Optical Response and Stability of MA2Z4 and Pentagonal Two-Dimensional Materials

Abstract

Janus two-dimensional (2D) materials exhibit rich physical properties and novel fundamental effects. The recent discovery and synthesis of MA2Z4 2D materials prompt further exploration of a new family of van der Waals materials. We investigate monolayer MA2Z4 and Janus MABZ4 (M = Mo, W; A, B = Si, Ge; Z = N, P) materials. Using first-principles density functional theory, we show that Janus structuring leads to out-of-plane nonlinear responses, enabling applications in optoelectronics such as shift current photovoltaics. We find that phosphorous-based MA2Z4 2D crystals exhibit enhanced nonlinear response beyond 2D MX2 materials. We also explore pentagonal 2D MX2 materials, and the preliminary results show metallic features in electronic band structures and strong optical response at low frequency as well as dynamic instabilities. In summary, our theoretical results suggest that 2D MA2Z4 and pentagonal materials offer further research opportunities for optoelectronic materials design with Janus structuring and non-conventional unit cells.