  Plasmons produce large confinement and enhancement of light that enable
applications as varied as cancer therapy and catalysis. Adding to these
appealing properties, graphene has emerged as a robust, electrically tunable
material exhibiting plasmons that strongly depend on the density of doping
charges. Here we show that adding a single electron to a graphene nanoisland
consisting of hundreds or thousands of atoms switches on infrared plasmons that
were previously absent from the uncharged structure. Remarkably, the addition
of each further electron produces a dramatic frequency shift. Plasmons in these
islands are shown to be tunable down to near infrared wavelengths. These
phenomena are highly sensitive to carbon edges. Specifically, armchair
nanotriangles display sharp plasmons that are associated with intense
near-field enhancement, as well as absorption cross-sections exceeding the
geometrical area occupied by the graphene. In contrast, zigzag triangles do not
support these plasmons. Our conclusions rely on realistic quantum-mechanical
calculations, which are in ostensible disagreement with classical
electromagnetic simulations, thus revealing the quantum nature of the plasmons.
This study shows a high sensitivity of graphene nanoislands to elementary
charges, therefore emphasizing their great potential for novel
nano-optoelectronics applications.
