Atomic Structures and Gram Scale Synthesis of Three Tetrahedral Quantum Dots

Luminescent semiconducting quantum
dots (QDs) are central to emerging
technologies that range from tissue imaging to solid-state lighting.
However, existing samples are heterogeneous, which has prevented atomic-resolution
determination of their structures and obscured the relationship between
their atomic and electronic structures. Here we report the synthesis,
isolation, and structural characterization of three cadmium selenide
QDs with uniform compositions (Cd35Se20(X)30(L)30, Cd56Se35(X)42(L)42, Cd84Se56(X)56(L)56; X = O2CPh, L = H2N-C4H9). Their UV-absorption spectra show a lowest energy electronic
transition that decreases in energy (3.54 eV, 3.26 eV, 3.04 eV) and
sharpens as the size of the QD increases (fwhm = 207 meV, 145 meV,
115 meV). The photoluminescence spectra of all three QDs are broad
with large Stokes shifts characteristic of trap-luminescence. Using
a combination of single-crystal X-ray diffraction and atomic pair
distribution function analysis, we determine the structures of their
inorganic cores, revealing a series of pyramidal nanostuctures with
cadmium terminated {111} facets. Theoretical and experimental studies
on these materials will open the door to a deeper fundamental understanding
of structure–property relationships in quantum-confined semiconductors.