Size-Dependent Lattice Dynamics of Atomically Precise Cadmium Selenide Quantum Dots.

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, to measure the size dependence of the phonon density of states in CdSe quantum dots. The fact that we have atomically precise structural models for these nanoparticles allows the calculation of the phonon density of states using density functional theory, providing both experimental and theoretical confirmations of the important role that the inertia of the surface capping species plays on determining the lattice dynamics.