Fast electron transfer across semiconductor-molecule interfaces: GaAs/Co(Cp)(2)(+/0)

The kinetics or majority electron transfer in the: dark from n-GaAs electro des to cobaltocenium (Co(Cp)(2)(+)) accepters in acetonitrile has been stud ied in detail, both experimentally and theoretically. The experimental resu lts were obtained from electrochemical impedance spectroscopy, quartz cryst al microbalance (QCM and EQCM) studies, and current-potential characteristi cs. The theoretical work involved calculating the adsorption energy and mol ecular configuration of the cobaltocenium accepters at the GaAs surface usi ng high level density functional theory (B3LYP and variations thereof) as w ell as semiempirical methods. The QCM experiments showed that both Co(Cp)(2 )(+) and Co(Cp)(2)(0) are physisorbed at GaAs surfaces, with adsorption ene rgies of about 0.2 and 0.4 eV, respectively. The theoretical results are co nsistent with these experimental results. They indicate that adsorption of the Co(Cp)(2)(+/0) redox system occurs on GaAs, with Co(Cp)(2)(0) somewhat more strongly adsorbed than Co(Cp)(2)(+); the Co(Cp)(2)(+/0) molecules were found to adsorb with the cyclopentadienyl rings parallel to the GaAs surfa ce. A model for the overall electron-transfer process was developed that in corporates Co(Cp)(2)(+) adsorption. Analysis of the detailed impedance spec tra over the range of 1 Hz to 600 kHz showed that the sequential electron-t ransfer steps in the model (i.e., electron transfer from the: GaAs conducti on band to adsorbed Co(Cp)(2)(+), followed by electron transfer from the ad sorbed Co(Cp)(2)(0) to free Co(Cp)(2)(+) in solution) are very fast and tha t the observed overall rate of electron transfer is limited by the rate of thermionic emission from the GaAs bulk region to the: surface. The implicat ions of these results for the theory of electron transfer at semiconductor- liquid interfaces, and the associated controversies surrounding theory and various experimental results for GaAs-metallocenium systems, are discussed.