electrodeposited Cu metal nanoparticles by 40–68 times. The OER activity of the CuO nanostructures is also stable, approaching about half of 20% IrOx/Vulcan XC-72 after an hour-long OER. In situ Raman spectroscopy at OER-relevant potentials recorded compelling evidence that CuIII active species may be responsible for the unusual OER activity of the CuO nanostructures, as indicated by its signature vibration at 603 cm−1. This hitherto unobserved peak is assigned, with the aid of the model compound NaCuIIIO2, to the Cu–O stretching vibration of CuIII oxide. This feature was not found on electrodeposited Cu metal, which exhibited correspondingly weaker OER activity. The enhanced catalysis of O2 evolution by the CuO nanostructures is thus attributed to not just their higher surface area, but also the higher population of CuIII active sites on their surface.
    No preview · Article · Sep 2015 · Catalysis Science & Technology
  • 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.
    No preview · Article · May 2015 · Applied Surface Science
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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.
    Full-text · Article · Apr 2015 · Nano Letters
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    Junpeng Lu · Sharon Xiaodai Lim · Chorng Haur Sow
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    ABSTRACT: One-dimensional (1D) nanomaterials have generated considerable interest amongst researchers because of their potential as fundamental building block in future nano- and micro-electronic devices. In this article, we present a review of a focused laser beam system as a versatile tool for the manipulation, structural transformation, micropatterning and chemical modification of 1D nanomaterials. This tool was found to be effective in patterning and modifying various physical and chemical properties of the pristine 1D nanomaterials. It also aids in the fabrication process of heterostructures and 1D nanomaterial based devices. Finally, we present the implementation of the focused laser beam setup as a valuable tool in the study of the origins and photoresponse mechanism of the 1D nanomaterial devices.
    Full-text · Article · Mar 2015 · Journal of Materials Science and Technology -Shenyang-
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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.
    Full-text · Article · Mar 2015
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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.
    No preview · Article · Feb 2015 · Nanotechnology
  • 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.
    No preview · Article · Feb 2015 · Journal of Applied Physics
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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.
    Full-text · Article · Jan 2015 · Small
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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
    Full-text · Article · Sep 2014 · Scientific Reports
  • 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.
    No preview · Article · Aug 2014
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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.
    Full-text · Article · Aug 2014 · Chemistry - A European Journal
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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.
    Full-text · Article · Jun 2014 · Nanoscale
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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.
    Full-text · Article · Jun 2014 · Electrochemistry Communications
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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.
    Full-text · Article · May 2014 · ACS Nano
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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.
    Full-text · Article · May 2014 · Lab on a Chip
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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.
    No preview · Article · Mar 2014 · Materials Letters
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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.
    No preview · Article · Feb 2014 · Nanoscale