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

capacitance values were also kept the same for the two curves, C 1 = 0.1 aFand C 2 = 1.1 aF, resulting in an f90% voltage drop on the tip–QD junctionand E Cf100 meV. The two curves differ in the ratio between the tunnelingrates: G 2 /G 1f 1 and 10 for the dashed and solid curves, respectively. Thedashed curve shows strong charging multiplets, typical for resonant tunnelingtaking place along with QD charging. The solid curve, on the other hand,exhibits only a signature of charging effect (e.g., one small charging peak inthe p multiplet). It is evident that the curve for G 2 /G 1f 1 resembles theexperimental spectrum obtained for the QD/linker molecule/Au system,whereas the G 2 /G 1 = 10 curve better corresponds to the QD/HOPG configuration,consistent with our above interpretation. Note also that the apparents–p level separation (both in theory and experiment) is smaller for the QD/HOPG configuration, due to the absence of charging contribution.A transition from charging-free tunneling to resonant tunneling in thepresence of charging is demonstrated by Fig. 15. Here, we plot two tunnelingspectra acquired on the same QD of radius 2.5 nm, with different tip–QDseparations. The dashed curve was measured with an STM setting V S = 1.5 Vand I S = 0.1 nA, whereas the solid curve was taken with I S = 0.8 nA, movingthe tip closer to the QD. The apparent gap in the density of states around zerobias is larger for the curve measured with the tip closer to the QD. This isattributed to the effect of voltage division between the two junctions (seeSection II). In these measurements C 1 is smaller than C 2 ; therefore, theapplied voltage V B largely drops on the tip–QD junction and tunnelingFigure 15 Tunneling spectra measured on a single QD (r f2.5 nm) with twodifferent tip–QD separations, exhibiting effects of both the apparent QD levelspacing and single-electron charging.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.