X-ray, micro-Raman, and infrared spectroscopy structural characterization of self-assembled multilayer silane films with variable numbers of stacked layers

The structure of a series of alcohol-terminated bifunctional long-tail organosilane films with varying numbers of superimposed monolayers (between 1 and 11), prepared on smooth, hydrophilic silicon substrates by the layer-by-layer self-assembly approach, has been investigated with the purpose of elucidating details of the molecular organization and the intra- and interlayer modes of binding in such films. To this end, experimental results obtained by synchrotron X-ray scattering, micro-Raman, and Fourier transform infrared (FTIR) spectroscopic techniques have been combined and compared. A comprehensive analysis of all data demonstrates that the studied multilayer films consist of stacks of uncorrelated discrete monolayers, the inner molecular order of which is preserved with the growing total number of superimposed layers. Similar to self-assembled films of long-tail silanes with terminal -COOH groups,(1) the intermolecular binding in the present films is characterized by partial intra- and interlayer covalent bond formation. The molecular hydrocarbon tails are perpendicularly oriented on the layer planes, forming a densely packed rotator phase like hexagonal lattice with a molecular surface area of similar to20 Angstrom(2) and a lateral correlation length of the order of 16 molecular diameters.