Electronic Structure, Surface Doping, and Optical Response in Epitaxial WSe2 Thin Films

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• 作者：Yi Zhang ; Miguel M. Ugeda ; Chenhao Jin ; Su-Fei Shi ; Aaron J. Bradley ; Ana Martín-Recio ; Hyejin Ryu ; Jonghwan Kim ; Shujie Tang ; Yeongkwan Kim ; Bo Zhou ; Choongyu Hwang ; Yulin Chen ; Feng Wang ; Michael F. Crommie ; Zahid Hussain ; Zhi-Xun Shen ; Sung-Kwan Mo
• 刊名：Nano Letters
• 出版年：2016
• 出版时间：April 13, 2016
• 年：2016
• 卷：16
• 期：4
• 页码：2485-2491
• 全文大小：460K
• 年卷期：0
• ISSN：1530-6992

文摘

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.