Simulation And Experimental Study Of Liquid-Phase Laser Irradiation Of Graphene

Graphene is a two-dimensional crystal with a honeycomb lattice structure formed by sp~2hybrid carbon atoms bonded by strong covalent bonds.Graphene has many excellent properties,and a very broad application prospect in the fields of Nano electronic devices,sensors,semiconductor materials,protective coatings and energy storage due to its unique crystal structure.Liquid-phase laser irradiation can produce graphene with good quality.Compared with the traditional chemical vapor deposition method,this method has many advantages,such as simple process,simple operation,clean pollution,and has attracted more and more attention of scholars.This dissertation uses a combination of simulation and experiment to explore the mechanism of graphene peeling and optimize the experimental process.Theoretical basis and practical experience for industrial promotion are provided in this dissertation.The main research work and results are as follows:(1)Based on the theory of thermodynamics and kinetics,the theoretical basis of the interaction between high energy laser and graphite was analyzed,including the temperature and pressure distribution of graphite during laser irradiation,the mechanism of graphite melting and sublimation under high temperature,high pressure environment,and the evolution of plasma.The feasibility of laser irradiation for stripping graphene in liquid medium was theoretically explained.(2)The molecular dynamics simulation method based on empirical potential was utilized to simulate the exfoliation process of graphene.The high temperature and high pressure environment under laser irradiation was simulated through the Fortite module in Material Studio.Calculated the most important face of graphite crystals through the BFDH model of the Morphology module.The research finds that the van der Waals force between the carbon atom surface network layers is the most important obstacle to be overcomed when peeling graphene from the graphite surface.High temperature can cause the carbon atom surface network on the graphite surface to slip,but it can not cause the exfoliation of graphene.In order to achieve the exfoliation of graphite,an external force needs to be applied.High temperature is beneficial for the peeling of graphene.The incident angle of the laser determines the direction of the external force.External forces in different directions produce different effects.Whether the external force can cause peeling depends mainly on the magnitude of the component of the external force in the vertical direction of the carbon atom surface.(3)Based on the simulation results,liquid-phase laser irradiation method was verified by experiments.Small-scale flake graphite was used as the carbon material.Deionized water was used as a liquid medium to prepare a graphite suspension.A copper foil was set as a collection substrate,and a small amount of graphene was prepared by nanosecond pulse laser irradiation.Products were analyzed by using a variety of characterization techniques.It was found that few-layer graphene were prepared,and the horizontal and vertical dimensions reached more than a few microns.Combined with the results of molecular dynamics simulation,the mechanism of liquid-phase laser irradiation to strip graphene was summarized:When a nanosecond pulse laser is applied to a solid target,a high temperature and high pressure plasma is induced instantly.Due to the constraint effect of the water medium,the plasma can not be diffused.Inside the plasma plume,heat conduction,expansion,and cooling occur until the collapse of plasma.The high temperature and high pressure environment generated in this process prompt the carbon atomic network on the surface of the graphite to overcome the van der Waals force between the layers and realize the peeling of the graphene.