Electric field enhancement effect on Raman spectra in two-dimensional MoSi[formula omitted]N[formula omitted], TiSi[formula omitted]N[formula omitted] and MoGe[formula omitted]As[formula omitted] monolayers

Abstract

Characterizing two-dimensional (2D) layered materials in the monolayer limit is key to discovering their new phenomena and unusual properties. We theoretically predict the electric field modulated Raman spectra of 2D MoSi2N4, TiSi2N4 and MoGe2As4 monolayers, with two kinds of laser lines (532 and 633 nm). Based on the Raman tensor, we have calculated the angle-dependent Raman intensities of the major Raman peak (A1′1) in MoSi2N4, TiSi2N4 and MoGe2As4 monolayers, locating at 1029 cm−1, 993 cm−1 and 317 cm−1, respectively. Applying the external electric field to MoSi2N4 monolayer, the symmetry of D3h reduces to C3v, which results in more peaks of Raman-active modes found in the Raman spectra. As a result, besides E′ and A1′ modes, three other major Raman peaks (A1′1, A1′2 and A1′3) are found in MoSi2N4, TiSi2N4 and MoGe2As4 monolayers, which can help us to distinguish the 2D MA2Z4 materials. We find that the enhancement factor by external electric field can be up to 10 times. Moreover, the non-resonant Raman spectra peaks of doubly degenerate mode (E′) and one-dimensional modes A1′ and A2″ in MoSi2N4, TiSi2N4 and MoGe2As4 monolayer are calculated. These theoretical results of Raman spectra of MoSi2N4 monolayer may provide theoretical guidelines for its utilization in crystal structure characterizations and optoelectric devices.