Plasmon Localization by H‑Induced Band Switching Z. Mamiyev,* ,†,‡ S. Sanna, §,∥ F. Ziese, §,∥ C. Dues, §,∥ C. Tegenkamp, †,⊥ and H. Pfnü r* ,†,‡ † Institut fü r Festkö rperphysik, Leibniz Universitä t Hannover, Appelstraße 2, 30167 Hannover, Germany ‡ Laboratorium fü r Nano- und Quantenengineering (LNQE), Leibniz Universitä t Hannover, Schneiderberg 39, 30167 Hannover, Germany § Institut fü r Theoretische Physik and ∥ Center for Materials Research (LaMa), Justus-Liebig-Universitä t Gießen, Heinrich-Buﬀ-Ring 16, 35392 Gießen, Germany ⊥ Institut fü r Physik, Technische Universitä t Chemnitz, Reichenhainer Straße 70, 09126 Chemnitz, Germany 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. ■ INTRODUCTION Metallic atomic wires show peculiar physical properties that closely resemble those of one-dimensional (1D) objects. 1,2 Indeed, a high anisotropy of electronic properties is coupled with the small lateral extension. 1D objects, however, need stabilization by interactions with embedding two-dimensional (2D) and three-dimensional (3D) environments. Inevitably, these modify to some extent the 1D properties. On the other hand, interactions in higher dimensions open possibilities for controlled modiﬁcations of the wires that open a wide and attractive playground for tailoring and tuning quasi-1D properties. The formation of chemical bonds by adsorption of gases is just one example of such modiﬁcations (see, e.g., ref 3). Plasmons are collective excitations of an electron gas in a partially ﬁlled topmost electronic band that exist in all dimensions. Particularly interesting are the plasmons of ultrathin metallic sheets or of wires with a few atoms in diameter, forming 2D sheet plasmons or 1D wire plasmons, which have inherent attractive properties: due to their ﬂat dispersion, which starts at zero energy for large wavelengths, much shorter wavelengths (below 10 nm) can be achieved compared with the standard 3D edge plasmons, called surface plasmon, allowing for better localization, as demonstrated, e.g., in graphene nanostructures. 4 Their 2D plasmonic properties were also studied in detail. 5−7 For 2D sheet plasmons, shielding of the plasmonic excitation by a metal substrate or interaction of stacks results in linearization of the dispersion, 8,9 which, for an unshielded single metallic sheet, starts as E k ∼ ∥ k ∥ is the in-plane wavevector. The linearization of dispersion allows for distortionless signal transport. Quasi-linear dis- persion of acoustic surface plasmons has been demonstrated for several surfaces of metals such as Au, Cu, and Be. 10−14 All of them have Shockley surface states that cross the Fermi level. Recent investigations on topological insulator surfaces yielded new plasmon modes such as plasmon−polaron modes due to coupling of surface electrons with acoustic bulk phonons. 15,16 Switching to 1D wire plasmons with their already built-in directionality of energy transport, the dispersion, according to theory, is intrinsically quasi-linear, since it starts at long wavelengths as E ∼ k ∥ ln k ∥ . 17 The tunability of plasma frequencies as well as of their quasi-linear dispersion makes Received: November 14, 2019 Revised: December 9, 2019 Published: December 9, 2019 Article pubs.acs.org/JPCC Cite This: J. Phys. Chem. C XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.jpcc.9b10688 J. 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