Manganese was doped upto 1% in MgAl2O4 spinel. As doping concentration increase,I found a gradual red shift in the transmission spectra. what is the basic cause of red shift?
Dear Haider,
Doping is addition of impurity to the material. Manganese can substitute the Mg atoms and so building MnAl2O4. So after doping the material will be composed of a mixer of two materials. The energy gap of the of the mixer will be smaller than the host material if the MnAl2O4 has a smaller band gap than MgAl2O4. I assume this is what happening in your experiment. In order to verify this hypothesis you need to get the energy gap of the two materials. Or you measure them separately.
There is other bend gap narrowing as a consequence of shallow doping because of the increase in the free carrier concentration either in the conduction or the valence band.
For more information about this phenomenon please see the paper in the link: Article A New and Simple Model for Plasma and Doping induced Band Ga...
Best wishes
The shift is due to the broad intense UV electronic ligand-metal charge-transfer absorption band O2- -> Mn2+ with maximum lower than the energy of forbidden gap of pure MnAl2O4. That is typical for transition metal ions-bearing oxygen-based crystals. .
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