Computational and infrared studies of the structure and behavior of spontaneously adsorbed amine reagents on the high-temperature superconductor yttrium-barium-copper oxide

Long carbon chain, primary amine molecules have been found to spontaneously adsorb on the surface of the high-temperature superconductor YBa₂Cu ₃O₇ (YBCO) and form persistent, well-ordered monolayers. These monolayers; can be used to modify the interfacial properties of YBCO and related high-temperature superconductors. Because research and product development in the area of high temperature superconductivity has been slowed by the interfacial; reactivity of these cuprate superconductors to common atmospheric species, an ability to modify the materials' interfacial properties is desired. A firm understanding of the structural order and properties of these monolayers is needed for exploring their full potential as controlling agents of interfacial properties. Unfortunately, a number of materials related problems have prevented the acquisition of reliable optical data from methods, such as sum frequency spectroscopy, that have been used to evaluate the structural features of monolayers. Therefore, computational methods were undertaken to predict the structure and temperature dependent behavior of these monolayers. Using molecular mechanic energy calculations, the best packing density for hydrocarbon and partially fluorinated molecules was found to be in a (√2x√2) R45° overlayer arrangement on the exposed copper sites of c-axis oriented YBa₂Cu ₃O₇. Plausible molecular orientations for the bound molecules were identified. The crystallinity of monolayers of different carbon chain lengths and terminating endgroups was examined with molecular dynamic simulations in which the distribution of molecular tilt angles was monitored. The distributions of tilt angles obtained from the simulations were correlated to the reflectance angle infrared spectra of the monolayers. In addition, through molecular dynamic simulation and reflectance-angle infrared spectroscopy, information on the temperature dependent behavior of the monolayers was obtained. Through further molecular dynamic simulations including water, the crystallinity of the monolayer and the terminating endgroup were found to affect the behavior of water on the surfaces. Additionally, ab initio studies of the amine headgroup on the superconductor surface provided information on the nature of the adsorbate binding. A better understanding of the YBCO/amine monolayer system has been gained through these computational methods and is the first system for which major structural aspects of the monolayer was computationally predicted before experimental evidence was available.