We present two different novel approaches to the synthesis of colloidal PbS NCs. Firstly an aqueous synthesis was developed to prepare 3nm PbS NCs overcoated with the higher energy band gap semiconductor, CdS. A further fluorescence activation step was also performed whereby Cd(OH)2 was used to overcoat the CdS layer around the PbS nanocrystal. The final overcoating procedure produced strong visible photoluminescence with two emission peaks, one from the band edge of the PbS nanocrystals and the other with energy higher than the PbS nanocrystal band edge. The total emission was tunable from red to orange to white, by varying the amount of CdS overcoating.
Two different post synthesis chemical treatments (hexane treatment and centrifuging) were used to isolate the two different emission peaks in the photoluminescence spectrum. We found two distinct fluorescent species were present in the synthesized solutions; one containing the PbS nanocrystal band edge emission and the other contained the emission with energy higher than the PbS nanocrystal band edge. Polarization and time-resolved photoluminescence spectroscopy was also used to indicate the presence of two distinctly different fluorescent species. Photoluminescence excitation spectroscopy showed both fluorescent species behaved like PbS nanocrystals and not CdS nanocrystals.
Secondly, a temperature dependent size tunable synthesis for PbS NCs (2- 4nm) in an organic solvent was developed. These PbS NCs were capped with oleic acid and produced strong near-IR photoluminescence from the PbS nanocrystal band edge with high quantum yields (Q.Y. = 70%). The photoluminescence lifetime of these PbS nanocrystals was extraordinarily long (1µs) and mono-exponential.
The oleic acid capped PbS NCs were blended with a conjugated polymer to alter its optical and electronic properties. These novel nanocrystal:polymer composites were examined with and without the nanocrystal surface capping ligand to explore the dynamics of the energy transfer processes between the conjugated polymer and the PbS nanocrystals. Quantitative steady-state photoluminescence spectroscopy showed a quenching of the photoluminescence in the conjugated polymer due to the PbS nanocrystals. Time-resolved photoluminescence spectroscopy showed a decrease in the excited state lifetimes of the conjugated polymer due to the presence of PbS nanocrystals in the composite materials when compared to pure conjugated polymer. We demonstrated exciton transfer from the conjugated polymer to the PbS nanocrystals when a nanocrystal surface capping ligand was present and charge transfer between the conjugated polymer and the PbS nanocrystals when the nanocrystal surface capping ligand was absent.
Finally we demonstrated the ability to obtain photoluminescence images and spectra from single Rhodamine B dye molecules and single CdSe nanocrystals using a custom built spectroscopic epi-fluorescent microscope. Single PbS nanocrystals were also possibly imaged, but conclusive evidence from the photoluminescence spectrum could not be obtained due to the extraordinarily long lifetime of the PbS nanocrystals.