Wolfgang E. S. Unger

Chemical analysis of plasma-polymerized films: The application of X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (NEXAFS) and fourier transform infrared spectroscopy (FTIR)

Abstract Selected FTIR, XPS and NEXAFS spectroscopy results obtained with films deposited with different plasma polymerization processes and different monomers (styrene, acetylene, ethylene and butadiene) are presented. In detail FTIR spectra, XPS surveys, XPS valence bands and core level signals including shake-up features as well as carbon K-edge absorption spectra are qualitatively and, in some cases, semi-quantitatively considered. Information on the film formation, the chemistry of the films and the film stability against air exposure are derived from spectroscopic features. With styrene chemically rather well defined plasma polymer layers can be formed with rather high deposition rates applying a pulse plasma process. Using ethylene, acetylene or butadiene as a monomer in the plasma deposition process the deposition rates are smaller. Using these monomers plasma polymer films can be obtained with a primary chemical structure which is similar to each other but more or less different from those of the respective conventional polymers. The main difference between these samples is their individual concentration of unsaturated carbon species. Finally, a technologically relevant example, i.e. a plasma deposited polymer barrier layer deposited on the inner wall of a poly(ethylene) vessel is briefly considered.

The European nanometrology landscape

This review paper summarizes the European nanometrology landscape from a technical perspective. Dimensional and chemical nanometrology are discussed first as they underpin many of the developments in other areas of nanometrology. Applications for the measurement of thin film parameters are followed by two of the most widely relevant families of functional properties: measurement of mechanical and electrical properties at the nanoscale. Nanostructured materials and surfaces, which are seen as key materials areas having specific metrology challenges, are covered next. The final section describes biological nanometrology, which is perhaps the most interdisciplinary applications area, and presents unique challenges. Within each area, a review is provided of current status, the capabilities and limitations of current techniques and instruments, and future directions being driven by emerging industrial measurement requirements. Issues of traceability, standardization, national and international programmes, regulation and skills development will be discussed in a future paper.

Homofunctionalized polymer surfaces formed by selective plasma processes

Abstract Several possibilities exist to produce a modified polymer surface with a high density of only one sort of functional group such as: (i) the plasma grafting of unfragmented monomer molecules and their polymerization forms OH, NH2, COOH groups, etc. in concentrations of approximately 25 groups per 100 C atoms; (ii) selective plasma bromination provides 10–25 CBr groups; (iii) the plasma oxidation of polymer surfaces in an O2 plasma followed by the chemical reduction of all O-containing groups to OH groups by diborane, vitride™ (Na complex) or LiAlH4 yields 9–14 OH groups per 100 carbon atoms; and (iv) the grafting of spacers with different endgroups onto OH or CBr groups produces 7–10 spacer molecules/100 C. This work was focused on the formation of thin plasma deposited polymer layers with a maximum of (homo)functional groups and with a minimum of chemical irregularities using the pulsed plasma technique. The monomers were allylalcohol, allylamine, acrylonitrile and acrylic acid. The further intent was to study the interactions of functional groups (OH, COOH, NH2) and deposited metals (Cr, Al, Ti). It was expected that more basic (NH2), weakly basic or neutral (OH) or more acidic (COOH) groups would show different interactions and chemical reactions with metal atoms.

Oxygen plasma induced degradation of the surface of poly(styrene), poly(bisphenol-A-carbonate) and poly(ethylene terephthalate) as observed by soft X-ray absorption spectroscopy (NEXAFS)

Biaxially stretched poly(ethylene terephthalate) foils, poly(ethylene terephthalate) and poly(styrene) spin coated films as well as self-supporting poly(bisphenol-A-carbonate) films were analysed by surface sensitive near edge X-ray absorption fine structure spectroscopy (NEXAFS) before and after low pressure oxygen DC plasma treatments. The degradation effects of the treatment are discussed in a common way in terms of probable reaction pathways. The results are, when possible, cross-checked by XPS. NEXAFS analysis provides data on the phenyl ring, ester and carbonate group degradation in a straightforward manner. It can be shown that the phenyl rings in partially crystalline poly(ethylene terephthalate) are, in comparison, most effectively protected against plasma attack. In the case of poly(styrene), where the rings are located as side chains, they are most efficiently degraded.

Comprehensive comparison of various techniques for the analysis of elemental distributions in thin films

In a recent publication by Abou-Ras et al., various techniques for the analysis of elemental distribution in thin films were compared, using the example of a 2-µm thick Cu(In,Ga)Se2 thin film applied as an absorber material in a solar cell. The authors of this work found that similar relative Ga distributions perpendicular to the substrate across the Cu(In,Ga)Se2 thin film were determined by 18 different techniques, applied on samples from the same identical deposition run. Their spatial and depth resolutions, their measuring speeds, their availabilities, as well as their detection limits were discussed. The present work adds two further techniques to this comparison: laser-induced breakdown spectroscopy and grazing-incidence X-ray fluorescence analysis.The present work shows results on elemental distribution analyses in Cu(In,Ga)Se2 thin films for solar cells performed by use of wavelength-dispersive and energy-dispersive X-ray spectrometry (EDX) in a scanning electron microscope, EDX in a transmission electron microscope, X-ray photoelectron, angle-dependent soft X-ray emission, secondary ion-mass (SIMS), time-of-flight SIMS, sputtered neutral mass, glow-discharge optical emission and glow-discharge mass, Auger electron, and Rutherford backscattering spectrometry, by use of scanning Auger electron microscopy, Raman depth profiling, and Raman mapping, as well as by use of elastic recoil detection analysis, grazing-incidence X-ray and electron backscatter diffraction, and grazing-incidence X-ray fluorescence analysis. The Cu(In,Ga)Se2 thin films used for the present comparison were produced during the same identical deposition run and exhibit thicknesses of about 2 μm. The analysis techniques were compared with respect to their spatial and depth resolutions, measuring speeds, availabilities, and detection limits.

Studies of amorphous carbon using X-ray photoelectron spectroscopy, near-edge X-ray-absorption fine structure and Raman spectroscopy

Abstract We report core level and valence band X-ray photoelectron spectroscopy (XPS), carbon K and oxygen K near-edge X-ray-absorption fine structure spectroscopy (NEXAFS), and Raman spectroscopy results of plasma-deposited amorphous carbon generated from fullerene C 60 . In comparison with evaporated C 60 , the C 1 s peak is broader and asymmetric for the amorphous carbon film and its shake-up satellites disappear. The valence band shows three fairly broad peaks. Only one prominent π * resonance occurs in the NEXAFS spectrum. Recognizable structures appear in the σ * region indicating the formation of new bonds. In the Raman spectrum the typical D and G bands were observed. The amorphization of C 60 and post-plasma functionalization of the surface after exposure to atmosphere cause changes in the electronic structure.

Chemical reactions at polymer surfaces interacting with a gas plasma or with metal atoms — their relevance to adhesion

Abstract The chemical and morphological stabilities of polymer segments in the near-surface layer were investigated by spectroscopic methods such as X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. Model studies were undertaken with Langmuir–Blodgett films, self-assembled monolayers and oligomer films. For thin polymer layers (30 to 500 nm), the changes in molecular-weight distributions of some polymers were investigated systematically by size exclusion chromatography, matrix-assisted laser desorption/ionization–time-of-flight mass spectrometry and thermal-field flow fractionation for oxygen- and helium-plasma exposures. The polymer surfaces were found to be relatively stable at exposure to an oxygen low-pressure plasma up to ca. 2 s. This is important information to get maximum adhesion to metals in composites. In correlation to their redox potentials, potassium, aluminium and chromium react with oxygen functional groups at the polymer/metal interface. In a dedicated study, chromium was found to attack aromatic rings and form different reaction products.

Plasma-based introduction of monosort functional groups of different type and density onto polymer surfaces. Part 2: Pulsed plasma polymerization

This new functionalization method consists of the deposition of very thin plasma polymer layers (20 to 100 nm) of functional group bearing monomers in pulsed plasma. With allylalcohol, a maximum of 30 OH groups per 100 C atoms was measured with a selectivity of about 90% and a significant stability at long-time exposure to air. Allylamine was used to produce primary amino groups, with a maximum of 18 NH2groups per 100 C atoms. Side reactions were observed during the storage in air, such as oxidation of the amino groups. Carboxylic groups could be produced using acrylic acid with a maximum concentration of 24 COOH groups per 100 C atoms. The most prominent side reaction was the decarbonylation/ decarboxylation of the acid group during plasma deposition. The variation of the density of functional groups using the pulsed-plasma polymerization of functional-group-bearing monomers was possible by the chemically-initiated radical copolymerization with either a chain-extending monomer, such as ethylene, or a cross linker, such as butadiene, in plasma. The density of functional groups could be adjusted continuously (0 to 30 OH, 0 to 18 NH2 and 0 to 24 COOH groups per 100 C atoms). The successful application of these densely functionalized polymer surfaces for producing biocompatible surfaces and for use in metal–polymer composites is proposed.

Influence of hydrogen on the analytical figures of merit of glow discharge optical emission spectroscopy—friend or foe?

It is well known that the presence of small quantities of hydrogen in an argon glow discharge (GD) causes serious alterations to the excitation and ionisation mechanisms in the GD plasma and hence to the analytical signals. This so-called “hydrogen effect” leads also to a change in the shape of the sputtering crater and its roughness. The present work shows how the manifold effects of hydrogen can be exploited in glow discharge optical emission spectroscopy (GD-OES) in order to improve analytical figures of merit such as analytical sensitivity, detection limits and depth resolution. Other problems caused by the presence of hydrogen, e.g. the occurrence of hydride bands, are demonstrated and discussed. It is shown that start-up phenomena leading to a falsification of the initial part of GD-OES depth profiles can be partially inhibited by controlling the GD source cleanness, and possibly by adding hydrogen.

Coupled Molecular Switching Processes in Ordered Mono- and Multilayers of Stimulus-Responsive Rotaxanes on Gold Surfaces

Interfaces provide the structural basis for function as, for example, encountered in nature in the membrane-embedded photosystem or in technology in solar cells. Synthetic functional multilayers of molecules cooperating in a coupled manner can be fabricated on surfaces through layer-by-layer self-assembly. Ordered arrays of stimulus-responsive rotaxanes undergoing well-controlled axle shuttling are excellent candidates for coupled mechanical motion. Such stimulus-responsive surfaces may help integrate synthetic molecular machines in larger systems exhibiting even macroscopic effects or generating mechanical work from chemical energy through cooperative action. The present work demonstrates the successful deposition of ordered mono- and multilayers of chemically switchable rotaxanes on gold surfaces. Rotaxane mono- and multilayers are shown to reversibly switch in a coupled manner between two ordered states as revealed by linear dichroism effects in angle-resolved NEXAFS spectra. Such a concerted switching process is observed only when the surfaces are well packed, while less densely packed surfaces lacking lateral order do not exhibit such effects.