Competing plasmonic and charge-transfer excitations in pyridine adsorbed silver and aluminum nanoparticles.

Based on time dependent density functional theory calculations, we reveal radical modifications in the optical absorption spectra of pyridine molecules adsorbed on Ag and Al nanoparticles. Transition density plots of the dominant peaks allowed us to classify the excitations into the plasmonic excitation of the nanoparticle or charge-transfer excitations between the pyridine molecule and Ag/Al nanoparticle. More precisely, in the case of Ag-pyridine structures, the optical response does not change much with respect to the 90° reorientation of pyridine, but the transition density plots change significantly. In the case of Al-pyridine structures, the optical response shows dramatic differences with respect to the pyridine orientations and the nature of the excitation of the main peaks exhibits a hybrid nature having both a plasmonic and charge-transfer contribution. We believe that the present theoretical results open exciting prospects in nanoantenna design.