I want to take an IR spectra to obtain the optical bandgap by tauc plot. Since the material is a flake with little transmittance, an ATR IR experiment is taken(with Ge, n=4.0). It should be basically flat at large wave number, but I got a sloping curve. The larger the wavenumber, the lower the absorbance. According to this trend, I think it is possibly due to that the larger the wavenumber, the shallower the evanescent wave. Since the accurate refractive index n of my material remains unknow, I tried several times but did't get a flat baseline. If I want to get optical bandgap from IR spectra, my focus will be a part of the curve rather than the peaks only. Since ATR's characteristic will change the shape of the curve, Is ATR IR suitable to obtain the optical bandgap? Thanks for any advice.
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