  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. These differences are governed by changes in the band structure due
to quantum confinement and are most profound if the underlying lattice symmetry
changes. 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 1,000 and 10,000 cm2/Vs 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 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 and black
phosphorus.
