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

QD solution. For an interdot distance of 18 Å, the red shift is much smaller atabout 18 meV (Fig. 20c). When the films were redissolved into colloidalsolution, the excitonic peak positions of the colloid solution were re-established.QDs could also contact each other through their oxide layers. However,the possibility of QDs contacting through strong chemical bonds orinterdot fusion of QDs can be excluded because in that case, the solid filmcould not be redissolved to yield the same spectrum as that of the initialcolloidal solution.These results suggest that electronic coupling between QDs does takeplace and are similar to the results for close-packed solid arrays of CdSe QDsthat were prepared with 16-A˚ -diameter dots [138]. As expected, it was foundthat the strength of the electronic coupling increases with decreasing QDdiameter and decreasing interdot distance. When the interdot distance in solidQD arrays is large, the QDs maintain their individual identity and theirisolated electronic structure, and the array behaves as an insulator. Quantummechanical coupling becomes important when the potential barrier anddistance between the dots is small [142–145]. A recent theoretical study onSi QDs showed that for small interdot distances in a perfect superlattice andalso in disordered arrays, one can expect the formation of delocalized,extended states (minibands) from the discrete set of electron and/or holelevels present in the individual QDs [145]. This effect is similar to theformation of minibands in a one-dimensional superlattice of quantum wells[37]. Randomly arranged QDs in a disordered array show the coexistence ofboth discrete (localized) and bandlike (delocalized) states [145], and transitionsare possible from completely localized electron states to a mixture oflocalized and delocalized states.Long-range energy transfer between QDs in an array has also beenobserved in close-packed CdSe QD arrays [146–148] and in close-packed InPQD arrays [134]; multilayer films were optically transparent and the QDs wererandomly ordered. Energy transfer from small QDs to large QDs wasobserved. The absorption spectra of uncoupled QDs in colloidal solutionare virtually identical to those from a QD solid film formed from the solution.The peaks of the emission spectra of the close-packed films are red-shiftedwith respect to the QD solution spectra. These observations suggest thatenergy transfer occurs within the inhomogeneous distribution of the QDs inthe solid film. The observed red shift, together with a narrowing of theemission spectra, becomes more prominent for samples with a broadened sizedistribution.Energy transfer between close-packed QDs in a mixed system consistingof 20% larger dots (37 A˚ ) and 80% smaller dots (28 A˚ ) was also investigated.The absorption and emission spectra of the two QD samples with differentsizes are presented in the bottom two plots in Fig. 21 and are labeled ACopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.