Cadmium sulfide spectra

Light detectors fall into two categories photoconductors and photodetectors. Photoconductors are devices whose resistance decreases upon exposure to light. Cadmium sulfide (CdS) and cadmium selenide (CdSe) are the most commonly used photoconductor materials in the visible spectrum. They are still mostly produced by sputtering but CVD is used increasingly (see Ch. 12, Sec. 4.0). 

A prime contender for leading thin film technology as applied to solar cells is cadmium telluride (CdTe). Its bandgap is almost ideal for use as a solar cell for energy conversion from the Sun s spectrum. Here, CdTe and cadmium sulfide (CdS) are used to produce a low cost thin film solar cell... 

Since the photophoretic force depends on the electromagnetic absorption efficiency Q y , which is sensitive to wavelength, photophoretic force measurements can be used as a tool to study absorption spectroscopy. This was first recognized by Pope et al. (1979), who showed that the spectrum of the photophoretic force on a 10 foa diameter perylene crystallite agrees with the optical spectrum. This was accomplished by suspending a perylene particle in a Millikan chamber with electro-optic feedback control and measuring the photophoretic force as a function of the wavelength of the laser illumination. Improvements on the technique and additional data were obtained by Arnold and Amani (1980), and Arnold et al. (1980) provided further details of their photophoretic spectrometer. A photophoretic spectrum of a crystallite of cadmium sulfide reported by Arnold and Amani is presented in Fig. 11. 

Cadmium sulfide suspensions are characterized by an absorption spectrum in the visible range. In the case of small particles, a quantum size effect (28-37) is observed due to the perturbation of the electronic structure of the semiconductor with the change in the particle size. For the CdS semiconductor, as the diameter of the particles approaches the excitonic diameter, its electronic properties start to change (28,33,34). This gives a widening of the forbidden band and therefore a blue shift in the absorption threshold as the size decreases. This phenomenon occurs as the cristallite size is comparable or below the excitonic diameter of 50-60 A (34). In a first approximation, a simple electron hole in a box model can quantify this blue shift with the size variation (28,34,37). Thus the absorption threshold is directly related to the average size of the particles in solution. 

When a solder such as the one just mentioned is irradiated in a cadmium-rich region with 1.9-MeV deuterons, the proton spectrum indicates traces of sulfur (around 30 ppm) (12). The presence of sulfur suggests that cadmium sulfide could have been used as a component of the brazing material. This possibility has been examined in our laboratory and has been confirmed (13, 14). Additional solder characterization was accomplished by using the se-... 

Watanabe et al. have reported similar action spectrum analysis of photoin-duced degradation of Rhodamine B with a cadmium sulfide suspension and pointed out a similar dye-sensitization mechanism Watanabe, T. Takizawa, T. Honda, K. J. Phys. Chem. 1977, 81, 1845. Photocatalytic reaction of MB in aerated titania suspensions was reported in 1937 by a Japanese photochemist Horio, M. Nihon Gakujutsu Kyokai Hokoku 1937, 12, 204 (in Japanese). As far as the author knows, this is the first report on titania photocatalysis. 

Briggs RJ, Ramdas AK (1976) Piezospectroscopic study of the Raman spectrum of cadmium sulfide. Phys Rev B 13 5518-5529... 

A further feature may be obtained by producing a less structured red form of cadmium sulfide. Hie red CdS colloid exhibits a red shifted emission spectrum compared to that of normal cadmium sulfide, and it also has a much longer lifetime. This gives a much longer lifetime for the electron-hole pair and larger yields of reduced product are expected from such systems. 

The luminescence of macrocrystalline cadmium and zinc sulfides has been studied very thoroughly The colloidal solutions of these compounds also fluoresce, the intensity and wavelengths of emission depending on how the colloids were prepared. We will divide the description of the fluorescence phenomena into two parts. In this section we will discuss the fluorescence of larger colloidal particles, i.e. of CdS particles which are yellow as the macrocrystalline material, and of ZnS particles whose absorption spectrum also resembles that of the macrocrystals. These colloids are obtained by precipitating CdS or ZnS in the presence of the silicon dioxide stabilizer mentioned in Sect. 3.2, or in the presence of 10 M sodium polyphosphate , or surfactants such as sodium dodecyl sulfate and cetyldimethylbenzyl-ammonium... 

In the presence of an excess of sulfide ions, [Cd2+]/[S2 ] = 5, a strong change in the absorption spectra at low water content is observed compared to that obtained for a ratio of [Cd2+]/[S2-] equal to 2. By increasing the water content, the sharp peak disappears and a similar behavior as in the case of excess of cadmium is observed, i.e., a red shift in the absorption spectrum. The sharp peak observed at low water content increases with the relative amount of sulfide ions (45). This peak is attributed to sulfide clusters (55) formed on the CdS particles because of the high local concentration of sulfide ions. The disappearance of this peak when increasing the water content could be explained by the fact that sulfide clusters, with negative charges, are repelled to the center of the droplets and redissolve themselves inside the water pool. 

Figure 35 shows the photolumincsccnce spectrum of CdS supported on PVG with a relatively high loading 196). Peaks are observed near 520, 560, and 680 nm. The 680-nm peak is associated with the sulfur vacancy since the presence of excess sulfide ions quenches the photoluminescence however, the presence of excess cadmium has no effect on the emission. The 520- and 560-nm photoluminescence are associated with the major bulk emission 197-199). The 520-nm emission is attributed to the band-to-band transition, and the 560-nm emission is attributed to a typical radiative clcctron-hole recombination at the particle surface. As shown in Fig. 35 (b), the addition of H2O to the catalyst has a significant effect on the spectrum. The 560-nm photoluminescence is completely quenched, as expected if the radiative recombination of electrons and holes occurs at the surfaces where H2O molecules easily interact with these electrons and holes, thereby reducing the energy and intensity of the photoluminescence. On the other hand, the 520-nm emission from the bulk emitting sites is not affected by the addition of H2O. The photoluminescence... 

Many other systems based on different nanoparticles have been introduced, such as copper indium disulfide (CuInS2) [263-265], copper indium diselenide (CuInSe2) [266,267], cadmium telluride (CdTe) [268], lead sulfide (PbS) [269,270], lead selenide (PdSe) [271], and mercury telluride (HgTe) [272]. Some of these systems show enhanced spectral response well into the infrared part of the solar spectrum [271,272]. In most cases the absorption of the nanocrystals was, however, quantitatively small as compared to the conjugated polymers. 

A lead sulfide photoconductor is the most widely used transducer for the near-1R region of the spectrum from 10,000 to 333 cm(I to 3 pm). It can be operated at room temperature. For mid- and far-lR radiation, MCT photoconductor transducers are used. They must be cooled with liquid nitrogen (77 K) to minimize thermal noise. The long-wavelength cutoff, and many of the other properties of these transducers, depend on the ratio of the mercury telluride to cadmium telluride. which can be varied continuously. 

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