Optical bandgap energy calculation by tauc plot method | UV-Visible Spectroscopy

Optical bandgap energy is an important physical parameter of a material that determines its electrical and optical properties. It is defined as the minimum energy required for an electron to move from the valence band to the conduction band of a material. The bandgap energy of a material can be determined by various experimental techniques, one of which is UV-Visible spectroscopy.

UV-Visible spectroscopy is a widely used technique for the characterization of materials based on their absorption and reflection properties in the UV and visible regions of the electromagnetic spectrum. It involves the measurement of the amount of light absorbed or reflected by a sample over a range of wavelengths.

One common method for determining the bandgap energy of a material from UV-Visible spectroscopy data is the Tauc plot method. This method involves plotting the absorption coefficient (α) of the material as a function of photon energy (hν) and fitting the resulting data to a straight line.

The Tauc plot method assumes that the absorption coefficient of a material follows a power law relationship with photon energy, such that α ~ (hν - Eg)^n, where Eg is the bandgap energy and n is a constant that depends on the nature of the electronic transitions involved. The Tauc plot method involves plotting (αhν)^2 as a function of hν and fitting the resulting data to a straight line. The bandgap energy can then be determined from the intercept of the straight line with the hν axis.

To perform the Tauc plot method for optical bandgap energy calculation using UV-Visible spectroscopy data, the following steps can be taken:

1. Acquire UV-Visible spectroscopy data for the material of interest, typically in the range of 200-800 nm.
2. Convert the wavelength data to photon energy data using the equation E = hc/λ, where E is the energy of a photon, h is Planck's constant, c is the speed of light, and λ is the wavelength.
3. Calculate the absorption coefficient α using the equation α = (1/T)d(logT)/dλ, where T is the transmittance of the sample at a given wavelength.
4. Plot (αhν)^2 as a function of hν and fit the resulting data to a straight line using linear regression analysis.
5. Determine the bandgap energy Eg from the intercept of the straight line with the hν axis.

It should be noted that the Tauc plot method assumes that the absorption coefficient of a material follows a power law relationship with photon energy, which may not be true for all materials. Additionally, the value of n can vary depending on the nature of the electronic transitions involved, and may need to be determined empirically for each material. However, despite these limitations, the Tauc plot method remains a useful and widely used technique for determining the bandgap energy of materials from UV-Visible spectroscopy data.