Synthesis Of Monolayer And Few-layer MoS₂ And Study On Room Temperature Ferromagnetism Of Co-doped ZnO Film

The main content of this dissertation is about synthesis of monolayer and few-layer MoS2 and the study on room temperature ferromagnetism of Co-doped ZnO film.There are five chapters in this dissertation:the first chapter narrates the basic conception of two-dimensional materials,the structure,properties,and application of MoS2,the introduction of diluted magnetic semiconductor,the main theories of the origin of room temperature ferromagnetism in diluted magnetic semiconductor,and the motivation of this dissertation;the second chapter introduces the characterization techniques of MoS2 layer number and grain size,as well as the commonly used MoS2 preparation methods;the third chapter presents the mechanical exfoliation of monolayer to quadlayer MoS2 and chemical vapor deposition of monolayer MoS2 film,and explores the processes as well as the affecting factors of MoS2 film growth;the fourth chapter illustrates the changes of magnetism and morphology of Co-doped ZnO film under different annealing conditions;the fifth chapter is summarization and outlook.The main contents of chapter three and four are shown below.In chapter three,we first prepared the monolayer,bilayer,trilayer,and quadlayer MoS2 by mechanical exfoliation method,and then we prepared the monolayer MoS2 film on sapphire substrate via chemical vapor deposition method with S and MoO3 as raw materials.We found that sulfur vapor concentration in the growth zone played a crucial role in the formation of monolayer MoS2 film,too high or too low of the sulfur vapor concentration was unfavorable for the monolayer MoS2 film growth.In the experiment,sulfur vapor concentration could be controlled by adjusting the locations of the sulfur source and the Ar flow rate.It was beneficial for monolayer MoS2 film growth when the sulfur source was placed downstream relative to the gas flow direction and the Ar flow rate was 50 sccm.Optical measurements showed that the as-synthesized film was uniform and continuous over a large area,and was of high quality comparable to the exfoliated monolayer MoS2.We also found that the MoS2/MoO2 microplates could be synthesized directly on SiO2/Si substrate without the assistance of metal oxides when the sulfur source was placed downstream relative to the gas flow direction and the Ar flow rate was 20 sccm.At last,by analyzing the products at different locations in the growth zone,we showed that the reaction process of MoO3 with S involves the stepwise reduction and sulfurization,that is,MoO3 is first partially reduced by vaporized S to form volatile suboxide MoO3-x species,which are further sulfurized to give rise to the formation of MoS2.Sulfur serves as the reductant and sulfurization agent at the same time.In chapter four,we showed that the room temperature ferromagnetism in Co-doped ZnO film is associated with multiple factors.Zn0.95Co0.05O films were deposited on Si single crystal substrates by magnetron sputtering and annealed in H2(5%)/N2(95%)atmosphere at different temperatures.We found that film changed from paramagnetic to ferromagnetic through annealing,and the magnitude of saturation magnetic moment increased with the rising annealing temperature and gradually reached its maximum at 700 ?.When annealing temperature was 620 ? or 650 ?,saturation magnetic moment would increase one order of magnitude,coincidentally,film started to be etched at 620 ? and changed into amorphous state at 650 ?.We thought that the appearance of oxygen vacancies was the reason that film changed from paramagnetic to ferromagnetic,and the enhanced gain boundary effect caused by etching as well as the formation of amorphous regions led to the sudden increase of saturation magnetic moment when annealing temperature were 620? and 650 ?,respectively.Then the H2/N2 annealed films were annealed in oxygen atmosphere for four times.We found that films were recrystallized,and few zinc-blende ZnO cubic grains were formed except the wurtzite ZnO hexagonal grains.Saturation magnetic moment of each the sequential annealed film decreased sharply after first oxygen annealing but resumed to the initial magnitude after second oxygen annealing and changed little with the next oxygen annealing.We thought that the decrease of oxygen vacancies,the vanishment of amorphous regions,and the weakened gain boundary effect resulted in the decrease of saturation magnetic moment after first oxygen annealing,and formation of Co-doped zinc-blende ZnO gave rise to the recovery of saturation magnetic moment after second oxygen annealing.