Alexei Nefedov

Charge-transfer-induced structural rearrangements at both sides of organic/metal interfaces

Organic/metal interfaces control the performance of many optoelectronic organic devices, including organic light-emitting diodes or field-effect transistors. Using scanning tunnelling microscopy, low-energy electron diffraction, X-ray photoemission spectroscopy, near-edge X-ray absorption fine structure spectroscopy and density functional theory calculations, we show that electron transfer at the interface between a metal surface and the organic electron acceptor tetracyano-p-quinodimethane leads to substantial structural rearrangements on both the organic and metallic sides of the interface. These structural modifications mediate new intermolecular interactions through the creation of stress fields that could not have been predicted on the basis of gas-phase neutral tetracyano-p-quinodimethane conformation.

Self-metalation of 2H-tetraphenylporphyrin on Cu(111): An x-ray spectroscopy study

The bonding and the temperature-driven metalation of 2H-tetraphenylporphyrin (2H-TPP) on the Cu(111) surface under ultrahigh vacuum conditions were investigated by a combination of x-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy with density functional theory calculations. Thin films were prepared by organic molecular beam epitaxy and subsequent annealing. Our systematic study provides an understanding of the changes of the spectroscopic signature during adsorption and metalation. Specifically, we achieved a detailed peak assignment of the 2H-TPP multilayer data of the C1s and the N1s region. After annealing to 420 K both XPS and NEXAFS show the signatures of a metalloporphyrin, which indicates self-metalation at the porphyrin-substrate interface, resulting in Cu-TPP. Furthermore, for 2H-TPP monolayer samples we show how the strong influence of the copper surface is reflected in the spectroscopic signatures. Adsorption results in a strongly deformed macrocycle and a quenching of the first NEXAFS resonance in the nitrogen edge suggesting electron transfer into the LUMO. For Cu-TPP the spectroscopic data indicate a reduced interaction of first-layer molecules with the substrate as demonstrated by the relaxed macrocycle geometry.

UV‐Triggered Dopamine Polymerization: Control of Polymerization, Surface Coating, and Photopatterning

UV irradiation is demonstrated to initiate dopamine polymerization and deposition on different surfaces under both acidic and basic pH. The observed acceleration of the dopamine polymerization is explained by the UV-induced formation of reactive oxygen species that trigger dopamine polymerization. The UV-induced dopamine polymerization leads to a better control over polydopamine deposition and formation of functional polydopamine micropatterns.

Probing electrons in TiO2 polaronic trap states by IR-absorption: Evidence for the existence of hydrogenic states

An important step in oxide photochemistry, the loading of electrons into shallow trap states, was studied using infrared (IR) spectroscopy on both, rutile TiO2 powders and single-crystal, r-TiO2(110) oriented samples. After UV-irradiation or n-doping by exposure to H-atoms broad IR absorption lines are observed for the powders at around 940 cm−1. For the single crystal substrates, the IR absorption bands arising from an excitation of the trapped electrons into higher-lying final states show additional features not observed in previous work. On the basis of our new, high-resolution data and theoretical studies on the polaron binding energy in rutile we propose that the trap states correspond to polarons and are thus intrinsic in nature. We assign the final states probed by the IR-experiments to hydrogenic states within the polaron potential. Implications of these observations for photochemistry on oxides will be briefly discussed.

Surface Faceting and Reconstruction of Ceria Nanoparticles

The surface atomic arrangement of metal oxides determines their physical and chemical properties, and the ability to control and optimize structural parameters is of crucial importance for many applications, in particular in heterogeneous catalysis and photocatalysis. Whereas the structures of macroscopic single crystals can be determined with established methods, for nanoparticles (NPs), this is a challenging task. Herein, we describe the use of CO as a probe molecule to determine the structure of the surfaces exposed by rod-shaped ceria NPs. After calibrating the CO stretching frequencies using results obtained for different ceria single-crystal surfaces, we found that the rod-shaped NPs actually restructure and expose {111} nanofacets. This finding has important consequences for understanding the controversial surface chemistry of these catalytically highly active ceria NPs and paves the way for the predictive, rational design of catalytic materials at the nanoscale.

Metal-Support Interactions of Platinum Nanoparticles Decorated N-Doped Carbon Nanofibers for the Oxygen Reduction Reaction.

N-doped carbon materials are discussed as catalyst supports for the electrochemical oxygen reduction reaction (ORR) in fuel cells. This work deals with the preparation of Pt nanoparticles (NPs) supported on N-doped carbon nanofibers (N-CNF) from a polyaniline nanofiber (PANI NF) precursor, and investigates the ORR activity of the produced materials. Initially, Pt NPs are deposited on PANI NFs. The PANI NF precursors are characterized by near-edge X-ray absorption fine structure (NEXAFS) and transmission electron microscopy (TEM) measurements. It is shown, that in the PANI NF precursor materials electrons from the Pt are being transferred toward the π-conjugated systems of the aromatic ring. This strong interaction of Pt atoms with PANI explains the high dispersion of Pt NPs on the PANI NF. Subsequently, the PANI NF precursors are carbonized at different heat-treatment conditions resulting in structurally different N-CNFs which are characterized by NEXAFS, X-ray photoelectron spectroscopy (XPS) ,and TEM measurements. It is shown that an interaction between N-groups and Pt NPs exists in all investigated N-CNFs. However, the N-CNFs differ in the composition of the N-species and the dispersion of the Pt NPs. A small mean Pt NP size with a narrow size distribution is attributed to the presence of pyrdinic N-groups in the N-CNFs, whereas, for the N-CNFs with mainly graphitic and pyrrolic N-groups, an increase in the average Pt NP size with a broad size distribution is found. The ORR activity in alkaline media investigated by Koutecky-Levich analysis of rotating disk electrode measurements showed a largely enhanced ORR activity in comparison to a conventional Pt/C catalyst.

Carbon nanowalls: the next step for physical manifestation of the black body coating

The optical properties of carbon nanowall (CNW) films in the visible range have been studied and reported for the first time. Depending on the film structure, ultra-low total reflectance up to 0.13% can be reached, which makes the CNW films a promising candidate for the black body-like coating, and thus for a wide range of applications as a light absorber. We have estimated important trends in the optical property variation from sample to sample, and identified the presence of edge states and domain boundaries in carbon nanowalls as well as the film mass density variation as the key factors. Also we demonstrated that at much lower film thickness and density than for a carbon nanotube forest the CNWs yield one order higher specific light absorption.

Advanced Applications of NEXAFS Spectroscopy for Functionalized Surfaces

Spectroscopic techniques based on the use of synchrotron radiation have significantly contributed to a better understanding of macroscopic as well as of microscopic properties of materials. One of them is near edge X-ray absorption fine structure (NEXAFS) spectroscopy, a source of valuable information on the electronic structure and orientation of molecular adsorbates on metal surfaces. NEXAFS spectroscopy has had its largest impact in connection with an electron-based detection of the photon absorption, which turns this technique in a highly surface sensitive method. The technique has been developed over the past 30 years and presently is routinely used to study the adsorption of organic molecules on a large variety of different substrates, including metals, oxides and polymers. In the last decades, in addition to above-mentioned systems, the spectroscopic characterization of nanostructures has become an important topic. The present contribution describes the basics of NEXAFS spectroscopy and demonstrates the potential of the method by discussing several case studies.

The Interaction of Formic Acid with Zinc Oxide: A Combined Experimental and Theoretical Study on Single Crystal and Powder Samples

We present azimuth- and polarization-dependent infrared spectroscopy results obtained under ultra-high vacuum conditions on surface species formed by the interaction of formic acid with the mixed-terminated ZnO(10