Extending Semiconductor Nanocrystals from the Quantum Dot Regime to the Molecular Cluster Regime

The size-dependent optical and electronic properties of semiconductor nanocrystal (NC) have been exploited over decades for various applications. This size dependence involves a transition from the regime of bulk colloids of ∼100 nm radius to quantum dots (QDs) of ∼10 nm radius, the details of which are material specific. To understand the transition from the QD regime (∼10 nm) to the molecular cluster regime (∼1 nm) of nanocrystals, we have carefully synthesized a set of CdSe nanocrystals with sizes ranging from 0.89 to 1.66 nm in radius. As the nanocrystals become small, the surface emission strongly increases in amplitude, and the core emission broadens and red-shifts. These effects are rationalized in terms of coupling to ligands via electron transfer theory. The core emission spectra arise from increased vibrational coupling of ligands for very small NC. The surface emission amplitudes arise from a size-dependent surface free energy. The transition from the QD to the molecular cluster regime is found to be at 1.2 nm radius, in contrast to the transition from the bulk to QD transition at the Bohr radius of 5.4 nm in CdSe. These size-dependent surface electronic phenomena may be used for light emission applications.