Theoretical Studies On The Kinetics Of Graphene Growth On Ni(111) Surface And Defect Healing Mechanism

Graphene,a single layer of two-dimensional(2D)honeycomb structure material with sp2-hybridized carbon atoms,has broad prospects in many applications such as field effect transistors,integrated circuits,sensors,transparent conductive films,functional composite materials,energy storage materials and catalyst carriers,due to its extremely high carrier mobility,excellent electronic,optical,thermal,and mechanical properties.The preparation of materials is the premise and foundation to realize their functional applications.Chemical vapor deposition(CVD)as one of the most promising methods for industrialized preparation of large-area graphene with high quality,has also attracted much attention.Experimental researchers have explored the effects of different growth conditions of the CVD method on the morphology,growth rate,number of layers,and growth quality of graphene,and made many breakthrough progress.However,it is still a big challenge to realize the preparation of large-area high-quality graphene.Therefore,in order to improve the graphene growth quality by CVD method,it is very important to understand its microscopic dynamic mechanisms at the atomic level.Theoretical simulation methods can deeply explore the dynamic processes of graphene nucleation by CVD method,clarify the mechanisms of carbon ring formation and defect healing in different environments,not only improve our understanding of the graphene growth behavior,but also provide theoretical reference and basis for optimizing the growth conditions in experiments to synthesize large-area high-quality graphene.In view of this,the related work in this paper employ quantum mechanics molecular dynamics(QM/MD)simulations method to study the graphene growth on Ni(111)surface and the defect healing mechanisms,focusing on the effects of environmental factors of the substrate surface on graphene growth.We investigate the effects of precursor morphology and deposition methods of active carbon species on the graphene growth behavior and quality,study the growth behaviors of carbon rings,especially the formation mechanisms of defective carbon rings,and the dynamic healing behaviors of defects at different hydrogen concentrations during annealing process,compare the effects of different metal substrates and hydrogen concentrations on the healing barriers of Stone-Wales(SW)defects by first principles study based on DFT,clarify the cooperative healing mechanism of H atoms and Ni substrate on defective carbon rings by combining dynamics and thermodynamics,which provide theoretical reference and basis for the experimental preparation of large-area high-quality graphene.The main research contents and conclusions are as follows:The first chapter mainly discusses the relevant background and research significance of our work,briefly introduces the properties,application and preparation methods of graphene,and focuses on describing the experimental and theoretical research progress of CVD method for the graphene preparation.The second chapter briefly introduces the method of DFTB-QM/MD calculation and the first principles calculation,as well as the corresponding DFTB+and VASP calculation software packages.In the third chapter,we simulate the molecular dynamic process of graphene growth on the Ni(111)substrate surface in the presence of different precursors C16 and C24 by employing the DFTB-QM/MD calculation method.Considering that our research focus on exploring the influence of precursors of Ni(111)surface,we ignore the discussion of the decomposition of hydrocarbons,and use C2 species as carbon source to deposit directly on the substrate surface instead.We explore the growth behaviors of the carbon rings at different precursors edges,and compare the effects of the precursor morphology on the quality of graphene formation.Our Studies show that the hexagon is preferentially formed at the precursor edges compared to substrate surface,and different edge structures play a critical role in the formation of polygons:the groove-like edge composed of deposited carbon atoms and the precursor is most conducive to the formation of hexagons,AC edge is in favor of growing more hexagons than ZZ edge,the types of polygons formed at the C-C bonds edge structure depend on the surrounding deposition environment.The C24 precursor has more ZZ edges than the C16 precursor,resulting in forming graphene with higher quality,which proves that the morphology of the precursor deposited on the metal surface affect the growth quality of graphene.This work provides the theoretical framework for the future studies to explore the influence factors of high-quality graphene growth.In the fourth chapter,based on the phenomenon observed in chapter 3 that more hexagons are formed around the precursor edges,we change the C2 deposition at random locations on the surface to sequential deposition along the edge of the carbon island,exploring the graphene growth behavior by sequential depositing C2 along edges,comparing the effects of different deposition methods on the quality of graphene.Our studies show that the edge sequential deposition makes full use of the ZZ edge of precursor to directly form pentagons or hexagons in the initial stage of nucleation,which avoids the complex processes of diffusion and cross-linking of carbon chains,inhibiting the formation of other nucleation sites,and greatly improves the efficiency of carbon ring formation,as well as provides more edge sites(such as ZZ,AC and groove-like edges)for C2 deposition to promote form more hexagons.In addition,we observe common defect healing phenomena during graphene growth by edge sequential deposition,which suppresses the formation of pentagons and further increases the number of hexagons.Comparing with random deposition,sequential depositing along edges significantly improves the graphene quality,which provides a theoretical basis for understanding the experimental method of graphene growth by controlling the nucleation area.In chapter 5,we pay our attention to exploring the defect healing process of graphene.We simulate the graphene growth based on Ni(111)substrate during the annealing period with the adsorption of different concentrations of H atoms,exploring the influence of H atoms on the graphene quality and clarifying the effects of H atoms and Ni atoms on defect healing mechanism.Our studies show that the diffusion of H atoms on the graphene surface is controlled by the concentration,as the concentration increases,the diffusion becomes more intense.The diffusion of H atoms plays a dual role in the cleavage and defect healing of graphene during the annealing process.The diffusion of H atoms at lower concentration only cleaves the unstable defective carbon rings,however,high-concentration H atoms not only cleave the hexagons,but also induce the defect healing phenomenon of side-chain condensation.The strong interaction of Ni-C also can independently achieve side-chain condensation,which proves that annealing process can improve the graphene quality based on Ni substrate,and cooperate with diffusion of H atoms can further realize the reconstruction of defective carbon rings.It can be seen that the key to improving the quality of graphene is to find an optimal H/C value,which provides a theoretical reference for improving the quality of graphene growth by adjusting the concentration of H atoms.In chapter 6,we use first principles calculations to explore the effects of H atoms and metal substrates on the healing energy barrier of SW defect,and discuss the healing mechanisms induced by H atoms and metal substrates,as well as cooperation effect.Studies show that the carbon sites undergoing C-C bond-breaking and bond-making during SW defect healing process are more likely to capture the H atoms.The diffusion of single H atom breaks the symmetry of the structure during C-C bond rotation and reduces the compression of the rotation bond,lowering the healing energy barrier.As the number of adsorbed H atoms increases,the reduction of the energy barrier becomes more obvious,especially when two H atoms are adsorbed on two adjacent C atoms which both undergo bond-breaking,the healing energy barrier can be reduced to 0.78 eV.The metal substrate stabilizes the reaction intermediate through the interaction between surface atoms and C atoms to reduce the healing energy barrier.The strong Ni-C interaction cooperates with the diffusion of the double H atoms adsorbed at adjacent sites can maximize the reduction of the SW defect healing energy barrier to 0.21 eV.This thermodynamic results are consistent with the dynamic phenomena of defect healing observed in chapter 5,which proves that the cooperation effect of the H atom-Ni substrate is beneficial to the healing of the defective carbon ring.Finally,in the last chapter,we summarize the main conclusions and innovations of this dissertation,and look forward to the future research works.