| In this thesis, metal adosprion and growth on graphene, quantum size effects on the growth morphology and properties of Mg adosprion on ultrathin Pb(111) films, and structures and properties of glycine and water molecular complexes and water clusters have been investigated by the quantum chemical methods ab initio and density functional theory (DFT). The main results and conclusions are summarized as follows:(1) Adsorption of rare earth (RE) adatoms (Nd, Gd, Eu, and Yb) on graphene was studied by first-principles calculations based on the density functional theory (DFT). The calculations show that the hollow site of graphene is the energetically favorable adsorption site for all the RE adatoms studied. The adsorption energies and diffusion barriers of Nd and Gd on graphene are found to be larger than those of Eu and Yb. Comparison with STM experiments for Gd and Eu epitaxially grown on graphene confirms these calculated adsorption energy and diffusion barrier differences, since fractal-like islands are observed for Gd and flat topped crystalline islands for Eu. The formation of flat Eu islands on graphene can be attributed to its low diffusion barrier and relatively larger ratio of adsorption energy to its bulk cohesive energy. The interactions between the Nd and Gd adatoms and graphene cause noticeable in-plane lattice distortions in the graphene layer. Adsorption of the RE adatoms on graphene also induces significant electronic dipole and magnetic moments.(2) Adsorption of the alkali, group III, and 3d-transition metal adatoms (Na, K, Al, In, V, Fe, Co, and Ni) on graphene was studied systematically by first-principles calculations. Bonding character and electron transfer between the metal adatoms and graphene were analyzed using the recently developed quasi-atomic minimal basis set orbitals (QUAMBOs) approach. The calculations showed that the interaction between alkali metal adatoms and graphene is ionic and has minimal effects on the lattice and electronic states of the graphene layer, agree with previous calculations. For group III metal adatoms adsorptions, mixed covalent and ionic bonding is demonstrated. In comparison, 3d-transition metal adsorption on graphene exhibits strong covalent bonding with graphene. The majority of the contributions to the covalent bonds are from strong hybridization between the dx2-y2 and dyz orbitals of the 3d-transition metal adatoms and pz orbitals of the carbon atoms. The strong covalent bonds cause large in-plane lattice distortions in graphene layer. Charge redistributions upon adsorptions also induce significant electric dipole moments and affect the magnetic moments.(3) We present a systematic study of metal adatoms adsorption on graphene by ab initio calculations. The calculations cover alkali metals, sp-simple metals, 3d-transition metals, noble metals, as well as rare earth metals. The correlation between the adatom adsorption properties and the growth morphology of the metals on graphene is also investigated. We show that the growth morphology is related to the ratio of the adsorption energy to the bulk cohesive energy (Ea/Ec) of the metals and the diffusion barrier (?E) of the metal adatom on graphene. The growth morphology also can be affected by long rang interactions such as dipole-dipole interaction and elastic interaction induced by metal adsorption on graphene. We also show that most of the metal nanostructures on graphene are thermally stable against coarsening.(4) The nucleation and growth of Fe on graphene is highly unusual that a constantly increasing island density indicates the presence of strong adatom predominantly repulsive interactions. We study these interactions by first-principles calculations to identify their origin. We find that the interactions consist of a short-range attraction and longer-range repulsions. Although electric dipole-dipole interaction can contribute partially to the repulsive force between Fe adatoms, another contribution to the repulsive energy originates from the elastic interaction mediated through graphene due to the lattice distortions induced by adatom adsorption.(5) We present systematic studies of manganese adatoms on graphene by first-principles calculations. The adsorption process is characterized by chemisorption at adsorption height of 2.1 ? and physisorption at adsorption height of 3.9 ?. We demonstrate that the chemisorptions barrier of manganese is about 0.032 eV and the desorption barrier of manganese is about 0.154 eV. The initial growth mode of manganese adatoms on graphene has also been studied. The transfer from two-Dimensional (2D) growth to three-Dimensional (3D) growth occurs at Mn4 on graphene. We have also investigated the magnetic properties of manganese adatoms on graphene. The results from our calculations show that the magnetisms of manganese clusters on graphene with smaller cluster size are ferromagnetic, different from their pure clusters or bulk. On the other hand, manganese clusters on graphene exhibit anti-ferromagnetic along with the cluster size arising since the manganese adatoms grown by 3D mode, similar to the bulk case.(6) We have carried out first-principles calculations of Mg/Pb(111) surface to study the oscillatory quantum size effect (QSE) exhibit in the adsorption energy, dipole moment, work function, and growth morphology with the thickness dependence of the energies of confined electrons. We note that the work function by adsorption of Mg atom exhibit bilayer-oscillation behavior due to quantum size effect, and play a key role in growth morphology. We find that the larger work function of the layer, the larger island density of Mg atom is, since the difference of work function between neighbored layers leads to movement of Mgδ+ after charge transfer from Mg atom to Pb(111) surface.(7) Ab initio calculations have been performed to study the structural trend, energetic stability, and fragmentation behavior of water clusters (H2O)2-30. We show that as size increasing, the cluster structures evolve from a mono-ring motif to multi-ring and ring-stacking motifs, and then to stuffed cage structures. We also show that losing water molecules one-by-one is the most favorable fragmentation channel with dissociation energy of 11.53 kcal/mol. Small water clusters such as water dimer, trimer, and tetramer, can also be observed in the fragmentation products since they often appear in the second best fragmentation channel.(8) The structures and interaction energies of stacking ring water clusters (H2O)8,10,12,15,16,18,20,24 were studied by ab initio calculations at the MP2/6-311++G(d,p) level of theory. We show that the orientation of the hydrogen bonds in each layer of the most stable stacking monocyclic ring clusters exhibit an alternative clockwise-anticlockwise pattern. Bonding and distortions of the water molecules in the water clusters were also analyzed. We also show that there is a strong bonding anisotropy in the stacking monocyclic ring water cluster isomers. The strength of the in-plane H-bonds is about 30-40% stronger than out of plane ones.(9) Structures and relative energies of neutral and zwitterionic Gly(H2O)n (n= 1?6) complexes are studied by ab initio calculations. The calculations at CCSD(T)/6-311++G(d,p) show that glycine are fully solvated by five discrete water molecules if polarizable continuum model (PCM) is used to model the rest of the solvent effects. However, even six discrete water molecules are not enough to solvate a glycine molecule if the PCM is not included. The structure of neutral Gly(H2O)n (n= 1-6) complexes changes from a ring motif (Gly(H2O)1,2) to a chain motif (Gly(H2O)3,4,5), and then to a cubic motif (Gly(H2O)6) as the number of discrete water molecules increases. However, for zwitterionic Gly(H2O)n (n =1-6) complexes, our ab initio results show that Gly(H2O) is not in a energy minimum at the MP2 level. The zwitterionic structures are mainly chain-like for Gly(H2O)2,3,4 and cubic-like for (H2O)5,6 with the cubic motif occurs earlier as compared to neutral Gly(H2O)n (n =1?6) complexes.
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