Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

where r is the quantum dot effective radius, m* is the carrier effective mass,and e matrix and e sc are the optical frequency dielectric constants of thesurrounding matrix and the semiconductor, respectively.A. Nanoporous TiO 2 ElectrodesElectron transfer to TiO 2 nanocrystals is of particular interest due to itspotential application in photovoltaic devices. Although quantum-confinementeffects are not typically observed in these systems, we mention chargetransfer in nanocrystalline TiO 2 here because of its similarities with chargetransfer in quantum-confined nanocrystal systems. In a photovoltaic ‘‘Gra¨tzelcell’’ [36–47], a monolayer of a ruthenium-based dye is adsorbed onto thesurface of a sintered thin film of colloidal TiO 2 . Because the internal surfacearea of an f10-Am-thick TiO 2 film can be nearly 1000 times larger than itsgeometrical area, a high optical density is achieved while still maintaining theintimate contact between the adsorbed dye and the semiconductor needed forultrafast electron transfer and efficient charge separation. Following electrontransfer to the TiO 2 , the electrons hop across grain boundaries until theyreach the back electrode, whereas the holes on the adsorbed dye are scavengedby an aqueous redox couple (typically I /I 3 ), which is regenerated at theopposite contact. Total solar power conversion efficiencies of over 10% can beachieved [36] and many variations on this theme are under investigation inorder to optimize th spectral response, open-circuit voltage, and especially toeliminate the aqueous electrolyte in these cells. This has led to the study ofcharge transfer to nanocrystalline TiO 2 from a variety of ruthenium dyes [36],semiconducting conjugated polymers [48–52], and even between chemicallysynthesized quantum dots and TiO 2 particles [53,54].The key advantage of using colloidally derived TiO 2 in these cells is thatthe exceptionally high surface-to-volume ratio provided by the nanocrystallinematerial yields an enormous amount of interfacial area to supportphotoinduced charge transfer from adsorbates to the semiconductor particles.Charge transfer in TiO 2 and other semiconductor nanocrystal systemsshare significant similarities because of the importance of the surface. Forinstance, adjusting the coupling between the particle surface and an adsorbatewill affect the rate of electron transfer [55,56]. In addition, midgap surfacestates exist, and their exact role in the charge transfer process must be assessed[57,58].Beyond their surface similarities, important differences between nanocrystallineTiO 2 electrodes and other semiconductor nanocrystal systemsremain. Because of the large effective carrier masses and comparatively largeparticle sizes, typical preparations of TiO 2 nanocrystals do not exhibit thestrong quantum-confinement effects characteristic of chemically synthesizedCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.