We present experimental observations of line-narrowing of the photoluminescence in close-packed thin films of CdSe nanocrystals with epitaxial ZnS shells, under excitation from a pulsed source of 100 ps duration, both at 77 K and at room temperature (RT). Atomic force microscopy (AFM) of these films reveals them to be optically flat to less than 1 of the emission wavelength and we propose that waveguiding and amplified spontaneous emission is the principal mechanism for line-narrowing. We observe that line-narrowing is assisted by the use of films with a bimodal distribution of nanocrystal sizes, where the excited state population is concentrated on the larger nanocrystals due to energy transfer.
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