Franz Faupel

Advances in top-down and bottom-up surface nanofabrication: techniques, applications & future prospects.

This review highlights the most significant advances of the nanofabrication techniques reported over the past decade with a particular focus on the approaches tailored towards the fabrication of functional nano-devices. The review is divided into two sections: top-down and bottom-up nanofabrication. Under the classification of top-down, special attention is given to technical reports that demonstrate multi-directional patterning capabilities less than or equal to 100 nm. These include recent advances in lithographic techniques, such as optical, electron beam, soft, nanoimprint, scanning probe, and block copolymer lithography. Bottom-up nanofabrication techniques--such as, atomic layer deposition, sol-gel nanofabrication, molecular self-assembly, vapor-phase deposition and DNA-scaffolding for nanoelectronics--are also discussed. Specifically, we describe advances in the fabrication of functional nanocomposites and graphene using chemical and physical vapor deposition. Our aim is to provide a comprehensive platform for prominent nanofabrication tools and techniques in order to facilitate the development of new or hybrid nanofabrication techniques leading to novel and efficient functional nanostructured devices.

Diffusion in metallic glasses and supercooled melts

Amorphous metallic alloys, also called metallic glasses, are of considerable technological importance.The metastability of these systems, which gives rise to various rearrangement processes at elevatedtemperatures, calls for an understanding of their diffusional behavior. From the fundamental point ofview, these metallic glasses are the paradigm of dense random packing. Since the recent discovery ofbulk metallic glasses it has become possible to measure atomic diffusion in the supercooled liquid stateand to study the dynamics of the liquid-to-glass transition in metallic systems. In the present article theauthors review experimental results and computer simulations on diffusion in metallic glasses andsupercooled melts. They consider in detail the experimental techniques, the temperature dependenceof diffusion, effects of structural relaxation, the atom-size dependence, the pressure dependence, theisotope effect, diffusion under irradiation, and molecular-dynamics simulations. It is shown thatdiffusion in metallic glasses is significantly different from diffusion in crystalline metals and involvesthermally activated, highly collective atomic processes. These processes appear to be closely related tolow-frequency excitations. Similar thermally activated collective processes were also found to mediatediffusion in the supercooled liquid state well above the caloric glass transition temperature. Thisstrongly supports the mode-coupling scenario of the glass transition, which predicts an arrest ofliquidlike flow already at a critical temperature well above the caloric glass transition temperature.

Design of a Perfect Black Absorber at Visible Frequencies Using Plasmonic Metamaterials

The design and fabrication of a plasmonic black absorber with almost 100% absorbance spanning a broad range of frequencies from ultraviolet (UV) to the near infrared (NIR) is demonstrated. The perfect plasmonic absorber is achieved by a combination of a metal film with suitable metal/dielectric nanocomposites. Our fabrication technique is simple, versatile, cost-effective, and compatible with current industrial methods for solar absorber production.

Diffusion of metals in polymers

Abstract First information on metal diffusion in polymers resulted from surface spectroscopies which mainly provided insight into chemical interactions of metals at polymer surfaces and into their growth mode. Medium energy ion scattering, electron microscopy, atomic force microscopy, and second-harmonic generation revealed a strong tendency of metals of low and intermediate reactivity to form clusters when deposited onto polymers. The interplay of diffusion and aggregation was also studied by Monte Carlo simulations. Metal diffusivities were obtained from radiotracer and Rutherford backscattering measurements. The available results show that reactive metals do not have any long-range mobility and are effective diffusion barriers. In contrast, isolated atoms of less reactive metals diffuse deep into polymers at elevated temperatures. However, the very pronounced aggregation tendency of these metals effectively impedes diffusion unless they are deposited at rates of the order of monolayers per minute or lower. Nevertheless, traces of noble metals always diffuse into polymers during the early stages of metal deposition, whereas no significant diffusion occurs from a continuous metal film. Even noble metal diffusivities are many orders of magnitude smaller than diffusivities of non-reactive gas molecules and largely decoupled from polymer dynamics. This is attributed to a pronounced reduction in the local chain mobility near metal atoms, e.g., by temporary metal-atom-induced crosslinking.

Plasmonic properties of Ag nanoclusters in various polymer matrices

Nanocomposite films containing Ag nanoparticles embedded in a polymer matrix of Teflon AF, poly(methyl methacrylate) (PMMA) and Nylon 6 were prepared by vapour phase co-deposition in high vacuum. A large variation of the particle plasmon resonance frequency in the visible region was obtained by increasing the Ag volume fraction from 4-80%. The metal volume fraction was measured by energy dispersive x-ray spectrometry (EDX) and the film thickness was measured by surface profilometry. The position, width and strength of the plasmon resonance depend strongly on the metal filling factor, cluster size and interparticle distance. The microstructure of the nanocomposites (shape, size, size distribution and interparticle separation of metal clusters) was determined by transmission electron microscopy. The effect of the surrounding dielectric medium on the optical properties of nanocomposites was investigated by comparing the Teflon AF/Ag, PMMA/Ag and Nylon/Ag composites.

Tunable multiple plasmon resonance wavelengths response from multicomponent polymer-metal nanocomposite systems

A method of vapor phase codeposition has been used at elevated target temperature coupled with selective masking technique to produce a multicomponent optical material with seven different ultraviolet-visible optical active planes. Such a system consists of different nanocomposites of Teflon AF/Ag, Teflon AF/Au, and Teflon AF/Cu with sharp interfaces. The fabricated system has regions of single nanocomposite plane together with multilayer structures facilitating tunable multiple particle plasmon resonance wavelength response from a single system. Distinct double and triple plasmon wavelength resonance absorptions could be obtained from such interfaces forming multilayers of either two or three nanocomposite structures.

Correlation between fractional free volume and diffusivity of gas molecules in glassy polymers

Despite its oversimplifications, the free-volume approach has proven to provide very useful correlative and even semipredictive capabilities. This article is concerned with the correlation between the diffusivity, D, of gas molecules in glassy polymers and the fractional free volume, FFV, determined by the Bondi method. The diffusivities were taken from a database, generated by the authors in connection with work on the new Landolt Bornstein series “Diffusion in Non-Metallic Solids”, which encompasses a very large variety of glassy polymers. For a given diffusant log D is a linear function of 1/FFV as predicted by the free volume theory. However, the deviations from this relationship are significant and strongly correlated between O2, N2, CO2, and CH4. The systematic deviations are opposite to the predicted effect of the polymer jumping unit size in the free-volume concept of Vrentas and Duda and are interpreted in terms of an increase in the activation energy with increasing chain stiffness. Correlation analysis further suggests that the diffusion mechanisms of H2 and particularly of He differ markedly from that of larger gas molecules. Furthermore the influence of the cohesive energy and the glass-transition temperature of the polymers is investigated.

Physico-chemical and antimicrobial properties of co-sputtered Ag?Au/PTFE nanocomposite coatings

In this work, we used co-sputtering of noble metals together with polytetrafluorethylene (PTFE) as a method for producing antibacterial metal/polymer nanocomposite coatings, where the precious metals are only incorporated in a thin surface layer. Moreover, they are finely dispersed as nanoparticles, thus saving additional material and providing a very large effective surface for metal ion release. Nanocomposite films with thickness between 100 and 300 nm were prepared with a wide range of metal filling between 10 and 40%. The antimicrobial effect of the nanocomposite coatings was evaluated by means of two different assays. The bactericidal activity due to silver release from the surface was determined by a modification of conventional disc diffusion methods. Inhibition of bacterial growth on the coated surface was investigated through a modified proliferation assay. Staphylococcus aureus and S. epidermidis were used as test bacteria, as these species commonly cause infections associated with medical polymer devices. The antibacterial efficiency of the coatings against different bacteria was demonstrated at extremely small noble metal consumption: Au: ~1 mg m−2 and Ag: ~0.1 g m−2. The maximum ability for having an antibacterial effect was shown by the Ag–Au/PTFE nanocomposite, followed by the Ag/PTFE nanocomposite.

Controlled syntheses of Ag?polytetrafluoroethylene nanocomposite thin films by co-sputtering from two magnetron sources

Co-sputtering from two independent magnetron sources was used to prepare polymer?metal nanocomposite films. Both gradient films with increasing metal fraction and homogeneous composite films were produced from polytetrafluoroethylene?(PTFE) and silver targets using a rotatable sample holder. The structure of the pure sputtered polymer as well as the composite structure was studied. Electrical properties of the composite material near the percolation threshold show the expected, sharp change in the resistivity from 107???cm atsmall silver content to 10?3???cm after percolation. The optical absorption in the visible region due to surface plasmon resonances also has a strong dependence on the metal content, showing a red shift of the absorption peak from 405?nm to more than 500?nm at higher silver content.

Review of Plasmonic Nanocomposite Metamaterial Absorber

Plasmonic metamaterials are artificial materials typically composed of noble metals in which the features of photonics and electronics are linked by coupling photons to conduction electrons of metal (known as surface _lasmon). These rationally designed structures have spurred interest noticeably since they demonstrate some fascinating properties which are unattainable with naturally occurring materials. Complete absorption of light is one of the recent exotic properties of plasmonic metamaterials which has broadened its application area considerably. This is realized by designing a medium whose impedance matches that of free space while being opaque. If such a medium is filled with some lossy medium, the resulting structure can absorb light totally in a sharp or broad frequency range. Although several types of metamaterials perfect absorber have been demonstrated so far, in the current paper we overview (and focus on) perfect absorbers based on nanocomposites where the total thickness is a few tens of nanometer and the absorption band is broad, tunable and insensitive to the angle of incidence. The nanocomposites consist of metal nanoparticles embedded in a dielectric matrix with a high filling factor close to the percolation threshold. The filling factor can be tailored by the vapor phase co-deposition of the metallic and dielectric components. In addition, novel wet chemical approaches are discussed which are bio-inspired or involve synthesis within levitating Leidenfrost drops, for instance. Moreover, theoretical considerations, optical properties, and potential application of perfect absorbers will be presented.