title: Direct observation of the transition from indirect to direct bandgap in atomically thin epitaxial MoSe2.
abstract: Quantum systems in confined geometries are host to novel physical phenomena. Examples include quantum Hall systems in semiconductors and Dirac electrons in graphene. Interest in such systems has also been intensified by the recent discovery of a large enhancement in photoluminescence quantum efficiency and a potential route to valleytronics in atomically thin layers of transition metal dichalcogenides, MX2 (M = Mo, W; X = S, Se, Te), which are closely related to the indirect-to-direct bandgap transition in monolayers. Here, we report the first direct observation of the transition from indirect to direct bandgap in monolayer samples by using angle-resolved photoemission spectroscopy on high-quality thin films of MoSe2 with variable thickness, grown by molecular beam epitaxy. The band structure measured experimentally indicates a stronger tendency of monolayer MoSe2 towards a direct bandgap, as well as a larger gap size, than theoretically predicted. Moreover, our finding of a significant spin-splitting of ∼ 180 meV at the valence band maximum of a monolayer MoSe2 film could expand its possible application to spintronic devices.
authors.affiliation: 1] Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA [2] Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA.
authors.forename: Yi
authors.identifier: 0000000312048717
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan.
authors.forename: Tay-Rong
authors.initials: TR
authors.lastname: Chang
authors.affiliation: 1] Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA [2] Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, California 94305, USA [3] Department of Physics and Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, UK.
authors.forename: Bo
authors.initials: B
authors.lastname: Zhou
authors.affiliation: 1] Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA [2] Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, California 94305, USA.
authors.forename: Yong-Tao
authors.initials: YT
authors.lastname: Cui
authors.affiliation: 1] Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA [2] Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, California 94305, USA.
authors.forename: Hao
authors.initials: H
authors.lastname: Yan
authors.affiliation: 1] Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA [2] Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, California 94305, USA.
authors.forename: Zhongkai
authors.initials: Z
authors.lastname: Liu
authors.affiliation: 1] Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA [2] Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, California 94305, USA.
authors.forename: Felix
authors.initials: F
authors.lastname: Schmitt
authors.affiliation: 1] Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA [2] Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, California 94305, USA.
authors.forename: James
authors.initials: J
authors.lastname: Lee
authors.affiliation: 1] Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA [2] Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, California 94305, USA.
authors.forename: Rob
authors.initials: R
authors.lastname: Moore
authors.affiliation: 1] Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA [2] Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, California 94305, USA [3] Department of Physics and Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, UK.
authors.forename: Yulin
authors.initials: Y
authors.lastname: Chen
authors.affiliation: Department of Physics, Northeastern University, Boston, Massachusetts 02115, USA.
authors.forename: Hsin
authors.initials: H
authors.lastname: Lin
authors.affiliation: 1] Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan [2] Institute of Physics, Academia Sinica, Taipei 11529, Taiwan.
authors.forename: Horng-Tay
authors.initials: HT
authors.lastname: Jeng
authors.affiliation: Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
authors.forename: Sung-Kwan
authors.initials: SK
authors.lastname: Mo
authors.affiliation: Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
authors.forename: Zahid
authors.initials: Z
authors.lastname: Hussain
authors.affiliation: Department of Physics, Northeastern University, Boston, Massachusetts 02115, USA.
authors.forename: Arun
authors.initials: A
authors.lastname: Bansil
authors.affiliation: 1] Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA [2] Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, California 94305, USA.
authors.forename: Zhi-Xun
authors.initials: ZX
authors.lastname: Shen