First-principles Studies On Design And Electronic Properties Of Novel Two-dimensional Materials

Since graphene was first isolated in 2004 by Geim and Novoselov using mechanical exfoliation, it has become the focus of condensed matter physics and material science. Due to the fascinating properties and extensive applications of graphene, there emerges an enthusiastic goal of investigating the other two-dimensional(2D) materials. Recently, with the development in computer technology and computational methods, ab initio calculations based on density functional theory have been widely applied in designing and electronic properties investigation on low dimensional materials. In this thesis, the embedment of macrocyclic molecules in graphene and its effect to graphene’s electronic properties were investigated firstly using density functional theory. Then we proposed a new potential 2D material conceived directly from the existing layered three-dimensional(3D) crystal. Based on this new 2D material, we constructed new 2D BN and graphene allotropes. At last, the band gap engineering of one 2D carbon allotrope by hydrogenation and halogenation was studied. The results are summarized as below:1. Tuning electronic structure and property through chemical modifications has become the focus of recent research on graphene. Here, using crown ether as an example, we demonstrate by first principles calculations that macrocyclic molecules match well with the graphene geometry and can be well embedded on it. The embedment of crown ether will not only change the electronic properties of graphene, but will also give graphene the ability to selectively bind metal atoms. The band gaps and band edges positions of crowned graphene vary with the density of the embedded crown ether on graphene and eventually approach the Dirac point at the dilute limit. The highest possible number of crown ether embedded on graphene leads to a new 2D materials: graphitic C2 O. It has a gap of 3.502 e V. Therefore, this is a new way to obtain a family of 2D materials ranging from wide-gap to low-gap semiconductors. Furthermore, our calculations show that the crown ether embedded on graphene can effectively bind metal atoms. This may provide a new method to modify the properties of graphene such as introduction of strong spin-orbit coupling and making graphene superconducting.2. The 2D semiconductor materials and the related device fabrication have become a new focus of electronics and materials science recently. Comparing with 3D semiconductor, the choice of the 2D materials is very limited. Here, we propose a single layer B2S3 as a new potential 2D material conceived directly from its existing layered 3D crystal. Using an advanced hybrid functional method, we demonstrated that 2D B2S3 has a gap of 3.750 e V, filling a missing energy range for 2D materials. Furthermore, by adding extra B atoms at the ‘vacancy’ sites of B2S3 structure toward 1:1 stoichiometry, we constructed new 2D BN and graphene allotropes that show large variation in the electronic structure. The BN allotrope exhibits a gap that is 0.987 e V lower than h-BN. Although the structure is significantly different to graphene, the new carbon allotrope contains a Dirac cone.3. Graphenylene, a new form of 2D carbon allotrope consisting of non-delocalized sp2-carbon atoms, is composed of hexatomic and tetratomic rings with porous structures and possesses the same point group, D6 h, as graphene. Recently, it has aroused considerable interest due to its thermodynamic stability and porous structure. Density functional theory is used to investigate the hydrogenation and halogenation of graphenylene. Hydrogen and fluorine atoms preferentially bind to graphenylene to form sp3 hybridized bonds at all the concentrations considered, the adsorption of chlorine and bromine is favorable only at lower concentrations, while iodine atoms are unstable at any concentrations. The change trend of the calculated binding energies indicated the clustering of hydrogen and halogen atoms on graphenylene is not preferable, which makes band gap tunable by hydrogenation and halogenation. The electronic structures of functionalized graphenylenes show that by controlling the concentration of adsorbate atoms, the band gap of graphenylene could be tuned in a wide range, from 0.0750 to 4.980 e V by hydrogenation and 0.0241 e V to 4.870 e V by halogenation.