Monday 11st November 2013 230pm, Hume-Rothery Lecture Theatre

Bottom-up synthesized graphene nanoribbons: inelastic transport and end states

Dr Mari Ijäs, Cambridge Graphene Center

Graphene nanoribbons have been proposed to function as components in future nanoscale electronical devices. In theoretical considerations, the edge termination defines many of the electronic properties of finite graphene nanostructures, thus requiring precise control of the edges for experimental realizations. Experimentally preparing well-defined nanostructures is, however, challenging. Top-down etching approaches typically yield ribbons with rough edges. In the bottom-up synthesis method, on the contrary, the structure of the precursor molecule, together with the mechanism of the polymerization step, defines the resulting nanostructure. After the seminal synthesis of armchair-terminated graphene nanoribbons  [1], subsequent studies have addressed their electronic [2-4] and transport properties [5]. Inelastic transport and how contacting the ribbons to the substrate affects their electronic properties has, however, remained unexplored. 

Here, the electronic states of finite 7-AGNRs are studied both theoretically and experimentally, concentrating on states localized at the zigzag termini of the finite ribbons. Theoretically, in zigzag-edged nanoribbons, a spin-split between the edge-localized states has been predicted [6]. In the present scanning tunneling spectroscopy measurements, a multipeak structure resembling this spin split is observed at the zigzag termini of the armchair ribbons. We show that it, however, actually arises from phonon-assisted inelastic tunneling. Moreover, contacting the ribbons to the substrate via a single-atom contact alters the transport properties through the ribbon by suppressing the inelastic transport channels, whereas the electronic states of the ribbon are not strongly modified [7]. 

[1] Cai et al., Nature 466, 470 (2010). 
[2] Ruffieux et al., ACS Nano 6, 6930 (2012)
[3] Talirz et al., JACS 135, 2060 (2013)
[4] Bronner et al., PRB 86, 085444 (2012)
[5] Koch et al., Nature Nanotech. 7, 713 (2012)
[6] Y.-W. Son et al., Nature 444, 417 (2006)
[7] J. van der Lit et al., Nature Commun. 4, 2023  (2013); M. Ijäs et al., PRB 88, 075429 (2013).