Leonhard Grill

Reactions of low coverages of Eu, Gd, Er and Pr on Si(111) - a search for 2D silicides

Abstract Electronic excitation and unoccupied electron states at one monolayer Gd, Er, Eu and Pr-Si(111) interfaces are studied in a quest for 2D silicides. The Er results are compared with those of other techniques that have established a 2D silicide, while Pr is shown to develop a homogeneous but not wholly ordered 2D phase. Gd-Si(111) annealed at 400–600°C has a distinctive ordered monolayer phase with a different inverse photoemission spectrum from the 3D bulk silicide and low reactivity with oxygen in contrast to the monolayer Eu interface that remains highly reactive and disordered. Further investigations required to confirm the existence of a 2D Gd silicide are discussed.

Initial stages of praseodymium growth on Si(111): morphology and electronic structure

Abstract An STM study of the Prue5f8Si(111)-(7 × 7) interface reveals a much more heterogeneous growth morphology than is suggested by diffraction techniques and spectroscopies which average over the surface. Deposition of 1 ML followed by annealing at 650°C gives the most orderly growth, but this falls short of 2D epitaxy. At high temperatures (> 1000°C), the small amount of rare earth remaining on the surface stabilises some novel Si(111) surface reconstructions.

Quantum tunneling in real space: Tautomerization of single porphycene molecules on the (111) surface of Cu, Ag, and Au

Tautomerization in single porphycene molecules is investigated on Cu(111), Ag(111), and Au(111) surfaces by a combination of low-temperature scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations. It is revealed that the trans configuration is the thermodynamically stable form of porphycene on Cu(111) and Ag(111), whereas the cis configuration occurs as a meta-stable form. The trans → cis or cis → trans conversion on Cu(111) can be induced in an unidirectional fashion by injecting tunneling electrons from the STM tip or heating the surface, respectively. We find that the cis ↔ cis tautomerization on Cu(111) occurs spontaneously via tunneling, verified by the negligible temperature dependence of the tautomerization rate below ∼23 K. Van der Waals corrected DFT calculations are used to characterize the adsorption structures of porphycene and to map the potential energy surface of the tautomerization on Cu(111). The calculated barriers are too high to be thermally overcome at cryogenic temperatures used in the experiment and zero-point energy corrections do not change this picture, leaving tunneling as the most likely mechanism. On Ag(111), the reversible trans ↔ cis conversion occurs spontaneously at 5 K and the cis ↔ cis tautomerization rate is much higher than on Cu(111), indicating a significantly smaller tautomerization barrier on Ag(111) due to the weaker interaction between porphycene and the surface compared to Cu(111). Additionally, the STM experiments and DFT calculations reveal that tautomerization on Cu(111) and Ag(111) occurs with migration of porphycene along the surface; thus, the translational motion couples with the tautomerization coordinate. On the other hand, the trans and cis configurations are not discernible in the STM image and no tautomerization is observed for porphycene on Au(111). The weak interaction of porphycene with Au(111) is closest to the gas-phase limit and therefore the absence of trans and cis configurations in the STM images is explained either by the rapid tautomerization rate or the similar character of the molecular frontier orbitals of the trans and cis configurations.

Image-potential resonances studied by selective electron scattering in thin Pb(1 1 1) films and clusters on Ge(111)

Abstract Image-potential resonance related peaks are observed in HREEL spectra by so-called selective electron scattering at the reciprocal lattice vector of a thin Pb(1xa01xa01) film. Upon a variation of the scattering conditions the spectral features change according to the electron scattering process and the effective masses, i.e. dispersion, of different orders are obtained. Image-potential resonances on the continuous film at 120 K and on lead islands at 300 K are equivalent. Electron scattering processes in the thin lead film can be distinguished from those in the substrate. In this way, the Stranski–Krastanov growth mode at room temperature is confirmed by separate electron scattering events on surface regions with different reciprocal lattice vectors. The appearance of an additional peak might be due to the presence of domain walls on the surface.

On-Surface Polymerization: From Polyarylenes to Graphene Nanoribbons and Two-Dimensional Networks

On-surface polymerization is a novel technique for the fabrication of one- and two-dimensional molecular networks confined on a surface and is a rapidly developing field in surface science. The molecular building blocks exhibit pre-defined connection sites at which, after thermal activation and diffusion on the surface, the molecules are linked in a clean environment. Depending on the position and number of these connection sites, activated molecules polymerize to yield chains or two-dimensional networks. The chemical composition of the resulting polymer is precisely defined by the precursor molecules. We review current developments in the field of on-surface polymerization and present different examples, including the fabrication of graphene nanoribbons. We introduce reductive Ullmann-type coupling as well as Scholl-type cyclodehydrogenation for fabrication of graphene nanoribbons of increasing width. The surface plays a crucial role during the activation and polymerization processes because it serves as a catalyst, promotes molecular diffusion, and has a huge influence on the final molecular architecture. One-dimensional polymers can act as molecular wires and their conductance has been studied at the level of individual chains. In addition, we discuss two-dimensional networks and describe recent progress in attempts to improve their quality using sequential activation or defect-healing.