The band gaps can be calculated via UV-Vis spectroscopy using Tauc Plots. By plotting the graph between (ahv)^(1/2) versus photon energy (hv) where, a (alpha) is the absorbance calculated from UV . (hv) can be calculated form wavelength using: (hv = 1240/wavelength);Extrapolating the straight line portion of the curves to zero absorption coefficient value gives the energy band gap value.
From UV-Vis spectrum, you are able to calculate the band gaps of material (allowed direct, allowed indirect, forbidden direct, and forbidden indirect transitions), by plotting the graph between (ahv)^(1/n) versus photon energy (hv).
where a: is the optical absorption coefficient, which can be calculated from absorbance(A), and thickness of the sample(t) using: (a=2.303A/t); and (hv) can be calculated form wavelength using: (hv = 1240/wavelength); The power factor (n) takes the values of (0.5, 2, 1.5, and 3) for allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions.
Extrapolation the straight line portion of the curves to zero absorption coefficient value gives the energy band gap value.
Hopefully, you are satisfied with the answer.
The optical band gap (Eg, in eV) can be calculated using absorption edge values (λ edge, in nm) from absorption spectra by using eq.:
1240/λ edge
Here you can find good explanation and band gap calculator,
http://www.instanano.com/characterization/theoretical/uv-vis-spectroscopy-band-gap-calculation/
Nishant Kumar ,Hay sir, could you tell me further about the thickness, my sample is liquid where can I find its thickness?
thanks
https://www.youtube.com/watch?v=u-chJAM73p0&ab_channel=nanotutes
Qaisar Nawaz
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
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