Synthesis And Physical Properties Of Layered Transition Metal Compounds Bulk And Thin Film

Layered transition metal compounds have long aroused extensive interests from scientific research and industrial application.Due to diversified chemical compositions and the unique two-dimensional structure,these compounds exhibit abundant physical properties,such as superconductivity,magnetism and charge-density-wave transition.Particularly,with regard to layered transition metal compounds with van der Waals gap,their structure and properties can be modulated by the introduction of exotic ions.The growth of thickness-controlled thin film provides a new dimension for investigation of these materials.It allows people to research thickness-dependent properties and novel properties in a two-dimensional limit.In this study,a series of high-quality layered transition metal compounds were synthesized by the self-flux method,chemical vapor transport method,electrochemical intercalation technique and molecular beam epitaxy thin film growth technique.More-over,their physical properties were determined.Specifically,a new layered transition metal compound with anti-PbO-type layers-Na4Cu3TaAs4 was developed,displaying a unique(?)superstructure.Also,organic ion tetrabutylammoniul(TBA+)was intercalated into transition metal dichalcogenide VSe2 to obtain(TBA)0.3 VSe2.The charge-density-wave transition temperature of VSe2 increased from 110 K to 165 K and hysteresis and a new 3a × 3a commensurate superstructure were observed.Addition-ally,metal-insulator transition,which is rare in transition metal dichalcogenide.At last,the thin-film growth technology based on molecular beam epitaxy technique was used to grow a new monolayer Cr2Se3 thin film with ferrimagnetism in a two-dimensional limit.The dissertation could be divided into four chapters as follows:1.IntroductionThis chapter briefly introduces the unique structure and physical properties of lay-ered transition metal compounds with anti-PbO-type layers,as well as transition metal dichalcogenide,in which the charge density wave transition is emphasized.In addi-tion,previous literature on magnetic thin film and molecular beam epitaxy technique is summarized.2.Structural and physical properties of the new layered transition metal material Na4Cu3TaAs4This chapter describes the synthesis of a new layered transition metal arsenide Na4Cu3TaAs4,together with the investigation of its structural and physical proper-ties.This material has a space group,with lattice parameters of a=5.9101(3)A,c=13.8867(12)A.Similar to the "111"-type iron-based superconductor NaFeAs,Na4Cu3TaAs4 contains two layers of Na sandwiched by anti-PbO-type(Cu/Ta)As lay-ers.On the transition metal sites are 75%Cu and 25%Ta,with Ta forming a well-defined(?)superstructure.Cu and Ta were determined to be in the+1 and+5 oxidation state respectively.The band structure of Na4Cu3TaAs4 measured by ARPES is in good agreement with DFT calculation.Both ARPES and resistivity measurement indicate that Na4Cu3TaAs4 exhibits metallic behavior with p-type carriers.Magnetic susceptibility measurement shows its nearly T-independent diamagnetism.3.Dimensional crossover induced metal-insulator transition in organic ion inter-calated VSe2In this chapter,we synthesized intercalated(TBA)0.3VSe2 through electrochem-ical intercalation of organic ions,and observed a dimensional crossover induced metal-insulator transition.In pristine VSe2,previous studies have revealed a three-dimensional charge-density-wave transition at TCDW?110 K with a metallic ground state.After intercalation of organic ions,the remarkable anisotropy of resistivity in-dicates a highly two-dimensional electronic state in(TBA)0.3VSe2,which is consistent with our DFT calculation.Interestingly,the dimensional crossover enhances the charge-density-wave transition with TCDW of 165 K and leads to an insulating ground state in(TBA)0.3VSe2.Moreover,a new commensurate superstructure with 3a × 3a period-icity is also confirmed in this insulating CDW state.Although the DFT calculation suggests that the new commensurate superstructure and the enhanced charge-density-wave temperature could be ascribed to the improved Femi surface nesting,whether the metal-insulator transition is driven by a perfect Fermi surface nesting is still elusive at the present stage.4.Thin-film growth technology based on molecular beam epitaxy technique and the growth and properties of monolayer thin film Cr2Se3In this chapter,we explored the thin film growth technology through molecular beam epitaxy and grew a new monolayer Cr2Se3 thin film on the x high oriented py-rolytic graphite(HOPG)substrate.Cr2Se3 thin film laid on the HOPG substrate closely,which is favorable to potential applications.Afterward,the X-ray diffraction result in-dicates the film adopts NiAs-type structure.Cr was determined to be in the+3 valence state by the X-ray photoelectron spectrum.High stability in air of this monolayer thin film provides convenience for its research and application.From magnetic suscepti-bility measurement,the monolayer Cr2Se3 thin film exhibits ferrimagnetism transition with a transition temperature of 80 K,obviously different from bulk Cr2Se3.