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

data in the inset of Fig. 19 represent the energy difference between two VB toCB optical transitions. Therefore, evolution of the complex quantum-dot VBedge states upon shell growth will inevitably affect the PLE spectra. Inparticular, a blue shift of the p-like component of the VB edge state uponshell growth will reduce the net observed PLE shift compared to the tunnelingdata, consistent with the experimental observations. This possibility ofdegeneracy lifting between the VB edge states of the core–shells gains supportfrom further comparing their tunneling spectra with that of the core (Fig. 18).In the negative bias side, associated with the tunneling through VB states,additional peaks appear for the core–shell particles.VI.QD WAVE–FUNCTION IMAGINGThe elegant artificial atom analogy for QDs, borne out from optical andtunneling spectroscopy, can be tested directly by observing the shapes of theQD electronic wave functions. Recently, probability densities of the CBground and first excited states for epitaxially grown InAs QDs embedded inGaAs were directly probed using cross-sectional scanning tunneling microscopy[71]. To access the embedded QD with the tip, the sample was cleaved ina plane perpendicular to the growth direction modifying the strain fieldcompared with that of the original embedded dots. Magnetotunneling spectroscopywith inversion of k-space data was also used to probe the spatialprofiles of states of such QDs [67]. This noninvasive probe revealed theelliptical symmetry of the ground state in an embedded QD.For colloidal free-standing nanocrystal QDs, the unique sensitivityof the STM to the electronic density of states on the nanometer scale seemsto provide an ideal probe of the wave functions. A demonstration of thiscapability is given by recent work on the InAs/ZnSe core–shells discussedearlier. Here, the different extent of the CB s and p states, implied by thespectroscopic results, can be directly probed by using the STM to image theQD atomiclike wave functions. To this end, bias-dependent current imagingmeasurement [74] were performed, as shown in Fig. 20 for a core–shellnanocrystal with 6 ML shell. The dI/dV–V spectrum is shown in Fig. 20a,and the bias values for tunneling to the s and p states are indicated. Atopographic image was measured at a bias value above the s and p states,V B = 2.1 V (Fig. 20b), simultaneously with three current images. At eachpoint along the topography scan, the STM feedback circuit was disconnectedmomentarily and the current was measured at three different V B values: 0.9 V,corresponding to the CB s state (Fig. 20c), 1.4 V, within the p multiplet (Fig.20d), and 1.9 V, above the p multiplet (Fig. 20e). With this measurementCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.