Bandgap modulation in the two-dimensional core-shell-structured monolayers of WS₂

Tungsten disulfide (WS₂) has tunable bandgaps, which are required for diverse optoelectronic device applications. Here, we report the bandgap modulation in WS₂ 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 WS₂ monolayers exhibit contrasts in optical images, Raman, and photoluminescence spectroscopy. The strain and doping effects in the WS₂, 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 WS₂ 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 WS₂ monolayers by the strain, accompanying a direct-to-indirect bandgap transition. Thus, the core-shell-structured WS₂ monolayers provide a practical method to fabricate lateral heterostructures with different optical bandgaps, which are required for optoelectronic applications.