Study of structural changes in two-dimensional materials based on cohesive energy analysis

Abstract

Due to the weak interlayer binding from vdW-interaction, layered structure could be formed with various physical properties depending on number of layer, stacking, and the kinds of stacked monolayer, which have attracted many researchers in academic and engineering fields. Reflecting this increasing interest in 2d materials, I considered two subjects constituting my thesis with a tremendous help of calculation using density functional theory. In first part, I analyzed the structure transformation near GB in polycrystalline graphene and obtaining energy curves corresponding structure transformation processed for quantitative understanding using DFT calculation. I categorized two main structure transformation inducing most of structure transformation observed in TEM experiments. first one is SW-transformation well known in pristine graphene, however the value of energy barrier is reduced than that in pristine graphene, which means the easier occurrence near grain boundary. the other one is evaporation of carbon dimer reducing two carbon atom in the graphene sheet. From the value of energy barrier obtained by DFT calculation, this structure transformation is also much easier to occur near grain boundary than similar structure transformation in pristine graphene with perfect hexagonal structure. From the observation of relaxed structure using DFT calculation, I understood that the energy barrier decrease due to atomic structure reconstruction only possible near grain boundary and the role of TEM imaging in structure transformation of grain boundary in graphene. in second part, I studied on Al(OH)3 with layered structure. Until now the vdW-interaction is usual interlayer binding mechanism in various materials with layered structure. Starting from scrutinizing two bulk phase of Al(OH)3 with layered structure, I tried to understand its monolayer structure and their binding with hydrogen bonding as interlayer binding mechanism. Using DFT calculation, I quantitatively find out interlayer binding by hydrogen bonds is weaker interaction comparing to the strong ionic bonding existed in single layer structure. So, energetically, I could consider Al(OH)3 as a layered material, However, it shows stronger binding than the layered structure bound by vdW-interaction. In band structural aspect, I also find out the surface state observed in conduction band bottom region in case of single layer or slab structure (finite number of layers), which is not found in the result for bulk structure. This surface state become a main reason of decreasing band gap of 2d structure necessarily having surface. Finally, I suggest Alkali-halide intercalation as a tailoring method for layered Al(OH)3 not only in crystal structure, but also in electronic band structure. Interlayer distance, the most important element in application of layered material, is affected by kinds of Alkali atom and halogen atom at the same time. Electronic band structure, especially in valence band region, also could be engineered by Alkali-halide intercalation, easily understood by hybridization among p-orbitals of halogen and oxygen atoms.