Surface Properties of ZnO

The semiconducting properties of ZnO have nevertheless proved to be advantageous because they permit the extensive application of modern surface spectroscopies (such as XPS, UPS, EELS, and LEED) for the investigation of the surface structures since the charging problems that are usually encountered with oxides are avoided (393). For this reason, ZnO is one of the most thoroughly investigated oxides. 

Because of the high purity, the excellent morphological definition, and the good optical properties of the microcrystals, the surface and catalytic properties of ZnO powders prepared by Zn combustion have been investigated extensively by spectroscopic techniques. The results of these investigations are well suited to comparison with results obtained with single crystals and less well-defined samples. 

ZnO films can provide substantial information on chemical and electronic properties of ZnO surfaces and interfaces, which occur in real thin film solar cell structures. In addition, general information on the interface formation of oxide materials can be extracted. In the following we describe ... 

Motivated by the application of ZnO in gas sensors and catalysis and by the more general desire to understand surface properties of ionically bonded solids, electronic properties of ZnO surfaces have been investigated for many years [20,76-80]. An overview of the early work on ZnO surface properties is included in the book of Henrich and Cox [81]. 

First sputtering processes for ZnO deposition were developed in the late 1960s for manufacturing surface acoustic wave devices [2]. The piezoelectric properties of ZnO films are crucial for that application and major efforts were made to develop ZnO sputtering processes which enabled c-axis oriented growth, high resistivity and unique termination of the ZnO crystallites [3,4]. 

For LP-CVD ZnO, similar variations in the structural properties of ZnO films occur with an increase of substrate temperature, but this takes place at a lower temperature, i.e., around 160°C, and with different crystallographic orientations. Indeed, the preferential orientation is (1120) for LP-CVD ZnO, instead of (0002) for the AP-CVD ZnO [3,16,23], Figure 6.21 shows XRD patterns of undoped LP-CVD ZnO films deposited with increasing substrate temperature, along with the corresponding SEM micrographs of their surface. 

As has been discussed above, there is a strong influence of grain size on the electrical and optical properties of ZnO films. Take as a first example, ZnO films grown by the LP-CVD process in the substrate temperature range between 155 and 180°C they have a microstructure as described in Sect. 6.2.2.1 with conical crystallites that form pyramids at the surface (see Fig. 6.6). This microstructure has, via the pyramidal structure of the surface, a pronounced influence on the optoelectronic properties of the films, specifically on their light scattering capability. 

Table 8.1. Surface properties of the etched ZnO Al films shown in Fig. 8.8. Root mean square roughness (ArrrlK), haze at a wavelength of 700 nm, and resulting short circuit current density, when applied in 1.1pm thick pc-Si I1 solar cells...

This book is devoted to the properties, preparation and applications of zinc oxide (ZnO) as an transparent electrode material. It focuses on ZnO for thin film solar cell applications and hopefully inspires also readers from related fields. The book is structured into three parts to serve both as an overview as well as a data collection for students, engineers and scientists. The first part, Chaps. 1-4, provide an overview of the application and fundamental material properties of ZnO films and their surface and interfaces properties. Chaps. 5-7 review thin film deposition techniques applied for ZnO preparation on lab scale but also for large area production. Finally, Chaps. 8 and 9 are devoted to applications of ZnO in silicon- and chalcopyrite-based thin film solar cells, respectively. One should note that the application of CVD grown ZnO in silicon thin film cells is discussed earlier in Chap. 6. 

Figure 6. Optimized surface texture of ZnO-layer (a) and optical properties transmission T and reflectivity R (b). ...

The absorption of ZnO from intact skin after topical application is non-detectable. The data on TiOg are controversial. Earlier studies suggested that a very small amount of titanium dioxide may penetrate the skin, but it is unlikely that this would have any biological significance (237). However, a recent in viuo human study, in which skin punch biopsies were collected after application of titanium dioxide, (256) showed that this sunscreen is solely deposited on the outermost surface of the stratum corneum and does not penetrate into the deeper stratum corneum layers, the epidermis or the dermis regardless of the surface properties of the particles (256). 

Studies in recent years on the surface properties of transition metal oxides have demonstrated that the surface structural stability, the surface electronic structure, and the surface chemical reactivity depend on the crystallographic orientation of the exposed surface and the presence of surface imperfection, such as steps and point defects (1 ). ZnO is one recent example. The natural surfaces of ZnO, which can be prepared in a relatively well-ordered state, include he Zn-polar (0001), the 0-polar (OOOT), and the nonpolar (IOIO) surfaces. (See Figure 1 for a schematic representation of these surfaces). These surfaces have been shown to possess different chemisorptive properties and reactivities. It was shown that CO2 was desorbed from a nonpolar surface at about 120 0, but from a Zn-polar surface at (2 ). 

In conclusion, the chemical properties of ZnO depend on the particular surface plane that is exposed. This surface specificity has now been demonstrated for the decomposition of 2-propanol, methanol, formaldehyde and formic acid, and adsorption and desorption of acetone, propene, water, CO, and CO2. These data have made possible better understanding of the results using ZnO powder. It will be intersting to se<5 how different are the catalytic properties of these surfaces. 

Hong et al. [231] have used this simple technique for the synthesis of ZnO-low-density polyethylene composites. ZnO nanoparticles and branched low-density polyethylene was melt-compounded in a high-shear mixer to prepare polymer nanocomposites with an improved resistance to thermal degradation. They also mixed submicron-sized ZnO particles with low-density polyethylene for a comparison and reported that the surface properties of nanoparticles (<100 nm) resulted in an increased thermal stability of nanocomposites. Ma et al. [232] also used melt blending for the synthesis of silane-modified ZnO-polystyrene resin nanocomposites. Incorporation of ZnO nanoparticles results in an increased flexural... 

Sakohara S., Ishida M., Anderson M.A. Visible luminescence and surface properties of nanosized ZnO colloids prepared by hydrolyzing zinc acetate. J. Phys. Chem. B 1998 102 10169-10175... 

It has been reported that alkoxide species are formed from alcohols at the surface density of 1 —2 X 10 molecules cm . Upon thermal desorption those alkoxides from Ca — C4 alcohols decomposed to aldehydes (or ketones) and olefins at 480 — 550 The selectivities to olefins were 0.2 to 0.4 except for ethanol for which the ratio was 0.9. Interactions and thermal desorption of several molecules for different surfaces of a ZnO single crystal have been studied in ultra-high vacuum. Fig. 3.30 shows the crystal planes examined. (Electronic properties and surfaces geometry of ZnO crystal have been briefly reviewed. ) The strength of the interaction of oxygen-containing... 

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