| Graphene is an isomer of carbon.Its carbon atoms are arranged in a regular hexagonal network.Since the first stripping of graphene by tape in 2004,a variety of preparation methods have been developed,including stripping,chemical vapor deposition,epitaxy growth and redox method.At present,one of the main directions of graphene research is to develop new synthetic technology,which can effectively utilize its excellent physical and chemical properties.Pulsed laser vapor deposition?PLD?is also a suitable gas phase growth technology for graphene due to its high vacuum,high controllability and fast deposition rate.In this paper,graphene thin films were prepared by PLD method using highly oriented pyrolytic graphite as target material.As for the present,graphene prepared by PLD method has many defects,tends to grow in multiple layers and has weak controllability of the number of layers,which are the key problems to be solved urgently.Starting from the basic growth mechanism,this paper systematically studied the key factors affecting the growth quality of graphene,and explored the growth conditions of large area and high quality graphene by combining theoretical and experimental analysis.The optical and electrical properties of monolayer and bilayer graphene were studied,and their related physical connotations were explored.It provides basic theoretical guidance for the PLD preparation of graphene-based composite devices and other two-dimensional materials?such as BN,MoS2,etc.?.Firstly,the growth process of graphene films prepared by PLD method was systematically studied.It was found that the growth of graphene films was affected by the migration and diffusion ability of particles.In the PLD process,graphene is first nucleated at the substrate step due to the principle of minimum energy.When the kinetic energy of graphene atom migration and diffusion is low,graphene tends to grow in island form,forming a three-dimensional nucleus,which is not conducive to the layered growth of graphene.Graphene tends to grow in layers only when the kinetic energy of migration and diffusion of graphene is high enough.Based on this,the relationship between the structure of graphene thin films and the laser energy density was studied in detail.The results showed that the appropriate laser energy density was conducive to the balance of carbon atom diffusion and condensation,as well as the growth of high-quality monolayer and bilayer graphene.In this paper,monolayer graphene with the highest crystal quality was prepared at a laser energy density of 5.66 J/cm2,with the largest monolayer graphene sheet reaching 20?m in diameter.Bilayer graphene was prepared at the laser energy density of 8.49 J/cm2.Controlled growth of monolayer and bilayer graphene was successfully achieved by adjusting the pulsed laser energy density.Secondly,the linear optical properties of graphene prepared by PLD method were studied.It was found that the absorption rates of monolayer and bilayer graphene prepared by laser energy density of 5.66 and 8.49 J/cm2 were 3.07%and 4.95%respectively,which were close to the theoretical values.Then the third-order nonlinearity of graphene was studied by Z-scan.It is found that the nonlinear absorption of graphene is saturated under the action of ps laser.The fs laser test showed that the nonlinear absorption of monolayer graphene changed from saturated absorption to reverse saturated absorption,which was derived from the ultra-short pulse width of fs laser and the two-photon absorption of monolayer graphene.Nonlinear absorption and refraction were observed simultaneously in bilayer graphene with pulse numbers of 500.The nonlinear absorption coefficient was 2.65 cm/GW,the nonlinear refractive index n2 was 3.7×10-5 cm2/GW,and the third-order nonlinear magnetization was8.73×10-12 esu.The non-linear absorption mechanism of graphene in time scale was studied by transient absorption spectra.It was found that there was an ultrafast saturated absorption process??mOD<0?in 1ps due to the rapid recombination of electrons and holes in monolayer and bilayer graphene.Finally,the basic electrical properties of graphene prepared by PLD method were studied.It was found that large area graphene were affected by grain boundaries,defects and other factors.Its carrier concentration n was in the order of 1013 and carrier mobility?was within 100 cm2/V.s.The extremely low temperature electrical transport properties of monolayer and bilayer graphene grown at laser energy densities of 5.66and 8.49 J/cm2 were studied.It was found that the carrier mobility of large-area monolayer graphene was smaller at extremely low temperature than at room temperature,ranging from 12.77 to 25.65 cm2/V.s.This was due to the electron scattering at the grain boundary,the interaction between electrons,and the interaction between electrons and phonons at extremely low temperature.The resistance of monolayer graphene decreased with the increase of temperature.At the same time,the magnetic field produced a negative magnetoresistance to the monolayer graphene,which reduced the resistance and enhanced the conductivity.Monolayer graphene exhibited obvious weak localization effect at extremely low temperature.The maximum decoherence length was 38.9 nm at the current of 0.1?A.On the contrary,the resistance of bilayer graphene increased with the increase of temperature.The magnetic field produced a positive magnetoresistance to bilayer graphene,which resulted in the resistance increasing with the increase of magnetic field.At the same time,the hall effect and weak localization effect of bilayer graphene cannot be observed at extremely low temperature because of its high resistance and low conductivity at extremely low temperature. |
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