Cadmium sulfide emission

Cadmium, determination by x-ray emission spectrography, 328 Cadmium sulfide, use in x-ray detection, 43... 

Karas BR, Strickert HH, Schreiner R, EUis AB (1981) Luminescent photoelectrochemical cells. 5. Multiple emission from teUurium-doped cadmium sulfide photoelectrodes and implications regarding excited-state communication. J Am Chem Soc 103 1648-1651... 

Zinc sulfides activated by copper are widely available, mass-produced, low cost materials. Their emission can be tuned over a wide range, from short UV to visible and so the phosphors work in most commonly encountered lighting conditions. However, the green emitting ZnS Cu phosphors, together with copper activated zinc-cadmium sulfide (Zn,Cd)S Cu, are the ones most often used in industrial apphcations. The different versions of ZnS Ag phosphors are exclusively used industrially to obtain a blue emission. Another very important industrial phosphor is the yellow to orange ZnS Mn, which finds application in monochromatic displays. ZnS Tb is a very efficient green phosphor. 

Multiple activation of zinc sulfide is also possible. Zinc-cadmium sulfide, doubly activated with silver and gold, which is used as a white-luminescing, one-component phosphor for monochromic cathode-ray tubes [5.334], and the yellow-luminescing ZnS Cu, Au, A1 phosphor, whose emission color corresponds to that of Zn, x Cdx S Cu [5.332], are known. 

The three basic types are photoconductive, photovoltaic, and photo-emissive, and all are sensitive to both heat and light. The resistance of a photoconductive cell is lowered when it is illuminated and, over a small range, its response is linear. Cells containing lead sulfide, which is sensitive at wavelengths greater than 700 nm, and cadmium sulfide or selenide, with a sharp response maximum at 710 nm, have been used but may not give a stable response and are largely restricted to specialized applications in other fields. Silicon photodiodes and transistors are sensitive from 340 to 1200 nm with a peak at 900 nm. 

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. 

From this discussion, it should be obvious that the two most important properties of cathode-ray phosphors are the response to electron-beaun excitation (brightness) and the decay time. We require a long-decay phosphor for radar applications and a short-decay phosphor for television usage. Nearly all the cathode-ray phosphors are based on the zinc and cadmium sulfides because they exhibit the highest efficiency to cathode-ray excitation. ZnS forms a series of solid solutions with CdS whose emission band can be shifted from the blue (ZnS Ag) to the red phosphor. 

The relationship between the size of the quantum dots and their electronic emission spectra has been elegantly shown with zinc selenide, cadmium sulfide, indium phosphide, and indium arsenide nanocrystals. The emission maxima of CdS, InP, and InAs quantum dots (Table 7.4) provide an excellent example of how these maxima are dependent on both... 

Nanocomposite films of KGM and cadmium sulfide (CdS) were prepared by one-step synthesis in order to create stealthy materials e.g. for military applications [73]. SEM and TEM indicated that hexagonal CdS nanoparticles with the sizes of 10-100 nm were evenly dispersed in KGM. FTIR showed evidences of chelations of Cd " to the hydroxyl groups of KGM. Infrared emissivity values of the samples were carried out on an infrared emissometer and the results indicated that both KGM and CdS nanoparticles had high emissivity, but that of KGM/CdS nanocomposite film was much lower. That could be attributed to the KGM wrapping on the surface of CdS nanoparticles. Strong synergism effect existed also between KGM/chitosan and CdS particles, as detected by the low infrared emissivity value [74]. 

There has been considerable interest in recent years in nanoparticles based on the cadmium sulfides and sulfoselenides. These materials, referred to as quantum dots, display unique optical and electrical properties that are quite different from the properties exhibited at pigmentary particle size, and are attributed to their semiconductor behaviour. The most apparent of these properties is their intense fluorescence, the emission wavelength of which may be tuned based on particle size. Quantum dots have potential for applications in medicine, displays, lasers and solar energy conversion. 

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 two fluorescence emissions are observed. The first is centered at 450 nm and is attributed to the direct recombination of charge carriers. The second emission band, observed at around 650 nm, depends on the particle size. This second emission band is very weak and is very often quenched by the presence of species absorbed at the interface. By analogy it could be attributed to cadmium ion vacancies. 

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... 

Pattabi et al. reported obtaining self-standing flexible nanocomposite films based on PVP capped CdS NPs embedded in a PVA matrix with photoluminescence properties (Pattabi et al. 2007) and electrical conductivity (Amma et al. 2014). CdS NPs were obtained from cadmium nitrate and hydrogen sulfide by a nonaqueous method and then were dispersed in a PVA matrix. With an average particle size of approximately 3-5 nm, they contributed to the obtaining of photoluminescence emission spectra with two peaks, at 502 and 636 mn (Pattabi et al. 2007). The electrical conductivity of the (PVP-CdS) PVA films was seen to increase with temperature (Amma et al. 2014). 

Figure 1. Nanometer-sized cadmium selenide sulfide (CdScxSj.J and cadmium selenide (CdSe). When the samples are irradiated with ultraviolet light (A = 365 nm), the emission colors range from violet to orange (right).

---