Chorng Haur Sow

National University of Singapore, Singapore, Singapore

Are you Chorng Haur Sow?

Claim your profile

Publications (167)591.21 Total impact

  • [Show abstract] [Hide abstract]
    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 08/2014; · 5.93 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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; · 6.23 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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; · 12.03 Impact Factor
  • [Show abstract] [Hide abstract]
    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; · 5.70 Impact Factor
  • [Show abstract] [Hide abstract]
    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; · 6.23 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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. · 3.39 Impact Factor
  • [Show abstract] [Hide abstract]
    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.
    J. Mater. Chem. C. 01/2014; 2(5).
  • [Show abstract] [Hide abstract]
    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. 01/2014; 118:150–153.
  • [Show abstract] [Hide abstract]
    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 01/2014; · 4.43 Impact Factor
  • [Show abstract] [Hide abstract]
    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. · 1.97 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We demonstrate that a direct focused laser beam irradiation is able to achieve localized modification on GeSe2 nanostructures (NSs) film. Using scanning focused laser beam setup, micropatterns on GeSe2 NSs film are created directly on the substrate. Controlled structural and chemical changes of the NSs are achieved by varying laser power and treatment environment. The laser modified GeSe2 NSs exhibit distinct optical, electrical and optoelectrical properties. Detailed characterization is carried out and the possible mechanisms for the laser induced changes are discussed. The laser modified NSs film shows superior photoconductivity properties as compared to the pristine nanostructure film. The construction of micropatterns with improved functionality could prove to be useful in miniature optoelectrical devices.
    11/2013;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: In this work, the electrochemical performance of NiFe2O4 nanofibers synthesized by an electrospinning approach have been discussed in detail. Lithium storage properties of nanofibers are evaluated and compared with NiFe2O4 nanoparticles by galvanostatic cycling and cyclic voltammetry studies, both in half-cell configurations. Nanofibers exhibit a higher charge-storage capacity of 1000 mAh g(-1) even after 100 cycles with high Coulmbic efficiency of 100 % between 10 and 100 cycles. Ex situ microscopy studies confirmed that cycled nanofiber electrodes maintained the morphology and remained intact even after 100 charge-discharge cycles. The NiFe2O4 nanofiber electrode does not experience any structural stress and eventual pulverisation during lithium cycling and hence provides an efficient electron conducting pathway. The excellent electrochemical performance of NiFe2O4 nanofibers is due to the unique porous morphology of continuous nanofibers.
    ACS Applied Materials & Interfaces 10/2013; · 5.01 Impact Factor
  • Source
    Bablu Mukherjee, Eng Soon Tok, Chorng Haur Sow
    [Show abstract] [Hide abstract]
    ABSTRACT: Single crystal GeSe2 nanobelts (NBs) were successfully grown using chemical vapor deposition techniques. The morphology and structure of the nanostructures were characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffractometry, and Raman spectroscopy. Electronic transport properties, photoconductive characteristics, and temperature-dependent electronic characteristics were examined on devices made of individual GeSe2 nanobelt. Localized photoconductivity study shows that the large photoresponse of the device primarily occurs at the metal-NB contact regions. In addition, the electrically Schottky nature of nanobelt-Au contact and p-type conductivity nature of GeSe2 nanobelt are extracted from the current-voltage characteristics and spatially resolved photocurrent measurements. The high sensitivity and quick photoresponse in the visible wavelength range indicate potential applications of individual GeSe2 nanobelt devices in realizing optoelectronic switches.
    Journal of Applied Physics 10/2013; 114(13). · 2.21 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We have synthesized high-quality micrometer-sized single-crystal GeSe nanosheets using vapor transport and deposition techniques. Photoresponse is investigated based on mechanically exfoliated GeSe nanosheet combined with Au contacts under a global laser irradiation scheme. The nonlinearship, asymmetric, and unsaturated characteristics of the I-V curves reveal that two uneven back-to-back Schottky contacts are formed. First-principles calculations indicate that the occurrence of defects create in-gap defective states, which are responsible for the slow decay of the current in the OFF state and for the weak light intensity dependence of photocurrent. The Schottky photodetector exhibits a marked photoresponse to NIR light illumination (maximum photoconductive gain ~ 5.3×102 % at 4V fixed bias) at a wavelength of 808 nm (energy 1.5 eV). The excellent photoresponse and high responsitivity (~3.5 A W-1) suggests its potential applications as photodetectors.
    ACS Applied Materials & Interfaces 09/2013; · 5.01 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We employ optical pump-terahertz probe spectroscopy to investigate the composition-dependent photoconductivity in ternary CdS x Se 1–x nanobelts. The observed carrier dynamics of CdS nanobelts display much shorter lifetime than those of ternary CdS x Se 1–x nanobelts. This indicates the implementation of CdS nanobelts as ultrafast switching devices with a switching speed potentially up to 46.7 GHz. Surprisingly, ternary CdS x Se 1–x nanobelts are found to exhibit much higher photoconductivity than binary CdS and CdSe. This is attributed to the higher photocarrier densities in ternary compounds. In addition, the presence of Se in samples resulted in prominent CdSe-like transverse optical (TO) phonon modes due to electron–phonon interactions. The strength of this mode shows a large drop upon photoexcitation but recovers gradually with time. These results demonstrated that growth of ternary nanostructures can be utilized to alleviate the high surface defect density in nanostructures and improve their photoconductivity.
    Nano Research 08/2013; 6(11):808-821. · 7.39 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Zn2SnO4 nanowires have been synthesized directly on stainless steel substrate without any buffer layers by the vapour transport method. The structural and morphological properties are investigated by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM). The electrochemical performance of Zn2SnO4 nanowires is examined by galvanostatic cycling and cyclic voltammetry (CV) measurements in two different voltage windows, 0.005- 3 V and 0.005- 1.5 V vs. Li and compared to that of Zn2SnO4 nanoplates prepared by hydrothermal method. Galvanostatic cycling studies of Zn2SnO4 nanowires in the voltage range 0.005-3V, at a current of 120 mA g-1, show a reversible capacity of 1000 (±5) mAh g-1 with almost stable capacity for first 10 cycles which thereafter fades to 695 mAh g-1 by 60 cycles. Upon cycling in the voltage range 0.005- 1.5 V vs. Li, a stable, reversible capacity of 680 (±5) mAh g-1 is observed for first 10 cycles with a capacity retention of 58 % between 10- 50 cycles. On the other hand, Zn2SnO4 nanoplates show drastic capacity fading up to 10 cycles and then showed a capacity retention of 80 % and 70 % between 10-50 cycles when cycled in the voltage range 0.005-1.5 V and 0.005 - 3 V, respectively. The structural and morphological evolutions during cycling and their implications on the Li-cycling behavior of Zn2SnO4 nanowires are examined. The effect of the choice of voltage range and initial morphology of the active material on the Li-cycleabilty is also elucidated.
    ACS Applied Materials & Interfaces 06/2013; · 5.01 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The frequency- and fluence-dependent transient photoconductivity in ternary CdSSe nanobelts is investigated using time-resolved terahertz spectroscopy. The carrier density and mobility are extracted by modeling the measured complex photoconductivity using the Drude–Smith model. Within the first few picoseconds of excitation, both the carrier density and mobility reach their maximum values and then decay gradually over tens to hundreds of picoseconds. The decay of free carriers is mainly attributed to fast surface trapping and structural-defect-mediated recombination. The surface trapping saturates rapidly with increasing excitation fluence attributable to the low trapping density on the nanobelt surface caused by self-passivation of surface defects during the growth process.
    The Journal of Physical Chemistry C 05/2013; 117:12379-12384. · 4.84 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Potassium ions are successfully intercalated into WO3 nanobundles with the integrity of the pseudo-orthorhombic structure remaining intact. The nanobundles display a five fold increase in the electrical conductivity. It changes from a value of 10-4 Sm-1 for pure WO3 to 40 Sm-1 upon potassium intercalation. The electrical conductivity also increases by ~200 times as temperature increases from 23℃ to 200℃ whereby analysis shows a thermal activation energy of ~1 eV. Density functional theory calculations show that K ions cause the reduction of the surrounding W atoms and lead to an increase in the electron population in the conduction band. Hence the conductivity of the K-WO3 nanobundles is greatly enhanced. The calculated band structure also shows a gap of 1 eV that is consistent with the measured thermal activation energy. Upon illumination of focused laser beam, individual and isolated nanobundle displays significant photon induced current (9 nA) without external bias at low laser power (2 mW), the amplitude and polarity of photocurrent could be controlled by location of laser spot.
    ACS Applied Materials & Interfaces 04/2013; · 5.01 Impact Factor
  • Bablu Mukherjee, Eng Soon Tok, Chorng Haur Sow
    [Show abstract] [Hide abstract]
    ABSTRACT: Single crystal GeSe2 nanobelts were grown using chemical vapor deposition techniques. Morphology of the nanostructures was characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometry (XRD) and Raman spectroscopy. Electronic transport properties, impedance spectroscopy, photoconductive characteristics and temperature-dependent electrical resistivity measurements were carried out on individual GeSe2 nanobelt devices. The photosensitivity of single GeSe2 nanobelt (NB) devices was examined with two different excitation wavelengths of laser beams with photon energies above band gap and at sub-band gap of the NB. A maximum photoconductive gain 10^6 % was achieved at a wavelength of 808 nm. The magnitude of the photocurrent and response time of the individual GeSe2 NB device indicate that the photoresponse could be attributed to the presence of isolated mid band gap defect levels. Temperature dependent photocurrent measurements indicate the rough estimation of the energy levels for the defect states. Localized photostudy shows that the large photoresponse of the device primarily occurs at the metal-NB contact regions.
    03/2013;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Desorption and readsorption of gas molecules from ambient air onto the surface of α-Fe2O3 quasi-1D nanostructures (nanoflakes and nanostrips) were studied in situ by X-ray photoelectron spectroscopy (XPS) and ex situ by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XPS revealed that carbon and oxygen species were physisorbed and chemisorbed as C–C/C–H, C–O, O–C═O, and O2 states on both surfaces of α-Fe2O3 quasi-1D nanostructures upon exposure in air. The physisorbed carbon species (C–C/C–H) and O2 desorbed from the surfaces when the two nanostructures were heated to 100 °C inside the vacuum chamber of XPS. Significant desorption of chemisorbed O–C═O and O–C occurred above 200 °C, which resulted in a reduction of Fe2O3 into Fe3O4 for both samples between 200 and 300 °C. Complete desorption of carbon and O–C/O–C═O/O2 species in O1s occurred at 400 °C, where Fe3O4 in nanoflakes (sample 1) was reduced further into FeO by excess metallic Fe from the bulk, while Fe3O4 in nanostrips (sample 2) was largely oxidized into Fe2O3 by the oxygen from the bulk of Fe2O3. Although no band bending was observed during the annealing and desorption of ambient gases, the valence band changed as the phase transformation occurred. After the annealed samples were exposed to air for two days, the same chemical states associated with C and O species were again detected on the surfaces of the two nanostructures. In addition, FeO (sample 1) was found to be oxidized into a mixture of Fe2O3 and Fe3O4 on the surface. The adsorption of gas molecules from ambient environment thus has a strong influence on the chemical and physical properties of nanostructures with large surface to volume ratio.
    The Journal of Physical Chemistry C 01/2013; 117(3):1509. · 4.84 Impact Factor