Effects of Surface Chemistry on Nonlinear Absorption of PbS Nanocrystals

A series of lead sulfide (PbS) nanocrystals grown in oleic acid (OA), poly(1-decene) (PD), poly(1-butene) (PB), and poly(vinyl alcohol) (PVA) were synthesized. The relationship between nonlinear optical properties and surface properties of PbS nanocrystals were analyzed using 532 and 1064 nm nanosecond open-aperture Z-scan measurements and femtosecond pump-probe experiments with a 390 nm pump and a 450-700 nm probe. The results indicated a clear difference in nonlinear optical properties between the OA and PVA composites in comparison to PB and PD. The materials with strong covalent bonding between the nanocrystal surface and organic phase, PVA and OA, did not show nonlinear absorption behavior in Z-scan measurements at 532 or 1064 nm. The PB and PD composites did, however, show strong reverse saturable absorption in Z-scans recorded at these wavelengths. The transient absorption measurements further separated the PVA and OA composites from the PD and PB samples. The decays for each of the samples indicated up to three contributing absorption decay components. The third component observed in the absorption decay lead to the primary differences in the samples and was attributed to relaxation from deep trap states. The value for its lifetime was determined to be >5 ns in PD and PB. This component was visible in the OA sample, but its lifetime was <2 ns and the results for the PVA showed no traces of this component. The differences in excited state lifetime seen in the OA and PVA composites in comparison to PD and PB mirrored the differences that were seen in the Z-scan experiment.