Excess Charge in Small Semiconductor Particles

Abstract

We address the question of the effect of excess charge on the spectroscopy of small semiconductor particles. A variety of reducing species was used to inject charge into small CdS particles. A blue-shift or broadening of the spectrum is often observed at the low energy edge of the absorption. The effect of the excess charges on the absorption spectra of the particles is independent of the source of the electrons or the pH of the solution. The spectral shifts are essentially independent of the loading of electrons into the particle. Time resolved conductivity measurements show that no protons are released to the bulk of the solution upon injection of electrons from H atoms below the point of zero charge. We infer that a dipole is created across the particle in that case.

To demonstrate the effect of excess charge at the particle surface we added electrons to capping molecules attached to the surface. The surface of the particles was modified to include various electron acceptors of known redox potentials. Injection of electrons into these acceptors at the surface generates the radical anion of the corresponding molecule and its absorption spectrum could easily be monitored. However, the same spectral effects on the absorption edge of the particles could be observed in this case as in the direct injection of the electrons into the particle. It is concluded that the effects result from the electric field associated with the excess charge.