Electrochemistry and electrogenerated chemiluminescence of semiconductor nanoparticles
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Electrochemical band gaps from semiconductor nanoparticles (NPs) such as CdSe and CdTe showed a size-confinement effect, and the values were in good agreement with the optical band gaps. For example, differential pulse voltammetry of 2.8 nm and 3.5 nm CdSe NPs showed electrochemical band gaps of 2.41 and 2.17 eV from the peak-to-peak separations. A large anodic wave from both CdSe and CdTe NPs was explained by the electrochemical oxidation of the particle surface of trioctylphosphine (TOP)-Se and TOP-Te species. Electrogenerated chemiluminescence (ECL)-potential curves also showed unique electronic properties of NPs. In general, ECL of NPs was sensitive to surface energy states. In the case of CdSe NPs, ECL onsets at positive potentials vii were suggested to result from electron transfer at hole traps in the NPs. The 2.0, 3.0, and 4.8 nm CdSe NPs showed size-dependent ECL behavior. The smallest particle, 2.0 nm, showed that most of the ECL signal came from the surface states, and surface state-ECL was observed at a wavelength that was red-shifted 200 nm from the band edge position. The ratio of surface state to band edge-ECL intensity was lower for larger particles and for densely-packed NP films. The surface stateECL was negligible at red-emitting CdSe and CdTe NPs due to changes in the luminescent surface states during the growth of the particles. Red-emitting Si NPs also showed ECL in good agreement with the photoluminescence (PL). CdSe/ZnSe core/shell NPs and oxygen-treated CdSe NPs showed a similar enhancement of PL intensity through the surface treatment. However, in the ECL measurements, oxygen treatment of the NPs decreased the surface state-ECL intensity with almost no change in the band edge-ECL, whereas the CdSe/ZnSe core/shell system increased the intensity of band edge-ECL. The ZnSe shell, with its wider band gap, can facilitate electron transfer from the surface energy states to the core of particle, which can not be expected in the oxygen-treated particle. ECL studies of the surface modified NPs demonstrate that ECL is an effective way to investigate the surface states of NPs.