Theoretical Studies On Structural Inducing Nucleation,Growth,and Exfoliation Of Graphene

As a new type of two-dimensional carbon material,graphene has excellent electrical,thermal,optical and mechanical properties,is thus widely used in many fields such as microelectronics devices,catalysis,high sensitivity sensors and battery electrodes.Among the various synthetic methods,the chemical vapor deposition(CVD)technique to prepare graphene is the most promising method for obtaining large-scale and single-crystal graphene.However,the large number of defects at the grain boundaries caused by the spontaneous nucleation will seriously reduce the electrical properties of graphene.In terms of this issue,the artificial introduction of "seeds" to induce the nucleation of graphene is beneficial to reduce the density of spontaneous nucleation,and allows graphene to grow up with better single crystal and electrical properties.On the other hand,the CVD-synthesized graphene on Cu and orther metallic substrates has to be transferred to the insulating/semi-insulating substrates for the actual characterizations and applications.Specially,the obvious interaction between graphene and metallic substrates causes a large number of defects and aggravates the difficulty of graphene transfer,which seriously affects the characterization results and limits the subsequent application of graphene.With the aid of the atomic intercalation technology,it is expected to reduce the electronic interaction between graphene and metal substrates,thus improving the exfoliative performance of graphene.However,it is difficulty to directly trace and describe the atomic processes and microscopic mechanism of graphene nucleation,growth as well as the atomic intercalations in experiments,owing to the complexity of structural evolutions and chemical reactions.In view of the limitations of experimental measurement and characterization,the theoretical simulation method is expected to provide an effective way for the basic research of material structure and reaction characteristics at the atomic scale.In this dissertation,we focus on the demands of improving the single crystal and electrical properties of graphene as the basic starting point,theoretically study the structural inducing effect of CuO and h-BN(simplified as BN)"seeds" on graphene nucleation,growth and defects transitions.The optimized mechanism of the inducing effect is further obtained by exploring the influence of H2 atmosphere and its concentration on the morphology of BN"seed".To improve the exfoliation performance of graphene,S and a series of halogen atoms(F,Cl,Br,and I)are selected as the intercalators to study the microscopic process of atomic intercalations.The formation and stability of different interfacial intercalation structures are investigated under different conditions,such as temperature,pressure and atomic concentrations.On this basis,the detailed microscopic mechanism and effects of atomic intercalation between graphene and substrate are summarized,which provides a theoretical basis for the synthesis of high-quality and single-crystal graphene with excellent electrical properties on Cu substrate.Finally,the formed new 2D materials and graphene-based heterostructures at graphene/Cu interface are discussed,and their application prospects are explored through the analysis of their electronic structures.Our main research contents and conclusions are as follows:In the first chapter,we mainly discuss the research background and topic significance of this thesis,and introduce the structure and properties,synthesis and transfer methods,and research status of graphene.In the second chapter,we briefly describe the basic principle of density functional theory and molecular dynamics,as well as the VASP and LAMMPS computing packages.In the third chapter,we mainly study the the micro-mechanism of graphene nucleation,growth and defects transitions around CuO "seed" on Cu(111)substrate,and verified the inducing effect of CuO by graphene growth experiments using a chemical vapor deposition(CVD)method.Studies have shown that the initial nucleation process of graphene is mainly dominated by active C atoms,and the adsorption energy of C and reaction barriers of C-C dimer formation could be reduced by artificially introducing CuO "seed" on a Cu(111)surface and the nucleation around CuO "seed" could thus be promoted.Besides,further calculation shows that the CuO "seed" does not decrease the formation energy barriers of defective C5 and C7 rings as compared to the perfect C6 rings.Therefore,the introduction of CuO "seed" would not significantly lead more defects at the initial stage of graphene nucleation.Further experiments determain the thickness of grown graphene around CuO is 1?2 layers,with good crystal orientation and structural integrity.In the fourth chapter,we mainly selecte the BN nanoclusters as nucleation"seed" to explore its inducing effect on graphene nucleation and influence on defects transitions.By combining theory with experiment,it is demonstrated that the edge of BN nanoclusters has good stability and its side position could act as the active sites for C adsorption and aggregation.The introduction of BN "seed"can effectively reduce the free energy and other energies during the growth of Ci clusters,and significantly improve the nucleation rate of graphene.Further studys also indicate that controlling the carbon source in a relatively low concentration is beneficial for amplifying the structural inducing effect of BN "seed".Besides,the BN "seed" can also significantly reduce the reaction barrier of C5 defects,which thus can be repaired during nucleation and growth processes and is conducive to improve the structural integrity of graphene.Relevant experiments show that the thickness of grown graphene around BN "seed" is 1 layer,also with good crystal orientation and structural integrity.In the fifth chapter,we continue to study the process of etching triangular BN"seed" with H2,along with the influence of BN "seed" morphology on the nucleation and growth of graphene on Cu substrates.Studies have shown that the sharp corners of normal triangular BN "seed" hinder the continuous C adsorption and aggregation around BN,and adversely affect the structural quality of induced graphene.Under high temperature and H2 atmosphere,the sharp corners of BN are easily etched,and the edge atoms are etched alternately in the order of N?B.As H concentration increases,the energy barrier of the rate-limiting step(N/B-Hn*?N/BHn(n=1,2,3))of the etching reaction decreases gradually.Since the atoms at the side edge are inactive than sharp corners,the most easily formed triangular BN "seed" tends to be transformed into a hexagonal morphology after H2 pre-etching treatment.The graphene nucleation rates are up to ten times higher than those before etching and 105 times higher than those on the pure Cu substrate in a wide range of carbon source concentration(corresponding to ??).Therefore,theoretical predictions show that the H2 pre-etching can optimize the edge structure and morphology of the BN "seed",which is more conducive to exerting its structure-inducing effect and realizing the rapid growth of grapheneStarting from chapter 6,our research focus on exploring the atomic intercalation methods and microscopic mechanisms for effective graphene exfoliation.In this chapter,we study the interaction form and microscopic intercalation processes of S atoms at different graphene/Cu interfaces,and explore the formation tendency and effects of interfacial sulfides under different conditions(temperature,pressure and S concentration).The results show that there is an obvious electronic interaction between the CVD grown graphene and Cu substrate,the electrons of Cu substrate could spontaneously transfer to graphene,making the graphene exhibits n-type Dirac characteristics.Specially,the electronic interaction at the graphene boundaries are even more serious.In view of the different edge structures of graphene,we propose two different S intercalating mechanisms,direct S intercalation and Hydrogen(H-)assisted S intercalation,occuring at the zigzag and armchair edges of graphene,respectively.Which clarified the difference of S/H atom capture and the energy barrier curves of S intercalation at different edges of graphene.The intercalated S atoms would reconstruct with the surface Cu atoms to form a series of 2D copper sulfides such as Cu2S,CuS,and CuS2 depending on the different experimental conditions.These sulfides have regular atomic arrangements and excellent stability,can significantly hinder the electron transfer between graphene and substrates,and reduce the exfoliation energy of the graphene from the Cu substrate,thereby promoting the effective exfoliation of graphene.In chapter 7,we study the intercalation mechanism of serial halogen atoms(F,Cl,Br and I)at graphene/Cu interface,and explore the formation tendency and stability of the interfacial halides under different conditions(temperature,pressure and intercalation concentration).On this basis,we further investigate the multifunctional features and potential applications of halogen atomic intercalations.Studies have shown that all halogen atoms have no better adsorption energies and structrural advantages than H atoms at the edge of graphene/Cu interface,the H-assisted operation is thus required for halogen atomic intercalations.In addition,as the increase of halogen atomic number(from F?I),the energy barrier of atomic intercalation increases gradually,while the corresponding exfoliation performance of graphene increases significantly.Especially for Cl and Br,the strippable 2D graphene/CuCl-3 and graphene/CuBr-3 heterojunctions and CuCl-4 and CuBr-1 materials can be synthesized at the interface by adjusting the intercalating concentrations.Both of these two heterostructures have a typical ?-type arrangement and good stability,the band edge locations(CMB and VBM)of CuCl-4 and CuBr-1 materials coincide on the potentials for photocatalytic water splitting and CO2 reduction.The potential advantages of CuCl-4 in photocatalytic applications are more obvious due to its more suitable light absorption wavelength and good stability.Finally,in chapter 8,we summarize the main conclusions and the innovation of this dissertation.The future research work is also prospected.