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

QDs, the nature of the QD capping species, QD orientation and packingorder, uniformity of QD size distribution, and the crystallinity and perfectionof the individual QDs in the array. Several studies of electronic coupling incolloidal QD arrays have been reported [37,134–138]. In these arrays, thesemiconductor QD cores are surrounded with insulating organic ligands andcreate a large potential barrier between the QDs. Thus, the electrons andholes are confined to the QD, and very weak electronic communication existsbetween dots in such arrays. Measurements of the photoconductivity ofclose-packed films of colloidally prepared CdSe QDs [137] with diameters>20 A˚ have shown that excitons formed by illumination are confined toindividual QDs and electron transport through the array does not occur.This lack of electronic coupling between QDs is also seen from the fact thatthe absorption spectra are the same for both colloidal solutions and closepackedarrays.However, arrays with very small CdSe QDs with a mean diameter of 16A˚ show that significant electronic coupling between dots in close-packedsolids can occur [138]. Recently, InP QDs with diameters of 15–23 A˚ wereformed into arrays that also show evidence of electronic coupling [139]. Thisconclusion is based on the differences in the optical spectra of isolated colloidalQDs compared to solid films of QD arrays (see Fig. 20). For closepackedQD solids, a large red shift of the excitonic peaks in the absorptionspectrum is expected if the electron or hole wave function extends outside theboundary of the individual QDs as a result of inter-QD electronic coupling.The first indication of a difference in the absorption spectra betweencolloidal QD solutions and QD films appears for 32-A˚ InP QDs with an inter-QD spacing of 11 A˚ (QDs were capped with TOPO); a very small shift ofabout 15 meV was apparent (see Fig. 20a). Above 32 A˚ , no difference wasobserved between the spectra of QD colloidal solutions and solid films.Figures 20b and 20c show results for 18-A˚ -diameter InP QDs, where theinterdot distance was varied between 9 and 18 A˚ by varying the length of alinear alkylamine molecule that was used as the colloidal stabilizer. Thesecolloids were initially prepared in trioctylamine, which was then replaced withhexylamine (interdot distance 9 A˚ ) or oleyamine (interdot distance f18) bya ligand-exchange method. The interdot distance for the different organicligands was estimated from previous published data for close-packed arraysof nanocrystals capped with different organic ligands [140,141]. However,because hexylamine is a weak stabilizer with a relatively low boiling point,QDs may partially lose the organic ligand so that the interdot distance may beshorter.Quantum-dot arrays with an interdot spacing of 9 A˚ have an absorptionspectrum that is relatively smooth with only weak structure (Fig. 20b) andwhich shows a red shift for the first exciton of about 140 meV relative to theCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.