Cadmium electronic properties

A review on mercury and cadmium pentelide halides has appeared,254 with emphasis on discussion of the structures, based on the Zintl Klemm concept, and their relationship to the electronic properties. 

Metal alkyls can be prepared in a simple manner from the main group halides (X = Cl, Br, I) and the appropriate alkyl Grignard reagent (RMgl) or the alkyllithium salt (RLi), as shown for the cadmium alkyls (Equation (2)).13 The elimination of impurities from the precursor source is of great importance, as any remaining impurities are invariably carried over into the growing semiconductor layers. Incorporation of impurities, even at levels as low as 1015 free carriers per cubic centimeter (one part in ca. 107), can drastically affect the electronic properties of the... 

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. 

Careful analysis of both powder X-ray dilfraction and EXAFS spectroscopic data located the cadmium sulfide as (CdS)4 cubes occupying the space within sodalite cages, with the Cd ions coordinated to framework oxygen atoms (Fig. 6.10). Furthermore, the clusters were observed to order between adjacent sodalite cages, to give superclusters or a superlattice structure. In subsequent work, a variety of compounds and elements have been prepared as well-defined clusters within zeolite frameworks, including metal oxides, selenides and phosphides, and these have been studied mainly with the view of determining the effects of cluster size on optical and electronic properties. 

Metallic particles can be prepared by reducing salts dissolved in the droplets. For example, copper particles can be produced using hydrazine as a reducing agent (Pileni, 1993). Composite particles with a core of one metal surrounded by another have also been reported (see Belloni, 1996). Preparation of nanoparticles of various metals has been reviewed by Capek (2004). In some cases, supercritical carbon dioxide (COj) has been used instead of a hydrocarbon as the continuous phase in the microemulsion, which provides a system where the phase behavior can be relatively easily controlled by changing pressure and eliminates the use of volatile solvents. There is considerable interest in making nanoscale particles of semiconductors such as cadmium sulfide (CdS) using water-in-oil microemulsions because their electronic properties are different from those of bulk crystals (see Pileni, 1993). 

In this chapter, special attention is given to the use of radiation for the synthesis of colloidal nano-particles and for the investigation of their chemical and electronic properties. Radiolytic preparation under strictly anaerobic conditions has been shown to be an especially useful method, which allows one to control particle formation in a reproducible manner. Most studies in the literature are concerned with particles of the noble metals, i.e. of Au, Pt, Ag and Cu. The radiolysis method enables one to prepare also colloids of the more electronegative metals, such as of cadmium and thallium. In addition, this method has also been used to initiate chemical reactions on the surface of nanoparticles in a controlled manner. 

Joswig, J.-O., Springborg, M., 8c Seifert, G. (2000). Structural and electronic properties of Cadmium Sulfide clusters. The Journal of Physical Chemistry B, 104, 2617-2622. 

Quantum-dots (QDs) are colloidal semiconductors. They exhibit unique optical and electronic properties that are closely related to the size and shape of each crystal. QDs are in the limelight for optical applications due to their high sensitivity and the stability towards photobleaching. However, they were not originally biocompatible because traditional QDs are made with toxic metal ions, such as cadmium. To increase the stability or the efficacy of delivery, chitosan derivatives have been utilized for coating or encapsulating QDs. Tan et al. developed chitosan... 

Solders are alloys that have melting temperatures below 300°C, formed from elements such as tin, lead, antimony, bismuth, and cadmium. Tin—lead solders are commonly used for electronic appHcations, showing traces of other elements that can tailor the solder properties for specific appHcations. 

Heterogeneous Photocatalysis. Heterogeneous photocatalysis is a technology based on the irradiation of a semiconductor (SC) photocatalyst, for example, titanium dioxide [13463-67-7] Ti02, zinc oxide [1314-13-2] ZnO, or cadmium sulfide [1306-23-6] CdS. Semiconductor materials have electrical conductivity properties between those of metals and insulators, and have narrow energy gaps (band gap) between the filled valence band and the conduction band (see Electronic materials Semiconductors). 

In this section, we will discuss organometallic derivatives of zinc, cadmium, mercury, and indium. The group IIB and IIIB metals have the d10 electronic configuration in the 2+ and 3+ oxidation states, respectively. Because of the filled d level, the 2+ or 3+ oxidation states are quite stable, and reactions of the organometallics usually do not involve changes in oxidation level. This property makes the reactivity patterns of these organometallics more similar to those of derivatives of the group IA and IIA metals than to those of derivatives of transition metals with vacancies in the d levels. The IIB metals, however, are... 

In certain solids such as titanium dioxide or cadmium sulfide, the energy of the band gap corresponds to that of light (visible, ultraviolet, or infrared), with the result that the solid, when illuminated, may become electrically conducting or acquire potent chemical redox characteristics because of the promotion of electrons to the conduction band (which is normally unoccupied). These properties have obvious practical significance and are considered at length in Chapter 19. 

Physical properties of cadmium are listed in Table 1. Its electronic structure is lT2T2 63T3/)63i7104T4p64i7105T, and its oxidation state in almost all of its compounds is +2, although a few compounds have been reported (1) in which cadmium exists in the +1 oxidation state. There are eight natural isotopes ... 

Cadmium is a member of Group 12 (Zn, Cd, Hg) of the Periodic Table, having a filled d shell of electrons 4valence state of +2. In rare instances the +1 oxidation state may be produced in the form of dimeric Cd2+2 species [59458-73-0], eg, as dark red melts of Cd° dissolved in molten cadmium halides or as diamagnetic yellow solids such as (Cd2)2+ (AlCl [79110-87-5] (2). The Cd + species is unstable in water or other donor solvents, immediately disproportionating to Cd2+ and Cd. In general, cadmium compounds exhibit properties similar to the corresponding zinc compounds. Compounds and properties are listed in Table 1. Cadmium(TT) [22537 48-0] tends to favor tetrahedral coordination in its compounds, particularly in solution as complexes, eg, tetraamminecadmium(II) [18373-05-2], Cd(NH3)2+4. However, solid-state cadmium-containing oxide or halide materials frequently exhibit octahedral coordination at the Cd2+ ion, eg, the rock-salt structure found for CdO. 

The (/-block elements tend to lose their valence s-electrons when they form compounds. Most of them can also lose a variable number of d-electrons and show variable valence. The only elements of the block that do not use their (/-electrons in compound formation are the members of Group 12 (zinc, cadmium, and mercury). The ability to exist in different oxidation states is responsible for many of the special properties of these elements and plays a role in the action of many vital biomolecules (Box 16.1). 

Colloids of semiconductors are also quite interesting for the transmembrane PET, as they possess both the properties of photosensitizers and electron conductors. Fendler and co-workers [246-250] have shown that it is possible to fix the cadmium sulfide colloid particles onto the membranes of surfactant vesicles and have investigated the photochemical and photocatalytic reactions of the fixed CdS in the presence of various electron donors and acceptors. Note, that there is no vectorial transmembrane PET in these systems. The vesicle serves only as the carrier of CdS particles which are selectively fixed either on the inner or on the outer vesicle surface and are partly embedded into the membrane. However, the size of the CdS particle is 20-50 A, i.e. this particle can perhaps span across the notable part of the membrane wall. Therefore it seems attractive to use the photoconductivity of CdS for the transmembrane PET. Recently Tricot and Manassen [86] have reported the observation of PET across CdS-containing membranes (see System 32 of Table 1), but the mechanism of this process has not been elucidated. Note, that metal sulfide semiconductor photosensitizers can be deposited also onto planar BLMs [251],... 

---