Kwong Kau Tiong

Anisotropy of Photoluminescence in Layered Semiconductors ReS2 and ReS2:Au

The optical anisotropy properties of rhenium disulphide (ReS2) and Au-doped rhenium disulphide (ReS2:Au) have been investigated by polarization-dependent photoluminescence (PL). Because the excitonic transitions show strong polarization dependences in the near-infrared region, we used polarization-dependent PL measurements in the range between 0° and 180° to characterize the unique polarization property of ReS2 and ReS2:Au, and identify the origin of the excitonic transitions. It was observed that the variation in the amplitude of PL excitonic transitions with different polarization characteristics follows the Malus rule. In comparison with the undoped ReS2, the PL spectra of Au-doped sample show not only the main excitonic transitions near the direct band edge but also some extra transitions owing to the doping effects.

Piezoreflectance and Raman Characterization of Mo1−xWxS2 Layered Mixed Crystals

A systematic optical characterization of a series of Mo1-xWxS2 (0 ≤ x ≤ 1) layered mixed crystals grown by chemical vapour transport method were carried out by using piezoreflectance (PzR) and Raman scattering measurements. From a detailed lineshape fit of the PzR spectra over an energy range from 1.6 to 5.0 eV, the energies of the band-edge excitonic and higher lying interband transitions were determined accurately. The transition energies and their splittings vary smoothly with the tungsten composition x indicating that the nature of the band structure is similar for the Mo1-xWxS2 series compounds. The peaks of the two dominant first-order Raman-active modes, and , and several second-order bands are observed in the range of 250-450 cm-1. The peaks corresponding to mode show a one-mode type behavior, while the peaks of mode demonstrate two-mode type behavior for the entire series of Mo1-xWxS2 crystals. These behaviors were discussed on the basis of the lattice vibrational properties of 2H-MoS2 and 2H-WS2 compounds.

Interplay Between Cr Dopants and Vacancy Clustering in the Structural and Optical Properties of WSe2

Here, we analyze the effect of Cr doping on WSe2 crystals. The topology and the chemistry of the doped samples have been investigated by atom-resolved scanning transmission electron microscopy combined with electron energy loss spectroscopy. Cr (measured to have formal valence 3+) occupies W sites (formal valence 4+), indicating a possible hole doping. However, single or double Se vacancies cluster near Cr atoms, leading to an effective electron doping. These defects organization can be explained by the strong binding energy of the CrW-Vse complex obtained by density functional theory calculations. In highly Cr-doped samples, a local phase transition from the 2H to the to 1T phase is observed, which has been previously reported for other electron-doped transition-metal dichalcogenides. Cr-doped crystals suffer a compressive strain, resulting in an isotropic lattice contraction and an anisotropic optical bandgap energy shift (25 meV in-plane and 80 meV out-of-plane).

Composition Dependent Band Gaps of Single Crystal Cu2ZnSn(SxSe1-x)4 Solid Solutions

Single crystals of Cu2ZnSn(SxSe1-x)4 (CZTSSe) solid solutions have deen grown by chemical vapor transport technique using ICl3 as a transport agent. Analyzing the X-ray diffraction patterns reveal that the as-grown CZTSSe solid solutions are crystallized in kesterite structure and the lattice parameters are determined. The S contents of the obtained crystals are estimated by Vegard’s law. The composition dependent band gaps of CZTSSe solid solutions are studied by electrolyte electroreflectance (EER) techniques. The band gaps of CZTSSe are evaluated by a lineshape fit of the EER spectra and are found to increase almost linearly with the increase of S content.

Structural and Band-Edge Properties of Cu(AlxIn1-X)S2 (0≤x≤1) Series Chalcopyrite Semiconductors

We have demonstrated structural and electronic properties of a series solar energy crystals Cu(AlxIn1-x)S2 (0<=x<=1) by using measurement techniques of X-ray diffraction, polarized thermoreflectance (PTR), and X-ray photoelectron spectroscopy (XPS). Single crystals of Cu(AlxIn1-x)S2 (0<=x<=1) (0 and E ^ polarizations. The PTR spectra clearly showed that the energy value of D increases with the increase of Al content x in the Cu(AlxIn1-x)S2 (0<=x<=1) series due to the enhanced strain in the lattice. The composition-dependent crystal-field-splitting energies can be evaluated and determined to be D(x)= (10±2)+( 139±5)×x meV. Based on the experimental analyses, the crystal structure and valence-band structure of the Cu(AlxIn1-x)S2 (0<=x<=1) (0<=x<=1) series are thus realized.

Photoluminescence and photoreflectance characterization of ZnxCd1−xSe/MgSe multiple quantum wells

Photoluminescence (PL) and photoreflectance (PR) were used to characterize ZnxCd1−xSe/MgSe multiple quantum well (MQW) structures grown on InP substrates by molecular beam epitaxy for mid-infrared (IR) device applications. The PL spectra yielded information of the fundamental excitonic recombination and ZnxCd1−xSe cap/spacer band edge emission of the samples. The PR spectra revealed multitude of possible interband transitions in MQW structures. The ground state transitions were assigned by comparing with the PL emission signals taken from the same structures. A comprehensive analysis of the PR spectra led to the identification of various interband transitions. The intersubband transitions were then estimated and found to be in a good agreement with the previous report of Fourier-transform IR absorption measurements [Li et al., Appl. Phys. Lett. 92, 261104 (2008)]. The results demonstrate the potential of using PL and PR as nondestructive optical techniques for characterization of ZnxCd1−xSe/MgSe MQWs for ...

Rapid Synthesis and Characterization of CuInS2 Prepared Using Microwave-Assisted Heating

In this work, the CuInS2 nanoparticles are successfully synthesized by microwave-assisted heating technique and further calcined at 400 °C. The morphological, structural, and optical properties of the synthesized CuInS2 nanoparticles are investigated by scanning electron microscope (SEM), X-ray diffraction (XRD), Raman scattering, and transmittance measurement, respectively. The SEM image shows the clear particle shape of the calcined CuInS2 nanoparticles. After calcination treatment, the fundamental (112) peak of the XRD spectrum and a broad Raman peak mixed with chalcopyrite and CuAu structures support the improved crystallinity of the calcined CuInS2 nanoparticles.

Luminescence Mechanism of ZnWO4 Nanopowder Synthesized by Microwave-Assisted Heating

The luminescence investigations on the calcinated zinc tungstate nanopowder (ZnWO4 NP) synthesized by microwave-assisted synthesis are presented using photoluminescence (PL) and time-resolved photoluminescence (TRPL) analyses. The X-ray diffraction (XRD) patterns exhibit that the significant wolframite structure of ZnWO4 NP can be detected at calcination temperatures above 300 °C. The 12 K PL and TRPL results demonstrated that the deformation of WO6 octahedra is responsible for the low-energy side of PL spectra and dominate the red-shifted PL spectra with increasing calcination temperatures.

Deposition and Characterization of Nanostructural IrOx by RF Sputtering

Large surface area nanostructural IrOx films were deposited on stainless steel substrates by reactive radio frequency magnetron sputtering using Ir metal target. The structural and spectroscopic properties of the nanostructural IrOx were characterized. The micrographs of field emission scanning electron microscopy showed the formation of folded leaves with chiffon-like structure for the as-deposited samples. X-ray photoelectron spectroscopy analysis provided the information of the oxidation states and the stoichiometry of IrOxNL. Raman spectra revealed the amorphous-like phase of the as-deposited nanostructural IrOx. The chiffon-like structure provides ultra-high surface area for electrical charge storage which makes the IrOxNL as an attractive candidate for the supercapacitor application.

High-Temperature Optical Characterization of Transition Metal Dichalcogenides by Piezoreflectance Measurements

A systematic optical characterization of transition metal dichalcogenide layered crystals grown by chemical vapour transport method as well as of natural molybdenite were carried out by using piezoreflectance (PzR) measurements. From a detailed lineshape fit of the room-temperature PzR spectra over an energy range from 1.6 to 5.0 eV, the energies of the band-edge excitonic and higher lying interband direct transitions were determined accurately. The possible assignments of the different origins of excitonic transitions are discussed. The near direct band edge A and B excitonic transitions detected in PzR spectra show a linear red-shift with the temperature increasing up to 525 K. The values of temperature-dependent energies of the excitonic transitions A and B are evaluated and discussed.