Doping of crystals

Crystallization from solution may be quite selective, making controlled doping of crystals very difficult to control or even impossible. In the melt this problem is much easier to handle and also much larger crystals may be grown. Because of their potential applications, various doped fluorides have been intensively studied ... 

The formation of defects of the first t) e is favored by reducing conditions for the doping of crystals while acceptors are created in oxidizing regimes. In order to investigate the lithium acceptor microstructure in ZnO, the photo - EPR method was used, which turned out to be very efficient in this respect. " ... 

Another example of the coloration phenomenon is demonstrated by transition metal doping of crystals exhibited by gemstones such as ruby. Table 2.6 lists some common gemstones, and the respective host crystal and dopants that give rise to their characteristic colors. Whereas crystals of pure corundum (a-alumina) are colorless. 

Acceptor dopants are introduced in the crucible either in elemental form or in the form of carbides. If a dopant is introduced in elemental form, it is placed in a special internal crucible with carbon or silicon carbide powder. This is required to prevent the dissolution of the crucible, in the case of aluminium doping, and to reduce the boron vapour pressure to the equilibrium value for the SiC-C system, in the case of boron doping. If elemental boron is placed in the vicinity of the substrate, this results in the formation of boron carbide on the crystal faces of SiC [46]. For moderate doping of crystals, grown at high temperatures, doped SiC sources also can be employed. 

Earlier during the discussion on the preparation of polymorphs, the doping of crystals was mentioned as a technique for encouraging the formation of one type of polymorph over another. Similarly, if a dopant is employed at levels that will disrupt the crystal lattice, the substance can be made to solidify as an amorphous material. Duddu and Grant [157] observed changes in the enthalpy of fusion of (-)-ephedrinium 2-naphthatenesulfonate when the opposite enantiomer, (+)-ephedrinium 2-naphthalenesulfonate, was added as a dopant. 

Single molecules also have promise as probes for local stmcture when doped into materials tliat are tliemselves nonfluorescent. Rlrodamine dyes in botli silicate and polymer tliin films exliibit a distribution of fluorescence maxima indicative of considerable heterogeneity in local environments, particularly for the silicate material [159]. A bimodal distribution of fluorescence intensities observed for single molecules of crystal violet in a PMMA film has been suggested to result from high and low viscosity local sites witliin tire polymer tliat give rise to slow and fast internal conversion, respectively [160]. 

Such defects facilitate movement of C02 within the crystal by transfer from HCOj to OH" and of Ag+ in the cation vacancies. This interpretation is supported [758,759] by the observed increase in reactivity resulting from doping of Ag2C03 with Cd2+, Y3+ or Gd3+, where incorporation of the additive is accompanied by the creation of cation vacancies. 

Accommodation of metal atoms of widely differing ionic radii into the same overall structure creates interesting possibilities for the doping of metal ions into a common matrix for spectroscopic examination under nearly constant crystal field effects. 

Electro-Optical Properties of Bimetallic Nanoparticle-Doped Liquid Crystal Displays... 

The second source of partitioning data is experimental equilibration of crystals and liquids followed by microbeam analysis of quenched run products. Starting materials can be natural rocks, or synthetic analogues. In either case it is customary to dope the starting material with the U-series element(s) of interest, in order to enhance analytical precision. Of course, doping levels should not be so high as to trigger trace phase saturation (e g.. 

Wenger OS, Giidel HU (2003) Influence of Crystal Field Parameters on Near-Infrared to Visible Photon Upconversion in Ti2+ andNi2+ Doped Halide Lattices 106 59-70 Wheatley AEH, see Linton DJ (2003) 105 67-139 Wilhelm M, see Haubner R (2002) 102 1-46... 

Doping is important for semiconductors in order to tune their optical and electrical properties for the potential applications in biotechnology and solar cells [65]. Ag-doped hexagonal CdS nanoparticles were successfully obtained by an ultrasound-assisted microwave synthesis method. Here, the doping of Ag in to CdS nanoparticles induced the evolution of crystal structure from cubic to hexagonal. Further support from photocatalytic experiment also clearly indicates the doping of Ag clusters into the CdS matrix. 

Now the possible explanation for the greater degradation of phenol by the cerium doped copper crystals than those of either cobalt or manganese ions could... 

Fig. 4. Arrhenius plot of the free hole concentration in a beryllium-doped germanium crystal grown in a hydrogen atmosphere. The shallow acceptor A(Be,H), present at a concentration of 1013 cm-3, is shown to dissociate under thermal annealing. 

Fig. 12. Derivative curves of EPR in a highly dislocated As-doped germanium crystal grown in a H2 atmosphere. The magnetic field is oriented along the [100] direction. T= 2 K, /= 25.16 GHz. Note the sign reversal of the new lines as compared to the As-donor hyperfine structure. Dislocation density 2 x 104 cm 2. (Courtesy Pakulis and Jeffries, reprinted with permission from the American Physical Society, Pakulis, E.J., Jeffries, C D. Phys. Rev. Lett. (1981). 47, 1859.)...

No material is completely pure, and some foreign atoms will invariably be present. If these are undesirable or accidental, they are termed impurities, but if they have been added deliberately, to change the properties of the material on purpose, they are called dopant atoms. Impurities can form point defects when present in low concentrations, the simplest of which are analogs of vacancies and interstitials. For example, an impurity atom A in a crystal of a metal M can occupy atom sites normally occupied by the parent atoms, to form substitutional point defects, written AM, or can occupy interstitial sites, to form interstitial point defects, written Aj (Fig. 1.4). The doping of aluminum into silicon creates substitutional point defects as the aluminum atoms occupy sites normally filled by silicon atoms. In compounds, the impurities can affect one or all sublattices. For instance, natural sodium chloride often contains... 

Acceptor doping, as in lithium oxide doping of nickel oxide, produces p-type thermistors. The situation in nickel-oxide-doped Mn304 is similar but slightly more complex. This oxide has a distorted spinel structure (Supplementary Material SI), with Mn2+ occupying tetrahedral sites and Mn3+ occupying octahedral sites in the crystal, to give a formula Mn2+[Mn3+]204, where the square parentheses enclose the ions in octahedral sites. The dopant Ni2+ ions preferentially occupy... 

Figure 9.21 Auger spectra of cobalt-doped MoS2 crystals taken at a) the basal plane and b) the edge regions (from Chianelli el ul. [72]).

Several ENDOR investigations on X-irradiated, Cu(II)-, and VO(II)-doped single crystals of triglycine sulfate (TGS) in its ferroelectric phase have been reported by Windsch... 

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