Theoretical Study On The Structures And Properties Of Several Two-Dimensional Nanomaterials

Materials have always been important to human society, as shown by the fact that our prehistoric eras are named after the new material that defined them, e.g. the Stone Age, the Bronze Age, and the Iron Age. As one kind of new materials, two-dimensional (2D) nanomaterials have already exhibited unique mechanical, optical, magnetic properties. With regard to the well-developed density functional theory (DFT) and simulation method, we used DFT to research the geometric, electronic and optical properties of the2D materials, and the colossal oxygen ionic conductivity at room temperature.In chapter one, we review the developments and potentials of several2D nanomaterials, including the energy crisis and energy materials, photocatalysis technology and photovoltaic materials.In chapter two, we introduce the development of the quantum chemistry as well as the basic concept of DFT, transition state search theory and Monte Carlo method. Finally, some software packages employed in our studies are described in detail.In chapter three, we investigate the unzipping mechanism of carbon nanotubes (CNTs) into narrow graphene nanoribbons (GNRs) upon oxidation. By treating possible adsorptive structures, we found that oxygen atoms prefer to form epoxy chains on the nanotubes, and direct formation of epoxy pairs is not favorable in energy. Upon further oxidation, epoxy pair tears CNT up from an edge position with an energy barrier of0.74eV, and the following steps of unzipping CNT become much easier because of the stress induced by the carbonyl pair, with a strikingly reduced energy barrier of only0.12eV (84%lower). Our results suggest that high-quality narrow GNRs may be produced more easily by direct oxidation of CNTs.In chapter four, we research the origins of distinctly different behaviors of Pd and Pt contacts on the graphene surface. In experiment, it was found that Pd could grow on graphene, while Pt cannot form a suitable coating on it. First-principles calculations showed that the electrons on the graphene transferring to the dxz+dyz orbitals are largely compensated by the electrons from the Pd dz2orbitals. This mechanism causes more interaction states and transmission channels between the Pd and graphene, but such a mechanism is not observed for the Pt layer.. Similar mechanisms were found when Pd grows on N-hybrid porous graphene and C3N4substrates.In chapter five, we study the2D silicon nanosheet (Si-NS) stabilized by (1) phenyl (-Ph) and (2) hexyl groups (-CnH2n+1). Geometric analysis shows that the most stable structure of the organic-modified Si-NS corresponds to the one in which the groups are regularly attached to both sides of the sheet. The electrostatic repulsion effect of the hexyl groups could be an important reason for the favorable structure. For alkyl-modified Si-NS, the electronic structure gives a direct band gap that is not sensitive to the length of the alkyl groups but sensitive to the strain effect which can be used to tune the gap continuously. Finally, both the first-principles molecular dynamics (FPMD) and the force-field molecular dynamics (FFMD) simulations show that the most stable structure of the organic-modified Si-NS could maintain its geometric configuration up to1000K.In chapter six, we examine the strain effect on the colossal oxygen ionic conductivity in selected sandwich structures of zirconia electrolytes. For KTaO3/YSZ/KTaO3sandwich structure with9.7%lattice mismatch, transition state calculations indicate that the strain effect changes the oxygen migration pathways from straight line into zigzag form and reduces the energy barrier by0.2eV. On the basis of our computed results, a possible oxygen ion diffusion highway is suggested. By First-principles molecular dynamics (FPMD) simulations, an activation barrier of0.33eV is obtained, corresponding to an oxygen ionic conductivity which is6.4×107times higher than that of the unstrained bulk zirconia at500K. A nearly linear relationship is identified between the energy barrier and the lattice mismatch in the sandwich structures.In chapter seven, state-of-the-art hybrid density functional theory investigations are presented to show that B and N hybrid porous graphene (BN-PG) outperforms the pristine PG with significantly enhanced visible-light absorption. Compared with PG (energy gap:3.2eV), the calculated energy gap of BN-PG shrinks to2.7eV and the optical absorption peak remarkably shifts toward the visible light region. The redox potentials of water splitting are well positioned in the middle of the band gap. Hybridizations among B_p, N_p and C_p orbitals are responsible for these findings. Valence and conduction band calculations indicate that the electrons and holes can be effectively separated, reducing charge recombination and improving the photoconversion efficiency. Moreover, the band gap and optical property of BN-PG can be further fine engineered by external strain.