Spectroscopic study of electronic states in an ensemble of close-packed CdSe nanocrystals

Nearly monodisperse CdSe quantum dots of 1.8 nm in size were synthesized an d capped with a surface monolayer of 1-thioglycerol, The optical properties of thin films made from these matrix-free close-packed quantum dots were s tudied at different temperatures and electric field strengths and compared with isolated quantum dots of the same size embedded in a PMMA matrix. The broadening and red shift of the optical transitions observed in absorption and excitation spectroscopy for the ensemble of close-packed quantum dots i s explained by the formation of collective electronic states between intera cting nanocrystals. The reversible collapse of these extended electronic st ates into localized states was demonstrated for the close-packed quantum do ts by applying an external electric field.