Zinc vacancies

A Stern-Volmer plot obtained in the presence of donors for the stilbene isomerization has both curved and linear components. Two minimal mechanistic schemes were proposed to explain this unforeseen complexity they differ as to whether the adsorption of the quencher on the surface competes with that of the reactant or whether each species has a preferred site and is adsorbed independently. In either mechanism, quenching of a surface adsorbed radical cation by a quencher in solution is required In an analogous study on ZnS with simple alkenes, high turnover numbers were observed at active sites where trapped holes derived from surface states (sulfur radicals from zinc vacancies or interstitial sulfur) play a decisive role... 

In recent years, there has been considerable effort to derive defect formation enthalpies of intrinsic defects in ZnO [108-110,113-117]. An example is shown in Fig. 1.13 [115]. Horizontal curves belong to neutral defects, curves with positive or negative slopes to charged donors or acceptors, respectively. The donor with the lowest formation enthalpy is the oxygen vacancy Vo, the acceptor with the lowest formation enthalpy the zinc vacancy Vzn-... 

The electron concentration in donor-doped TCOs becomes compensated with increasing oxygen partial pressure. The nature of the compensating defect thereby depends on the material. As mentioned earlier, compensation of n-type doping in ZnO occurs by introduction of zinc vacancies. In contrast, compensation in 1 03 is accomplished by oxygen interstitials [117], Their importance in Sn-doped 1 03 has been already pointed out by Frank... 

Fig. 1.15. Electron concentration (dashed line) of Sn-doped indium oxide and Al-doped ZnO in dependence on oxygen partial pressure for a dopant concentration of 1 % [117]. With increasing oxygen partial pressure the donors become compensated by oxygen interstitials (In20s) or by zinc vacancies (ZnO). Reprinted with permission from [117]. Copyright (2007) by the American Physical Society...

Self-diffusion in materials occurs by repeated occupation of defects. Depending on the defects involved one can distinguish between (1) vacancy, (2) interstial, and (3) interstitialcy mechanisms [107], As an example, different diffusion paths for oxygen interstitials are illustrated in Fig. 1.16 [129]. For a detailed description of diffusion paths for oxygen vacancies, zinc vacancies and zinc interstitials the reader is also referred to literature [129,130]. 

Erhart and Albe also calculated zinc diffusion in ZnO [130]. The results are displayed in Fig. 1.18 together with a comparison to experimental data. Depending on chemical potential and Fermi level position either zinc vacancy or zinc interstitial diffusion can dominate. In the case of n-type material, where the Fermi level is close to the conduction band, zinc diffusion is mostly accomplished via the vacancy mechanism. 

The ZnS nanotubes and nanorods were characterized by UV-visible absorption spectroscopy and PL spectroscopy. The inset in Fig. 2a shows the absorption spectrum of the ZnS nanotubes. The band appearing at 318nm is blue-shifted relative to that of the bulk ZnS (350 nm) [17]. Nanowires of ZnS of diameter 5 nm were reported to show an absorption maximum around 326 nm [18], An absorption band at 320 nm has been reported in the case of ZnS quantum dots [19], The PL spectrum of ZnS nanotubes given in the inset of Fig. 2b exhibits two bands, a weak blue emission at 485 nm and a strong green emission around 538 nm. The 485 nm band is attributed to zinc vacancies in the ZnS lattice. Emission bands at 470 nm [20] and 498nm [21] have been reported in ZnS nanobelts. The 538 emission band is similar to that reported for ZnS nanobelts [22] and is considered to result from vacancy or interstitial states [22,23]. 

ZnS has a band gap of 3.64eV. When doped with Cu , it emits radiation at 670 nm. When zinc vacancies are produced by the incorporation of CP ions, the radiation is centered on 440 nm. 

Fig. 3. Illustrations of (a) the geometrical arrangement of zinc and oxygen species on the ZnO(lOlO) non-polar surface, and (b) the electronic energy levels adjacent to the surface. Evac, Ec, Ef, and Ev denote, respectively, the energy level of electrons in vacuum, at the bottom of the bulk conduction band, at the Fermi level, or at the top of the valence band. Discrete energy levels associated with oxygen or zinc vacancies are denoted by Ey0 anc respectively. Bands of surface states associated with...

Also in other semiconductors such donor-acceptor pair emissions have been found. A well-known example is ZnS Cu,Al where Alzn is the donor and Cuzo the acceptor. This material is used as the green-emitting phosphor in color television tubes. The blue emission of ZnS Al is due to recombination in an associate consisting of a zinc vacancy (acceptor) and Alzn (donor). Since these centres occur as coupled defects, their distance is restricted to one value only (this is sometimes called a molecular center). Due to strong electron-lattice coupling the emissions from ZnS consist of broad bands. 

The formation energies of native defects in ZnO have been calculated by several groups of theorists and the results generally agree [86,88-91). The results for oxygen and zinc vacancies, interstitials, and antisites in ZnO are shown in Figure 3.17 for the... 

In 2000, Pingbo et al. [59] synthesized nanocrystalline ZnS Mn using reaction via microemulsion method. They recorded PL spectra for different Mn concentrations and compared the luminescence with and without surface-modification. They also analized fluorescence lifetime for different emission wavelengths and reported that nanosecond decay is due to zinc vacancies and millisecond decay is attributed to Mn. They also studied the photo aging of their samples.The XPS measurements indicated the effect of surface-modification on the surface structure. 

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