Polymer-on-metal interfaces

In the many reports on photoelectron spectroscopy, studies on the interface formation between PPVs and metals, focus mainly on the two most commonly used top electrode metals in polymer light emitting device structures, namely aluminum [55-62] and calcium [62-67]. Other metals studied include chromium [55, 68], gold [69], nickel [69], sodium [70, 71], and rubidium [72], For the cases of nickel, gold, and chromium deposited on top of the polymer surfaces, interactions with the polymers are reported [55, 68]. In the case of the interface between PPV on top of metallic chromium, however, no interaction with the polymer was detected [55]. The results concerning the interaction between chromium and PPV indicates two different effects, namely the polymer-on-metal versus the metal-on-polymer interface formation. Next, the PPV interface formation with aluminum and calcium will be discussed in more detail. 

It is perhaps obvious that the nature of the interface between a molecular solid (polymer) and a (clean) metal surface is not necessarily equivalent to the interface formed when a metal is vapor-deposited (essentially atom-by-atom ) on to the (clean) surface of the polymer or molecular solid. Atoms of all metals are active in the form of individual atoms , even gold atoms. In the context of the new polymer LEDs, some of the works discussed in chapter 7 involve the study of the early stages of formation of the interface in the latter configuration (metal-on-polymer interfaces). Very little has been reported on conjugated polymer-on-metal interfaces, however, primarily because of the difficulties in preparing monolayers of polymer materials on well defined metal substrates appropriate for study (via PES or any other surface sensitive spectroscopy). The issues discussed below are based upon information accumulated over two decades of involvement with the surfaces of condensed molecular solids and conjugated polymers in ultra-thin form, represented by the examples in the previous chapter. 

Most of the illustrative examples will come from polyimide-metal interface studies directed at investigating the role of interfacial chemistry in adhesion at these interfaces and the non-equivalence of polymer-on-metal and metal-on-polymer interfaces. 

The metal-on-polymer interface has been the most studied Interface as metals can conveniently be deposited by evaporation in situ 1n a controllable fashion in a UHV system (26-33). In the case of polyimide, Cu and Cr have been the most studied metals but other metals including N1, Co, Al, Au, Ag, Ge, Ce, Cs, and Si have been studied. The best experimental arrangement includes a UHV system with a load lock Introduction chamber, a preparation chamber with evaporators, heating capabilities, etc., and a separate analysis chamber. All the chambers are separated by gate valves and the samples are transferred between chambers under vacuum. Alternative metal deposition sources such as organometall1c chemical vapor deposition are promising and such techniques possibly can lead to different interface formation than obtained by metal evaporation(34). 

Some details of the electronic structure of a specific metal-on-polymer interface have been presented to illustrate the importance of such interfaces in determining... 

Salaneck, W. R., and Bredas, J. L., The metal-on-polymer interface in polymer light-emitting-diodes, Adv. Mater., 8, 48 (1996). 

The aim of this chapter is to present a simple but general band structure picture of the metal-semiconductor interface and compare that with the characteristics of the metal-conjugated polymer interface. The discussion is focused on the polymer light emitting diode (LED) for which the metal-polymer contacts play a central role in the performance of the device. The metal-polymer interface also applies to other polymer electronic devices that have been fabricated, e.g., the thin-film field-effect transistor3, but the role of the metal-polymer interface is much less cruical in this case and... 

In the following discussion of the electronic properties of metal-semiconductor interfaces, the properties of the electron-injecting contact are taken as the example for contacts. Most of our studies of metals on polymers have involved low work function metals with the... 

The often used FPL etdi of an aluminum-lithium alloy bonded with polysulfone leads to interfacial (at the metal oxide/polymer interface) failure (38) which is a surprisingly uncommon type of failure. The results leading to this assignment are shown as XPS C Is and O Is narrow scan spectra in Figure 15. This definitive assignment of failure mode is based on the fact that one failure surfece has an oi gen photopeak similar to the pretreated adherend before bonding and the other failure surfece has an 0 gen photopeak similar to the adhesive. 

Some of the telecommunication cable plant components are made with met2ils embedded in polymers, and the metals are exposed to the corrosive environment only at very small areas (e.g., edges or sm2dl defect sites on the polymer). Test methods are used to ev duate the penetrability of the metal-to-polymer interface and the ensuing progressive corrosion of the metal. 

Studies of the interface characteristics are of great importance in connection with various device applications. Investigations of initial stages of both metal-on-polymer and polymer-on-metal interface formation are of importance in the context of how the interfaces may affect the actual device performance. In other words, when a few isolated atoms of an active metal are applied in some way to a polymer surface, they will react in a very different way than when an isolated polymer chain is applied in some way to the clean surface of an otherwise three-dimensional metal substrate with or without the presence of an oxide layer [lOJ. 

C. Fredriksson and S. Stafstrom, Metal/conjugated polymer interfaces sodium, magnesium, aluminum and calcium on truns-polyacetylene, J. Chem. Phys. /0/ 9137 (1994). 

Friedrich et al. also used XPS to investigate the mechanisms responsible for adhesion between evaporated metal films and polymer substrates [28]. They suggested that the products formed at the metal/polymer interface were determined by redox reactions occurring between the metal and polymer. In particular, it was shown that carbonyl groups in polymers could react with chromium. Thus, a layer of chromium that was 0.4 nm in thickness decreased the carbonyl content on the surface of polyethylene terephthalate (PET) or polymethylmethacrylate (PMMA) by about 8% but decreased the carbonyl content on the surface of polycarbonate (PC) by 77%. The C(ls) and 0(ls) spectra of PC before and after evaporation of chromium onto the surface are shown in Fig. 22. Before evaporation of chromium, the C(ls) spectra consisted of two components near 284.6 eV that were assigned to carbon atoms in the benzene rings and in the methyl groups. Two additional... 

On the other hand, Doblhofer218 has pointed out that since conducting polymer films are solvated and contain mobile ions, the potential drop occurs primarily at the metal/polymer interface. As with a redox polymer, electrons move across the film because of concentration gradients of oxidized and reduced sites, and redox processes involving solution species occur as bimolecular reactions with polymer redox sites at the polymer/solution interface. This model was found to be consistent with data for the reduction and oxidation of a variety of species at poly(7V-methylpyrrole). This polymer has a relatively low maximum conductivity (10-6 - 10 5 S cm"1) and was only partially oxidized in the mediation experiments, which may explain why it behaved more like a redox polymer than a typical conducting polymer. 

In this chapter we describe the basic principles involved in the controlled production and modification of two-dimensional protein crystals. These are synthesized in nature as the outermost cell surface layer (S-layer) of prokaryotic organisms and have been successfully applied as basic building blocks in a biomolecular construction kit. Most importantly, the constituent subunits of the S-layer lattices have the capability to recrystallize into iso-porous closed monolayers in suspension, at liquid-surface interfaces, on lipid films, on liposomes, and on solid supports (e.g., silicon wafers, metals, and polymers). The self-assembled monomolecular lattices have been utilized for the immobilization of functional biomolecules in an ordered fashion and for their controlled confinement in defined areas of nanometer dimension. Thus, S-layers fulfill key requirements for the development of new supramolecular materials and enable the design of a broad spectrum of nanoscale devices, as required in molecular nanotechnology, nanobiotechnology, and biomimetics [1-3]. 

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