title: Universal Mechanism of Band-Gap Engineering in Transition-Metal Dichalcogenides.
abstract: van der Waals two-dimensional (2D) semiconductors have emerged as a class of materials with promising device characteristics owing to the intrinsic band gap. For realistic applications, the ideal is to modify the band gap in a controlled manner by a mechanism that can be generally applied to this class of materials. Here, we report the observation of a universally tunable band gap in the family of bulk 2H transition metal dichalcogenides (TMDs) by in situ surface doping of Rb atoms. A series of angle-resolved photoemission spectra unexceptionally shows that the band gap of TMDs at the zone corners is modulated in the range of 0.8-2.0 eV, which covers a wide spectral range from visible to near-infrared, with a tendency from indirect to direct band gap. A key clue to understanding the mechanism of this band-gap engineering is provided by the spectroscopic signature of symmetry breaking and resultant spin-splitting, which can be explained by the formation of 2D electric dipole layers within the surface bilayer of TMDs. Our results establish the surface Stark effect as a universal mechanism of band-gap engineering on the basis of the strong 2D nature of van der Waals semiconductors.
authors.affiliation: Department of Physics, Pohang University of Science and Technology , Pohang 37673, Korea.
authors.forename: Mingu
authors.initials: M
authors.lastname: Kang
authors.affiliation: Department of Physics, Pohang University of Science and Technology , Pohang 37673, Korea.|Advanced Light Source, E. O. Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.
authors.forename: Beomyoung
authors.initials: B
authors.lastname: Kim
authors.affiliation: Department of Physics, Pohang University of Science and Technology , Pohang 37673, Korea.
authors.forename: Sae Hee
authors.initials: SH
authors.lastname: Ryu
authors.affiliation: Department of Physics, Pohang University of Science and Technology , Pohang 37673, Korea.
authors.forename: Sung Won
authors.initials: SW
authors.lastname: Jung
authors.affiliation: Department of Physics, Pohang University of Science and Technology , Pohang 37673, Korea.
authors.forename: Jimin
authors.initials: J
authors.lastname: Kim
authors.affiliation: Department of Physics, Pohang University of Science and Technology , Pohang 37673, Korea.|Advanced Light Source, E. O. Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.
authors.forename: Luca
authors.initials: L
authors.lastname: Moreschini
authors.affiliation: Advanced Light Source, E. O. Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.
authors.forename: Chris
authors.initials: C
authors.lastname: Jozwiak
authors.affiliation: Advanced Light Source, E. O. Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.
authors.forename: Eli
authors.initials: E
authors.lastname: Rotenberg
authors.affiliation: Advanced Light Source, E. O. Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.
authors.forename: Aaron
authors.initials: A
authors.lastname: Bostwick
authors.affiliation: Department of Physics, Pohang University of Science and Technology , Pohang 37673, Korea.
authors.forename: Keun Su
authors.identifier: 0000-0002-7033-1177
authors.initials: KS
authors.lastname: Kim
keywords: Band-gap engineering; giant Stark effect; transition-metal dichalcogenides; two-dimensional semiconductors