In recent years, energy transfer ͑ET͒ using semiconductor quantum dots ͑QDs͒ is getting increased attention. However, it has been postulated that ET between QDs is based on the Förster model, which is a wellestablished model of ET mechanism in organic dye systems, without verification. In this work, we have investigated ET mechanism in colloidal CdS QDs measuring photoluminescence dynamics of a bilayer structure consisting of differently sized CdS QDs. In the bilayer structure, the distance between the monolayer of donor QDs and that of acceptor QDs was controlled precisely by a spacer layer that is layer-by-layer assembly of polyelectrolytes. The bilayer structure enabled us to systematically measure the spacer-layer dependence of photoluminescence dynamics reflecting the ET process between QDs. It is demonstrated that ET between the donor and acceptor QDs is conclusively dominated by the dipole-dipole interaction, which verifies the appropriateness of the Förster model.Since the first report of quantum size effects in semiconductor doped glasses 1 and also in colloidal solutions 2 in the early 1980s, semiconductor quantum dots ͑QDs͒ have attracted considerable attention. So far, many studies have been conducted from a scientific viewpoint to understand the intrinsic nature of physical/chemical properties of QDs, as well as from interest in the application to new functional materials. 3-6 A turning point in QD studies was the development of monodispersed colloidal QDs having high photoluminescence ͑PL͒ yield with use of rapid injection of organometallic precursors into hot coordinating solvents ͑hot-injection method͒. 7,8 The breakthrough in synthesizing the new class of colloidal QDs have led to an explosive increase in QD studies and opened up possibilities for various applications such as biomolecular imaging, 9 QD lasing, 6,10 and QD solar cells. 11,12 Randomly dispersed QDs have been a major target in most of the studies so far. The dynamical process of resonant ET between CdSe QDs was reported in recent years. 13,14 This opened up a new aspect in photophysics of semiconductor QDs and stimulated studies on QD-based energy transfer ͑ET͒ processes employing QDs as energy donors in QDbioconjugate systems 9 and QD-organic dye systems, 15,16 as well as ET between QDs. 17,18 All of the studies, however, have postulated that ET employing QDs is based on the Förster model 19 that is a typical ET mechanism between organic molecules. It should be noted that the appropriateness of the Förster model for explanation of ET in QDs has not been verified until now. For the experimental clarification of the ET mechanism, it is essential to measure PL dynamics in a well-designed sample structure in which the distance between QDs was precisely controlled.How can we control the distance between QDs? We have focused on a layer-by-layer ͑LBL͒ assembly. 20-22 LBL is a simple and powerful technique allowing the realization of a multilayer structure that is controlled at a molecular level. This technique is based on the sequential a...

In the published version of our paper, the expression for k ET is incorrect ͑the left hand column of page 4͒. This equation should bewhere PVA and bilayer are the decay time of the bright exciton state in the PVA film sample and that in the bilayer structure, respectively.It is noted that they are just typographical errors and that the calculations in the paper are based on the correct equation.

Thin films of colloidal CdS and ZnS-CdS alloy quantum dots (QDs) were fabricated by layer-by-layer (LBL) self-assembly to synthesize blue luminescent QD thin films. The assembly of negatively charged colloidal QDs and positively charged poly(diallyldimethylammonium chloride) results in QD/polymer multilayers. The relative intensity of the band-edge photoluminescence (PL) to that of the defect-related PL was decreased during the LBL assembly process. With an additional dipping treatment in a Cd(ClO 4 ) 2 aqueous solution with pH 10, the band-edge PL intensity was recovered. The PL decay profiles before and after the dipping treatment were considerably different. From this fact, we conclude that the enhancement of the band-edge PL intensity originates from the reduction of nonradiative recombination processes at the QD surface.

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