Bandgap broadening at grain boundaries in single-layer MoS2

Two-dimensional semiconducting transition-metal dichalcogenides have attracted considerable interest owing to their unique physical properties and future device applications. In particular, grain boundaries (GBs) have been often observed in single-layer MoS2 grown via chemical vapor deposition, which can significantly influence the material properties. In this study, we examined the electronic structures of various GBs in single-layer MoS2 grown on highly oriented pyrolytic graphite using low-temperature scanning tunneling microscopy/spectroscopy. By measuring the local density of states of a series of GBs with tilt angles ranging from 0° to 25°, we found that the bandgaps at the GBs can be either broadened or narrowed with respect to the intrinsic single-layer MoS2. The bandgap broadening shows that the GBs can become more insulating, which may directly influence the transport properties of nanodevices based on polycrystalline single-layer MoS2 and be useful for optoelectronics.