arXiv subjects

B. Mortazavi

Publications and source records attributed to B. Mortazavi.

6 recordsLinked to original sources

arXiv2020

First-Principles Prediction of Graphene-Like XBi (X=Si, Ge, Sn) Nanosheets

Research progress on single-layer group III monochalcogenides have been increasing rapidly owing to their interesting physics. Herein, we predict the dynamically stable single-layer forms of XBi (X=Ge, Si, or Sn) by using density functional theory calculations. Phonon band dispersion calculations and ab-initio molecular dynamics simulations reveal the dynamical and thermal stability of predicted nanosheets. Raman spectra calculations indicate the existence of 5 Raman active phonon modes 3 of which are prominent and can be observed in a possible Raman measurement. Electronic band structures of the XBi single-layers investigated with and without spin-orbit coupling effects (SOC). Our results show that XBi single-layers show semiconducting property with the narrow band gap values without SOC. However only the single-layer SiBi is an indirect band gap semiconductor while GeBi and SnBi exhibit metallic behaviors by adding spin-orbit coupling effects. In addition, the calculated linear-elastic parameters indicate the soft nature of predicted monolayers. Moreover, our predictions for the thermoelectric properties of single-layer XBi reveal that SiBi is a good thermoelectric material with increasing temperature. Overall, it is proposed that single-layer XBi structures can be alternative, stable 2D single-layers with their varying electronic and thermoelectric properties.

cond-mat.mtrl-sci↗

arXiv2019

Enhancement in hydrogen storage capacities of light metal functionalized Boron Graphdiyne nanosheets

The recent experimental synthesis of the two-dimensional (2D)boron-graphdiyne (BGDY) nanosheet has motivated us to investigate its structural, electronic,and energy storage properties. BGDY is a particularly attractive candidate for this purpose due to uniformly distributed pores which can bind the light-metal atoms. Our DFTcalculations reveal that BGDY can accommodate multiple light-metal dopants (Li, Na, K, Ca)with significantly high binding energies. The stabilities of metal functionalized BGDY monolayers have been confirmed through ab initio molecular dynamics simulations. Furthermore, significant charge-transfer between the dopantsand BGDY sheet renders the metal with a significant positive charge, which is a prerequisite for adsorbing hydrogen (H2) molecules with appropriate binding energies.This results in exceptionally high H2 storage capacities of 14.29, 11.11, 9.10 and 8.99 wt% for the Li, Na, K and Ca dopants, respectively. These H2storage capacities are much higher than many 2D materials such as graphene, graphane, graphdiyne, graphyne, C2N, silicene, and phosphorene. Average H2 adsorption energies for all the studied systems fall within an ideal window of 0.17-0.40 eV/H2. We have also performed thermodynamic analysis to study the adsorption/desorption behavior of H2, which confirmsthat desorption of the H2molecules occurs at practical conditions of pressure and temperature.

cond-mat.mes-hall↗

arXiv2019

N-, B-, P-, Al-, As-, Ga-graphdiyne/graphyne lattices: First-principles investigation of mechanical, optical and electronic properties

Graphdiyne and graphyne are carbon-based two-dimensional (2D) porous atomic lattices, with outstanding physics and excellent application prospects for advanced technologies, like nanoelectronics and energy storage systems. During the last year, B- and N-graphdiyne nanomembranes were experimentally realized. Motivated by the latest experimental advances, in this work we predicted novel N-, B-, P-, Al-, As-, Ga-graphdiyne/graphyne 2D lattices. We then conducted density functional theory simulations to obtain the energy minimized structures and explore the mechanical, thermal stability, electronic and optical characteristics of these novel porous nanosheets. Acquired theoretical results reveal that the predicted carbon-based lattices are thermally stable. It was moreover found that these novel 2D nanostructures can exhibit remarkably high tensile strengths or stretchability. The electronic structure analysis reveals semiconducting electronic character for the predicted monolayers. Moreover, the optical results indicate that the first absorption peaks of the imaginary part of the dielectric function for these novel porous lattices along the in-plane directions are in the visible, IR and near-IR (NIR) range of light. This work highlights the outstanding properties of graphdiyne/graphyne lattices and recommends them as promising candidates to design stretchable energy storage and nanoelectronics systems.

physics.comp-ph↗

arXiv2018

Electrical and thermal transport in coplanar polycrystalline graphene-hBN heterostructures

We present a theoretical study of electronic and thermal transport in polycrystalline heterostructures combining graphene (G) and hexagonal boron nitride (hBN) grains of varying size and distribution. By increasing the hBN grain density from a few percents to $100\%$, the system evolves from a good conductor to an insulator, with the mobility dropping by orders of magnitude and the sheet resistance reaching the M$\Omega$ regime. The Seebeck coefficient is suppressed above $40\%$ mixing, while the thermal conductivity of polycrystalline hBN is found to be on the order of $30-120\,{\rm W}{\rm m}^{-1}{\rm K}^{-1}$. These results, agreeing with available experimental data, provide guidelines for tuning G-hBN properties in the context of two-dimensional materials engineering. In particular, while we proved that both electrical and thermal properties are largely affected by morphological features (like e.g. by the grain size and composition), we find in all cases that nm-sized polycrystalline G-hBN heterostructures are not good thermoelectric materials.

cond-mat.mes-hall↗

arXiv2017

Record Low Thermal Conductivity of Polycrystalline MoS2 films: Tuning the Thermal Conductivity by Grain Orientation

We report a record low thermal conductivity in polycrystalline MoS2 obtained by varying grain sizes and orientations in ultrathin films. By optimizing the sulphurisation parameters of nanometre-thick Mo layer, we could grow MoS2 films with tuneable morphologies. The thermal conductivity is extracted from a Raman laser power-dependent study on suspended samples. The lowest value of thermal conductivity of 0.27 Wm-1K-1, which reaches a similar value as teflon, is obtained in a polycrystalline sample formed by a combination of horizontally and vertically oriented grains, with respect to the bulk (001) monocrystal. Analysis by means of molecular dynamics and finite element method simulations confirm that such grain arrangement leads to lower grain boundary conductance. We discuss the possible use of these thermal insulating films in the context of electronics and thermoelectricity.

cond-mat.mes-hall↗

arXiv2016

Thermal conductivity of MoS2 polycrystalline nanosheets

We report a technique for transferring large areas of the CVD-grown, few-layer MoS2 from the original substrate to another arbitrary substrate and onto holey substrates, in order to obtain free-standing structures. The method consists of a polymer- and residue-free, surface-tension-assisted wet transfer, in which we take advantage of the hydrophobic properties of the MoS2. The methods yields better quality transferred layers, with fewer cracks and defects, and less contamination than the widely used PMMA-mediated transfer and allows fabrication of few-layer, fee-standing structures with diameters up to 100 micro-m. We report thermal conductivity measurements by means of contactless Raman thermometry on the so-fabricated samples. The measurements revealed a strong reduction in the in-plane thermal conductivity down to 0.5 W/mK. The results are explained using finite elements method simulations for a polycrystalline film.

cond-mat.mtrl-sci↗