Marie D'ANgelo

Dirac nodal line (DNL) semimetals are a novel class of topological materials in which the valence and conduction bands touch along lines in the reciprocal space, with linear dispersion. These materials attract a growing attention, but the experimental realizations for two-dimensional systems are sparse. This article reports the first experimental realization of a two-dimensional hexagonal monolayer Cu₂Ge, grown by evaporation of Ge on a Cu(111) substrate. Through a combination of low-energy electron diffraction XPS and ARPES measurements, it is shown that the surface presents all characteristics expected from calculations for a free-standing Cu₂Ge monolayer. More specifically, the preservation of the two concentric nodal lines around the Γ point indicates weak interactions between the Cu₂Ge surface and its Cu(111) substrate, making it an ideal system for the study of DNL materials.

The adsorption of phthalocyanine (H₂Pc) on the 6H-SiC(0001)-(3 × 3) surface is investigated using X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure spectroscopy (NEXAFS), and density functional theory (DFT) calculations. Spectral features are tracked from the submonolayer to the multilayer growth regime, observing a significant modification of spectroscopic signals at low coverage with respect to the multilayer films, where molecules are weakly interacting. Molecules stay nearly flat on the surface at the mono and submonolayers. Previously proposed adsorption models, where the molecule binds by two N atoms to corresponding Si adatoms, do not reproduce the experimental spectra at the submonolayer coverage. We find instead that another adsorption model where the molecule replaces the two central H atoms by a Si adatom, effectively forming Si-phthalocyanine (SiPc), is both energetically more stable and yields in combination a better agreement between the experimental and simulated spectra. This suggests that the 6H-SiC(0001)-(3 × 3) surface may be a candidate substrate for the on-surface synthesis of SiPc molecules.

Crystal surfaces, and more particularly semiconductor and oxide surfaces, are of great interest for both technological applications and fundamental physics. As a consequence, they have been intensively studied in recent decades and now their control at the atomic scale has been achieved. It is thus possible to go to next step and look at the interaction of these surfaces with specific atoms or molecules to improve or change their properties. In this topical review article, we describe in detail some selected examples of modifications of the electronic properties of semiconductor and oxide surfaces that can be expected with the adsorption of atoms such as hydrogen, alkalis or noble metals. By selecting an appropriate adsorbate on a specific substrate the electronic properties can be tuned to match desired characteristics.