Browsing by Author "Puska, Martti J., Prof., Aalto University, Finland"

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Electron transport in graphene nanostructures
(2013) Saloriutta, Karri
School of Science | Doctoral dissertation (article-based)
Since its first synthesis and characterization in 2004 graphene has been the focus of an intense research effort. Charge carriers in graphene are massless Dirac fermions that behave fundamentally differently than electrons in conventional semiconductor heterostructures. For applications the most interesting factor is the very high quality of the graphene lattice leading to ballistic transport over micron length scales. The transport of electrons in graphene has thus been widely studied for applications in electronics. Although bulk graphene is gapless, a gap can be generated by breaking the graphene into finite strips, graphene nanoribbons. Our work concerns the study of electron transport in graphene and graphene nanoribbons using first principles density functional theory (DFT) and semiempirical tight-binding (TB) methods. The TB and DFT approaches are complementary in that DFT makes it possible to study small graphene structures with an accurate accounting for effects such as ionic relaxation, charge transfer and magnetism while TB can be used for fast calculations of large, disordered samples. By combining DFT and TB very accurate TB parameterizations can be generated. The parameterizations can also be generalized for magnetic systems by using the Hubbard model. We have used these methods to study the effect various structural defects such as vacancies, adatoms and disordered edges have on the transmission properties of graphene nanostructures. We have shown that even single defect calculations are enough for estimating the conducting properties of large samples with the aid of a scaling approach to transport. This makes it possible to characterize different defects in graphene based on a scattering cross section that can be calculated directly by DFT for small systems.