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Evolution of the valley position in bulk transition-metal chalcogenides and their monolayer limit

Because of their great potential for electronic and spintronic devices, energy storage and catalytic applications, two-dimensional (2D) graphene-like layered materials with novel properties have been one of the focused topics in physics and material science recently. With a layered honeycomb lattice, transition metal dichalcogenides MX2 (M = Mo, W; X = S, Se, Te) are such examples. In addition, having two well separated inequivalent valleys in their electronic structures, the valley index in the compounds acts in analogy to the spin quantum number, thus opening up a new research direction called ‘valleytronics’. Finally, the strong spin-orbital interaction in these compounds could further make MX2 thin films show polarized spin texture thus become promising materials for a new type of valley-coupled spintronics applications.
Dispite these bright perspectives, however, most current understanding of their electronic structure near band valleys is (surprisingly) based on either theoretical investigations or optical measurements, leaving the important details of the band structure elusive. As an example, the exact position of the conduction band valley of these compounds are still controversial nowadays.
Conventionally, angle resolved photoemission spectroscopy (ARPES) is the most effective method for visualizing the band structures of solid. However, it cannot probe the unoccupied bands - as most MX2 samples are slightly p-type, their conduction bands are not populated and thus remains undetectable by ARPES; also, although the thin films of these materials are promising for device fabrications, they are typically too small (a few micrometers in size obtained by exfoliation technique) to be investigated by conventional ARPES (which typically requires millimeter-size samples).
To overcome these difficulties, we developed new methods to carry out the ARPES measurements. For the former, we used in-situ surface electron doping method to fill the unoccupied conduction bands; for the latter, we made use of the Spectromicroscopy beamline at Elettra to achieve the ARPES measurement on MX2 thin films with sub micrometer resolution (micro-ARPES).
With these news methods, we systematically investigated the band structures and their evolution in representative bulk chalcogenides MoS2, WS2 and WSe2. For the first time, we were able to directly visualize their electronic structures evolution and explain the discrepancy in reported theoretical studies on the band locus positions. Thus establish a solid background to understand the underlying physics of these materials, and provide guidance for new material design and novel electronic/spintronics device development.
Secondly, by micro-ARPES measurements, we were able to freely scan over exfoliated MX2 flake and studied the momentum-resolved electronic structure on different thickness layers, and clearly observed the distinct transition from indirect to direct band gap in the electronic structure of MoS2 (see Fig. 1) and WSe2 (Fig. 3) as the thickness of the flake decreased from multiple layers to a monolayer with unprecedented details.

Figure 1. Band valley evolution from multi-, bi- to mono-layer MoS2 nano-flakes. a) 2D photoemission spectra intensity contrast map of MoS2 flakes with different magnifications demonstrating the processes to locate the target mono-layer flake. Panel (iv) gives the optical image of the same flake for comparision, where the mono-, bi- and multi-layer MoS2 flakes can be clearly seen. Points P1-P3 indicate the three measurement positions for mono-, bi- and multi-layer MoS2 flakes. b) Constant energy plots measured at mono-layer (point P1), bilayer (point P2) and multilayer (point P3) spots, with the energy positions at E-EVBM = 0 eV and E-EVBM = - 0.5 eV, respectively. c) Band dispersions along the high symmetry K-Γ-K and M-Γ-M directions from point P1-P3, showing the band valley evolution with different flake thicknesses.
 

Figure 2. Same measurements as in Fig. 1 for WSe2.

Our experimental band structure thus directly addresses the recent controversy in the theoretical studies on MX2 materials, and by comparing to ab initio calculations, we discovered and successfully explained the discrepancy between previous theories and experiments – which would not be possible without the direct and systematic study of the band structures
These exciting results thus establish a solid basis to understand the underlying physics of these materials, which can further provides important guidance for new materials design and novel valleytronics device development, and also demonstrates the power of the in situ study of the band structure of sub-micron size semiconductors used in functional devices.

 

This research was conducted by the following research team:


Hongtao Yuan1,2, Zhongkai Liu1,2,3,4, Gang Xu1, Bo Zhou5,6, Sanfeng Wu7, Dumitru Dumcenco8,9, Kai Yan1,2, Yi Zhang6, Sung-Kwan Mo6, Pavel Dudin10, Victor Kandyba11, Mikhail Yablonskikh11, Alexei Barinov11, Zhixun Shen1,2, Shoucheng Zhang1,2, Yingsheng Huang8, Xiaodong Xu7, Zahid Hussain6, Harold Y. Hwang1,2, Yi Cui1,2,12, and Yulin Chen3,4,5,10

 

Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California, United States
Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California, United States
School of Physical Science and Technology, ShanghaiTech University, Shanghai, China
CAS-Shanghai Science Research Center, Shanghai, China
Physics Department, Clarendon Laboratory, University of Oxford, Oxford, United Kingdom
Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Department of Physics, Department of Materials Science and Engineering, University of Washington, Seattle, Washington, United States
Department of Electronic and Computer Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan (ROC)
Electrical Engineering Institute, Ecole Polytechnique Federale de Lausanne (EPFL), Lausanne, Switzerland
10 Diamond Light Source, Didcot, Oxfordshire, United Kingdom
11 Elettra-Sincrotrone Trieste, Trieste, Basovizza, Italy
12 Department of Materials Science and Engineering, Stanford University, Stanford, California, United States
 


Contact person:

Yulin Chen, email: Yulin.Chen@physics.ox.ac.uk

 

Reference

Hongtao Yuan, Zhongkai Liu, Gang Xu, Bo Zhou, Sanfeng Wu, Dumitru Dumcenco, Kai Yan, Yi Zhang, Sung-Kwan Mo, Pavel Dudin, Victor Kandyba, Mikhail Yablonskikh, Alexei Barinov, Zhixun Shen, Shoucheng Zhang, Yingsheng Huang, Xiaodong Xu, Zahid Hussain, Harold Y. Hwang, Yi Cui, and Yulin Chen
Nano Letters, 16, 4738 (2016), DOI: 10.1021/acs.nanolett.5b05107
Last Updated on Wednesday, 14 September 2016 15:52