Preparation of ultrasmall, uncapped PbS quantum dots via electroporation of vesicles

Electric-field-induced transient pore formation (electroporation) in synthetic unilamellar dioleoylphosphatidylcholine vesicles of 178-nm diameter is utilized for the preparation of subnanometer-size PbS quantum dots. With Pb2+ ions originally entrapped in the vesicles and S2- ions placed in the bulk, their reaction is initiated by the opening of pores and occurs in the bulk. The ensuing self-aggregation of PbS is slowed to the hour and day time scales by its adsorption at the exterior surface of the vesicles. The growth of the particles in the molecular size regime is found to exhibit novel, time-dependent, oscillating red and blue shifts of the characteristic UV absorption band. On the basis of similarities between the oscillating trend of the experimentally observed transition energy and that of the calculated highest occupied molecular orbital-lowest unoccupied molecular orbital gap of (PbS)n clusters with n = 1-9, the wavelengths of the sequential spectral peaks can be assigned to the PbS monomer (237.5 nm), dimer (282 nm), tetramer (232 nm), hexamer (281 nm), octamer (234.5 nm), and nonamer (278-280 nm). Growth beyond the octamer is associated with the customary monotonic red shift of the absorption band. Under the experimental conditions used, a stable system is reached with unchanging spectral features after 20 days. This solution is estimated to contain 1.82 x 10(-5) M (PbS)9 particles, each with a greatest dimension of <9 A.