The strong sensitivity of plasmonic excitations on nanostructures
to their environment is studied, going to the ultimate limit of single
atomic chains. As a first step, we investigated how metallicity in
self-assembled arrays of Au chains on Si(557) is modified by the simplest
possible adsorbate, namely, atomic hydrogen. Both experimental studies
and ab initio simulations were carried out combining plasmon spectroscopy
with atomistic first-principles density functional calculations (DFT).
While metallicity, in general, is only distorted by H-induced disorder,
we also observed band gap opening in the measured plasmon dispersion
at large momenta, k
∥, that limits
the plasmonic excitation to an energy of 0.43 eV in the presence of
H. In the long-wavelength limit, disorder leads to plasmonic standing
wave formation on short sections of wires and finite excitation energies
for k
∥ → 0. DFT shows that
Si surface bands strongly hybridize with those of Au so that H adsorption
on the energetically most favorable sites at the Si step edge and
the restatom chain not only causes a significant shift of bands but
also strongly changes the character of hybridization. Together with
H-induced changes in band order, this causes band gap opening and
reduced overlap of wave functions. These mechanisms were identified
as the main reasons for plasmon localization. Interestingly, although
the whole electronic system is modified by H adsorption, there is
no direct interaction between H and the Au chains.