  Polyoxometalates (POMs) are redox-active molecular oxides, which attract
growing interest for their integration into nano-devices, such as high-density
data storage non-volatile memories. In this work, we investigated the
electrostatic deposition of the negatively charged [H7P8W48O184]33- POM onto
positively charged 8-amino-1-octanethiol self-assembled monolayers (SAMs)
preformed onto gold substrates or onto an array of gold nanodots. The
ring-shaped [H7P8W48O184]33- POM was selected as an example of large POMs with
high charge storage capacity. To avoid the formation of POM aggregates onto the
substrates, which would introduce variability in the local electrical
properties, special attention has to be paid to the preformed SAM seeding
layer, which should itself be deprived of aggregates. Where necessary, rinsing
steps were found to be crucial to eliminate these aggregates and to provide
uniformly covered substrates for subsequent POM deposition and electrical
characterizations. This especially holds for commercially available gold/glass
substrates while these rinsing steps were not essential in the case of template
stripped gold of very low roughness. Charge transport through the related
molecular junctions and nanodot molecule junctions (NMJs) has been probed by
conducting-AFM. We analyzed the current-voltage curves with different models:
electron tunneling though the SAMs (Simmons model), transition voltage
spectroscopy (TVS) method or molecular single energy level mediated transport
(Landauer equation) and we discussed the energetics of the molecular junctions.
We concluded to an energy level alignment of the alkyl spacer and POM lowest
occupied molecular orbitals (LUMOs), probably due to dipolar effects.
