posted on 2015-02-10, 00:00authored bySukanta Dolai, Poulami Dutta, Barry
B. Muhoberac, Charles D. Irving, Rajesh Sardar
We
have designed a new nonphosphinated reaction pathway, which
includes synthesis of a new, highly reactive Se-bridged organic species
(chalcogenide precursor), to produce bright white light-emitting ultrasmall
CdSe nanocrystals of high quality under mild reaction conditions.
The detailed characterization of structural properties of the selenium
precursor through combined 77Se NMR and laser desorption
ionization–mass spectrometry (LDI-MS) provided valuable insights
into Se release and delineated the nanocrystal formation mechanism
at the molecular level. The 1H NMR study showed that the
rate of disappearance of Se precursor maintained a single-exponential
decay with a rate constant of 2.3 × 10–4 s–1 at room temperature. Furthermore, the combination
of LDI-MS and optical spectroscopy was used for the first time to
deconvolute the formation mechanism of our bright white light-emitting
nanocrystals, which demonstrated initial formation of a smaller key
nanocrystal intermediate (CdSe)19. Application of thermal
driving force for destabilization resulted in (CdSe)n nanocrystal generation with n = 29–36
through continuous dissolution and addition of monomer onto existing
nanocrystals while maintaining a living-polymerization type growth mode. Importantly, our ultrasmall CdSe nanocrystals
displayed an unprecedentedly large fluorescence quantum yield of ∼27%
for this size regime (<2.0 nm diameter). These mixed oleylamine
and cadmium benzoate ligand-coated CdSe nanocrystals showed a fluorescence
lifetime of ∼90 ns, a significantly large value for such small
nanocrystals, which was due to delocalization of the exciton wave
function into the ligand monolayer. We believe our findings will be
relevant to formation of other metal chalcogenide nanocrystals through
expansion of the understanding and manipulation of surface ligand
chemistry, which together will allow the preparation of “artificial solids” with high charge conductivity
and mobility for advanced solid-state device applications.