P. Gröning

Porous graphenes: two-dimensional polymer synthesis with atomic precision.

We demonstrate, by surface-assisted coupling of specifically designed molecular building blocks, the fabrication of regular two-dimensional polyphenylene networks with single-atom wide pores and sub-nanometer periodicity.

The deposition of anti-adhesive ultra-thin teflon-like films and their interaction with polymers during hot embossing

The chemical and physical interactions of ultra-thin teflon-like films at interfaces are a surface science problem with many technological implications. Such films are the material of choice for protective layers and anti-adhesive coatings. During the replication of microstructures in polymers by hot embossing, interfacial forces between the master and the replica need to be reduced by an anti-adhesive layer, in order to ensure a clean demolding process. In this work, we investigated two different teflon-like films, one obtained by ion sputtering, and the other by plasma polymerization. Using both deposition methods, we deposited thin fluorinated films on nickel substrates and conducted depth-resolved X-ray Photoelectron Spectroscopy (XPS) measurements for a detailed comparison. In a subsequent step, nickel surfaces covered by both anti-adhesive coatings were hot embossed into two different polymers. The chemical composition of both the anti-adhesive film and the polymer replicas was monitored, as a function of the number of embossings made with the same Polytetrafluoroethylene (PTFE)-treated nickel stamp. During the embossing process, a transfer of material was found to occur from the teflon-like film to the embossed polymer, consisting of fluorinated entities or small polymer chains. The influence of the operating parameters on these phenomena was also investigated and resulted in a better understanding of the film/polymer interactions under pressure and at high temperature.

Vacuum arc discharges preceding high electron field emission from carbon films

Field emission measurements on chemical vapor deposition diamond and laser ablated a‐C films show an activation step after reaching a certain critical electric field. At this field a vacuum arc of some hundred ns duration initiates. While high current arcing leads to the evaporation of the spot surface melting, amorphization or cracking of the film is encountered for lower currents. In any case, much higher electron emission can be observed after this activation procedure due possibly to tip formation resulting in an electric field enhancement. By using a 1 GΩ resistance the discharge current can be limited nevertheless, an activation is observed.

Field emission from DLC films

Abstract Field emission measurements on diamond like carbon (DLC) films with different amounts of sp3 and sp2 carbon were carried out. Depending on the amount of sp2 carbon in the film, activated and non-activated Fowler-Nordheim like emission could be observed. The emission spots were investigated using a combination of AFM and STM, by simultaneously measuring the topography and the conductivity of the samples. In the case of sp2 rich DLC films we could observe that the emission originates from highly conducting inclusions of sp2 carbon in a matrix of insulating sp3 carbon. These inclusions are already existing on the sp2 sample by the deposition process itself and are formed by the activation on the sp3 rich sample.

Field emitted electron energy distribution from nitrogen-containing diamondlike carbon

We report on energy resolved field emission measurements of 0.3 μm thick nitrogen containing diamondlike carbon (DLC). The film used in this study was deposited by filtered arc deposition onto a highly p-doped Si(100) wafer. The film showed homogeneous field emission over the entire wafer area with an onset field for 1 nA emission current of 20–25 V/μm. The energy resolved field emission measurements were carried out with an applied electric field of 20–22 V/μm. The field emitted electrons originate from the Fermi energy, indicating that no field penetration occurs. The energy distribution has a FWHM of 0.63 eV at an applied field of 21 V/μm. The spectra could be deconvoluted using standard tunneling theory. The results of the deconvolution indicate an electric field strength of 6500 V/μm  at the emission site.

Creation of nanostructures to study the topographical dependency of protein adsorption

Abstract Nanostructures of sizes comparable to protein dimensions are created on Si and Ti surfaces by local anodic oxidation (LAO) using the atomic force microscope (AFM). The characterization of the surface by X-ray photoelectron spectroscopy (XPS) reveals that this method assures a modification of the topography of the surface without a change of its chemical composition. Surfaces structured by LAO therefore represent ideal systems to study the dependence of protein adsorption on topography. We are able to visualize the created nanostructures with an AFM and successively adsorb the proteins in situ, rinse and image the new surface. The densities of adsorbed proteins on the nanostructured and neat surfaces are compared and we find that the protein arrangement depends on the underlying nanostructures, showing that proteins can “sense” the topography of surfaces at the nanometer scale. This result can be considered as the nanoscale analogous of the adsorption found for cell systems on micrometer structures.

Interaction of low-energy ions (< 10 eV) with polymethylmethacrylate during plasma treatment

Abstract Using in-situ X-ray photoelectron spectroscopy (XPS) we investigated the chemical modification of the polymethyl-methacrylate (PMMA) surfaces after plasma and low-energy ion beam treatment. A comparison between plasma and ion beam treatment has shown, that for noble gases both treatments produce absolutely the same modifications of the chemical composition on the PMMA surface. In reactive gases (O2, N2) molecular ions were found to decompose polar bonds at the polymer surface, but they do not contribute to the incorporation of reactive gas atoms. Reactive atomic ions and radicals are responsible for this incorporation. We found that in the case of PMMA less than three ions are needed to decompose the ester group (OCO) completely. Therefore, we conclude that the decomposition of the ester group by ions is a chemical and not a physical effect due to the kinetic energy of the ions.

Protein adsorption on topographically nanostructured titanium

We study the protein adsorption on surfaces in order to investigate their predominant role in biocompatibility. Nanostructures are created by local anodic oxidation on titanium using the atomic force microscope. A remarkable specificity of the actin filament adsorption on the nanostructure height is noticed. F-actin is observed to have a low adsorption on nanostructures of a height of 4 nm and the adsorbed proteins appear to be randomly oriented. In contrast high protein adsorption is observed for structure height between 1 and 2 nm, moreover the filaments adsorb preferentially parallel to the nanostructured pattern.

Properties of thin anti-adhesive films used for the replication of microstructures in polymers

The aim of this work was to investigate the degradation of the anti-adhesive properties of protective PTFE films during the hot embossing of thermoplasts, and to identify possible failure causes which reduce the lifetime of these films. A comparative XPS study of different deposited films and embossing parameters resulted in a better understanding of the film/polymer interactions at high temperatures. The loss of fluorine was found to be a major cause of the film quality degradation. It resulted from the diffusion of fluorinated entities or small polymer chains from the film to the embossed material during the embossing process. This led us to make some suggestions for the optimization of the working parameters in the hot embossing technique.

AFM measurements of the topography and the roughness of ECR plasma treated polypropylene

Abstract Polypropylene (PP) samples have been treated in an ECR-rf plasma with several gases and at different treatment times and rf-potentials. Modifications of the surface topography have been analyzed by AFM and the results were correlated with previous XPS measurements of the surface chemistry. Plasma treatments with reactive gases (N 2 , O 2 ) lead to the incorporation of new chemical species in the PP surface, whereas plasma treatments with noble gases (He, Ar, Xe) induce a desorption of hydrogen and a graphitization. The untreated PP sample has a rough surface with a granular structure. Plasma treatments with reactive gases induce weak morphology changes, but no new defined structures. Moreover, the modifications of the surface roughness are very sensitive to the treatment conditions. Noble gas plasma treatments, on the contrary, create a completely new surface morphology, which consists of a network of chains of 40–100 nm in diameter oriented in a random way. The size and the shape of these structures are very sensitive to the nature of the gas and to the treatment conditions (ion energy and dose, total energy deposition).