Synthesis And Electronic And Magnetical Properties Of Two-dimensional Cobalt Sulfur Selenium Compounds And Related Alloys

Two dimensional materials have unique sheet structure,ultra-high specific surface area,a large number of surface atoms and unique properties.Therefore,they have become one of the most studied nano materials in the past decade.With the gradual deepening of the research on two-dimensional（2D）materials,people have gradually discovered many comprehensive physical properties of the two-dimensional material family.From graphene conductor to Mo S₂semiconductor to insulating h-BN,there have been many exciting new physical discoveries.So far,chemical vapor deposition（CVD）is considered as a reliable method to synthesis ultra-thin 2D materials because of its ablity to controllable layer number and high yield.Two dimensional transition metal dihalogenated compounds（2D TMDs）exhibit exciting properties,such as ferromagnetism,superconductivity and novel electronic applications.They have great prospects application in device miniaturization,energy,quantum information science and optoelectronics.In recent years,several 2D ultra-thin magnetic materials with excellent properties have been successfully synthesized by CVD,and there are still many high-performance 2D magnetic materials need to be synthesized and studied.The cobalt sulfide（CoS₂）bulk material has a Curie temperature of 125 K,but the corresponding 2D CoS₂nanosheets have not been synthesized.At the same time,many ternary and quaternary two-dimensional alloy materials formed by doping or alloying have been reported in recent years,so as to richen the new functions of two-dimensional materials,such as magnetism,ferroelectricity and quantum state.The development of controllable synthesis of 2D TMDs alloys with different composition and structure,can provide great opportunities for future optical and electronic applications.In view of this,based on the synthesis method of chemical vapor deposition（CVD）,controllable synthesis of high-quality two-dimensional metal type CoS₂and CoS_2xSe_2-2xmetal alloys,and the study of their electrical and magnetic properties are achieved in this paper.The main research results are as follows:（1）High quality ultra-thin CoS₂nanosheets were synthesized by chemical vapor deposition for the first time.By adjusting the growth temperature and carrier gas flow rate,the thickness of CoS₂nanosheets can be thinned down to 4.5 nm and the maximum transverse size was 168 nm.X-ray diffraction,X-ray photoelectron spectroscopy,scanning electron microscopy and transmission electron microscopy showed that the obtained CoS₂nanosheets were high-quality single crystals with hexagonal phase.Electrical transport studies show that two-dimensional CoS₂nanosheets show excellent conductivity,up to 5.43×10⁵S m^-1,high breakdown current density up to 5×10⁷A/cm².Using the vibrating sample magnetometer option test of the comprehensive physical property test system,an obvious hysteresis loop is observed under the in-plane magnetic field.From the qualitative analysis of the field-cooled and zero-field-cooled curves,the Curie temperature of CoS₂nano sheet can be determined to be about 125 K.（2）CoS_2xSe_2-2x（x=0-1）nanosheets were prepared by using Co₃O₄powder,sulfur powder and selenium powder as reaction sources and controlling the dosage ratio of sulfur powder and selenium powder in the CVD process.The effects of different growth conditions on the morphology,size and thickness of CoS_2xSe_2-2xnanosheets were discussed.Through a series of material characterization methods,it is proved that the nanosheets synthesized through this method are hexagonal CoS_2xSe_2-2xnanosheets with high crystalline quality.By optimizing the experimental conditions,the thickness of this material can be adjusted and down to 1.8 nm,and the transverse size can reach 170μm.Two dimensional CoS_2xSe_2-2xnanosheets show excellent conductivity up to 8.86×10⁵S m^-1,breakdown current density up to 8.51×10⁷A/cm²,with strong thickness dependence.Using the vibrating sample magnetometer option of the comprehensive physical property test system to test CoS_2xSe_2-2x（x=0.88）nanosheets,an obvious hysteresis loop is observed under the in-plane magnetic field,and the Curie temperature is fetermined to be about 140 K.At the same time,we also tested CoSe₂nanosheets.The results show that CoSe₂is a room temperature ferromagnetic metal material with Curie temperature up to 280 K.In two-dimensional magnetic materials,280 K is a high Curie temperature,which provides a new platform for the birth of new devices,such as sensors and magneto-optic electronics.