I have synthesized the CZTS nanoparticles, which were analysed using PL spectroscopy. The band gap calculated using PL was found to be 1.75 eV which is higher than the actual band gap of the CZTS. Is it possible that the band gap could be higher. If Yes, then suggest me the possible mechanism behind it. Thank you!
I agree with Professor Martin Stutzmann. The band gap of a material in nanoscale increases compared to its bulk size.
For simplicity, you may consider the energy eigenvalue of a particle in a potential well problem, where, En is inversely proportional to the square of the width of the well (a). En=(n2pi2ħ2)/2ma2
So, (En+1-En) increases with the decrease of a.
How nano are your nanoparticles? Any idea about the actual particle size? Is there no single paper on quantum effects in CZTS?
Dear Roman Kudrin,
The particle size is around 20-30 nm. The actual particle size is not known due to lack of HRTEM facility. There is no clear cut explanation about the quantum effect in CZTS yet published. Although, the earlier literature reported that the band gap is lower than that of the actual band gap energy i.e. 1.4 to 16 eV. But in our case this energy gap is increased upto 1.78 eV.
Quantum effects, in general, are expected to increase the bandgap and not to decrease it. So, your situation might have been explained by quantum confinement if the particle size would be on the order of Bohr exciton radius, which is something like 6 nm for CZTS (or even less). Your particles are larger, which means, there is hardly any quantum influence. I would suggest it is rather something about stoichiometry of your material, see here for example:
http://www.sciencedirect.com/science/article/pii/S0925838813018343
A high order degree in the Cu-Zn plane is another factor that can give a band gap higher than that usually reported (see Valentini et. al Appl. Phys. Lett. 108, 211909 (2016)). We are still working to develop a consistent and comprehensive picture including both stoichiometry and ordering effects. It is possible that nanoparticles can reach a perfect order state which is almost impossible to reach for an extended sample? Who knows...
Another point is that often CZTS nanoparticles grow in the hexagonal wurtzite phase. It is still debated if this phase has a larger or a smaller gap than the tetragonal one (see Cattley C. A. et al , Chem. Commun., 2013, 49, 3745) . I suggest you to check the crystal structure of your NPs.
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