The electronic level structure of semiconductor quantum-dot/nanorod core/shell heterostructure was probed using scanning tunneling spectroscopy (STS). We exploit the high resolution of the STM and its ability to measure local density of states to find local variations inside a single core/shell heterostructured NC. So far, no method was reported for direct measurement of the band-offsets in colloidal nanocrystals and only indirect information could be derived from optical measurements. Here we demonstrate that STS along with theoretical modeling can be used to determine band-offsets in such nanostructures. Applying this approach to CdSe/CdS quantum-dot/nanorod core/shell nanocrystals portrays its type-I band structure where both the hole and electron ground-state are localized in the CdSe core, in contrast to previous reports which predicted electron delocalization.  The generality of the approach is further demonstrated in ZnSe/CdS nanocrystals where their type-II band alignment, leading to electron-hole separation, is manifested.

In addition to the local level structure, we have also investigated the electrical transport through NR arrays. We used a controlled evaporation method to achieve long-range self-organization of CdSe NRs on silicon substrates. Appling this method for the alignment of NRs between Au electrodes enabled us to measure the dark- and photo-conductivity of such CdSe NR arrays. We found that the photoconductivity of such arrays is considerably enhanced by exchanging the trioctylphosphine (TOP) capping ligands by diamine molecules or upon annealing. The corresponding current-voltage characteristics were highly non-linear, showing, in the aligned NR arrays, a notable decrease in the differential conductivity at a certain capping-molecule dependent applied electrical field. This transition smears, however, when the degree of NR alignment is reduced. These findings are well described by an exciton field-ionization model, which also accounts for the correlation we observe between the photocurrent I(V) curves and the voltage dependence of the fluorescence quenching in seeded-grown CdSe/CdS core/shell NR films.