Usually if we go from bulk (3D) structure to 2D, 1D, 0D structure, band gap of the materials increases accordingly. But, in my case exfoliated WO3 ( expected to have 2D sheet like structure/ nanoplatelets) is showing less band gap compared to bulk as calculated from Tauc plot. How it is happening? Please anyone help me.
A close examination of the absorbance spectra is mandatory. By just neglecting signal distorsion or losses due to possible light scattering effects, a critical point might stay in the true lineshape in Urbach tail region, where structural/chemical defects may sensibly enhance the band-to-band transition contribution (rising the tail), which may cause large errors in extracting a significant GAP by TAUC method
Dear Priyabrata Sahoo, It's true that thickness variation (as stated in the question) changes the band gap energy of a material. However, it is not the only factor. The bandgap energy of a material is greatly affected by its crystal structure also. The crystal structure, in turn, depends on the methods and conditions of preparation. The crystallite growth, particle size and grain agglomerations are all affected by the method of sample preparation. The conditions of sample preparation include temperature, pressure, state of precursor used, etc.
@kamal prasad sapkota Thank you for your answer sir. Now it is clear.
@Giuseppe curro.I am attaching origin file of the plot which is obtained through DRS. It would be helpful if you can see the graph and explain to me. Thanks for your time.
dear Priyabrata, I can not open the file you attached. Please post it in a std format, e.g. jpg for pictures or pdf for docs
Dear Pryiabrata, The plots are quite clear and confirm my suspicion about a large contribution from b-to-b transitions. The Urbach tail just below the absorpion jump is clearly much larger in the exfoliated case than in bulk sample, and this may have a strong influence in extracting the GAP value just for an increased difficulty in locating a baseline and deciding for a "linear" (or better to say "constant slope") range along the raising ramp.
Moreover, it seems that a small but visible change in the ramp slope occurs as well... I'm not sure but I would guess that some dispersion due to disorder in orientation of the WO3 leaflets may be the cause (that is, you are observing a convolution of more contributions).
I suggest you to post the same spectra (with just labels and no data columns) so some other collegue may give you further help and ideas.
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