Jörn Kanzow

Radiotracer measurements as a sensitive tool for the detection of metal penetration in molecular-based organic electronics

The metallization of organic thin films is a crucial point in the development of molecular electronics. However, there is no method established yet to detect trace amounts of metal atoms in those thin films. Radiotracer measurements can quantify even very small amounts of material penetrating into the bulk, in our case less than 0.01% of a monolayer. Here, the application of this technique on two different well-characterized organic thin film systems (diindenoperylene and pentacene) is demonstrated. The results show that Ag is mainly adsorbed on the surface, but indicate that already at moderate deposition temperatures Ag can penetrate into the organic thin films and agglomerate at the film/substrate interface.

Positron Annihilation Spectroscopy in Polymers

During the last decades positron annihilation has become a very powerful tool for the investigation of polymers. In particular, positron annihilation lifetime spectroscopy (PALS) yields valuable information on free volume and other properties. The present invited paper gives examples from recent research of the Kiel group. Generally, the so-called standard model, developed by Tao and Eldrup, is used to determine the size of free volume holes from the ortho-positronium (o-Ps) lifetime o-Ps. Despite its success, the model resorts to several assumptions, including a spherical hole shape. Although the deviations from spherical shape are significant for holes above the size of positronium, average hole sizes Vh, determined by the standard model from o-Ps, show a good correlation with diffusivities D of inert gas molecules when plotted as 1/Vh vs logD, as predicted by the free volume approach. The correlation can further be improved by taking into account the cohesive energy density of the polymers. The o-Ps intensity Io-Ps is often taken as a measure of the hole density. However, Io-Ps is also affected by the Ps formation probability and drops during mechanical milling of polymers due to formation of free radicals by chain scission, for instance. IoPs is also seen to change during phase separation in polymer blends. This can be explored to detect both, the binodal and the spinodal decomposition, already at the initial stage which is not easily accessible by other techniques. PALS was also used to study thermosets. Here we show in-situ results on the cross-linking of an epoxy resin. Finally, we demonstrate the benefits of the positron beam technique which allows investigations of polymer thin films and surfaces. For example, very recent results, obtained at the positron beam in Munich, on the structure and dynamics of epoxy films as function of film thickness will be presented.

Ag-Diffusion in the Organic Semiconductor Diindenoperylene

The metallization of organic thin films is a crucial point in the development of organic electronic devices. There is no method established yet to detect trace amounts of metal atoms in the organic thin films after metal deposition. Radiotracer measurements are probably the most sensitive tool to study diffusion and to quantify even very small amounts of material penetrating into the bulk. So far, this has been shown for metals and polymers, but not for thin ordered molecular organic films. Here, the first application of this technique on a well-characterized organic thin film system, diindenoperylene using thermally evaporated Ag containing 110mAg radiotracers is shown. The results show that Ag is mainly adsorbed on the surface, but indicate that already at moderate substrate temperatures small concentrations of Ag can penetrate into the organic thin films and agglomerate at the interface during metallization.

Nanocomposites of vapour phase deposited Teflon AF containing Ni clusters

Two component nanocomposites of polymer containing 3d transition metal clusters are of immense interest because of their potential future applications in magnetic high frequency or data storage media, in particular: However, nanoscale engineering of such airstable 3d clusters protected in polymer layers is a major technological challenge, given the readily reactive nature of such nanoparticles. In this invited paper, we introduce relatively simple vapour phase deposition as a powerful solvent free technique to prepare polymer nanocomposites containing 3d transition metal clusters. The different vapour phase production routes investigated show that the technique lends itself to the formation of metallic clusters of the appropriate size while reactions of the reactive Ni atoms with the growing Teflon AF matrix, contrary to common belief, do not take place. In addition to other results, a promising route of vapour phase tandem evaporation to produce nanoclusters of reactive Ni protected between the capping layers of Teflon AF is particularly presented. Independent control over homogeneously distributed particle size ( similar to 3 - 10 nm) is discussed, while increasing a cluster volume filling from similar to 1 to 20 % by following co-deposition to many sequential thermal evaporation of the two components. TEM electron diffraction investigations have shown that crystal structure of such nanoclusters to be fee, confirming the bulk Ni crystal parameters.

Free Volume Evolution in Bulk Metallic Glass during High Temperature Creep

Metallic glasses lack long-range translational symmetry and have excess volume trapped within their amorphous structure, which has a direct bearing on their physical properties including deformation characteristics. Moreover, the trapped excess free volume is directly correlated to the defect concentration facilitating the possibility to model the temperature and time dependence of the free volume changes during creep as a trade off between defect generation and annihilation. Using differential scanning calorimetry (DSC) analysis the residual free volume of a metallic glass can be characterised based on the glass transition peak height (Δ c p ). In the present work constant strain rate tests were carried out at the ‘onset’ ( T g on = 685 K) and ‘point of inflection’ ( T g p = 705 K) of the calorimetric glass transition to study the time dependent flow behaviour in Zr 55 Cu 30 Al 10 Ni 5 bulk metallic glass. Modelling based on DSC analysis and positron lifetime spectroscopy on samples creep deformed to different plastic strain values corroborate the stress decrease after the peak stress (‘stress overshoot’) occurring in bulk metallic glasses with increasing plastic strain to be associated with a small increase in free volume.

Radiotracer Diffusion Measurements of Noble Metal Atoms in Semiconducting Organic Films.

The application of organic field effect transistors (OFETs) for large scale low-cost electronic devices has lead to intense research. Diindenoperylene (DIP) thin films on SiO 2 are a prominent system due to their high structural out-of-plane order. While bottom contact OFET structures can be realized easily, preparation of top contacts might cause diffusion of metal atoms (typically Ag or Au) deep into the organic film changing the injection properties at the interface. These properties are of great importance for device fabrication. Therefore, only by understanding the diffusion behaviour of metals into the organic layer, formation of well defined interfaces and control of their properties will become possible. For a better understanding of the diffusion of noble metal atoms into crystalline organic films, a radiotracer technique has been used to obtain diffusion profiles for Ag and Au diffusion in crystalline DIP films. For Ag diffusion in DIP, the decrease in Ag concentration of four orders of magnitude within the first few nanometers indicates that most of the metal atoms remain near the surface while small amounts can penetrate deep into the thin film and can even accumulate at the interface between organic film and the silicon substrate. A comparison with diffusion profiles obtained for polymers indicates that the interplay between diffusion and immobilization by aggregation also determine the diffusion behaviour of metals in organic crystalline materials. Latest experiments support this interpretation of the diffusion profiles. Single atoms are highly mobile in the organic crystalline material due to the weak interaction between the metal and the organic material. Therefore, most of the single atoms that penetrate into the material do so during the initial phase of the deposition. When more and more atoms arrive at the surface, cluster formation sets in. Due to the high cohesive energy of the metal the atoms can not leave the cluster and become immobilized. After deposition of a closed surface layer no further metal diffusion should be observed. With the knowledge about the diffusion processes gained by the radiotracer measurements control of process parameters and development of barrier layers in sub-monolayer range should be possible.