Energy transfer properties thick films

Chemical properties of deposited monolayers have been studied in various ways. The degree of ionization of a substituted coumarin film deposited on quartz was determined as a function of the pH of a solution in contact with the film, from which comparison with Gouy-Chapman theory (see Section V-2) could be made [151]. Several studies have been made of the UV-induced polymerization of monolayers (as well as of multilayers) of diacetylene amphiphiles (see Refs. 168, 169). Excitation energy transfer has been observed in a mixed monolayer of donor and acceptor molecules in stearic acid [170]. Electrical properties have been of interest, particularly the possibility that a suitably asymmetric film might be a unidirectional conductor, that is, a rectifier (see Refs. 171, 172). Optical properties of interest include the ability to make planar optical waveguides of thick LB films [173, 174]. 

The top-down unsymmetric substituted spiro compoimds behave differently with regard to their fluorescence properties. In the case of Spiro-SPO 116, the absorption and fluorescence characteristics are mainly determined by the sexiphenyl chain, with an absorption maximum at 331 nm and an intense fluorescence peak with a maximum at 380 nm in dichloromethane. For this compound, the threshold for ASE in neat films is quite low [91]. For a 122-nm-thick film, a threshold of 1 p,J/cm was measured, with a peak wavelength of 419.0 nm. We attribute the low threshold to an efficient absorption by the oxadiazole moieties that have a higher extinction coefficient than the sexiphenyl moieties at the pump wavelength (337 nm) and a subsequent energy transfer to the radiative sexiphenyl chains. 

When an oxide film is too thick for electron tunneling to take place, the intrinsic electronic properties of the film control the kinetics of the charge transfer at the film-electrolyte interface. Most passive films are semiconducting, but their electronic properties can vary greatly from one to another. Figure 6.30 gives the energies of the valence and conduction bands for different oxides, relative to the vacuum level. It also indicates the standard potentials of several redox reactions for the species present in the electrolytes, on the electrochemical scale. 

Tab.l Typical data for passive films taken from Ref. [1], density p, dielectric permittivity e, band gap energy g, flat band potential Ufb, equilibrium potential of oxide electrode Uqx (Reaction 2 [16]), donor concentration N, difference of electronegativity Ax, transference number of cations f+, formation factor dd/dU, and initial oxide thickness do. Because of the strong dependence of properties on the preparation technique, the microstructure and the sensitivity of thin films, the reliability of these data is less than for bulk, crystalline solids... 

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