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

II.GENERAL COMPARISON BETWEEN TUNNELINGAND OPTICAL SPECTROSCOPY OF QDsTunneling and optical spectroscopy are two complementary methods for thestudy of the electronic properties of semiconductor QDs. In photoluminescenceexcitation (PLE) spectroscopy, a method that has been widely used toprobe the electronic states of QDs, one monitors allowed transitions betweenthe VB and CB states [44,47]. Size selection is achieved by opening a narrowdetection window on the blue side of the inhomogeneously broadened PLpeak. Pending a suitable assignment of the transitions, the intraband levelseparations can be extracted from spacings between the PLE peaks. Intunneling spectroscopy, on the other hand, it is possible to separately probethe CB and VB states, and, practically, there are no selection rules [9]. Here,one measures the dI/dV versus V characteristics of single QDs that yield directinformation on the tunneling density of states (DOS). For a discrete QD levelstructure, the spectra exhibit a sequence of peaks corresponding to resonanttunneling through the states.Seemingly, it should be possible to directly compare the PLE andtunneling spectra. However, in tunneling spectroscopy, the QD is chargedand, therefore, the level structure may be perturbed compared to the neutraldot monitored in PLE. Furthermore, even if charging does not intrinsicallyperturb the level structure significantly, the peak spacing and peak structure inthe tunneling experiment depend extrinsically on the DBTJ parameters. Wenow present a simple theoretical framework for the interpretation of thedependence of the tunneling spectra on the junction parameters starting witha qualitative explanation.As shown in Fig. 1, a DBTJ is realized by positioning the STM tip overthe QD. The QD is characterized by a discrete level spectrum with degeneraciesreflecting the symmetry of the system. The DBTJ is characterized by acapacitance and a tunneling resistance for each junction. The capacitance andtunneling resistance (inversely proportional to the tunneling rate) of the tip–QD junction (C 1 and R 1 ) can be easily modified by changing the tip–QDdistance, usually through the control over the STM bias and current settings(V s and I s ). On the other hand, the QD–substrate junction parameters (C 2 andR 2 ) are practically stable for a specific QD. They can be controlled in differentexperiments by the choice of the QD–substrate linking chemistry.Adding a single electron to a QD requires a finite charging energy, E c ,which in the equivalent circuit of the DBTJ is given by e 2 /[2(C 1 +C 2 )]. In atypical STM realization of a DBTJ with nanocrystals, E c is on the order off100 meV, similar to that expected for an isolated sphere, e 2 /2er, with a radiusr of a few nanometers and a dielectric constant e f10. The capacitance valuesdetermine also the voltage division between the junctions, V 1 /V 2 = C 2 /C 1 .Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.