Dear Chadli Ilham

Peace be upon you,

You are very welcome,

Have a nice day and good times

To calculate the optical band gap of your samples, you should estimate firstly the absorption coefficient by using the transmission and reflection spectra.

Experimentally, the absorption coefficient (α) can be calculated  from this simple relation:

α = 1/t  ln [(1-R)2 / T]

where t  is the sample thickness, T and R are the transmission and  reflection.But if you don't have T and R and you have Absorbance, then:

absorption coefficient  (α) = 2.303 A / t 

where (A) is absorbance  and (t) is thickness of thin film.

Moreover, You can get  the value of Eg  by usually use the Tauc's relation and plots. Tauc's relation is given by the following equation:  

 αhν = A (hν - Eg )n

Where  (hν) is the photon energy, where:

hν(eV) = 1240 / [incident wavelength (nm)]

Now, If you plot a graph between (αhν)1/n  versus (hν), then you can get a straight line. This line intersects the X-axis at (αhν)1/n = 0 . The values of Eg have been estimated from this intercept.  The value of n is dependent on the electronic transition type. Where:

n=1/2 for direct allowed transition,

 n= 2 for indirect allowed transition, 

n=3  for direct forbidden transition and

n=3/2 indirect forbidden transition. You should try to select the suitable n according to your samples and their preparations. These Plots are known as Tauc's Plots.

The appended papers may help you; they are examples of direct and indirect allowed transitions.

Regards and Good luck

Your,

Ahmed Saeed Hassanien

https://www.researchgate.net/publication/283338024_Effect_of_Se_addition_on_optical_and_electrical_properties_of_chalcogenide_CdSSe_thin_films

https://www.researchgate.net/publication/281495242_Influence_of_composition_on_optical_and_dispersion_parameters_of_thermally_evaporated_non-crystalline_Cd50_S50-xSex_thin_films

https://www.researchgate.net/publication/304150361_Study_of_optical_properties_of_thermally_evaporated_ZnSe_thin_films_annealed_at_different_pulsed_laser_powers

And others.

Dear Chadli Ilham! If you need to calculate the optical band gap of the material thick enough for to be nontransparent - you may use the reflectance spectra. Depending on whether your material is direct-gap or indirect-gap you should use different approximation of the diffuse reflectanse spectrum to find the band gap. Try to find info on Kubelka-Munk function. I do believe it helps. And you may like to read the next papers to see how people are doing this:

1) J. DEAN, D.G. THoMAs. Intrinsic Absorption-Edge Spectrum of Gallium Phosphide. PHYS. REV. V. 150, N2, 1966, P.690

2) A. Hafdallah, F. Yanineb, M.S. Aida, N. Attaf. In doped ZnO thin films. Journal of Alloys and Compounds 509 (2011) 7267–7270

3) Rajinder Singh, · Manesh Kumar, · Heena Khajuria, · Jigmet Ladol, · Haq Nawaz Sheikh. Hydrothermal synthesis of magnetic Fe3O4– nitrogen‑doped graphene hybrid composite and its application as photocatalyst in degradation of methyl orange and methylene blue dyes in presence of copper (II) ions. Chemical Papers. https://doi.org/10.1007/s11696-018-0385-y

Kubelka-Munk function for non-specular materials based on spectroscopic measurements is done

http://www.fhi-berlin.mpg.de/acnew/department/pages/teaching/pages/teaching__wintersemester__2016_2017/annette_trunschke__diffuse_reflectance_spectroscopy__161111.pdf

I am interested

Sir V. B. Zaytsev thank you very much ,

I think, these references are very useful in my research.

Hello dear Chadli Ilham,

There is the Tauc equation, which is quite simple to use, take a look, maybe it will suit you.

https://en.wikipedia.org/wiki/Tauc_plot

http://sphinxsai.com/2014/vol6_6_ICMCT/4/(3353-3356)ICMCT14.pdf

https://ri.ufs.br/handle/riufs/9245

Dear Chadli Ilham

Peace be upon you,

You are very welcome,

Have a nice day and good times

To calculate the optical band gap of your samples, you should estimate firstly the absorption coefficient by using the transmission and reflection spectra.

Experimentally, the absorption coefficient (α) can be calculated  from this simple relation:

α = 1/t  ln [(1-R)2 / T]

where t  is the sample thickness, T and R are the transmission and  reflection.But if you don't have T and R and you have Absorbance, then:

absorption coefficient  (α) = 2.303 A / t 

where (A) is absorbance  and (t) is thickness of thin film.

Moreover, You can get  the value of Eg  by usually use the Tauc's relation and plots. Tauc's relation is given by the following equation:  

 αhν = A (hν - Eg )n

Where  (hν) is the photon energy, where:

hν(eV) = 1240 / [incident wavelength (nm)]

Now, If you plot a graph between (αhν)1/n  versus (hν), then you can get a straight line. This line intersects the X-axis at (αhν)1/n = 0 . The values of Eg have been estimated from this intercept.  The value of n is dependent on the electronic transition type. Where:

n=1/2 for direct allowed transition,

 n= 2 for indirect allowed transition, 

n=3  for direct forbidden transition and

n=3/2 indirect forbidden transition. You should try to select the suitable n according to your samples and their preparations. These Plots are known as Tauc's Plots.

The appended papers may help you; they are examples of direct and indirect allowed transitions.

Regards and Good luck

Your,

Ahmed Saeed Hassanien

https://www.researchgate.net/publication/283338024_Effect_of_Se_addition_on_optical_and_electrical_properties_of_chalcogenide_CdSSe_thin_films

https://www.researchgate.net/publication/281495242_Influence_of_composition_on_optical_and_dispersion_parameters_of_thermally_evaporated_non-crystalline_Cd50_S50-xSex_thin_films

https://www.researchgate.net/publication/304150361_Study_of_optical_properties_of_thermally_evaporated_ZnSe_thin_films_annealed_at_different_pulsed_laser_powers

And others.

Good morning,

Pr. A.S.Hassanien, ans R.S. Silva,

thank you for enriching this scientific discussion.