Growth And Study On The Properties Of Transition Metal Sulfides

In recent years,with the discovery of graphene,two-dimensional?2D?materials have attracted extensive attention due to their rich physical properties and broad application prospects.Although graphene has many appealing properties,such as high mobility and electrical conductivity,good flexibility and excellent thermal conductivity,low light absorption.However,due to the zero band gap of graphene,its application in electronic circuit devices is severely limited.Transition metal dichalcogenides?TMDs?compounds are a class of 2D materials?such as MoS ₂,WS₂,MoSe₂,etc.?,which have a similar single layer structure with graphene,and have received significant attention.Most of them belong to semiconductors,and their band gap covers the ultraviolet to infrared spectrum,with high carrier mobility and unique electrical switching ratio,and is a candidate for electronic devices.Bandgap is one of the basic properties of a semiconductor and determines the electronic and optical properties of the material.However,many TMDs are limited by their inherent band gap,affecting their optical and electrical applications.Therefore,an important work in the study of TMDs is how to obtain materials with adjustable band gaps and enrich their applications in optoelectronic devices.In addition,chemical vapor deposition?CVD?technology is the main method for preparing these TMDs.For some high-performance materials,due to difficulties in their preparation?such as easy vertical growth?,how to use CVD to grow these materials to enrich their applications is an urgent problem to be solved.Based on the above problems,the Nb_xW_1-xS₂ monolayer alloy material with adjustable band gap was successfully grown on the silicon substrate by the doping method using traditional CVD technique.Combined with density functional theory?DFT?,we systematically investigated the optical and electrical properties of single-layer alloy materials;Meanwhile,HfS₂ nanosheets with non-growth were successfully synthesized on different substrates by CVD method.The HfS₂ nanosheets were characterized by atomic force microscopy?AFM?and Raman spectra.The main research results are summarized as follows:?1?Nb_xW_1-xS₂ alloy material was successfully prepared by CVD method.Exploring the effects of different growth parameters?growth temperature,gas flow rate,growth time?on the quality of alloy samples,and obtaining the optimal growth conditions.The morphology and atomic structure of the alloy materials were characterized by AFM,scanning electron microscopy?SEM?and spherical aberration-corrected scanning transmission microscopy?STEM?.We have found that Nb successfully replaced the W atom in the WS₂ lattice.X-electron spectroscopy?XPS?revealed that the binding energy peaks of the S and W elements of the alloy Nb_xW_1-xS₂ move toward the lower energy than the pure WS₂,indicating that the Nb atoms are acted as p-type dopants.?2?Through the study of the optical and electrical properties of the alloy material,it is found that the photoluminescence?PL?value of the single-layer Nb_xW_1-xS₂ is red-shifted compared to the pure single-layer WS₂,and the band gap is shifted from1.98?WS₂?to 1.69eV.It also found that The PL intensity of Nb_xW_1-xS₂ is drastically decreased as the concentration of doped Nb increases.The DFT calculation results confirmed that shift direction of PL and Nb substitution site in the experiment.We have separately fabricated Nb_xW_1-xS₂ and WS₂ field effect transistors?FETs?by electron beam lithography followed by lift-off process.It has been found that the conductivity of Nb_xW_1-xS₂ FET is significantly improved compared to WS₂.Most importantly,the conductivity type of the Nb_xW_1-xS₂ alloy changes from the n-type of WS2 to the degenetrated p-type.?3?Hexagonal HfS₂ nanosheets were synthesized on the four different substrates?silicon wafer,mica,sapphire,molten glass?by CVD method,and the HfS ₂ samples obtained on the sapphire and molten glass substrate were confirmed by Raman characterization.The molten glass is more suitable for the growth of HfS₂ nanosheets than the sapphire substrate,and the crystal quality of the sample is getting better as the temperature increases.