Controllable Growth And Transfer Method Of Ultra-flat Graphene

Graphene with rich physical and chemical properties has great application propects in the fields of transparent conductive films,semiconductors and et al.Curently,among the methods of graphene preparing,Chemical vapor deposition（CVD）method is considered to be the most likely way to realize the future application due to its lower cost,simpility of operating and the ability to produce high-quality graphene on a large scale.However,the CVD graphene often has wrinkles,which are known as the liner defects that degrade the properties of graphene and affect the macroscopic uniformity of the film.Therefore,to completely eliminate wrinkles and realize the large-scale preparation of wrinkle-free and ultra-flat graphene is important and difficult in this field.In addition,CVD graphene uaually grows on the metal substrates,therefore the as-grown graphene need to be transferred further.While,the traditional transfer process usually introduces new defects such as folds and cracks,which is another reason to restrict the real application of CVD graphene.Therefore,it’s also urgent to realize the nondestructive transfer of large-area high-quality films.This paper is to carry out research on the above difficulties respectively.The main conclusions are as follows:1)Proton-assisted growth of ultra-flat graphene filmsFirstly,we analyze the generation process of wrinkles in principle,and realize that the wrinkles could be completely eliminated only by decoupling graphene.Then,the proton permeation model is proposed,that is,a large number of freely penetrating protons and electrons recombine again between graphene and substrate,forming a layer of hydrogen interlayer,and achieving the complete decoupling of graphene.Futhermore,the change state of hydrogen atoms in the graphene/substrate system is simulated by molecular dynamics simulation to verify our conjecture.Based on the above analyses and simulation results,the hydrogen plasma technology is developed.Firstly,we use this method to post-treat the winkled graphene films,the wrinkles can be gradually weakened and even be eliminated completely under higher temperatures.Then,we success in growing a wafer scale,ultra-flat graphene film on the flat single-crystal copper substrate with this hydrogen plasma-assisted approach.Next,we use a variety of means to characterize the feature of the ultra-flat graphene comprehensively,which is without wrinkles and in a state of decoupling,including the use of atomic force microscope（AFM）to observe the surface morphology,Raman spectroscopy（Raman）to analyze the residual stress and distribution,scanning tunneling microscope（STM）to observe Moirépatterns,scanning tunneling spectrum（STS）and angular resolution photoelectron spectrum（ARPES）to detect the change of bonding force between graphene and substrate,in situ variable temperature Raman spectroscopy to characterize the changes in the rate of thermal expansion and so on.Finally,we make a further expansion and compare the effects of different types of graphene samples and deuterium（D₂）and helium（He）plasmas,which not only verify the accuracy of the proton permeation model,but also emphasize the importance and universality of this method.2)Spin-heating assisted transfer of damage-free graphene filmsFirstly,it is found that new defects are easily introduced in the process of dehydration during the traditional wet transfer method,which affect the high quality and macroscopic homogeneity of samples.Then,the improved spin-heating assisted method is used to take place of the natural dehydration process,which can remove of water droplets between the substrate and graphene uniformly and thoroughly,resulting in a large-area transfer film without folds and cracks.Raman and electrical distribution show that the quality and uniformity of the graphene film are significantly improved.Most importantly,based on the previous chapter,we succeed in achieving the"flat to flat"transfer of the sample,perfectly retain the characteristics of the original growth graphene,and the resulting 2-inch film is still free of wrinkles and breakage.The ultra-flat and wrinkle-free nature of graphene film make it easy to clean when removing the protective medium PMMA.In addition,we make the sample into field effect transistor（FET）devices with different line widths,including 20μm,100μm and500μm.The quantum hall effect is not only realized in electrical measurement,but also in the room temperature.And this is the largest quantum hall effect device.Moreover,the threshold value of the hall platform appears almost unchanged for these different line-width devices.The homogenization and high quality of ultra-flat graphene are fully proven by those results.In short,we first develope a proton-assisted method to successfully growth wafer-level,ultra-flat graphene films,and then carry out a comprehensive and in-depth study on the characteristics and causes of wrinkle-free.And this new method is expected to expand to other materials and other fields,such as hydrogen energy storage.In addition,the optimized spin-heating assisted transfer method perfectly preserve the characteristics of the as-grown graphene,and we finally achieve large scale"flat to flat"non-destructive transfer.Its high quality proves that eliminating wrinkles will be an important basis for the real application of 2D material devices in the future.