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.
We have studied phosphor doping of core / shell nanocrystals (NCs) where the impurity emitter resides in the shell.
We have found that a two step synthesis can be used to create these non-toxic materials that efficiently transfer energy
from the core to the doped shell. These core / shell NCs retain ample brightness when solubilized in water. We
explored the functionalization of these materials in water as well to create ratiometric chemical sensing agents. First,
we used a method of controlled polymerization to synthesize amphiphilic polymers to solubilize the intrinsically
hydrophobic NCs into water. The polymer has a build in "chemical handle" which we use to functionalize the polymers
closely bound to the NC with a fluorescent dye in aqueous solution. We have found that there exists efficient Förster
resonant energy transfer from the shell doped phosphors to the surface bound dyes. Conjugation of the NC to an
environmentally sensitive dye such as fluorescein has also demonstrated that non-toxic doped NCs can be used to
develop ratiometric sensing / biological imaging agents. Last, we have found that the same technique can be applied to
functionalize non-emissive magnetic nanocrystals as well.