On the kinetics and thermodynamics of excitons at the surface of semiconductor nanocrystals: Are there surface excitons?

- The surface of semiconductor nanocrystals is one of their defining features.
- The kinetics of surface trapping can be monitored by pump/probe spectroscopy.
- The thermodynamics of surface trapping is revealed by photoluminescence spectroscopy.
- We produce the first microscopic picture of how excitons are coupled to the surface.
- We discuss the possibility of surface excitons in nanocrystals.

The surface of semiconductor nanocrystals is one of their defining features by virtue of their nanometer size. Yet the surface is presently among the most poorly understood aspects of nanocrystal science. This perspective provides an overview of spectroscopic work that has revealed the first insights into the nature of the surface, focusing upon CdSe nanocrystals. We focus on two aspects of surface processes in nanocrystals: the kinetics of surface trapping and the thermodynamics of core/surface equilibria. We describe femtosecond pump/probe spectroscopic experiments which reveal the signatures of carrier trapping at the surface. We also describe temperature dependent steady-state photoluminescence experiments which reveal new aspects of the surface. This work suggest that the surface emission is largely driven by homogeneous broadening via phonon progressions. The implications are that the surface electronic state bears similarity to the quantized excitonic core of the nanocrystal.