  Material properties depend sensitively on the atomic arrangements and atomic
bonding, but these are notoriously difficult to measure in nanosized atomic
clusters due to the small size of the objects and the challenge of obtaining
bulk samples of identical clusters. Here we have combined the recent ability to
make gram quantities of identical semiconductor quantum-dot nanoparticles with
the ability to measure lattice dynamics on small sample quantities of
hydrogenated materials using high energy resolution inelastic x-ray scattering
(HERIX), to measure the size-dependence of the phonon density of states (PDOS)
in CdSe quantum dots. The fact that we have atomically precise structural
models for these nanoparticles allows the calculation of the PDOS using Density
Functional Theory (DFT), providing both experimental and theoretical
confirmations of the important role that the inertia of the surface capping
species plays on determining the lattice dynamics.
