J. D. Swalen

Anisotropic orientation in molecular monolayers by infrared spectroscopy

Fourier transform IR measurements of Langmuir–Blodgett monolayers have been made with both grazing incidence reflection and transmission techniques. The reflection arrangement at incident angles from 85° to 88° has the electric field component perpendicular to the substrate surface while the standard transmission arrangement has the electric field component in the plane of the film. These techniques were used to explore the anisotropic arrangement of cadmium arachidate molecules perpendicular and parallel to the monolayer plane. The implication of these results for characterizing the orientational anisotropy of chemical groups in thin polymer films is also discussed. In addition, the question of uniaxial and biaxial orientation in the monolayer plane was addressed because of the importance of intermolecular packing on both the extent and rate of polymerization in unsaturated fatty acid monolayer components which form synthetic membranes.

Surface plasmon spectroscopy of organic monolayer assemblies

Abstract Because the surface plasmon resonance is a sensitive probe of metallic surfaces we have measured quantitatively the changes that occur in surface plasmon resonances from coatings of thin layers of cadmium arachidate of different but known thicknesses on silver films. An ATR (attenuated total reflection) experimental arrangement was used. The dispersion of the refractive index of the organic layer was derived by a least square fit of the measured reflectivity curves and it agrees well with other data obtained by different methods. The influence of a possible intermediate layer between the silver surface and the first organic monolayer due to the preparation process is also discussed.

Order–disorder transitions in Langmuir–Blodgett monolayers. I. Studies of two‐dimensional melting by infrared spectroscopy

A two step melting process in Langmuir–Blodgett (LB) monolayer assemblies of cadmium arachidate (CdA) has been studied by infrared spectroscopy in conjunction with thermal measurements. These techniques have allowed, for the first time, the investigation of detailed structural changes which occur during heating, melting, and annealing cycles. Prior to the melting point, a pretransitional disordering of the hydrocarbon tails was observed. This was followed by a catastrophic breakup of the cadmium lattice at a temperature corresponding to the melting point as measured by differential scanning calorimetry (DSC).

Corona poling and real‐time second‐harmonic generation study of a novel covalently functionalized amorphous nonlinear optical polymer

Thin films for optical second‐harmonic generation (SHG) were prepared from a newly designed and synthesized amorphous polymer that incorporated a high density of active nonlinear optical groups (p‐nitroaniline as attached side groups). For alignment of the nonlinear groups a very high electric field was applied by a corona discharge to the polymer films above Tg (125 °C). The subsequent freezing process resulted in a polymer film initially exhibiting a very high second‐order nonlinear coefficient, d33=31 pm/V, measured by the Maker‐fringe technique, plus excellent thermal, mechanical, and optical properties. The dynamics of polar alignment and decay, studied by in situ poling (or depoling) and SHG measurements, indicated a multiple exponential behavior with the average relaxation time somewhat longer than expected from extrapolation of the dielectric relaxations data according to the Williams–Landel–Ferry equation. The frozen‐in SHG behavior at room temperature (∼100 °C below Tg) relaxed after 5 days to a...

The orientation of Langmuir–Blodgett monolayers using NEXAFS

Carbon K‐shell NEXAFS (near edge x‐ray absorption fine structure) spectra of oriented hydrocarbon chains in Langmuir–Blodgett (LB) monolayers were measured and used to study the orientation of these molecules. The LB monolayers were assembled from arachidic acid or cadmium or calcium arachidate on the oxidized Si(111) surface. The observed NEXAFS resonances are assigned to transitions to excited states which are localized on individual CH2 groups or C–C bonds. From a detailed analysis using curve‐fitting techniques of the angular dependence of the various spectral peaks, the hydrocarbon chains of the cadmium arachidate monolayer is estimated to lie within 15° of the surface normal, the hydrocarbon chains of the calcium arachidate monolayer is estimated to be tilted by 33±5° from the surface normal, while the arachidic acid monolayer is not ordered at all. The determined chain orientations are discussed in terms of a microscopic model involving lateral interactions between the zig–zag hydrocarbon chains.

Highly efficient and stable nonlinear optical polymers via chemical cross‐linking under electric field

Extending the novel method of Eich et al. [J. Appl. Phys. 66, 3241 (1989)] to prepare highly stable second‐order nonlinear optical (NLO) polymers via chemical cross‐linking under electric field, we have obtained a new polymer exhibiting large and stable second‐order optical nonlinearities after relaxation at 80 °C. This was achieved by forming a network polymer from two NLO‐active monomers, bifunctional N,N‐(diglycidyl)‐4‐nitroaniline and trifunctional N‐(2‐aminophenyl)‐4‐nitroaniline. Here, every NLO moiety is connected to the network by a single covalent bond. After full cure under corona poling at 120 °C, the sample exhibited at ambient conditions d33≂50 pm/V and d31≂16 pm/V at 1064 nm fundamental wavelength, as estimated from the Maker fringe data. Upon heating to 80 °C, the nonlinearities decreased somewhat initially, but leveled off and remained stable at 80 °C. This stable sample gave d33≂42 pm/V and d31≂14 pm/V at ambient conditions. Furthermore, the linear electro‐optical coefficient of this samp...

Temperature dependence of the third‐order nonlinear optical susceptibilities in polysilanes and polygermanes

Optical third‐harmonic generation was used to measure third‐order nonlinear susceptibilities χ(3) for thin films of planar zigzag polysilanes and polygermanes. A χ(3) of 11.3×10−12 esu was measured, the largest value ever reported for a polymer with transparency throughout the visible. The value of χ(3) was found to vary substantially with thermally induced reversible changes in polymer backbone conformation and also with film thickness.

Electron mean‐free pathlengths through monolayers of cadmium arachidate

The inelastic mean‐free pathlengths λ, of 1042–1402 eV electrons through cadmium arachidate (C19H39COO)2Cd, monolayers have been determined from attenuation measurements of the core level photoemission (XPS) signals from gold, silver and indium substrates. Both fixed angle‐variable overlayer thickness experiments and variable angle‐fixed overlayer thickness experiments were performed. We derive a preferred value of 3.6 nm at 1117 eV and an E0.5 energy dependence over the narrow energy range considered. Values determined for bulk cadmium arachidate and arachidic acid from absolute intensity comparisons with gold and silver are close to the monolayer values. The results are discussed in the light of the controversy existing concerning other values of λ’s determined for organic and polymeric materials.

Raman spectra of thin organic films enhanced by plasmon surface polaritons on holographic metal gratings

The variation in the Raman intensity of a thin film of polystyrene coated onto a Ag metal holographic grating has been measured as a function of the angle of incidence of the exciting laser beam. The Raman signal reaches its maximum intensity at the angle at which the laser excites the plasmon surface polariton mode of the metal directly by coupling through the grating vector. This demonstrates that plasmon surface polariton fields can be exploited to enhance the Raman spectra of very thin films on metal surfaces. The advantages of using holographic gratings in this type of experiment are briefly discussed.

Surface plasmon enhanced Raman spectra of monolayer assemblies

This paper describes the systematics of using plasmon surface polariton (PSP) fields to enhance the Raman spectra of monolayer assemblies deposited on silver metal gratings by the Langmuir–Blodgett deposition technique. The intensity of the incident light is enhanced by grating coupling to the PSP mode, likewise, the intensity of the outcoupled Raman light is enhanced by emission via radiative PSP modes. It is shown how the various spectral regions corresponding, e.g., to C–C and C–H stretching vibrations are individually enhanced by selecting the appropriate scattering angle for the light collected from the grating. Results are presented that demonstrate how the gaps in the PSP dispersion curve at the center and the boundary of the Brillouin zone can be shifted to preselected frequency ranges using assemblies of different thickness. It is also shown that the dispersion gap width and intensities of the Raman vibrational bands depend on the number of monolayers making the assembly.