Synthess Of Large Area Graphene Films By APCVD

Transparent conductive films(TCF)are one of the most widely used electrical components all over the world.LCD monitors,flat panel TV's,solar cells,and even touch screen contain transparent conductive electrodes.TCFs will grow rapidly with the increasing importance of electrical displays in our lives since it was synthesized.ITO controls most of the market because of its outstanding electrical and optical properties.Commercially available ITO boasts low sheet resistance and relatively high transmittance.However,there are several major obstacles that hinder the continued use and expansion of ITO as a transparent electrode: The price of ITO became expensive due to the high cost of indium and supply fluctuates as market demand varies.ITO is brittle thus cannot be utilized as a flexible electrode.ITO also can diffuse into organic layers and unstable in presence of acid.Thus,much research has been focused around developing other viable materials to replace or partial substitution of ITO.Graphene,which is a two-dimensional hexagonal array of carbon atoms,has emerged as a promising alternative to ITO due to its excellent electronic,optical,chemical and mechanical properties,has become one of the research hotspots.Traditional micromechanical exfoliation can obtain high quality mono-layer graphene flakes,but it`s morphology and productivity cannot be controlled,this technique has significant limitations for practical applications.Chemical Vapor Deposition(CVD)is more suitable for mass production of large area,high quality graphene films.However,there are still a lot of challenges.A current CVD route typically requires high temperature and low pressure to ensure the formation and quality of graphene.However,the high temperature process requires expensive and precise equipment and makes the direct deposition of graphene films in electronic device manufacturing processes infeasible due to the several physical damages to substrates under the graphene.It is therefore indispensable to synthesis graphene at reduced growth temperatures,permitting large area fabrication with relatively low cast.In this thesis,high quality graphene films for transparent electrode applications and amorphous graphene films was synthesized by Oxygenfree Atmospheric Pressure Chemical Vapor Deposition(O-APCVD)and the key factors that determine the electrical and optical properties of the graphene films was investigate.The effects of temperature and hydrogen content in the growth step were also studied.Conclusions are as follows:(1)Pretreatment of copper foils plays an important role in high quality single-layer graphene films synthesized.Using ammonium persulfate solution as etchant to clean the commercial copper foils and then annealing at 1000? in the present of hydrogen can achieve smoother surface of copper foils.The nucleation density is relatively low in the graphene synthesis process.This route can achieve high quality graphene films synthesized.(2)Oxygen content and operation temperature have great influence on the graphene films preparation process.With the operation temperature decreases,the volatilization rate and decomposition rate of toluene decreased,and the controllability during preparation process was enhanced.Single-layer graphene domain with the dimeter of 20?m to 37?m can be synthesized at 150? when using O-APCVD method,but the surface coverage decreased.Continued graphene films with highly single-layer rate can be synthesized at 280?.The thickness of the prepared graphene film is 0.49 nm,showing a signature of monolayer graphene.(3)Hydrogen partial pressure is also an important factor affecting the quality of graphene.With the increase of hydrogen partial pressure in the graphene films synthesis process,the comprehensive quality of graphene decreases,and the low hydrogen partial pressure is favorable for the synthesis of high quality graphene films.The transmittance of graphene film is 97.3% when the hydrogen flow rate as low as 5sccm.The resistance of prepared films at 280? is 435?/?,showing excellent electrical properties.(4)The magnesium foils were chosen as substrate to catalyze the amorphous graphene films synthesis for the first time.Using 0.025mol/L oxalic acid solution as etchant to clean the commercial magnesium foils and then annealing at 490? in the present of hydrogen can achieve smoother surface of magnesium foils.The grain size obtained after the pretreatment was range from 80 ?m to 200?m.(5)Atmosphere composition and operation temperature have great influence on the graphene films preparation process.Amorphous graphene films can be synthesized with the neglect of H2,and the number of graphene layers can be controlled by varying the growth temperature.Continued amorphous graphene films with highly single-layer rate can be synthesized at 400? with the Ar? flow rate of 5sccm.(6)The I_D/I_G of synthesized amorphous graphene is about 1.0,showing high degree of defect.The value of ID/IG slightly increase with the decrease of temperature.The calculated crystal domain size is about 16.2nm.The sample prepared at 400? shows the electrical behavior of insulator.The resistance of sample obtained at 490? is as high as 1.3×10~5 ?/?,which is much higher than that of copper-based graphene.