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

probed in the optical measurements. In the optical spectra, allowed valenceband (VB) to conduction band (CB) transitions are detected, whereas intunneling spectroscopy, the CB and VB states can be separately probed. Inaddition, the tunneling spectra may show effects of single-electron charging ofthe QD. Such interplay between single-electron charging and resonant tunnelingthrough the QD states can provide unique information on the degeneracyand, therefore, the symmetry of the levels.The interplay between single-electron tunneling (SET) effects andquantum-size effects in isolated nanoparticles can be experimentally observedmost clearly when the charging energy of the dot by a single electron, E c , iscomparable to the electronic level separation DE L , and both energy scales arelarger than k B T [7,52,53]. These conditions are met by semiconductor nanocrystalsin the strong quantum-confinement regime, even at room temperature,whereas for metallic nanoparticles, E c is typically much larger than DE L .SET effects are relevant to the development of nanoscale electronic devices,such as single-electron transistors [54,55].However, for small colloidal nanocrystals, the task of wiring up theQD between electrodes for transport studies is exceptionally challenging. Tothis end, various mesoscopic tunnel junction configurations were employed,such as the double-barrier tunnel junction (DBTJ) geometry, where a QD iscoupled via two tunnel junctions to two macroscopic electrodes [7,8,56,57].Klein et al. achieved this by attaching CdSe quantum dots to two lithographicallyprepared electrodes and have observed SET effects [58]. In thisdevice, a gate voltage can be applied to modify the transport properties. Analternative approach to achieve electrical transport through single QDs is touse scanning probe methods. Alperson et al. observed SET effects at roomtemperature in electrochemically deposited CdSe nanocrystals using conductiveatomic force microscopy (AFM) [59].A particularly useful approach to realize the DBTJ with nanocrystalQDs is demonstrated in Fig. 1. Here, a nanocrystal is positioned on a conductingsurface providing one electrode and the STM tip provides the secondelectrode. Such a configuration has been widely used to study SET effectsin metallic QDs and in molecules [7,60–63]. In this geometry, in additionto the QD level structure, the parameters of both junctions, in particular thecapacitances (C 1 and C 2 ) and tunneling resistances (R 1 and R 2 ) stronglyaffect the tunneling spectra [64,65]. Therefore, a detailed understanding ofthe role played by the DBTJ geometry and the ability to control it areessential for the correct interpretation of tunneling characteristics of semiconductorQDs, as well as for their implementation in electronic nanoarchitectures,as demonstrated by Su et al. for semiconducting quantumwells [66].Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.