I know that a graph..but for calculating absorption coeff. we have to put thickness in the formula?So tell me if there exist any method please tell.
If you do not know the film thickness, you take transmission measurements in the wide wavelength range 200 nm to 2000 nm so that you get interference fringes. Then, apply Swanepoel method to calculate the thickness and absorption coefficient both. Then, you can use Tauc plot to calculate the band gap. You can also get other optical parameters, i.e, real and imaginary part of the refractive index. You need not to take reflection into consideration in this method. Seperate measurement of thickness is not required. The only requirement is to get interference fringes in transmission.
Dear Bulkesh.
To estimate the band gap of TiO2 you need to record the UV-vis absorption spectrum and then create the Tauc plot.
The Tauc plot is a method that is widely used for the determination of band gap. Shown next is the procedure for determining the band gap from a diffuse reflectance spectrum using the Tauc plot: The following relational expression proposed by Tauc, Davis, and Mott is used. (hνα)1/n = A(hν - Eg) (1) where: h: Planck's constant, ν: frequency of vibration, α: absorption coefficient, Eg: band gap, A: proportional constant. The value of the exponent n denotes the nature of the sample transition.
For direct allowed transition••••••••••••• n = 1/2
For direct forbidden transition•••••••••• n = 3/2
For indirect allowed transition•••••••••• n = 2
For indirect forbidden transition•••••••• n = 3
The acquired diffuse reflectance spectrum is converted to Kubelka-Munk function. Thus, the vertical axis is converted to quantity F(R∞), which is proportional to the absorption coefficient. The α in the Tauc equation is substituted with F(R∞). Thus, in the actual experiment, the relational expression becomes: (hνF(R∞))2 = A(hν - Eg) (2) Using the Kubelka-Munk function, the (hνF(R∞))2 can be plotted against the hν. The curve that plots the value of (hν - (hνF(R∞))2) on the horizontal axis hν and vertical axis (hνF(R∞))2 is drawn. A line is drawn tangent to the point of inflection on the curve and the hν value at the point of intersection of the tangent line and the
horizontal axis is the band gap Eg value.
You can also calculate optical bandgap using first derivative of optical transmission,,, This method becomes more useful, when you do not have fringes in the transmission... Because, if there are no fringes, you can not apply Swanepoel method...
Here are some references:
https://www.researchgate.net/publication/260675597_Photo-induced_optical_changes_in_GexAs40Se60x_thin_films
https://www.researchgate.net/publication/233160174_Effect_of_UV_Exposure_on_Some_Optical_Properties_of_As-Se_Based_Chalcogenide_Glasses
Article Photo-induced optical changes in GexAs40Se60–x thin films
Article Effect of UV Exposure on Some Optical Properties of As-Se Ba...
just read the article and you will find the procedure to find the band gap of TiO2
Cathodoluminescence is another suitable method for the determination of Near Band Edge.
by envelope method u can measure the thickness of the film that helps in calculating the band gap by tauc plot method
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