Measurement of direct and indirect bandgaps in synthetic ultrathin MoS₂ and WS₂ films from photoconductivity spectra

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