Dielectric Langmuir-Blodgett Films

L.M. Blinov, S.P. Palto, and S.G. Yudin, Stark Spectroscopic Technique for Probing the Local Field in Dielectric Langmuir-Blodgett Films, T. Molec. Electronics 5 45 (1989). 

The monolayer also provides an environment of variable dielectric so that intermolecular association between photoactive molecules can readily occur. For example, molecular association of pyrene within a Langmuir-Blodgett film is clearly seen through time-resolved fluorescence measurements on the picosecond timescale [92], Attenuated total reflectance studies of dyes in cast films can similarly reveal their positions and photophysical interactions [93], Photochromism in a monolayer assembly has been attributed to excitation of ion-pair charge transfer complexes formed within the array [94]. 

One core chiral system that shows dramatic amplification of its chiral structure is the substituted helicenes of Katz and coworkers [83]. In essence, this research cuts the helix into a number of six-helicene subunits that self-assemble (Figure 10). Only when these subunits, which look like lock washers, are prepared in optically pure form the material associates into supramolecular helical columns [84]. The assemblies have been synthesized with different amounts of substitution around the exterior. Depending on the helicenes substitution, the material exhibits hexagonally ordered soft-crystalline [84] or liquid-crystalline phases [85]. The liquid-crystalline versions of these molecules switch when electric fields are applied to neat and solution-phase samples and have been characterized as a dielectric response [85-87]. Upon association, these materials have enormous changes in their CD intensities and optical rotations [74]. In addition, this supramolecular chirality also significantly enhances the second-order nonlinear optical behavior of these materials in Langmuir-Blodgett films [88]. 

Local Piezoelectric Response and Surface Potential of Dielectric AND Ferroelectric Langmuir-Blodgett Films Studied by Electrostatic Force Microscopy... 

The optical properties of materials are determined by the so-called dielectric function. This dielectric function was determined for PPP as a result of first principles band structure calculations. In Fig. 30.3 we depict one of the main results, namely the dependence of the imaginary part of the dielectric function (which is proportional to the optical absorption coefficient) on the orientation parallel (cc) and perpendicular ( , e ) to the chain axis. From comparison with the experiment one can see that the optical absorption in the visible and ultraviolet range is mainly determined by the dielectric function parallel to the polymer chain. This is shown in Fig. 30.4, where the calculated absorption coefficient along the chain is compared with experimental data. The observed n-n transition is therefore the result of optical excitation with the dipole moment parallel to the PPP chains. Another experimental proof for this are the absorbance properties of hexaphenyl single crystals [43]. The absorbance spectra for thin films with the hexaphenyl chains oriented perpendicular and parallel to the substrate plane are shown in Fig. 30.5 together with the photoluminescence (PL) and electroluminescence (EL) emission spectra. The above-described anisotropy of the optical absorption is also valid for photoluminescence emission and can be used to obtain polarized EL spectra from Langmuir-Blodgett films made from soluble PPPs [44]... 

The monolayer formed by 6-dodecyl-2-naphthol [192] on neutral water shows no fluorescence due to the DP form at 420 nm. The absence of a water-cluster-base [33-37], and a reduction of apparent surface dielectric constant have been offered as explanations of this observation. A similar observation of the absence of excited DP form fluorescence was made in Langmuir-Blodgett (LB) films of... 

The Surface Plasmon Resonance (SPR) technique is presented as a sensitive system of measurement, and the underlying physical mechanism is described. The method is ideal for determining the linear and non-linear optical properties of dielectric media in thin film form, specifically those prepared by the Langmuir-Blodgett (LB) method. The LB method of film deposition and its inherent advantages as a process for non-linear optical device fabrication are discussed briefly. 

A comparative study of ultrathin dielectric (an azo-compound) and ferroelectric (copolymer P(VDF-TrFE)) Langmuir-Blodgett (LB) films has been carried out by Electrostatic Force Microscopy (EFM). Films were poled locally by a strong d.c. field applied between a conductive tip of an Atomic Force Microscope (AFM) and the bottom A1 electrode. The electrically poled domain was studied by EFM using a weak a.c. electric field and a lock-in amplifier technique. Two modes, a contact and non-contact ones, allowed for the measurement of field a in the air gap between the film and the tip and the piezoelectric distortion of the film due to the d.c. field aligned spontaneous polarization. Simultaneously the topographic relief of the same area was imaged. The results confirm unequivocally a possibility to switch ferroelectric LB film locally by an AFM tip. 

Here, the target of measurement by infrared ER spectrometry is a thin film deposited on a flat surface of a dielectric material. Multilayered adsorbed species (adsorbates) may also be a target, but, if a quantitative analysis of their observed spectra is to be performed, each layer in the adsorbate film should have a flat and parallel structure. For this reason, the discussion in this chapter assumes that thin films like a Langmuir-Blodgett (LB) film [4], which has an ideally flat layer, are the target of study. A dielectric substrate is made of a nonmetallic material and includes semiconductors the surface of water is also regarded as a dielectric substrate. Monolayers spread onto the surface of water are often analyzed by ER spectrometry. 

The interaction with an external surface in any material may induce a discontinuity and anisotropy in the energetic balance of the system, imposing effects different from those observed in bulk. In ferroelectric materials, it has been observed that finite size effects may also appear. However, it was verified that ultrathin films of P (VDF-TrFE) copolymers prepared via the Langmuir-Blodgett (LB) technique exhibit a F-P transition with a Tc nearly equal to the bulk value, but, in addition, another transition appears at a lower temperature, associated with the surface layers only [41]. LB films are highly crystalline, therefore, interfacial layers in this particular case do not present a large number of defects. Nevertheless, dielectric experiments have reported the presence of a dielectric segmental relaxation in P (VDF-TrFE) LB films, attributed to the motion of amorphous chains [42]. 

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