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

D CB = 0.31 eV is thus obtained. On the negative bias side, tunneling throughfilled dot levels takes place, reflecting the tunneling DOS of the QD valenceband. Again, two groups of peaks are observed. The multiplicity in this case,in contrast with the CB, cannot be clearly assigned to a specific angularmomentum degeneracy. In a manner similar to that described for the CBstates, we extract a value of D VB = 0.10 eV for the level separation between thetwo observed VB states.In the region of null current around zero bias, the tip and substrateFermi energies are located within the QD bandgap, where the tunneling DOSis zero. Tunneling is onset when the bias is large enough to overcome both thebandgap and charging energy. E g is thus extracted by subtracting E c from theobserved spacing between the highest VB and lowest CB peaks and is equal to1.02 eV.The tunneling conductance spectra for single InAs nanocrystals spanninga size range of 10–35 A˚ in radius are presented in Fig. 10. Two groups ofpeaks are observed in the positive bias side (CB). The first is always a doublet,consistent with the expected s symmetry of the 1S e level, whereas the secondhas higher multiplicity of up to six, consistent with 1P e . The separationbetween the two groups, as well as the spacing of peaks within each multiplet,increase with decreasing QD radius. This reflects quantum-size effects on boththe nanocrystal energy levels and its charging energy, respectively. In somecases (e.g., Fig. 8), one can observe a small peak or shoulder just before theonset of the p multiplet, which may be related to the situation of tunneling intothe p level without fully charging the s level, as discussed in Section II. On thenegative bias side, generally two groups of peaks, which exhibit similarquantum-confinement effects are also observed. Here, we find variations inthe group multiplicities between QDs of different size as well as variations ofpeak energy spacings with each group. This behavior is partly due to the factthat E c is very close to D VB , the level spacing in the valence band, as shown inFig. 11b. In this case, sequential single-electron tunneling may be eitheraddition to the same VB state or excitation with no extra charging to the nextstate. An atomic analogy for this situation can be found in the changing orderof electron occupation when moving from the transition to the noble metalswithin a row of the periodic table.C. Comparison Between Optical and Tunneling SpectraThe comparison between tunneling and PLE data can be used to decipher thecomplex QD level structure. This correlation is also important for examiningpossible effects of charging and tip-induced electric field in the tunnelingmeasurements on the nanocrystal level structure.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.