Effects of Surface Chemistry on Nonlinear Absorption, Scattering, and Refraction of PbSe and PbS Nanocrystals

Oleic acid capped lead sulfide (PbS) and lead selenide (PbSe) were synthesized, then subjected to a postsynthesis washing in a 1:1 ethanol/hexane solution. The relationship of third order nonlinear optical properties to nanocrystal surface chemistry as affected by washing was analyzed using nanosecond 532 nm Z-scan and measurement of near-IR radiative emission. The results indicated a significant change in optical nonlinearities which emerged only after the nanocrystals were washed in the ethanol/hexane mixture. Transmission electron microscopy of the oleic acid capped PbSe and PbS nanocrystals showed a cubic shape, narrow size distribution and an average size of ∼10 nm. Neither size nor shape of the nanocrystals were modified by the washing process, indicating that all optical differences were related to changes in surface chemistry and the formation of deep trap states. The “as grown” nanocrystals showed high emission efficiency, weak saturable absorption, and a self-defocusing refractive index, n2, det...