The interest in the modification of the surfaces chemical structure goes beyond the fundamental aspects of chemical science, toward relevant applications in the field of heterogeneous catalysis, photocatalysis, electrocatalysis, sensing, and microelectronics. Often this is performed by deposing on the surface organic molecules prone to form self-assembled monolayers. It is, however, evident how the possibility to form well-ordered molecular assemblies would pave the way toward new, relevant applications.
In the article "Writing with Molecules: Tip-Induced Local Chemisorption of N-Heterocyclic Olefins on Cu(111)" (doi: 10.1021/jacs.5c06188), recently published in the Journal of the American Chemical Society (Impact Factor 15.6, 2024 Journal Impact Factor, Journal Citation Reports (Clarivate Analytics, 2025)), it was shown by means of Scanning Tunnelling Microscopy (STM) measurements that, thanks to the electric field generated at positive bias potential, it is possible to write at the nanoscale on a copper surface, immobilizing molecules of N-heterocyclic olefins. This work was jointly carried out the University of Münster (synthesis of organic molecules), the Fritz Haber Institute of Berlin (deposition and spectroscopic study), and the Department of Materials Science, University of Milano-Bicocca (DFT simulations).
N-heterocyclic olefins are peculiar organic molecules, sporting a cycle with C and N atoms bound to a functional group formed by a double C-C bond and a terminating CH2 group. The C-C double bond is electron-rich and highly polarizable, thanks to the effect of the heterocycle, and can react to form a stable chemical bond with the metal surface.
Sergio Tosoni, associated professor at the Department of Materials Science of the University of Milano-Bicocca, contributed to this research with first-principles simulations to clarify the mechanism at the base of the selective immobilization at positive bias voltage. The simulations showed that chemisorbed olefins are more stable (and less mobile) compared to the physisorbed ones. Chemisorption, however, remains an activated process. The stabilization of chemisorbed species results stronger in presence of an external electric field coherent with the one generated by the STM tip at positive bias potential, thanks to the interaction of the molecular dipole moment with the tip-induced electric field. One can thus hypothesise that the positive bias voltage, above a given threshold, will overcome the kinetic barrier between physisorbed and chemisorbed species. The correct assignment of the molecular binding modes at the surface was supported by crosschecking experimental HREELS spectra with simulated spectra, generated with a method developed in this Department.