  The size-dependent structure of CdSe nanoparticles, with diameters ranging
from 2 to 4 nm, has been studied using the atomic pair distribution function
(PDF) method. The core structure of the measured CdSe nanoparticles can be
described in terms of the wurtzite atomic structure with extensive stacking
faults. The density of faults in the nanoparticles ~50% . The diameter of the
core region was extracted directly from the PDF data and is in good agreement
with the diameter obtained from standard characterization methods suggesting
that there is little surface amorphous region. A compressive strain was
measured in the Cd-Se bond length that increases with decreasing particle size
being 0.5% with respect to bulk CdSe for the 2 nm diameter particles. This
study demonstrates the size-dependent quantitative structural information that
can be obtained even from very small nanoparticles using the PDF approach.
