R.G. Pritchard

The Application of Inelastic Electron Tunnelling Spectroscopy to Epoxide Adhesives

Abstract The theory of inelastic electron tunnelling spectroscopy (IETS) and the suitability of this technique for examining adhesive-aluminium oxide interfaces are discussed. IET spectra are presented of an epoxide resin (the diglycidyl ether of bisphenol A), two aliphatic amine hardeners (di-[1 -aminopropyl-3-ethoxy] ether and triethylene tetramine), and mixtures of resin and hardener before and after subjection to the usual heat curing schedules. These show that the curing reaction does not take place within an IETS junction; a possible reason for this is the epoxide resin is physically adsorbed on to the aluminium oxide surface whilst the hardeners may be chemically adsorbed through the amine groups.

An inelastic electron tunnelling spectroscopy (IETS) study of poly(vinylacetate) poly(methyl methacrylate) and poly(vinylalcohol) adsorbed on aluminium oxide

Abstract IETS is used to investigate the adsorption of poly(vinylacetate) (PVA), poly(methylmethacrylate) (PMMA), and poly(vinylalcohol) (PVOH) on aluminium oxide. These polymers are of interest in the field of adhesion science, and until now synthetic macromolecules have not been studied in this way. Both commercially available polymers and those synthesized in our laboratory have been used. On the basis of IET spectra presented here, and existing i.r. spectra it is believed that PMMA and PVA undergo ester cleavage at the oxide surface leading to their subsequent adsorption. For PMMA this is thought to be via carboxylate anions generated on the polymer side groups, while PVA is expected to be adsorbed as PVOH. Bonding of PVOH to the oxide is not fully understood, but may occur by the formation of an AlOC bridge. Another possibility for the above polymers, that of intermolecular hydrogen bonding between polar polymer side groups, and adsorbed hydroxyl species present on the oxide surface, cannot be ruled out.

An adhesive study by electron tunnelling: Ethyl α-cyanoacrylate adsorbed on an oxidized aluminium surface

Abstract The interfacial properties of an adhesive system, ethyl α-cyanoacrylate/aluminium, are investigated by the adsorption of a thin adhesive layer upon the Al oxide insulator of an AlPb tunnel junction. Such junctions exhibit electrical properties determined in part by the physical and chemical nature of the adsorbed layer. Inelastic electron tunnelling spectra, and resistance and capacitance data are presented which indicate that exposure of the oxide to pure adhesive vapour produces an inhomogeneous adsorbed layer, which increases in overall thickness with increasing vapour exposure time. By retarding vapour polymerization, a more uniform layer is adsorbed, whose thickness is less strongly dependent on exposure time. Tunnelling spectra agree well with those obtained using bulk i.r. spectroscopy (which are also presented here), allowing IET vibrational mode assignments to be inferred. Shifts in CO and CO-stretching frequencies are observed in the tunnelling spectrum of the uniform layer which have been attributed by other workers using a reflectance i.r. technique to adhesive hydrogen bonding at the oxide surface. The absence of CC and CH 2 modes in the tunnelling spectra is indicative of cyanoacrylate polymerization at the monomolecular level. A chemical model of the interface based on this, and on peak intensity data, is presented.

The application of inelastic electron tunnelling spectroscopy to adhesive bonding

Abstract Inelastic electron tunnelling spectroscopy is a way of obtaining the vibrational spectra of molecules adsorbed on a metal oxide. Hence it is appropriate to use this technique to examine adhesives and adhesion promoters. Here, the principles and methods of inelastic electron tunnelling spectroscopy are presented, with the results of studies on polyvinylacetate, polymethylmethacrylate and some silane adhesion promoters.

Conductance changes in inelastic electron tunnelling junctions during infusion doping

Abstract The conductance of inelastic electron tunnelling (IET) junctions, typically Al/aluminium oxide/dopant/Pb (about 3000 A thick), was measured during infusion doping. The measurements, necessarily rapid and accurate, were made by employing a digital multimeter and microcomputer. These measurements indicate an increase in the insulating barrier thickness during doping, which has been confirmed by corresponding changes in the junction capacitance. An experimental technique was also developed, allowing contamination-free infusion-doped IET spectra to be obtained routinely. A model is proposed which to a first approximation predicts the behaviour of junction conductance as a function of time during doping. Thus the dynamics of the infusion doping process have been clarified.

Inelastic electron tunnelling spectra of some plasma polymers on aluminium oxide

Abstract Inelastic electron tunnelling spectra have been obtained for plasma-polymerized polyvinylacetate, polyethylacrylate, polyacrylic acid, polymethylmethacrylate and polyacrylonitrile on hydrated aluminium oxide. Spectra of plasma-polymerized polyvinyl-acetate and polymethylmethacrylate are very similar to those of conventional free radical polymers. All polymers exhibit spectral peaks at about 1440 cm−1 and 1600 cm−1 which are due to the carboxylate anion, and this indicates that ester groups are cleaved on the oxide to produce ionic attractions at the interface which are advantageous for adhesive bonding.

Inelastic Electron Tunneling Spectroscopy of Some Aminosilane Coupling Agents

Abstract Inelastic electron tunneling spectra have been obtained for some aminosilanes (3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, the mixed and separated isomers of aminophenyltrimethoxysilane and N-2-aminoethyl-3-aminopropyl trimethoxysilane) adsorbed on aluminium oxide. The silanes have been applied from the vapour and from solutions in either benzene, water or acidic aqueous alcohol. Spectra indicate diferent levels of hydrolysis and condensation for the amines, which depend upon the doping conditions, and that no isotopic exchange occurs when the doping medium is D2O. The mode of attachment to the oxide surface is different for the three isomers of aminophenyltrimethoxysilane.

An examination of the interaction of silanes containing carboncarbon double bonds with aluminium oxide by inelastic electron tunnelling spectroscopy

Abstract Inelastic electron tunnelling spectra were obtained for some silanes with CC bonds doped from solution in acidic aqueous alcohol and from their vapours. Spectra show a high level of hydrolysis of the silanes doped from solution, but only partial hydrolysis from vapour doping. Vinyltrichlorosilane is fully hydrolysed with both methods of doping. A silane containing a methacrylate group is partially saponified.

Examination of the interaction of 3-glycidoxypropyltrimethoxysilane with aluminium oxide by inelastic electron tunnelling spectroscopy

Abstract Inelastic electron tunnelling spectra of 3-glycidoxypropyltrimethoxysilane show the absence of features due to the epoxide group, which are apprently replaced by C=C and C=O groups. This is supported by infrared by epectroscopy and some chemical tests. Two reaction sequences are proposed to account for the observed changes.

Ester Polymers and their Interaction with Alumina Studied by Inelastic Electron Tunnelling Spectroscopy

Inelastic electron tunnelling spectroscopy (IETS) allows one to record the vibrational spectrum of a monolayer of organic molecules adsorbed upon the metal oxide of a metal/metal oxide/metal thin film sandwich - an IET junction (1–3). In this work the junctions were aluminium, aluminium oxide, adsorbed organic layer, and lead. If a d.c. bais voltage is applied to the junction, a net current flows due to electrons tunnelling from one metal to the other. Approximately 1% of these electrons tunnel inelastically and so lose energy to molecular oscillators in the oxide or adsorbed layer. This produces a small increase in conductance of the IET junction at a bias voltage V given by the relationship V = hυ/e where h is Planck’s constant, e is the electronic charge and υ the frequency of the oscillator. This increase is more easily seen as a peak in a plot of the second derivative of the voltage with respect to current (d2V/dI2) against bias voltage V. IETS is usually performed with the junction at a temperature 4.2 K, mainly to reduce thermal broadening of the IET lines. Both Raman and infra-red (IR) vibrational modes may be activated.