In Situ Kinetics of Self-Assembly by Surface Plasmon Resonance Spectroscopy

Abstract

Using surface plasmon resonance (SPR) spectroscopy in the Kretschmann configuration, we followed the self-assembly of organic ultrathin films which resulted from exposure of gold surfaces to solutions of CH 3 (CH 2 ) n - 1 SH ( n = 8, 12, 16, and 18) in ethanol and heptane. We monitored film growth in situ and continuously for up to 72 h with an overall thickness resolution of <1 Å. Film dielectric constants, necessary for accurately calculating average film thicknesses from SPR spectra, were determined unambiguously for fully formed films by comparing spectra from organic films in different solvents. In addition, we introduce a novel two-color SPR experiment with which we can obtain both film thickness and film dielectric constant without changing solvents. We studied the chain length dependent and concentration dependent kinetics of film formation in ethanol and found that there are at least three distinct kinetics steps. The kinetics of the first, most rapid, step and the third, slowest, step can be described well with Langmuir adsorption models. The kinetics of the second step are zeroth order and depend on alkanethiol chain length, concentration, and partial film thickness. The formation kinetics in heptane can be described with a single step Langmuir adsorption model. We find that films formed by continuous self-assembly in solution are always thicker than films for which the process of self-assembly has been interrupted by rinsing with neat solvents. We also present evidence that, by rinsing the partially formed film before reimmersion into thiol solution, we can change the final film thickness and structure of a hexadecanethiolate film. Our results are most consistent with a formation mechanism involving adsorption of both chemisorbed and physisorbed molecules during film formation in ethanol, with the overall formation kinetics determined by the relative solubility of the alkanethiol in the solvent.