Control of band polarity in two-dimensional VX2 (X = S, Se, and Te)

Bipolar magnetic semiconductor (BMS) has special electronic structures; i.e., its conduction band minimum (CBM) and valence band maximum (VBM) are completely spin-polarized in opposite directions. In this work, the band structures of 2H-VX2 (X = S, Se, and Te) are examined through first-principles calculations, and the results show that both 2H-VS2 and 2H-VSe2 are BMSs, while 2H-VTe2 is a unipolar magnetic semiconductor (UMS); i.e., its CBM and VBM show the same spin direction. Most interestingly, we find that electronic orbitals near the Fermi level of 2H-VX2 are occupied by dz2 and dxy orbitals, which can be effectively modulated by the biaxial strain. With appropriate strain modulations, 2H-VX2 can be BMS, UMS, or half-metal (HM). Our investigation reveals strain effects on the band structure of 2H-VX2, which greatly enhances their significance in spintronics.