Enhanced exciton emission behavior and tunable band gap of ternary W(SxSe1−x)2 monolayer: temperature dependent optical evidence and first-principles calculations

Abstract

Up to date, the electronic and optical properties of WS₂ and WSe₂ have been widely explored. However, the synthesis and characterization of their ternary alloy nanosheets have been rarely reported. Here, we fabricated single layer W(S_xSe_1−x)₂ nanosheets by a one-step chemical vapor deposition (CVD) method. It is demonstrated that exciton emission behavior of single layer W(S_xSe_1−x)₂ nanosheets (0 ≤ x ≤ 1) can be remarkably tuned by changing the sulfur content. The theoretical calculations proved that single layer W(S_xSe_1−x)₂ alloy has a direct gap, and the band gap can be tuned by the sulfur content, which is in accordance with the spectral experiments. Moreover, we present temperature-dependent photoluminescence (PL) measurements in monolayers of W(S_xSe_1−x)₂ alloys from 80 K to 320 K. The neutral exciton (X) and charged exciton (trion, T) can be observed at all measured temperatures. In sulfur-rich ternary W(S_xSe_1−x)₂ alloys, the trion dominates the PL spectra at low temperatures while the exciton dominates the PL spectra at higher temperatures. In selenium-rich ternary alloys, however, the exciton is dominant in PL spectra at all measured temperatures. As the sulfur content gradually increases, the intensity ratio of the trion to exciton becomes dramatically larger. There is an obvious upward trend of the trion intensity in W(S_xSe_1−x)₂ monolayers, which results from the significant growth of the two-dimensional electron gas (2DEG) concentration. On the other hand, the strong exciton–trion coupling mediated by an optical phonon also contributes to this improvement. These results indicate that exciton emission behavior of W(S_xSe_1−x)₂ monolayers is controllable with the sulfur content. It highlights the importance of further detailed characterization on exciton features in ternary alloy nanosheets, and can enable spectral tunability for potential optoelectronic applications.