Evolution of the Bandgap in WS₂, from Monolayers to Multilayers, with Different Film Fabrication Methods

The appeal of transition metal dichalcogenides (TMD) stems from their 2D character as well as the versatility enabled by the tuning of the electronic bandgap by adjusting composition and layer number. While theoretical investigations have predicted the evolution of the gap in TMDs, including WS₂[1], with a common consensus that it increases as the number of layers decreases, no systematic experimental work has been done to study how the gap changes with the number of layers. Furthermore, the effects of the preparation methods on the electronic properties of the TMD films are still unknown. In this study, we explore the optoelectronic properties of mechanically transferred monolayer, bilayer, trilayer, and quadrilayer WS₂ in ABAB stacking and compare them with directly deposited CVD-grown high-quality and large-scale (1 cm²) monolayers and multilayer (&gt; 10 layers). The transferred layers are obtained by exfoliation from a WS₂ single crystal [2]. Using ultraviolet and inverse photoelectron spectroscopies (UPS/IPES), we determine Fermi level position, conduction band minimum (CBM), valence band maximum (VBM), and thus the electronic bandgap (E_G) of the materials as a function of the number of layers. We find that the bandgap of the mechanically transferred and stacked WS₂ decreases from 2.43 eV for monolayer, to 2.19 eV for bilayer, to ~1.97 eV for trilayer and quadrilayer, signaling a bulk transition at trilayer from the experimental perspective. The 0.45 eV bandgap reduction comes mainly from a CBM shift away from E_vac by 0.37 eV, as well as a VBM shift toward E_vac by 0.08 eV. We further compare the electronic properties of the transferred monolayer and multilayers with the CVD-grown samples. The bandgap of the CVD-grown monolayer WS₂ is found to be 2.56 eV, slightly larger than the bandgap of the mechanically transferred monolayer. This is consistent with the optical bandgap (E_opt) trend measured by UV-Vis absorption and photoluminescence (PL) emission. The difference becomes more negligible with multi-layer films: the bandgap for CVD-grown multilayer is 1.90 eV, consistent with the bandgap of mechanically transferred WS₂ (≥ 3 layers). By combining UPS/IPES with UV-Vis measurements, we also determine the exciton binding energy (E_B) for monolayer WS₂, which is 0.55 eV for the CVD-grown monolayer and 0.43 eV for the mechanically transferred monolayer. This series of experiments helps incorporate WS₂ in devices such as LEDs and photovoltaics with a more comprehensive band alignment in practice. The resilience of WS₂ in different fabrication methods, as demonstrated in comparable optoelectrical properties of layers either mechanically transferred or CVD-grown, will be important to help scale up the use of WS₂ in other applications.<br/><br/>[1] A. Kuc, N. Zibouche, and T. Heine, “Influence of quantum confinement on the electronic structure of the transition metal sulfide TS2,” <i>Phys. Rev. B - Condens. Matter Mater. Phys.</i>, vol. 83, no. 24, p. 245213, Jun. 2011, doi: 10.1103/PhysRevB.83.245213.<br/>[2] F. Liu <i>et al.</i>, “Disassembling 2D van der Waals crystals into macroscopic monolayers and reassembling into artificial lattices,” <i>Science (80-. ).</i>, vol. 367, no. 6480, pp. 903–906, Feb. 2020, doi: DOI: 10.1126/science.aba1416.