arXiv subjects

Chee Kwan Gan

Publications and source records attributed to Chee Kwan Gan.

30 records · Page 2Linked to original sources

arXiv2013

Strain dependence of the heat transport properties of graphene nanoribbons

Using a combination of accurate density-functional theory and a nonequilibrium Green function's method, we calculate the ballistic thermal conductance characteristics of tensile-strained armchair (AGNR) and zigzag (ZGNR) edge graphene nanoribbons, with widths between 3-50 Å. The optimized lateral lattice constants for AGNRs of different widths display a three-family behavior when the ribbons are grouped according to N modulo 3, where $N$ represents the number of carbon atoms across the width of the ribbon. Two lowest-frequency out-of-plane acoustic modes play a decisive role in increasing the thermal conductance of AGNR-N at low temperatures. At high temperatures the effect of tensile strain is to reduce the thermal conductance of AGNR-N and ZGNR-N. These results could be explained by the changes in force constants in the in-plane and out-of-plane directions with the application of strain. This fundamental atomistic understanding of the heat transport in graphene nanoribbons paves a way to effect changes in their thermal properties via strain at various temperatures.

cond-mat.mtrl-sci↗

arXiv2013

Interlayer breathing and shear modes in few-trilayer MoS2 and WSe2

Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have recently attracted tremendous interest as potential valleytronic and nano-electronic materials, in addition to being well-known as excellent lubricants in the bulk. The interlayer van der Waals (vdW) coupling and low frequency phonon modes, and how they evolve with the number of layers, are important for both the mechanical and electrical properties of 2D TMDs. Here we uncover the ultra-low frequency interlayer breathing and shear modes in few-layer MoS2 and WSe2, prototypical layered TMDs, using both Raman spectroscopy and first principles calculations. Remarkably, the frequencies of these modes can be perfectly described using a simple linear chain model with only nearest-neighbour interactions. We show that the derived in-plane (shear) and out-of-plane (breathing) force constants from experiment remain the same from two-layer 2D crystals to the bulk materials, suggesting that the nanoscale interlayer frictional characteristics of these excellent lubricants should be independent of the number of layers.

cond-mat.mtrl-sci↗

arXiv2011

First-principles study of graphene edge properties and flake shapes

We use density functional theory to determine the equilibrium shape of graphene flakes, through the calculation of the edge orientation dependence of the edge energy and edge stress of graphene nanoribbons. The edge energy is a nearly linear function of edge orientation angle; increasing from the armchair orientation to the zigzag orientation. Reconstruction of the zigzag edge lowers its energy to less than that of the armchair edge. The edge stress for all edge orientations is compressive, however, reconstruction of the zigzag edge reduces this edge stress to near zero. Hydrogen adsorption is favorable for all edge orientations; dramatically lowering all edge energies and all edge stresses. It also removes the reconstruction of the zigzag edge. Using the new edge energy data, we determine the equilibrium shape of a graphene sheet (with unreconstructed edges) to be hexagonal with straight armchair edges in the presence and absence of hydrogen. However, zigzag edge reconstruction produces graphene flakes with a six-fold symmetry, but with rounded edges. This shape is dominated by near zigzag edges. The compressive edge stresses will lead to edge buckling (out-of-the-plane of the graphene sheet) for all edge orientations, in the absence of hydrogen. Exposing the graphene flake to hydrogen dramatically decreases the buckling amplitude.

cond-mat.mes-hall↗

arXiv2011

First-principles study of heat transport properties of graphene nanoribbons

We combine density-functional theory and the nonequilibrium Green's function method to study the thermal conductance of graphene nanoribbons with armchair and zigzag edges. Zigzag ribbons have higher thermal conductance than armchair ribbons of comparable widths due to an anisotropy in phonon dispersion for graphene nanoribbons: low-frequency bands in zigzag ribbons are more dispersive than those in armchair ribbons. The edges with and without hydrogen-passivation modify the atomic structure and ultimately influence the phonon thermal transport differently for the two ribbon types.

cond-mat.mes-hall↗

arXiv2007

Nonequilibrium Green's function method for thermal transport in junctions

We present a detailed treatment of the nonequilibrium Green's function method for thermal transport due to atomic vibrations in nanostructures. Some of the key equations, such as self-energy and conductance with nonlinear effect, are derived. A self-consistent mean-field theory is proposed. Computational procedures are discussed. The method is applied to a number of systems including one-dimensional chains, a benzene ring junction, and carbon nanotubes. Mean-field calculations of the Fermi-Pasta-Ulam model are compared with classical molecular dynamics simulations. We find that nonlinearity suppresses thermal transport even at moderately high temperatures.

cond-mat.stat-mech↗

arXiv2006

First-principles study of wurtzite InN$(0001)$ and $(000{\bar 1})$ surfaces

Density-functional calculations are used to study various plausible structures of the wurtzite InN $(0001)$ and $(000{\bar 1})$ surfaces. These structures include the unreconstructed surfaces, surfaces with monolayers of In or N, several possible coverages and locations of In or N adatoms and vacancies. The stable structure of the $(0001)$ surface under N-rich conditions is the unreconstructed, In-terminated, surface, while under In-rich conditions the stable surface has a 3/4 monolayer of In atoms. The stable structure of the InN $(000{\bar 1})$ surface corresponds to a full monolayer of In atoms in the atop sites (directly above the N atoms) over the entire range of accessible In (or N) chemical potential. The atomic structures of the low-energy structures are also discussed.

cond-mat.mtrl-sci↗

arXiv2006

First-principles calculation of the thermodynamics of In$_x$Ga$_{1-x}$N alloys: Effect of lattice vibrations

The thermodynamics properties of the wurtzite and zinc-blende \InGaN alloys are calculated using first-principles density-functional calculations. Special quasi-random structures are used to describe the disordered alloys, for $x= 1/4, 1/2$, and 3/4. The effect of lattice vibrations on the phase diagram, commonly omitted from semiconductor alloy phase diagram calculations, are included through first-principles calculations of phonon spectra. Inclusion of lattice vibrations leads to a large reduction in the order-disorder critical temperature ($\sim 29$% and $\sim 26$% for the wurtzite and zinc-blende structures, respectively) and changes the shape of the solubility and spinodal curve through changes in the entropies of the competing phases. Neglect of such effect produces significant errors in the phase diagrams of complex ordered semiconductor compounds. The critical temperature for phase separation is 1654 K (1771 K) for the wurtzite (zinc-blende) structures. The predicted phase diagrams are in agreement with experimental measurements on MOCVD \InGaN\ films.

cond-mat.mtrl-sci↗

arXiv2004

Linear scaling computation of the Fock matrix. IX. Parallel computation of the Coulomb matrix

We present parallelization of a quantum-chemical tree-code [J. Chem. Phys. {\bf 106}, 5526 (1997)] for linear scaling computation of the Coulomb matrix. Equal time partition [J. Chem. Phys. {\bf 118}, 9128 (2003)] is used to load balance computation of the Coulomb matrix. Equal time partition is a measurement based algorithm for domain decomposition that exploits small variation of the density between self-consistent-field cycles to achieve load balance. Efficiency of the equal time partition is illustrated by several tests involving both finite and periodic systems. It is found that equal time partition is able to deliver 91 -- 98 % efficiency with 128 processors in the most time consuming part of the Coulomb matrix calculation. The current parallel quantum chemical tree code is able to deliver 63 -- 81% overall efficiency on 128 processors with fine grained parallelism (less than two heavy atoms per processor).

cond-mat.other↗

arXiv1997

Nonequilibrium relaxation of the two-dimensional Ising model: Series-expansion and Monte Carlo studies

We study the critical relaxation of the two-dimensional Ising model from a fully ordered configuration by series expansion in time t and by Monte Carlo simulation. Both the magnetization (m) and energy series are obtained up to 12-th order. An accurate estimate from series analysis for the dynamical critical exponent z is difficult but compatible with 2.2. We also use Monte Carlo simulation to determine an effective exponent, z_eff(t) = - {1/8} d ln t /d ln m, directly from a ratio of three-spin correlation to m. Extrapolation to t = infinity leads to an estimate z = 2.169 +/- 0.003.

cond-mat.stat-mech↗

arXiv1997

Extended series expansions for random sequential adsorption

We express the coverage (occupation fraction) $θ$, in powers of time $t$ for four models of two-dimensional lattice random sequential adsorption (RSA) to very high orders by improving an algorithm developed by the present authors [J. Phys. A 29, L177 (1996)]. Each of these series is, to the best of our knowledge, the longest at the present. We analyze the series and deduce accurate estimates for the jamming coverage of the models.

cond-mat↗

arXiv1996

Series expansion studies of random sequential adsorption with diffusional relaxation

We obtain long series (28 terms or more) for the coverage (occupation fraction) $θ$, in powers of time $t$ for two models of random sequential adsorption with diffusional relaxation using an efficient algorithm developed by the authors. Three different kinds of analyses of the series are performed for a wide range of $γ$, the rate of diffusion of the adsorbed particles, to investigate the power law approach of $θ$ at large times. We find that the primitive series expansions in time $t$ for $θ$ capture rich short and intermediate time kinetics of the systems very well. However, we see that the series are still not long enough to extract the kinetics at large times for general $γ$. We have performed extensive computer simulations employing an efficient event-driven algorithm to confirm the $t^{-1/2}$ saturation approach of $θ$ at large times for both models, as well as to investigate the short and intermediate time behaviors of the systems.

cond-mat↗

arXiv1995

An algorithm for series expansions based on hierarchical rate equations

We propose a computational method to obtain series expansions in powers of time for general dynamical systems described by a set of hierarchical rate equations. The method is generally applicable to problems in both equilibrium and nonequilibrium statistical mechanics such as random sequential adsorption, diffusion-reaction dynamics, and Ising dynamics. New result of random sequential adsorption of dimers on a square lattice is presented.

cond-mat↗