posted on 2023-12-12, 19:10authored byYuri Tanuma, Gregor Kladnik, Luca Schio, Marion van Midden Mavrič, Bastien Anézo, Erik Zupanič, Gregor Bavdek, Ruben Canton-Vitoria, Luca Floreano, Nikos Tagmatarchis, Hermann A. Wegner, Alberto Morgante, Christopher P. Ewels, Dean Cvetko, Denis Arčon
We deposit azafullerene
C59N• radicals
in a vacuum on the Au(111) surface for layer thicknesses between 0.35
and 2.1 monolayers (ML). The layers are characterized using X-ray
photoemission (XPS) and X-ray absorption fine structure (NEXAFS) spectroscopy,
low-temperature scanning tunneling microscopy (STM), and by density
functional calculations (DFT). The singly unoccupied C59N orbital (SUMO) has been identified in the N 1s NEXAFS/XPS spectra
of C59N layers as a spectroscopic fingerprint of the molecular
radical state. At low molecular coverages (up to 1 ML), films of monomeric
C59N are stabilized with the nonbonded carbon orbital neighboring
the nitrogen oriented toward the Au substrate, whereas in-plane intermolecular
coupling into diamagnetic (C59N)2 dimers takes
over toward the completion of the second layer. By following the C59N• SUMO peak intensity with increasing
molecular coverage, we identify an intermediate high-spin-density
phase between 1 and 2 ML, where uncoupled C59N• monomers in the second layer with pronounced radical character are
formed. We argue that the C59N• radical
stabilization of this supramonolayer phase of monomers is achieved
by suppressed coupling to the substrate. This results from molecular
isolation on top of the passivating azafullerene contact layer, which
can be explored for molecular radical state stabilization and positioning
on solid substrates.