Design,Preparation And Electron Emission Performance Of Graphene Based Cathodes

The cathode material is the core part and plays a decisive role in the performance of the field emission devices.As a novel two-dimensional carbon material,Graphene is very suitable for field emitter due to its unique structure,such as high aspect ratio,abundant sharp edges,extremely high carrier mobility,excellent thermal conductivity,mechanical properties and stable physical and chemical properties.However,up to now,the too high turn-on electric field,low emission current density,and the poor stability still cannot meet the requirement of the devices.The reasons are mainly as follows:the work function of Graphene emitter is still large;due to the morphology and distribution,the field enhancement factor of Graphene is relatively small,resulting in higher turn-on electric field;the less density of emission sites leads to the small emission current;the interface contact between Graphene emitter and the metal electrode affects the electron transport and the mechanical contact with the substrate,which limits the current density and stability.Therefore,this thesis will follow above key factors and propose corresponding solutions.In this work,we focused on the design and preparation of Graphene cathode,and improving the field emission properties of Graphene based emitters.Aiming to decrease the turn-on and threshold electric filed,increase the field emission current and the electron emission stability,so as to promote its application in large current density field emission devices.The main achievements of this dissertation can be concluded as follows:1.N dopped and supported by CNTs arrays to improve field emission performance of Graphene synthesized by CVD(a)Material modification:N dopped Graphene to reduce the work function and increase the carrier concentration.Compared with the pristine Graphene,N dopped Graphene showed excellent performance with decreasing turn-on voltage from 6.5V/?m to 1.5V/?m,and increasing the maximum current density from 0.07 mA/cm~2 to 0.9 mA/cm~2.(b)Structure optimization:Support Graphene on CNTs arrays to increase the field enhancement factors by the protrusions and edges.Graphene was transferred onto the surface of the well aligned CNTs arrays.Excellent field emission performances were obtained from the protrusions and edges which were generated by supported on the CNTs.The result showed lower turn-on field of 1.8V/?m and the maximum current density of 2.8mA/cm~2.2.Material modification.Synthesize defected Graphene by RF-PECVD to increase the density of emission sitesIn respect of the less density of emission sites from Graphene,we directly synthesized defected Graphene by PECVD method.These defects increases the density of emission sites,especially sp~3 defects effectively reduces the work function of graphene.The results showed that Graphene grown by RF-PECVD method is very different from thermal CVD method.Although it is also laid flat on the substrate,it has abundant defects,and the defect types changed according to the growth parameters,including vacancy,grain boundary and sp3 defect.The result showed a higher maximum current density of1.32 mA/cm~2,much lower turn-on field and threshold field of 1.4V/?m and 3V/?m,respectively,which can be compared with previously reported field emission of Carbon Nanowall.3.Structure optimization.A novel petal-like Graphene emitter directly grown on Si-Cu film substrate to improve the field enhancement factor and the interface contactAccording to the small field enhancement factor,poor electrical and mechanical contact between Graphene and substrate,we optimized and prepared a novel petal-like Graphene emitter directly grown on Cu film sputtered on Si substrate.This unique structure makes Graphene random oriented distribution and better adhesion with substrate,resulted in increasing the field enhancement factor and better electrical and mechanical contact at the interface.The theoretical simulation and experimental results indicated the turn-on field and threshold field of 1.5V/?m and 2.2V/?m,respectively.The maximum current density was10.5mA/cm~2 corresponding to the electric field of 4V/?m.The emitter also showed good emission stability with a current fluctuation below 2%within 10h duration test.4.Introduced Graphene as buffer layer to improve interface contact.Prepared Graphene based hybrid emitter to improve the field emission stability.There were some problems in the field emission of ZnO and CNTs,such as the poor carrier mobility in ZnO,and much joule heat was generated between emitter and substrate during the field emission process due to high contact resistance.The heat accumulation resulted in the damage of the combination,as well as reducing the stability and lifetime of the device.In terms of the above problems about the electron transport and thermal conduction at the interface,we introduced Graphene as the buffer layer,the Graphene-ZnO and Graphene-CNTs hybrid cathodes were prepared by hydrothermal method and wet transfer method respectively,which greatly improved the electrical and thermal contact between emitter and substrate.The results showed that the turn-on and threshold electric field was 1.6V/?m and 3.7V/?m respectively for Graphene-ZnO hybrid emitter,compared to that of 2.8V/?m and 4.5V/?m respectively for ZnO emitter.Duo to the ohmic contact,the Graphene-ZnO hybrid emitter also showed better emission uniformity and stability,with a current fluctuation below 3%within 5h duration test,while the current degeneration for ZnO emitter was more than 9%.Similarly,for the Graphene-CNT hybrid emitter,the maximum current increased from 950?A to 1670?A after transfer CNTs on the Graphene buffer layer,which was attributed to the improved conductivity.Duo to the reduced resistance and improved heat conduction at the interface,the Graphene-CNTs hybrid emitter showed better emission stability,with a current fluctuation below 2%in 5h duration,while the current degeneration for CNTs emitter was more than 12%.