Year

2014

Degree Name

Master of Science - Research

Department

Institute for Superconducting and Electronic Materials

Abstract

In this thesis, a systematic study of structural, electronic, charge transfer properties in novel two dimensional and single layered materials using first principles calculations is presented. The work aims to model newly discovered layered materials, R2O2Bi compounds (R = Y, La, and rare earth metals) and various single layered materials such as graphene, Boron Nitride (BN), MoS2 and 6,6,12-graphyne which is a new form of single layer of graphene.

R2O2Bi is a recently synthesized group of tetragonal ThCr2Si2 lattice type compounds, with unusual metallic and poor metallic behaviour. These compounds consist of a two dimensional square lattice layer formed by the rarely occurring Bi2- anions, which sandwich a (R2O2)2+ fluorite layer. Using first principles calculations, it is found that the compounds with various R are all metallic, and the anti-ferromagnetic (AFM) state is the most favourable ground state for R = Gd, Sm, and Nd compounds. The electrons from the px and py orbitals of the Bi2- ions contribute to the density of states (DOS) at the Fermi level, leading to the metallic nature of these compounds. When the systems change to ferromagnetic (FM) states, the DOS at the Fermi level is further enhanced and makes the compounds even more metallic. The calculations of R2O2Bi compounds are in agreements with the experimental data. The effects of replacing Bi2- with Te2- has also been studied. Our calculation reveals that the Te2- changes the compounds from metallic to semiconducting/insulating, with variable band gaps for various R. It is found that the Te2- lowers the energy of the p orbital electrons of Bi2-, which become fully localized.

Due to the unique physical and chemical properties of two dimensional single-layer materials, it is highly important to explore the potential applications of these materials as gas sensors. Using first principles calculations, the adsorption mechanisms for various gases on a few single-layer materials are explored in this study. Calculations were performed for a series of gas molecules (CO2, CO, H2O, NO2, NO3, NH3, H2, O2, and H2S) on supercells of graphene, and on single-layer BN and MoS2. Calculation results show that the charge transfer for H2O, CO, and NO2 is in agreement with what has been observed experimentally for graphene. It is also predicted that the sensitivity of graphene based gas sensors can be significantly improved for sensors fabricated using graphene ribbons, as more charge transfer take place for the gas bound to the edges rather than the surfaces of the graphene. Charge transfer is observed for CO, NH3, NO2, H2O, and H2S molecules absorbed on single-layer BN, with strong absorption energy for H2S. For single-layer MoS2, charge transfer takes place for H2, H2S, and CO2. Results reveal that graphene is most suited to use as a NO2 sensor, while single layer MoS2 could be more useful for detecting either H2S or H2. The types of charge transfer, band structure, DOS and partial DOS, adsorption energies, and equilibrium distances are also discussed.

6,6,12-graphyne is a carbon allotrope which features a Dirac-cone-like band structure that is the same as that of graphene. Using first principles calculations, the H2O adsorption mechanisms and their effects on 6,6,12-graphyne are explored in this study. The optimal adsorption position of H2O gas was determined by using different positions and orientations of the H2O molecule. Calculation results show that the H2O can strongly stick to the graphyne, with electrons being transferred to the graphyne. H2O adsorption at a particular adsorption site changes the system there from the Dirac state to that of a conventional metal, while the graphyene retains the Dirac state for most adsorption sites.

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