Photoluminescence (PL) is the spontaneous emission of light from a material following optical excitation. It is a powerful technique to probe discrete energy levels and to extract valuable information about semiconductor sample composition, quantum well thickness or quantum dot sample monodispersity.
An electron that has been excited above the conduction band of a material will eventually fall and recombine to the hole that has been excited below the valance band after losing some energy through releasing a phonon to the lowest available non-radiative energy level. The efficiency of photoluminescence signal is determined by the the nature of optical excitation, properties of material; i.e. radiative and non-radiative recontamination and defects.