Tailoring a Molecule’s Optical Absorbance Using Surface Plasmonics

Understanding
the interaction of light with molecules physisorbed
on substrates is a fundamental problem in photonics, with applications
in biosensing, photovoltaics, photocatalysis, plasmonics, and nanotechnology.
However, the design of novel functional materials in silico is severely
hampered by the lack of robust and computationally efficient methods
for describing both molecular absorbance and screening on substrates.
Here, we employ our hybrid G0[W0 + ΔW]-BSE implementation,
which incorporates the substrate via its screening ΔW both at the quasiparticle G0W0 level and when solving the Bethe–Salpeter
equation (BSE). We show that this method can be used to both efficiently
and accurately describe the absorption spectra of physisorbed molecules
on metal substrates and thereby tailoring the molecule’s absorbance
by altering the surface plasmon energy. Specifically, we investigate
how the optical absorption spectra of three prototypical π-conjugated
molecules, benzene (C6H6), terrylene (C30H16), and fullerene (C60), depend on
the Wigner–Seitz radius rs of the
metallic substrate. To gain further understanding of the light–molecule/substrate
interaction, we also study the bright excitons’ electron and
hole densities and their interactions with infrared-active vibrational
modes. Our results show that (1) benzene’s bright E1u1 exciton at 7.0
eV, whose energy is insensitive to changes in rs, could be relevant for photocatalytic dehydrogenation and
polymerization reactions, (2) terrylene’s bright B3u exciton at 2.3 eV hybridizes with the surface plasmon, allowing
the tailoring of the excitonic energy and optical activation of a
surface plasmon-like exciton, and (3) fullerene’s π–π*
bright and dark excitons at 6.4 and 6.8 eV, respectively, hybridize
with the surface plasmon, resulting in the tailoring of their excitonic
energy and the activation of both a surface plasmon-like exciton and
a dark quadrupolar mode via symmetry breaking by the substrate. This
work demonstrates how a proper description of interfacial light–molecule/substrate
interactions enables the prediction, design, and optimization of technologically
relevant phenomena in silico.