Synthesis of PbS/CdS Core−Shell QDs and their Nonlinear Optical Properties

We synthesized different sizes of PbS/CdS core−shell quantum dots (CSQDs) by using a cation-exchange method and studied the changes in their absorption and luminescence. Blue shifts in the absorption and photoluminescence (PL) peaks were observed with varying PL quantum yields. The PbS/CdS CSQDs dispersed in hexane or embedded in polystyrene films showed higher free-carrier absorption (FCA) as compared to pure PbS QDs. Saturable absorption (SA) was found to be significant in the smallest nanoparticles in both hexane and polymer films. A size-dependent FCA coefficient was observable, and smaller PbS/CdS nanoparticles were found to exhibit greater FCA. Free surface ligands were found to contribute to an enhancement in the nonlinear scattering above a critical irradiance threshold. We found that the FCA cross-section and the nonlinear refractive index are independent of the thickness of PbS/CdS-embedded polymer within experimental error.