  Small-angle neutron and X-ray scattering, neutron backscattering and neutron
time-of-flight spectroscopy are applied to reveal the structure of the ligand
shell, the temperature-dependent diffusion properties and the phonon spectrum
of PbS nanocrystals functionalized with oleic acid in deuterated hexane. The
nanocrystals decorated with oleic acid as well as the desorbed ligand molecules
exhibit simple Brownian diffusion with a Stokes-Einstein temperature-dependence
and inhibited freezing. Ligand molecules desorbed from the surface show strong
spatial confinement. The phonon spectrum of oleic acid adsorbed to the
nanocrystal surface exhibits hybrid modes with a predominant Pb-character.
Low-energy surface modes of the NCs are prominent and indicate a large
mechanical softness in solution. This work provides comprehensive insights into
the ligand-particle interaction of colloidal nanocrystals in solution and
highlights its effect on the diffusion and vibrational properties as well as
their mechanical softness.
