Using X-rays to characterize the process of self-assembly in real time

We have performed in situ X-ray reflectivity studies of the growth of octadecyltrichlorosilane (OTS) monolayers on oxidized Si(111) from solutions in heptane. We find that for all concentrations, the film grows through the formation of islands of vertical molecules. The coverage follows a simple Langmuir form as a function of time, except for very low concentration solutions at early times, where a time offset is required to fit the curve. We have also examined films removed from solution, and we find that rinsing removes molecules and causes the remaining molecules to tilt. Thus, samples studied using the ‘interrupted growth’ technique are not representative of the actual growth process.