Local electronic properties of functionalized graphene studied with scanning tunneling microscopy and spectroscopy
Final Report Abstract
We have studied the electronic and structural properties of graphene nanoribbons (GNRs) which are thin strips of graphene with widths ranging from few nanometers down to sub-1 nm. The electronic properties of GNRs crucially depend on their width and edge geometry, so that fabrication methods are required that allow for controlling these parameters. Instead of using conventional lithographic etching of graphene or unzipping of carbon nanotubes which both lack the necessary level of control over the GNR properties, we have fabricated GNRs with atomic precision using a unique bottom-up synthesis technique. This approach incorporates a substrate assisted self-assembly of pre-designed molecular precursors in an ultra-high vacuum environment. Through co-deposition of two molecular precursors that have been previously used to achieve N=7 and N=13 armchair graphene nanoribbons, we have demonstrated the first molecular selfassembly of a width-modulated heterojunction with single-atom thickness and sub-2 nm width based on armchair GNRs. We have studied the resulting 7-13-GNR heterojunction with scanning tunneling microscopy (STM) and spectroscopy (STS) and identify peculiar electronic structures in the different GNR segments along the junction, which are reminiscent of intra-ribbon bandgap engineering. We have performed first-principles calculations to support the experimental findings. A novel molecular precursor is used for the bottom-up synthesis of N=11 armchair graphene nanoribbons. Unlike commonly employed molecules that are based on conjugated systems of sp2-bonded carbon atoms, we demonstrate the fabrication of GNRs through cyclodehydrogenation of “out-of-plane” sp3-bonded carbons. With scanning tunneling microscopy we observed the conversion process from sp3 to sp2 via thermal annealing. Our approach adds a new chemical route for achieving armchair GNRs. As on-surface polymerization techniques in the process of GNR growth can be hindered by molecular diffusion barriers or undesired side products, we have developed an alternative insolution polymerization procedure that has the potential to have better polymer yield and higher selectivity with regard to polymer length. Using combined in-solution polymerization and on-surface cyclodehydrogenation in ultra-high vacuum, we have successfully synthesized N=9 armchair graphene nanoribbons. We characterized the resulting GNRs with regard to width and edge termination using STM.
Publications
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“Molecular bandgap engineering of bottom-up synthesized graphene nanoribbon heterojunctions”. Nature Nanotechnology 10, 156-160 (2015)
Yen-Chia Chen, Ting Cao, Chen Chen, Zahra Pedramrazi, Danny Haberer, Dimas G. de Oteyza, Felix R. Fischer, Steven G. Louie & Michael F. Crommie
