In-situ Surface Studies In Epitaxial Growth Of Two-Dimensional Atomic Crystals On Metal Surfaces Modulated By Subsurface Species

Controlled epitaxial growth of two dimensional(2D)materials is an important subject in basic researches and applications of 2D atomic crystals.Many efforts have been made for the controllable growth of 2D atomic crystals by changing growth precursors,carrier gases,substrate surface structures,and growth temperature.Instead,we propose a way to control epitaxial growth of 2D atomic crystals via introduction of subsurface species underneath substrates.In this thesis,we combine in-situ imaging techniques(Photoemmision Electron Microscopy(PEEM)/Low Energy Electron Microscopy(LEEM)),in-situ photoelectron spectroscopic technique(X-ray Photoelectron Spectroscopy(XPS))and density functional theory(DFT)to study the modulation of 2D atomic crystals growth by subsurface species.The main results are as follows.(1)Modulation of growth dynamics and morphology of 2D atomic crystals by subsurface species.We have in-situ studied the morphology change of h-BN growth on the clean Ru(0001)surface and the Ru(0001)with subsurface Ar atoms at different temperatures,respectively.The abnormal "compact to fractal" morphological transition have been observed on the two surfaces.We confirm that the growth processes follow reaction-limited-aggregation(RLA)mechanism on both surfaces.Furthermore,decreases in reaction and diffusion barriers are induced by subsurface Ar atoms,which enhance the morphological transition.Compared with the epitaxial growth of graphene on both surfaces,we predict that the occurrence of this transition is determined by the strength of "shielding effect".This study helps to understand the growth dynamics of 2D materials and enables us to control the morphology of grown 2D structures.(2)Modulation of the epitaxial structure of 2D atomic crystals by subsurface species.h-BN overlayers are synthesized by ammoniation on the Ni(111)surface containing subsurface B species.It has been found that subsurface B species weaken the interaction between h-BN and Ni(111)surface and further produce the non-epitaxial h-BN structures.The ratio between non-epitaxial h-BN and epitaxial h-BN structures increases with the content of subsurface B species.The "non-epitaxial to epitaxial"structural transition by annealing has been observed,which suggests the epitaxial h-BN structures are energetically preferred.This work proposes a new way to synthesize h-BN by ammonization based on the reaction of surface segregated B species with NH3 and provides a new method to modulate the epitaxial features of 2D atomic crystals.(3)Construction of the 2D atomic crystal heterostructures based on segregation of subsurface C species.Stacked or in-plane h-BN/graphene heterostructures have been obtained with combination of CVD growth of h-BN graphene and growth via subsurface C species segregation on Ni foils.We have found that the decoupling effect of graphene interlayer on h-BN/Ni interfaces,as well as the restriction effect of h-BN island edges on segregated graphene under h-BN.(4)The interaction of 2D atomic crystals with intercalated species under 2D atomic crystal surfaces.The etching processes of graphene on Pt(111)in H2 and O2 have been conducted,respectively.In 10-5 Torr H2,significant edge-etching and interior-etching of graphene islands are both observed above 1023 K,with a determined reaction barrier at 5.7 eV.The similar etching phenomena are found in 10-7 Torr O2 above 973 K,while only edge-etching occurs between 823 and 923 K.We confirm that etching of graphene edges is facilitated by Pt-aided hydrogenation or oxidation of edge carbon atoms,while interior-etching is attributed to the intercalation-reaction at high temperatures.