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Title : Indirect Bandgap Puddles in Monolayer MoS₂ by Substrate-Induced Local Strain
Authors : By Bong Gyu Shin, Gang Hee Han, Seok Joon Yun, Hye Min Oh, Jung Jun Bae, Young Jae Song, Chong-Yun Park, and Young Hee Lee
Abstract : Monolayer transition metal dichalcogenides (TMdCs) have attracted considerable attention due to their unusual direct bandgap for which a wide range of bandgaps can be obtained based on the chosen material. However, the bandgaps that are optically or electronically measured exhibit variety due to dielectric screening effect or external strain. For monolayer TMdCs, atoms are exposed to an environment so that extrinsic effects are inevitable. Here, we report an unusually large bandgap modulation of 1.23-2.65 eV from monolayer MoS2 on Si substrate, which is induced by a substrate, using scanning tunneling microscopy and spectroscopy. The direct bandgap was converted to an indirect bandgap at a bending strain of approximately 1.5%, which was provoked by the inherent local morphology corrugation from naturally formed oxides on Si substrate. The percolated indirect ‘bandgap puddles’ in direct bandgap MoS2, mostly by the valence band maximum modulation, were strongly correlated with the local strain of the MoS2 monolayer. Approximately 80% of the surface area revealed an indirect bandgap in contrast with the general belief of a direct bandgap in monolayer MoS2, which was confirmed further by the degraded photoluminescence compared to that from suspended MoS2.