Toward direct band gaps in typical 2D transition-metal dichalcogenides junctions via real and energy spaces tuning

Most of the van der Waals homo- and hetero-junctions of group VIB two-dimensional (2D) transition-metal dichalcogenides (TMDs; MoS2, WS2, MoSe2, and WSe2) show indirect energy band gaps which hinders some of their applications especially in optoelectronics. In the current work, we demonstrate that most of the bilayers and even few-layers consisting of group VIB TMDs can have direct gaps by efficient weakening of their interlayer interactions via real and/or energy spaces tuning, which is based on insights from quantitative analyses of interlayer electronic hybridizations. Real space tuning here means introducing large-angle rotational misalignment between layers, which has been realized in a very recent experiment; and, energy space tuning means introducing energy mismatch between layers which can be introduced efficiently by different means thanks to the small vertical dielectric constant of 2D semiconducting TMDs. The efficient tuning in both real and energy spaces proposed here paves an avenue for indirect-direct gap regulation of homo- and hetero-junctions of TMDs and other 2D semiconductors. Notably, both tuning can be permanently preserved and hence our work is of great significance for the diverse applications of 2D semiconductors. Most van der Waals homo- and hetero-junctions of 2D transition-metal dichalcogenides of group VIB have indirect bandgaps. Here, the authors demonstrate a way of inducing direct gaps in these systems by tuning interlayer interactions via rotational misalignment or energy mismatch between layers.