Noncontact Layer Stabilization of Azafullerene Radicals: Route toward High-Spin-Density Surfaces

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.


Figure S2. DFT calculations of close-packed C59N
• monomers on Au(111).The system is fully optimized (fixed slab and allowed all atoms of C59N to vary) for the 0° case.Other points are generated by rotating C59N about its centre of mass from the 0° case without subsequent geometric relaxation.These energies confirm that there is an energetic preference for the azafullerene to orient with the carbon dangling bond towards an Au-atom in the layer below.Each spectrum normalized to highest spectral peak (i.e., not constant normalization across all four spectra).Fermi level is aligned at 0 eV.The unbound carbon next to nitrogen ('radical C') shows highly dispersed levels around the Fermi level, indicating strong interaction with gold electronic states.

SA. The C 59 N radical state within the 1st monolayer
At our deposition temperatures, the C59N are surface mobile and efficiently form extended hexagonally packed monolayer islands attached to Au step edges, as imaged in lowtemperature STM (Fig. 1 in the main text).We below include several experimental and theoretical arguments that support partial retention of the radical character of C59N molecules already within the first monolayer.
(1) Nitrogen tilt angle with respect to the surface: In the lower part of the Fig. S11a-c separate N 1s NEXAFS spectra for the photon polarization in TM (p-pol, Ip) and TE (spol, Is) are shown for films with a thickness between 3.5 Å and 7.3 Å.From the linear dichroism of the LUMO (i.e.SUMO) and LUMO+1 peak intensity (Is/Ip) we deduce the orientation of N site within the fullerene cage relative to the surface 1 .The resulting N site orientations are given in terms of average tilt angles (θ) from the surface normal.In the Fig. S11a the Is/Ip=0.2ratio yields the average nitrogen orientation mainly toward the Au(111) substrate (θ = 30 o ), which is almost constant throughout the monolayer coverage range (i.e., up to 7.3 Å, in Fig. S11c).Such azafullerene orientation clearly demonstrates the interaction of the Au(111) and C59N molecules in the first monolayer.The spectra for the supramonolayer (1+ML) film shown in Fig. S12b are substantially less dichroic in agreement with the random C59N radical orientation within the 2 nd layer.On the other hand, for the 2 ML film (Fig. 3   (2) Radical peak intensity (ILUMO) as a function of film thickness: In the upper panel of Fig. S12 we show the N 1s NEXAFS spectra in the "magic angle" for three characteristic C59N coverages.For the monolayer, the NEXAFS intensity of the radical peak (LUMO at 400.3 eV) relative to LUMO+1 is 24±4%, whereas it is 34% in the supramonolayer radical film and less than 14% in the dimerized 2 ML film.This proves that on Au(111) the C59N monomers partially retain their radical state despite their coupling to Au surface (which dictates the observed N orientation toward the surface as argued above).However, from the present data it is impossible to resolve the nature of the radical state quenching (ILUMO decrease from 34% to 24 %) since Au surface displays numerous sites where C59N-Au interaction is particularly strong (monoatomic steps, adatoms/vacancies and specific sites of the Au(111) herringbone reconstruction and possibly also Au sites where C59N adsorbs with its sp 3 carbon of the fullerene cage exactly above the Au atomsee DFT results below).(3) Thermal annealing of C59N layers: Annealing of ML films dramatically affects the radical state of C59N (Fig. S13d,e).Namely, when the 1 ML film is thermally annealed at 240 o C, the radical peak at 400.6 eV is mostly quenched (<11%) and the overall spectrum then closely resembles that of the in-plane oriented (C59N)2 dimers observed for the 2 ML film (Fig. 3 in the main text).Importantly, the C 1s XPS still shows that after annealing the sample remains as a monolayer film.In addition, the linear dichroism reflecting the N orientation toward Au( 111) is reversed (Is/Ip=1.2,Fig. S12a,d), also confirming that the annealed monolayer here is made of non-radical, in-plane oriented (C59N)2 dimers.Fig. S12e shows the thermally annealed 2 ML film which also turns into 1 ML film of (C59N)2 dimers, proving that the 2 nd layer dimer adsorption is substantially weaker than that for the (C59N)2 on Au(111).In this case the first monolayer thus becomes fully sacrificial, no longer demonstrating spin.As such this shows that temperature is a useful tool that allows us to control the spin state of this first monolayer.
Finally, the slightly broader radical peak seen in the N 1s NEXAFS for the 1 ML compared to the 1+ ML is consistent with local variations in C-Au distances.In agreement with the calculations and the STM showing some azafullerenes height variation, which depends on the local stacking match with the Au (Fig. 1b in the main text).
(4) C59N interaction to the Au(111) surface from DFT calculations: The surface interaction of the first monolayer is next theoretically investigated with DFT calculations for low surface density C59N on Au (25% surface coverage, i.e., avoiding the interaction between neighboring C59N present in the islands).In this case the lowest energy structure has the previously unbonded carbon of the C59N directly above a Au atom in the layer below, with the nitrogen atom at approximately 30° inclination to the surface normal.In this case there is the formation of a Au-C sigma bond (s-p character), removing the radical state from around the Fermi level and replacing it with a bonding and anti-bonding pair deeper into the valence and conduction bands respectively (Fig. S13a).The Au-C bond length is 2.2 Å, and the system has no net spin.By taking the difference in total charge density between this system, and the calculated charge density for the C59N and Au calculated separately and then simply summed, we obtain the change in charge density distribution caused by the proximity of the C59N to the Au, which we plot in Fig. S13b.This shows a dipolar redistribution of charge around the azafullerene surface associated with the bond formation.In order to investigate the effect of height variation on the system spin, we carried out a sequence of single point energy calculations stepping the azafullerene away from the surface, holding all atoms fixed in the system except for the carbon atom facing the gold surface.In each case the system spin is allowed to fully relax.The result is shown in Fig. S14, showing that the C59N recovers a non-zero partial net spin already when ~0.6 Å further from the surface than its fully relaxed surface bound configuration.The experimental STM images in Fig. 1b of the main text and in Fig. S1 show significant variation of about ~1.5 Å in azafullerene distance from the surface.This can be caused by various factors but notably the misfit in the lattice spacing between the azafullerenes and the underlying gold layer, which necessarily imposes that not every C59N can sit directly above an Au atom.Thus in light of the calculations we expect that a weak radical spin will be seen in those further from the surface, as detected in the experimental NEXAFS of the first monolayer.There are some parallels with the Au2-C59N configuration used for the TDDFT NEXAFS simulations (see Fig. 3b in the main text).In this case the Au forms a non-metallic bound pair and cannot covalently bond to the C59N, and the result is the lower energy peak visible in the N 1s NEXAFS associated with the radical on its carbon neighbor, similar to that observed in experiment.
In conclusion, DFT calculations demonstrate that some of the adsorbed C59N molecules in the first monolayer will strongly bind to Au and lose their radical character.However, mismatch between the azafullerene and Au lattices will cause many of the C59N molecules to move away from the Au, weakening their bonding and recovering a weak radical state, as detected in the N 1s NEXAFS.
In summary, we can now provide a full and complete description of the behavior of the first azafullerene layer on Au(111).In the very early deposition stages, it gives non-radical C59N that form a weak sigma bond to underlying Au.These molecules are highly surface mobile, rapidly forming a close-packed monolayer islands primarily attached to surface steps on the Au.The slight broadening of the N 1s NEXAFS peak comes from the variation in Au-C distances caused by the lattice mismatch between Au and C59N.Lifting numerous C59N monomers away from the surface for these entities blocks the possibility of covalent Au-C bonding.As a result, a radical signal is experimentally observed in N 1s NEXAFS, with the linear dichroism demonstrating that the preferred orientation of N toward the surface is still preserved.Heating the monolayer to 240°C allows the fullerenes to overcome an activation barrier for rotation and dimerization, resulting in an almost complete radical quenching as the contact layer of radical monomers undergoes a phase transition to stable monolayer of (C59N)2 dimers, with the inter-fullerene C-C bond lying parallel to the surface.These results demonstrate that 1 ML is rather inhomogeneous where some of the adsorbed azafullerene molecules retain their radical state and the others do not.

Figure S1 .
Figure S1.(a) STM image of C59N island on Au(111) substrate and (b) its cross-section

Figure S4 .
Figure S4.X-ray secondary electron cut-off measurements of the surface work function

Figure S5 .
Figure S5.C 1s shake-up spectrum of C59N films on Au(111).Please note a shoulder in the

Figure S6 .
Figure S6.Carbon and nitrogen NEXAFS for the 2.1 ML film shown in the magic angle

Figure S10 .
Figure S10.Solution 13 C nuclear magnetic resonance (NMR) spectrum of as-prepared (C59N)2 in the main text) Is/Ip=1.2 proving that N is preferentially oriented in-plane (θ = 60 o or 120 o ) as expected for the C59N in-plane coupling into (C59N)2 dimers.For each film we have also measured the C 1s XPS binding energy, which yields the Au screening shift and serves as an indicator of the film proximity from the Au(111).

Figure S12 .
Figure S12.N 1s NEXAFS of C59N film with different thicknesses and their change after

Figure S13 .
Figure S13.(a) Bonding orbital between C59N and Au when positioned in ground state

Figure S14 .
Figure S14.Total system spin (μB) as a function of C59N distance from its equilibrium