금속원자흡착 또는 당김으로 인한 그라핀의 전기적 성질변화
- 금속원자흡착 또는 당김으로 인한 그라핀의 전기적 성질변화
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- Graphene is a one-atom-thick 2D structural material. Since the first report of successful separation of single layer graphene by mechanical exfoliation from graphite by Novoselov and Grim’s group in 2004, many theoretical and experimental works about graphenes have been reported. And graphene has been the focus of the research in condensed matter and materials science fields. Graphene has very high mobility (200,000 cm2v-1s-1), and is expected to replace silicon-based electric materials. To realize such expectation, it is very important to understand the metal-doping effect and mechanical deformation effect on the properties of graphene. In this thesis, I studied the electronic properties of graphene modified by metal-doping or strain or mechanical sliding with the use of pseudopotential density functional method.
First, I investigated the electronic properties of epitaxial graphenes with Na adsorption or intercalation. It is found that the charge transfer and the Na binding energy show strong coverage dependence. Calculated energetics shows that Na prefers the intercalation between the buffer and top graphene layers to the adsorption on the top graphene layer. The buffer layer is inert to Na adsorption on top graphene layer but it is charged when Na atoms are intercalated. And we extended the research on metal-doping effects to graphene nanoribbon structures. Strong site dependence is observed in metal adsorption on graphene nanoribbons (GNRs), and the adsorbed metal atoms are found to spontaneously form atomic chains in a particular form of GNRs. Such doped GNRs exhibit intriguing magnetic properties such as spin compensation as metal atoms switch from one edge to another at alternating gate voltages. The metal atoms can thus be used as reagents that can identify the edge atomic structures of GNRs and also as gate-driven spin valves that control the spin current in GNRs.
Next I studied about the electronic properties of graphene under strain. The semimetallic nature is shown to persist upon 30% except a very narrow strain range where a tiny energy gap opens, and the group velocities under uniaxial strain exhibit a strong anisotropy. The work function is also predicted to increase substantially as both the uniaxial and isotropic strain increases. Utilizing the work function change in strained graphene, I studied the asymmetrically strained bilayer graphene. If different homogeneous strains are applied to the two layers of bilayer graphene, transverse electric fields across the two layers are generated, and the electric fields give rise to band-gap opening. The results demonstrate a simple mechanical method of realizing pseudoelectromagnetism in graphene and suggest a maneuverable approach to fabrication of electromechanical devices based on bilayer graphene.
Finally, the sliding effects on bilayer graphenes were studied. It is well known that the bilayer graphene is arranged according to Bernal stacking, and it has parabolic energy bands near Fermi level. It was found that the parabolic bands are separated to two linear Dirac cones, and the separating direction and distance is depend on the sliding direction and the displacement distance.
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