Quantum confinement: In a quantum dot (size L), energy levels are discrete: Eₙ = n²π²ℏ²/(2m*L²) — the particle in a box model.
Smaller dots → larger energy gaps → shorter wavelength (blue-shifted) emission. This is why CdSe quantum dots glow from red (large) to blue (small).
Effective mass m* accounts for the crystal environment. The absorption spectrum shows allowed transitions (selection rule Δn=0 for 3D, approximately). Real quantum dots also have Coulomb (exciton) corrections ~ 1.8e²/(εL).