In-plane optical anisotropy of quasi-one-dimensional layered semiconductor Nb&lt;sub&gt;4&lt;/sub&gt;P&lt;sub&gt;2&lt;/sub&gt;S&lt;sub&gt;21&lt;/sub&gt; single crystal

Yongfeng Li ,Qi Qi Qi Qi,Hao Yin ,Xie Yao-Feng Xie Yao-Feng,Hongjun Gao ,Kai Zhang ,Hailong Chen ,Hui Guo ,Yuhui Li ,Han Ye-Chao Han Ye-Chao,K. Zhu ,Yang Hai-Tao Yang Hai-Tao,Liu He-Yuan Liu He-Yuan,Cheng Qiu-Zhen Cheng Qiu-Zhen,Xian Guo-Yu Xian Guo-Yu,Che Bing-Yu Che Bing-Yu,Pan Lu-Lu Pan Lu-Lu,Jinbo Pan

doi:10.7498/aps.72.20231539

Abstract

Transition-metal phosphorous chalcogenide MPS (M = 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 Nb4P2S21 will possess the strong anisotropic optical and photoelectric properties. Therefore, the single crystal and low-dimensional materials of Nb4P2S21 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 Nb4P2S21 single crystals. Herein, the millimeter-sized Nb4P2S21 single crystals are successfully prepared by the chemical vapor transport method. The chemical composition, the crystal structure and the anisotropic optical properties of the Nb4P2S21 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 Nb4P2S21 single crystal is 1.8 eV. Interestingly, the Nb4P2S21 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 Nb4P2S21 each have only a weak shift, indicating a weak interlayer interaction in the Nb4P2S21 single crystal. In order to make an in-depth study of the optical properties of Nb4P2S21 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–1 and at 489 cm–1 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 Nb4P2S21 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 Nb4P2S21 single crystal, which can further promote their applications in the low-dimensional angle-dependent optoelectronics.

Full Text