Kian Ping Loh

National University of Singapore, Tumasik, Singapore

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Publications (313)1873.35 Total impact

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    ABSTRACT: We present evidence of a drastic renormalization of the optical conductivity of graphene on SrTiO3 resulting in almost full transparency in the ultraviolet region. These findings are attributed to resonant excitonic effects further supported by ab-initio Bethe-Salpeter equation and density functional theory calculations. The (\pi,\pi*)-orbitals of graphene and Ti-3d t2g orbitals of SrTiO3 are strongly hybridized and the interactions of electron-hole states residing in those orbitals play dominant role in the graphene optical conductivity. These interactions are present much below the optical band gap of bulk SrTiO3. These results open a possibility of manipulating interaction strengths in graphene via \textit {d}-orbitals which could be crucial for optical applications.
    Physical Review B 01/2015; 91(3):035424. · 3.66 Impact Factor
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    ABSTRACT: Surface modification or decoration of ultrathin MoS2 films with chemical moieties is appealing since nanointerfacing can functionalize MoS2 films with bonus potentials. In this work, a facile and effective method for microlandscaping of Au nanoparticles (NPs) on few-layer MoS2 films is developed. This approach first employs a focused laser beam to premodify the MoS2 films to achieve active surface domains with unbound sulfur. When the activated surface is subsequently immersed in AuCl3 solution, Au NPs are found to preferentially decorate onto the modified regions. As a result, Au NPs can be selectively and locally anchored onto designated regions on the MoS2 surface. With a scanning laser beam, microlandscapes comprising of Au NPs decorated on laser-defined micropatterns are constructed. By varying the laser power, reaction time and thickness of the MoS2 films, the size and density of the NPs can be controlled. The resulting hybrid materials are demonstrated as efficient Raman active surfaces for the detection of aromatic molecules with high sensitivity.
    Small 01/2015; · 7.51 Impact Factor
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    ABSTRACT: Porous graphene oxide can be used as a metal free catalyst in the presence of air for the oxidative coupling of primary amines. Herein, we explore GO-catalysed carbon-carbon or/and carbon-heteroatom bond formation strategy to functionalize primary amines in tandem to produce a series of valuable products, i. e. α-aminophosphonates, α-aminonitriles and polycyclic hetero compounds. Furthermore, when decorated with nano-Pd, the Pd-coated porous graphene oxide can be used as a bifunctional catalyst for tandem oxidation and hydrogenation reactions in the N-alkylation of primary amines, achieving good to excellent yields under mild conditions.
    Journal of the American Chemical Society 12/2014; · 11.44 Impact Factor
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    ABSTRACT: The photoelectrochemical splitting of water has attracted much attention due to its potential application for the conversion of solar energy into hydrogen fuel. Herein, we report the synthesis of a perylene derivative (perylene tetracarboxylic di-(propyl imidazole), abbreviated as PDI) which is coordinated with Co (II) ions to form a co-ordination polymer [PDI-Co(Cl)2(H2O)2]n (abbreviated as PDI-Co). The PDI-Co complex combines the photoactivity of the perylene dye with the electrocatalytic activity of the "Co(II)" centre for photoelectrochemical hydrogen evolution reaction (HER). To improve charge transfer interactions, the PDI-Co complex is immobilized on reduced graphene oxide (rGO) via non-covalent interactions to form the rGO-PDI-Co complex. The composite shows good performance in multiple cycle testing and the turnover number (TON vs CoII) of this hybrid material for HER (754 after 5 hrs) is considerably higher than previously reported dye-sensitized cobalt-based catalysts.
    ACS Applied Materials & Interfaces 12/2014; · 5.90 Impact Factor
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    ABSTRACT: Layer-by-layer-stacked chemical vapour deposition (CVD) graphene films find applications as transparent and conductive electrodes in solar cells, organic light-emitting diodes and touch panels. Common to lamellar-type systems with anisotropic electron delocalization, the plane-to-plane (vertical) conductivity in such systems is several orders lower than its in-plane conductivity. The poor electronic coupling between the planes is due to the presence of transfer process organic residues and trapped air pocket in wrinkles. Here we show the plane-to-plane tunnelling conductivity of stacked CVD graphene layers can be improved significantly by inserting 1-pyrenebutyric acid N-hydroxysuccinimide ester between the graphene layers. The six orders of magnitude increase in plane-to-plane conductivity is due to hole doping, orbital hybridization, planarization and the exclusion of polymer residues. Our results highlight the importance of interfacial modification for enhancing the performance of LBL-stacked CVD graphene films, which should be applicable to other types of stacked two-dimensional films.
    Nature Communications 11/2014; 5:5461. · 10.74 Impact Factor
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    ABSTRACT: Diffusion barriers prevent materials from intermixing in electronic devices. Most diffusion barrier materials are specific for a certain combination of materials and/or change the energetics of the interface because they are insulating or add to the contact resistances. This paper presents graphene (Gr) as an electronically transparent diffusion barrier in metal/semiconductor devices, where Gr prevents Au and Cu from diffusion into the Si, and unintentionally dope the Si. We studied the electronic properties of the n-Si(111)/Gr/M Schottky barriers (with and without Gr and M = Au or Cu) by I(V) measurements and at the nano-scale by ballistic electron emission spectroscopy (BEEM). The layer of Gr does not change the Schottky barrier of these junctions. The Gr barrier was stable at 300 °C for 1 h and prevented the diffusion of Cu into n-Si(111) and silicide formation. Thus, we conclude that the Gr is mechanically and chemically stable enough to withstand the harsh fabrication methods typically encountered in clean room processes (e.g., deposition of metals in high vacuum conditions at high temperatures), it is electronically transparent (it does not change the energetics of the Si/Au or Si/Cu Schottky barriers), and effectively prevented diffusion of the Cu or Au into the Si.
    ACS Applied Materials & Interfaces 10/2014; · 5.90 Impact Factor
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    ABSTRACT: It is important to understand the structure of redox-active self-assembled monolayers (SAMs) down to the atomic scale since these SAMS are widely used as model systems in studies of mechanisms of charge transport or to realize electronic functionality in molecular electronic devices. We studied the supramolecular structure of SAMs of n-alkanethiolates with ferrocenyl (Fc) end groups (S(CH2)nFc, n = 3 or 4) on Au(111) by scanning tunneling microscopy (STM). In this system, the tilt angle of the Fc units with respect to the surface normal (α) depends on the value of n because the Au-S-C bond angle is fixed. The ordered domains of the SAMs were imaged by STM after annealing at 70 °C at ultra-high vacuum conditions. High resolution electron energy loss spectroscopy (HREELS) and cyclic voltammetry (CV) show that this annealing step only removed physisorbed material and did not affect the structure of the SAM. The STM images revealed the presence of row defects at intervals of 4 nm, i.e., six molecules. We determined by near edge X-ray absorption fine structure spectroscopy (NEXAFS) that the Fc units of the SAMs of SC3Fc are more parallel to the Au(111) plane with a tilt angle α = 60.2 ° than the Fc units of SC4Fc SAMs (α = 45.4 °). These tilt angles are remarkably close to the tilt angles measured by X-ray diffraction data of bulk crystals (bc-plane). Based on our data, we conclude that the molecules are standing up and the SAMs pack into lattices that are distorted from their bulk crystal structures (because of the build-up stain due to the differences in size between the Fc units and thiolate anchoring groups).
    Langmuir 10/2014; · 4.38 Impact Factor
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    ABSTRACT: Graphene-based nanomaterials are increasingly being explored for use as biomaterials for drug delivery and tissue engineering applications due to their exceptional physicochemical and mechanical properties. However, the two-dimensional nature of graphene makes it difficult to extend its applications beyond planar tissue culture. Here, graphene–cell biocomposites are used to pre-concentrate growth factors for chondrogenic differentiation. Bone marrow-derived mesenchymal stem cells (MSCs) are assembled with graphene flakes in the solution to form graphene-cell biocomposites. Increasing concentrations of graphene (G) and porous graphene oxide (pGO) are found to correlate positively with the extent of differentiation. However, beyond a certain concentration, especially in the case of graphene oxide, it will lead to decreased chondrogenesis due to increased diffusional barrier and cytotoxic effects. Nevertheless, these findings indicate that both G and pGO could serve as effective pre-concentration platforms for the construction of tissue-engineered cartilage and suspension-based cultures in vitro.
    Small 10/2014; · 7.51 Impact Factor
  • Physical Review B 09/2014; 90(11). · 3.66 Impact Factor
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    ABSTRACT: Heteroepitaxy of two-dimensional (2D) crystals, such as hexagonal boron nitride (BN) on graphene (G), can occur at the edge of an existing heterointerface. Understanding strain relaxation at such 2D laterally fused interface is useful in fabricating hetero-interfaces with a high degree of atomic coherency and structural stability. We use in-situ scanning tunneling microscopy to study the 2D heteroepitaxy of BN on graphene edges on a Ru(0001) surface, with the aim of understanding the propagation of interfacial strain. We found that defect-free, pseudomorphic growth of BN on a graphene edge "substrate" occurs only for a short distance (< 1.29 nm) perpendicular to the interface, beyond which misfit zero-dimensional dislocations occur to reduce the elastic strain energy. Boundary states originating from a coherent zigzag-linked G/BN boundary is observed to greatly enhance the local conductivity, thus affording a new avenue to construct one-dimensional transport channels in G-BN hybrid interface.
    Nano Letters 08/2014; · 12.94 Impact Factor
  • Bao Yang, Hai Xu, Jiong Lu, Kian Ping Loh
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    ABSTRACT: Grain boundaries consisting of dislocation cores arranged in a periodic manner have well-defined structures and peculiar properties and can be potentially applied as conducting circuits, plasmon reflectors and phase retarders. Pentagon-heptagon (5-7) pairs or pentagon-octagon-pentagon (5-8-5) carbon rings are known to exist in graphene grain boundaries. However, there are few systematic experimental studies on the formation, structure and distribution of periodic grain boundaries in graphene. Herein, scanning tunneling microscopy (STM) was applied to study periodic grain boundaries in monolayer graphene grown on a weakly interacting Cu(111) crystal. The periodic grain boundaries are formed after the thermal reconstruction of aperiodic boundaries, its structure agree well with the prediction of the coincident-site-lattice (CSL) theory. Periodic grain boundaries in quasi-freestanding graphene give sharp LDOS peaks in the tunneling spectra as opposed to the broad peaks of the aperiodic boundaries. This suggests that grain boundaries with high structural quality can introduce well-defined electronic states in graphene and modify its electronic properties.
    Journal of the American Chemical Society 08/2014; · 11.44 Impact Factor
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    ABSTRACT: A mono- to multilayer thick MoS2 film has been grown by using the atomic layer deposition (ALD) technique at 300 °C on a sapphire wafer. ALD provides precise control of the MoS2 film thickness due to pulsed introduction of the reactants and self-limiting reactions of MoCl5 and H2S. A post-deposition annealing of the ALD-deposited monolayer film improves the crystallinity of the film, which is evident from the presence of triangle-shaped crystals that exhibit strong photoluminescence in the visible range.
    Nanoscale 07/2014; · 6.74 Impact Factor
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    ABSTRACT: This study describes that the current rectification ratio, (R ≡ |J|(-2.0 V)/|J|(+2.0 V) for supramolecular tunneling junctions with a top-electrode of eutectic gallium indium (EGaIn) that contains a thin (0.7 nm) supporting outer oxide layer (Ga2O3), increases by up to four orders of magnitude when voltage pulses of > +1.0 V up to +2.5 V are applied for a period of time of up to one hour. The increase in R is caused by the presence of water and ions in the supramolecular assemblies which react with the Ga2O3/EGaIn layer and increase the thickness of the Ga2O3 layer. This increase in the oxide thickness from 0.7 nm to ~2.0 nm changed the nature of the contact monolayer—top-electrode contact from ohmic to non-ohmic contact. These results unambiguously expose the experimental conditions that allow for a safe bias window of ±1.0 V (the range of biases normally studies of charge transport using this technique are conducted) to investigate molecular effects in molecular electronic junctions with Ga2O3/EGaIn top-electrodes where electrochemical reactions are not significant. Our findings also show that the interpretation of data in studies involving applied biases of >1.0 V may be complicated by electrochemical side reactions which can be recognized by changes of the electrical characteristics as a function voltage cycling or in current retention experiments.
    Nanoscale 07/2014; · 6.74 Impact Factor
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    ABSTRACT: Functionalized graphene oxide derivatives are found to be efficient and reusable carbocatalysts for the one-pot multicomponent Strecker reaction of aldehydes and ketones under neat and open-air conditions.
    ChemCatChem 07/2014; · 5.18 Impact Factor
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    ABSTRACT: In this work, we study the light transmission through multilayer graphene sheets that are spatially separated. By adopting the transfer matrix method modified for two-dimensional material, we find that the graphene multilayers manifest a broad passband in the transmission spectrum. The two edges of the passband are determined by the interband threshold of electron transition in graphene and the cutoff wavelength of Bloch mode in the multilayer structure. By varying the interlayer space of the graphene multilayers, the structure can be used to increase the phase velocity of normally incident light or to make the phase at rest like epsilon-near-zero material. The study shows that the graphene multilayers may find great applications in optical switches, filters, and phase modulators.
    Journal of Applied Physics 06/2014; 115(21):213102-213102-8. · 2.19 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; · 12.03 Impact Factor
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    ABSTRACT: Highly porous, N-doped graphene foam is synthesized by chemical vapour deposition process on nickel foam. The voids of the graphene foam can be filled with curved graphene sheets by impregnating the nickel foam template with micron-sized nickel powder. Subsequent etching of nickel produces a graphene "eggshells"-in-graphene foam structure. The reversible capacity of such graphene foam when used as anode in lithium ion battery is improved by the presence of graphene "eggshells", as compared to the unfilled foam. The improvement is attributed to the higher rate of lithium diffusion, better buffering of strain associated with lithiation/delithiation and higher volumetric energy density of the unique eggshell-in-graphene foam structure.
    ACS Applied Materials & Interfaces 05/2014; · 5.90 Impact Factor
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    ABSTRACT: Graphene exhibits extraordinary electronic and mechanical properties, and extremely high thermal conductivity. Being a very stable atomically thick membrane that can be suspended between two leads, graphene provides a perfect test platform for studying thermal conductivity in two-dimensional systems, which is of primary importance for phonon transport in low-dimensional materials. Here we report experimental measurements and non-equilibrium molecular dynamics simulations of thermal conduction in suspended single-layer graphene as a function of both temperature and sample length. Interestingly and in contrast to bulk materials, at 300 K, thermal conductivity keeps increasing and remains logarithmically divergent with sample length even for sample lengths much larger than the average phonon mean free path. This result is a consequence of the two-dimensional nature of phonons in graphene, and provides fundamental understanding of thermal transport in two-dimensional materials.
    Nature Communications 04/2014; 5:3689. · 10.74 Impact Factor
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    ABSTRACT: The nonlinear optical property of few-layered MoS<sub>2</sub> nanoplatelets synthesized by the hydrothermal exfoliation method was investigated from the visible to the near-infrared band using lasers. Both open-aperture Z-scan and balanced-detector measurement techniques were used to demonstrate the broadband saturable absorption property of few-layered MoS<sub>2</sub>. To explore its potential applications in ultrafast photonics, we fabricated a passive mode locker for ytterbium-doped fibre laser by depositing few-layered MoS<sub>2</sub> onto the end facet of optical fiber by means of an optical trapping approach. Our laser experiment shows that few-layer MoS<sub>2</sub>-based mode locker allows for the generation of stable mode-locked laser pulse, centered at 1054.3 nm, with a 3-dB spectral bandwidth of 2.7 nm and a pulse duration of 800 ps. Our finding suggests that few-layered MoS<sub>2</sub> nanoplatelets can be useful nonlinear optical material for laser photonics devices, such as passive laser mode locker, Q-switcher, optical limiter, optical switcher and so on.
    Optics Express 03/2014; 22(6):7249-60. · 3.53 Impact Factor

Publication Stats

5k Citations
1,873.35 Total Impact Points

Institutions

  • 2006–2013
    • National University of Singapore
      • • Department of Chemistry
      • • NUS Nanoscience and Nanotechnology Initiative (NUSNNI)
      • • Department of Physics
      Tumasik, Singapore
    • Institute Of High Performance Computing
      Tumasik, Singapore
  • 2010
    • University of Kansas
      • Department of Physics and Astronomy
      Lawrence, KS, United States
  • 2004–2010
    • Nanjing University
      • Department of Chemistry
      Nan-ching, Jiangsu Sheng, China
  • 2003–2010
    • Nanyang Technological University
      • School of Electrical and Electronic Engineering
      Singapore, Singapore
  • 2002
    • National Institute for Materials Science
      Tsukuba, Ibaraki, Japan