Band alignment and optical features in Janus-MoSeTe/X(OH)2 (X = Ca, Mg) van der Waals heterostructures
Van der Waals heterostructures can be effectively used to enhance the electronic and optical properties and extend the application range of two-dimensional materials. Here, we construct for the first time the MoSeTe/X(OH)$_2$ (X = Ca, Mg) heterostructures and investigate their electronic and optical properties as well as the relative orientation of these layers with respect to each other and the effects of electric field. Our results show that in the MoSeTe/X(OH)$_2$ heterostructures, Janus MoSeTe monolayer is bonded to X(OH)$_2$ layer via the weak van der Waals forces. Owing to different kinds of chalcogen Se and Te atoms in both sides of Janus MoSeTe, there exist two main stacking types of the MoSeTe/X(OH)$_2$ heterostructures, that are MoSeTe-Se/X(OH)$_2$ and MoSeTe-Te/X(OH)$_2$ heterostructures. Interestingly, the Se- and Te-interface can induce straddling type-II and type-I band alignments. The MoSeTe-Se/X(OH)$_2$ heterostructure exhibits a type-II band alignment, making it a potential ability to separate the photogenerated electrons and holes. Whereas, the MoSeTe-Te/Ca(OH)$_2$ heterostructure displays the type-I band alignment, which may result in a ultrafast recombination between electrons and holes, making MoSeTe-Te/Ca(OH)$_2$ heterostructure suitable material for optoelectronic application. The MoSeTe/X(OH)$_2$ heterostructures show isotropic behavior at low energy region while anisotropic at high photon energy region. The dielectric function of MoSeTe-Te/Ca(OH)$_2$ heterostructure is high at low photon energy relative to other heterostructures verifying it to have good optical absorption. Furthermore, the band gap values and band alignment of the MoSeTe/X(OH)$_2$ heterostructures can be modulated by applying electric field, which induces the semiconductor-to-metal and type-I(II) to type-II(I) band alignment. These results demonstrate that the MoSeTe/X(OH)$_2$ heterostructures are promising candidates for optoelectronic and photovoltaic nanodevices.