Electronic Structure of Spatially Aligned Graphene Nanoribbons on Au(788)

Abstract

We report on a bottom-up approach of the selective and precise growth of subnanometer wide straight and chevron-type armchair nanoribbons (GNRs) on a stepped Au(788) surface using different specific molecular precursors. This process creates spatially well-aligned GNRs, as characterized by STM. High-resolution direct and inverse photoemission spectroscopy of occupied and unoccupied states allows the determination of the energetic position and momentum dispersion of electronic states revealing the existence of band gaps of several electron volts for straight 7-armchair, 13-armchair, and chevron-type GNRs in the electronic structure.

Large-scale solution synthesis of narrow graphene nanoribbons

“…However, the 1.3 eV bandgap is substantially lower than the values reported in the studies of individual GNRs synthesized by the surface-assisted approach on gold substrates 27,28,33,34 . For example, a significantly larger bandgap of 3.1 ± 0.4 eV was recently reported for the same GNRs 5 synthesized on Au(788) by the surface-assisted approach 34 ; this value is much higher than the value of B1.3 eV found in this work for GNRs 5, and even larger or comparable to the bandgap of the precursor polymer 4 (B2.9 eV) determined from the absorption onset in the optical spectrum (Fig. 4c).…”

“…Ribbons that are only a few benzene rings wide and have atomically smooth armchair edges were synthesized on a surface of either Au (111) or Ag (111) single crystal by coupling molecular precursors into linear polyphenylenes followed by cyclodehydrogenation. This work demonstrates that bottom-up techniques could yield narrow atomically engineered GNRs that are currently unachievable by any top-down approach, stimulating their detailed characterization [27][28][29][30][31][32][33][34] , as well as further research and development of new synthetic methods for GNRs.…”

“…4c). A possible explanation for these differences is the fact that the characterization of solution-synthesized GNRs (or polymer precursors), such as UV-vis-NIR and UPS/IPES, is performed on bulk samples where GNRs (or precursor polymers) are heavily aggregated, whereas prior measurements were performed on isolated GNRs on a gold substrate 34 . Aggregation effects were previously shown to affect the optical absorption spectra of PAH molecules visibly reducing their apparent bandgaps 57 .…”

Bottom-Up Graphene-Nanoribbon Fabrication Reveals Chiral Edges and Enantioselectivity

“…This type of distribution is unusual compared to previous diffusion-controlled bottom-up GNR assemblies, which typically exhibit Γ-distribution in GNR lengths. 21,23 The fact that 198 out of 352 GNRs are 1 nm long (i.e., one-DBPM long) is evidence that once polymerization starts, isolated and misaligned DBPMs can only undergo debromination and molecular flattening, without the possibility of subsequent diffusion or polymerization. …”

Exciton-dominated optical response of ultra-narrow graphene nanoribbons

“…It is thus perfectly suited to detect the anisotropic absorption of GNRs, provided that unidirectional alignment over macroscopic length scales can be achieved. This is obtained by depositing molecular precursors on the regularly stepped Au(788) surface, which results in a high degree of GNR alignment along the 011 ½ direction, with an average estimated nanoribbon length of 20 nm and a coverage around 0.8 monolayers (MLs) 13,22 , corresponding to an average distance between ribbon axes of B1.4 nm. Both inter-ribbon interactions and finite-size effects are thus expected to be negligible (see Supplementary Note 1).…”