The film was composed of cupric oxide phase. Please let me know if there is any other way to do it.
Using Tauc's plot or differential absorption one can determine band edge using UV-Vis absorption. Another way is to use PL spectroscopy.
The cupric oxide band gap is about 2 eV, while the Si – about 1 eV. So it’s unlikely that you can see the cupric oxide optical absorbency when it is deposited on Si. For measuring it`s band gap width by UV spectroscopy it would be better to deposit it onto some wider band gap material – quartz for example.
When looking at the band gap of materials with UV/Vis you want to be looking at the lower absorption edge (the right edge of the absorption, to higher wavelenghts / lower energies). The band gap is typically taken to be at the point where the absorption intensity of your materials reaches zero.
The theory of optical absorption gives the relationship between the absorption coefficient α and the photon energy hν for direct allowed transition as a = [(hν − Eg)^1/2]/hν
This equation gives the band gap (Eg) when straight portion
of (αhν)2 against hν plot is extrapolated to the point α=0.
V.R. Shinde et al. / Materials Chemistry and Physics 96 (2006) 326–330
When you measure by UV spectroscopy the bandgap width of cupric oxide film on Si surface it is very possible that Si will absorb all the light at the wavelength where one can expect the cupric oxide band gap edge could be found. That`s why it would be better to use another substrate in this case.
Using Tauc's plot or differential absorption one can determine band edge using UV-Vis absorption. Another way is to use PL spectroscopy.
To study the optical properties of thin film deposited on Si substrates can take PL mesurment. The energy band gap which estemated by PL could be near to that can found by optical absorption.
In thin films is not totally correct to use UV-Vis spectroscopy because there ara a lot of interferences. I recomend you to use ellipsometry. I used it for the Band Gap calculation of TiO2 nanoparticles. By UV-Vis measurements I obtained 3,5-3,8 eV but by ellipsometry 3,2-3,3 (according to theorical values).
Ellipsometry will give a reliable value for n,k and thickness.Band gap value is best approximated by slightly greater than PL value.
Sebastian @ The theory of optical абсорtion, which gives the relationship between the absorption coefficient α and the photon energy hν for direct allowed transition as a = [(hν − Eg)^1/2]/hν, ignores the presence of electron-phonon interaction. This interaction is very strong at room temperature. In this case, as well as at low temperatures, the shape of the absorption edge is described by Urbach's rule, i.e, has exponential dependence on energy.
Nagapradeep @ Of course, you can calculate the band gap energy by the photoluminescence measurements. For example of such studies presented in attached paper. The short-wavelength emission may be acossiated with the band gap energy.
As, U.V range is 100-400nm.
Therefore it can works above the 3.5eV band gap materials.
As, your thin film is about 2ev band gap... so in between semiconductor and insulator range.
May be four probe method will be useful ....
If you have CuO nanoparticles with sizes of 5-6 nm, we will be observe the photoluminescence spectrum in the spectral range 350-500 nm (see attached paper). Usually there is the UV-Visible spectrometer. Using this technique allows to study the photoluminescence spectra.
Using UV- Vis NIR spectroscopy in reflectance or specular reflectance mode you can determine band gap of films that are coated over opaque substrates.
The problem with CuO is that it may give features near the 340 nm which is a source change region. But still you can observe notable behaviour near absorption edge
Here you can find good explanation and band gap calculator,
http://www.instanano.com/characterization/theoretical/uv-vis-spectroscopy-band-gap-calculation/
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