BP/GaN and BP/GaP core/shell nanowires: theoretical insights into photovoltaic and gas-sensing abilities.

Abstract

DFT-based calculations were undertaken to, first, fully optimize and study the structural and electrical properties of bare BP nanowire (NW) in its hexagonal wurtzite (WZ) phase. The bare BP NW was found to have an indirect bandgap of 1.362 eV. Hence, the optimization of BP/GaN and BP/GaP core/shell nanowires (CSNWs) was performed to check if an indirect-to-direct band transition occurred. Both the CSNWs showed direct bandgaps of 0.225 eV and 1.252 eV, respectively. The Shockley-Queisser limits for the bare BP NW and BP/GaP CSNW were calculated and compared to gauge their respective photovoltaic efficiencies. The bare BP NW and BP/GaP CSNW yielded almost identical SQ efficiencies of 33.80% and 33.55%, respectively. However, as far as the nano- and micro-photovoltaic cell applications are concerned, the BP/GaP CSNW would be preferable, owing to its direct bandgap. Furthermore, the adsorption of some small oxide gases like carbon monoxide (CO), carbon dioxide (CO2), nitrogen dioxide (NO2) and sulfur dioxide (SO2) gases on BP/GaN and BP/GaP CSNWs was studied. On the basis of the charge transfer and work function mechanisms, NO2 and SO2 gases showed selectivity to be detected by both the CSNWs. However, the very highly escalated desorption times for these gases would reduce the repeatability of sensors. Conversely, both BP/GaN and BP/GaP CSNWs could find applications in the fabrication of entrapment devices for NO2 and SO2. The current-voltage (I-V) curves for the CSNWs before and after adsorption were also plotted and analyzed. The occurrence of negative differential conductance (NDC) can be observed in both the CSNWs. The CO2, NO2 and SO2 gases show significantly higher values of current than the pristine BP/GaN CSNW for voltages beyond 0.5 V. Thus, these gases are good proponents to be detected by BP/GaN CSNWs with negligible selectivity amongst them. However, CO@BP/GaN is a stand-out case with a characteristically unique NDC region at 0.9 V. In the case of BP/GaP CSNWs, CO and CO2 gases can be selectively detected, with a unique NDC region for CO at 0.9 V. Thus, the BP/GaP CSNW, in particular, stands out as an extremely versatile material that can be used to fabricate nano-photovoltaic and nano-sensing devices of the next generation.