Twist Induced Tunability of Electronic and Optical Properties in a Van der Waal Janus Heterostructure

Tremendous research attention is currently in focus onto two-dimensional (2D) nano-materials and their heterostructure due to their rich physical properties and diverse technological applications [1, 2, 3]. Recent discoveries of 2D Janus materials with broken symmetry have shown promising potential for the current semiconductor industry due to their distinctive physical and chemical characteristics (strong Rashba effect, out-of-plane piezoelectric polarisation) [4, 5]. The 2D van der Waals heterostructures (vdWH) are smart artificial materials composed of two similar or different types of monolayers [6]. We have systematically studied the stability, electronic, and optical properties of nine different Janus vdWHs (MoS₂/MoSeTe, MoS₂/WSeTe, WS₂/MoSeTe, WS₂/WSeTe, MoSe₂/MoSeTe, MoSe₂/WSeTe, WSe₂/MoSeTe, WSe₂/WSeTe, and MoSeTe/WSeTe) through first-principles based calculations. Our results show that when TMD monolayers are stacked vertically with Janus monolayers, an intrinsic electric field appears because of the lack of mirror symmetry and charge accumulation, which leads to the origin of versatile properties in Janus vdWHs. In addition, the interlayer twist also induces an electronic phase crossover from indirect to direct bandgap semiconducting state at some specific rotation angles.

It is interesting to note that the Janus MoS₂/MoSeTe (θ = 19.11°, 40.89°), WS₂/WSeTe (θ = 19.11°, 30°), MoS₂/WSeTe (θ = 19.11°), WSe2/MoSeTe (θ = 16.10°), WS₂/MoSeTe (θ = 40.89°) vdWHs have direct bandgap with type-II band alignment, which is an essential parameter for photocatalytic water splitting applications. Furthermore, the Janus vdWHs are more efficient optical absorption from the UV to the visible light range. Our findings not only provide a compelling platform for exploring the behavior of the Janus vdWHs but also provides theoretical guidance for the designing of novel 2D nanodevices.