Synthesis Of Graphene And Surface Potential Control Of Graphene With Applications

Graphene is a two-dimensional honeycomb-shaped carbonaceous crystalline material composed of a single layer of carbon atoms arranged face-to-face.Since its inception,graphene has shown potential applications in many fields due to its excellent electrical,optical,mechanical and thermal properties.Surface potential is one of the important physical parameters of graphene materials,and the convenient and effective regulation of graphene surface potential can help to promote graphene from the laboratory to the future full-scale applications.In this paper,three kinds of graphene optoelectronic devices are selected to be studied around the measurement,regulation and matching of graphene surface potential,which are related to the modulation of terahertz wave,flexible display and field emission light source.These three aspects cover the application of light in communication,entertainment,and lighting,fully reflecting the potential impact of graphene as a new material on our lives in the future.1.The process of synthesizing large-area monolayer graphene by Chemical Vapor Deposition（CVD）has been investigated,and large-area monolayer graphene with a continuous film structure has been prepared,which lays the foundation for the subsequent device application of graphene.Aiming at the problem of a large number of lattice defects in the synthesis of graphene by the traditional CVD method,processes and methods to reduce the nucleation density of graphene and to expand the size of single-crystal graphene were investigated,and the quality of the produced graphene was improved.Firstly,based on the growth of single-crystal graphene by the copper box method,it is proposed to increase the air pressure in the growth stage to obtain a smaller graphene nucleation density.After counting the roughness of the inner surface of the copper box under different growth air pressures,it was found that the roughness of the inner surface of the copper box was lower when the growth air pressure was higher,which was more favorable for the growth of single-crystal graphene.Finally,single-crystal graphene with a diameter of 450μm was obtained within 20 min of growth time,which can meet the preparation requirements of most graphene transistors and has a higher growth efficiency compared with the traditional low-pressure copper box method.In addition,the distribution of the gas flow rate inside the quartz boat and the nucleation density of graphene at different positions inside the quartz boat were investigated by multiphysics simulation and experiment,and it was found that the gas flow rate at the bottom of the quartz boat was slower and the nucleation density was lower.Finally,a very low nucleation density was obtained at the bottom of the quartz boat by covering the quartz sheet on the quartz boat,and millimeter-sized single-crystalline graphene was synthesized on a common copper foil by 60 min growth.2.Two types of terahertz Electromagnetically Induced Transparency（EIT）hypersurfaces,static and dynamic,are designed,and a single-layer graphene composite with a metallic structure of the hypersurface is used to realize the dynamic tunability of the EIT.Firstly,a static EIT hypersurface structure with SRR ring embracing CW is proposed by taking advantage of the phenomenon that two open-mode resonant structures,Cut Wire（CW）and Split Ring Resonator（SRR）,are capable of generating coherent phase cancellation at the center frequency.It is found through simulations and theoretical calculations that the resonance of one of the structures can be weakened to make the coherent phase cancellation more complete and to improve the amplitude of the EIT transparent peaks when the gap of the CW is increased or the gap of the SRR is decreased.The results of the simulation analysis were confirmed after testing the experimental samples with different structural parameters,whose transparent peak amplitude could reach up to 90%and produced a group delay of 44.89 ps.Secondly,dynamically tunable terahertz EIT hypersurfaces were prepared by combining a metal structure with a single layer of graphene using a micro-nano process.Simulations and theoretical calculations revealed that the resonance state of the CW metal structure can be tuned by changing the graphene figure of merit and thus the conductivity of graphene,thus regulating the amplitude of the transparent peak of the EIT and the group velocity of the terahertz wave.In the photomodulation experiments of EIT,with the enhancement of femtosecond pump light power,graphene conductivity decreases,the amplitude of the EIT transparent peak increases,and the group delay of the structure increases.Finally,the two metal-graphene composite hypersurfaces exhibit ultrafast optical modulation properties of 8 ps and 11 ps as measured by varying the delay time of the pump light pulse and terahertz wave pulse.3.Due to the field shielding effect in graphene sheet arrays,the field emission of vertically oriented graphene sheet arrays was firstly simulated and analyzed by using MAGIC software,and it was found that the field emission current was maximum in a fixed area when the spacing of graphene sheets was four times of their own height.Hybrid paper emitters with overlapping distribution of Reduced Graphene Oxide Sheets（r GS）and Carbon Nanotubes（CNTs）were designed and prepared.In the field emission test of the hybrid paper with different thicknesses of CNTs layers,it was found that the field shielding between graphene sheets in the hybrid paper was the weakest when the thickness of the CNTs layer was 5μm,which corroborated the previous simulation results.The field emission data of the hybrid paper was also utilized to calculate a 0.14 e V cut in the graphene sheet tip figure of merit due to the Schottky effect,confirming the modulation of the graphene tip figure of merit by the change in field strength.When the r GS/CNTs hybrid paper is rolled,the turn-on field can be as low as 0.1 V/μm,and the maximum emission current density exceeds 2 A/cm².The hybrid paper-roll emitters are ultimately encapsulated as cathodes to make a field-emitting cathodoluminescent lamp（CCL）,which can be driven by 110 V or 220 V household power supplies without the need for transformers or other external circuits.or other external circuitry.4.In order to address the problems of high resistance and low power function of the thin layer of graphene after transfer when graphene is used as the anode of organic light-emitting diode（OLED）,firstly,the breakage of large-area monolayer graphene in the transfer process was reduced by using the plastic border-assisted method,and furthermore,the infiltration and conductivity of the modified PEDOT:PSS were improved by adding dimethyl sulfoxide and fluorocarbon surface In addition,by adding dimethyl sulfoxide and fluorocarbon surface activator to PEDOT:PSS,the wettability and conductivity of ordinary PEDOT:PSS were improved,so that the improved PEDOT:PSS could be uniformly spin-coated on the graphene surface as a hole-injection layer of graphene to achieve a better power function match with the overlaying organic layer,and finally,graphene anode OLEDs spin-coated with the improved PEDOT:PSS were successfully lit up,with a luminescence uniformity superior to copper nanowire anode OLEDs having the same superstructure.When a layer of Mo O₃ is added between the PEDOT:PSS and the overlaying organic layer as a matching layer,better luminescence effect is obtained at the same voltage in the test,and the device can work normally under bending condition when using the flexible material as the substrate.5.The surface potential distributions of graphene on different substrates were non-destructively scanned using Kelvin Probe Force Microscopy（KPFM）.The surface potential distributions of single-crystal graphene and polycrystalline graphene on copper foils were compared,and it was found that the surface potential distribution of monocrystalline graphene on copper foils was more uniform than that of polycrystalline graphene after a lot of tests,and this feature provided a new way of thinking for the rapid judgment of the monocrystalline nature of graphene.The difference in the surface work function of single-layer and multilayer graphene on copper foil and silicon oxide substrate is also compared,and it is found that the surface work function of graphene decreases with the increase of the number of graphene layers,which is of reference significance for the design of field-emission cathode that requires a lower work function.