A series of alkyl phosphates with alkyl chain lengths ranging from C10 to C18 have been synthesized. Self-assembled monolayers (SAMs) of these molecules were prepared on titanium oxide surfaces by immersion of the substrates in alkyl phosphate solutions of 0.5 mM concentration in n-heptane/isopropanol. The SAMs were characterized by means of dynamic water contact angle (dCA) measurements, variable-angle spectroscopic ellipsometry (VASE), X-ray photoelectron spectroscopy (XPS), and polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS). A higher degree of order and packing density within the monolayers was found for alkyl phosphates with alkyl chain lengths exceeding 15 carbon atoms. This is reflected in a lower dCA hysteresis, as well as a film thickness measured by VASE and XPS close to the expected values for SAMs with an average alkyl chain tilt angle of 30 degrees to the surface normal. Additionally a shift of the symmetric and antisymmetric C-H stretching modes in the PM-IRRAS spectra to lower wave numbers was observed. These findings imply a higher two-dimensional crystallinity of the films derived from alkyl phosphates with a longer alkyl chain length.

Multiple transmission-reflection (MTR)a recently developed infrared spectroscopy sampling method for surfaceshas been applied to the study of silane monolayer formation on silicon oxide. Thanks to the excellent signal:noise ratio of data obtained by MTR, spectra of silane monolayers on a silica substrate could be readily obtained. This system has been previously difficult to investigate by standard sampling methods. The data is particularly important for gaining insights into the nature of the silica-silane interaction. The results support a model in which the inherent strain caused by the mismatch of alkyl-chain van der Waals radius and Si–O–Si bond distance is relieved in silane monolayers by the formation of a structure resembling snow moguls or closely packed umbrellas.

Surface-chemical gradients composed of self-assembled monolayers (SAM) of decanethiol (DT) and a partially fluorinated decanethiol (PFDT) on gold, exhibiting gradual changes in surface concentration of one or both components, have been prepared by a simple, controlled-immersion process. Infrared spectroscopic studies on a single-component PFDT gradient indicate a change in average molecular orientation with increasing surface coverage, whereas on a two-component gradient, the orientation remains invariant over the entire length of the gradient. X-ray photoelectron spectroscopic measurements on a single-component PFDT gradient show a systematic decrease in the fluorine (F 1s) binding energy with increasing surface coverage, whereas a single-component DT gradient shows an increase in the carbon (C 1s) binding energy. In two-component (DT-PFDT) gradients, the molar ratios of the two components at any particular location on the sample surface determine the magnitude of the binding-energy shifts at that location. Such shifts, which are on the order of 1 eV, are shown to be a consequence of work-function changes in the underlying gold upon SAM formation. These results are discussed in light of the surface-potential measurements on a DT-PFDT gradient by Kelvin Probe Force Microscopy and XP spectra acquired on “floating” and grounded samples.

A new infrared spectroscopic measurement involving multiple transmissions and reflections for molecular monolayers adsorbed on silicon surfaces has been established. Compared to the well-known multiple internal reflection (MIR) method, the distinctive advantage of multiple transmission-reflection infrared spectroscopy (MTR-IR) is the convenient measurement using standard silicon wafers as samples, while in the MIR setup special fabrication of geometric shapes such as 45 degrees bevel cuts on an attenuated total reflection silicon crystal is needed. Both p- and s-polarized spectra can be obtained reproducibly with the same order of sensitivity as by the MIR spectroscopy. Optimal conditions for spectral acquisition have been obtained from theoretical calculations. The ability of this methodology to gather high quality infrared spectra of adsorbed monolayers is demonstrated and the analysis of the surface packing and molecular orientation is discussed.