First-principles Investigation For The Electronic Properties Of Graphene With Deformations And Defects

Abstract:This dissertation presents briefly the progress on graphene and the first principle calculations based on the nonequilibrium Green's function method and the density-functional theory (DFT) at first. The effects of typical deformations and defects on the electronic properties of graphene nano structure are investigated systematically. The interesting and innovative characteristics are found, and the physical mechanisms for each characteristic are revealed and explained with the right theories. Specifically, the innovative results as following:The altering regularities of electronic structure in wrapped graphene nanoribbons are investigated firstly. It is found that the zigzag-edge graphene nanoribbons (ZGNRs) are not sensitive to the wrapping deformation, only when the wrapped angle near360�, the electronic structure has a mutation; within240�wrapping deformation, the band gap of armchair-edge graphene nanoribbons (AGNRs) still meet the3P rules:?3p+1??3p?3p-1. The band gap of the3p-1family of AGNRs decreases slowly with increasing the wrapped angle, when wrapped angle is larger than300�, but the band gap increases rapidly, which undergoes a phase transition from metal to semiconductor; the band gap of the3P+1family of AGNRs decreases with the wrapped angle increasing, after the angle exceeds300�, the band gap decreases rapidly, a transition from semiconductor to semimetal; the band gap of the3P family increases slightly with increasing the angle.Then, the electronic transport properties of twisted AGNRs and ZGNRs are investigated. Results show that electronic transport properties are sensitive to twisting deformations for semiconductor-type AGNRs, but are robust against twisting deformations for quasi-metallic AGNRs and ZGNRs. The electronic conduction becomes weaker gradually for moderate-gap semiconductor-type AGNRs, but gets stronger for wide-gap semiconductor-type AGNRs when the twisted angle increases to120�While for quasi-metallic AGNRs and ZGNRs, the electronic conduction is strong and obeys Ohm's law of resistance strictly.The spin-polarized electronic properties for ZGNRs with an extended line defect are investigated. An ab initio scanning tunneling microscope image for the defective ZGNR is also predicted. Results show that such a ZGNR can exhibit a strong spin polarization and ferromagnetic state, this intrinsic feature is sensitive to the position of the line defect. The spin-filter effect can be significantly tuned and improved by the external electric field. The spin-filter efficiency for our proposed device is predicted to be able to reach up to60%?81%in a large bias range of0?0.6V at Eext=3.0V/nm.Finaly, the ways to solve an outstanding problem on how to properly pattern graphene to induce bandgaps are reported, in particular, related altering regularities and mechanisms are investigated systematically. It has been found that, regardless of the shape of the perforated hole, the periodicity along the zigzag-edge orientation of graphene nanomeshes (GNMs) plays crucial role in bandgap opening, and its variations can lead to a converting between semimetallic and semiconducting behavior. And also shown is that changing the periodicity along the armchair-edge orientation only modifies the dispersion gradually. For the GNM with circular, triangular and rectangular holes, the respective altering regularities of bandgaps with varying the size, shape, and density of hole are found. Importantly, the general numerical fittings for GNMs to quantitatively reflect the relationship between the bandgap and the structural parameters of GNMs are obtained. Furthermore, the underlying mechanisms for opening bandgap are revealed and the altering regularities of bandgaps are clearly explained.The findings stated above might be useful in the design for developing graphene-based nanoelectronics and spintronics.