Bond and electronic relaxation dynamics of graphene, gold clusters, and water ice

• Main Author: Zhang, Xi
• Other Authors: Sun Changqing
• Format: Theses and Dissertations
• Language: English
• Published: 2013
• Subjects: DRNTU::Science::Physics::Electricity and magnetism
• Online Access: http://hdl.handle.net/10356/52459
• Institution: Nanyang Technological University

Undercoordinated atoms and nonbonding electrons exist widely in nanomaterials and in network-structural materials with their impact under-estimated. Bonds around under-coordinated sites and nonbond lone pairs follow irregular relaxation dynamic rules with the rules remaining unclear. A quantum theory was proposed to calculate the under-coordinated effects on the electronic structure of materials by incorporating bond order-length-strength (BOLS) correlation theory to tight-binding Hamiltonian (BOLS-TB), adopting mean-field Hubbard term to calculate the electron-electron interactions. Consistency between the BOLS-TB calculation and density functional theory (DFT) calculation on graphene nanoribbons (GNRs) verified that i) the physical origin of the band gap expansion lays in the enhancement of edge potentials and hopping integrals due to the shorter and stronger bonds between undercoordinated atoms; and ii) nonbond electrons at the edge of zigzag-edged GNRs and atomic vacancies accompanied with the broken bond contribute to the Dirac-Fermi polaron (DFP) with a local magnetic momentum; iii) the formation of triple bond at the edge of armchair-edged and reconstruct-edged GNRs annihilates the nonbond electron and prevent formation of DFPs. Size-dependent surface bond contraction, potential well depression, electron and energy entrapment and valence band polarization of Au nanoclusters and nanocages were also verified by DFT calculations and BOLS-TB analysis. Results of transition state calculations of the carbon monoxide oxidization confirmed that smaller clusters (Au13 and cage Au12) reduced the activation energy much more than larger clusters (Au55 and cage Au42), since the significantly red shift of the valence band of ultra-lowly coordinated clusters makes the valence 5d electrons ultra-highly catalytic. The hidden force opposing H2O compression behind the repulsion between nonbonding lone pair and bonding pair of hydrogen bond was revealed by theoretical analysis and molecular dynamics (MD) and ab initio MD calculations: i) the compression shortens and strengthens the intermolecular ‘‘O2- : H+/p’’ lone-pair and stretching phonons (<400 cm-1) are thus stiffened; ii) the repulsion pushes the bonding electron pair away from the H+/p and hence elongates and weakens the ‘‘H+/p–O2-” bond, making stretching phonons (>3000 cm-1) softened upon compression. Three springs model was proposed to build up the fundamental physical model of ‘‘O2- : H+/p–O2-” bond, in order to explain the complicated and anomalous behavior of water and ice. Current progress in bond relaxation dynamics around the under-coordinated sites and nonbond lone pairs paves a path to the mysteries of nanomaterials and net-work structural materials.