Van der Waals density functional studies of hydrogenated and lithiated bilayer graphene

In this thesis, we use first principles density functional theory (DFT) to study the energetics, structural and electronic properties of hydrogenated and lithiated bilayer graphene material systems. The newly developed four variants of the non-local van der Waals (vdW) exchange-correlation functionals (vdW-DF, vdW-DF2, vdW-DF C09x and vdW-DF2 C09 x) are employed to explore all the possible configurations of hydrogen adsorption at 50% and 100% coverage on a 1x1 unit cell. The results obtained are also compared with the GGA PBE functional.;For 50% hydrogen coverage, 16 unique configurations are identified in the unrelaxed state. Formation energy analysis reveals six possible energetically favourable configurations with three low-energy competing configurations. It is found that the properties of hydrogenated bilayer graphene greatly depend on the hydrogen configuration. For instance, the formation of a hydrogen dimer within the layers decouples the structure, whereas the dimer formation outside surfaces does not have a significant in uence on the van der Waals forces; thus the bilayers remain coupled. In this coupled configuration, the vdW-DF C09x functional predicts the lowest formation energy and shortest interlayer separation, whereas the GGA PBE functional gives the highest formation energy and largest interlayer distance. The reasons behind the variation of these functionals are discussed. Two of the three low-energy competing configurations exhibit semimetallic behaviour, whereas the remaining configuration is a wide band gap material. The wide band gap structure is found to undergo a hydrogen-induced spontaneous phase transformation from hexagonal to tetrahedral (diamond-like) geometry. We conclude that this wide band gap configuration represents a viable template for synthesizing nanodiamonds from graphene by hydrogenation. At 100% coverage, ten unique hydrogen configurations are identified from a 1x1 unit cell. All exchange-correlation functionals predict nine of the structures to have negative formation energies. From these nine structures, three low-energy competing structures are noted and found to be wide band gap semiconductors, whereas the other configurations exhibit either a semimetallic or metallic character. Although a 1x1 unit-cell is able to present a clear picture for the interaction between hydrogen and graphene, our results reveal that it limits the occurrence of other interesting physics. The cell size was increased to 2x1, to identify other low-energy configurations that are not possible in 1x1 cell. The identified configurations have shown physically interesting hydrogen arrangements such as chair-like, zigzag-like and boat-like configurations. Furthermore, our results reveal that hydrogenation reduces the elastic properties of the pristine structures.;We further perform a systematic investigation of the effects of lithium (Li) on AA and AB stacking sequences of bilayer graphene. Two Li atoms are considered to examine the effects of the Li-Li interaction on bilayer graphene, and a total of 12 unique configurations for AB and 9 for AA stackings are identified. The vdW-DF consistently predicts the highest formation energies, whereas vdW-DF2 C09x gives the lowest. Unlike in the case of the pristine structures, it is noted that for lithiated bilayer graphene, GGA PBE gives comparable results to the other functionals. One of the Li intercalated configurations undergoes a spontaneous translation from the AB to AA stacking, and is found to be the most energetically stable configuration. We therefore conclude that Li favours the AA stacking, and that configuration represents a feasible template for experimentally synthesizing and characterizing a Li-based anode material. We noticed that all identified Li configurations exhibit metallic behaviour. Lastly, we found that the intercalated Li dimer weakly interacts with the graphene layers, whereas the intercalated isolated Li atom exhibits strong interaction.