First principles study of structural, electronic, optical, and electron transport properties of novel hybrid monolayers SixGeySn1-x-y with NH3 sensing application

Abstract

At the nanoscale, two-dimensional (2D) materials are showing great promise in improving device mobility, on current density, on/off current ratio, subthreshold swing, and contact resistance, etc. Furthermore, contemporary three-dimensional integration of logic, memory, and optoelectronic devices into a single nanoscaled chip requires 2D materials for transistor channel, thermal insulator, light emitters, and photodetectors etc. Since the emergence of graphene, researchers have been investigating novel 2D materials that can combine structural stability with superior electronic, thermal and optical properties. Group-IV elemental monolayers (graphene, silicene, germanene, and stanene) offer many fascinating characteristics such as tunable energy bandgap, very high charge carrier mobility, superconductivity, enhanced optical conductivity. However, these materials have limited their application in digital electronics due to their semimetallic property. Other two-dimensional materials have been studied, including transition metal dichalcogenides, hexagonal boron nitrides, and phosphorene, but they do not surpass graphene in terms of other electrical, thermal, and optical characteristics. Stable two-dimensional materials with graphene-like characteristics and a considerable energy bandgap are of great scientific interest. In this work, the structural, electronic, optical, and electron transport properties of three different atomically thin novel hybrid monolayers comprising of Si, Ge, and Sn atoms with varying proportions are studied using first principles calculations within the framework of density functional theory that combine superior electronic and optical properties with considerable energy bandgap. The fabrication of similar hybrid materials is practically realizable but the study of different properties of these novel monolayers is yet to explore. 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 bandstructures of these hybrid 2D monolayers, namely Ge0.25Sn0.25Si0.50, Si0.25Ge0.25Sn0.50, and Sn0.25Si0.25Ge0.50 show 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×m0 – 0.101×m0), where m0 is the rest mass of electron, and very high acoustic phonon limited mobility (~106 cm2V-1s-1) of the charge carriers inside the hybrid monolayers ensure the presence of relativistic-massless Dirac fermion. In order to further investigate the electronic properties, we have calculated 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 (~104 cm-1) over a wide range of frequency, high refractive indices (~1.8) suggest these hybrid monolayers as potential candidates for optoelectronic applications. Three different armchair nanoribbons have been designed using these novel monolayers to study the effect of the adsorption of NH3 molecules on these hybrid nanoribbons. Calculated electron transport properties ensure the applications of these nanoribbons as NH3 sensor at the molecular level. Electron transport properties are also investigated in the presence of point defects to understand the effect of defects on the transport properties of these nanoribbons. Thus, these results suggest that the proposed SixGeySn1-x-y hybrid monolayers can be a potential candidate for nanoelectronics, optoelectronics and, sensor-based applications.