Using scanning tunneling spectroscopy (STS) and optical reflectance contrast measurements, we examine band-gap properties of single layers of transition metal dichalcogenide (TMDC) alloys: $\mathrm{Mo}{\mathrm{S}}_2$, $\mathrm{M}{\mathrm{o}}_{0.5}{\mathrm{W}}_{0.5}{\mathrm{S}}_2$, $\mathrm{M}{\mathrm{o}}_{0.25}{\mathrm{W}}_{0.75}{\mathrm{S}}_2$, $\mathrm{M}{\mathrm{o}}_{0.1}{\mathrm{W}}_{0.9}{\mathrm{S}}_2$, and $\mathrm{W}{\mathrm{S}}_2$. The quasiparticle band gap, spin-orbit separation of the excitonic transitions at the $K/K^{\prime }$ point in the Brillouin zone, and binding energies of the $A$ exciton are extracted from STS and optical data. The exciton binding energies change roughly linearly with tungsten concentration. For our samples on an insulating substrate, we report quasiparticle band gaps from $2.17\pm 0.04\mathrm{eV}\left(\mathrm{Mo}{\mathrm{S}}_2\right)$ to $2.38\pm 0.06\mathrm{eV}\left(\mathrm{W}{\mathrm{S}}_2\right)$, with $A$ exciton binding energies ranging from 310 to 420 meV.

• Received 15 April 2016

DOI:https://doi.org/10.1103/PhysRevB.94.075440