The oxidation rate of silver films by atomic oxygen has been measured by using a gravimetric method. The rate of the reaction has been followed as a function of temperature, film thickness, and the atomic oxygen concentration. The oxidation can be divided into three phases. During the first phase, the process is first-order in atomic oxygen concentration, and the product is AgO. The process is described in terms of a moving boundary model, where the diffusion rate of oxygen atoms is high in the oxide layer and nil in the elemental silver layer. The diffusion coefficient for oxygen atoms in silver oxide is 2.5 X 10~13 cm2/s at 298 K. The second phase is much slower, and it is independent of the atomic oxygen concentration. The stoichiometric formula for the product is Ag02, and its formation is inhibited by increasing the temperature from 290 to 313 K. Upon prolonged exposure to atomic oxygen, phase 3, the silver films are oxidized to a stoichiometric formula Ag03. These products are stable to several hundred kelvin above ambient temperature. Silver films have a rapid affinity for atomic oxygen, and complete penetration of the films as thick as 200 nm has been demonstrated.
The authors show that a carbon nanotube p-n diode is a very sensitive probe of optical transitions in individual single-walled carbon nanotubes. In the photocurrent spectra, an alternating sequence of resonant peaks from dissociation of excitons and exciton-phonon bound states, for the lowest and higher electronic subbands, is observed. At an intermediate energy, the onset of continuum is observed that allows measurement of exciton binding energies. Both the binding energy and the onset of continuum follow the inverse diameter relation as expected from general theory of optical transitions in nanotubes.
Polymeric coatings on copper surfaces are known to degrade at a faster rate than identical materials on other metals such as aluminum, particularly during thermal aging. We have studied the interfacial reactions occurring at copper surfaces coated with poly(esterimide) and polyimide wire enamels. Thin coatings (100 Å–1 μm) were heat treated at temperatures from 200 to 240 °C. Interfacial reactions were studied by x-ray photoelectron spectroscopy (XPS), Auger profiling, and reflectance infrared spectroscopy. In addition to copper oxide growth at the interface, thermal oxidative degradation of the polymer leads to thinning of the coating. This reaction is catalyzed by the copper (oxide) surface and material loss occurs primarily at the polymer/copper (oxide) interface. Migration of mobile copper species into the bulk of the coating is observed by its appearance at the surface and by depth profiling. For polyimide films prepared from poly(amide-acid) precursors, an interfacial reaction occurs during initial contact, prior to curing or aging, and infrared and XPS measurements show incomplete curing of the film. The formation of a copper carboxylate is postulated.
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