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

Figure 18 Tunneling conductanceF spectra of an InAs core QD and two core–shellnanocrystals with two and six ML shells with nominal core radii f1.7 nm. Thespectra were offset along the V direction to center the observed zero current gaps atzero bias.an InAs QD of radius similar to the nominal core radius f1.7 nm. The generalappearance of the spectra of core and core–shell nanocrystals is similar. Thegap in the density of states around zero bias, associated with the QD bandgap,is nearly identical, as observed in the optical absorption measurements. Incontrast, the s–p level separation is substantially reduced. Both effects areconsistent with a model in which the s state is confined to the InAs core region,whereas the p level extends to the ZnSe shell. In this case, the p state is redshiftedupon increasing shell thickness, whereas the s level does not shift,yielding a closure of the CB s–p gap.Optical spectroscopy also provides evidence for the reduction of the s–pspacing upon shell growth, as manifested in PLE spectra presented in Fig. 19.The three spectra, for cores (solid line) and core–shells with 4 ML and 6 MLshell thickness (dotted and dashed lines, respectively), were measured usingthe same detection window (970 nm), corresponding to the excitonic bandgapenergy for InAs cores 1.7 nm in radius. The peak labeled III, which asdiscussed earlier, corresponds in the cores to the transition from the VB edgestate to the CB 1P e state, is red-shifted monotonically upon shell growth. Thedependence of the difference between peak III and the bandgap transition Ion shell thickness is depicted in the inset of Fig. 19 (circles), along with the1S e 1P e level spacing extracted from the tunneling spectra (squares).Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.