Surface Charging in CdSe Quantum Dots: Infrared and Transient Absorption Spectroscopy

The surface states of selenium-rich CdSe particles are composed of occupied and unoccupied selenium p orbitals, which play a significant role in photophysical and photochemical properties of CdSe quantum dots. The empty p orbitals on the surface are involved in the "recently reported phenomenon of thermal "surface charging". Surface charging occurs when electrons are thermally promoted from the valence band to empty surface orbitals at slightly higher energies. At room temperature, this equilibrium results in some fraction of the particles having a hole in the valence band; i.e., the surface acceptor states make the particle p-type. Photo-excitation of surface charged particles results in a positive trion which can undergo a rapid Auger process, making the nanocrystal nonluminescent. The extent of surface charging can be quantified by the amplitude of the trion decay component in the transient absorption kinetics. The presence of occupied p orbitals is quantitatively characterized by an 800 cm(-1) feature in the infrared spectra, which is assigned to a Se-O stretch involving an oxidized surface selenium. We show that, because the extents of surface charging and surface selenium-oxygen bonds are both indicative of the density of unligated surface seleniums, the intensity of the 800 cm(-1) IR feature correlates with the extent of surface charging.