nm-Semiconductor Particles and Molecular Aggregates as Redox Species

Abstract

It is well established that the electronic levels in semiconductors shift when the latter are prepared in the form of nm-size particles [1]. Correspondingly the optical behavior of such small particles depends on their size and shape. One can expect that the redox behavior will also change with the size of nm-particles. Knowledge in the latter field is still in an infant state. In this paper experiments will be discussed that shed some light on the redox behavior of nm-semiconductor particles, firstly as donors in the excited electronic state and secondly as electron acceptors. Transport of excess charge carriers through a chain of nm- size semiconductor particles will be briefly discussed since this latter process is important for some of the suggested device applications for nm- particles. Presently available experimental results concerning electron transfer with semiconducter nm-particles suffer from the fact that the detailed structure and chemical nature of the interface is not really known for these systems. The experimental systems are complicated and the measurements can suffer from systematic faults. Experimental results obtained at closely related systems can serve as guidelines, e.g. electron transfer measurements on covalently linked molecular donor-spacer-acceptor systems and on quantum well-barrier-quantum well systems. Progress in this field appears highly desirable, since very interesting practical applications have been suggested for systems prepared from nm-size semiconductor particles involving electron transfer reactions and electron transport. Some of these systems have shown promising features. One prominent example is an electrode prepared from nm-size anatase TiO₂ colloidal particles. This electrode is prepared with nm-particles that retain to a large degree their individual properties but are glued together such that they can facilitate efficient transport of excess charge carriers.