Molecular rectification in self-assembled monolayers of donor-acceptor dyads - an STM/STS study

Matino, Francesca1; Arima, Valentina1; Piacenza, Manuel1; Maruccio, Giuseppe1; Phaneuf, Ray2; della Sala, Fabio1; Cingolani, Roberto1; Rinaldi, Ross1 1Italy; 2United States

Self assembled donor-acceptor dyads are of current interest in the field of nanoscale molecular devices as they are biomimetic to the natural photosynthetic conversion system. In particular, metallo-porphyrins in combination with fullerenes have been recently demonstrated to act as donor/acceptor pairs. Here we perform an ultra high vacuum (UHV) Scanning Tunneling Microscopy (STM) and Spectroscopy (STS) study on single dyads, consisting of a metallo- [Cobalt(II) or Zinc(II)] tetraphenylporphyrin (CoTPP/ZnTPP) and a fulleropyrrolidine (PyC2C60) assembled by axial ligation on Au(111) functionalized with 4-aminothiophenols (4-ATP).
The distribution of the dyads on the surface is influenced by the different affinities of the central metal atom for the amino group of the 4-ATP and for the pyridil group of the PyC2C60, resulting into small clusters in the case of PyC2C60s assembled on ZnTPPs and isolated dyads on CoTPPs. A different bias–dependent behaviour was also observed. The majority of the structures assembled on CoTPPs become less bright when imaged at negative voltages. On the other hand, the small clusters observed on ZnTPPs at positive bias disappear or at least modify their shape at negative bias, meanwhile other structures emerge from the background. We ascribe again this different response to the different organization and orientation of the metallo-porphyrin layers.
This behaviour is consistent with the measured STS curves. The I-V curves acquired on the dyads show a highly rectifying behaviour, while metallo-porphyrins characteristics are symmetric. The normalized conductance spectra can be explained by a simple model for molecular rectification. First-principles density functional theory calculations have been performed to clarify the molecular structure and the electronic properties of the two dyads. We compared the density of states to measured I/V curves. Theoretical results show that the I/V asymmetry originates from tunneling to the lowest unoccupied molecular orbitals of the fullerene.

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