Study On Preparation Of Hexagonal Boron Nitride By Space Limited Chemical Vapor Deposition

Hexagonal boron nitride(h-BN)has a layered structure similar to graphene,and the layers are bound and alternately arranged by boron and nitrogen atoms in strong covalent bonds.Due to the strong covalent sp2 bond in the plane,it has very high mechanical properties,thermal conductivity and chemical stability.These excellent properties enable h-BN to be used in many applications,such as dielectric substrates for transistors,flexible nanoelectronics and nanocapacitors,corrosion resistance,and deep ultraviolet luminescent materials.Therefore,the preparation of high-quality h-BN films is considered to be a prerequisite for their wide application.As a bottom-up synthesis method,chemical vapor deposition(CVD)has been commonly used to fabricate atomic-scale two-dimensional materials,which has great potential for growing h-BN films.However,there are many key technical problems that need to be solved in basic research and application development such as controllable preparation and related growth mechanism.For example,the preparation of large-area,multi-layer,high-quality h-BN and the clean and lossless transfer of h-BN on metal substrates are still key problems to be solved in this field.To this end,we adopted a growth method combining CVD and spatial confinement,introduced a molecular flow growth model,and prepared high-quality h-BN on Cu foil substrate.The controllable growth of h-BN single and double layers is realized through the double confined space.The clean and lossless transfer of h-BN on Cu foil substrate was also explored.The specific research content of this paper is as follows:(1)In this work,h-BN was deposited on Cu foil substrate by one-step CVD method,and the successful preparation of h-BN domains and thin films was achieved by the spatial confinement method.According to the perspective of hydrodynamics,Cu foil is placed on top of the sapphire substrate to establish a confined space growth model based on molecular flow.Based on this research method,different experimental parameters involved in the preparation of h-BN were studied,such as Cu foil electrochemical polishing treatment conditions,precursor heating temperature,growth time,etc.By changing the growth parameters to regulate the growth of h-BN,the optimal process conditions for the growth of h-BN films by low-pressure chemical vapor deposition(LPCVD)were obtained,and the controllable preparation of monolayer h-BN domains and films was realized.We found that the edge topography of h-BN domains changes with the heating temperature of the precursor.In addition,we further use the method of combining growth and hydrogen etching to explore the formation mechanism of h-BN in complex structural morphology.(2)Based on one-step growth,we construct a sapphire sandwich double confined space,and realize the controllable synthesis of large-area bilayer h-BN by two-step CVD method.Through systematic characterization,the synthetic hBN has two layers,and the growth mechanism of double layer h-BN is explored.We found that in situ annealing before the second step can effectively promote selective growth of h-BN,and the local redox process can create preferred nucleation sites for the growth of bilayer h-BN.The results enrich the understanding of the synthesis mechanism and lay a foundation for the controlled synthesis of high-quality multilayer h-BN films.(3)On the basis of the controllable synthesis of single and bilayer h-BN films,we further study their non-destructive transfer schemes.By comparing the various transfer methods of h-BN,a water-soluble polyvinyl alcohol solution(PVA)was finally selected to assist the transfer by dry method,and the transferred h-BN was characterized,and the results showed that the high-quality h-BN after transfer could be maintained.Furthermore,by optimizing the parameters involved in the transfer process,the clean and non-destructive transfer of single and bilayer h-BN films is realized.This transfer method not only arbitrarily and efficiently transfers h-BN to the target substrate,but also avoids substrate etching and polymethyl methacrylate(PMMA)residue.H-BN continued to grow on the used copper foil,and it was tested and characterized,and it was found that the quality of the h-BN grown again was not affected at all,indicating that the metal substrate could be reused,effectively reducing the cost of the experiment.