History of Zinc Oxide

First Discovery

Before zinc was recognized as a unique element, it was used to make brass by the Romans in the first century A.D. It wasn't until the 14th centrury that zinc was recognized as a metal in Zewar, India. ZnO was created as a byproduct of the smelting process [1]. The white powder proved to be useful as a remedy for sore eyes.

The zinc smelting technique was then taken to China in the 16th or 17th century where it was used to produce brasses with high zinc contents. Europeans imported zinc from China shortly afterwords and it was listed as an element on the periodic table by Antoine Lavoisier in 1789 [2].

Zinc Oxide Pigments

Before the electrical and optical properties were utilized, the white color of ZnO powder was used in water colors and oil based paints in the late 1800's. It provides a whiter and colder color than lead or titanium white. 

First Use of ZnO in Electronics

Perhaps the first use of ZnO for electronic applications was in build your own radio sets in the 1920's [3]. A Schottky barrier was created by contacting a ZnO crystal with a copper wire, providing the rectification needed to convert the AC radio waves to DC signals. 

The first wide spread use of ZnO in electronics was for varistors, allowing reliable surge protection. Without this advance, use of electronics in the home would not have been possible without constantly replacing fried components. 

Investigation into the Properties of ZnO

Material science really blossomed in the 20th century and ZnO was one of the first materials investigated in detail. Electron diffraction data on ZnO first appeared in 1935 [4]. Three years later, the first ever scanning transmission electron microscope image was taken of a ZnO crystal [5].

Temperature dependent Hall measurements of ZnO were first reported in 1954 confirming the inherently n-type nature of this material [6]. Around this same time the light emission from ZnO was gaining interest after some initial work in Germany in the 1930's [7].

Since these early reports there have been literally tens of thousands of articles published about ZnO and ZnO related materials. Nearly every major journal interested in materials science publishes several articles on ZnO every month. The above figure shows the number of publications per year on ZnO. Also shown are publications relating to GaN, another wide band gap semiconductor which currently is used as the laser gain medium in Sony Playstation 3 and BluRay disc players. Research into GaN has someone leveled off while the ZnO research still is climbing. 

ZnO shows, perhaps, the most variety in self assembled nanostructured forms. The above graph shows the Web of Science publications for nano related articles, which is climbing steadily at an impressive rate. ZnO research only makes up a small portion of the modern nano-research, but nevertheless, it is one of the materials which will play an increasing role in the nanotechnology of the future.

ZnO Sunscreen

ZnO is perhaps the most widely applied topical drug of all time. It is the active ingredient in most sun screens and finds its way into many ointments for pain and itch relief. ZnO microcrystals are very efficient light absorbers in the UVA and UVB spectral range due to the wide bandgap. Next time you go out in the sun, don't forget your ZnO! 

Research in the 21st Century

ZnO research in the 21st century is focused on several topics. Achieving reliable, stable p-type doped ZnO has so far eluded researchers. Some progress has been made, with some reports of UV laser diodes based on ZnO p-n junctions. However, wide scale deployment of such devices is still hindered by stability issues. Furthermore, it seems that standard materials deposition processes which are easily scaled for mass production, like magnetron sputtering, do not produce p-type material of device quality. 

Transparent electronics is another hot research topic and ZnO has found its way into transparent thin film transistors for flexible electronics on plastics and polymer substrates. The conductivity of ZnO can be greatly enhanced by doping with Al, B,  Ga, or other impurities. Transparent conductive oxides are necessary for flat panel displays and highly efficient solar cells. Currently, tin-doped indium oxide, commonly known as ITO is the industry standard for transparent conducting applications. Unfortunately, the abundance of In is low and the cost is high, which hinders the goal of achieving solar cells below the target cost of 1$/Watt. Doped ZnO is a cheaper alternative but there are still several issues to overcome before it can compete with the quality and durability of ITO coatings. 

Keep the research coming!!!!

[1] Vijaya Deshpande, A note on ancient zinc smelting in India and China, Indian Journal of History of Science 31, 275 (1996) 275.
[2] J.R. Partington, A short history of chemistry, 3rd Edition, Dover, 1989.
[3] C. Jagadish and S. Pearton (Eds.), Zinc Oxide Bulk, Thin Films and Nanostructures: Processing, Properties, and Applications,  Amsterdam, Elsevier, 2006.
[4] H.J. Yearian, Intensity of di.raction of electrons by ZnO, Physical Review 48, 631 (1935)
[5] M von Ardenne, Das Elektronen-Rastermikroskop. Praktische Ausfhrung, Z. tech. Phys. 19 407 (1938) 
[6] S.E. Harrison, Conductivity and Hall e.ffect of ZnO at low temperatures, Physical Review 93 52 (1954) 52
[7] F.A. Kroger and H.J. Vink, The origin of the fluorescence in selfactivated ZnS, CdS, and ZnO, J. Chem. Phys. 22, 250 (1954)