Numerical characterization of thermal transport in hexagonal tungsten disulfide (WS2) nanoribbons.

In this study, we have investigated the thermal transport characteristics of single-layer tungsten disulfide, WS2nanoribbons (SLTDSNRs) using equilibrium molecular dynamics simulations with the help of Green-Kubo formulation. Using Stillinger-Weber (SW) inter-atomic potential, the calculated room temperature thermal conductivities of 15 nm × 4 nm pristine zigzag and armchair SLTDSNRs are 126 ± 10 W m-1K-1and 110 ± 6 W m-1K-1, respectively. We have explored the dependency of thermal conductivity on temperature, width, and length of the nanoribbon. The study shows that the thermal conductivity of the nanoribbon decreases with the increase in temperature, whereas the thermal conductivity increases with an increase in either the width or length of the ribbon. The thermal conductivity does not increase uniformly as the size of the ribbon changes. We have also observed that the thermal conductivity of SLTDSNRs depends on edge orientations; the zigzag nanoribbon has greater thermal conductivity than the armchair nanoribbon, regardless of temperature or dimension variations. Our study additionally delves into the tunable thermal properties of SLTDSNRs by incorporating defects, namely vacancies such as point vacancy, edge vacancy, and bi-vacancy. The thermal conductivities of nanoribbons with defects have been found to be considerably lower than their pristine counterparts, which aid in enhanced values for the thermoelectric figure of merit (zT). We have varied the vacancy concentration within a range of 0.1% to 0.9% and found that a point vacancy concentration of 0.1% leads to a 64% reduction in the thermal conductivity of SLTDSNRs. To elucidate these phenomena, we have calculated the phonon density of states for WS2under different aspects. The findings of our work provide important understandings of the prospective applications of WS2in nanoelectronic and thermoelectric devices by tailoring the thermal transport properties of WS2nanoribbons.

Novel hybrid monolayers SixGeySn1-x-y: first principles study of structural, electronic, optical, and electron transport properties with NH3 sensing application.

The structural, electronic, optical, and electron transport properties of three different atomically thin novel hybrid monolayers comprising of Si, Ge, and Sn atoms in varying proportions are studied using first principles calculations within the framework of density functional theory. The fabrication of similar hybrid materials is practically realizable but the different properties of these novel monolayers are yet to be explored. The proposed hybrid buckled honeycomb monolayers with sp2-sp3 like orbital hybridization are mechanically and dynamically stable, confirmed by the analysis of in-plane elastic constants, phonon dispersion curve and cohesive energy of the monolayers. The electronic band structures of these hybrid two-dimensional (2D) monolayers, namely Ge0.25Sn0.25Si0.50, Si0.25Ge0.25Sn0.50, and Sn0.25Si0.25Ge0.50, show a considerable direct energy bandgap ranging from 120 meV to 283.8 meV while preserving the linear energy-momentum relation at the K point of the Brillouin zone. The calculated significantly low effective mass (0.063-0.101m0) and very high acoustic phonon limited mobility (∼106 cm2 V-1 s-1) of the charge carriers inside the hybrid monolayers ensure the presence of relativistic-massless Dirac fermions. In order to further investigate the electronic properties, we have calculated the atom projected density of states and differential charge density. Optical properties, e.g. dielectric function, electron loss function, absorption coefficient, refractive index, reflectivity, and optical conductivity, are also explored for parallelly and perpendicularly polarized incident light. These hybrid monolayers show anisotropic optical response for parallel and perpendicular polarization as a function of frequency of the incident light. Polarization tunable plasma frequency, high absorption coefficient over a wide range of frequency, and high refractive indices suggest these hybrid monolayers as potential candidates for optoelectronic applications. We have also designed three different armchair nanoribbons to study the effect of the adsorption of NH3 molecules on these hybrid nanoribbons. Our calculated electron transport properties ensure the applications of these nanoribbons as an NH3 sensor at the molecular level. Thus, our results suggest that the proposed SixGeySn1-x-y hybrid monolayers can be a potential candidate for nanoelectronics, optoelectronics and sensor based applications.

Functionalization of electronic, spin and optical properties of GeSe monolayer by substitutional doping: a first-principles study.

Substitutional doping has traditionally been used to modulate the existing properties of semiconductors and introduce new exciting properties, especially in two-dimensional materials. In this work, we have investigated the impact of substitutional doping (using group III, IV, V, and VI dopants) on the structural, electronic, spin, and optical properties of GeSe monolayer by using first-principles calculations based on density functional theory. Our calculated binding energies, formation energies and phonon dispersion curves of the doped systems support their stability and hence the feasibility of physical realization. Our results further suggest that switching between metallic and semiconducting states of GeSe monolayer can be controlled by dopant atoms with a different number of valence electrons. The band gap of the semiconducting structures can be tuned within a range of 0.2864 eV to 1.17 eV by substituting with different dopants. In addition, most of the doped structures maintain the low effective mass, 0.20m0to 0.59m0for electron and 0.21m0to 0.52m0for hole, which ensures the enhanced transport properties of GeSe based electronic devices. Moreover, when Ge is substituted with group V dopants, a magnetic moment is introduced in an otherwise non-magnetic GeSe monolayer. The optical absorption coefficient of the doped structures can be significantly improved (>2×) in the visible and infrared regions. These intriguing results would encourage the applications of doped GeSe monolayer in next-generation electronic, optoelectronic and spintronic devices.

SiGe/AsSb bilayer heterostructures: structural characteristics and tunable electronic properties.

Tunable band gap along with high carrier mobility are attractive characteristics for high speed nano electronic device applications. In this work we studied the structural and electronic properties of atomically thin silicon germanide (SiGe) and antimony arsenide (AsSb) heterobilayers using first principle calculations within density functional theory. Monolayer SiGe is a semimetal with a Dirac cone at the K point of the Brillouin zone (BZ) which combines superior properties of germanene and synthesis advantages of silicene. The study shows that a considerable band gap (90-459 meV) is introduced in SiGe when modulated by monolayer AsSb without degrading the carrier mobility. Moreover AsSb introduces negligible lattice mismatch in optimized heterobilayers which is favorable for synthesis purposes. We studied the density of states and space charge distribution to investigate the mechanism of the band gap opening and interlayer binding. Finally we modulated the band gap at K the point of the BZ efficiently by applying biaxial strain and also by changing the interlayer spacing. The calculated electron effective mass as a function of strain reveals that linear energy dispersion relation is preserved and the effective mass remains significantly small within the strained structure. The results predict that SiGe/AsSb heterobilayers can be an excellent choice in Si and Ge-based nano electronics and spintronic applications.

Thermal transport characterization of stanene/silicene heterobilayer and stanene bilayer nanostructures.

Recently, stanene and silicene based nanostructures with low thermal conductivity have incited noteworthy interest due to their prospect in thermoelectrics. Aiming at the possibility of extracting lower thermal conductivity, in this study, we have proposed and modeled stanene/silicene heterobilayer nanoribbons, a new heterostructure and subsequently characterized their thermal transport by using an equilibrium molecular dynamics simulation. In addition, the thermal transport in bilayer stanene is also studied and compared. We have computed the thermal conductivity of the stanene/silicene and bilayer stanene nanostructures to characterize their thermal transport phenomena. The studied nanostructures show good thermal stability within the temperature range of 100-600 K. The room temperature thermal conductivities of pristine 10 nm × 3 nm stanene/silicene hetero-bilayer and stanene bilayer are estimated to be 3.63 ± 0.27 W m-1 K-1 and 1.31 ± 0.34 W m-1 K-1, respectively, which are smaller than that of silicene, graphene and some other 2D monolayers as well as heterobilayers such as stanene/graphene and silicene/graphene. In the temperature range of 100-600 K, the thermal conductivity of our studied bilayer nanoribbons decreases with an increase in the temperature. Furthermore, we have investigated the dependence of our estimated thermal conductivity on the size of the considered nanoribbons. The thermal conductivities of both the nanoribbons are found to increase with an increase in the width of the structure. The thermal conductivity shows a similar increasing trend with the increase in the ribbon length, as well. Our results suggest that, the low thermal conductivity of our studied bilayer structures can be further decreased by nanostructuring. The significantly low thermal conductivity of the stanene/silicene heterobilayer and stanene bilayer nanoribbons realized in our study would provide a good insight and encouragement into their appealing prospect in the thermoelectric applications.

Numerical characterization of the electronic and optical properties of plumbene/hBN heterobilayer using first-principles study.

We present a novel plumbene/hexagonal boron nitride (hBN) heterobilayer with intriguing structural, electronic, and optical properties. Three different stacking patterns of the bilayer are proposed and studied under the framework of density functional theory using first-principles calculations. All the stacking configurations display direct band gaps ranging from 0.399 eV to 0.432 eV in the presence of spin orbit coupling (SOC), whereas pristine plumbene possesses an indirect band gap considering SOC. Based on binding energy calculations, the structures are found to be stable and, consequently, feasible for physical implementation. All three structures exhibit low effective mass, ∼0.20m0 for both electrons and holes, which suggests improved transport characteristics of the plumbene/hBN based electronic devices. The projected density of states reveals that the valence and conduction band peaks around Fermi energy are dominated by the contributions from the plumbene layer of the heterobilayer. Therefore, the hBN layer is a viable candidate as a substrate for plumbene since charge carriers will only travel through the plumbene layer. Biaxial strain is employed to explore the dependence of the electronic properties like bandgap and effective mass of the heterobilayer on applied strain. We find that applied biaxial compressive strain can induce switching from the semiconducting to metallic state of the material. In addition, we explore various optical characteristics of both pristine plumbene and plumbene/hBN. The optical properties of the heterobilayer signify its potential applications in solar cells as well as in UV photodetectors.

Large band gap quantum spin Hall insulators in plumbene monolayer decorated with amidogen, hydroxyl and thiol functional groups.

Two-dimensional Quantum Spin Hall (QSH) insulators featuring edge states that are topologically protected against back-scattering are arising as a novel state of quantum matter. One of the major obstacles to finding QSH insulators operable at room temperature is the insufficiency of suitable materials demonstrating the QSH effect with a large bulk band gap. Plumbene, the latest group-IV graphene analogous material, shows a large SOC-induced band gap opening but the coupling between topological states at different momentum points makes it a topologically trivial insulator. Pristine plumbene can be chemically functionalized to transform it from a conventional insulator to a topologically non-trivial insulator with a considerable bulk band gap. In this work, three new QSH phases in plumbene have been theoretically predicted through functionalization with amidogen (-NH2), hydroxyl (-OH) and thiol (-SH) groups. The derived electronic properties show non-trivial topological states in plumbene with very high bulk band gaps ranging from 1.0911 eV to as high as 1.1515 eV. External strain can be used to further enhance and tune these bulk gaps, as demonstrated in this work. We also propose a H-terminated SiC (0001) surface as a suitable substrate for the practical implementation of these monolayers to minimize lattice mismatch and maintain their topological order. The robustness of these QSH insulators against strain and substrate effects and the large bulk gaps provide a promising platform for potential applications of future low dissipation nanoelectronic devices and spintronic devices at room temperature.

Dimensional crossover of thermal transport in few-layer graphene.

Graphene, in addition to its unique electronic and optical properties, reveals unusually high thermal conductivity. The fact that the thermal conductivity of large enough graphene sheets should be higher than that of basal planes of bulk graphite was predicted theoretically by Klemens. However, the exact mechanisms behind the drastic alteration of a material's intrinsic ability to conduct heat as its dimensionality changes from two to three dimensions remain elusive. The recent availability of high-quality few-layer graphene (FLG) materials allowed us to study dimensional crossover experimentally. Here we show that the room-temperature thermal conductivity changes from approximately 2,800 to approximately 1,300 W m(-1) K(-1) as the number of atomic planes in FLG increases from 2 to 4. We explained the observed evolution from two dimensions to bulk by the cross-plane coupling of the low-energy phonons and changes in the phonon Umklapp scattering. The obtained results shed light on heat conduction in low-dimensional materials and may open up FLG applications in thermal management of nanoelectronics.

Analysis and design of InAs nanowire array based ultra broadband perfect absorber.

An ultra-broadband perfect absorber has a wide range of applications which include solar energy harvesting, imaging, photodetection etc. In this regard, InAs nanowire (NW) based structure is investigated in this work for achieving an ultra broadband perfect absorber. Finite difference time domain (FDTD) based numerical analysis has been performed to optimize the InAs nanowire based structure to obtain an efficient light absorber by varying different dimensional parameters. Mie theory and guided mode resonance based theoretical analysis is developed to validate the results and to get an insight into the tunability of the nanowire based structure. Moreover, the theoretical analysis elucidates the underlying physics of light absorption in nanowires. To achieve ultra broadband absorption, multi radii InAs nanowire based arrays are investigated and it is found that they exhibit superior performance compared to single radius NW based structures. The computed light absorption efficiency (LAE) and short circuit current density values are enhanced to 97% and 40.15 mA cm-2 at 10° angle of incidence for the optimized quad radii NW array within the wavelength range of 300 nm to 1000 nm and 300 nm to 1200 nm, respectively. Moreover, the absorption spectra for these structures are polarization independent and exhibit robust performance for varying angle of incidence. In addition, arrangement of the NW array (hexagonal or square) has negligible effect on the absorption spectra. Such ultra-broadband absorption capability of the proposed structure compared to existing works suggests that the InAs nanowire based structure is very promising as light absorber with prospects in the fields of photo detection, solar power generation, perfect cloaking, photochemistry and other thin film photonic devices.

Thermal transport characterization of carbon and silicon doped stanene nanoribbon: an equilibrium molecular dynamics study.

Equilibrium molecular dynamics simulation has been carried out for the thermal transport characterization of nanometer sized carbon and silicon doped stanene nanoribbon (STNR). The thermal conduction properties of doped stanene nanostructures are yet to be explored and hence in this study, we have investigated the impact of carbon and silicon doping concentrations as well as doping patterns namely single doping, double doping and edge doping on the thermal conductivity of nanometer sized zigzag STNR. The room temperature thermal conductivities of 15 nm × 4 nm doped zigzag STNR at 2% carbon and silicon doping concentration are computed to be 9.31 ± 0.33 W m-1 K-1 and 7.57 ± 0.48 W m-1 K-1, respectively whereas the thermal conductivity for the pristine STNR of the same dimension is calculated as 1.204 ± 0.21 W m-1 K-1. We find that the thermal conductivity of both carbon and silicon doped STNR increases with the increasing doping concentration for both carbon and silicon doping. The magnitude of increase in STNR thermal conductivity due to carbon doping has been found to be greater than that of silicon doping. Different doping patterns manifest different degrees of change in doped STNR thermal conductivity. Double doping pattern for both carbon and silicon doping induces the largest extent of enhancement in doped STNR thermal conductivity followed by single doping pattern and edge doping pattern respectively. The temperature and width dependence of doped STNR thermal conductivity has also been studied. For a particular doping concentration, the thermal conductivity of both carbon and silicon doped STNR shows a monotonic decaying trend at elevated temperatures while an opposite pattern is observed for width variation i.e. thermal conductivity increases with the increase in ribbon width. Such comprehensive study on doped stanene would encourage further investigation on the proper optimization of thermal transport characteristics of stanene nanostructures and provide deep insight in realizing the potential application of doped STNR in thermoelectric as well as thermal management of stanene based nanoelectronic devices.

Stanene-hexagonal boron nitride heterobilayer: Structure and characterization of electronic property.

The structural and electronic properties of stanene/hexagonal boron nitride (Sn/h-BN) heterobilayer with different stacking patterns are studied using first principle calculations within the framework of density functional theory. The electronic band structure of different stacking patterns shows a direct band gap of ~30 meV at Dirac point and at the Fermi energy level with a Fermi velocity of ~0.53 × 106 ms-1. Linear Dirac dispersion relation is nearly preserved and the calculated small effective mass in the order of 0.05mo suggests high carrier mobility. Density of states and space charge distribution of the considered heterobilayer structure near the conduction and the valence bands show unsaturated π orbitals of stanene. This indicates that electronic carriers are expected to transport only through the stanene layer, thereby leaving the h-BN layer to be a good choice as a substrate for the heterostructure. We have also explored the modulation of the obtained band gap by changing the interlayer spacing between h-BN and Sn layer and by applying tensile biaxial strain to the heterostructure. A small increase in the band gap is observed with the increasing percentage of strain. Our results suggest that, Sn/h-BN heterostructure can be a potential candidate for Sn-based nanoelectronics and spintronic applications.