Bandgap modulation in the two-dimensional core-shell-structured monolayers of WS.

Tungsten disulfide (WS) has tunable bandgaps, which are required for diverse optoelectronic device applications. Here, we report the bandgap modulation in WS monolayers with two-dimensional core-shell structures formed by unique growth mode in chemical vapor deposition (CVD). The core-shell structures in our CVD-grown WS monolayers exhibit contrasts in optical images, Raman, and photoluminescence spectroscopy. The strain and doping effects in the WS, introduced by two different growth processes, generate PL peaks at 1.83 eV (at the core domain) and 1.98 eV (at the shell domain), which is distinct from conventional WS with a primary PL peak at 2.02 eV. Our density functional theory (DFT) calculations explain the modulation of the optical bandgap in our core-shell-structured WS monolayers by the strain, accompanying a direct-to-indirect bandgap transition. Thus, the core-shell-structured WS monolayers provide a practical method to fabricate lateral heterostructures with different optical bandgaps, which are required for optoelectronic applications.