There are good explanatory videos on Youtube on how to analyze UV-vis absorbance data and how to calculate optical bandgap using the Tauc plot. For instance, https://www.youtube.com/watch?v=vXxNrb8rmaE and https://www.youtube.com/watch?v=a9fSg2TREag explain well. As for the formula, it is (alpha*h*nu)^1/n =  beta*(h*nu - Eg), where alpha is the energy-dependent absorption coefficient, h is the Planck constant, nu is the photon’s frequency, beta is a constant, and Eg is the band gap energy.

Gunel Huseynova

Dear Ma'am,

I have confusion because I want to find a bandgap of a thin film which I cannot put in the cuvette, In such a case how to calculate alpha .... In the video, they are taking the path length of the light as the standard cuvette length. For liquids, this works but in the case of solids, how to calculate alpha?

Dear Sutapa Badyakar Is your thin film deposited on a glass or some other rigid substrate? In that case, there should be a holder for putting the sample. There are many interesting papers on how to analyze thin films' bandgap using Tauc plot. I can recommend several and you can also find even more. For example, google please the papers titled "Determination of the Optical GAP in Thin Films of Amorphous Dilithium Phthalocyanine Using the Tauc and Cody Models", "Assessing Tauc Plot Slope Quantification: ZnO Thin Films as a Model System", Evaluation of the Tauc Method for Optical Absorption Edge Determination: ZnO Thin Films as a Model System, andArticle Nanotwinning and structural phase transition in CdS quantum dots

Sutapa Badyakar

Tauc used an equation in 1968 to calculate the absorption edges (bandgaps) of amorphous Ge and Si from their absorption data.

(αhν)γ=A(hν-Eg)

In this equation, α is the absorption coefficient, h is Planck’s constant, υ is the frequency of the incident photon, A is a proportionality constant (which is determined by the index of refraction, electron, and hole effective masses; however, it is usually taken as 1 for amorphous materials), and Eg is the bandgap energy. The important term is the exponent γ, which denotes the nature of the electronic transition, that is, when γ=2 it is a direct allowed transition, and when it is equal to 1/2, it is an indirect allowed transition. For γ=2/3 it is a direct forbidden transition, and for γ=1/3 an indirect forbidden transition. Typically, the allowed transitions dominate the basic absorption processes, giving either direct or indirect transitions.

Thus, the basic procedure for a Tauc analysis is to acquire optical absorbance data for a sample in question that spans a range of energies from below the bandgap transition to above it.

Plotting the (αhν)γ versus (hν) is a matter of testing γ=2 or γ=1/2 to compare which provides the better fit and thus identifies the correct transition type.

I have provided the Template file (Origin file) in the video description. Thanks

https://youtu.be/LLESVSsZmlg