Electronic properties of single semiconductor nanocrystals: optical and electrostatic force microscopy measurements

We review the room temperature optical and electronic properties of single semiconductor nanocrystals, which are made by chemical synthesis. Confocal luminescence spectroscopy of single nanocrystals reveals a blinking behavior apparently due to an intermittent photoionization. To investigate this further, the dielectric constant and electrostatic charge of single CdSe nanocrystals was measured. The static dielectric constant among single CdSe nanocrystals is uniform, and its value is consistent with the value of the dielectric constant of the bulk material. However, the electrostatic polarization among individual nanocrystals is non-uniform, with a significant fraction of the nanocrystals possessing a partial positive charge (Q∼0.5 e), and/or a permanent dipole (P∼35 D). A small fraction of the nanocrystals has a positive polarization, which blinks ‘on’ and ‘off’ over time. In addition, photoexcitation with frequencies greater than the band gap of the nanocrystal results in photoionization.