Plasmon Localization by H-Induced Band Switching

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(parallel to), 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(parallel to) -> 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.