Scanning Raman spectroscopy of few- and single-layer MoS2flakes

Abstract

The dichalcogenide MoS 2 , which is an indirect-gap semiconductor in its bulk form, was recently shown to become an efficient emitter of photoluminescence as it is thinned to a single layer, indicating a transition to a direct-gap semiconductor due to confinement effects. With its layered structure of weakly coupled, covalently bonded twodimensional sheets, it can be prepared, just as graphene, using mechanical exfoliation techniques. With these techniques, few- and single-layer flakes can be prepared. Raman spectroscopy is a sensitive tool to determine the number of layers of a flake, as two characteristic Raman modes in MoS 2 shift to higher or lower frequency with the number of layers. In addition to previously reported Raman modes in MoS 2 , we observe an interlayer shear mode at very low frequencies, which also shifts with the number of layers. We use scanning Raman spectroscopy to map and characterize MoS 2 flakes.