Doctor of Philosophy
Institute for Superconducting and Electronic Materials
Feng, Liya, Simulation of crystal, electronic and magnetic structures, and gas adsorption of two dimensional materials, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials, University of Wollongong, 2014. https://ro.uow.edu.au/theses/4152
In this thesis, functionalization of monolayer honeycomb lattices and their allotropes is investigated, including graphene, silicene, germanene, and MoS2, with theoretical calculations based on density functional theory (DFT) implemented by the Vienna Ab-initio Simulation Package (VASP) code.
The atomic structure of graphene is that of an sp2 bonded planar lattice. The band structure indicates that graphene is a semimetal with a Dirac cone at the K-point. Silicene and germanene have mixed sp2-sp3 buckled hexagonal structures and display similar electronic properties to those of graphene. Single-layer MoS2 features a hexagonal packed layer composed of Mo atoms sandwiched between two layers of S atoms and appears to be a nonmagnetic semiconductor.
In order to alter the metallic properties, various adatoms are introduced on the graphene surface. A single H atom on top of a carbon atom in graphene pulls the bonded C atom out of the plane and creates a magnetic moment of 1 μ B for graphene. Single F and O adatoms on graphene are adsorbed at the top site and bridge site, respectively, without any contribution of magnetic moment. The adsorption of a single F atom on graphene was found to be metallic. Single H and O adatoms on graphene transform the system into a semiconductor.
Under the inspiration of the graphene allotropes, new types of silicene allotropes, octasilicene, silicyne, and silicdiyne, are constituted and found to be buckled metallic materials. To fill unsatisfied valence shells and replace the dangling bonds with valence bonds in silicene allotropes, hydrogen atoms were introduced on silicene allotropes. Consequently, all the hydrogenated silicene allotropes appear to be semiconductors. The type of adsorption in which the dangling bonds are completely replaced by valence bonds is proved to be more stable, suggesting that the Si atom favors sp3 hybridization.
Fluorinated germanene keeps the chair configuration characteristic of pure germanene, which is the most stable configuration, but there is also a metallic structure for both types of fluorinated germanene.
The O atom was used to study the functionalization of single-layer MoS2. The top of an S atom site is the most energetically favorable position, and the oxygen atom at this site results in a direct gap.
The electronic structures of a series of isostructural compounds, M2SiO4 (M = Mn, Co, and Ni), belonging to the olivine family, are studied by first principles calculations. The results show that the ground states of all the compounds are antiferromagnetic, which is consistent with the experimental results. The effects of the various M site ions on the crystal structure, spin states, orbitals, and electronic states, for ferromagnetic and antiferromagnetic magnetic states are investigated.
Two- and three-dimensional Ising–like models for a triangular spin-chain lattice are proposed to study the magnetic properties of a newly discovered triangular compound, Sr3Co2O6, using Monte Carlo simulation. Simulations based on both of the two models show a multistep curve at low T, magnetization plateaus at M = 1/3 MO, where MO is the saturated magnetization, at intermediate T and paramagnetic behaviour at high T. The width of the plateau becomes larger as interactions increase.
Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.