title: Electronic Structure, Surface Doping, and Optical Response in Epitaxial WSe2 Thin Films.
abstract: High quality WSe2 films have been grown on bilayer graphene (BLG) with layer-by-layer control of thickness using molecular beam epitaxy. The combination of angle-resolved photoemission, scanning tunneling microscopy/spectroscopy, and optical absorption measurements reveal the atomic and electronic structures evolution and optical response of WSe2/BLG. We observe that a bilayer of WSe2 is a direct bandgap semiconductor, when integrated in a BLG-based heterostructure, thus shifting the direct-indirect band gap crossover to trilayer WSe2. In the monolayer limit, WSe2 shows a spin-splitting of 475 meV in the valence band at the K point, the largest value observed among all the MX2 (M = Mo, W; X = S, Se) materials. The exciton binding energy of monolayer-WSe2/BLG is found to be 0.21 eV, a value that is orders of magnitude larger than that of conventional three-dimensional semiconductors, yet small as compared to other two-dimensional transition metal dichalcogennides (TMDCs) semiconductors. Finally, our finding regarding the overall modification of the electronic structure by an alkali metal surface electron doping opens a route to further control the electronic properties of TMDCs.
authors.affiliation: National Laboratory of Solid State Microstructures, School of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University , Nanjing 210093, China.|Stanford Institute of Materials and Energy Sciences, SLAC National Accelerator Laboratory , Menlo Park, California 94025, United States.|Advanced Light Source, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.
authors.forename: Yi
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: Department of Physics, University of California at Berkeley , Berkeley, California 94720, United States.|CIC nanoGUNE , 20018 Donostia-San Sebastian, Spain.|Ikerbasque, Basque Foundation for Science , 48011 Bilbao, Spain.
authors.forename: Miguel M
authors.initials: MM
authors.lastname: Ugeda
authors.affiliation: Department of Physics, University of California at Berkeley , Berkeley, California 94720, United States.
authors.forename: Chenhao
authors.initials: C
authors.lastname: Jin
authors.affiliation: Department of Physics, University of California at Berkeley , Berkeley, California 94720, United States.|Department of Chemical and Biological Engineering, Rensselaer Polytechnic Institute , Troy, New York 12180, United States.
authors.forename: Su-Fei
authors.initials: SF
authors.lastname: Shi
authors.affiliation: Department of Physics, University of California at Berkeley , Berkeley, California 94720, United States.
authors.forename: Aaron J
authors.initials: AJ
authors.lastname: Bradley
authors.affiliation: Department of Physics, University of California at Berkeley , Berkeley, California 94720, United States.|Departamento de Física de la Materia Condensada, Universidad Autónoma de Madrid , E-28049 Madrid, Spain.
authors.forename: Ana
authors.initials: A
authors.lastname: Martín-Recio
authors.affiliation: Advanced Light Source, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.|Max Plank POSTECH Center for Complex Phase Materials, Pohang University of Science and Technology , Pohang 790-784, Korea.
authors.forename: Hyejin
authors.initials: H
authors.lastname: Ryu
authors.affiliation: Department of Physics, University of California at Berkeley , Berkeley, California 94720, United States.
authors.forename: Jonghwan
authors.initials: J
authors.lastname: Kim
authors.affiliation: Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University , Stanford, California 94305, United States.|State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences , Shanghai 200050, China.
authors.forename: Shujie
authors.initials: S
authors.lastname: Tang
authors.affiliation: Advanced Light Source, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.
authors.forename: Yeongkwan
authors.initials: Y
authors.lastname: Kim
authors.affiliation: Advanced Light Source, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.|Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University , Stanford, California 94305, United States.|Department of Physics and Clarendon Laboratory, University of Oxford , Parks Road, Oxford, OX1 3PU, United Kingdom.
authors.forename: Bo
authors.initials: B
authors.lastname: Zhou
authors.affiliation: Max Plank POSTECH Center for Complex Phase Materials, Pohang University of Science and Technology , Pohang 790-784, Korea.|Department of Physics, Pusan National University , Busan 609-735, Korea.
authors.forename: Choongyu
authors.initials: C
authors.lastname: Hwang
authors.affiliation: Department of Physics and Clarendon Laboratory, University of Oxford , Parks Road, Oxford, OX1 3PU, United Kingdom.
authors.forename: Yulin
authors.initials: Y
authors.lastname: Chen
authors.affiliation: Department of Physics, University of California at Berkeley , Berkeley, California 94720, United States.|Materials Sciences Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.|Kavli Energy NanoScience Institute at the University of California Berkeley and the Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.
authors.forename: Feng
authors.initials: F
authors.lastname: Wang
authors.affiliation: Department of Physics, University of California at Berkeley , Berkeley, California 94720, United States.|Materials Sciences Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.|Kavli Energy NanoScience Institute at the University of California Berkeley and the Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.
authors.forename: Michael F
authors.initials: MF
authors.lastname: Crommie
authors.affiliation: Advanced Light Source, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.
authors.forename: Zahid
authors.initials: Z
authors.lastname: Hussain
authors.affiliation: Stanford Institute of Materials and Energy Sciences, SLAC National Accelerator Laboratory , Menlo Park, California 94025, United States.|Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University , Stanford, California 94305, United States.
authors.forename: Zhi-Xun
authors.initials: ZX
authors.lastname: Shen
authors.affiliation: Advanced Light Source, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.
authors.forename: Sung-Kwan
authors.initials: SK
authors.lastname: Mo
keywords: ARPES; MBE; STM/STS; Transition metal dichalcogenides; WSe2; exciton binding energy