Zinc oxide nanostructure
ZnO nanostructures have found uses in environmental, technological and biomedical purposes including ultrafast optical functions, dye-sensitised solar cells, lithium-ion batteries, biosensors, nanolasers[2] and supercapacitors.
[1] In vapor deposition processes, zinc and oxygen are transported in gaseous form and react with each other, creating ZnO nanostructures.
Other vapor molecules or solid and liquid catalysts can also be involved in the reaction, which affect the properties of the resultant nanostructure .
VS processes can create a variety of ZnO nanostructures but their morphology and properties are highly dependent on the reactants and reaction conditions such as the temperature and vapor partial pressures.
These are known as vapor-liquid-solid (VLS) processes, and use a catalytic liquid alloy phase as an extra step in nanostructure synthesis to accelerate growth.
This reaction can be highly controlled to produce more complex nanostructures by modifying the size and arrangement of gold seeds, and of the alloys and vapor constituents.
[1] A large variety of ZnO nanostructures can also be synthesised by growth in an aqueous solution, which is desirable due to its simplicity and low processing temperature.
[1] Altering the growth solution and its concentration, temperature and structure of the seed layer can change the morphology of the synthesised nanostructures.
[7] Another method to synthesise ZnO nanostructures is electrodeposition, which uses electric current to facilitate chemical reactions and deposition on electrodes.
Its low temperature and ability to create precise thickness structures makes it a cost-effective and environmentally friendly method.
[1] Doping ZnO nanostructures with other elements and molecules leads to a variety of material characteristics, because the addition or vacancy of atoms changes the energy levels in the band gap.
Zn interstitials occur when extra zinc atoms are located inside the crystal ZnO lattice.
[14] Both oxygen vacancies and Zn interstitials increase the number of electron charge carriers, thus becoming an n-type semiconductor.
Since these defects occur naturally as a by-product of the synthesis process, it is difficult to make p-type ZnO nanostructures.
[15] Defects and dopants are usually introduced during the synthesis of the ZnO nanostructure, either by controlling their formation or accidentally obtained during the growing process through contamination.
At temperatures where the lattice is mobile, oxygen molecules and gaps can be moved using electric fields to change the nature of the material.
A solution may be to dope with different materials and to develop on the nanoscale with nanostructures, such as porous surfaces, that allow for volume changes during the chemical process.
Just as for the batteries, multiple combinations of carbon structures, graphene, metal oxides with ZnO nanostructures have improved capacitance of these materials.
Since used ZnO biosensors will eventually dissolve and release Zn ions, they may be absorbed by the cells and the local effect of this is not yet known.
