Optically bifunctional heterostructured nanocrystals

We show that an attempt to synthesize nanocrystal alloys of Cd1-xZnxSySe1-y in a single-step reaction leads naturally to a heterostructured nanocrystal system with CdSe-rich core and CdS-ZnS hybrid shell because of differences in chemical reactivities of the different constituents. These nanocrystals exhibit very high (similar to 65%) photoluminescence (PL) efficiency. The band-edge excitonic energy and the corresponding band-edge emission can be tuned over a range of the visible spectrum by changing the composition. These samples show very strong two-photon absorption cross section (sigma(2) = 1.923 x 10(-47) cm(4 center dot)s/photon or 1923 GM), establishing a very rare example of an optically bifunctional material with tunability and a very high quantum efficiency for linear optical properties, namely, PL, and strong nonlinear properties, namely, two-photon absorption.