Optical Excitations and Field Enhancement in Short Graphene Nanoribbons

Abstract

The optical excitations of elongated graphene nanoflakes of finite length are investigated theoretically through quantum chemistry semi-empirical approaches. The spectra and the resulting dipole fields are analyzed, accounting in full atomistic details for quantum confinement effects, which are crucial in the nanoscale regime. We find that the optical spectra of these nanostructures are dominated at low energy by excitations with strong intensity, comprised of characteristic coherent combinations of a few single-particle transitions with comparable weight. They give rise to stationary collective oscillations of the photoexcited carrier density extending throughout the flake, and to a strong dipole and field enhancement. This behavior is robust with respect to width and length variations, thus ensuring tunability in a large frequency range. The implications for nanoantennas and other nanoplasmonic applications are discussed for realistic geometries. TOC GraphicKeywords: nanoplasmonics, ZINDO, UV-vis spectrum, carbon nanostructures, transition density 2In the last few years remarkable interest has grown for nanoplasmonics and the perspectives it offers to merge electronics and photonics at the nanoscale. 1,2 A wide range of potential applications can be designed, including sensing and spectroscopic techniques, 3-6 fabrication of nanoantennas 7-11 and light emitters, 12,13 as well as beyond-THz optical devices or solar-energy conversion systems. [14][15][16] While great attention has been devoted to metal nanoparticles, due to the relative ease to produce them and to their possibility to support surface modes, 17-19 new materials and metamaterials are now being explored, 20,21 which, in addition to enhanced optical responses, are able to optimize circuit integration and reduce losses. Graphene has proved to have unique electronic and mechanical properties, 22 and has been more recently investigated also for photonics and optoelectronics. [23][24][25] In the field of (nano)plasmonics, so far most emphasis has been devoted to spectroscopy of plasmons in extended graphene, either doped or undoped, [26][27][28][29][30][31][32] and to the large lifetimes of their excitations compared to conventional metals. 30,33 Some interesting predictions, mostly based on macroscopic models, have been proposed for plasmonics in spatially-modulated graphene of micron and sub-micron size range. 32,[34][35][36][37][38] Here we focus on graphene in a completely different regime, typical of nanoscale structures, where an optical gap opens as a consequence of quantum confinement, and we show that field enhancement effects can also be seen. This regime has become particularly exciting in view of the recent production of controlled graphene wires by chemical routes. [39][40][41][42] We consider the case of self-standing elongated graphene nanoflakes (GNFs) with H-terminated edges, which we analyze by applying a fully-microscopic quantum-chemical approach. These flakes can be thought of as finite portions of armchair-edged graphene n...

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