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

charging indicated that the tunneling rate G 2 ~ 1/R 2 was on the order of orsmaller than G 1 ~ 1/R 1 . Otherwise, for positive sample bias, an electrontunneling from the tip to the QD would escape to the substrate before the nextelectron could tunnel into the QD. Consequently, merely resonant tunnelingthrough the QD states without charging would take place. By varying the tip–QD distance, we were able to modify the voltage division between junctionsup to the distance that allowed us to obtain meaningful (well above the noiselevel) tunneling spectra. Bakkers and Vanmaekelbergh also reported an STMstudy of CdS and CdSe QDs, focusing on the role of voltage division [64]. Wedemonstrated that by working without linker molecules, charging-free resonanttunneling, as well as a transition back to tunneling accompanied by QDcharging can be achieved for a single QD by controlling R 2 [20].Figure 13 shows InAs nanocrystals deposited without any linker moleculesdirectly on highly oriented pyrolitic graphite (HOPG) [86]. In Fig. 14a,we plot a tunneling spectrum measured on an InAs QD, f2 nm in radius,along with a representative spectrum measured on a QD of similar radius,but anchored to a gold substrate via linker molecules, as described in theprevious section (dashed line). There is a profound difference between thesetwo spectra. In the spectrum measured in the QD/linker molecule/Au geometry,resonant tunneling accompanied by QD charging is clearly seen, as discussedearlier. In contrast, the charging multiplets are absent in the spectrummeasured in the QD/HOPG geometry, and each multiplet is replaced by asingle peak, indicating charging-free resonant tunneling through the s- andp-like CB states. Typically, the peaks observed in the QD/HOPG configurationare broadened as compared to those seen for the QD/DT/AuFigure 13 A 30-nm 30-nm STM topographic showing two single InAs QDspositioned near a monolayer step on HOPG. In this case, no linker molecules separatethe QD from the substrate, thus reducing the QD–substrate tunneling barrier.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.