Organic radicals are thought of as ideal organic material for spintronics, due to their open-shell electronic structure. They could be used in novel memory devices or serve as qubits in quantum computing devices. They possess one or more unpaired electrons in their ground state and are considered unstable species, since unpaired electrons are highly reactive.

Azafullerenes are a class of radical organic molecules, C59N, where one N atom replaces a C atom in the fullerene skeleton. Due to the valence difference between N and C, C59N is a closed-cage radical where the odd electron resides on the C atom neighbouring the N. C59N radicals are highly reactive due to the singly occupied valence orbital, and in bulk they readily form diamagnetic dimers (C59N)2.

Although the dimer breaks into two radical monomers upon heating and molecular sublimation under UHV conditions, after deposition on substrates the spin-silent dimer is re-formed, when a suitable partner radical is found. The main goal of this study was to protect the molecular radical, i.e. the spin-½ magnetic molecular state.

Recently, a novel strategy for azafullerene spin protection by [10]cycloparaphenylene ([10]CPP) nanohoop encapsulation has been proposed. Thanks to molecular shape matching, C59N may be trapped within the host [10]CPP ring via pi-pi interaction, forming a bulky-belt protection of the radical orbital. Also, in this case the readily formed dimers (C59N•⊂[10]CPP)2 can be broken into spin-active monomers, with substantially longer stability. The organic radical with spin-½ molecular charge state shows up in the spin-active signal of EPR experiments, which are principally conducted on bulk, powder-phase samples.

In our experiment, we used the STM to study the azafullerene molecules deposited in vacuum on a previously grown template layer of [10]CPP nanohoops. Our measurements revealed a guest-host on-surface complexation of C59N and [10]CPP, where the latter encapsulates azafullerene thanks to molecular shape matching between the two species. The C59N•⊂[10]CPP complexes self-assemble into an extended hexagonal nanolattice composed of stable spin-½ radicals.

Spectroscopic fingerprint of complexation

We studied in detail the dynamics of encapsulation as a function of coverage of both species and substrate temperature. X-ray photoemission and absorption measurements provided strong evidence for the electronic coupling of C59N and [10]CPP, with the radical state of the azafullerene being effectively protected.

“The obtained 2D lattice of stable spin-½ radicals could serve as a prototype system in various fields of physics, be it for studying quantum phenomena in the realization of qubits, or for use as a basic building block for memory devices,” says Bavdek. “Thanks to NFFA-Europe, we were able to conduct a complementary study of our system using state-of-the-art instruments and equipment.”

https://doi.org/10.1038/s41467-024-55521-2

The scanning tunneling microscope at OSMOS laboratory (joint project of CNR IOM and Elettra, Trieste) was used for C59N and [10]CPP thin film characterization as well as for studying the C59N•⊂[10]CPP complexation dynamics. The organic materials were evaporated from laboratory-made Knudsen cells on the Au(111) surface, previously cleaned by Ar+ ion bombardment and crystallografically ordered by thermal annealing.

XPS, NEXAFS and ResPES measurements were performed at Aloisa Beamline, Elettra – Sincrotrone Trieste, which operates under CNR IOM. Laboratory-made Knudsen cells were used for vacuum deposition of organic molecules on the Au(111) surface. We exploited standard techniques, such as Ar+ ion sputtering and thermal annealing, to obtain clean and well-ordered Au(111) surface.

Dr. Gregor Bavdek is a researcher and assistant professor at the Department of Physics and Technical Studies, Faculty of Education, University of Ljubljana. His main research topics include nanoscale physics at surfaces, adsorption geometries of organic molecules, self-assembling nanoarchitectures, electronic structure and charge dynamics in thin films exploiting UV and X-ray photoemission spectroscopy, near-edge X-ray absorption spectroscopy, resonant photoemission measurements and scanning tunneling microscopy.

Dr. Denis Arčon is head of the Department for Condensed Matter Physics at “Jožef Stefan Institute”, Ljubljana, Slovenia. He is also a full professor at Department of Physics, Faculty of Mathematics and Physics, University of Ljubljana. He is specialized in magnetic resonance measurements, including nuclear magnetic resonance, electron paramagnetic resonance and muon spin relaxation. His research work includes quantum magnetism, primarily focused on low-dimensional and frustrated antiferromagnets, metal-to-insulator transitions, in particular, the role of electron correlations on superconductivity and magnetic properties of doped fullerides.

Dr. Gregor Kladnik is a researcher and assistant professor at the Department of Physics, Faculty of Mathematics and Physics, University of Ljubljana. His principal research activity is focused on studying electronic structure and charge transfer dynamics at organic and metallic heterojunctions and theoretical molecular simulations using DFT. His experimental work includes synchrotron based spectroscopic techniques, such as X-ray photoemission and near-edge X-ray absorption spectroscopy as well as resonant photoelectron spectroscopy.

Dr. Dean Cvetko is a full professor at the Department of Physics, Faculty of Mathematics and Physics, University of Ljubljana, and researcher at the Department of Low and Medium Energy Physics at “Jožef Stefan Institute”, Ljubljana, Slovenia. His research work includes electron dynamics and charge transport in metallic and heteroorganic interfaces, surface phenomena in thin metallic and organic films and heterojunctions. He mainly uses X-ray photoemission, near-edge absorption and resonant photoemission spectroscopy.