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Strain-induced direct-indirect bandgap transition and phonon modulation in monolayer WS2

Nano Research 2015年第8期8卷 2562-2572页

作者： Yanlong Wang Chunxiao Cong Weihuang Yang Jingzhi Shang Namphung Peimyoo Yu Chen Junyong Kang Jianpu Wang Wei Huang Ting Yu Nanyang Technological University-Nanjing Tech Center of Research and Development Nanjing Tech University Nanjing 211816 China Division of Physics and Applied Physics School of Physical and Mathematical Sciences Nanyang Technological University 637371 Singapore Fujian Key Laboratory of Semiconductor Materials and Applications Department of Physics Xiamen University Xiamen 361005 China Key Laboratory of Flexible Electronics (KLOFE) and Institute of Advanced Materials (IAM) Jiangsu National Synergetic Center for Advanced Materials (SICAM) Nanjing Tech University (NanjingTech) Nanjing 211816 China Key Laboratory for Organic Electronics & Information Displays (KLOEID) and Institute of Advanced Materials (IAM) Nanjing University ot Posts & Telecommunications 9 Wenyuan Road Nanjing 210046 China

In situ strain photoluminescence （PL） and Raman spectroscopy have been employed to exploit the evolutions of the electronic band structure and lattice vibrational responses of chemical vapor deposition （CVD）-grown monolayer tungsten disulphide （WS2） under uniaxial tensile strain. Observable broadening and appearance of an extra small feature at the longer-wavelength side shoulder of the PL peak occur under 2.5% strain, which could indicate the direct-indirect bandgap transition and is further confirmed by our density-functional-theory calculations. As the strain increases further, the spectral weight of the indirect transition gradually increases. Over the entire strain range, with the increase of the strain, the light emissions corresponding to each optical transition, such as the direct bandgap transition （K-K） and indirect bandgap transition （F-K, ≥2.5%）, exhibit a monotonous linear redshift. In addition, the binding energy of the indirect transition is found to be larger than that of the direct transition, and the slight lowering of the trion dissociation energy with increasing strain is observed. The strain was used to modulate not only the electronic band structure but also the lattice vibrations. The softening and splitting of the in-plane E＇ mode is observed under uniaxial tensile strain, and polarization-dependent Raman spectroscopy confirms the observed zigzag-oriented edge of WS2 grown by CVD in previous studies. These findings enrich our understanding of the strained states of monolayer transition-metal dichalcogenide （TMD） materials and lay a foundation for developing applications exploiting their strain-dependent optical properties, including the strain detection and light-emission modulation of such emerging two-dimensional TMDs.

关键词： monolayer WS2 strain light-emission tuning indirect transition trion crystallographic orientation

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Identification of twist-angle-dependent excitons in WS2/WSe2 heterobilayers

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National Science Review 2022年第6期9卷 56-63页

作者： Ke Wu Hongxia Zhong Quanbing Guo Jibo Tang Jing Zhang Lihua Qian Zhifeng Shi Chendong Zhang Shengjun Yuan Shunping Zhang Hongxing Xu School of Physics and Technology and Key Laboratory of Artificial Micro-and Nano-Structures of Ministry of Education Wuhan University School of Physics Huazhong University of Science and Technology The Institute for Advanced Studies Wuhan University Key Laboratory of Materials Physics of Ministry of Education School of Physics and MicroelectronicsZhengzhou University

Stacking atomically thin films enables artificial construction of van der Waals heterostructures with exotic functionalities such as superconductivity, the quantum Hall effect, and engineered light-matter *** particular, heterobilayers composed of monolayer transition metal dichalcogenides have attracted significant interest due to their controllable interlayer coupling and trapped valley excitons in moiré superlattices. However, the identification of twist-angle-modulated optical transitions in heterobilayers is sometimes controversial since both momentum-direct(K–K) and-indirect excitons reside on the low energy side of the bright exciton in the monolayer constituents. Here, we attribute the optical transition at ～1.35 eV in the WS2/WSe2heterobilayer to an indirect Γ–K transition based on a systematic analysis and comparison of experimental photoluminescence spectra with theoretical calculations. The exciton wavefunction obtained by the state-of-the-art GW-Bethe-Salpeter equation approach indicates that both the electron and hole of the excitons are contributed by the WS2layer. Polarization-resolved k-space imaging further confirms that the transition dipole moment of this optical transition is dominantly in-plane and is independent of the twist angle. The calculated absorption spectrum predicts that the so-called interlayer exciton peak coming from the K–K transition is located at 1.06 eV, but with a much weaker amplitude. Our work provides new insight into the steady-state and dynamic properties of twist-angle-dependent excitons in van der Waals heterostructures.

关键词： heterobilayers twist angle exciton indirect transition transition dipole moment

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UV-Vis and Photoluminescent Spectra of TiO_2 Films

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Journal of Wuhan University of Technology(Materials Science) 2003年第3期18卷 36-39页

作者：赵青南 Centre for Materials Research and Analysis Wuhan University of Techuology

TiO 2 films have been deposited on glass substrates using DC reactive magnetron sputtering at different oxygen partial pressures from 0.10Pa to *** transmittance (UV vis) and photoluminescence (PL) spectra of the films were *** results of the UV vis spectra show that the deposition rate of the films decreased at oxygen partial pressure P(O 2)≥0.15Pa,the band gap increased from 3.48eV to 3.68eV for direct transition and from 3.27eV to 3.34eV for indirect transition with increasing the oxygen partial *** PL spectra show convincingly that the transtion for films was indirect,and there were some oxygen defect energy levels at the band gap of the *** increasing the O 2 partial pressure,the defect energy levels *** the films sputtered at 0.35 and 0.65Pa there were two defect energy levels at 2.63eV and 2.41eV,corresponding to 0.72eV and 0.94eV below the conduction band for a band gap of 3.35eV,*** the films sputtered at 0.10Pa and 0.15Pa,there was an energy band formed between 3.12eV and 2.06eV,corresponding to 0.23eV and 1.29eV below the conduction band.

关键词： TiO 2 films indirect transition defect energy level sputtering

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列宁国家资本主义思想的演进及其启示

马克思列宁主义研究

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马克思列宁主义研究 2016年第7期

作者：李红梅

列宁的国家资本主义思想经历了一个历史演进的过程。列宁曾一度设想通过国家资本主义“直接过渡”到社会主义；随着社会主义建设实践的展开，列宁对国家资本主义的认识发生了深刻的变化，转而设想通过国家资本主义实现“迂回过渡”。探... 详细信息

列宁的国家资本主义思想经历了一个历史演进的过程。列宁曾一度设想通过国家资本主义“直接过渡”到社会主义；随着社会主义建设实践的展开，列宁对国家资本主义的认识发生了深刻的变化，转而设想通过国家资本主义实现“迂回过渡”。探讨列宁国家资本主义思想的演进过程，对于中国特色社会主义建设具有重要的启迪和指导意义。

关键词：列宁国家资本主义直接过渡迂回过渡 Lenin the State Capitalism direct transition indirect transition

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