On-surface Synthesis of a Ferromagnetic Molecular Spin Trimer

TL;DR

Open-shell carbon nanographenes can host multiple localized spins, and establishing ferromagnetic exchange between them remains challenging. This work designs TTAT by covalently fusing three pristine and N-doped triangulenes into a triangular aza-nanographene, predicted to host three unpaired π electrons and a neutral $S=3/2$ ground state, on Au(111). Using low-temperature STM/STS, bond-resolved STM/AFM with CO tips, and complementary DFT and multiconfigurational CASCI/CASSCF calculations, the authors observe a weak underscreened Kondo resonance and inelastic spin steps near ±15 mV, assignable to transitions between quartet and doublet states, and map three localized SOMOs at the triangulene corners. Dyson orbital analysis and Kondo-orbital simulations reproduce the experimental $dI/dV$ maps, confirming three localized spins coupled via symmetric ferromagnetic exchange with $J\approx 10$ meV, yielding a quartet ground state; neutral charge state is maintained on Au(111) with $\Delta E\approx 15$ meV between $S=3/2$ and $S=1/2$ excitations. Overall, TTAT provides a molecular ferromagnetic spin-trimer platform for exploring entanglement and spin dynamics in carbon-based nanostructures, with potential relevance to spintronics and quantum information.

Abstract

Triangulenes are prototypical examples of open-shell nanographenes. Their magnetic properties, arising from the presence of unpaired $π$ electrons, can be extensively tuned by modifying their size and shape or by introducing heteroatoms. Different triangulene derivatives have been designed and synthesized in recent years, thanks to the development of on-surface synthesis strategies. Triangulene-based nanostructures with polyradical character, hosting several interacting spin units, can be challenging to fabricate but are particularly interesting for potential applications in carbon-based spintronics. Here, we combine pristine and N-doped triangulenes into a more complex nanographene, \textbf{TTAT}, predicted to possess three unpaired $π$ electrons delocalized along the zigzag periphery. We generate the molecule on an Au(111) surface and detect direct fingerprints of multi-radical coupling and high-spin state using scanning tunneling microscopy and spectroscopy. With the support of theoretical calculations, we show that its three radical units are localized at distinct parts of the molecule and couple via symmetric ferromagnetic interactions, which result in a $S=3/2$ ground state, thus demonstrating the realization of a molecular ferromagnetic Heisenberg-like spin trimer

Figures (5)

• Figure 1: (a) Schematic representation of the formation of the aza-triradical (TTAT) in (c) from four [3]triangulenes combined such that the majority sublattice of the central triangulene results opposite to the one of the three external triangulenes. Introducing an N atom in the center of the inner triangulene (in the majority sublattice) results in five electrons occupying four states. The final structure, therefore, hosts three unpaired electrons, expected to couple ferromagnetically by Hund's exchange. (b) In-solution and on-surface reaction steps leading to the synthesis of TTAT, with the C-C bonds formed during each step indicated in blue and red, respectively. (d) STM constant-current image ($V=0.9$ V, $I=30$ pA) after deposition of the precursor on Au(111) and subsequent annealing at 330 $^{\circ}$C. The white dotted square highlights an intact and planar molecule, corresponding to TTAT. (e) STM constant-current image of TTAT measured with a CO-functionalized tip ($V=200$ mV, I$=30$ pA). (f) Constant-height bond-resolved STM current scan ($V=5$ mV) and (g) constant-height bond-resolved AFM image (oscillation amplitude $A=60$ pm), performed with CO-functionalized tips.
• Figure 2: (a) Low-energy $dI/dV$ spectra of TTAT measured with a CO-functionalized tip at the positions indicated in the inset. The spectra display a zero-bias resonance and inelastic spin excitation features at $V \approx$$\pm 15$ mV. The black dashed lines represent fits to the data using the perturbative model by Ternes Ternes2015, for the case of three $S=1/2$ spins coupled with a ferromagnetic exchange $J=9$ meV. Weaker steps at $\pm 35$ mV are attributed to the excitation of frustrated rotational modes of the CO molecule Delatorre2017. Spectroscopy parameters: $V=50$ mV, $I=1$ nA, $V_\mathrm{mod}=2$ mV. (b,c) $d^{2}I/dV^{2}$ maps at $V=-3.3$ mV and $V=15$ mV, obtained by numerical differentiation from a grid of $dI/dV$ spectra. The maps probe the spatial distribution of the zero-bias resonance (b) and the inelastic signal (c) without elastic background effects. (d) Simulated Kondo and (e) spin excitation $dI/dV$ maps, computed from the Kondo orbitals (d) and Natural Transition Orbitals (NTOs), respectively (see text and Supplementary Information).
• Figure 3: (a) $dI/dV$ spectra measured at the points indicated in b), revealing molecular orbital resonances ($V=1$ V, $I=500$ pA, $V_\mathrm{mod}=10$ mV). (b) Constant-height (CH) $dI/dV$ map recorded at $V=-100$ mV with a CO-functionalized tip, corresponding to an orbital with non-vanishing signal over the inner N-doped triangulene (open feedback parameters: $V=-100$ mV, $I=300$ pA, $V_\mathrm{mod}=10$ mV) . (c) Constant-current (CC) $dI/dV$ maps recorded at different bias values around 0, with a CO-terminated tip ($I=300$ pA, $V_\mathrm{mod}=10$ mV). (d) Simulated $dI/dV$ maps, obtained using Dyson orbitals, corresponding to the processes of adding and removing electrons. (e) Dyson orbitals isosurfaces.
• Figure 4: (a) Natural orbitals computed by CASSCF(7,10). The numbers at the bottom of each orbital indicate the electron occupation. We find three orbitals with occupation close to 1, spatially distributed over the triangulene edges. (b) Diagram representing the energy and the total spin of the many-body ground state and first two excited states of TTAT, as computed by CASSCF(7,10). (c) Schematic representation of the most relevant Slater determinants for each of the many-body states in (b), displaying the electronic occupation of the three natural orbitals highlighted in the dashed box in (a). The number below each Slater determinant refers to its weight in the corresponding many-body state.
• Figure 5: (a) Representation of the three singly-occupied orbitals using a maximally localized basis set, which shows that each spin is mostly located on a triangulene corner. (b) Spin-spin correlation between each pair of spins computed using the orbital representation in (a). This picture describes the magnetic state of TTAT in terms of a symmetric Heisenberg ferromagnetic trimer, as illustrated in (c). According to this model the experimental quartet-doublet energy gap of $15$ meV corresponds to an exchange coupling $J=10$ meV Haraldsen2005.