Graphene Films Preparation,Defect Control and Study of Optical and Electric Property

Author:Zhang Dong Jiu

Supervisor:cheng hai feng


Degree Year:2015





Andre Geim and Konstantin Novoselov won the Nobel Prize in Physics "for pioneering experiments in graphene"in 2010.Subsequently,graphene became a focal point of research in numerous fields.Graphene was introduced as a new member of the carbon crystal family,possessing a unique single atomic layer structure and a two-dimensional crystal structure.A variety of excellent properties are concentrated in graphene,such as ultra high carrier mobility,superior electronic conductivity and thermal conductivity,excellent permeability and mechanical strength.The carbon crystal structure is capable of forming fullerenes,carbon nanotubes,graphene,graphite,and diamond.Herein,a complete system was developed from zero to three dimensions of the carbon paradigm.First,the chemical vapor deposition method for graphene film preparation is discussed.After changing deposition parameters that influence the microstates of graphene,a 1.82 mm diameter single crystal graphene was prepared with a carrier mobility of 1.05×105 cm2/(V·S).For low temperature graphene films(500℃)grown on an insulating substrate(Al2O3,MgO,quartz glass)glycerol is utilized as the gas source.Liquid carbon source is implemented for graphene doping in which we successfully prepared graphene films of different doping types and doping level.Graphene-based optical limiting devices were formed via a wet chemical etching transfer method and reached a certain optical limiting value.Graphene-based resistive memory was fabricated with a Ag/mGraphene/Pt/SiO2/Si structure.The device demonstrates a high resistance switching characteristic with a facile fabrication process.By controlling the nucleation sites of the substrate and the concentration of silicon etching nanoparticles,defective graphene film is obtained.Nucleation site density of the substrate demonstrates a linear relation with the graphene film defect density;as the growth time increases,the defect density increases until reaching a limiting value before the films cover the substrate.A low concentration of SiOx nanoparticles(less than 3mg/ml)of etching the thin film can be obtained above 1300 K,and 60°,120°etching angles were recorded with an edge to edge angle of 90°.However,a high concentration of SiOx nanoparticles causes the film to deviate from an etched form to a non-linear type with dendritic nanocrystalline shape.Through deliberate gas source selection,an effective air pressure,temperature adjustment,optimized raw material proportion,facilitated the synthesis of 2.50% boron-doped graphene with triangular shape and 8.00% nitrogen-doped graphene with rectangular shape.Silicon-doped content was varied from 3.00%-10.02%,which helped graphene etching from defective sites.When B and N doping at the same time,as the doping amount increases,films were more likely to transverse growth to the longitudinal growth.When B,N,and O were simultaneously doped,as the doping amount of the film increases,the more distinct the layered film became.After the UV absorption spectrum of various films were characterized,we observed a change in the bandgap by varying the elements when B and N were doped at the same time.The bandgap sharply changes with an increase of doping amount when B,N,O elements were doped at the same time and when the carbon content was less than 20.00%,demonstrating a band gap larger than 3.0 eV.Different with B elements doping,other element doping together or single in graphene always changed graphene film from p to n.Based on defective graphene films,we fabricated several patterned optical limiting devices.The influence of the doping amount and element identity on the nonlinear optical properties are discussed.Z-scan method was implemented for optical limiting properties characterization.The mechanism of limiting characteristics are as follows:the nonlinear absorption coefficient of films vary with doping performance;as the doping level changes,nonlinear scattering in films were also changed,especially when the presence of impurities increased,which distinctly causes more scattering;refraction between every film system was more important,along with film thickness increase,realization of film appeared as the limiting characteristic changed stepwise.Superlattice structure contributes to enhanced nonlinear laser limiting properties of graphene films that exhibit nonlinear characteristics with a periodic micro structure of graphene,which was better than the other thin films that demonstrated a higher threshold value.Herein,a Ag/mGraphene/Pt/SiO2/Si multilayer structure resistive memory device demonstrates excellent bipolar resistance.Nonvolatile bipolar switch characteristics are shown in the device,high and low resistance rates of switch ratio are more than 103,cyclic stability of more than 102 times and data hold time is more than 103 s.AFM,TEM,SEM,EELS and Raman were implemented to characterize composition,morphology and structure of the device characterization.Results demonstrate that the device is a Ag/mGraphene/Pt/SiO2/Si multilayer structure while each layer is also a multilayer structure but there is no influence between each of the layers,in which elements bonding mode was also confirmed.EELS and Raman spectrum were used to ensure the mechanism of the device.In carbon-based materials,binding energy shifts from sp2 and sp3 in which changes in the bonding mechanism occur with other atoms and was identified as the mechanism of resistance for the given device.