Tunable Electronic Properties of Novel 2D Janus MSiGeN4 (M = Ti, Zr, Hf) Monolayers by Strain and External Electric Field

Since the recent successful experimental synthesis of MoSi2N4 [Science, 369 (2020), 670], the "MA(2)Z(4) family" has been of particular interest to the scientists in the field of materials science due to its outstanding physical properties. In this paper, the first-principles calculations are performed to study the structural, elastic, and electronic properties of novel two-dimensional (2D) Janus MSiGeN4 monolayers (M = Ti, Zr, Hf). The calculations of phonon spectra indicate that monolayers MSiGeN4 are dynamically stable and can be experimentally synthesized. The obtained Young's modulus and Poisson's ratio of the Janus structures MSiGeN4 are much larger than that of other binary 2D materials and meet the mechanical stability criteria suggested by Born and Huang. In the calculations using either PBE or HSE06 functionals, the Janus MSiGeN4 structures exhibit indirect semiconductor characteristics with larger band gaps than that of similar septuple-atomic-layer materials, such as MoSiGeN4 and WSiGeN4. In addition, the influences of biaxial strain and external electric field on the electronic structure of MSiGeN4 are investigated. It is found that the biaxial strain tunes the electronic characteristics more significantly than the external electric field. The obtained results can provide insights into novel Janus monolayers with potential applications in electronic devices.