How can I know the band gap using UV-Vis absorption data?

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Xiao-Ye Wang

It is generally determined from the low-energy absorption onset according to the equation Eg = 1243/onset

b.c. Rai

Band gap is the energy gap in energy band diagram between lowest of CB and top of VB. when divided by h, we get frequency. The formula given in previous answer follows: Eg in eV and onset in nm (wavelength).

Bahram Roughani

UV-Vis absorption data (in particular room temperature data) may or may not be sufficient for determining the band gap. Techniques such as Photoluminescence (PL) done at low temperatures, or modulation spectroscopy such as photoreluctance or elecroreflectance might provide better and more accurate answers. In modulation spectroscopy one is looking at the rate of the change of absortion (a derivative function) which provides greater sensitivity and sharper spectra corresponding to transitions associated with band-to-band activities, as well as band-to-impurities transitions, etc. UV-Vis absorption data my not provide the most accurate value for the band gap, but should one have a guess as where (energy value) to look for such transitions, one may then point out features in teh absorption structure that may suggest a value for teh band gap.