Intrinsic Defects in ZnO

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-... 

In comparison to the research in n-type oxide semiconductors, little work has been done on the development of p-type TCOs. The effective p-type doping in TCOs is often compensated due to their intrinsic oxide structural tolerance to oxygen vacancies and metal interstitials. Recently, significant developments have been reported about ZnO, CuA102, and Cu2Sr02 as true p-type oxide semiconductors. The ZnO exhibits unipolarity or asymmetry in its ability to be doped n-type or p-type. ZnO is naturally an n-type oxide semiconductor because of a deviation from stoichiometry due to the presence of intrinsic defects such as Zn interstitials and oxygen vacancies. A p-type ZnO, doped with As or N as a shallow acceptor and codoped with Ga or Zn as a donor, has been recently reported. However, the origin of the p-type conductivity and the effect of structural defects on n-type to p-type conversion in ZnO films are not completely understood. 

Intrinsic point defects are deviations from the ideal structure caused by displacement or removal of lattice atoms [106,107], Possible intrinsic defects are vacancies, interstitials, and antisites. In ZnO these are denoted as Vzn and Vo, Zn and 0 , and as Zno and Ozn, respectively. There are also combinations of defects like neutral Schottky (cation and anion vacancy) and Frenkel (cation vacancy and cation interstitial) pairs, which are abundant in ionic compounds like alkali-metal halides [106,107], As a rule of thumb, the energy to create a defect depends on the difference in charge between the defect and the lattice site occupied by the defect, e.g., in ZnO a vacancy or an interstitial can carry a charge of 2 while an antisite can have a charge of 4. This makes vacancies and interstitials more likely in polar compounds and antisite defects less important [108-110]. On the contrary, antisite defects are more important in more covalently bonded compounds like the III-V semiconductors (see e.g., [Ill] and references therein). 

Another contribution to variations of intrinsic activity is the different number of defects and amount of disorder in the metallic Cu phase. This disorder can manifest itself in the form of lattice strain detectable, for example, by line profile analysis of X-ray diffraction (XRD) peaks [73], 63Cu nuclear magnetic resonance lines [74], or as an increased disorder parameter (Debye-Waller factor) derived from extended X-ray absorption fine structure spectroscopy [75], Strained copper has been shown theoretically [76] and experimentally [77] to have different adsorptive properties compared to unstrained surfaces. Strain (i.e. local variation in the lattice parameter) is known to shift the center of the d-band and alter the interactions of metal surface and absorbate [78]. The origin of strain and defects in Cu/ZnO is probably related to the crystallization of kinetically trapped nonideal Cu in close interfacial contact to the oxide during catalyst activation at mild conditions. A correlation of the concentration of planar defects in the Cu particles with the catalytic activity in methanol synthesis was observed in a series of industrial Cu/Zn0/Al203 catalysts by Kasatkin et al. [57]. Planar defects like stacking faults and twin boundaries can also be observed by HRTEM and are marked with arrows in Figure 5.3.8C [58],... 

Recently, experimental and theoretical evidence for a model of the active site of industrial methanol synthesis that combines the role of ZnO and defects in Cu has been presented [58]. Planar defects have been shown to lead to changes in surface faceting of the Cu nanoparticles (Figure 5.3.8C) associated with formation of steps and kinks that were assumed to represent high-energy surface sites of special catalytic activity. For a series of Cu/ZnO-based catalysts, a linear correlation of the defect concentration with the intrinsic activity of the exposed Cu surface was observed. In addition, (partial) surface decoration of Cu with ZnOx by SMSI has been... 

ZnO exhibits naturally n-type conductivity through electron doping via defects, originating from Zn, and VQ in the ZnO lattice [14], The concentration of the intrinsic defects was estimated to be normally of the order of 1018cm-3. The intrinsic defect levels that led to n-type doping was reported to lie approximately 0.05 eV [14] or about 0.2 eV [15] below the conduction band. In the concentration range of the order of 1018 cm-3 [16], however, the intrinsic defect donor band was considered to most likely arise from intrinsic lattice defects. 

Irradiation of the ZnO - Li samples by protons leads to the appearance of a photosensitive EPR signal (f- signal) which corresponds to the center with an axial symmetry and g-factor gy = 1.9948 and gi= 1.9963. This signal is very important for understanding the processes of defect formation, since it leads to gives rise to two alternative interpretations for the microscopic structure of the intrinsic defects that determine the deviation from the stoichiometry in ZnO. ... 

Example of application of triangle method. A study was made by Goodwin and Weisz (unpublished) of the intrinsic activity of ZnO when sintered at various temperatures to achieve various amounts of defect structure. In such a series of catalyst samples the surface area and pore structure, and therefore diffusivity, changes over a wide range. Measured activities have to be scrutinized for diffusion effects. Methanol decomposition rates at 270°C. were measured in a Schwab reactor, on particles of two different sizes, Ri = 0.13 cm., and = 0.4 cm. Therefore, = 3. For the various samples the measured activities per gram of... 

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... 

The opto-electrical property of the ZnO/Pt IPMC was characterized using photoluminescence (PL). In order to understand the PL quenching phenomenon, measurements of the PL spectrum as a function of the potential were carried out with potential variation of 0-2.0 V. Fig 3.14 (a) shows the variation of PL spectra of the ZnO/PT IPMC recorded at the room temperature using an excitation wavelength of 280 nm. The spectra of the sample displays a broad emission band with some vibronic structure from 350 to 500 nm and the maximum emission wavelength is Xmax = 468 nm. The blue emission is believed to originate from intrinsic defects, particularly interstitial zinc [Fang et al. (2004)]. The maximum PL intensity is observed... 

Recently, it was shown that transient absorption decay for hematite nanoparticles was very fast, 70% of the transient absorption disappeared within 8 ps and no measurable transient absorption remained beyond 100 ps [43]. This represented a much faster decay than many other semiconductors, which is consistent with the observed poor charge transfer properties in hematite. It should be mentioned that this decay was independent of the excitation power, which suggests alternative relaxation mechanisms compared to those observed for Ti02 and ZnO for instance [43]. Since the relaxation was independent of pump power, probe wavelength, pH and surface treatment the fast decay was interpreted to be due to intrinsic mid-bandgap states and trap states rather than surface defects. This is in agreement with earlier investigations [44]. 

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