Singlet/Triplet Reversal in Strongly-Coupled GaSe Nanoparticle Aggregates

GaSe nanoparticles have been synthesized in the absence of tightly binding edge ligands. These particles are ligated primarily with TOP and TOPO ligands that are easily displaced by far less bulky alkyl aldehydes. Ligand replacement results in nanoparticle aggregates in which the lowest energy optical transitions are strongly coupled, resulting in a large red shift of the absorption spectrum (about 2600 cm−1) and a reversal of the singlet and triplet states. The reversal of the spin states results in changes in the polarization spectroscopy and a dramatic decrease in the radiative lifetime. Specifically, the exciton singlet states are linear oscillators, and time-resolved fluorescence polarization spectroscopy gives an initial anisotropy very close to the linear oscillator limit of 0.4. The radiative lifetime of this fluorescence is about 12 ns, compared to about 82 ns for the monomers. Upon aggregation, the fluorescence quantum yield increases from 4.7 to 61%.