Research On The Optical And Electrical Properties Of Transition Metal Phosphorus Compound Crystals And Low-dimensional Materials

The rich chemical composition and diverse lattice structures of transition metal phosphorus compounds endow them with rich physical and chemical properties,giving them a broad range of applications and attracting a wide attention in recent years.The transition metal phosphorus compounds include various materials such as superconductors,metals,semimetals,semiconductors,showing the optical anisotropy,the nonlinear optics,the photoelectric detection and other optical properties,as well as showing the negative magnetoresistance,the giant magnetoresistance,the superconductivity and other electrical properties.The Bi-based materials,which are also in the P group,also exhibit topological properties such as quantum anomalous Hall effect and axion insulation state.Therefore,studying the controllable preparation of high-quality transition metal phosphorus compound crystals and their optical and electrical properties will help to deeply understand the physical mechanisms behind the novel physical properties of this type of materials,and further promote their application in optical,electrical and spin electronic devices.This article mainly focuses on the preparation and optical properties and electrical transport properties of binary and ternary transition metal phosphorus semiconductor single crystals,and also explore the low-dimensional ternary transition metal Bi-based topological insulator material and its properties.Firstly,we have successfully prepared PtP₂ single crystals with a high quality by using the tin flux method.Three-dimensional weak localization effect and negative magnetoresistance are observed in the high-quality PtP₂ single crystals for the first time.The research results indicate that the three-dimensional weak localization effect in PtP₂single crystal originates from the electron-electron interaction.Combining the crystal structure and optical measurement results,we find the defects in PtP₂ single crystal can suppress the negative magnetoresistance effect.The specific heat and magnetic measurement results show that the transition point in the temperature-dependent resistivity mainly originates from the transition process from a disordered paramagnetic state to a weakly ordered magnetic state.This work establishes the relationship between defects and physical properties in PtP₂ single crystals,provides a new platform for the study of correlated electronic states and provides new ideas for the development of new high-quality semiconductor materials.Secondly,since the introducing sulfur into the transition metal phosphides can broaden their band gap range and enhance their semiconductor properties,the semiconductor Nb₄P₂S₂₁ single crystal with a quasi-one-dimensional chain structure was grown by chemical vapor transport.For the first time,the optical anisotropy of Nb₄P₂S₂₁single crystal was studied through the polarized Raman spectroscopy and the angle-dependent femtosecond transient absorption spectroscopy.The polarized-dependent Raman scattering spectra reveal that the intensity of Raman peak at 202 cm^-1 show a 2-fold symmetry in the parallel and vertical polarization configurations,and the intensity of Raman peak at 489 cm^-1 show a 4-fold symmetry in the parallel and vertical polarization configurations.Moreover,the results of the ultrafast carrier dynamics indicate that the hot carrier amount and relaxation rate of Nb₄P₂S₂₁crystals are anisotropic after photoexcitation.This work correlates the quasi-one-dimensional structure and optical anisotropy of Nb₄P₂S₂₁,promoting its application in new polarization beam splitters,polarization-sensitive photodetectors and other fields.Thirdly,since Bi and Te elements lie in the same main group as P and S,have small electronegativity and poor activity,the successful growth of two-dimensional transition metal bismuth telluride compounds has the important significance for the growth of transition metal phosphorus chalcogenide compounds.Here,for the first time,we report a direct chemical vapor deposition approach to epitaxially grow high-quality,two-dimensional intrinsic magnetic topological insulator Mn Bi₂Te₄ nanosheets on mica.By controlling the growth temperatures,an evolution of the thickness and lateral size of the as-grown Mn Bi₂Te₄ nanosheets has been observed.Atomic-resolution scanning transmission electron microscope imaging demonstrates the perfect atomic structure of the Mn Bi₂Te₄,confirming its high quality.The anomalous Hall effect induced by the surface net magnetic moments and the transition from antiferromagnetic state to tilted antiferromagnetic state to ferromagnetic state have been observed in the eight-layer Mn Bi₂Te₄ nanosheets.The low temperature magnetic phase diagram of Mn Bi₂Te₄nanosheets is provided based on the multi-step spin transformation process.This work establishes the relationship between magnetism and electrical transport and promotes the integration and practical application of topological electronic devices based on high-quality two-dimensional Mn Bi₂Te₄.