Transport and chemical reaction dynamics in Langmuir-Blodgett films studied with Fourier transform infrared spectroscopy
Abstract
Langmuir-Blodgett (LB) films are ultrathin organic films which have potential applications in membranes, microelectronics, and nonlinear optics. The ion exchange dynamics of LB films of calcium stearate and various acid solutions (acetic, hydrochloric, sulfuric), and the effect on the film structure were studied by attenuated-total-reflection Fourier-transform-infrared-spectroscopy (ATR-FTIR). The effective diffusivity of water in a CaSt$\sb2$ film was found to be 3 $\times$ 10$\sp{-12}$ cm$\sp2$/s. The ion exchange dynamics as a function of film thickness and film structure show that the ion exchange process is limited by the mass transfer rate. Also, there are possibly two different mass transfer processes involved, one related to the transverse transport into the film, and the other involving lateral transport in the film. The counter ion (Cl$\sp-$, SO$\sb4\sp{-2}$, CH$\sb3$-COO$\sp-$) had little effect on the exchange dynamics. Also, no accumulation of the counter ion within the film was detected. The crystal structure of the CaSt$\sb2$ film were altered by the ion exchange process. The degree of crystallinity increased with the conversion of calcium stearate to stearic acid. A film which had been melted and cooled was as crystalline as unmelted LB films after the ion exchange was completed. These results show the ion exchange process involves not only mass transfer and chemical reaction but a recrystallization. The results have implications on the use of LB films as water barriers and ion-exchange materials. In another project, it was found that CdSt$\sb2$ films decarboxylate when exposed to X-rays during X-ray photoelectron spectroscopic analysis. There was a loss of carboxylate as well as hydrocarbon signal. Even 15 minutes of X-ray exposure can cause significant alterations in the structure and composition of CdSt$\sb2$ LB films. Experiments on octadecylamine LB films showed enhanced stability but they still were affected by X-ray exposure. These results impact the characterization techniques of LB films. Finally, ATR-FTIR was used to probe the growth of native oxides on GaAs after various surface treatments and evaluate the robustness of the treatments. An (NH$\sb4)\sb2$S treatment reduced the oxide growth compared to an NH$\sb4$OH etched surface. Also, it was found that exposure to humid air causes a rapid oxidation of the GaAs surface. This rapid oxidation is likely promoted by a pre-existing thin oxide layer. This work should help the development of GaAs as an electronic device material.
Degree
Ph.D.
Advisors
Franses, Purdue University.
Subject Area
Chemical engineering
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