Plasmon Localization by H-Induced Band Switching

Abstract

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.

View on Si(111)−(5×2)−Au with plasmon spectroscopy

“…This phase at its completion has a gold concentration of 0.48 ML. Exceeding this concentration by more than 0.01 ML results in a significant decrease of plasmon energies [41]. At a surplus concentration of 0.03 ML the formation of a √ 3-ordered island was observed after annealing.…”

“…This strategy is applied here in order to discriminate between band structures calculated from KK and EBH models. Moreover, we exploit the high sensitivity of this technique to chain ordering and confinement in the low-k regime to study the influence of disorder on wire length and structural arrangement [35,36].…”

Adsorbate‐Induced Modifications in the Optical Response of the Si(553)–Au Surface

“…[26] Changes in the geometry, electronic structure, and optical properties of the Si(553)-Au surface as a function of adsorbate deposition have been predicted with the help of DFT calculations and observed experimentally. [28,33,[35][36][37] Atomic or molecular adsorption is known to induce a surface charge redistribution between the step edge and Au-chain, which finds experimental support, e.g., by reflectance anisotropy spectroscopy (RAS) measurements during hydrogenation. [28,33,38] In this work, we explore the impact of two different types of adsorbates that have been the subject of a previous ab initio calculation study: molecular H and toluene-3,4-dithiol (TDT), an aromatic divalent thiol with two -SH ligands.…”

“…Changes in the geometry, electronic structure, and optical properties of the Si(553)–Au surface as a function of adsorbate deposition have been predicted with the help of DFT calculations and observed experimentally. [ 28,33,35–37 ] Atomic or molecular adsorption is known to induce a surface charge redistribution between the step edge and Au‐chain, which finds experimental support, e.g., by reflectance anisotropy spectroscopy (RAS) measurements during hydrogenation. [ 28,33,38 ]…”

“…We demonstrate that optical measurements are sufficiently sensitive to monitor the changes in surface properties as induced by molecular H 2 , [28,42] which we use instead of atomic H, in contrast to previous experiments. [33,35,37] Corresponding DFT-based calculations model the experiment. We should note that due to computational limitations the DFT-based calculations of the optical response are limited to the optical anisotropy.…”