CHIN KOK CHUNG
Email Address
phyckc@nus.edu.sg
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Publication Raman scattering characterization of CVD graphite films(2008-05) Obraztsov, A.N.; Tyurnina, A.V.; Obraztsova, E.A.; Zolotukhin, A.A.; Liu, B.; Chin, K.-C.; Wee, A.T.S.; PHYSICS; CHEMISTRY; NUS NANOSCIENCE & NANOTECH INITIATIVERaman spectroscopic study has been performed for thin graphite films grown on nickel substrates by chemical vapor deposition from a mixture of hydrogen and methane activated by a direct current discharge. Depending on the growth conditions, the CVD films are composed of graphene layers parallel to the substrate surface or of plate-like crystallites with the predominant orientation of their graphene layers perpendicular to the substrate surface. A comparison of the Raman spectra for the CVD films and for the highly oriented pyrolytic graphite has been performed. The mechanisms governing the Raman scattering process in the films are discussed. An important role of a double resonance mechanism in the Raman spectra of these graphite-based materials has been revealed. The Raman band positions and intensities and their dependence on excitation wavelength confirm a high degree of the structural order in the CVD graphite films. © 2008 Elsevier Ltd. All rights reserved.Publication Selective area growth of aligned carbon nanotubes by ion beam surface modification(2003-03-28) Gohel, A.; Chin, K.C.; Lim, K.Y.; Tay, S.T.; Liu, R.; Chen, G.S.; Wee, A.T.S.; INSTITUTE OF ENGINEERING SCIENCE; PHYSICS; NUS NANOSCIENCE & NANOTECH INITIATIVEO 2 + ion beams are used to modify the surface of 50 nm thick Fe catalyst films deposited on Si(100) substrates to demonstrate selective area growth of aligned multiwall carbon nanotubes (MWNT). Aligned MWNTs were grown on the modified Fe/Si substrate using the hot filament plasma enhanced chemical vapor deposition (HF-PECVD) method. A higher growth rate and density of MWNTs was observed on the ion-modified areas, facilitating selective area growth of aligned MWNTs at temperatures as low as 560 °C. Deposition of graphitic sheets at the initial growth process on the unmodified areas inhibits the deposition of MWNTs. © 2003 Elsevier Science B.V. All rights reserved.Publication Laser pruning of carbon nanotubes as a route to static and movable structures(2003-02-17) Lim, K.Y.; Sow, C.H.; Lin, J.; Cheong, F.C.; Shen, Z.X.; Thong, J.T.L.; Chin, K.C.; Wee, A.T.S.; ELECTRICAL & COMPUTER ENGINEERING; PHYSICS; NUS NANOSCIENCE & NANOTECH INITIATIVEA simple focused laser beam pruning method used to fabricate unique two- and three-dimensional static structures without a pre-patterned substrate and under ambient conditions was reported. By exploiting th elastic nature of carbon nanotubes (CNTs), microscopic, 3D, movable CNT structures, readily controlled by laser light, were constructed. Thus, it could be useful for potential micro-optomechanical and field emission device applications.Publication Optical limiting properties of amorphous SixNy and SiC coated carbon nanotubes(2004-01-01) Chin, K.C.; Gohel, A.; Elim, H.I.; Ji, W.; Chong, G.L.; Lim, K.Y.; Sow, C.H.; Wee, A.T.S.; PHYSICS; NUS NANOSCIENCE & NANOTECH INITIATIVE; CHEMICAL & BIOMOLECULAR ENGINEERINGDifferent thicknesses of amorphous silicon carbide (a-SiC) and amorphous silicon nitride (a-SixNy) coatings were deposited on randomly aligned multiwall carbon nanotubes (MWNTs) by plasma enhanced chemical vapor deposition (PECVD). Their optical limiting properties were studied as a suspension in water. Optical limiting experiments show that they had slightly reduced optical limiting properties compared to pure MWNTs, the extent depending on the thickness and coating material. However, the linear transmission spectra of these coated MWNTs show a broader band transmission than pure MWNTs. This suggests that the coated MWNTs are more ideal as protective material for optical sensors as compared to pure MWNTs because they are effective over a wider optical transmission band. © 2003 Elsevier B.V. All rights reserved.Publication Optical limiting studies of new carbon nanocomposites and amorphous Si xNy or amorphous SiC coated multi-walled carbon nanotubes(2004-06) Elim, H.I.; Chen, W.; Wei, J.I.; Zhong, Z.; Lin, J.; Meng, G.C.; Chin, K.C.; Gohel, A.; Wee, A.T.S.; Chen, G.X.; Hong, M.H.; PHYSICS; NUS NANOSCIENCE & NANOTECH INITIATIVE; CHEMICAL & BIOMOLECULAR ENGINEERINGBy using fluence-dependent transmission measurement with nanosecond laser pulses, we have studied optical limiting (OL) properties of new carbon nanocomposites as well as amorphous SixNy or amorphous SiC coated carbon nanotubes suspended in distilled water. The observed nonlinearity at 532 nm contributed to OL performance of the carbon nanocomposites or carbon nanoballs (CNBs) is suggested to have its origin in the optically induced heating or scattering effects. It is found that when the linear transmittance of the CNBs is less than or equal to 70%, the intensity-dependent transmission of the CNBs is comparable to that of C60. While at 80% linear transmittance, CNBs possess better OL behavior than that of C60. These findings strongly support a potential application of CNBs for all laser protection devices. We have also observed OL effects in the amorphous silicon nitride (a-SixNy) and amorphous silicon carbide (a-SiC) coated multi-walled carbon nanotubes (MWNTs) at wavelengths of 532 and 1064 nm, and found that their OL performances are slightly poorer than that of their parent MWNTs. The possible sources of thickness-dependent OL effects of a-SixNy and a-SiC coated MWNTs are discussed.Publication Large-scale synthesis of Fe3O4 nanosheets at low temperature(2007-07-05) Chin, K.C.; Chong, G.L.; Poh, C.K.; Van Hui, L.; Sow, C.H.; Lin, J.; Wee, A.T.S.; PHYSICS; NUS NANOSCIENCE & NANOTECH INITIATIVEWe introduce a simple approach to synthesize high-quality Fe 3O4 nanosheets at low temperature. By oxidizing pure Fe substrates in acidic solution on a hot plate maintained at 70 °C, we have grown the nanosheets in large-scale. The samples were characterized by SEM, XRD, TEM, micro-Raman, and VSM. Flowing O2 gas into the solution during oxidation enhances the growth rate of the nanosheets and thereby shortens the growth time from 24 h to 15 min. The magnetic property of the sample was also investigated. This technique can potentially be extended for large-scale synthesis of other metal-oxide nanosheets such as ZnO. © 2007 American Chemical Society.Publication Fabrication of size-tunable gold nanoparticles array with nanosphere lithography, reactive ion etching, and thermal annealing(2005-06-09) Tan, B.J.Y.; Sow, C.H.; Koh, T.S.; Chin, K.C.; Wee, A.T.S.; Ong, C.K.; PHYSICS; NUS NANOSCIENCE & NANOTECH INITIATIVETwo-dimensional ordered arrays of gold (Au) nanoparticles were fabricated using two different variants of the nanosphere lithography technique. First, ordered arrays of polystyrene nanospheres on Si substrate were used as deposition masks through which gold films were deposited by electron beam evaporation. After the removal of the nanospheres, an array of triangular Au nanodisks was left on the Si substrate. After thermal annealing at increasing temperature, systematic shape transition of the nanostructures from original triangular Au nanodisks to rounded nanoparticles was observed. This approach allows us to systematically vary the size and morphology of the particles. In the second and novel technique, we made use of reactive ion etching to simultaneously reduce the dimension of the masking nanospheres and create arrays of nanopores on the substrate prior to the deposition of the Au films. These samples were subsequently annealed, which resulted in size-tunable and ordered Au nanoparticle arrays with the nanoparticles nested in the nanopores of the templated substrate. With the nanoparticles anchored in the nanopores, the substrate could be useful as a template for growth of other nanomaterials. © 2005 American Chemical Society.Publication Large area, rapid growth of two-dimensional ZnO nanosheets and their field emission performances(2008-03) Chin, K.C.; Poh, C.K.; Chong, G.L.; Lin, J.; Sow, C.H.; Wee, A.T.S.; PHYSICS; NUS NANOSCIENCE & NANOTECH INITIATIVEWe demonstrate an improved and rapid method to synthesize uniform two-dimensional ZnO nanosheets at low temperature. These nanosheets can be prepared within an hour. The compositions and morphologies of the nanosheets produced were characterized by SEM, XRD, Raman and photoluminescence analysis. The nanosheets showed reasonably strong electron field emission properties with good reliability. Emission current density close to 0.3 mA/cm2 was obtained. The effects of dimensions and morphologies on the field emission performance of the nanosheets are also discussed. © 2007 Springer-Verlag.Publication Modified carbon nanotubes as broadband optical limiting nanomaterials(2006-11) Chin, K.C.; Gohel, A.; Elim, H.I.; Chen, W.; Ji, W.; Chong, G.L.; Sow, C.H.; Wee, A.T.S.; PHYSICS; NUS NANOSCIENCE & NANOTECH INITIATIVECarbon nanotubes have been shown to be effective broadband optical limiters for nanosecond laser pulses. In this paper, we review the recent developments of carbon nanotube-based optical limiters, in particular the effects of modifying carbon nanotubes for device applications. The techniques used to modify carbon nanotubes mainly include thin film coating, doping, and blending with optical absorbing dye. These modifications can greatly enhance the optical limiting performance of carbon nanotubes, with the goal of fabricating an optimal optical limiter system. © 2006 Materials Research Society.Publication Characterization and field emission performance of electrochemically synthesized FeOOH nanowalls(2007-09) Chin, K.C.; Cui, H.; Sow, C.H.; Sheu, F.S.; Van, L.H.; Gao, X.; Wee, A.T.S.; PHYSICS; BIOLOGICAL SCIENCES; NUS NANOSCIENCE & NANOTECH INITIATIVEWe report a direct method for the synthesis of Iron(III) oxyhydroxide (FeOOH) nanowalls using an electrochemical technique at room temperature. The length of the nanowalls can be varied depending on the number of repetitive potentiostatic pulse cycles during the electrochemical process. The samples were characterized by ex-situ techniques such as SEM, XPS, FTIR, and TEM. Field emission performances of these nanowalls are also reported for the first time. The measured turn-on electric field is about 4.8 V/μm, with emission current density of 0.12 mA/cm 2 at 7.3 V/μm. This technique provides a simple alternative method for large area synthesis of FeOOH nanowalls. Copyright © 2007 American Scientific Publishers All rights reserved.