Graphene And Its Stacks Design

Graphene has attracted enormous attention due to its outstanding properties since its separation from graphite bulk.The two-dimensional(2D)layered structure and excellent mechanical properties of graphene have brought new opportunities and challenges in nano/micro-mechanics,which will help researchers to understand the relationship between microscopic physical structure and macroscopic mechanical behavior more deeply.The study of mechanical behavior of graphene through experimental tests is one of the important research fields of nano/micro-mechanics However,current experiments testing methods lack high-quality samples that meet the needs,effective mechanical loading methods,and effective measurement methods,which greatly limits the study of the mechanical behavior of 2D materials.Therefore,it is necessary to develop a new experimental mechanics platform for 2D materials Using Raman microscopy as the measurement method,obtaining a variety of samples that meet the needs and loading the designed samples effectively are the foundations of the platform.The growth control of high-quality graphene and reliable graphene transfer technologies are the main contents in this thesisFrom the perspective of graphene preparation,the graphene samples of specific isotopes,thickness,size,defect density and domain orientation can be realized through parameter control.Morever,non-destructive transfer technologies affect the final morphology the graphene film.Therefore,firstly,we obtained high-quality single-crystal monolayer graphene by oxygen-assisted chemical vapor deposition(CVD),and invented a new transfer method which is suitable for transferring onto flexible substrates to establish the accurate correlation between graphene structure and Raman signals.Secondly,we designed cracked monolayer graphene film with zigzag edges by controlled CVD process,combined with directional etching transfer method to achieve in-situ observation of the fracture process.Thirdly,we used isotope labeling to trace the growth of bilayer graphene to achieve the decoupling of two graphene layers,which can help to study the behavior of graphene interface.Fouthly,we promoted the concept and demo of graphene tape,which has the ablity to transfer graphene to arbitrary surfaces regardless of the professional skills and devices.The main results are concluded as belows:(1)Large domain size monolayer graphene synthesis:an oxygen-assised CVD process was developed to achieve millimeter-sized monolayer graphene efficient synthesis.By deeply oxidizing polycrystalline copper foil at high temperature,the graphene nucleation density is reduced by 105,the average domain size is increased by 103,and the graphene coverage is increased from 64%to 100%in the same growth time.The average size of the single graphene domains obtained in the experiment is 1.5 mm,and the maximum reaches 2.8 mm with a growth rate of about 16 μm/min.Oxygen treatment not only reduces the nucleation barrier of graphene,increases the growth rate of graphene domains,but also achieves the cleaning growth.Large-domain monolayer graphene can greatly reduce the density of linear defects and improve the macroscopic properties.The single crystal samples can be used to establish the relationship between graphene structure and Raman signals.By adjusting oxidation and growth time,a graphene film with preset cracks(10 μm to 30 μm long)with a zigzag edgeis obtained.This crack graphene film can be used in crack observation.(2)Bilayer graphene synthesis:by combining the oxygen-assisted CVD process and isotope-labeling technology,the growth mechanism and stacking structure of bilayer graphene was studied with Raman microscopy and atomic force microscopy.The results suggest that there are mainly two modes in the growth process of bilayer graphene-simultaneous growth mode(63%)and sequential growth mode(37%).Moreover,the ratio of AB-stacked bilayer graphene and twisted bilayer graphene is around 4:1.It was also found that the crystal orientation change in twisted bilayer graphene occurred in the upper layer,which was attributed to the surface roughness of the copper substrate.On rough copper surfaces,graphene crystal orientation changes at copper grain boundaries or surface defects.However,for copper substrates covered with graphene,surface reconstruction makes the copper surface smoother,and the thermal stress in newly graphene layer can be released by rotation.The research on the growth mechanism and stacking structure of bilayer graphene provides powerful guidance for the development of large-area,pure bilayer,AB-stacking graphene.Isotope-labeled bilayer graphene can decouple the two-layers Raman signals to obtain the relationship between graphene strain and peak shifts of two layers respectively,and is helpful in graphene interface behavior studying.(3)Improved graphene transfer methods:two improvements to the traditional method are proposed.The first is the use of bipolar electrochemical technology to achieve directional etching of the copper substrate.Directional etching avoided the generation of the copper frame and effectively released the stress concentration at the etching fronts.Through the directional etching method,the surface roughness of the transferred graphene is reduced by 1/3,the sheet resistance is reduced by 40%,and the carrier mobility is increased by more than 50%.Combine directional etching method with preset crack monolayer graphene to observe the in-situ crack propagation was our exclusive technology.The second is a polymer-free graphene water transfer printing(WTP)method improved by the heptane liquid protection layer,a convex liquid surface,and an etchant containing anti-wrinkle agents.WTP method can achieve high-quality graphene transfer on rigid,flexible,rough,three-dimensional substrates,and even on 100-micronmeters trenches.WTP graphene has fewer impurities and lower defect density than traditional transfer methods,with a 50%decrease in sheet resistance and a 130%increase in average carrier mobility.Moreover,the cyclic bending test shows that the graphene’s electromechanical properties are very stable.WTP method is currently the most applicable method in graphene/flexible substrate transfer.It can not only maintain the graphene structure,but also avoid doping and pollution caused by polymer.By applying WTP method in high-quality single-crystal graphene,the correlation between structure and Raman signals can be established.By applying WTP method in isotopic bilayer graphene,the displacement loading of the graphene/flexible substrate can be realized,and the interface behavior can be studied.(4)Graphene tape:a product of graphene tape was designed with WTP method to bridge the gap between graphene synthesis and applications.Graphene tape decomposes the graphene transfer process into two parts—preparation of tape including several complicated transfer processes and super simple and reliable using process.Graphene tape gets rid of the dependence of transfer equipment and technology on graphene industrial applications,and realizes efficient and high-quality graphene transfer.Through the finite element simulation of the graphene tape model,the process of graphene being stripped was simulated,and the reaction force and strain distribution in the material during the stripping process was obtained.Graphene tape can transfer graphene onto various objects,even on the inner surface.Most importantly,the graphene tape can achieve one-month storability which is currently not possible for any other transfer methods,achieving high-quality and efficient transfer of graphene.