Nanoscale mapping of local intrinsic strain-induced anomalous bandgap variations in WSe2 using selective-wavelength scanning photoconductivity microscopy

Abstract

Nanoscale defects can locally tailor the optoelectronic properties of two-dimensional materials such as bandgap. However, it is still challenging to directly map and analyze the defect-induced bandgap variations in a quantitative manner. Here, we report the nanoscale mapping of local intrinsic strain-induced anomalous bandgap variations in as-grown WSe₂ by using a selective-wavelength scanning photoconductivity microscopy. In this method, a conducting nanoprobe is utilized to map the spatial distributions of strain, sheet-conductance, and charge trap density (N_eff) of epitaxially-grown WSe₂ on sapphire while illuminating monochromatic light of selective wavelengths. Under un-illuminated conditions, the maps showed WSe₂ domain structures with boundaries having slightly lower sheet-conductance and higher N_eff than those in domains, presumably due to low misorientation angles between adjacent epitaxially-grown domains. By measuring wavelength-dependent photoconductance maps and fitting each pixel value of the maps, we could successfully obtain the map of local bandgap. The map clearly showed bandgap variations inside domains as well as rather low bandgaps at boundaries. By comparing with local strain map, we found that tensile strain effectively reduced bandgap following two different power-law relationships. Such anomalous bandgap variations in WSe₂ could be explained by strain-induced weakening of d-orbital couplings. Furthermore, in a monolayer-bilayer WSe₂ interface, bilayer WSe₂ exhibited lower bandgap than the underlying monolayer WSe₂, while interfacial boundaries exhibited the strain-induced bandgap values in between those of monolayer and bilayer WSe₂. Since our method allows us to directly map the intrinsic strain-induced bandgap variations with a nanoscale resolution down to tens-of-nm, it can be a powerful tool for basic research and practical applications of optoelectronic devices based on two-dimensional materials.