Photophysical and Redox Properties of Molecule-like CdSe Nanoclusters

Advancing our understanding of the photophysical and electrochemical properties of semiconductor nanoclusters with a molecule-like HOMO–LUMO energy level will help lead to their application in photovoltaic devices and photocatalysts. Here we describe an approach to the synthesis and isolation of molecule-like CdSe nanoclusters, which displayed sharp transitions at 347 nm (3.57 eV) and 362 nm (3.43 eV) in the optical spectrum with a lower energy band extinction coefficient of ∼121 000 M–1 cm–1. Mass spectrometry showed a single nanocluster molecular weight of 8502. From this mass and various spectroscopic analyses, the nanoclusters are determined to be of the single molecular composition Cd34Se20(SPh)28, which is a new nonstiochiometric nanocluster. Their reversible electrochemical band gap determined in Bu4NPF6/CH3CN was found to be 4.0 V. There was a 0.57 eV Coulombic interaction energy of the electron–hole pair involved. The scan rate dependent electrochemistry suggested diffusion-limited transport of nanoclusters to the electrode. The nanocluster diffusion coefficient (D = 5.4 × 10 –4 cm2/s) in acetonitrile solution was determined from cyclic voltammetry, which suggested Cd34Se20(SPh)28 acts as a multielectron donor or acceptor. We also present a working model of the energy level structure of the newly discovered nanocluster based on its photophysical and redox properties.