High electron mobility, quantum Hall effect and anomalous optical response in atomically thin InSe

Encapsulated few-layer InSe exhibits a remarkably high electronic quality, which is promising for the development of ultrathin-body high-mobility nanoelectronics. A decade of intense research on two-dimensional (2D) atomic crystals has revealed that their properties can differ greatly from those of the parent compound 1 , 2 . These differences are governed by changes in the band structure due to quantum confinement and are most profound if the underlying lattice symmetry changes 3 , 4 . Here we report a high-quality 2D electron gas in few-layer InSe encapsulated in hexagonal boron nitride under an inert atmosphere. Carrier mobilities are found to exceed 10 3 cm 2 V −1 s −1 and 10 4 cm 2 V −1 s −1 at room and liquid-helium temperatures, respectively, allowing the observation of the fully developed quantum Hall effect. The conduction electrons occupy a single 2D subband and have a small effective mass. Photoluminescence spectroscopy reveals that the bandgap increases by more than 0.5 eV with decreasing the thickness from bulk to bilayer InSe. The band-edge optical response vanishes in monolayer InSe, which is attributed to the monolayer's mirror-plane symmetry. Encapsulated 2D InSe expands the family of graphene-like semiconductors and, in terms of quality, is competitive with atomically thin dichalcogenides 5 , 6 , 7 and black phosphorus 8 , 9 , 10 , 11 .