nm-Semiconductor Particles and Molecular Aggregates as Redox Species

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

Luminescence and Configurations of Perylene Dimers in a Langmuir-Blodgett Film

The photoemission electron microscope (PEEM) makes it possible to image a surface via its work function. On a CO-covered Pt(100) surface, we prepared oxygen islands which appear dark in the PEEM image due to their higher work function. As the surface is heated to temperatures above 650 K we observe the conversion of these dark islands into very bright ones with work functions much lower than even that of the clean surface arising from an inverted dipole moment of oxygen atoms beneath the surface. We found an activation energy for this conversion of about 15 kcal/mol. Partially transformed oxygen islands were used to study the reactivity and the formation of this species further. CO and H2 both react with subsurface oxygen rather slowly, while at the interface between the subsurface and the chemisorbed phase, both adsorbents accelerate the conversion of parts of the remaining oxygen atoms towards the subsurface state. In contrast, additional oxygen adsorption does not contribute to a further transformation. We propose a qualitative microscopic model for the formation of subsurface oxygen based on experimental evidence