Abstract
Narrow graphene nanoribbons exhibit substantial electronic bandgaps and optical properties fundamentally different from those of graphene. Unlike graphene--which shows a wavelength-independent absorbance for visible light--the electronic bandgap, and therefore the optical response, of graphene nanoribbons changes with ribbon width. Here we report on the optical properties of armchair graphene nanoribbons of width N=7 grown on metal surfaces. Reflectance difference spectroscopy in combination with ab initio calculations show that ultranarrow graphene nanoribbons have fully anisotropic optical properties dominated by excitonic effects that sensitively depend on the exact atomic structure. For N=7 armchair graphene nanoribbons, the optical response is dominated by absorption features at 2.1, 2.3 and 4.2 eV, in excellent agreement with ab initio calculations, which also reveal an absorbance of more than twice the one of graphene for linearly polarized light in the visible range of wavelengths.
Highlights
Narrow graphene nanoribbons exhibit substantial electronic bandgaps and optical properties fundamentally different from those of graphene
We focus on the optical properties of armchair Graphene nanoribbons (GNRs) of width N 1⁄4 7 (7-AGNR, 0.74 nm wide with respect to edge C atoms, Fig. 1b) as resulting from the bottom-up fabrication process on gold substrates[12]
It is 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 1⁄2011 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
Summary
Narrow graphene nanoribbons exhibit substantial electronic bandgaps and optical properties fundamentally different from those of graphene. Recent studies showed that the bottom-up fabrication of GNRs can be achieved in solution[17,18] where, aggregation issues need to be addressed, the process can be scaled up Similar as for their electronic properties, GNRs are expected to exhibit optical properties that are significantly different from the parent material graphene: Whereas graphene, owing to its linear band dispersion, reveals a wavelength-independent light absorbance of 2.3%19 in the visible range, GNRs are expected to exhibit characteristic absorption bands related to band gap openings as well as to pronounced excitonic effects[8,9,10,11] that become dominant for quasi onedimensional (1D) systems[20]. We catch the GNR in the act of its formation by following the signatures of fundamental optical excitations from the isolated molecular components through polymerization via intermolecular C–C coupling to the final GNR structure obtained after intramolecular cyclodehydrogenation
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