Molybdenum disulfide (MoS₂) is a typical representative of the family of layered transition metal dichalcogenides (TMDs). MoS₂ has rich crystal structures including 2H, 3R, 1T, 1T', 1T’’ and 1T’’’, depending on the coordination of [MoS₆] polyhedral (Figure 1).

Among the diverse polymorphs, the octahedral coordinated metastable phases (1T, 1T' and 1T''' MoS₂) attract global attentions, because of the peculiar physical properties which are theoretically predicted, such as superconductivity, quantum spin Hall effect and two-dimensional ferroelectricity. However, the metastable phases are thermodynamically unstable and they will be converted to stable 2H phase easily at high temperature. Thus it is a great challenge to develop the preparation of the metastable polymorph and to study its physicochemical property.

Recently, the Photovoltaic Materials and Devices Group from Shanghai Institute of Ceramics, Chinese Academy of Sciences has cooperated with Peking University, Shanghai Jiao Tong University, and Huazhong University of Science and Technology. They have successfully prepared the metastable 1T''' MoS₂ crystals and firstly determined the crystal structure.

In view of the noncentrosymmetric structure of 1T''' MoS₂, the team further studied its nonlinear optical properties. The second harmonic response of the 1T''' MoS₂ crystals was tested by laser with different wavelengths, and was found having the strongest response intensity under excitation light with a wavelength of 850 nm (Figure 2). Furthermore, the team tested the polarization second harmonic response, and observed a hexagonal petal image, corresponding to its intrinsic crystal structure.

This discovery may bring new insights into the mechanism study of the metastable 2D layered materials.

The result is published in J. Am. Chem. Soc. 2019 (DOI: 10.1021/jacs.8b12133).

The research work is supported by the National Key Research and Development Program, the National Natural Science Foundation, the Chinese Academy of Sciences, and the Shanghai Municipal Science and Technology Commission. 

Figure 1. a. b. c. d. The schematic atomic structure of 1T’’’, 1T’, 1T and 2H MoS₂.

Figure 2. The SHG responses of 1T’’’ MoS₂.

Reference:

Structural Determination and Nonlinear Optical Properties of New 1T'''-Type MoS₂ Compound

https://pubs.acs.org.ccindex.cn/doi/10.1021/jacs.8b12133

Contact:

Prof. Fuqiang Huang

Shanghai Institute of Ceramics

huangfq@mail.sic.ac.cn