Aluminum/polymer 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. 

Metal thin films deposited on polymers are widely used in various industrial domains such as microelectronics (capacitors), magnetic recording, packaging, etc. Despite much attention that has been paid in the recent literature on the adhesive properties of metals films on polyimide (PI)( 1 - 5 ) and polyethyleneterephtalate (PET)((L) it appears that a better knowledge of the metal/polymer interface is needed. In this paper we focus ourself on the relationship between the adhesion and the structural properties of the aluminum films evaporated (or sputtered) on commercial bi-axially stretched PET (Du Pont de Nemours (Luxembourg) S.A.). A variety of treatment (corona, fluorine,etc.) have been applied in order to improve the adhesion of the metallic layer to the polymer. The crystallographic... 

Electrified interfaces are predominantly built up by mobile charged species which in the case of metal/polymer interfaces may be identified with ions embedded in the polymeric matrix. Therefore, electrochemically driven reactions prevail in environments which allow the presence of such species. Furthermore, the conservation of charge may require the presence of ion-transfer reactions and this condition is almost always satisfied for reactive metals such as iron, copper, zinc, and aluminum. 

The rate of electron transfer reactions (ETRs) is strongly influenced by the surface composition of the metal. As most materials are covered by oxides, their electronic properties will determine the rate of ETR. Therefore, metals that are covered by electron conducting or semiconducting oxides such as iron or zinc will show a higher ETR rate at the substrate-polymer interface in comparison to materials that form highly insulating oxides such as aluminum. 

Subsequent investigations showed that identical hydration reactions occurred on both bare aluminum surfaces as well as bonded surfaces, but at very different rates of hydration [45]. An Arrhenius plot of incubation times prior to hydration of bare and buried FPL surfaces clearly showed that the hydration process exhibited the same energy of activation ( 82 kJ/mol) regardless of the bare or covered nature of the surface (Fig. 6). On the other hand, the rate of hydration varies dramatically, depending on the concentration of moisture available to react at the oxide/polymer interface or the oxide surface. The epoxy-covered surfaces have incubation times (and rate constants) three to four orders of magnitude longer than bare, immersed specimens, reflecting the limited amount of moisture absorbed by the epoxy and free to react with the oxide. 

In the present paper. Static Secondary Ion Mass Spectrometry (SSIMS) is used to investigate the interfacial chemistry between vacuum-deposited Al and Cu on PET by following the initial stages of metallization in the submonolayer and monolayer regimes. From the SIMS intensity variations with the deposited metal flux, information on the initial growth mechanisms of the metal layer Is expected. Two metals, copper and aluminum, have been chosen In order to investigate the influence of the metal reactivity on the metal-polymer interface formation. Aluminum with its electropositive sp band is known to react strongly with the carbonyl functionalities of the whereas copper is an inert metal and its Interaction is believed to be much weaker. ... 

Theoretical Studies of Metal/Conjugated Polymer Interfaces Aluminum and Calcium Interacting with 7i-Conjugated Systems... 

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. 

In failure analysis the possibility to record all elements is advantageous, not least in combination with 3D imaging (i.e., a TOF-SIMS instrument with dual-beam capability is the instrument of choice). An example is the investigation of black spots in OLEDs where a fluorine-based polymer was sandwiched between a metallic cathode consisting of Ba and A1 and a poly(3,4-ethylenedioxythiophene)/ITO anode. From the recorded raw data, depth profiles can be reconstructed as well as two-dimensional (2D) images in any depth or a 3D representation of all interesting signals. It was found that aluminum was oxidized at the Al/polymer interface [220]. 

Electrical measurements performed by Nguyen et al. [187] indicate that the aluminum/PPV interface shows a rectifying behavior when aluminum is deposited as a top electrode, as well as a bottom electrode, after polymer conversion and before polymer conversion, respectively. XPS analyses indicate chemical reactions between the polymer and the metal in the presence of ojqrgen to form metal-carbon complexes [187]. 

C. Fredriksson and J. L. Bredas, Metal/conjugated polymer interfaces a theoretical investigation of the interaction between aluminum and trans-polyacetylene oligomers, J. Chem. Phys. 98 4253 (1993). 

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