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

electron and hole states: nQ F , where n is the main quantum number, andQ=S, P, D, . . . denotes the lowest L in the evelope wave function. The rightpanel shows the calculated relative oscillator strength for the optically activetransitions, usually between electron and hole states with the same Q. Thecalculated level separations closely reproduce the observed strong transitions.With respect to the comparison between tunneling and PLE spectra, particularfocus will be given to the first three strong transitions: the bandgaptransition 1S 3/2 (h)1S 1/2 (e), and excited transitions E 3 and E 5 . The first wasidentified as the 2S 3/2 (h)1S 1/2 (e) transition, and the second follows the1P 3/2 (h)1P e transition [in fact, 1P e is split into 1P 1/2 (e) and 1P 3/2 (e), whichare nearly degenerate].Although the agreement between the PLE data and the theory isrelatively good, it relies on the comparison of differences between transitions.A further independent probe of the level structure and symmetry may behighly beneficial, in particular to provide separate information on the CB andVB states. Such information is obtained by scanning tunneling microscopyand spectroscopy.B. Scanning Tunneling SpectroscopyFor the tunneling measurements we link the nanocrystals to a gold film viahexane dithiol (DT) molecules [25,75], enabling the realization of a DBTJ.Figure 8a (left inset) shows a scanning tunneling microscope (STM) topographicimage of an isolated InAs QD, 32 A˚ in radius. Also shown in Fig. 8a isa tunneling current–voltage (I–V) curve that was acquired after positioningthe STM tip above the QD and disabling the scanning and feedback controls(Fig. 8a, right inset). A region of suppressed tunneling current is observedaround zero bias, followed by a series of steps at both negative and positivebias. In Fig. 7b, we present the dI/dV versus V tunneling conductance spectrum,which is proportional to the tunneling DOS. A series of discrete singleelectrontunneling peaks is clearly observed, where the separations aredetermined by both the single-electron charging energy (addition spectrum)and the discrete level spacings (excitation spectrum) of the QD. The I–Vcharacteristics were acquired with the tip retracted from the QD to a distancewhere the bias predominantly drops on the tip–QD junction. Under theseconditions, as discussed in Section II, CB (VB) states appear at positive(negative) sample bias, for which electrons tunnel from the tip to the QD, andthe excitation peak separations are nearly equal to the real QD level spacings[60]. Similarly, at negative sample bias, the VB states can be resolved, aselectrons tunnel from the dot to the tip.On the positive bias side of Fig. 8b, two closely spaced peaks are observedright after current onset, followed by a larger spacing and a group of sixCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.