Point defect: also extrinsic

Fig. 2 illustrates (he room-temperature photoluminescence (PL) spectra recorded from the as-prepared ZnO colloidal solution and the ZnO nanostructure formed after deposition of the colloid on the silicon substrate. An UV band at 385 nm was detected from all ZnO products. In addition, a broad orange-red photoluminescence band centered at around 620 nm could be also observed in some materials. The UV photoluminescence peak at 385 nm is well known to be related to the exciton emission, ihe mechanism of visible emission is suggested mainly due to the present of various point defects, either extrinsic or intrinsic, which can easily form recombination centers. Photoluminescence measurements show that the deposited ZnO nanostructures have the stronger UV emission than the ZnO nanoparticles in the colloidal solutions. The better UV emission characteristic of deposited ZnO is suggested to be due to the lower defect density and oxygen vacancies in ZnO nanocrystals in the first case. Similar results have also been reported previously [8]. In addition, the aqueous surrounding can change the surface states of ZnO nanocrystals. It is well known that surface states may... 

Previous sections of this chapter have shown that it is possible to introduce defects into a perfect crystal by adding an impurity. Such an addition causes point defects of one sort or another to form, but they no longer occur in complementary pairs. Impurity-induced defects are said to be extrinsic. We have also noted that when assessing what defects have been created in a crystal, it is important to remember that the overall charge on the crystal must always be zero. 

There are many factors which contribute to dielectric loss and in the case of the complex ceramic compounds discussed above, to achieve a satisfying understanding of the relative magnitudes of the various loss mechanisms is challenging. There will be contributions to loss intrinsic to the idealized structural chemistry of the material and it is now clear that this is complicated by a domain structure. There will also be contributions of an extrinsic nature, particularly those associated with departures from the ideal structure, point defects and... 

Nonequihbrium concentrations of point defects can be introduced by materials processing (e.g. rapid quenching or irradiation treatment), in which case they are classified as extrinsic. Extrinsic defects can also be introduced chemically. Often times, nonstoichiometry results from extrinsic point defects, and its extent may be measmed by the defect concentration. Many transition metal compounds are nonstoichiometric because the transition metal is present in more than one oxidation state. For example, some of the metal ions may be oxidized to a higher valence state. This requires either the introduction of cation vacancies or the creation of anion interstitials in order to maintain charge neutrality. The possibility for mixed-valency is not a prerequisite for nonstoichiometry, however. In the alkah hahdes, extra alkah metal atoms can diffuse into the lattice, giving (5 metal atoms ionize and force an equal number... 

Fig. 3.3. Illustration of the main proton transfer mechanisms (a) defect mechanism in a densely packed structure (b) loosely packed structure with a high concentration of mobile species (c) quasi-liquid state with a proton jump contribution In (a) the conductivity is favoured by intrinsic (interstitial rabbits) or extrinsic (impurity elephant) point defects. An orientation defect (hippopotamus in the wrong orientation) can also favour disorder of rabbits (Oj for Zr02 CaO, H for KHSO4) (b) the tree sublattice is a perfectly stable loosely packed structure and a high rabbit disorder can exist without affecting the host lattice (e.g. NH4 in p-AljOj) (c) only the mobile species sublattice is considered here these entities are moving with different speeds in different directions and some are hopping such may be the image of a quasi liquid or surface liquid (V205.nH20, HUP).

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