Magneto-photoluminescence properties of colloidal CdSe nanocrystal aggregates

Magnetic fields up to 10 T were used to study the relative excited-state population of exciton fine-structure states in aggregates of 0-D CdSe nanocrystals. Following excitation by 400 nm light, energy-resolved, intensity-integrated photoluminescence (PL) spectra of CdSe quantum dot aggregates were collected at 1.6 K. Temperature-dependent PL measurements revealed multiple exciton relaxation channels, which were attributed to direct and longitudinal optical phonon-assisted recombination of isolated and aggregated nanocrystals. The relative populations of the 1S_(e)- 1S_3/2(h) exciton fine-structure levels in both isolated and aggregated nanocrystals were actively controlled by application of a magnetic field. The efficient applied magnetic field-assisted population control led to an increase in the total PL of the aggregate that resulted from a reduction in phonon-assisted recombination and an increase in direct radiative e-h recombination. An increase in the relative radiative recombination yield was attributed to field-induced mixing of the "dark" and "bright" exciton states. This behavior was quantified using a novel branching ratio analysis that related the relative population of the "bright" exciton to the total excited state population. The data present promise for studying spin-dependent energy transfer in nanocrystal superstructures.