I prepared ZnO via a different method but I found something. I think it's a strange optical band gap.
If you mean «less» in term of nm... it is normal. When you have nanometer-sized materials, the gap will become larger because of the quantum confinement. Therefore the transition will be more energetic (blue-shifted). If you are talking about «less» in term of band gap energy in eV, I don't understand. If you determined Eg with the fluorescence wavelength, maybe it is not fluorescence from the excitonic transition but maybe from some trap-state transitions which give you an apparent smaller gap.
Thanks but i measure only the optical absorption and by using tauc's relation for analysis
i have this result for two sample prepared by two different method and i guess that it may be trap defect or others but there is a sharp peak in optical absorption spectra and at low wave length gradually decrease
Ok, I see... I don't know anything about your synthesis conditions, but maybe the problem is there. Maybe some elements that you are using in your synthesis are leaking in the core and creates alloys with a smaller gap. For example, thiols are often used in nanomaterials synthesis. If some of the thiols are thermolysed at some point in the synthesis, then maybe some sulfur have leaked in the core. Also, assuming that you used thiols as ligands, the sulfur atom is bonded to the rest of the crystal, which may affect its properties. Usually S-based nanocrystals have smaller bandgaps than O-based nanocrystals which could potentially explain the situation. Indeed, I assumed you used thiols but it would be the same for any other ligands. Also, you may have formed an other cryatal structure that have different optical properties than the bulk crystals you usually work with. An unstable phase in the bulk doesn't mean it is unstable in nanomaterials. For both cases, maybe XRD analysis will help you find the answer. If there is some leaking in your NCs, you will probably see a shift in the peaks position. If it's an other crystal structure, then, the difference will be obvious. Good luck!
The difference lies in the size of the nano particles and bulk ZnO. As size of ZnO decreases the quantum confinement becomes more prominent and absorption of ZnO is shifted to blue side of the electromagnetic radiation. As wavelength decreases, the band gap energy increases according to the following equation
Eg = hc/λ
If you confirmed the formation of ZnO by different experimental tools and you found a decrease in the band gap relative to bulk Zno structure, then, the decrease in the band gap may arise from the formation of crystal defects like oxygen vacancies. You could confirm this interpretation by measuring the fluorescence spectroscopy.
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