Despite the ubiquitous importance of nanoparticles in the modern scientific and technological materials landscape, it is notoriously difficult to obtain good information about how the atomic bonding in these systems changes as a function of nanoparticle size. Such information is critical to understand properties such as catalytic activity or optical response of nanoparticles.

A team of scientists from the Billinge group, and from Prof. Jon Owen’s group at Columbia University, led by grad student Chenyang Shi, together with researchers from Brookhaven and Argonne National Laboratories, addressed this question by examining the size dependence of of lattice dynamics, which is uniquely sensitive to the bonding strength, on novel atomically precise cadmium selenide quantum dots.  This work was made possible due to two factors: (1) the development of x-ray synchrotron based milli-eV resolution inelastic X-ray scattering technologies (suitable for studying small samples of ligand coated nanoparticles), combined with (2) the development in the Owen group of synthetic methods to make macroscopic quantities of atomically identical quantum dot nanoparticles.

This study, published in Physical Review Letters, gives unprecedented insights into the bonding in these quantum dot systems by directly measuring the atomic dynamics in the clusters as a function of particle size, and comparing this to quantum mechanical density functional theory calculations on the known structures of the particles.  An interesting blue-shift in the mode frequencies as a function of decreasing particle-size was observed, which the calculations indicate is due to the inertia of the surface ligands, going beyond the chemistry of the ligand-particle interactions. This opens the door to engineering details of the bonding in nanoparticles without changing surface chemistries.

The work in the Billinge group was funded by the US Department of Energy, Office of Basic Energy Sciences and in the Owen group by US National Science Foundation.  Structural experiments were carried out at the XPD beamline at the NSLS-II, facility and DFT calculations in the Computational Science Initiative, both at Brookhaven National Laboratory, and IXS experiments at beamline 30-ID at the Advanced Photon Source at Argonne National Laboratory.