Effects of surface chemistry and shape on nonlinear absorption, scattering, and refraction of PbSe nanocrystals

Oleic acid capped lead sulfide (PbSe) cubes, spheres, rods and star-shaped nanocrystals with sizes ranging from 5 to 20 nm were synthesized, then subjected to a post-synthesis washing in a 1: 1 ethanol/hexane solution. The relationship of third order nonlinear optical properties to nanocrystal surface chemistry as affected by this washing was analyzed using nanosecond 532 nm Z-scan. The results indicated a significant change in optical properties emerged only after the nanocrystals, regardless of shape, were washed in the ethanol/hexane mixture. The "as grown" nanocrystals displayed high emission efficiency with no nonlinear absorption, which converted to a highly-absorbing and strongly scattering media with quenching of optical emission following washing. Transmission electron microscopy showed that neither the size nor shape of the nanocrystals was changed by washing, indicating that all differences were related to changes in surface chemistry. Finally, the Z-scan of washed nanocrystals displayed a strong correlation between nonlinear scattering and nonlinear transmittance which was also not dependent upon shape. It is postulated that the washing allowed formation of deep trap states that permitted nonlinear optical absorption.