Browsing by Author "Uppstu, Christer"
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- Electronic states in finite graphene nanoribbons: Effect of charging and defects
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2013) Ijäs, M.; Ervasti, M.; Uppstu, Christer; Liljeroth, P.; van der Lit, J.; Swart, I.; Harju, A.We study the electronic structure of finite armchair graphene nanoribbons using density-functional theory and the Hubbard model, concentrating on the states localized at the zigzag termini. We show that the energy gaps between end-localized states are sensitive to doping, and that in doped systems, the gap between the end-localized states decreases exponentially as a function of the ribbon length. Doping also quenches the antiferromagnetic coupling between the end-localized states leading to a spin-split gap in neutral ribbons. By comparing dI/dV maps calculated using the many-body Hubbard model, its mean-field approximation and density-functional theory, we show that the use of a single-particle description is justified for graphene π states in case spin properties are not the main interest. Furthermore, we study the effect of structural defects in the ribbons on their electronic structure. Defects at one ribbon terminus do not significantly modify the electronic states localized at the intact end. This provides further evidence for the interpretation of a multipeak structure in a recent scanning tunneling spectroscopy (STS) experiment resulting from inelastic tunneling processes [van der Lit et al., Nat. Commun. 4, 2023 (2013)]. Finally, we show that the hydrogen termination at the flake edges leaves identifiable fingerprints on the positive bias side of STS measurements, thus possibly aiding the experimental identification of graphene structures. - Quantum confined electronic states in atomically well-defined graphene nanostructures
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2011) Hämäläinen, Sampsa; Sun, Z.; Boneschanscher, M.P.; Uppstu, Christer; Ijäs, M.; Harju, A.; Vanmaekelbergh, D.; Liljeroth, PeterDespite the enormous interest in the properties of graphene and the potential of graphene nanostructures in electronic applications, the study of quantum-confined states in atomically well-defined graphene nanostructures remains an experimental challenge. Here, we study graphene quantum dots (GQDs) with well-defined edges in the zigzag direction, grown by chemical vapor deposition on an Ir(111) substrate by low-temperature scanning tunneling microscopy and spectroscopy. We measure the atomic structure and local density of states of individual GQDs as a function of their size and shape in the range from a couple of nanometers up to ca. 20 nm. The results can be quantitatively modeled by a relativistic wave equation and atomistic tight-binding calculations. The observed states are analogous to the solutions of the textbook “particle-in-a-box” problem applied to relativistic massless fermions. - Suppression of electron-vibron coupling in graphene nanoribbons contacted via a single atom
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2013) van der Lit, J.; Boneschanscher, M.P.; Vanmaekelbergh, D.; Ijäs, M.; Uppstu, Christer; Ervasti, M.; Harju, A.; Liljeroth, P.; Swart, I.Graphene nanostructures, where quantum confinement opens an energy gap in the band structure, hold promise for future electronic devices. To realize the full potential of these materials, atomic-scale control over the contacts to graphene and the graphene nanostructure forming the active part of the device is required. The contacts should have a high transmission and yet not modify the electronic properties of the active region significantly to maintain the potentially exciting physics offered by the nanoscale honeycomb lattice. Here we show how contacting an atomically well-defined graphene nanoribbon to a metallic lead by a chemical bond via only one atom significantly influences the charge transport through the graphene nanoribbon but does not affect its electronic structure. Specifically, we find that creating well-defined contacts can suppress inelastic transport channels.