When we dope ZnO with Co or Fe or Ni its band gap mostly decreases, but sometimes it increases with it. What's the reason behind it?
Dear Sohail Ahmad ,
This is a difficult question. Many factors are involved in the process. However, one thing that could happen is that the doping can cause the particle size shrinking. Smaller the particle size, bigger the band gap - This is very common for nanomaterials.
Sometimes, for TiO2 for example, doping with another metals can constrain the growth of the nanoparticles, making it smaller. This also can happen to ZnO materials.
@ Sohil,
Below i have mentioned few reasons for increase in the bandgap.
1. The bandgap of a material mainly depends on the crystallinity. With increase in the crystallinity the bandgap reduces and vice versa.
2. When the dopant is introduced into the host matrix (ZnO), the impurity (dopant) generated free electrons shifts the Fermi level into the conduction band. Hence the electron in the valance band requires more energy to reach the conduction band above the Fermi level. This is called as B-M shift. Due to this phenomenon the bandgap of the material increases.
Bandgap = Bandgap of the material + B-M shift
3. If bandgap of the doping material is more than that of the host material (ZnO), then there is a chance of increase in the bandgap.
In fact, a lot depends on how you define the gap width. This can be the optical gap width, kinetic (from the temperature dependence of the resistance) or X-ray spectral (from the difference between the boundaries of the XPS and XAS spectra). It is possible that in fact the real width does not change. Doping can give rise to localized impurity levels that determine the activation energy of conduction. This can be taken as a change in the width of the fundamental gap.
But it is also possible to propose mechanisms for changing precisely the width of the gap. For example, it may depend on the form in which the dopant ions enter - in the form of a substitutional or interstitial impurity. So this issue needs to be addressed specifically.
There are multiple factors involved which alter the band gap when you dope transition metal to ZnO. Please refer to the following articles:
Article Effect of Cu-N co-doping on the dielectric properties of ZnO...
Article Effect on the dielectric properties due to In–N co-doping in...
Article Effect of synthesis methods on dielectric performance of ZnO...
Hope these articles will answer your question!
Actually, the Bandgap depends on a lot of factors such as the crystallinity of the host matrix, Dopants and so forth...you can follow this to get external idea..
Article Photocatalytic degradation of chlorazol yellow dye under sun...
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