P-N hetero junction Nano composite band-gap is very narrow compare to their individual nano particle (other than oxygen deficiency )
Generally, there are two main possible reasons behind that:
1- Structural effect: this is due to the formation of interfaces and the interfacial phases.
2- Size effect: this effect appears when the size of the composite particles is larger than that on its components.
It is possible to reconstruct main lattice due to its doping. The doping process by adding donor or acceptor impurities alters the Fermi level and replaces the CB in the n-type or replaces the VB in the P-type semiconductor which could narrow the distance between CB and VB
Other than oxygen deficiency, the intimate interfacial interaction could cause the band gap narrowing.
The band gap of a semiconductor is the gap between bonding molecular orbitals(filled of electrons) or HOMO and antibonding molecular orbitals (usually empty of elements) or LUMO. When a semiconductor is dopped with other specious, some new bonds are formed and the number of molecular orbitals will be increased. On increasing the number of molecular orbitals the gap between the bonding and antibonding molecular orbitals is decrased. That is, the band gap of dopped semiconductor will be narower than the pure one.
The band gap energy increases due to particle size and quantum confinement effect The band gap energy decreases with increase in temperature
When crystallite size approaches the nanoscale, the number of atoms in the prepared sample will decrease which will cause a decrease in orbitals to overlap as well as energy level. This will leads to the creation of narrow valence and conduction bands. As the valance band and the conduction bands are narrower for a nanoparticle, the band gap between the conduction and valance bands will be more.
Dopants atom orbitals act as intermediate state or bridge the gap between valence and conduction band of electrons of the composite materials, resulting shorter energy for electronic transitions (lower bandgap)
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