Dear Prathiba,

yes, you can that from UV-Visible Spectrophotometer data, after finding absorption coefficient you can using Excel program and by using Tauc relation to plot between hv(eV) on x-axis and (alpha*hv)^n , and take n=2 for allowed direct transition, n=1/2 for  allowed indirect transition, and from the graph you can testing the linear region which intersect at hv=0, the better linear region it determine the true energy band gap and can you regression factor in Excel program which varied between 0 to 1. with best regards

Thank u dear Bilal Taher for your answer.. If I understand it right, all semiconductors would have both type of band gaps...and more linearity of the hv vs (alpha*hv)^n curve determines which type of band gap it belongs to...

Dear Prathiba,

you can use easy method to determine direct energy gap firstly by observation values of absorption coefficient if greater than ten to power 4 (greater than 10^4 cm^-1, this mean the materials have direct energy gap, and you directly use the relation  hv vs (alpha*hv)^2.

Dear Bilal Taher,

please also justify explain the difference between direct band gap and indirect band gap and when one should be used?

some time material have direct band gap higher then indirect band gap, so which one should be used for photocatalytic application?

Dear Nasir

Direct band gap mean the electronic transition from top valance band to bottom conduction band (with same position of K- vector (crystal momentum) delta K=0), also indirect band gap mean the transition obtain also from from top valance band to bottom conduction band (but with not same position of K- vector (crystal momentum) delta K don't equal 0), The band gap is called "direct" if the momentum of electrons and holes is the same in both the conduction band and the valence band; an electron can directly emit a photon. In an "indirect" gap, a photon cannot be emitted because the electron must pass through an intermediate state and transfer momentum to the crystal lattice which called "phonon". Direct band gap were used in different application such as solar cell or detector or sensors, etc, because its give high efficiency and performance such as (GaAs, CuInS2, CuGaSe2, ...etc.). you can get on better information from through semiconductor books or from the net. With best regards

Bilal Taher,

You gave good explanation for bandgaps. Is there any technique available, other than UV-Vis. Spectroscopy, to determine exact bandgap.

Regards

Dear Umar,

I think this method is very good, also you can use PL (photo luminescence) technique to determine exact bandgap.

Regards

Dear Bilal Taher

Thanks alot

You can easily find both direct and indirect band gap from optical data using well-known Tauc law.

Dear Bilaal Yaseen Taher

Sir, please give me some references for this statement, "Easy method to determine direct energy gap firstly by observation values of absorption coefficient if greater than ten to power 4 (greater than 10^4 cm^-1, this mean the materials have direct energy gap, and you directly use the relation  hv vs (alpha*hv)^2".

Thank you