I have carried out diffused reflectance UV spectroscopy for determination of band gap for powder samples. Using K-M method,I have obtained sharp peak at particular wavelength.Instead of getting smooth curve to determine bandgap.Is it due to impurities or any other errors?
The spike looks like a lamp change or filter change artifact to me. Some of my equipment has adjustable lamp changes. I deal with this by subracting a baseline. Also think you may be saturating your equipment. I wouldn't trust the ragged top of that peak at all.
looks like an excitonic band for this material, check the litterature
So for calculations should I ignore the spike or include the spike in the band gap calculation?
I am quite sure it is not the lamp change because you usually see a vertical shift at the specific wavelength, typically 350 nm in standard conditions when the lamp is changing. Here you have a relatively smooth decrease then increase of the signal, so this is a real peak.
try to do some extrapolationtaking the beginning of your curve in the 2-3 eV and 3.25-3.4 eV (before the signal becomes flat at the top), and you may have some good curve to extract the bandgap value.
But what could be an explanation for such spike?Because in my other samples the spike broadens.
It can be excitonic absorption. Cf Wikipedia for a quick start. An exciton is a bound state of an electron and an electron hole which are attracted to each other by the electrostatic Coulomb force.
Try searching "exciton absorption zno" in Google, you will see such features.
I also found this article, exhibitibg different peaks or band features. It is REALLY part of your material http://scitation.aip.org/content/aip/journal/jap/85/11/10.1063/1.370601
check also this website http://www.cat.gov.in/technology/laser/lmpd/resnanonew.html
The spike looks like a lamp change or filter change artifact to me. Some of my equipment has adjustable lamp changes. I deal with this by subracting a baseline. Also think you may be saturating your equipment. I wouldn't trust the ragged top of that peak at all.
Changes of a range in the form of a sharp failure (peak) are hardly a range of absorption of an exciton. Probably it is instrument effect: change of the filter, lamp or feature of work of system of registration, perhaps effect of saturation.
This link may help,
http://www.instanano.com/characterization/theoretical/uv-vis-spectroscopy-band-gap-calculation/
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