Tiem Leong Yoon

Surface and interface phonon polaritons of wurtzite GaN thin film grown on 6H-SiC substrate

Surface and interface phonon polaritons of wurtzite GaN thin film grown on 6H-SiC substrate are investigated experimentally and theoretically. Two strong absorption peaks that correspond to the surface and interface phonon polaritons are clearly observed at 710 and 916 cm−1, respectively. This observation is in good agreement with the results simulated using an anisotropy model. Finally, the obtained surface phonon polariton (SPP) mode is compared with the reported result. It is revealed that the SPP mode in the wurtzite GaN thin film is shifted toward higher frequency as compared to that in the wurtzite bulk GaN.

AERONET data-based determination of aerosol types

Aerosols are among the most interesting topics investigated by researchers because of their complicated characteristics and poor quantification. Moreover, significant uncertainties are associated with changes in the Earths radiation budget. Previous studies have shown numerous difficulties and challenges in quantifying aerosol influences. In addition, the heterogeneity from aerosol loading and properties, including spatial, temporal, size, and composition features, presents a challenge. In this study, we investigated aerosol characteristics over two regions with different environmental conditions and aerosol sources. The study sites are Penang and Kuching in Malaysia, where a ground-based AErosol RObotic NETwork (AERONET) sun-photometer was deployed. The types of aerosol, such as biomass burning, urban/industrial, marine, and dust aerosols, for both study sites were identified by analyzing aerosol optical depth and angstrom exponent. Seasonal monsoon variation results in different aerosol optical properties, characteristics, and types of aerosols that are dominant in Penang and Kuching. Seasonal monsoon flow trend patterns from a seven-day back-trajectory frequency plotted by the Hybrid Single-Particle Lagrangian Integrated Trajectory model illustrated the distinct origins of trans-boundary aerosol sources. Finally, we improved our findings in Malaysian sites using the AERONET data from Singapore and Indonesia. Similarities in the optical properties of aerosols and the distribution types (referred to as homogeneous aerosol) were observed in the Penang-Singapore and the Kuching-Pontianak sites. The dominant aerosol distribution types were completely different for locations in the western (Penang-Singapore) and eastern (Kuching-Pontianak) parts of the South China Sea. This is a result of spatial and temporal heterogeneity. The spatial and temporal heterogeneities for the western and eastern portions of South China Sea provide information on the natural or anthropogenic processes that take place.

First-principles LDA

Yttrium niobate () phosphor is studied experimentally and through first-principles calculations, in which the structural and electronic properties of are investigated using the local-density approximation LDA+U method. The absorption and luminescence experiments that were conducted on the host lattice show the band gap to be 4.1 eV. The LDA+U calculations allow us to obtain a band gap of 4.28 eV. The density of states obtained from the calculation shows that O 2p states contribute to the valence band. The lower conduction band is mainly composed of Nb 4d states, while the upper conduction bands involve contribution mainly from Y 4d states. The partial DOS of each atom in the niobate system is then compared to the ultraviolet (UV) and vacuum ultraviolet (VUV) spectra from A photoluminescence excitation (PLE) experiment to explain the nature of the bands observed.

+U

Abstract Mechanical and electronic properties of s-triazine sheet are studied using first-principles calculations based on density functional theory. The in-plane stiffness and bulk modulus for s-triazine sheet are found to be less than that of heptazine. The reduction can be related to the nature of the covalent bonds connecting the adjacent sheets and the number of atoms per unit cell. The Poisson’s ratio of s-triazine sheet is half the value to that of graphene. Additionally, the calculated values of the two critical strains (elastic and yielding points) of s-triazine sheet are in the same order of magnitude to that for heptazine which was calculated using MD simulations in the literature. It is also demonstrated that s-triazine sheet can withstand larger tension in the plastic region. These results established a stable mechanical property for s-triazine sheet. We found a linear relationship of bandgap as a function of bi-axial tensile strain within the harmonic elastic region. The reduced steric repulsion of the lone pairs (px-, py-) causes the pz-like orbital to shift to high energy, and consequently an increase in the bandgap. We find no electronic properties modulation of the s-triazine sheet under electric field up to a peak value of 10 V/nm. Such noble properties may be useful in future nanomaterial applications.

calculations and luminescence study of YNbO4

The plasmon resonance properties of gold nanoparticles (AuNPs) in liquid ionic medium with different concentration was investigated using a UV-visible/NIR spectrophotometer. The optical absorption spectra of AuNPs suspended in NaCl solutions in the range of 400 – 800 nm were measured; specifically, 10 nm bare AuNPs separately suspended in NaCl solution with molar concentrations of 0.01M and 1M resulted in different shapes and widths in the plasmon resonance peak. The optical spectral resonances were analyzed using the Mie theory, Lorenz-Drude size-dependent dielectric functions and effective medium theory. Analysis of the spectral features by means of theoretical models showed that the damping of the plasmon resonance of AuNPs in ionic medium is size dependent. The experimental results indicated that the effect of chemical interface damping is more prominent compared with the inherent size effects of AuNPs.

Theoretical studies on mechanical and electronic properties of s-triazine sheet

There is a unique SU(4) ⊗ SU(2)L ⊗ SU(2)R gauge model which allows quarks and leptons to be unified at the TeV scale. It is already known that the neutrino masses arise radiatively in the model and are naturally light. We study the atmospheric, solar and LSND neutrino anomalies within the framework of this model.

Effect of chemical interface damping and aggregation size of bare gold nanoparticles in NaCl on the plasmon resonance damping

The search for the ground state structures of Boron clusters has been a difficult computational task due to the unique metalloid nature of Boron atom. Previous research works had overcome the problem in the search of the Boron ground-state structures by adding symmetry constraints prior to the process of locating the local minima in the potential energy surface (PES) of the Boron clusters. In this work, we shown that, with the deployment of a novel computational approach that incorporates density functional theory (DFT) into a guided global optimization search algorithm based on basin-hopping, it is possible to directly locate the local minima of small Boron clusters in the PES at the DFT level. The ground-state structures search algorithm as proposed in this work is initiated randomly and needs not a priori symmetry constraint artificially imposed throughout the search process. Small sized Boron clusters so obtained compare well to the results obtained by similar calculations in the literature. The electronic properties of each structures obtained are calculated within the DFT framework.

Solutions of the atmospheric, solar and LSND neutrino anomalies from TeV scale quark - lepton unification

Aerosol optical depth (AOD) from AERONET data has a very fine resolution but air pollution index (API), visibility and relative humidity from the ground truth measurements are coarse. To obtain the local AOD in the atmosphere, the relationship between these three parameters was determined using multiple regression analysis. The data of southwest monsoon period (August to September, 2012) taken in Penang, Malaysia, was used to establish a quantitative relationship in which the AOD is modeled as a function of API, relative humidity, and visibility. The highest correlated model was used to predict AOD values during southwest monsoon period. When aerosol is not uniformly distributed in the atmosphere then the predicted AOD can be highly deviated from the measured values. Therefore these deviated data can be removed by comparing between the predicted AOD values and the actual AERONET data which help to investigate whether the non uniform source of the aerosol is from the ground surface or from higher altitude level. This model can accurately predict AOD if only the aerosol is uniformly distributed in the atmosphere. However, further study is needed to determine this model is suitable to use for AOD predicting not only in Penang, but also other state in Malaysia or even global.

Guided basin-hopping search of small boron clusters with density functional theory

Abstract We employ first-principles calculations to study the mechanical, geometrical, electronic and magnetic properties of Fe atom embedded s-triazine () system under the influence of external environment. Our results show that the binding energy of can be modulated by an applied tensile deformation and perpendicular electric field. The non-magnetic semiconducting property of pure s-triazine sheet () is found to change upon embedding of Fe atom in the porous site of the sheet. It is revealed that the system exhibit half-metallic electronic character with a magnetic moment in the order similar to that of an isolated Fe atom. Furthermore, electronic and magnetic properties of the systems are preserved up to a maximum value of 10 V/nm in electric field strength and 6% tensile strain. Interestingly, we find that the half-metallic electronic character can be tuned into semiconductor via adsorption of atoms and molecules into the system. The magnetic moment of with adsorbed atoms/molecules is also modified. Our findings may serve as a guide for future applications of structures in spintronics devices.

Multiple regression method to determine aerosol optical depth in atmospheric column in Penang, Malaysia

Melting dynamics of hafnium clusters are investigated using a novel approach based on the idea of the chemical similarity index. Ground state configurations of small hafnium clusters are first derived using Basin-Hopping and Genetic Algorithm in the parallel tempering mode, employing the COMB potential in the energy calculator. These assumed ground state structures are verified by using the Low Lying Structures (LLS) method. The melting process is carried out either by using the direct heating method or prolonged simulated annealing. The melting point is identified by a caloric curve. However, it is found that the global similarity index is much more superior in locating premelting and total melting points of hafnium clusters.