Thomas Strunskus

Metal/polymer interfaces with designed morphologies

The morphology of a metal/polymer interface is important for many properties, e.g. its adhesional strength. Starting from the basic processes occurring in the initial stages of metal/polymer interface formation, it is possible to obtain different morphologies by variation of the preparation conditions. In this report we present selected examples from our own work of how metal/polymer interfaces with different morphologies can be prepared by evaporating noble metals (Au, Ag, Cu) onto chemically different polymers, i.e. bisphenol-trimethyl cyclohexane polycarbonate (TMC-PC), pyromellitic dianhydride-oxydianiline (PMDA-ODA) polyimide (PI), and on Teflon AF 1601. The interfaces were characterized using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The combination of these techniques allows one to determine morphological parameters such as the concentration and distribution of metal clusters at the surface and in the near-surface region. Using low deposition rates and elevated temperatures, spread-out metal/polymer interfaces can be formed, whereas the use of high deposition rates and moderate temperatures results in relatively sharp interfaces. Another approach to obtain a defined morphology is to form large metal clusters of 10-30 nm diameter on the polymer surface and embed them into the polymer in a controlled manner by a subsequent annealing process. First experiments on the macroscopic adhesion of Au and Cu on TMC-PC showed that the initially low peel strength could be increased substantially by subsequent annealing above the glass transition temperature.

Functional Polymer Nanocomposites

While extensive research has been carried out in the field of structural polymer-based nanocomposites much less investigations have been concerned with polymer nanocomposites for functional applications. Among the functional nanomaterials, nanocomposites consisting of metal nanoparticles dispersed in a dielectric matrix are of particular interest due to their novel functional properties offering hosts of new applications. Here, polymers are attractive as matrix, and several approaches have been reported to incorporate metal nanoparticles into polymers. The present review is concerned with the preparation of polymer-based nanocomposites by vapor phase co-and tandem deposition and the resulting functional properties. The techniques involve evaporation and sputtering, respectively, of metallic and organic components and inter alia allow the preparation of composites which contain alloy clusters of well defined composition. Emphasis is placed on soft-magnetic high frequency materials with cut-off frequencies well above 1 GHz and on optical composites with tuned plasmon resonances suitable for ultra thin color filters, Bragg reflectors, and other devices. In addition, antibacterial coatings and sensors for organic vapors are addressed. The latter take advantage of the steep drop of the electrical resistivity at the percolation threshold. First results are also reported on the incorporation of photo-switchable molecules into nanocomposites near the percolation threshold. Moreover, a novel approach to produce magnetic nanorods is presented.

Real-Time Monitoring of Morphology and Optical Properties during Sputter Deposition for Tailoring Metal−Polymer Interfaces

The reproducible low-cost fabrication of functional metal-polymer nanocomposites with tailored optoelectronic properties for advanced applications remains a major challenge in applied nanotechnology. To obtain full control over the nanostructural evolution at the metal-polymer interface and its impact on optoelectronic properties, we employed combined in situ time-resolved microfocus grazing incidence small angle X-ray scattering (μGISAXS) with in situ UV/vis specular reflectance spectroscopy (SRS) during sputter deposition of gold on thin polystyrene films. On the basis of the temporal evolution of the key scattering features in the real-time μGISAXS experiment, we directly observed four different growth regimes: nucleation, isolated island growth, growth of larger aggregates via partial coalescence, and continuous layer growth. Moreover, their individual thresholds were identified with subnanometer resolution and correlated to the changes in optical properties. During sputter deposition, a change in optical reflectivity of the pristine gray-blue PS film was observed ranging from dark blue color due to the presence of isolated nanoclusters at the interface to bright red color from larger Au aggregates. We used simplified geometrical assumptions to model the evolution of average real space parameters (distance, size, density, contact angle) in excellent agreement with the qualitative observation of key scattering features. A decrease of contact angles was observed during the island-to-percolation transition and confirmed by simulations. Furthermore, a surface diffusion coefficient according to the kinetic freezing model and interfacial energy of Au on PS at room temperature were calculated based on a real-time experiment. The morphological characterization is complemented by X-ray reflectivity, optical, and electron microscopy. Our study permits a better understanding of the growth kinetics of gold clusters and their self-organization into complex nanostructures on polymer substrates. It opens up the opportunity to improve nanofabrication and tailoring of metal-polymer nanostructures for optoelectronic applications, organic photovoltaics, and plasmonic-enhanced technologies.

Azobenzene-containing triazatriangulenium adlayers on Au(111): structural and spectroscopic characterization.

Adlayers of different azobenzene-functionalized derivatives of the triazatriangulenium (TATA) platform on Au(111) surfaces were studied by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), gap-mode surface-enhanced Raman spectroscopy (gap-mode SERS), and cyclic voltammetry (CV). The chemical composition of the adlayers is in good agreement with the molecular structure, i.e., different chemical groups attached to the azobenzene functionality were identified. Furthermore, the presence of the azobenzene moieties in the adlayers was verified by the vibration spectra and electrochemical data. These results indicate that the molecules remain intact upon adsorption with the freestanding functional groups oriented perpendicularly to the TATA platform and thus also to the substrate surface.

Influence of reactive gas admixture on transition metal cluster nucleation in a gas aggregation cluster source

We quantitatively assessed the influence of reactive gases on the formation processes of transition metal clusters in a gas aggregation cluster source. A cluster source based on a 2 in. magnetron is used to study the production rate of titanium and cobalt clusters. Argon served as working gas for the DC magnetron discharge, and a small amount of reactive gas (oxygen and nitrogen) is added to promote reactive cluster formation. We found that the cluster production rate depends strongly on the reactive gas concentration for very small amounts of reactive gas (less than 0.1% of total working gas), and no cluster formation takes place in the absence of reactive species. The influence of discharge power, reactive gas concentration, and working gas pressure are investigated using a quartz micro balance in a time resolved manner. The strong influence of reactive gas is explained by a more efficient formation of nucleation seeds for metal-oxide or nitride than for pure metal.

Chemistry, diffusion and cluster formation at metal-polymer interfaces

This paper shows how structure and formation of metal-polymer interfaces depend strongly on the preparation process and the interfacial chemistry. Emphasis is placed on results obtained from transmission electron microscopy (TEM), X-ray photo electron spectroscopy (XPS), radiotracer measurements and computer simulations on the early stages of interface formation during noble-metal deposition onto fully cured polymers. Noble metal atoms deposited onto polymers diffuse on and inside the polymer and tend to agglomerate to clusters. XPS results show that no significant diffusion occurs from larger clusters and is therefore also not expected from a continuous metal film. Thus the extent of diffusion into the polymer appears to be determined only by the initial stage of the deposition process and increases strongly at low deposition rates and elevated temperatures, where a large fraction of isolated metal atoms is able to diffuse into the polymer before being trapped by other atoms at or near the surface. Our results point to a strong interplay between chemical interaction, diffusion and agglomeration.

Anomalous surface compositions of stoichiometric mixed oxide compounds.

Surface-oxide films are present in many types of oxidecontaining materials, such as grain boundaries in ceramics, interfaces in ceramic-ceramic and metal-oxide systems, and affect their materials and transport properties. In heterogeneous catalysis, the properties of the outermost surface layer are of prime importance because they control the catalytic performance. Although bulk mixed-metal oxide catalysts are widely used in industrial selective oxidation processes, not much is known about their outermost surface composition. Models based on surfaces derived from a truncation of the bulk structure have dominated discussion on catalytic reaction mechanisms and active sites (reviewed, for example, in Ref. [6]). This view has been questioned by several recent studies reporting the surface enrichment and depletion phenomena in solid-oxide solutions (e.g., CoxNi1 xO ), the identification of TiO2-rich overlayers on reconstructed SrTiO3(001) model surfaces, [8] and evidence for the formation of amorphous oxide overlayers in which there is surface enrichment of one of the components under selective oxidation reaction conditions. However, the development of realistic concepts on reactant activation, surface reaction mechanisms, and the design of advanced catalytic materials are still hampered by the lack of detailed knowledge of the surface composition and structure of bulk mixed-metal oxides. For such studies, X-ray photoelectron spectroscopy (XPS) with laboratory sources is of limited value because its average sampling depth of 1–3 nm results in a signal where the outermost surface layer only contributes on the order of 30%. Synchrotron radiation allows for increasing the surface sensitivity of XPS by decreasing excitation and, hence, photoelectron kinetic energies. Exclusive information on the outermost surface layer, however, is only given by low-energy ion scattering (LEIS) because ions penetrating below the surface become largely neutralized. The surfaces of stoichiometric bulk mixed-metal molybdates and vanadates have also been characterized through their interactions with probe molecules, for example, CH3OH, [12–15] which allows CH3O* and intact CH3OH* intermediates on different surface cations to be discriminated by IR spectroscopy. For such materials, combined methanol chemisorption and oxidation kinetic studies suggested a strong surface enrichment of MoOx or VOx. [12,14,15] In methanol oxidation studies, similar catalytic turnover frequencies were found over bulk mixed-metal oxides and related supported metal oxides (e.g., Fe2(MoO4)3 and MoO3/Fe2O3), which supports the idea of surface MoOx enrichment of the bulk phases. These observations, however, are qualitative as exposed metal oxide ions of low catalytic activity would not be detected by the test reaction. Thus, we have undertaken a study of the outermost surface compositions of such compounds by LEIS and excitation-energy resolved XPS (ERXPS). The LEIS was applied in sputter series taking advantage of its destructive character, the ERXPS is a version utilizing information from different sampling depths. LEIS sputter series from stoichiometric bulk mixed oxides and related supported metal oxides are given in Figure 1 and [*] Dr. S. V. Merzlikin, Prof. Dr. W. Gr nert Lehrstuhl f r Technische Chemie, Ruhr-Universit t Bochum Postfach 102148, 44780 Bochum (Germany) Fax: (+49)234-32-14115 E-mail: w.gruenert@techem.rub.de Homepage: http://www.techem.rub.de Dr. N. N. Tolkachev N. D. Zelinsky Institute of Organic Chemistry Russian Academy of Sciences, Moscow (Russia)

Size dependent characteristics of plasma synthesized carbonaceous nanoparticles

Low temperature plasmas with their strong non equilibrium character offer unique possibilities for the production of nanoparticles. This contribution deals with size dependent properties of nanoparticles synthesized in a capacitively coupled discharge operated in mixtures of argon and acetylene. X-ray absorption measurements show that the particle properties dramatically change during the growth process. For nanoparticles under 10 nm in diameter, near edge x-ray absorption fine structure spectroscopy shows a sp2 rich graphite-like material. The bonding situation changes with the increasing size of the dust particles, showing the formation of a sp2 poor mantle around the sp2 rich core. This phenomenon can be explained in terms of the nucleation and growth process of nanoparticles, i.e., due to differences in the heating of small nanoparticles (nuclei) and due to differences in the gas phase species involved in the nucleation phase and the surface growth phase.

Reversible light-controlled conductance switching of azobenzene-based metal/polymer nanocomposites

We present a new concept of light-controlled conductance switching based on metal/polymer nanocomposites with dissolved chromophores that do not have intrinsic current switching ability. Photoswitchable metal/PMMA nanocomposites were prepared by physical vapor deposition of Au and Pt clusters, respectively, onto spin-coated thin poly(methylmethacrylate) films doped with azo-dye molecules. High dye concentrations were achieved by functionalizing the azo groups with tails and branches, thus enhancing solubility. The composites show completely reversible optical switching of the absorption bands upon alternating irradiation with UV and blue light. We also demonstrate reversible light-controlled conductance switching. This is attributed to changes in the metal cluster separation upon isomerization based on model experiments where analogous conductance changes were induced by swelling of the composite films in organic vapors and by tensile stress.

Plasmonic properties of vapour-deposited polymer composites containing Ag nanoparticles and their changes upon annealing

The effect of temperature on microstructure and optical properties of nanocomposite films containing Ag nanoparticles embedded in a polymer matrix of Teflon AF was investigated in detail. Temperature effects were studied in two modes: the effect of temperature during preparation of the nanocomposites and post-deposition heat treatment. Substrate heating during deposition leads to a decrease in the condensation coefficient up to the glass transition temperature of the polymer and increases beyond this temperature. During heat treatment after deposition metal diffuses into the polymer and leads to a change in the microstructure of the nanocomposites resulting in larger metal cluster size, an increase in the interparticle distance and more spheroidal shaped clusters. An increase in the substrate temperature during deposition can lead to both blue shifts and red shifts of the plasmon peak position. Changes in the microstructure upon heat treatment above the glass transition temperature are caused by diffusion of metal clusters into the polymers. In this regime, the peak wavelength of the plasmon band shifts towards shorter wavelength with increasing temperature.