I measure the UV-vis-NIR absorption spectrum of nanotubes in a thin PMMA film on a sapphire substrate, but the signal from nanotubes is buried under the interference pattern of the thin film. Is it legitimately to obtain the absorption spectrum of nanotubes just by subtraction like
nanotubes=(nanotubes@PMMA on supphire)-(PMMA on supphire),
or I should take into account something else?
From the interference pattern, the thickness of the film is estimated as 400 nm.
I think what you are expecting is right. Provided the atmosphere is not changing so much like humidity, temperature of your chamber. So better try to do these measurements back to back and then subtract. As what i feel If you will take the absorbance of PMMA on supphire at different time your result may little vary. Back to back measurement of this as background and sample as nanotubes@PMMA on supphire will surely workout.
As the interference appears it means the incident waves are which reflected from the bottom of the films are interfering with those incident thus in this case you need to use the relation T=(1-R1)(1-R2)exp(-ad)/(1-2R1R2exp(-2ad) with R1 being that of substrate and R2 of the film. If you do not measure the reluctance then u need to use swanepoel transmittance technique to average the interference patterns through selection of maximal and minimal envelope functions for the transmittance in the interfering range of spectrum.
What if you did this :
1. Prepare a reference sample (PMMA on sapphire)
2. Prepare a sample with X1% concentration of nanotubes in PMMA on sapphire
3. Prepare a sample with X2% concentration of nanotubes in PMMA on sapphire
4. Prepare a sample with X3% concentration of nanotubes in PMMA on sapphire
Then take measurements and try fitting to lambert Beer law.
There is no way of estimation. If interference occurs, you have to use the exact formulas for transmittance provided e.g. by Abeles. A way to calculate the absorbance would be to use dispersion analysis to model the absorption index and the underlying dielectric function and from there calculating the absorbance. There is also an older procedure relaying on an iterative estimation of the absorption index and involving the Kramers-Kroning relation to generate an index of refraction, also relying on Abeles formalism. The latter formalism must be involved in the calculation - there is no short-cut...
https://www.researchgate.net/project/Signal-enhancement-in-Transmission-and-Transflection-measurements-of-films
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Article The Measurement of the Optical Constants of Thin Solid Films...
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