  Dipole plasmon resonances are ubiquitous in nano-particles with delocalized
charge carriers. Doped semi-conductor colloidal nano-crystals constitute a
novel paradigm for plasmon excitations in a finite electron system and offer
the possibility to tune the carrier density and thus the dipole resonance from
visible to infra-red, which cannot be achieved with metallic clusters.
Restricting ourselves to highly n-doped ZnO nano-crystals, we explain the
observed smooth transition from small sizes dominated by quantum effects to
large sizes where the resonance reaches its classical value. A schematic two
interacting highly degenerate level quantum model, validated by a full Random
Phase Approximation calculation, yields nicely the experimentally observed
trends.
