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Peking University discovered the photoelectric response of ultra-wide spectrum at room temperature
[ China Instrument Network Instrument Development ] Recently, Dr. Wu Dong and his collaborators of the Wang Nanlin Research Group of the Quantum Materials Science Center of Peking University discovered the photoelectric response of ultra-wide spectrum at room temperature in the two-dimensional charge-density wave (CDW) material 1T-TaS2. . They studied the 1T-TaS2 material system in the near-common CDW state at room temperature and found that when the applied bias (electric field) is close to the threshold of CDW slip, the material is irradiated with light from the visible light to the wide energy range of terahertz. Become very sensitive. This discovery opens up a new way to achieve uncooled, ultra-wideband (including terahertz bands) and sensitive photodetection research. The work was published in the recent Science article in the form of Research Article: Science Advances, aao 3057 (2018). Wu Dong and Wang Nanlin are co-authors of the work.
Fig. 1T-TaS2 Ultra-wide spectrum photoelectric response characteristics. A. The effect of illumination on the IV curve. B. Switching effect of photocurrent. C. Photocurrent response of the device at λ = 1550 nm under different bias voltages. D. The magnitude of the optical responsivity (R) of the different bands near the threshold voltage.
The ability to convert optical signals into electrical signals is extremely important in the field of photonics. This effect is widely used in imaging, communication, quantum information and even space science. However, based on the ultra-wide spectrum of the same device, especially photodetection including far-infrared and terahertz bands has been a difficult point in this field. Limited by the structure of the electron band, conventional semiconductors cannot directly respond to light smaller than their energy gap energy. For example, silicon-based photodetectors achieve light response by single-particle excitation through a band gap (~1.1 eV), and are therefore transparent to spectra below their band gap energy, such as far-infrared and terahertz. On the other hand, for ordinary metals, although the band gap is zero, due to the existence of a large number of free carriers, the photoexcitation signal that may occur is often directly shielded, and thus is not suitable as a photodetector material. In general, the electronic excitation of metals and semiconductors belongs to the category of single particle excitation.
Unlike ordinary metals and semiconductors, some special states such as superconducting and charge density waves have not only single-particle excitation but also collective excitation modes. The charge density wave is one of the most studied collective aggregation phenomena in solids. There are two different collective excitation modes, namely, Amplitude mode and Phase mode. The amplitude excitation appears as a longitudinal vibration of the charge density (a change in CDW amplitude), similar to the excitation of an optical phonon, without a direct effect on the charge transport properties. The phase excitation mode corresponds to the translation of the CDW as a whole, corresponding to the flow of charge. In the ideal case where no defects or impurities are present, this translational motion is unimpeded and should have zero resistance characteristics. In practical materials, CDW is pinned to a limited frequency due to the natural presence of impurities and defects and thus has a limited resistance. However, when the applied electric field exceeds the threshold of the phase excitation mode, the charge density wave material exhibits a significant nonlinear current-voltage behavior, which is manifested by a sharp drop in resistance, commonly referred to as slippage of CDW.
1T-TaS2 is a representative two-dimensional CDW material. Electron-related effects and electrophonon interactions make this system rich in phase structure. As the temperature decreases, it goes from simple metal to non-common-CDW phase (phase transition temperature ~550K), near-common-CDW phase (phase transition temperature ~350K), and finally enters the commensurate-CDW insulation phase (< 183K). The electrical properties of 1T-TaS2 exhibit a significant nonlinear dependence on the applied electric field. Wu Dong et al. found that when the applied bias voltage is less than the threshold value causing the charge density wave slip, if the applied light is excited, it will lead to a change in the resistance state, that is, the phase mode slip is induced in advance. Corresponding to this, it is equivalent to a large photoelectric response effect, and exists in a wide energy range from ultraviolet to terahertz, which can be used to fabricate photoelectric response detectors in a wide energy region.
Prof. Ma Yongchang from Tianjin University of Technology, Professor Zhao Ning from Tsinghua University and the Critical Research Group of the Center for Quantum Materials Science participated in the collaborative research of this work. The work was supported by the National Natural Science Foundation of China, the National Major Scientific Research Program, and the 2011 Plan Collaborative Innovation Center for Quantum Matter Science.
(Original title: Science Advances reports Quantum Materials Science Center Wang Nanlin Research Group on the research of ultra-wide-spectrum photoelectric response of charge-density wave materials at room temperature)