Transition from Coulomb to molecular charging in tunneling spectra of gold nanocrystals

We show that alkanethiol-passivated gold nanocrystals can be identified by phcore size in scanning tunneling microscopy (STM) experiments by correlating the core size distribution determined by mass spectrometry with the height distribution measured by STM. This establishes a mapping from STM height to core mass or core diameter 2R. In the 1.3--2 nm range of core-diameters, I-V gaps determined by room-temperature scanning tunneling spectroscopy are found to scale approximately as R^-3, which suggests that the quantized density-of-states plays a key role, even when individual quantum levels are not resolved. below ∼ 1.3 nm diameter, the tunneling I-V gaps are smaller than would be indicated by the R^-3 scaling. We discuss the behavior in terms of a crossover from classical Coulomb charging to molecule-like charging.