Measurement of direct and indirect bandgaps in synthetic ultrathin MoS₂and WS₂films from photoconductivity spectra

Exploring the thickness-dependent electronic properties of ultrathin transition metal dichalcogenides is crucial for novel optoelectronic devices. Particularly important is experimental information regarding the bandgap width. This information is scarce and often inconsistent among the several measurement techniques that were employed for this task, such as optical absorption, scanning tunneling spectroscopy, and photoconductivity. Here, we present photoconductivity measurements in large-area synthetic MoS2 and WS2 films (one to five monolayers and the bulk crystal) grown on insulating layers (SiO2, Al2O3, or HfO2). The excitonic peaks of MoS2 and WS2 were detected in both the photocapacitor and traditional in-plane geometries. Their contribution to the photoconductivity is explained by the electric field-assisted dissociation mechanism. We have separated the excitonic and free carrier components in the photocurrent spectra and extracted the direct and indirect bandgaps using the Tauc plot, revealing their dependencies on the number of monolayers.