White-light emission from ultrasmall cadmium selenide nanocrystals studied by ultrafast fluorescence upconversion spectroscopy

Cadmium Selenide nanocrystal synthetic strategies have been adapted to produce CdSe nanocrystals from less than 17 A to greater than 100 A with cadmium oxide (CdO) as the cadmium source for the reaction. Ultrasmall CdSe nanocrystals that emit white light have been synthesized and subjected to a myriad of structural and optical characterization techniques. Atomic number contrast scanning transmission electron microscopy (ZSTEM) indicated that these ultrasmall nanocrystals are crystalline, consist of approximately four lattice planes and likely have defects resulting from incomplete lattice structures. Single molecule fluorescence spectroscopy indicated that the broad emission is an intrinsic property of the dots rather than an ensemble behavior resulting from a mixture of nanocrystal sizes or morphologies. The broad emission likely results from multiple trap states for both the electron and hole. A first attempt at assigning these states has been made through use of ultrafast fluorescence upconversion spectroscopy.