Microscopic basis for the band engineering of Mo1−xWxS2-based heterojunction

Transition-metal dichalcogenide layered materials, consisting of a transition-metal atomic layer sandwiched by two chalcogen atomic layers, have been attracting considerable attention because of their desirable physical properties for semiconductor devices and a wide variety of pn junctions, which are essential building blocks for electronic and optoelectronic devices, have been realized using these atomically thin structures. Engineering the electronic/optical properties of semiconductors by using such heterojunctions has been a central concept in semiconductor science and technology. Here, we report the first scanning tunneling microscopy/spectroscopy (STM/STS) study on the electronic structures of a monolayer WS 2 /Mo 1−x W x S 2 heterojunction that provides a tunable band alignment. The atomically modulated spatial variation in such electronic structures, i.e., a microscopic basis for the band structure of a WS 2 /Mo 1−x W x S 2 heterojunction, was directly observed. The macroscopic band structure of Mo 1−x W x S 2 alloy was well reproduced by the STS spectra averaged over the surface. An electric field of as high as 80 × 10 6 Vm −1 was observed at the interface for the alloy with x = 0.3, verifying the efficient separation of photoexcited carriers at the interface.