Near-Infrared Absorption of Monodisperse Water-Soluble PbS Colloidal Nanocrystal Clusters

Highly monodisperse PbS colloidal nanocrystal clusters with well controllable size and size distribution, high crystallinity and high water solubility have been successfully synthesized through a modified polyol process. Thiourea stock solution was rapidly injected into diethyene glycol solution containing lead precursor at an elevated temperature to produce PbS clusters. The high reaction temperature allows for control over size and size distrubution and yields highly crystalline products. The superior water solubility is achieved by using poly(acrylic acid) as the capping agent. The caboxylate groups of which partially bind to the nanocrystal surface and partially extend into the surrounding water. The uncoordinated caboxylate groups also bring high density of charges to the surface of CNCs at the same time. Moreover, the CNCs exhibit visible or near-infrared absorption even though the overall sizes of particles are larger than the excitation Bohr radius. This is due to the fact that the clusters are composed of primary nanocrystals and the whole cluster shows optical property similar to the tiny primary nanocrystals.