Sensitization of the Photoconductivity and Photo-emf of Heavy Metal Azides Stained with Organic Dyes

Yu. V. Gavrishchenko, G. G. Savel’ev, Yu. A. Zakharov

(Presented by the scientific seminar of the Department of Radiation Chemistry)

Elucidation of the mechanism of optical sensitization of photoelectric phenomena in solids is usually reduced to the study of processes occurring under illumination. Recently, however, when measuring the dark conductivity of samples before and after application of a dye [1], dark electron transitions between the substrate and the adsorbed dye were discovered; these must be taken into account in constructing a mechanism of optical sensitization.

To study dark electron transitions between the substrate and the adsorbed dye, as well as the mechanism of optical sensitization of lead, silver, and thallium azides stained with organic dyes, we measured their photoconductivity and photo-emf. Since sensitization of heavy metal azides had previously scarcely been studied, we carried out exploratory investigations with typical photographic dyes: thiocarbocyanine, indocarbocyanine, imidocarbocyanine, 3,3′-diethylthiazolinocarbocyanine iodide, 3,3′-diethyloxacarbocyanine iodide, 3,3′-di(β-oxyethyl)-9-methylthiocarbocyanine chloride, 3,3′-di(β-oxyethyl)-9-ethyl-5,5′-dimethoxythiocarbocyanine chloride, sodium eosin, erythrosin, cryptocyanine, and trypaflavine.

The heavy metal azides were obtained by precipitation from aqueous solutions, followed by staining them in an alcoholic dye solution with a concentration of \(10^{-3} \div 10^{-5}\) mole/liter. The photoconductivity was measured in a constant-field regime under modulated illumination with a modulation frequency of 100 Hz [2]. The photo-emf was measured by the dynamic-capacitor method [3]. As light sources, a high-pressure mercury lamp of the DRSh-250 type and a 127 V, 500 W incandescent lamp with an SF-4 quartz monochromator were used.

On the basis of measurements of the spectral distribution of the capacitor photo-emf of lead and silver azides [4, 5], it was shown that these azides have a hole type of conductivity; moreover, lead azide has an anti-barrier surface potential, while silver azide has a barrier potential. In the present work the photo-emf of thallium azide was studied, and it was found that pure thallium azide has a hole type of conductivity and a barrier surface potential.

Heavy metal azides stained with organic dyes have sensitization maxima of photoconductivity in the spectral region of dye absorption.

The results obtained are summarized in the table.

In azides stained with cryptocyanine, trypaflavine, 3,3′-diethylthiazolinocarbocyanine iodide, and 3,3′-diethyloxacarbocyanine iodide, no sensitization of photoconductivity was observed.

At the same time, in stained samples the photoconductivity changes in the wavelength region corresponding to the intrinsic absorption of the azides. This phenomenon can be explained by depletion or enrichment with electrons of the near-surface layer as a result of a change in the surface potential.

Table 1

The change in surface potential upon staining the samples indicates the presence of electron transitions between the dye and the substrate and can be determined from the change in the magnitude of the capacitor photo-emf in the region of intrinsic absorption, where the main component is the Dember emf [6]. Figures 1a, b, c show the spectra of the capacitor photo-emf of pure and stained azides. When lead and thallium azides are stained with indocarbocyanine and thiocarbocyanine, and also silver azide with thiocarbocyanine, the maximum in the wavelength region \(< 400\) nm decreases; however, for silver azide stained with indocarbocyanine this maximum increases. This is readily explained if it is assumed that, upon staining, electrons pass from lead and thallium azides into the dyes, while silver azide gives up its electrons to thiocarbocyanine but captures them when stained with indocarbocyanine. Thus, the energy level of thiocarbocyanine molecules upon adsorption on azides lies below the Fermi level of the azides, while the energy level of indocarbocyanine must lie below the Fermi level of lead azide but above the Fermi level of silver azide. These data agree with measurements of the thermoelectronic work function in lead and silver azides, which is equal to 4.5 eV and 4.7 eV, respectively [5].

If sensitization occurs by energy transfer, then a considerable gradient of carrier concentration should be formed in the substrate, and the sign of the capacitor photo-emf should be the same as in the region of intrinsic absorption; however, this condition is notրաժեշտ

necessary, but not sufficient for establishing the mechanism of sensitization, since it is also necessary to take into account the sign of the surface potential. If the sign of the capacitor photo-emf in the sensitization region is opposite to the sign in the intrinsic region, then sensitization occurs only through electron transfer.

Lead azide has an anti-barrier surface potential; moreover, when stained with indocarbocyanine, the capacitor photo-emf has a positive sign in the dye absorption region, i.e., sensitization occurs through electron transfer; whereas when stained with thiocarbocyanine the capacitor photo-emf has a negative value, i.e., sensitization occurs through energy transfer.

Fig. 1. Spectral dependence of the photo-emf of lead (a), silver (b), and thallium (c) azides; 1—unstained azide; 2—stained with indocarbocyanine; 3—stained with thiocarbocyanine.

Silver and thallium azides, stained with indocarbocyanine and thiocarbocyanine, have a barrier surface potential, and for this reason one cannot judge from the photo-emf spectra whether, in this case, electron transfer or energy transfer occurs.

In the present work, sensitization of the photolysis of heavy-metal azides by adsorbed dyes is not considered; however, in two cases there are literature data on sensitization of the photolysis of lead azide by thiocarbocyanine [5] and of silver azide by erythrosin [7]. The sensitization of the photocurrent observed by us in these samples indicates a significant role of electronic defects in the photochemical decomposition of lead and silver azides.

The authors express their gratitude to Doctor of Chemical Sciences S. V. Natanson for kindly providing the dyes for carrying out the experiments.

REFERENCES

1. S. J. Dudkowski, A. G. Kepka, L. I. Grossweiner. J. Chem. Phys. Sol., vol. 28, No. 3, 485 (1967).

2. S. M. Ryvkin. Photoelectric Phenomena in Semiconductors, p. 40, Moscow, 1963.

3. A. M. Meshkov, I. A. Akimov. Instruments and Experimental Techniques, No. 3, 141, 1964.

4. G. G. Savel’ev, Yu. V. Gavrishchenko, Yu. A. Zakharov. Izv. Vyssh. Uchebn. Zaved., Fizika (in press).

5. Yu. A. Zakharov, G. G. Savel’ev, Yu. V. Gavrishchenko, S. M. Ryabykh, G. A. Kislin. “Proceedings of the All-Union Conference on Radiation Physics,” Tomsk, 1967 (in press).

6. I. A. Akimov. Optiko-mekhanicheskaya promyshlennost’, No. 5, 4, 1966.

7. F. Bowden, A. Yoffe. Fast Reactions in Solids. IL Publishing House, Moscow, p. 148, 1962.