Friday, February 11, 2011

Zinc Oxide Nanomaterials

Zinc oxide is one of the most important II-VI semiconductor material with direct wide band gap (3.37 eV or 375 nm), good transparency, high electron mobility (>100 cm2/Vs) photoconductivity, strong room-temperature luminescence large exciton and biexciton energies of 60 meV and 15 meV respectively. It has attracted increasing attention due to its excellent optical and electrical properties, inexpensiveness, relative abundance, chemical stability towards air, ability to produce significant quantum confinement effect. 

Since ZnO is an important trace element for humans, it is environment friendly and suitable for in vivo applications. Zinc oxide has high refractive index, high thermal conductivity, antibacterial and UV-protection properties. Consequently, it is added in various materials and products including plastics, ceramics, glass, cement, rubber, lubricants, paints, ointments, adhesives, sealants, pigments, foods, batteries, ferrites, fire retardants etc. and non-toxicity, making it a suitable additive for textiles and surfaces that come in contact with humans.

Nanosized ZnO has immense importance due to its multifunctionality. Thermal stability, irradiation resistance and flexibility to form different nanostructures are the advantages that highlight its promising applications in solar cells and electronic devices, ultraviolet-light detectors and photo diodes and in catalysis.Structures like nanowires, nanobelts and nanorings are of great interest in photonics research, optoelectronics, nanotechnology, and biomedicine. Therefore, the controlled synthesis of various ZnO nanostructures such as nanocrystals, nanowires, nanobelts and other complex nano architectures has been extensively explored.

Zinc oxide nanostructure growth is heavily researched presently. The substance is likely to have the largest variety of nanostructures (and their associated properties) among all known materials. Its hexagonal lattice can easily match catalyst lattice structure and facilitate controlled growth patterns. Positive zinc surfaces and negative oxygen surfaces create electric dipoles that facilitate polarization growth along certain directions and planes under applied voltage and temperature. Different techniques such as sol-gel, spray pyrolysis, thermal evaporation, wet chemical processes etc are used for the synthesis of ZnO nanomaterials.

ZnO quantum dots exhibit emission bands in the ultraviolet and visible regions as shown by its photoluminescence spectra. The UV emission band at around 370 nm is usually attributed to the interband transition or the exciton combination in ZnO . Even though the emissions in the visible region are associated with the electronic defects due to surface states or trapping effects in the QDs, there are still
controversies related to the unambiguous electron transitions. In the visible region, the blue emission in ZnO QDs has considerable importance in biological fluorescence labeling. Several studies indicate that ZnO is one of the most efficient oxide-based phosphors in both photoluminescence (PL) and electroluminescence (EL).

Considerable studies have been done on the properties of metal incorporated fluorescent materials like Co, Ni and Fe doped ZnO aiming to develop efficient magnetic semiconductors. The properties of inner-transition metal doped ZnO nanoparticles have also been subjected to tremendous amount of studies. The combination of magnetic and optical properties provides the composites significantly important applications in biomedical fields including drug targeting, bioseparation and diagnostic analysis.

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