AVS 46th International Symposium
    Surface Science Division Friday Sessions
       Session SS1+AS+BI-FrM

Paper SS1+AS+BI-FrM1
Simple Viscosity Model Analysis of Hydronium Ion Motion in Nanometer Organic Films

Friday, October 29, 1999, 8:20 am, Room 606

Session: Organic Films/Self-Assembled Monolayers
Presenter: K. Wu, Pacific Northwest National Laboratory
Authors: K. Wu, Pacific Northwest National Laboratory
M.J. Iedema, Pacific Northwest National Laboratory
J.P. Cowin, Pacific Northwest National Laboratory
Correspondent: Click to Email
Nanometer organic films such as methylcyclohexane and 3-methylpentane were vapor-deposited at 30 K on Pt(111) with a molecular beam. Pure hydronium ions were soft-landed on top of the films at a kinetic energy less than 1.2 eV. The voltage change and desorption of the organic films were simultaneously monitored by a Kelvin probe and a mass spectrometer. Ion dosing on the organic films was a capacitive charging process, therefore the film voltage change actually reflected the ion motion in the organic films, assuming the dielectric constants of the organic films do not change much with temperature. When the films were warmed, ions gradually moved into the films. The experimental results were analyzed by a simple viscosity model. To a large extent, the temperature (or time) evolution of the film voltage could be well predicted by the model. The film voltage fall-off temperature width from the theory was, however, about half of that from the experiment. Further experimental evidence showed that the ion self-generated electric field strength had a strong effect on the ion motion. For example, when the electric field strength was higher than 0.05 V/Angstrom, the theoretical prediction seriously deviated from the experimental result, indicating that a high electric field might trigger non-linear ion motion and made the Stokes-Einstein equation not accurate. When films were prepared at higher temperatures, methylcyclohexane could crystallize on Pt(111), making ions more difficult to transport in the crystalline films. But, 3-methylpentane never crystallized before ion motion in it completed, indicating that it's a good glass material. With this general approach, we could attack many important issues such as ion motion across liquid-liquid interfaces, hydration of ions and so on.