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

Figure 1 Experimental realization of the DBTJ using the STM (left) and an equivalentelectrical circuit (right). Linker molecules (not shown) can be used to providechemical binding of the QD to the substrate, thus enhancing the QD–substrate tunnelbarrier (R 2 ).The elegant artificial atom analogy, borne out from optical and tunnelingspectroscopy for QDs, can be tested directly by observing the shapesof the electronic wave functions. Recently, the probability density of theground and first excited states for epitaxially grown InAs QDs embedded inGaAs was probed using magnetotunneling spectroscopy with inversion of thefrequency domain data [67]. The unique sensitivity of STM to the localdensity of states can also be used to directly image electronic wave functions,as demonstrated for ‘‘quantum corrals’’ on metal surfaces [68], carbonnanotubes [69], and d-wave superconductors [70]. This technique was alsoapplied to image the quantum-confined electronic wave functions in MBEand in colloidal-grown QDs [71,72]. The information extracted from suchimaging measurements provides a detailed test for the theoretical understandingof the QD electronic structure.In this review, we concentrate on the application of tunneling andoptical spectroscopy to colloidal-grown QDs, with particular focus on ourown contributions in the study of InAs nanocrystals and InAs cores coated bya semiconducting shell (core–shells). Section II gives a general comparisonbetween tunneling and optical spectroscopy of QDs, and in Section III, thespecific application of this approach to InAs nanocrystals will be presented. InSection IV, we discuss the effects of the tunnel junction parameters on themeasured tunneling spectra. Section V focuses on the synthesis of core–shellQDs with InAs cores and presents their optical and tunneling properties. Thewave-function imaging of electronic states in QDs is discussed in Section VIand concluding remarks are given in Section VII.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.