Optically variable films

By casting films from suspensions of cellulose microcrystals, cellulose films with the optical properties of chiral nematic liquid crystals can be prepared. The films can be tailored to give different colors of reflected light by altering the salt content of the suspension for a given source of cellulose and set of hydrolysis conditions. Possible areas of application include optical variable films and ink pigments for security papers [81]. 

This variation of the ellipsometric angles is compared with a two-layer optical model the open squares correspond to a hard a C H film with refractive index h = 2.1 — i0.13 and variable film thickness d. At point 1, the film thickness is d = 31.62 nm while d = 31.71 nm at point 1. A polymer-like film with refractive index n = 1.62 — 0 on top of a 31.62 nm thick hard film is represented by open circles. In both model curves, film thickness increases by Sd = 0.01 nm between consecutive model points in the direction of decreasing values for A. 

Revol, J.-F. (1998). Solid films of cellulose with chiral nematic order and optically variable properties,/. Pulp Pap. Set, 24,146-149. 

Descriptors Polymers Optical variables measurement Powders Plastic films Molecular structure Morphology Proteins Casein Polypeptides Sodium compounds... 

In the earliest SFG experiments [Tadjeddine, 2000 Guyot-Sionnest et al., 1987 Hunt et al., 1987 Zhu et al., 1987], a first-generation data acquisition method was used, and, because of the limited signal-to-noise ratios, IR attenuation by the electrolyte solution was a substantial handicap. So, in earlier SFG studies, as in IRAS studies, measurements were performed with the electrode pressed directly against the optical window [Baldelli et al., 1999 Dederichs et al., 2000]. With the in-contact geometry, the electrolyte was a thin film of uncertain and variable depth, probably of the order of 1 p.m. However, the thin nonuniform electrolyte layers can strongly distort the potential/coverage relationship and hinder the ability to study fast kinetics. 

Plasma Synthesis The use of plasma methods has lead to a new range of materials having unique properties. An example is the family of amorphous elemental hydrides (eg cr-C H Of -Si H or-P H) which contain a variable proportion of H from almost zero to 50 atomic %. The carbon films, known variously as "hard carbon", "diamond-like carbon", " a-carbon" etc (9 ) - These layers are of considerable interest because of their optical and abrasion-resistant properties etc (Table I). The properties of these Gr-carbon films, can be tailored by modifying the plasma parameters. 

Films have been deposited using selenourea and an ammonia-complexed solution at 65°C [96]. Zincblende CdSe was obtained with an optical spectrum corresponding to a bandgap of 1.84 eV (the bulk room-temperature bandgap of zincblende CdSe is ca. 1.8 eV). Analysis of electrical conductivity measurements indicated charge transfer occurred via a variable hopping mechanism through fairly deep states (a level 0.29 eV below the conduction band was found from these measurements). 

X-ray diffraction showed the film to be CuiO, with no detectable amount of CuO and with a crystal size, estimated from the peak widths, of 20 nm. Optical transmission measurements of the films gave a value of (indirect) bandgap of 2.28 eV (literature room-temperature bandgap 2.1 eV but is rather variable). 

Figure 13.5 shows a diagram of photoresist film coated on a non-reflective substrate and irradiated from the front. The thickness of the photoresist is h. It is convenient to introduce a dimensionless variable z measuring the position in the film, with z — 0 on the top of the film and z = 1 on the photo-resist/substrate interface. The photoresist with density p consists of crosslinkable units with molecular weight M0, molar absorptivity e, and concentration m. At the onset of irradiation, concentration of the cross-linkable units is uniform through the film, m — m0 — p/M0, and the initial optical density of the film is D0, which is given by... 

When optical anisotropies form spontaneously in the polymeric film during deposition, the situation is more complicated. Significant effects are observed in optical and spectroscopic properties, such as LED emission [17] and waveguide propagation [45-50,52,64], For these films, accurate evaluation of the optical constants is more difficult and must be based on variable incidence angle measurements, as in the case of surface plasmon resonance [45-47], waveguide propagation [48-50,52], ellipsometry [64,67], and reflectance/transmittance [68]. 

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