Sandhya Chandola

Kurzzusammenfassung: N‐Heterocyclische Carbene (NHCs) sind etablierte Liganden für verschiedene Oberflächen, welche für ihre starke Bindung, ihre Fähigkeit zur Bildung selbstorganisierter Monolagen und ihre modulare Struktur bekannt sind. Trotz ihrer technologischen Bedeutung steckt die Modifizierung von Halbleiteroberflächen mit NHCs jedoch noch in den Kinderschuhen. Während sich frühere Studien auf Silizium konzentrierten, besitzen III‐V‐Verbindungshalbleiter eine direkte Bandlücke und hohe Elektronenbeweglichkeiten, was sie ideal für die (Opto‐)Elektronik macht. Diese Studie untersucht die Adsorption verschiedener NHCs auf GaAs, einem prototypischen III‐V‐Material, mittels Rastertunnelmikroskopie, Dichtefunktionaltheorie, Röntgenphotoelektronenspektroskopie, Beugung niederenergetischer Elektronen und Reflektionsanisotropiespektroskopie. Es werden kovalente Bindungen an die Oberfläche und die Bildung gut geordneter NHC‐Monolagen beobachtet, einhergehend mit außergewöhnlich großen Reduktionen der Austrittsarbeit. Die einzigartigen strukturellen Eigenschaften von NHCs ermöglichen eine präzise Kontrolle der Filmstruktur, der Ordnung und der elektronischen Eigenschaften.

N‐Heterocyclic carbenes (NHCs) are established ligands for various surfaces, known for their strong binding, ability to form self‐assembled monolayers, and modular structure. However, semiconductor surface modification with NHCs is still in its infancy despite its technological importance. Although previous studies focused on silicon, III‐V compound semiconductors offer direct bandgaps and high electron mobilities, making them ideal for (opto‐)electronics. This study examines the adsorption of different NHCs on GaAs, a prototypical III‐V material, using scanning tunneling microscopy, density functional theory, X‐ray photoelectron spectroscopy, low‐energy electron diffraction, and reflectance anisotropy spectroscopy. Covalent binding to the surface and the formation of well‐ordered NHC monolayers are observed, along with exceptionally large work function reductions. The unique structural features of NHCs enable precise control over film structure, ordering, and electronic properties.

The early growth stages of palladium (Pd) on Si(001) are studied in situ by reflectance anisotropy spectroscopy and Raman spectroscopy in ultrahigh vacuum. Two vicinal Si(001) samples with different degrees of surface ordering are investigated, revealing a distinct Volmer–Weber‐type growth behavior. Ex situ atomic force microscopy shows an inhomogeneous surface covered with islands on the less‐ordered substrate, while a complex layer with circular structures related to a nucleation‐controlled phase transition as previously reported is found on the well‐ordered sample. By Raman spectroscopy, a silicide‐like reacted phase at the Si–Pd interface is identified which is promoted by annealing up to 600 °C. The appearance of several additional phonon modes due to the interface reaction is in very good agreement with vibrational modes obtained from ab initio calculations for Pd incorporation into Si surface layers.

We report the first evidence of Raman scattering from surface phonons of a pristine metal surface. Our study reveals a Raman-active surface vibrational resonance on Cu(110) with a surprisingly large scattering efficiency. With the incident photon energy close to the energy of the Cu(110) surface state electronic transition, the Raman scattering from the surface optical resonance can be significantly enhanced, while any contribution from bulk phonons is absent.

The conductivity of substrate‐supported metallic nanowires can be adjusted, e.g., by strain or adsorbates. In this article, the effect of atomic hydrogen and toluene‐3,4‐dithiol (TDT) adsorption on the quasi‐1D structures of the Si(553)‐(5 × 2)–Au reconstruction is investigated. Reflectance anisotropy spectroscopy (RAS) and infrared ellipsometry reveal optical signatures of the surface. Spectral modifications related to the adsorbate exposure suggest the activation of adsorption‐induced interband transitions. Density functional theory (DFT) calculations reproduce the spectral modifications and explain their origin. Preferential adsorption on sites located at the step edges of the structure occurs independent of the type of adsorbate and induces a charge transfer between electronic states related to the step edges and the Au dimer rows. This charge redistribution modifies the electronic structure close to the Fermi level, enhances the dimerization of the Au chains, and strongly influences the low‐energy region of the RAS spectra. While previous studies employed atomic H as a chief adsorbate, it is shown here that even molecular H deeply modifies the Si(553)–Au optical response. Structural modification of Au–Si(553) by H and TDT adsorbates as suggested from recent ab initio calculations are verified.

Two-dimensional rare-earth silicide layers deposited on silicon substrates have been intensively investigated in the last decade, as they can be exploited both as Ohmic contacts or as photodetectors, depending on the substrate doping. In this study, we characterize rare-earth silicide layers on the Si(111) surface by a spectroscopic analysis. In detail, we combine Raman and reflectance anisotropy spectroscopy (RAS) with first-principles calculations in the framework of the density functional theory. RAS suggests a weakly isotropic surface, and Raman spectroscopy reveals the presence of surface localized phonons. Atomistic calculations allow to assign the detected Raman peaks to phonon modes localized at the silicide layer. The good agreement between the calculations and the measurements provides a strong argument for the employed structural model.

N-Heterocyclic carbenes (NHCs) are promising modifiers and anchors for surface functionalization and offer some advantages over thiol-based systems. Because of their strong binding affinity and high electron donation, NHCs can dramatically change the properties of the surfaces to which they are bonded. Highly ordered NHC monolayers have so far been limited to metal surfaces. Silicon, however, remains the element of choice in semiconductor devices and its modification is therefore of utmost importance for electronic industries. Here, a comprehensive study on the adsorption of NHCs on silicon is presented. We find covalently bound NHC molecules in an upright adsorption geometry and demonstrate the formation of highly ordered monolayers exhibiting good thermal stability and strong work function reductions. The structure and ordering of the monolayers is controlled by the substrate geometry and reactivity and in particular by the NHC side groups. These findings pave the way towards a tailor-made organic functionalization of silicon surfaces and, thanks to the high modularity of NHCs, new electronic and optoelectronic applications.