In-plane optical anisotropy of quasi-one-dimensional layered semiconductor Nb₄P₂S₂₁ single crystal

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.