As Mohammad mentioned, if the the crystallite size is large there are more atoms and also more atomic orbitals for overlap. Therefore, the number of molecular orbitals, both bonding and antibonding, are increased and the gap between HOMO and UMO( which the band gap) will be decreased. So for smaller particle size a larger band gap is expected.

It is because of quantum confinement effect. The bands are basically composed of discrete energy levels and the distance between valence band and conduction band is called band gap. So, when they are few atoms in a small particle, there are few of those discrete energy levels to make bands. That made the bands narrower and consequently increases the gap between them.

This can affect the resistive properties of films.

thanks sarika sigh, mohammad nazari and Michail Georgievich zuevfor your kind reply.

yes Michail georgievich zuev. it shows that high resistance. Is this any possible to occurquantum confinement effect? how is possible chief reasercher?

As Mohammad mentioned, if the the crystallite size is large there are more atoms and also more atomic orbitals for overlap. Therefore, the number of molecular orbitals, both bonding and antibonding, are increased and the gap between HOMO and UMO( which the band gap) will be decreased. So for smaller particle size a larger band gap is expected.

Band gap need not always follow a trend while comparing particle size and band gap.

It decreases or increases depending on the electronic property of the oxide.

Will you please mention the name of the oxide material you are using?

The apposite should happened based on the quantum size effect.

That's the band gap generally increases with decreasing the size

Generally, the quantum confinement leads to the increase in band gap due to the decrease in the number of orbitals participating in the formation of VB and CB through orbital overlap. However, the trend doesn't always need to be followed by a material. there are other parameters which can affect the band gap of material, you need to figure out the predominant phenomena or effect that lead to decrease in band gap. the  usual suspects are point defects like vacancies and interstitials. In oxide materials, the quantum size effects show up only below a certain grain size, the  reason being that the defect induced effects are predominant.

you can read this article:

https://www.researchgate.net/publication/305213896_Grain_size_disposed_structural_optical_and_polarization_tuning_in_ZnO?_iepl%5BviewId%5D=nBaM1TZToxOvnOFC0X1FB2Ss&_iepl%5BprofilePublicationItemVariant%5D=default&_iepl%5Bcontexts%5D%5B0%5D=prfpi&_iepl%5BtargetEntityId%5D=PB%3A305213896&_iepl%5BinteractionType%5D=publicationTitle

Article Grain size disposed structural, optical and polarization tuning in ZnO

saying it in the simplest way, decreasing the crystallite size you always cause a (direct) GAP increase, whichever the semiconductor material you are using. You can understand this by just recalling the usual explaination, that is decreasing the particle size you are moving from a bulk-like with bands to an atomic-like, with single discrete levels, electronic distribution... and the energy gap from the last occupied to the first unoccupied level simply increases. Hence, you are not facing a Gap reduction, but the occurrence of intra-gap defect states and bands which show up as the gap size would be reduced. The kind of defects, their energy distribution and their effect on local carrier populations (Fermi level) depends on kind, size and density of structural/chemical defects you intrinsecally have in your material (be aware that we call defect anything that breaks the crystal periodicity). As a matter of facts, the same grain boundaries of your nanocrystalline material introduce defects whose energy may lie within the gap and hence may act at least as hopping sites for electrons to enter the conduction band or holes into the valence band.

Thanks to all for yours valubale sugggestions.

you may have impurities or defects

May be due the effect of quantum confinement

Dear Marimuthu Govindasamy

The quantum confinement leads to the increase in band gap due to the decrease in the number of orbitals participating in the formation of VB and CB through orbital overlap. The number of molecular orbitals, both bonding and antibonding, are increased and the gap between HOMO and UMO( which the band gap) will be decreased. So for smaller particle size a larger band gap is expected.

The band gap increases inversely with the size of the nanoparticle

the bandgap if decrease or increase sometimes it is not related to the grain size , but depend on method of preparation and the preparation condition.

for the same phase at all deposition temperatures, you should check the Urbach tail.

Dear Marimuthu,

Reason

Two reasons are there. First (i) impurity concentration which brings narrowing effects and the second (ii) large crystalline sizes increases the bonding and antibonding and consequently band gap is decreased. This can be explained by the gap of valance and conduction band.

The energy band gap is strongly dependent upon the crystal structure of the material. Consequently, it depends on the preparative conditions and the preparation process. It is observed experimentally that shrinkage of the bandgap occurs when the impurity concentration is particulary high. This effect is called the bandgap narrowing effect which is ascribed to the emerging of the impurity band formed by the overlaped impurity states.

Exact reason, If the the crystallite size is large there are more atoms and also more atomic orbitals for overlap. Therefore, the number of molecular orbitals, both bonding and antibonding, are increased and the gap between the band gap will be decreased (valance and conduction band).

The size- and shape-dependent band gap energy of semiconductor compound nanomaterials (SCNs) for example, Energy band gap increases as particle size of the semiconductor nanomaterials decreases. Particle size reaches the nanoscale, the number of overlapping of orbitals or energy level decreases and the thickness of band becomes thinner. This will cause a rise in energy band gap between the valence band and the conduction band. This describes the higher energy band gap in SCNs than their corresponding bulk counterpart.

For the the optical band gap of the thin films may found to increase with the decrease in crystallite size, which is in accordance with the quantum confinement effect. The dislocation density and microstrain were found to vary inversely with the crystallite size, whereas the lattice constant was found to increase with an increase in crystallite size.

Hope it is helpful to you.

Ashish

Thank you Marimuthu.

Ashish

They are all good.