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 nonlinearitics 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 similar to 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, n(2), determined to be -(3.0 +/- 0.9) x 10(-14) and -(2.9 +/- 0.6) x 10(-14) cm(2)/W for PbS and PbSe nanocrystals, respectively. After post-synthesis washing, the nanocrystals were converted to a reverse saturable absorbing and strongly scattering media with a self-focusing refractive index of (11.4 +/- 2.0) x 10(-14) and (12.7 +/- 0.9) x 10(-14) cm(2)/W for PbS and PbSe nanocrystals, respectively. The fitted experimental Z-scan data for the samples after washing gave high values for the nonlinear absorption coefficient, beta(eff), in the range of 130-150 cm/GW. The appearance of reverse saturable absorption in the nanocrystals was due to the presence of trap states at the nanocrystal surface that were not present in the as-grown nanocrystals and did not show similar optical nonlinearity.