Matrix-Embedding Effects on Nanodiamond Phonons.

A complete picture of the unique lattice dynamics of nanocrystals is slowly being painted with contributions from a variety of techniques. How these unique dynamics change when the nanocrystals are embedded in a matrix, however, is not well explored. We systematically compare the phonon spectra of diamond nanocrystals before and after embedding in a tin telluride matrix. Time-of-flight neutron spectroscopy captures phonons from elementally light nanocrystals dispersed in a heavy matrix across ∼0.5-250 meV, a challenge for other techniques. Classical molecular dynamics simulations aid interpretation. Upon embedding, surface phonons are quenched, core phonons soften, and line widths narrow due to new boundary conditions and tensile strain imposed by the matrix. Temperature-dependent measurements reveal suppressed anharmonic surface dynamics, with subtle differences between nanodiamonds in agglomerates vs isolated grains. These results are significant for understanding the vibrational and thermodynamic properties of a range of nanocrystal composites such as thermoelectric nanocomposites.