Carrier recombination in CdS nanocrystals under single-electron and high-density excitation

Room-temperature green luminescence has been studied in laser-modified (darkened) CdS nanocrystals with the average radii a ranging above the exciton Bohr radius (from 3.3 nm to 100 nm). Under high picosecond-pulse excitation, the electron-hole plasma decay kinetics exhibited a decrease of lifetime with decreasing crystallite size, enabling the surface recombination with the velocity of 1.3*103 cm s-1 to be revealed. Under low cw excitation, a striking increase in the luminescence intensity of nanocrystals with one excited electron-hole pair was discovered for average radii below 10 nm. The bulk-like bimolecular recombination with the constant of 3*10-10 cm3 s-1 was shown to account for the observed size dependence.