Structures And Motion Of Grain Boundaries In Two-dimensional Hexagonal Boron Nitride

Two-dimensional(2D)materials,such as graphene,hexagonal boron nitride(h-BN),and transition-metal dichalcogenides(TMDs),have attracted a great deal of interest because of their remarkable and technologically useful properties.However,large-area 2D materials are typically polycrystalline with differently oriented grains.Grain boundaries(GBs)is an inevitable structural imperfection in polycrystalline two-dimensional(2D)materials,behave as one-dimensional line defects that connect differently oriented grains.Extensive studies have confirmed that the inherent structures of GBs(for example,misorientation angles and interior atomic arrangements)determine the mechanical,thermal,and electrical properties of 2D materials.Thus,characterizing and tailoring GBs at atomic scale is one of the utmost goals for advancing fundamental research and industrial applications of 2D materials.In this thesis,we proposed a novel bicrystal method to build up interfaces in 2D materials,which enabled us to experimentally achieve the quasi-full-parameter-space GBs in 2D h-BN.We then systematically studied the atomic structures of GBs in 2D h-BN,unravel the long-term controversies on the registry of GBs.We also revealed several new GB configurations.Furthermore,we explored the atomic mechanism of GB motion in 2D materials and build up its frameworks.Finally,we characterized the structures of GBs in 2D h-BN films grown by chemical vapor deposition(CVD,the most practical method for preparing 2D materials),and reveal their formation mechanism.1.Via modifying the long-standing bicrystal strategy,we proposed a novel bicrystal nanowelding approach to build up interfaces between two dimensional materials with atomic precision.This method enabled us,for the first time,to experimentally achieve the quasi-fullparameter-space grain boundaries(GBs)in 2D hexagonal boron nitride(h-BN).It further helps us unravel the long-term controversy and confusion on the registry of GBs in h-BN,including(1)Discriminate the relative contribution of the strain and chemical energy on the registry of GBs semi-quantitatively,and resolve the dislocation cores at different GB parameters for the first time;(2)Identify a new dislocation core-Frank partial dislocation;(3)Identify four new antiphase boundaries;and(4)Confirm the universal GB faceting and reveal the mechanism of GB faceting.2.Via abbreviation-corrected high-resolution TEM(HRTEM),we revealed,for the first time,the atomic-scale mechanism of GB motion in grains with fixed ends(polycrystal).(1)We demonstrated that shear-coupled motion and GB sliding are both necessary for sustaining the motion of symmetric GBs.(2)We propose a new mechanism of the motion in symmetric GBs with two sets of dislocations.(3)For the motion of asymmetric GBs,we found that GB faceting-defaceting process and dislocation reaction are confirmed to play important roles.(4)Grain rotation during grain shrinking was found to be highly dependent on the misorientation angles between two grains and the coupling mode of facets that dominate in grain shrinking.(5)We discussed the characteristics of GB motion in a binary lattice,and revealed that combination and dissociation of partial dislocation participate in the motion of 38°<|?|<60° GBs;(6)We also studied the motion of inversion domain boundaries and grain growth in inversion domains.3.We build a full picture of GB structures in CVD-grown monolayer h-BN by conducting dark-field imaging analysis at the grain size scale and aberration-corrected HRTEM imaging to get the atomic structures.Our results show that overlapped GBs(OLGBs)are found to be dominant in CVD-grown h-BN based on statistical analysis.We also explored the structure of GBs formed between two rotationally aligned grains,which shows a folded structure feature instead of the perfect connection between two grains.To obtain a deep understanding of the formation mechanism of OLGBs in h-BN,we carried out density functional theory(DFT)calculations and revealed the energetic competition between covalently bonded GB(CBGBs)and OLGBs responsible for the formation of GBs,and the role of hydrogen during the formation of GBs.Finally,I summarized the thesis on the characterization of GBs in h-BN and the study on the mechanism of GB motion.Furthermore,I also analyzed the weakness of current work and prospected the scientific problems for further research.