Coulomb staircases in double-barrier tunneling junctions consisting of a scanning-probe–vacuum-gap–alkanethiol-protected Au nanoparticle/Au (111) electrode have been measured as a function of the set point current of scanning tunneling spectroscopy. The tunneling resistances of the scanning probe-Au core of a nanoparticle $\left(R_1\right)$ and the Au core-Au (111) electrode $\left(R_2\right)$ are evaluated by fitting a theoretical Coulomb staircase into the experimental tunneling current-voltage characteristics measured by scanning tunneling spectroscopy. When a vacuum gap exists between the scanning probe and alkanethiol Au nanoparticles, $R_1$ is inversely proportional to the set point current. On the contrary, in the case of $R_1<R_2$, the top of the tip of the scanning probe tends to penetrate the octanethiol-protecting molecule of an Au nanoparticle. $R_2$ is found to be independent of the set point current, and $R_2$ of octanethiol- and hexanethiol-protected Au nanoparticles are evaluated as $7.6\mathrm{G}\Omega \pm 10\%$ and $460\mathrm{M}\Omega \pm 10\%$, respectively.

• Received 22 November 2004

DOI:https://doi.org/10.1103/PhysRevB.72.205441