Electron spin resonance conducting films

EPR measurements were first performed on wurtzite GaN in 1993 by Carlos and co-workers [2-4] and on cubic GaN by Fanciulli and co-workers at about the same time [5], The primary resonance in the wurtzite films is slightly anisotropic (gy = 1.9510 and gi = 1.9483) with a width 0.5 mT at 4.2 K and generally acknowledged to be due to a band of delocalised effective mass (EM) donor electrons. The average g value is consistent with the expectations of a 5-band k.p analysis and is also similar to that obtained by Fanciulli [5] for a much broader line (—10 mT) in their conduction electron spin resonance experiments on zincblende films. With this exception all of the work discussed in this Datareview is on the wurtzite phase. 

Recently, both polycrystalline and microcrystalline ( 200 A crystallite size) Si films have been investigated by ESR (Hasegawa ft a/., 1981b, 1983b). A dangling bond resonance in microcrystalline silicon films is observed at g = 2.0049. The spin density of this resonance is typically between 10 and 10 spins cm. In P-doped samples a conduction-electron spin-resonance signal is observed at = 1.997. 

Polyaniline films prepared potentiostatically in sulfuric acid solution are investigated by FT-IR, UV-Vis optical spectroscopies at various dopant concentrations. Formation of charge carriers in the films are studied Ity means of electron spin resonance (ESR) spectroscopy. These results will in turn be correlated on the basis of conductivity measurement. The formations of polarons and bipolarons at different doping levels are also discussed. 

Built-up multilayers can be obtained only for transfer achieved at a surface far beyond the plateau regime, generally around 35 mN/m. Hence the resulting films are just a disordered superposition of oken monocrystals, with a random orientation in the plane of the support, as shown in Fig. 2b. As a consequence, the normal to the support behaves as an axis of full rotation. Such a macroscopic configuration is easy to investigate. This was done by the use of two different resonance methods, namely, linear dichroism and electron spin resonance, with the following result TCNQ radical anions lie completely flat on the substrate in the dimeric form [9]. Obviously, such a structure is not compatible with an in-plane conductivity, for which TCNQ molecular planes should be perpendicular to the substrate. In addition, the complete charge transfer between pyridinium and TCNQ prevents any kind of electron conductivity in the film and, as expected, the electrical properties of the pristine film are those of an insulator. 

Correlated transport studies (including temperature dependent dc conductivity, audio-frequency (lOHz-105 Hz) conductivity and dielectric constant, electric field dependence of conductivity, thermopower, electron spin resonance, microwave conductivity and dielectric constant) have been carried out on oriented emeraldine hydrochloride films [15], the... 

The role of the dopant potential on the stability and magnetic and optical properties of polarons and bipolarons in conducting polymers is shown with the aid of calculations of singlet and triplet states of a bipolaron [167] and by spectroelectrochemical and conductivity measurements [168-170]. The X-band optically detected magnetic resonance of PHT and PDDT shows that the distant intrachain polaron recombination is temperature-independent and identical in films and solutions. However, the triplet polaronic excitation decay is observable in films, but not in solutions [171], Electrochemical in situ conductivity and EPR measurements of PT films were performed in several solutions [172]. The results indicate that polarons merely seem to initiate the electrical conductivity. The electronic delocalization of polarons is restricted to a relatively short chain length at low potentials. As the polaron concentration increases (spin density maximum), bipolarons are generated immediately (probably too fast for the detection of polarons by EPR). Thus the bipolarons prevail in the fully conducting polymer films and as a consequence should be mainly responsible of the intrinsic conductivity [172]. Asymmetrically disub-stituted PBT display well-defined redox processes which are correlated to the consecutive formation of radical cations, dimerized radical cations, and dications [173]. 

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