Showing papers by "Kok Wai Cheah published in 2002"

Synthesis and near-infrared luminescence of 3d-4f bi-metallic Schiff base complexes

Abstract: The reaction of the zinc(II) Schiff base complex ZnL [H2L=N,N′-bis(3-methoxysalicylidene)ethylene-1,2-diamine] with one equivalent of Ln(NO3)3·xH2O (Ln=Nd, Ho, Er or Yb) gives the neutral 3d-4f bi-metallic Schiff base complexes [Zn(NO3)(μ-L)Ln(NO3)2(H2O)], which in solution exhibit emission corresponding to the Ln(III) ions (Ln=Nd, Er and Yb) in the near-infrared region.

Synthesis, structures and optical spectroscopy of photoluminescent platinum-linked poly(silylacetylenes)

Abstract: Synthetic routes to a series of novel oligomeric and polymeric platinum(II)-containing silylacetylenes have been developed. The CuI-catalyzed reaction of trans-[Pt(PEt3)2PhCl] with the alkynylsilane Ph2Si(CCH)2, in a basic medium, afforded the mononuclear trans-[Ph(Et3P)2PtCCSiPh2CCH] (1) and the dinuclear trans-[Ph(Et3P)2PtCCSiPh2CCPt(PEt3)2Ph] (2). Complex 1 can be utilized as a key starting material for access to the triplatinum(II) system trans-[Ph(Et3P)2PtCCSiPh2CCPt(PBu3)2CCSiPh2CCPt(PEt3)2Ph] (3). Attempted oxidative coupling of 1 only led to the isolation of the triacetylenic species trans-[Ph(Et3P)2PtCCCCCCPt(PEt3)2Ph] (5), involving cleavage of Si–C(alkyne) bonds. Soluble and thermally stable platinum(II) poly-yne silane polymer trans-[–Pt(PBu3)2CCSiPh2CC–]n (4) was prepared in good yield by CuI-catalyzed polymerization of trans-[Pt(PBu3)2Cl2] and Ph2Si(CCH)2. The single-crystal X-ray structural analyses and DFT calculations were performed on 1, 2 and 5. We report the optical absorption and photoluminescence spectra of these new metal-based oligomeric and polymeric silylacetylenes and the results are compared with platinum(II) poly-ynes with purely acetylenic and (hetero)aromatic conjugated units.

Synthesis, crystal structures, luminescence and magnetic properties of lanthanide complexes containing the 1,8-bis(2-hydroxybenzamido)-3,6-dioxaoctane ligand

Abstract: The reaction of 1,8-bis(2-hydroxybenzamido)-3,6-dioxaoctane (H2L) with lanthanide nitrates in MeCN afforded new binuclear lanthanide complexes (Ln = La–Nd, Sm–Yb or Y, 1–13) in high yields, and the molecular structures of 1 and 2 were established by X-ray crystallography A photophysical study of the ligand H2L and of the binuclear complexes (Ln = Nd, Sm, Eu, Tb and Dy) has been performed and the quenching effect of coordinated water molecules was observed The H2L and Tb complex exhibit an up-conversion process under OPO excitation at 660 nm The magnetic properties of Pr and Gd complexes are also reported

Structure and electronic properties of SiO2/Si multilayer superlattices: Si K edge and L3,2 edge x-ray absorption fine structure study

Abstract: We report an x-ray absorption fine structure study at the Si K and L3,2 edges of a series of Si/SiO2 superlattices (SL). The SL system comprises four periods of elemental silicon with a spacing of 1, 1.4, 2.2, and 2.6 nm sandwiched by a 1.5 nm silicon oxide and capped by a 3 nm silicon oxide layer. These systems exhibit electroluminescence and photoluminescence. X-ray absorption near edge structure (XANES) at both the Si K and L3,2 edge confirms that the Si layers are amorphous. Polarization dependent measurement at the Si K edge reveals that a distinct Si/SiO2 interface exists with strong Si–O bonding oriented preferentially closer to the surface normal. High resolution XANES at the Si L3,2 edge shows a noticeable blueshift of the edge threshold as the lattice spacing decreases, in good accord with quantum confinement. The results and their implications for the origin (quantum confinement and interface/oxide defects) of luminescence in these superlattice systems are discussed.

Gallium vacancy and the residual acceptor in undoped GaSb studied by positron lifetime spectroscopy and photoluminescence

Abstract: Positron lifetime, photoluminescence (PL), and Hall measurements were performed to study undoped p-type gallium antimonide materials. A 314 ps positron lifetime component was attributed to Ga vacancy (V-Ga) related defect. Isochronal annealing studies showed at 300 degreesC annealing, the 314 ps positron lifetime component and the two observed PL signals (777 and 797 meV) disappeared, which gave clear and strong evidence for their correlation. However, the hole concentration (similar to2x10(17) cm(-3)) was observed to be independent of the annealing temperature. Although the residual acceptor is generally related to the V-Ga defect, at least for cases with annealing temperatures above 300 degreesC, V-Ga is not the acceptor responsible for the p-type conduction. (C) 2002 American Institute of Physics.

Synthesis of oxygen-free nanosized InN by pulse discharge

Abstract: Oxygen-free nanosized InN was synthesized by pulse discharge. NH3/N2 mixture and bulk indium were used as nitrogen source and discharge cathode, respectively. Optical emission spectra show that N2 and NH3 were highly dissociated in the discharge and oxygen could be eliminated effectively. A comparison was made between samples produced in the discharge of N2, NH3 and N2/NH3, respectively. XPS was used to determine the composition of the synthesized materials. Only In-N was produced when N2/NH3 was used as the discharge gas, whereas indium oxide was found in the case using N2 or NH3 as the discharge gas. The size of the produced InN was between 5 and 200 nm. A broad band with a blue shift of 150 nm from that of bulk InN was detected in the photoluminescence of the produced nanosized InN.

Synthesis of Nano-ZnS Modified Porous Si with White Light Photoluminescence Properties

Abstract: Porous silicon (PS) was modified with nano-ZnS using a chemical method. Scanning electron microscopy (SEM) images show that after the modification, smaller nano-ZnS particles 10–50 nm in diameter appear in the PS host. The formation of nano-ZnS in PS was also confirmed by energy dispersive spectroscopy (EDS). The EDS results revealed that the same ratio of Zn and S atoms existed in PS after the deposition of ZnS. Photoluminescence spectra of nano-ZnS/PS structures exhibited a distinct shift of emission from red to blue with increasing ZnS coverage. With appropriate doping, it is possible to obtain white light photoluminescence from nano-ZnS/PS.

[Lanthanide ion luminescence probe: low temperature phase transition of EuS4N complex].

Abstract: Fluorescence spectrum of EuS4N complex at low temperature is very different from that at room temperature When temperature changes, shapes of both excitation and emission spectra change dramatically at about 160 K, which is taken as an indicator of the structural change of EuS4N Peak splitting of 5D0-->7F0 also indicates that there are two different types of coordination structure for Eu3+ ions at low temperature, while only one at room temperature Thermal effect for the structure transition is also deduced from the relation between fluorescence intensity and temperature changing manner These results were in good accordance with directly measurement on sample temperature The reason for the structure change is also discussed from the structure of the EuS4N complex This study proved that lanthanide luminescent probe, as a complementarity to X-ray diffraction technique, should be an effective tool in low temperature crystal structure study

Influence of Different Atmospheres on the Life Time of Porous Silicon Light-Emitting Devices

Abstract: In present report, we investigated the degradation processes in porous silicon light-emitting devices (LED) in different atmospheres (O2, N2, air and vacuum) by photoluminescence (PL), electroluminescence (EL), lifetime (LT) and I-V characteristic measurements as well as by Energy Dispersive X-ray Spectroscopy (EDS). The contacts were made by evaporation of Au and Au/Cu alloy. The LEDs emit in visible range at forward and reverse bias. As a rule, full width at half maximum of EL spectrum is wider than that of PL spectrum. The bias direction of applied voltage during degradation change EL, PL, I-V characteristics, and LT of the LEDs. At forward bias, LT degradation is less than that in reverse bias. The degradation of LEDs during forward bias did not produce any change in the spectral shape of EL and PL. At reverse bias, degradation led to red shift in the peak of EL and PL. The results show that the lifetime of LEDs with Au contact is longer than Au-Cu. Operating in different atmospheres, the LT in vacuum is longest and is more than 100 hours in reverse bias at room temperature. Possible mechanisms of degradation of LEDs are discussed. It is proposed that degradation is connected mainly with two processes: oxidation and metal diffusion. It is shown that the oxygen and metal in ionic state can diffuse quickly. Hence, in forward bias, the diffusion of metal would dominate, and in reverse bias, diffusion of oxygen dominates.