Transparent Zinc Oxide

Similar to transparent titanium dioxide microparticles of zinc oxide are manufactured by using sol/gel processes or precipitation in the presence of protective colloids to limit particle growth [5.288], 

An industrial process [5.289] operates with solutions of zinc sulfate and zinc chloride in the ratio 1 2. Basic zinc carbonate is precipitated by feeding simultaneously the zinc salt solution and a mixed solution of sodium hydroxide and sodium carbonate into a reactor charged with water. The precipitated product is intensively washed several times and then spraydried. 

It is used for cosmetics and paints as transparent UV-light shielding chemical. 

Trade names include Sachtotec Micro-Zinkoxid (Sachtleben, Germany) and Z-cote HP1 (SunSmart Inc., USA). 

Luminescent inorganic compounds are also known as luminophors and phosphors. Depending on the nature of the exciting energy, the resulting luminescence is named as follows  

The most significant property of transparent zinc oxide is again the absorption of UV radiation. The main application of zinc oxide is in sunscreens. The usual industrial manufacturing process is either direct combustion of high-purity zinc metal in a plasma with oxygen [5.196] according to the reaction  

This process produces primary particles of ca. 15 nm, but it is time-consuming and expensive. 

Luminescence originates from electronically excited states in atoms and molecules and the emission process is governed by quantum mechanical selection rules. Forbidden transitions generally are slower than allowed optical transitions. Emission originating from allowed optical transitions, with decay times of the order of ps or faster is called fluorescence the term for emission with longer decay times is phosphorescence. The time in which the emission intensity decreases to 1/e or 1/10 (for exponential decay and hyperbolic decay, respectively) is called the decay time. 

Luminescent materials have changed the world. Energy saving lamps, many kinds of displays and modern medical equipment all rely on luminescent materials and it is hardly imaginable that large scale application of luminescent materials started only slightly more than 100 years ago (for an overview see e.g. Ref [5.198]). 

In this section, historic aspects of luminescent materials will be discussed first, followed by a short treatment of luminescence mechanisms and luminescence excitation schemes. Thereafter, devices based on luminescent materials and the way in which luminescent materials determine their operational performance will be discussed. Preparational aspects of luminescent materials will be described and then this section will end with an outlook. 

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For increased solubiHty to prevent bloom, shorter-chain carboxyHc acids or zinc carboxylates can be substituted. The use of chain-branched carboxyHc acids reduces the tendency for the formulations to lose sulfur cross-links or revert upon prolonged heating (7). Translucent articles such as crepe soles can use a zinc carboxylate or employ zinc carbonate as a transparent zinc oxide. 

Izaki, M. and Omi, T. (1995) Transparent zinc oxide films prepared by electrochemical reaction. Appl. Phys. Lett., 68, 2439-2440. 

Izaki M., Omi T. Transparent zinc oxide films chemicaUy prepared from aqueous solution. J. Elec-trochem. Soc. 1997 144 L3-L5... 

Accelerated sulphur systems also require the use of an activator comprising a metal oxide, usually zinc oxide, and a fatty acid, commonly stearic acid. For some purposes, for example where a high degree of transparency is required, the activator may be a fatty acid salt such as zinc stearate. Thus a basic curing system has four components sulphur vulcanising agent, accelerator (sometimes combinations of accelerators), metal oxide and fatty acid. In addition, in order to improve the resistance to scorching, a prevulcanisation inhibitor such as A -cyclohexylthiophthalimide may be incorporated without adverse effects on either cure rate or physical properties. 

Transparent semiconductor oxide films, such as tin oxide (Sn02) and zinc oxide (ZnO), produced by MOCVD are also being considered for photovoltaic applications. ]... 

Zinc oxide is usually white and opaque at the nanoscale, it becomes transparent. 

A dusting agent which is soluble in rubber and thus does not impair the vulcanised bond between rubber components of a composite product. It is also an activator combining the functions of zinc oxide and stearic acid, of particular value in transparent rubbers since it does not produce the same opacity as zinc oxide. 

Norris, B. J. Anderson, J. Wager, J. F. Keszler, D. A. 2003. Spin-coated zinc oxide transparent transistors. J. Phys. D.Appl. Phys. 36 L105-L107. 

Other materials such as gold (< = 4.9 eV), aluminum (< = 4.2 eV), indium-doped zinc oxide, magnesium indium oxide, nickel tungsten oxide, or other transparent conductive oxide materials, have been studied as anodes in OLEDs. Furthermore, the WF of ITO can be varied by surface treatments such as application of a very thin layer of Au, Pt, Pd, or C, acid or base treatments, self-assembly of active surface molecules, or plasma treatment. 

S. Major and K.L. Chopra, Indium-doped zinc-oxide films as transparent electrodes for solar-cells, Sol. Energy Mater., 17 319-327, 1988. 

X. Jiang, F.L. Wong, M.K. Fung, and S.T. Lee, Aluminum-doped zinc oxide films as transparent conductive electrode for organic light-emitting devices, Appl. Phys. Lett., 83 1875-1877, 2003. 

Manufacturers have already begun to take advantage of some of these nanoparticle properties. Sunscreens, which protect users from burns by absorbing or deflecting harmful rays, are often made from chemicals such as titanium dioxide or zinc oxide that are particularly effective. These sunscreens often leave a whitish residue—which used to be common on the nose of a pool or beach lifeguard—but when companies embedded nanoparticles of titanium dioxide or zinc oxide instead of bulkier particles, the creams become transparent yet maintained or even increased their effectiveness. With no embarrassing residue, these sunscreens have become popular. 

Fig. 2.1. A lens for high-resolution acoustic microscopy in reflection. The central transparent part is a single crystal of sapphire, with its c-axis accurately parallel to the axis of the cylinder. The sandwich structure at the top is the transducer, with the yellow representing an epitaxially grown layer of zinc oxide between two gold electrodes. The pink shaded areas within the sapphire represent the plane-wavefronts of an acoustic pulse they are refracted at the lens cavity so that they become spherical in the coupling fluid. A lens for use at 2 GHz would have a cavity of radius 40f[Pg.8]

One of the most famous nanoparticles is the buckyball, a molecule made up of 60 carbon atoms arranged in a soccer-ball shape. Some skin creams now include buckyball molecules. Another widely used nanoparticle is zinc oxide. For years, swimmers applied zinc oxide as a thick sunscreen paste, but the new nano-sized version of the compound is so small that it looks transparent when slathered on as sunblock. The makers of these products say the nanoparticles can reflect light and fight off damage to skin cells better than normal-sized particles. A few cosmetics manufacturers are experimenting with nanoparticles in eye shadow and lipsticks to see if the tiny molecules can produce different colors or new visual effects like iridescence, where colors seem to constantly change. 

In this book the chemical, structural, optical, electrical, and interface properties of zinc oxide are summarized with special emphasis on the use of ZnO as transparent conductive electrode in thin film solar cells. This application has a number of requirements, which can be fulfilled by ZnO ... 

In Table 1.1 a number of properties of zinc oxide are summarized in comparison to other transparent conducting oxides and to silicon. 

Recently, field-effect transistors based on zinc oxide were reported [173,174], opening the opportunity to design microelectronic devices that are transparent and/or work at high temperatures [175]. More details on thin film transistors employing transparent conducting oxides are given in Chap. 2. 

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