  We report the synthesis and the electrical properties of fullerene-based
molecular junctions on silicon substrate in which the highly \pi-conjugated
molecule C60 (\pi quantum well) is isolated from the electrodes by alkyl chains
(\sigma tunnel barriers). Initially, the Si/SiO2/\sigmaC60 architecture was
prepared either by sequential synthesis (3 different routes) or by direct
grafting of the presynthesized C60-\sigma-Si(OEt)3 molecule. We described the
chemical synthesis of these routes and the physico-chemical properties of the
molecular monolayers. Then, the second \sigma tunnel barrier was added on the
Si/SiO2/\sigma C60 junction by applying a hanging mercury drop electrode
thiolated with an alkanethiol monolayer. We compared the electronic transport
properties of the Si/SiO2/\sigma C60//Hg and Si/SiO2/\sigma C60//\sigmaHg
molecular junctions, and we demonstrated by transition voltage spectroscopy
that the fullerene LUMO - metal Fermi energy offset can be tailored from ~ 0.2
eV to ~ 1 eV by changing the length of the alkyl chain between the C60 core and
the Hg metal electrode (i. e. from direct C60//Hg contact to 14 carbon atoms
tunnel barrier).
