Emergence of -s, -p-dband inversion in zincblende gold iodide topological insulator and its thermoelectric properties.

We employfirst-principlescalculations to investigate the topological states (TS) and thermoelectric (TE) transport properties of three dimensional (3D) gold iodide (AuI) which belongs to the zincblende family. We explore, semi-metal (SM) to topological conductor (TC) and topological insulator (TI) phase transitions. Under pristine conditions, AuI exhibits Dirac SM nature but, under the influence of mild isotropic compressive pressure the system undergoes electronic quantum phase transition driving it into non-trivial topological state. This state exhibits Dresselhaus like band spin splitting leading to a TC state. In order to realize TI state from the SM state, we break the cubic symmetry of the system by introducing a compressive pressure along (001) crystal direction. The non-trivial TI nature of the system is characterized by the emergence of robust surface states and theZ2invariantν0= 1 which indicates a strong TI nature. A novel facet of the phase transition discussed here is, the -sand -p, -dorbital band inversion mechanism which is unconventional as compared to previously explored TI families. This mechanism unravels new path by which TI materials can be predicted. Also, we investigated the lattice and electronic contributions to the TE transport properties. We characterize the TE performance by calculating the figure of merit (zT) and find that, at room temperature (300 K) and for a fixed doping concentration (i.e.,n= 1 × 1019 cm-3) the zT is 0.55 and 0.53 for electrons and holes respectively. This is quite remarkable since, higher values of zT are generally predicted at higher temperature scales whereas, zT values as in the present case are desired at room temperatures for various energy applications. The manifestation of non-trivial TS governed by the unconventional band inversion mechanism and the TE properties of AuI make it a unique multi-functional candidate with probable thermoelectric and spintronic applications.

Electron density modulation of a metallic GeSb monolayer by pnictogen doping for excellent hydrogen evolution.

The catalyst assisted water-splitting method as an eco-friendly and cleaner pathway for energy generation has gained much interest in recent times. In this regard, numerous two-dimensional electrocatalysts such as mono/binary compounds synthesized from group IV, III-V and V elements with compatible activity towards hydrogen evolution, oxygen evolution, oxygen reduction and CO2 reduction have been reported. Motivated by the novel approach of material design and the need for better and cheaper electrocatalytic materials, we have investigated the ground state properties of the GeSb monolayer using state-of-the-art density functional theory. The computed electronic properties reveal the metallic nature of the pristine GeSb monolayer, indicating its potential for utilization as an electrocatalyst. The site-dependent catalytic response of the GeSb monolayer indicates that the Sb-site is more sensitive towards hydrogen adsorption amongst the considered sites. The computed adsorption and Gibbs free energies follow the trend of E < E < E. Finally, we have investigated the role of arsenic (As) and bismuth (Bi) doping on the catalytic activity of the GeSb monolayer. We notice that the electron density modulation occurs at the Sb-site due to incorporation of substitutional doping which results in a 72% enhancement in the catalytic activity of the monolayer on As substitution. The present study envisages that the electron density modulation can be utilized as a pathway for tailoring the catalytic activity of a system for the hydrogen evolution reaction.

ab initio Energetics and Thermoelectric Profiles of Gallium Pnictide Polytypes.

The state-of-the-art Density Functional Theory (DFT) is utilized to investigate the structural, electronic, vibrational, thermal and thermoelectric properties of gallium pnictides GaX (X = P, As, Sb) in cubic zincblende (ZB) and hexagonal wurtzite (WZ) phases. The lattice parameters, bulk modulus, energy band nature and bandgap values, phonon, thermal and thermoelectric properties are revisited for ZB phase while for WZ phase they are predictive. Our results agree reasonably well with the experimental and theoretical data wherever they are available. The phonon dispersion curves are computed to validate the dynamic stability of these two polytypes and for further investigating the thermal and thermoelectric properties. Our computed thermoelectric figure of merit ZT gives consistent results with highest observed magnitude of 0.72 and 0.56 for GaSb compound in ZB and WZ phases respectively. The first time calculated temperature variation of lattice thermal conductivity for WZ phase shows lower value than ZB phase and hence an important factor to enhance the figure of merit of considered gallium pnictides in WZ phase. Present results validate the importance of GaX in high temperature thermoelectric applications as the figure of merit ZT shows enhancement with significant reduction in thermal conductivity at higher temperature values.