Preparation of zinc oxide ZnO films and nanostructures by oxidation

4 Preparation of zinc oxide (ZnO) films and nanostructures by oxidation 

Zinc oxide (ZnO) is a cheap but important material with many unique properties and potential applications [123-125]. Firstly, it is a gas/pressure-sensing ceramic and can be used as non-linear varistors and sensors [126]. Secondly, piezoelectric ZnO can find applications in surface and bulk acoustic wave devices [127-129]. Thirdly, it is a semiconducting oxide with a wide band gap of 3.37 eV at room temperature and a large exciton energy of 60 meV, 

ZnO films can be prepared by many techniques, including chemical vapour deposition (CVD) [135], electron beam evaporation [136], molecular beam epitaxy (MBE) [137], pulsed laser deposition (PLD) [138], sol-gel [139], spray pyrolysis [140], sputtering [141] and chemical bath deposition [142]. Thermal oxidation of metallic Zn [143-149], ZnS or ZnSe [150-153] films has been used to prepare ZnO films. The Zn precursor films were deposited onto the substrates (glass, sapphire or Si) by magnetron sputtering, thermal/ electron-beam evaporation or filtered cathodic vacuum arc technique. Oxidation treatment was normally conducted in air or oxygen atmospheres in a wide temperature range typically from 300 to 1000°C. 

Besides these techniques, thermal oxidation of Zn (in the form of metal, film or powder) might gain popularity in the fabrication of ZnO lowdimensional structures due to its simplicity in processing control. To date, whiskers [187,188], nanowires [189-194], multi-layer nanosheets [195], tetrapodnanostructures [196,197], micro-rods [198],micro-belts/combs [199] and micro-spheres/cages [200] have been obtained by thermal oxidation in ambient atmospheres or in pure oxygen. 

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