Structure and Growth of Vapor-Deposited n-Dotriacontane Films Studied by X-ray Reflectivity

Abstract

We have used synchrotron X-ray reflectivity measurements to investigate the structure of n-dotriacontane (n-C(32)H(66) or C32) films deposited from the vapor phase onto a SiO(2)-coated Si(100) surface. Our primary motivation was to determine whether the structure and growth mode of these films differ from those deposited from solution on the same substrate. The vapor-deposited films had a thickness of approximately 50 A thick as monitored in situ by high-resolution ellipsometry and were stable in air. Similar to the case of solution-deposited C32 films, we find that film growth in vacuum begins with a nearly complete bilayer adjacent to the SiO(2) surface formed by C32 molecules aligned with their long axis parallel to the interface followed by one or more partial layers of perpendicular molecules. These molecular layers coexist with bulk particles at higher coverages. Furthermore, after thermally cycling our vapor-deposited samples at atmospheric pressure above the bulk C32 melting point, we find the structure of our films as a function of temperature to be consistent with a phase diagram inferred previously for similarly treated solution-deposited films. Our results resolve some of the discrepancies that Basu and Satija (Basu, S.; Satija, S. K. Langmuir 2007, 23, 8331) found between the structure of vapor-deposited and solution-deposited films of intermediate-length alkanes at room temperature.

Crystalline-to-plastic phase transitions in molecularly thin n-dotriacontane films adsorbed on solid surfaces

“…We have investigated three possible explanations for this step up in the ellipsometric signal dP. Based on recent synchrotron x-ray reflectivity measurements on similar films, 12 we proposed that this step is due to a "chainmelting" transition in which gauche defects are introduced collectively into the molecules. This transition produces a slight increase in the density within the monolayer islands, which is observed as a step up in the ellipsometric signal.…”

“…This value is very close to the shrinking of 40.3 Å observed with XRR. 12 Considering the almost constant step height we measured at monolayer coverages of ϳ50% and ϳ70%, a linear scaling may be regarded as the upper limit for a dP correction.…”

“…A formation of up to four gauche defects is already beyond the limit of consistency with observed changes in film thickness. 12 For this reason, we also discard this third possibility of a collective change in molecular polarizability to explain the dP step of ϳ0.04°and retain the first scenario discussed ͑compression of the film thickness with the corresponding increase in the film refractive index probably due to the formation of one to two gauche defects͒.…”

“…At the substrate-alkane interface, there is a complete bilayer of molecules with their long axis parallel to the surface. 12 On top of this bilayer, molecules with their long axis perpendicular to the surface coexist with monoclinic and orthorhombic bulk particles. 13,14 The mesa-shaped orthorhombic particles are surrounded by an area depleted of perpendicular layers.…”

Effects of pluronic F68 micellization on the viability of neuronal cells in culture

“…Prior to F68 deposition, we cleaned the silicon substrate by immersing it into a solution of sulfuric acid and hydrogen peroxide (70% H 2 SO 4 1 30% H 2 O 2 ) at 90 C for 30 min, providing an hydrophilic substrate surface. 18 Previous analysis of X-ray reflectivity curves, 17 indicated that the thickness of the SiO 2 coating was 12 Å . This cleaning procedure does not remove the native oxide layer and provides very reproducible substrates.…”