First-principles insights on electronic and transport properties of novel ternary AlM X[sub]3[/sub] and quaternary Janus Al [sub]2[/sub] M [sub]2[/sub] X [sub]3[/sub] Y [sub]3[/sub] (M = Ge, Sn; X / Y = S, Se, Te) monolayers

In this paper, we design and investigate the stability, electronic characteristics, and transport parameters of a series of 12 two-dimensional materials, including six ternary AlMX 3 and six quaternary Janus Al 2 M 2 X 3 Y 3 (M = Ge, Sn; X / Y = S, Se, and Te; X not equal Y) monolayers by using first-principles calculations based on the density functional theory. The phonon spectra and Born elastic stability criteria are used to consider the stability of the studied materials. Our examinations indicate that, except for Janus Al 2 Sn 2 S 3 Te 3 monolayer, the other structures are structurally stable. While the ternary AlMX 3 monolayers are found to be indirect semiconductors, quaternary Janus Al 2 M 2 X 3 Y 3 structures can be either direct semiconductors or metallic (Al 2 Ge 2 S 3 Te 3 ). Interestingly, Mexican hat -like dispersion can be found in the topmost valence band of ternary AlMX 3 monolayers, which are not observed in their corresponding Janus structures. All studied semiconducting materials exhibit a high electron mobility, up to 1 . 51 x 10 3 cm 2 V -1 s -1 for Al 2 Sn 2 Se 3 Te 3 monolayer. The electron mobilities of the studied structures are high directional isotropic along the x and y axes and much higher than those of the hole mobility. The unique electronic and transport features of ternary AlMX 3 and quaternary Janus Al 2 M 2 X 3 Y 3 monolayers can be suitable for applications in electronic nanodevices.