Infrared Ellipsometry of Langmuir−Blodgett Films on Gold. Toward Interpreting the Molecular Orientation

Abstract

Langmuir-Blodgett (LB) films (nine double layers) of 2-[4-(N-dodecanoylamino)phenyl]-5-(4-nitrophenyl)-1,3,4-oxadiazole on gold were analyzed by means of infrared (IR) ellipsometry followed by theoretical interpretation of the measured spectra in a uniaxial anisotropic layer model. The same material in a KBr pellet was investigated by IR transmission spectroscopy. The evaluation provided data for vibrational band parameters, necessary for elucidation of the molecular orientation. The LB film was then annealed at 130°C since structural changes due to the thermal treatment were expected to occur, and hence the films before and after annealing can serve as model systems with different molecular orientations. The data revealed that the molecules were hydrogen bonded both in a pellet and as LB films. These bonds were only partly disrupted due to the thermal treatment and play the role of an additional factor stabilizing the film structure. After annealing, a change of about 14°was calculated from ellipsometric spectra for the head NO2 group orientation with respect to the surface plane, which would provoke a commensurable decrease of the aliphatic chain tilt angle. The presented results demonstrate the capabilities of infrared spectroscopic ellipsometry not only to determine the film optical constants and thickness, but also to probe some aspects of the molecular orientation. IntroductionLangmuir-Blodgett (LB) films have attracted substantial interest over the last two decades since they have potential for some practical applications in sensors, nonlinear optics, and electronic devices and keep serving as model systems for studying ordered films. 1-3 Extensive research including infrared (IR) spectroscopy, secondharmonic generation (SHG), atomic force microscopy (AFM), ellipsometry, and X-ray diffraction studies has been performed 1,2 aiming to elucidate the inherent molecular structure of the film and the relative orientation of the aliphatic chains with respect to the substrate surface. A large segment of the existing experimental data have been accumulated by IR studies. In attempts to better characterize the intrinsic molecular orientation, both transmission measurements on IR transparent substrates and reflection measurements on metallic, dielectric, or semiconductor surfaces have been carried out. 4-7 While normal incidence IR transmission spectra of LB films display bands due to vibrational transition moments projected into directions within the surface plane, reflection spectra on metallic substrates exhibit only bands due

Reflection and Absorption Techniques for Optical Characterization of Chemically Assembled Nanomaterials

“…[128] Using a similar approach, the thickness and structure of a Langmuir±Blodgett film prior and subsequent to annealing have also been elucidated. [346] There are only a few published reports on the use of internal reflection geometry (see Fig. 9) in infrared ellipsometry.…”

Influences of thick film inhomogeneities on the ellipsometric parameters

“…Applications of this method include studies of molecular composition and orientation of molecules in thin films [2][3][4][5], as well as interface structure and bonding between an organic film and inorganic substrate [6]. However, investigations of optical properties of thick inhomogeneous films, such as polyethylene or biological samples, could lead to certain difficulties, because depolarization effects might take place during the measurements.…”

Progress in spectroscopic ellipsometry: Applications from vacuum ultraviolet to infrared

“…In addition to molecular identification, it may also be possible to interpret the molecular orientation from IR-SE measurements. 40 IR-SE can also identify biological materials. 41,42 IR ellipsometry has demonstrated submonolayer sensitivity.…”