Atomically thin semiconducting transition-metal dichalcogenides enable new insight into physics of many-body effects mediated by Coulomb and electron-phonon interactions. We report photoluminescence (PL) and reflectivity contrast (RC) measurements of excitons ($X$) and trions ($T$) and Raman spectra of phonons in monolayers (MLs) of $\mathrm{Mo}{\left({\mathrm{S}}_y{\mathrm{Se}}_{1-y}\right)}_2$ alloys with sulfur mole content up to $y=0.5$. We find the phonon energy crossing the trion binding energy with the sulfur mole content. Binary ${\mathrm{MoSe}}_2$ and ternary $\mathrm{Mo}{\left({\mathrm{S}}_y{\mathrm{Se}}_{1-y}\right)}_2$ alloys exhibit contrasting behavior in the temperature evolution of excitons and trions PL intensity from $T=7$ to 295 K. In ${\mathrm{MoSe}}_2$, the trion dominates PL spectra at low temperatures but the exciton dominates PL at higher temperature. In contrast, in ternary $\mathrm{Mo}{\left({\mathrm{S}}_y{\mathrm{Se}}_{1-y}\right)}_2$ alloys trions dominate PL spectra at all measured temperatures, with the trion to exciton PL intensity ratio increasing with sulfur content. We attribute the strong increase of the trion PL intensity in $\mathrm{Mo}{\left({\mathrm{S}}_y{\mathrm{Se}}_{1-y}\right)}_2$ MLs with increase of sulfur mole content to two effects: (i) strong increase of exciton-trion coupling mediated by the optical phonon, which is realized by tuning phonon energy through trion binding energy, and (ii) significant increase of two-dimensional electron gas concentration. We also demonstrate that increasing sulfur content in $\mathrm{Mo}{\left({\mathrm{S}}_y{\mathrm{Se}}_{1-y}\right)}_2$ alloys increases total PL intensity at high temperature. With the temperature growth from 7 to 295 K, the total PL in ${\mathrm{MoSe}}_2$ decreases about three orders of magnitude more than in $\mathrm{Mo}{\left({\mathrm{S}}_{0.5}{\mathrm{Se}}_{0.5}\right)}_2$.

• Received 18 January 2017
• Revised 2 May 2017

DOI:https://doi.org/10.1103/PhysRevB.95.195427