Sabine-Antonia Savu

Nanoscale Assembly of Paramagnetic Organic Radicals on Au(111) Single Crystals

The successful thin-film deposition of a pyrene-substituted nitronyl nitroxide radical under controlled conditions has been demonstrated. The electronic properties, chemical environment at the interface, and morphology of the thin films have been investigated by a multitechnique approach. Spectroscopic and morphological analyses indicate a Stranski-Krastanov growth mode and weak physisorption of molecules onto the metallic surface. Electron spin resonance (ESR) spectroscopy shows that evaporation processes and deposition do not affect the paramagnetic character of the molecules. Useful concepts for the engineering of new, purely organic-based magnets, which may open the way to fruitful exploitation of organic molecular-beam deposition for assembly on solid surfaces in view of future technological applications, are presented.

Chemisorption, morphology, and structure of a n-type perylene diimide derivative at the interface with gold: influence on devices from thin films to single molecules.

We have investigated thin films of a perylene diimide derivative with a cyano-functionalized core (PDI-8CN2) deposited on Au(111) single crystals from the monolayer to the multilayer regime. We found that PDI-8CN2 is chemisorbed on gold. The molecules experience a thickness-dependent reorientation, and a 2D growth mode with molecular stepped terraces is achieved adopting low deposition rates. The obtained results are discussed in terms of their impact on field effect devices, also clarifying why the use of substrate/contact treatments, decoupling PDI-8CN2 molecules from the substrate/contacts, is beneficial for such devices. Our results also suggest that perylene diimide derivatives with CN bay-functionalization are very promising candidates for single-molecule electronic devices.

Nanoscale assembly, morphology and screening effects in nanorods of newly synthesized substituted pentacenes

We report our investigation on the nanorods of two newly synthesized substituted pentacenes, δ4-substituted (2,3-X2-9,10-Y2) pentacene with X = Y = methoxy group (MOP) and X = F, Y = methoxy (MOPF), by using X-ray photoemission spectroscopy (XPS), near edge X-ray absorption fine structure (NEXAFS), and atomic force microscopy (AFM). The nanorods were deposited on Au(111) single crystals. Energy dependent photoemission spectra show complex features, including a rich satellite structure that we have analyzed in detail by using a best-fit procedure applying constraints based on stoichiometry, electronegativity, and bond strength. This analysis reveals the presence of surface core level shifts due to the high electronegativity of the fluorine atoms. The distinctive features of growth and morphology of the nanorods are subjected to a template effect by the substrate lattice geometry, leading to morphological well-organized assemblies. Fluorine atoms play an important role not only in the electronic structure but also in the morphology of the nanorod assemblies.

Pentacene-based nanorods on Au(111) single crystals: Charge transfer, diffusion, and step-edge barriers

AbstractWe investigate nanorod assemblies of two δ4-substituted pentacenes, namely (2,3-X2-9,10-Y2)-substituted pentacenes with X = Y = OCH3 (MOP) and with X = F, Y = OCH3 (MOPF), grown on Au(111) single crystals. By using a multi-technique approach based on ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, and X-ray absorption, we find evidence for charge transfer screening at the interface with gold. Furthermore, the MOP and MOPF nanorods show a rough surface morphology, which was investigated with atomic force microscopy. We use molecular simulation techniques to investigate the energetic barriers to diffusion and to traverse step-edges to estimate their influence on the nanorod roughness. We find that barriers to surface diffusion on a terrace are anisotropic and that their direction favors the formation of nanorods in these materials.

Direct observation of step-edge barrier effects and general aspects of growth processes: morphology and structure in diindenoperylene thin films deposited on Au(100) single crystals

We investigate thin films of diindenoperylene deposited on Au(100) single crystals by using real time in situlow energy electron microscopy (LEEM) in synergy with X-ray photoelectron spectroscopy (XPS), and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. We find evidence for Stranski–Krastanov growth, with island nucleation prevalently at step bunches, and a lower probability of island nucleation at terraces. We also directly observe the effect of the Ehrlich–Schwoebel barrier at the substrate and in the film morphology, in particular inside the islands that show a non-homogeneous thickness due to a rough front growth, because of the additional energy required for the diffusing molecules to surmount a downward step.