title: A Unified Understanding of the Thickness-Dependent Bandgap Transition in Hexagonal Two-Dimensional Semiconductors.
abstract: Many important layered semiconductors, such as hexagonal boron nitride (hBN) and transition-metal dichalcogenides (TMDs), are derived from a hexagonal lattice. A single layer of such hexagonal semiconductors generally has a direct bandgap at the high-symmetry point K, whereas it becomes an indirect, optically inactive semiconductor as the number of layers increases to two or more. Here, taking hBN and MoS2 as examples, we reveal the microscopic origin of the direct-to-indirect bandgap transition of hexagonal layered materials. Our symmetry analysis and first-principles calculations show that the bandgap transition arises from the lack of the interlayer orbital couplings for the band-edge states at K, which are inherently weak because of the crystal symmetries of hexagonal layered materials. Therefore, it is necessary to judiciously break the underlying crystal symmetries to design more optically active, multilayered semiconductors from hBN or TMDs.
authors.affiliation: Department of Emerging Materials Science, DGIST , Daegu 711-873, Korea.
authors.forename: Joongoo
authors.initials: J
authors.lastname: Kang
authors.affiliation: College of Materials Science and Engineering and Key Laboratory of Automobile Materials of MOE, Jilin University , Changchun 130012, China.
authors.forename: Lijun
authors.initials: L
authors.lastname: Zhang
authors.affiliation: Beijing Computational Science Research Center, Beijing 100094, China.
authors.forename: Su-Huai
authors.initials: SH
authors.lastname: Wei