Metal Atoms Interaction With Graphene Under Electron Beam Irradiation

Graphene,as an ultra-thin two-dimensional atomic crystal material formed by sp2 hybrid carbon atoms,with its unique physical and chemical properties are promising for application in electrochemical energy storage,flexible electronic devices,optical communication,and future semiconductor devices.The subtle structural changes are known to have profound influences on the physical and chemical properties of graphene.The development of new methods which is expected to control the growth and engineer the nanostructure of graphene,which is used for expanding the application for graphene.In order to achieve such control,it is of key importance to understand the intrinsic relationship of graphene structure-performance.In-situ electron microscopy has become an indispensable tool for the study the graphene's structure-properties relationship at the atomic level.The electron beam can not only be used to record the dynamic structural evolution of materials but also to drive the fabrication of nanomaterials,which also provide the most evidence for revealing the structure-performance relationship.In this thesis,the interaction between metal atoms and graphene under electron beam irradiation have been investigated in a transmission electron microscopy.The dynamic process and related mechanisms of how metal atoms catalytic growth graphene,bridging to construct hetero sp2 hybrid nanostructures,and inducing the structure evolution at atomic scale have also been investigated.The main contents are as follows:1.The formation and stability of Cr atoms doping defects has been investigated.The Cr atoms decorated on graphene was prepared by subliming and decomposing the Cr(acac)3.We directly observe two types of Cr atoms doping defects,which formed by single Cr atom embedded in graphene mono-and di-vacancies in a TEM.The experimental and First-principle calculations suggest that migration in graphene vacancies is difficult for Cr atoms under electron beam irradiation at 80 kV,which is consistent with calculated large binding energy for the site exchange between Cr and C atoms.2.Single Cr atoms catalytic growth of Graphene has been investigated.We examine Cr atoms migrates at graphene edge while under electron beam irradiation in a TEM with an acceleration voltage of 80 kV.The captured migrations show new hexagonal structure formed at the graphene edge,leaving a zig-zag edge termination.Another example shows Cr atoms catalytically growing graphene to heal the hole using carbon atoms from the carbon nanotube.Molecular Dynamics simulation show that Cr is relative stable but still capable of diffusing,and hence more efficient as a nucleation catalyst.The work augments Cr is a highly efficient catalyst for sp2 carbon growth.3.Cr atoms as molecular anchor and drive transformation of sp2 nanostructure has been investigated.We show how Cr atoms can be used as molecular anchor to connect carbon nanotubes and fullerenes to graphene,and fullerenes attached to graphene edge through bridging Cr atoms.Crucially,while under electron irradiation,the Cr atoms can drive transformations such as catalytic healing of a hole in graphene with simultaneous transformation of a single wall carbon nanotube into a fullerene.The atomic resolution of the electron microscopy along with density functional theory based total energy calculations provide insight into the dynamic transformations of Cr atom linkers.The work augments the potential of transmission electron microscopes as nano laboratories.4.In situ electron driven Au nanocrystal transformation to form a two dimensional single-layer thickness Au nanoribbon has been investigated.The formed Au nanoribbon shows a hexagonal close-packed structure with an average lattice constant of 2.71±0.01 A.Image simulations and experimental images further confirm that Au nanorbbon is a single atom thick freestanding gold structure.The 2D Au nanoribbons exhibit significant stability under electron irradiation(at 80 kV)over 5 minutes,this because the threshold value of the knock-on energy for Au atoms are higher than 80 keV electron beam.Meanwhile,Au atoms at the Au-Graphene interface can form very stable bonding configurations which have the largest bonding energy.