Aluminium oxide monolayer bonding

Cohen et al. [361] studied, amongst other things, the influence of heat on monolayers of OTS and of arachidic acid adsorbed on surfaces of aluminium oxide formed on aluminium surfaces. They used wetting studies and infrared spectroscopy and concluded that, whereas arachidic acid layers deteriorated in an irreversible manner at about 100°C, OTS layers survived intact to about 140°C. This result is attributed by these authors to polymerisation of the latter material but could also be due to a rather different bonding mechanism between the organic material and the aluminium oxide. 

Inelastic electron tunnelling spectroscopy (lETS) has been used to study some silanes on aluminium oxide. The technique records vibrational spectra of an absorbed monolayer. Silanes can be applied to the oxidised metal from solution or vapour, and devices are completed by evaporation of a top electrode which is usually of lead, because of its superconductivity. The device is cooled to the temperature of liquid helium (4.2 K) to minimise thermal broadening. Most electrons (>99%) pass through the device elastically, but a small number excite vibrational modes. It is these that are detected and displayed as a spectrum. Both IR and Raman modes can be observed the selection rule for lET spectroscopy is one of orientation, in that bonds which are aligned perpendicular to the surface give the most intense peaks. 

Early work by Schonhorn [119] showed that orientated monolayers of amphipathic molecules, such as stearic acid, could be employed as extremely effective adhesives in the bonding of polyethylene to aluminium indeed, so effective were these adhesives that joint strengths often exceeded the cohesive strength of the polyethylene Multilayer adsorption lowered the joint strength not because of less intrinsic adhesion of the amphipathic molecule to the substrate, but because the relatively thick layer possessed low cohesive strength and thus behaved as a weak boundary layer. Chemisorption was thought to occur for the stearic acid on the aluminium oxide [120,121], and this... 

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