Insights about the Surface of Colloidal Nanoclusters from Their Vibrational and Thermodynamic Properties

We use first principles density functional theory to
study the
vibrational properties of colloidal quantum dots with up to 1000 atoms.
This represents a size regime where the vibrations are molecular but
are reminiscent of bulk phonons. We describe how these vibration types
coexist and interact. Some counterintuitive results, such as blue
shifts with increasing passivant mass, are highlighted. We compare
the vibrational properties of zinc-blende and wurtzite CdSe nanoclusters,
and find significant differences for the “optical” modes
that we attribute to the intrinsic anisotropy of the wurtzite structure.
We suggest studying the low temperature specific heat to unveil the
otherwise hard to access surface properties.