arXiv subjects

Thong Leng Lim

Publications and source records attributed to Thong Leng Lim.

9 recordsLinked to original sources

arXiv2019

Generation of ground state structures and electronic properties of ternary Al$_x$Ti$_y$Ni$_z$ clusters (x+y+z=6) with a two-stage DFT global search approach

The structural and electronic properties of ternary AlxTiyNiz clusters, where x, y, and z are integers and x + y + z = 6 are investigated. Both SVWN and B3LYP exchange-correlation functionals are employed in a two-stage density functional theory (DFT) calculations to generate these clusters. In the first stage, a minimum energy cluster structure is generated by an unbiased global search algorithm coupled with a DFT code using a light exchange-correlation functional and small basis sets. In the second stage, the obtained cluster structure is further optimized by another round of global minimization search coupled with a DFT calculator using a heavier exchange-correlation functional and more costly basis set. Electronic properties of the structures are illustrated in the form of a ternary diagram. Our DFT calculations find that the stability of the clusters increases with the increment in the number of constituent nickel atoms. These results provide a new insight to the structure, stability, chemical order and electronic properties for the ternary alloy nanoclusters.

cond-mat.mtrl-sci↗

arXiv2018

The generation of ground state structures and electronic properties of ternary AlkTilNim clusters (k+l+m=4) from a two-stage DFT global searching approach

Structural and electronic properties of ternary clusters AlkTilNim, where k, l, and m are integers and k + l + m = 4 are investigated. These clusters are generated and studied by performing a two-stage density functional theory (DFT) calculations using the SWVN and B3LYP functional exchange correlations. In the first stage, an unbiased global search algorithm coupled with a DFT code with a light exchange-correlation and smaller basis sets are used to generate the lowest energy cluster structures. It is then followed by further optimization using another round of DFT calculation with heavy exchanged correlations and large basis set. Electronic properties of the structures obtained via the two-stage procedure are then studied via DFT calculations. The results are illustrated in the form of ternary diagram. Our DFT calculations find that the stability of the cluster increases with the increase in the number of nickel atoms inside the clusters. Our findings provide new insight into the ternary metallic cluster through the structure, stability, chemical order and electronic properties studies.

physics.atm-clus↗

arXiv2018

Molecular dynamics simulation of melting of finite and infinite size silicene

We report the melting temperature of free-standing silicene by carrying out molecular dynamics (MD) simulation experiments using optimzed Stillinger-Weber (SW) potential by Zhang {\it et al.}. The melting scenario of a free-standing silicene is well captured visually in our MD simulations. The data are systematically analyzed using a few qualitatively different indicators, including caloric curve, radial distribution function and a numerical indicator known as `global similarity index'. The optimized SW potential consistently yield a melting temperature of 1500~K for the simulated free-standing, infinite silicene.

cond-mat.mtrl-sci↗

arXiv2017

Effects of atoms and molecules adsorption on electronic and magnetic properties of s-triazine with embedded Fe atom: DFT investigations

We employ first-principles calculations to study the mechanical, geometrical, electronic and magnetic properties of Fe atom embedded s-triazine ($\mathrm{Fe}$@${\mathrm C_6}{\mathrm N_6}$) system under the influence of external environment. Our results show that the binding energy of $\mathrm{Fe}$@${\mathrm C_6}{\mathrm N_6}$ can be modulated by an applied tensile deformation and perpendicular electric field. The non-magnetic semiconducting property of pure s-triazine sheet (${\mathrm C_6}{\mathrm N_6}$) is found to change upon embedding of Fe atom in the porous site of the sheet. It is revealed that the $\mathrm{Fe}$@${\mathrm C_6}{\mathrm N_6}$ system exhibits half-metallic electronic character with a magnetic moment in the the order similar to that of an isolated Fe atom. Furthermore, electronic and magnetic properties of the $\mathrm{Fe}$@${\mathrm C_6}{\mathrm N_6}$ 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 of $\mathrm{Fe}$@${\mathrm C_6}{\mathrm N_6}$ system can be tuned into a semiconductor via adsorption of atoms and molecules into the $\mathrm{Fe}$@${\mathrm C_6}{\mathrm N_6}$ system. The magnetic moment of $\mathrm{Fe}$@${\mathrm C_6}{\mathrm N_6}$ with adsorbed atoms/molecules is also modified. Our findings may serve as a guide for future applications of $\mathrm{Fe}$@${\mathrm C_6}{\mathrm N_6}$ structures in spintronics devices.

cond-mat.mtrl-sci↗

arXiv2017

First-principles investigation of graphitic carbon nitride monolayer with embedded Fe atom

Density-functional theory calculations with spin-polarized generalized gradient approximation and Hubbard $U$ correction is carried out to investigate the mechanical, structural, electronic and magnetic properties of graphitic heptazine with embedded $\mathrm{Fe}$ atom under bi-axial tensile strain and applied perpendicular electric field. It was found that the binding energy of heptazine with embedded $\mathrm{Fe}$ atom system decreases as more tensile strain is applied and increases as more electric field strength is applied. Our calculations also predict a band gap at a peak value of 5 tensile strain but at expense of the structural stability of the system. The band gap opening at 5 tensile strain is due to distortion in the structure caused by the repulsive effect in the cavity between the lone pairs of edge nitrogen atoms and $\mathrm{d}_{xy}/\mathrm{d}_{x^2-y^2}$ orbital of Fe atom, hence the unoccupied $\mathrm{p}_z$-orbital is forced to shift towards higher energy. The electronic and magnetic properties of the heptazine with embedded $\mathrm{Fe}$ system under perpendicular electric field up to a peak value of 10 $\mathrm{V/nm}$ is also well preserved despite obvious buckled structure. Such properties may be desirable for diluted magnetic semiconductors, spintronics, and sensing devices.

cond-mat.mtrl-sci↗

arXiv2017

Adsorption of atoms and molecules on s-triazine sheet with embedded manganese atom: First-principles calculations

The mechanical, structural, electronic and magnetic properties of s-triazine sheet (C6N6) with embedded Mn atom (Mn-C6N6) is investigated under the influence of external environment using density functional theory. Our results show that Mn-C6N6 system is structurally and mechanically stable. The binding energy of Mn embedded in C6N6 sheet can be modulated under the influence of symmetric deformation and perpendicular electric field respectively. The semiconducting property of pure C6N6 sheet is maintained upon embedment of Mn atom in the porous site. It is also found that small increment in bi-axial tensile strain enhances the band gap (from 0.630 eV at zero strain to 0.802 eV at 5% strain) while the magnetic moment of the embedded Mn atom is preserved. The electronic and magnetic properties of the Mn-C6N6 systems are maintained up to 10 V/nm in electric field strength. We also explore the geometries, electronic and magnetic properties of Mn-C6N6 with adsorbed atoms and molecules. The Mn-C6N6 with adsorbed O atom and O2 molecule systems shows half-metallic character whereas the remaining systems preserve their semiconducting property. The total magnetic moment per unit cell in most of the systems is found to reduce as compared to that of the Mn-C6N6 sheet. The reduction in magnetic moment can be related to the strong interactions among the Mn atom and the surrounding atoms which lead to the formation of low-spin configurations. Overall, our results indicate that the Mn-C6N6 systems with and without adsorbed atoms and molecules can serve as potential candidates for future spintronics and catalysis applications.

cond-mat.mtrl-sci↗

arXiv2017

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

Mechanical and electronic properties of $s$-triazine 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 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 repulse of the lone pairs ($\mathrm{p}_x$-, $\mathrm {p}_y$-) causes the $\mathrm {p}_z$-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.

cond-mat.mtrl-sci↗

arXiv2017

Molecular dynamics simulations and photoluminescence measurements of annealed ZnO surfaces

The effect of thermal annealing on wurtzite ZnO, terminated by two surfaces, (0 0 0 $\bar 1$) (which is oxygen-terminated) and (0 0 0 1) (which is Zn-terminated), is investigated via molecular dynamics simulation using reactive force field (ReaxFF). As a result of annealing at a threshold temperature range of 700~K $ < T_{\mbox{\small t}} \leq 800$~K, surface oxygen atoms begin to sublimate from the (0 0 0 $\bar 1$) surface, while no atom leaves the (0 0 0 1) surface. The ratio of oxygen leaving the surface increases with temperature $T$ (for $T \geq T_{\mbox{\small t}}$). The relative luminescence intensity of the secondary peak in the photoluminescence (PL) spectra, interpreted as a measurement of amount of vacancies on the sample surfaces, qualitatively agrees with the threshold behavior as found in the MD simulations. Our simulations have also revealed the formation of oxygen dimers on the surface and evolution of partial charge distribution during the annealing process. Our MD simulation based on the ReaxFF is consistent with experimental observations.

cond-mat.mtrl-sci↗

arXiv2016

Molecular dynamics simulation of melting of finite and inifinite size graphene

We investigate the melting phenomena of pristine, free-standing infinite and finite size graphene sheets via molecular dynamics simulation using AIREBO potential as implemented in the LAMMPS package. In our simulations, the temperature of the systems under investigation are systematically heated up using two independent heating protocols so that the resultant melting temperatures from both schemes can be checked against each other for consistency. The melting temperature of infinite graphene sheet is obtained by following three independent computational experiments. In the first experiment, we simulate the melting of various finite size graphenes, and then determine the melting temperature of infinite graphene sheet as the temperature at which the finite graphenes asymptotically grow in size. In the second experiment, we simulate the melting of infinite single-wall carbon-nanotubes (SWCNTs) with different radius, and then determine the melting temperature of infinite graphene sheet as the temperature at which the radius of SWCNTs asymptotically grows in size. In the third experiment, we heat up an infinite graphene that is formed by constructing a rectangular supercell which is subjected to periodic boundary condition at it sides. Melting temperature for infinite graphene obtained based on the first approach yields $\sim$ 5799 K $\pm$ 22 K. The second approach yield $\sim$5302 K $\pm$ 36 K, whereas $\sim$5355 $\pm$ 140 K from the third. There is an apparent disparity between the results from the first experiment and that of the second and third experiments due to differences in the technical details in these MD simulations. We cautiously conclude that, based on the consistency of the data of the second and third experiments, that a free-standing infinite graphene sheet melts at the temperature of 5302 K $\pm$ 36 K, using AIREBO forcefield.

cond-mat.mtrl-sci↗