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.