Fingerprinting plasmon-exciton coupling in small plasmonic nanoparticles from an ab initio GW-BSE approach

In conventional semiconductors, plasmonic and excitonic collective excitations are found at different energies, but the confinement and reduced screening in quasi-0D metallic nanoparticles allow them to coexist. We study such systems with an ab initio GW plus Bethe-Salpeter equation (BSE) approach to capture both plasmonic and excitonic effects on an equal theoretical footing and naturally isolate their contributions. With the inclusion of excitonic effects, we find a continuum of strongly bound dark excitons and significant modulation (~2 eV) of the plasmon states. We analyze excitonic effects on the wavefunctions and energies of excited states and identify the smoothness of the excited-state electron-hole overlap density as a key metric of plasmonicity. We use this insight to explain plasmon damping in the presence of catalytic metal dopants beyond conventional explanations in terms of interband screening.