Layers, visible spectra

The thickness of thin film layers separated by uniform, parallel interfaces can be determined from optical interference patterns that result. These measurements can be made from about 400 nm out through the visible spectrum and on into the near-infrared (NIR) region. Since film thickness measurements rely not on the absolnte magnitude of the reflected light, but on the variation of that signal with wavelength, the choice of nnits is less important. Typically %R is used, but in some cases raw intensity is also satisfactory. We will treat thickness determinations in more detail in the applications section of this chapter. 

The optical reflectance spectra were dependent on the nanocrystallite structure and dimensions, porosity and the layer thickness (Fig. 9.2). The maximal photosensitivity in the visible wavelength range of the spectra (30-35 mA/Lm) was typical of the sNPS layers with the nanocrystallite dimensions of 15 nm, and it decreased with increasing size of the nanocrystallites. The maximal sensitivity to the ultraviolet irradiation was obtained for sNPS layers with nanocrystalhte dimensions of 20-25 nm. sNPS layers obtained by electrochemical etching as well as by chemical etching showed the photoluminescence typical of this material a broad peak in the visible spectrum with the intensity sufficient for observation of the photoluminescence with a naked eye. sNPS samples obtained by chemical or electrochemical etching had intensive emission with the maximum at A, 640 nm and 700 nm. 

Birefringence itself can provide a novel mechanistic tool. Reduction in the symmetry of tetragonal crystals due to selective formation of product in one orientation can be observed with a polarizing microscope, even when neither starting material nor product has an absorption in the visible spectrum. This technique has been used to show confinement of reaction within individual lamellae of a layered crystal [40]. By using dispersion rather than absorption, this technique extends the range of reactions that can be studied optically. 

The simplest spectroscopic experiments are based on measurements of the amount of light transmitted by a sample. Such transmission-mode experiments are possible with chemically modified electrodes if the substrate electrode is transparent. In fact, tin oxide or indium tin oxide ( ITO ) electrodes are quite transparent over the entire visible spectrum [46]. If the modifying layer contains visible-range chromophores, then examining potential-induced chemical changes... 

Figure 5.17. The two molecules discussed in [299]. These molecules can be deposited by the LB technique in alternate layers and form a non-centrosymmetric structure which can be used to generate a second harmonic in the visible spectrum. It is believed that the single hydrocarbon chain of one material interdigitates with the double chain of the other material, thus forming a stable structure.

Molecules of chromophores were attached to appropriate ionic polymers to allow their confinement in desired layers. Good spectral overlap between doner emission and acceptor absorption spectra permits efficient energy transfer in the supra-molecular assembly. Figure 36 summarizes the spectral characteristics of the four photoactive species [184]. The coumarin polymer (Coum-PAH) absorbs in the blue region of the visible spectrum (imax =412 nm), and its emission maximum, 485 nm,... 

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