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BIT Achieves Important Progress in Weir Semimetal Optoelectronic Response

release date :2019-12-24 08:43:00  |   [ close window ]ViewCount:

  

  

  Recently, Professor Yao Yugui, and Special Associate Researcher and Team Member, Wang Qinsheng and collaborators of School of Physics, Beijing Institute of Technology, with Researcher Sun Dong from the International Center for Quantum Materials of Peking University have achieved important progress in the research of WTe2 photoelectric response of the Class II Weir semimetal materials. With scanning photocurrent spectroscopy, they have systematically studied the photoelectric response of WTe2, a Class II Weir semimetal material, and they have found that there is an abnormal photocurrent response at the material boundary (Figure 1). And the boundary symmetry determines the presence or absence of this abnormal photocurrent response. In this job, the use of the crystal field of the material's low symmetry boundary is applied as an effective means of photo-generated electron-hole pair separation, and the possible influence of topological edge states on the photoelectric response is discussed, which provides new ideas for the application of topological semimetal materials in photoelectric detection and other aspects. Relevant results have been published on Nature Communications.

  

  Fig.1 Photocurrent response of WTe2 device at the boundary

  

  The effective separation of photogenerated electron-hole pairs is the foundation of a series of optoelectronic applications such as photodetection, solar cells, and photocatalysis. In order to effectively separate photo-generated electron-hole pairs, that is, to produce an asymmetric distribution of the space of non-equilibrium photo-generated carrier momentum, it is required to break the spatial inversion symmetry of the device or material. For traditional Si, GaAs, graphene and other materials with a center inversion symmetry, the external bias voltage is usually applied to introduce the structure such as PN junction, Schottky junction, etc., to build internal electric field, or the photothermoelectric effect at the interface of materials with different Seebeck coefficients is applied to break the spatial inversion symmetry of the device. However, these means are usually subject to device processing and application conditions. Another method to break the symmetry of spatial inversion is to apply a material with a non-central inversion crystal structure as a photoelectric response material. However, such materials (such as LiNbO3 and BaTiO3, etc.) usually have large band gaps, which limit their applications in the long wave band. Weir semimetals with non-center inversion symmetry are a new type of quantum materials that have received widespread attention. Zero-bandgap characteristics of Weir semimetals make them ideal materials for wide-band photodetectors. And theoretical research shows that the divergence of the Bailey curvature near the Weir point will greatly enhance the photoelectric response near the Weir point, which makes the Weir semi-metals more advantageous in the application of mid-far infrared detection. Therefore, it is of great significance to study the photoelectric response properties of the external semimetal materials.

  

  The research team studied the photocurrent response behavior of Class II Weir semimetal tungsten telluride WTe2 by scanning photocurrent spectroscopy. It was found that although the in-plane photocurrent response in the WTe2 bulk material disappears due to limitation of the symmetry of the material crystal. However, the specific boundary of the WTe2 sample shows boundary photocurrent responses under wide spectral range excitation (532nm - 10.6um). A systematic study of the results of scanning the photocurrent spectrum of WTe2 shows that photocurrent is present only at the boundary of lower symmetry. In the non-boundary areas and at the highly symmetrical boundary, no photocurrent response is generated. This proves that the crystal symmetry generated by the photocurrent can be limited by the boundary destruction, thereby generating a photocurrent at the sample’s boundary. Combined with Shockley-Ramo theory (for semi-metallic materials, the macroscopic photocurrent response of the device is determined by the local photocurrent and the virtual electric field distribution of the device), by analyzing the local photocurrent distribution and virtual electric field distribution at the boundary, which successfully explains mechanism of this photocurrent response. In addition, the boundary of the Weir semimetal material has a non-mediocre Fermi arc surface state, and the topological surface state in WTe2 only is present at the boundary with a low symmetry. Therefore, although the generation of boundary photocurrent can be explained by the crystal boundary symmetry, the effect of non-mediocre topological surface states in the generation of boundary photocurrent, and the microscopic mechanism of local photocurrent generation are still worth studying carefully. Based on these considerations, this work preliminarily discusses the possible effects of topological surface states on the boundary photocurrent response and points out the possibility of applying topological boundary states to enhance the boundary photocurrent response. It provides new ideas for designing new photodetectors based on Weir semimetal materials. The work was published in Nature Communications 10, 5736 (2019).

  

  This work was supported by the National Natural Science Foundation of China, the National Key R & D Plan of the Ministry of Science and Technology, and the Strategic Pilot Plan of the Chinese Academy of Sciences, Beijing Natural Science Foundation and Young Teacher Academic Startup Program of BIT. The research team would like to thank Professor Sun Kai of the University of Michigan, Researcher Jia Shuang and Researcher Chen Jianhao of Peking University, and Researcher Shi Youguo of the Chinese Academy of Sciences for their great support and cooperation.

  

Article link: https://www.nature.com/articles/s41467-019-13713-1

Editor: News Agency of BIT

Translation: News Agency of BIT

  

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