S. L. Lim

National University of Singapore, Tumasik, Singapore

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Publications (19)46.19 Total impact

  • S.L. Lim · F. Xu · N.N. Phuoc · C.K. Ong
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    ABSTRACT: CoFe2 nanowires with high aspect ratios were electrodeposited into the anodic aluminum oxide (AAO) template. The coercivities and remanence measured along the longitudinal axis of the nanowires are found to increase with the length of nanowires. The magnetostatic interaction between the nanowires is responsible for the decrease of coercivity and remanence of an array of nanowires as compared to an isolated single nanowire. By considering the magnetostatic interaction between the nanowires, a simple model as a function of length is used to calculate the coercivity in an array of the nanowire and this model shows good agreement with experimental data.
    No preview · Article · Sep 2010 · Journal of Alloys and Compounds
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    Nguyen N. Phuoc · S. L. Lim · F. Xu · Y. G. Ma · C. K. Ong
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    ABSTRACT: A systematic investigation of the dependences of the exchange bias and the ferromagnetic resonance frequency on the pore size of the antidot arrays fabricated by depositing Permalloy-FeMn multilayer thin films onto self-organized porous anodic aluminum oxide membranes was carried out. The magnetic and microwave properties of the antidot arrays with different pore sizes ranging from 30 to 80 nm are characterized and compared with that of the continuous thin films. It was found that the exchange bias field and the ferromagnetic resonance frequency are increased with the increase of the pore size, which may tentatively be interpreted in the framework of the random field model. It was also found that by using the antidot arrays in the best condition (the FeNi thickness is 20 nm and the pore size is 80 nm), one can enhance the exchange bias field from 65 to 135 Oe and the ferromagnetic resonance frequency from 3.1 to 4.1 GHz accordingly.
    Preview · Article · Dec 2008 · Journal of Applied Physics
  • H.J. Liu · S.L. Lim · C.K. Ong
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    ABSTRACT: The half-metallic La0.67Sr0.33MnO3 (LSMO) thin film was fabricated by pulsed laser deposition on porous anodic aluminum oxide template. The mesh-like pattern was obtained with the lateral size of 30–50 nm. This laterally mesoscopic-patterned LSMO film exhibits typical low field magnetoresistance. The coercive field of 268 Oe is larger than that of the continuous polycrystalline film on Al2O3 substrate of 115 Oe, which is consistent with the larger coercive field of magnetic nanowires compared with magnetic bulk materials or thin films. This work provides a cost-effective method to fabricate mesoscopic-pattern film with the lateral size down to several tens nanometers.
    No preview · Article · Apr 2008 · Materials Letters
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    ABSTRACT: We report a nonvolatile, write-once-read-many-times (WORM) memory device based on a simple organic-inorganic heterojunction. The organic-based hybrid used is 9,9-dihexylfluorene and Eu-complexed benzoate, which contains both electron-donor (9, 9″ -dihexylfluorene) and electron-acceptor (europium complex) groups. Under current-voltage testing, the device is able to switch from one initial nonconducting state to a conducting state once a threshold voltage is reached. Diode rectifying characteristics, with a current ratio of four orders of magnitude, is also observed after the device is turned on, which is essential to address one memory cell in large passive matrix circuits.
    No preview · Article · Jan 2008 · Journal of The Electrochemical Society
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    ABSTRACT: We present an electrical-bistability device based on MIM-sandwiched structure. Poly(N-vinylcarbazole) (PVK) mixed with gold nanoparticles (GNPs) serve as the active layer between two metal electrodes. After applying a voltage, the as-fabricated device can transit from low conductivity state to high conductivity state. By simply using a reverse bias, the high conductivity state can return to the low conductivity state. An on/off current ratio as high as 10<sup>5 </sup> at room temperature has been achieved. The memory effect is attributed to electric-field-induced charge transfer complex formed between the PVK and the GNPs. The device shows a good stability under stress test for both states and exhibits a high potential on Flash-type memory applications
    Preview · Article · Mar 2007 · IEEE Electron Device Letters
  • Y G Ma · S L Lim · C K Ong
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    ABSTRACT: CoAlO composite antidot arrays were fabricated on self-organized porous anodic aluminium oxide (AAO) membranes. The effects of pore size and film thickness on the magnetism and magnetotransport properties of the CoAlO films were investigated. On increasing the pore dimensions in the arrays, an anisotropic to isotropic magnetism transition was observed. The result is discussed based on the competitive contributions from the external field induced uniaxial anisotropy and the topology-induced shape anisotropy superimposed by the stray fields from the pore channels. Magnetoresistance showed corresponding variations with increasing pore sizes, as evidenced by a magnetoresistance variation from typically anisotropic to nearly isotropic behaviour. When deposited on large-pored AAO membranes, the antidot arrays showed no obvious anisotropy at different film thicknesses. It led to negligible magnetoresistive loops in the thick films of high structural continuity. The possible reasons for spin-independent electron scatterings are discussed.
    No preview · Article · Feb 2007 · Journal of Physics D Applied Physics
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    ABSTRACT: We report a non-volatile, write-once-read-many times (WORM) memory device based on a simple organic-inorganic heterojunction. The organic film used is 9, 9-dihexylfluorene and Eu-complexed benzoate (PF6Eu (DBM)), which contains both electron-donor (9, 9-dihexylfluorene) and electron-acceptor (europium complex) groups. The inorganic n-type silicon substrate is used as the bottom electrode, while the Al is used as the top electrode. Under current-voltage testing, the device is able to switch from one initial non-conducting state (OFF) to a conducting state (ON) once a threshold voltage is reached under the first positive sweep. The “OFF” state is not recoverable with subsequent negative sweep after the device is turned “ON”. The ON/OFF current ratio is around 4×104. Diode rectifying characteristics is also observed for the turned-on device with a current ratio of 7×104, which is essential to address one memory cell in large passive matrix circuits. Reliability test is carried out and the device is able to sustain its “ON” state for at least 12 hours without any external bias.
    No preview · Article · Jan 2006 · MRS Online Proceeding Library
  • W.C. Wang · K.G. Neoh · E.T. Kang · S.L. Lim · D Yuan
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    ABSTRACT: Photo-induced reduction of gold and platinum metal salt solutions was carried out using viologen graft copolymerized on low-density polyethylene (LDPE) films and viologen-containing poly(vinylidene fluoride) (PVDF-PVBV) microporous membranes. The effects of the UV irradiation time and concentration of the metal salt solutions on the metal ion reduction process and the resultant metal deposition on the polymeric substrates were investigated. The metal-polymer composites were characterized using X-ray photoelectron spectroscopy (XPS), scanning and transmission electron microscopy (SEM and TEM), atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and UV-visible absorption spectroscopy. The amount of metal uptake, the state of the metal, and the size of the metal particles were found to be strongly dependent on the UV irradiation time and the type and concentration of the metal salt solution. The microporous structure and the high viologen content of the PVDF-PVBV membrane constitute an effective matrix for metal ion reduction and preparation of metal nanoparticles.
    No preview · Article · Dec 2004 · Journal of Colloid and Interface Science
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    ABSTRACT: Molecular modification of a fluorinated polyimide (FPI), 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride + 4,4‘-bis(4-aminophenoxy)diphenyl sulfone, via ozone pretreatment and thermally induced graft copolymerization with N-isopropylacrylamide (NIPAAm) in a N-methyl-2-pyrrolidone solution, was carried out. The resulting FPIs with grafted NIPAAm polymer side chains [FPI-g-P(NIPAAm) copolymers] were cast into microfiltration membranes by phase inversion in water at temperatures ranging from 4 to 55 °C. The surface composition of the membranes was characterized by X-ray photoelectron spectroscopy. The surface composition, mean pore size, and morphology of the membrane varied with the temperature of the aqueous coagulation bath. For the copolymer membrane cast below the lower critical solution temperature of the NIPAAm polymer (32 °C), the rate of water permeation increased substantially at a permeate temperature above 32 °C. For the flux of 2-propanol through the membrane cast above 32 °C, a reversed permeate temperature dependence was observed.
    No preview · Article · Jul 2003 · Industrial & Engineering Chemistry Research
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    ABSTRACT: High-resolution X-ray photoelectron spectroscopic (XPS) measurements of the various intrinsic redox states of polyaniline (PANI), using a monochromatized Al—Kα source, were carried out. The presence of the imine, amine and positively charged nitrogen species corresponding to a particular intrinsic redox state and protonation level of the polymer was resolved quantitatively and unambiguously. The result confirmed the peak assignments of former XPS core-level studies using the lower resolution non-monochromatized Mg—Kα X-ray source. Thus, the high-resolution XPS using a monochromatized Al—Kα X-ray source is a truly unique tool for the convenient and quantitative analysis of the various intrinsic redox states of PANI.
    No preview · Article · Dec 2000 · Colloid and Polymer Science
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    S.Y Xu · X.J Huang · C.K Ong · S.L Lim · Y.L Chang · Z Yang · Z.W Li · H.B Nie
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    ABSTRACT: Yttria-stabilized zirconia (YSZ) substrates were used to fabricate Nd–Fe–B thin films from a Nd13.5Fe80.0B6.5 target by pulsed-laser deposition at temperature of 500–690°C and under varied ambient Ar pressure. Grown on (1 0 0) YSZ, the main phase in as-deposited Nd–Fe–B films is tetragonal Nd2Fe14B, which tends to orient its c-axis perpendicular to the substrate surface. YSZ is found to be chemically compatible with Nd–Fe–B film, yielding a sharp film/substrate interface and a short inter-diffusion length (30–40 nm) of Y and Zr in the film. Large fluctuations of magnetic properties in terms of 4πMs (0.3–1.5 T), Mr/Ms ratio (0.4–0.7) and Hc (200–1050 Oe) associated with fluctuation in surface morphology and surface particle density were observed, indicating that magnetic properties of the films depend greatly on their microstructure rather than chemical composition. YSZ substrate appears to be a suitable candidate for the study of structure dependence on the magnetic property of Nd–Fe–B films. Optimization of the fabrication parameters for Nd–Fe–B film on YSZ may lead to novel applications, where YSZ serves as both diffusion barrier and lattice matching buffer on substrates such as Si.
    Full-text · Article · Dec 2000 · Journal of Magnetism and Magnetic Materials
  • S. L. Lim · K. L. Tan · E. T. Kang · W. S. Chin
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    ABSTRACT: Comparative ab initio restricted Hartree–Fock (RHF) and density functional theory (DFT) calculations were performed to investigate the geometric and electronic structures of various neutral aniline oligomers. These oligomers were selected to model polyaniline (PANI) in different intrinsic oxidation states, with an aim to study the applicability and extendibility of the theoretical methods to conjugated polymers. In general, we found that DFT calculations produce results that are in good agreement with observations from x-ray diffraction, ultraviolet-visible absorption, ultraviolet photoelectron and x-ray photoelectron spectroscopy. The DFT method has reproduced well the ∼4.0 eVπ–π&ast; transition of leucoemeraldine base and the ∼2.0 eV Peierls gap transition of pernigraniline base. The valence band structure and the ∼1.2 eV energy separation of nitrogen core levels of emeraldine base are also correctly predicted. On the other hand, large discrepancies with experimental measurements are predicted by the RHF method. Single-point MP2 calculations show that the DFT-optimized structures are all at lower energy than the RHF-optimized ones.
    No preview · Article · Jun 2000 · The Journal of Chemical Physics
  • Z. H. Ma · S. L. Lim · K. L. Tan · S. Li · E. T. Kang
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    ABSTRACT: X-ray photoelectron spectroscopy was employed for the in situ study of interactions between thermally evaporated Mg atoms and electroactive polyaniline (PANI) films of various intrinsic oxidation states. Quantitative changes in the N1s core-level spectra and the Mg2p core-level spectra, as well as the changes in surface chemical stoichiometry of these films throughout the Mg evaporation process were carefully monitored. Although the nitrogen sites appeared to be more attractive to the in-coming Mg atoms and the emeraldine base (EB) and nigraniline base (NA) films underwent an apparent decrease in intrinsic oxidation state ([=N-]/[-NH-] ratio) as a result of Mg evaporation, there is no direct interaction between the two species. On the other hand, the adsorbed oxygen from the bulk of the polymer film played a dominant role in the interfacial interactions between the polymer and the metal. Characterization of the delaminated indium-tin-oxide (ITO) and polymer surfaces from EB film and its camphorsulfonic acid-doped counterpart cast on ITO substrates had also been carried out to evaluate the interaction of PANI with the high work function metal.
    No preview · Article · May 2000 · Journal of Materials Science Materials in Electronics
  • Z. H. Ma · S. L. Lim · K.L. Tan · S. Li · E.T. Kang
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    ABSTRACT: X-ray photoelectron spectroscopy was employed for an in situ study of the interactions between thermally evaporated magnesium atoms and polyaniline films of various intrinsic oxidation states. Quantitative changes in the N1s core-level spectra, the Mg2p core-level spectra and the surface chemical stoichiometries of these films throughout the magnesium evaporation process were carefully monitored. Although the nitrogen sites appeared to be more attractive to the incoming magnesium atoms, and the emeraldine and nigraniline films underwent an apparent decrease in the intrinsic oxidation state ([N]/[NH] ratio) as a result of magnesium evaporation, there is no direct interaction between the two species. On the other hand, the adsorbed oxygen from the bulk of the polymer film played a dominant role in the interfacial interactions between the polymer and the metal.
    No preview · Article · May 2000 · Surface Science
  • S J Wang · SY Xu · L P You · S L Lim · C K Ong
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    ABSTRACT: We demonstrated a new approach to fabricate high quality superconducting YBa2 Cu3 O7- (YBCO) thin films on natively oxidized Si(100) wafers with an yttria-stabilized zirconia (YSZ) buffer layer. Perfect YSZ thin films were deposited on the Si wafer under ambient pressure of 10-7 mbar at the initial stage followed by 10-4 mbar, yielding a YSZ film with a root mean square roughness of only 0.3-0.5 nm. Clear and sharp interfaces were observed between YSZ and Si, and YBCO and YSZ. No obvious amorphous SiOx layer or BaZrOx layer were observed at these two interfaces, respectively. The missing SiOx layer was believed to be a result of the special deposition process. The epitaxial correlations of the YBCO/YSZ/Si multilayer were found to be YBCO(001) YSZ(100) Si(100) and YBCO[100] YSZ[011] Si[011]. The diffusion length of Si into the YSZ layer was only 40 nm. The perfect crystalline structure of the films as well as the absence of diffused Si in the YBCO layer resulted in a transition temperature Tc (0) as high as 87 K in the YBCO/YSZ/Si multilayers. The method may also be applied in the fabrication of other perovskite thin films on native Si wafers.
    No preview · Article · Mar 2000 · Superconductor Science and Technology
  • H. C. Fang · C. K. Ong · SY Xu · K. L. Tan · S. L. Lim · Y. Li · J. M. Liu
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    ABSTRACT: Barium hexaferrite (BaM) thin films were deposited on (0001) LiTaO3 substrates by pulsed laser deposition. Effects of the substrate temperature and oxygen gas pressure on the formation and quality of these films were studied. Films deposited at a substrate temperature of 800 °C and an oxygen pressure around 0.23 mbar showed the best c axis normal to the film plane with locked in-plane orientation. The saturation magnetization Ms and anisotropy field Ha measured by vibrating sample magnetometer were almost the same as those reported on bulk barium ferrite. Decreasing oxygen pressure hinders the formation of the Ba layer in BaM magnetoplumbite structure and gives rise to the spinel phase, which greatly decreases coercivity Hc of the films and finally destroys the whole BaM structure. Effects of the lattice mismatch and substrate-induced strains on the film structure were also studied. It was found that barium ferrite thin films grown on LiTaO3 substrates tend to choose a matching mode with compressional strains rather than shear strains. © 1999 American Institute of Physics.
    No preview · Article · Aug 1999 · Journal of Applied Physics
  • S. L. Lim · K. L. Tan
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    ABSTRACT: X-ray photoelectron spectroscopy was employed for the in situ study of the interactions between thermally evaporated copper atoms and neutral polyaniline (PANI) films of various intrinsic oxidation states ([N−]/[−N−] ratios), as well as with the HClO4-protonated PANI film. Quantitative changes in the N 1s core-level spectra, evolution of the Cu 2p core-level signals and the Cu LMM Auger line shapes, and angle-resolved changes in surface stoichiometry of these films with the progressive deposition of Cu atoms were carefully monitored. For the neutral films, the incoming Cu atoms appear to affect predominantly the nitrogen sites, resulting in a decrease in the intrinsic oxidation state of the polymer. Nevertheless, the effect was observed to be rather weak, possibly through a slight disruption of the π-electron conjugation of the polymer backbone. However, for the protonated film, interactions with incoming copper atoms resulted in the decomposition of the perchlorate dopant and the formation of copper chloride species. Diffusion of Cu atoms into the bulk of the polymer was observed for all types of films studied. The interactions of the Cu atoms with the PANI surfaces were also compared with those of the Al atoms.
    No preview · Article · Aug 1998 · Langmuir
  • S. L. Lim · K. L. Tan · E. T. Kang
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    ABSTRACT: Aluminum was thermally deposited onto H2SO4-protonated, HClO4-protonated and neutral emeraldine (EM) base films. The interfacial interactions of Al atoms with these EM films were studied in situ by X-ray photoelectron spectroscopy (XPS). The Al atoms interact preferentially with the dopant anions in the cases of the protonated EM films, and with the nitrogen atoms of the conjugated polymer backbone in the case of the neutral EM film. For the H2SO4-protonated EM film, interactions with Al resulted in the formation of aluminum sulfate species, whereas, for the HClO4-protonated film, the reaction resulted in the decomposition of the perchlorate dopant and the formation of aluminum chloride species. For the neutral EM film, the incoming Al resulted in a decrease in the intrinsic oxidation state of the polymer. The migration of bulk-adsorbed oxygen to the surface in response to Al deposition was observed in all of the EM films, and the formation of aluminum oxides, such as Al2O3, suggests that the adsorbed oxygen plays an important role in the interface formation.
    No preview · Article · Feb 1998 · Synthetic Metals
  • S. L. Lim · K. L. Tan · E. T. Kang
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    ABSTRACT: X-ray photoelectron spectroscopy was employed for the in situ study of the interactions between thermally deposited Al atoms and polyaniline films of different intrinsic oxidation states. Quantitative changes in the N 1s core-level line shape, the evolution of the Al 2p core-level spectra, and the changes in surface stoichiometry of these films throughout the Al deposition process were carefully tracked. Although the nitrogen sites appear to be more attractive to the incoming Al atoms and the emeraldine and nigraniline films undergo an apparent decrease in intrinsic oxidation state ([=N–]/[–NH–] ratio) as a result of Al deposition, there is no direct interaction between the two species, and the adsorbed oxygen from the bulk of the polymer film plays a dominant role in the interfacial interactions between the polymer and the metal. © 1998 American Vacuum Society.
    No preview · Article · Jan 1998 · Journal of Vacuum Science & Technology A Vacuum Surfaces and Films