Scanning tunneling microscopy and spectroscopy of single molecules and nanocrystals in double-barrier tunnel junctions

Electron transport in single molecules and nanocrystals was studied in the regime of weak electronic coupling to the electrodes. To decouple the electronic states of the adsorbates from the metal substrate, they were adsorbed on thin insulating layers, such as oxide or alkali halide, grown or deposited on the metal substrate. The presence of the two tunneling barriers allows observation of a number of interesting physical phenomena, such as bipolar transport, charging, and vibronic coupling.; Single copper phthalocyanine (CuPc) molecules were adsorbed on a thin aluminum-oxide film grown on a NiAl(110) surface. Aluminum-oxide decouples the molecule from the metallic substrate and allows charging of the molecule with the tunneling electrons. The spatial behavior of the charging and its dependence on the applied sample bias was studied with the STM. Peculiar topographic features were explained with the help of scanning tunneling spectroscopic measurements and their charging nature was revealed.; Thin layers of NaBr film were grown on the NiAl(110) surface as an alternative insulator to decouple the electronic states of the adsorbates from the metal substrate. Single CuPc molecules were deposited and observed on one-, two-, three-, and four-layer NaBr films. An increase in the lifetime of the charged state of the CuPc molecule was observed. Differential conductance spectra of molecules adsorbed on films of different thicknesses provides evidence that the vibronic coupling is weaker for thicker films, which can be attributed to the screening of the charge on the molecule due to the polarization of the surrounding NaBr.; Bipolar conductance was observed for fullerene nanocrystals adsorbed on thin NaBr layers on NiAl(110) surface. Due to the differences in tunneling rates and dielectric constants of the two tunneling barriers, the same electronic state appears differently in the differential conductance spectra for opposite bias polarities.; These experiments illuminate the effect of the electron-phonon coupling and charging on the electron transport in nanoscale systems.