I suggest you this old question/answer
https://www.researchgate.net/post/What_is_Quantum_confinement_effect
When sizes are comparable to or smaller than the Bohr radius, the dimensions of the nanoparticle itself defines the spatial extent of the electron - hole pair (exciton) state (but not the strength of e - h Coulomb coupling), and hence the size of the spatial confinement of electronic wave-functions, which is known as “quantum confinement”. As the nanoparticle size is reduced, the electronic excitations shift to higher energy, and the oscillator strength is concentrated within just a few transitions, , making their electronic states become discrete.
Quantum confinement of both the electron and hole in all three dimensions leads to an increase in the effective band gap of the material with decreasing crystallite size. This size dependency is clearly remarkable in the emission.
Quantum confinement not only produces size-dependent band gap of nanocrystals, but also size-dependent extinction coeffcient, which indicates how much light is absorbed by the material at a certain wavelength, per mass unit or molar concentration.
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