Dear colleague Dr. Suhail

Have a nice time and good day.

Annealing may affect the optical band gap to be high or low. What is the reason? Firstly, the optical gap increases or decreases according to the density of localized states; low or high respectively. XRD, SEM and TEM are useful techniques to study the effect of annealing on the focused nano material. Optical band gap is related to the density of localized states. The lower the density of localized states the higher the optical band gap. If the annealing has a positive effect on nano material the density of localized states will be annealed out and decreases consequently the optical band gap increases and vice versa.

Ex. if the intensity if XRD increases and the FWHM decreases with the increasing of annealing, this is a document that the increasing of annealing gives more enhancement of order and the localized states or defects decreases by the increasing of annealing which means the increasing of crystallite size. TEM can confirm the XRD data and SEM gives you what happen for the surface morphology. SEM can give you also if the sample under consideration has a porous nature or not; porous nature and the optical band gap are strongly affect by annealing.

Thank you very much Eldenglawey sir for adding a very detailed answer.

Thank you

Suhail

Hi Suhail, I think after heat treatment, an escalation in the particle size and degree in crystallinity may perceived as an outcome of temperature rise. An increase in the calcination temperature yields a blue shift in the absorption maxima of the absorption spectrum and a shrinkage of bandgap being a result of the growth in particle size. 

Thank you Kunal Sir.