In-plane optical anisotropy of quasi-one-dimensional layered semiconductor Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystal
Authors: Cheng Qiu-Zhen , Huang Yin , Li Yu-Hui , Zhang Kai , Xian Guo-Yu , Liu He-Yuan , Che Bing-Yu , Pan Lu-Lu , Han Ye-Chao , Zhu Ke , Qi Qi , Xie Yao-Feng , Pan Jin-Bo , Chen Hai-Long , Li Yong-Feng , Guo Hui , Yang Hai-Tao , Gao Hong-Jun ,   State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China,   Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China,   School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100190, China,   Songshan Lake Materials Laboratory, Dongguan 523808, China
Published: November 2023

Transition-metal phosphorous chalcogenide <i>M</i>PS (<i>M</i> = transition metal), an emerging type of two-dimensional (2D) van der Waals material with the unique optical and opto-electronic properties, has received much attention. The quasi-one-dimensional chain structure of Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> will possess the strong anisotropic optical and photoelectric properties. Therefore, the single crystal and low-dimensional materials of Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> have potential applications in new polarization controllers, polarization-sensitive photoelectronic detectors, etc. However, there is still a lack of research on the anisotropic optical properties of the high-quality Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystals. Herein, the millimeter-sized Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystals are successfully prepared by the chemical vapor transport method. The chemical composition, the crystal structure and the anisotropic optical properties of the Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystals are carefully analyzed. The energy dispersive X-ray spectroscopy results show that the element distribution is uniform and the element ratio is close to the stoichiometric ratio. The X-ray diffraction and the transmission electron microscopy results show a good crystallinity. The absorption spectra shows that the optical band gap of the Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystal is 1.8 eV. Interestingly, the Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystal can be mechanically exfoliated to obtain few-layer material. The thickness-dependent Raman spectra show that the Raman vibration peaks of bulk and few-layer Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> each have only a weak shift, indicating a weak interlayer interaction in the Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystal. In order to make an in-depth study of the optical properties of Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystals, the polarized-dependent Raman spectra and the femtosecond transient absorption (TA) spectra by using pump pulses and probe pulses with a wavelength of 400 nm and a wavelength range of 500–700 nm are recorded. Importantly, the polarized-dependent Raman scattering spectra with the angle-dependent measurements reveal that the intensity of Raman peak at 202 cm<sup>–1</sup> and at 489 cm<sup>–1</sup> show a 2-fold symmetry and a 4-fold symmetry in the parallel and vertical polarization configurations, respectively. Moreover, the results of ultrafast carrier dynamics with the in-plane rotation angles of Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystals in the parallel polarization configurations, clearly indicate that both the hot carrier number and the relaxation rate after photoexcitation have the in-plane anisotropic properties. These results are useful in understanding the in-plane anisotropic optical properties of Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystal, which can further promote their applications in the low-dimensional angle-dependent optoelectronics.