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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.