Interaction between shallow NV$^-$ and spin active azafullerenes on hydrogenated and fluorinated (001) diamond surfaces

TL;DR

The paper investigates how a spin-active azafullerene $C_{59}$N$^\bullet$ interacts with a shallow NV$^-$ center in diamond, focusing on (2$\times$1)-(001) surfaces with H and F terminations to understand charge-transfer dynamics driven by surface electron affinity and band bending. Using density functional theory with PBE-GGA (plus Grimme dispersion) in large diamond slabs, the authors map how surface termination controls NV charge stability and the adsorption-induced electronic reconfiguration. They find that a hydrogenated surface (NEA) promotes electron transfer from the NV$^-$ center to $C_{59}$N$^\bullet$, yielding $C_{59}$N$^-$ and NV$^0$ and quenching the NV spin, whereas fluorination (PEA) preserves both NV$^-$ and $C_{59}$N$^\bullet$ as a neutral radical, maintaining spin activity. These results connect to observed photoluminescence quenching and suggest surface fluorination as a viable strategy to mitigate charge-transfer issues in diamond-based quantum sensing with adsorbed spin-active molecules.

Abstract

The interaction between surface-lying nitrogen-substituted fullerenes (radical azafullerene, C$_{59}$N$^\bullet$) with sub-surface negative nitrogen-vacancy complexes (NV$^-$) in diamond is investigated using first principles calculations. We consider (2$\times$1) reconstructed (001) oriented diamond surfaces with both H- and F-surface termination. The charge stability of NV$^-$, when in close proximity to both the nearby surface and the spin active azafullerene is discussed, in the context of diamond band bending arising from surface-induced changes in electron affinity (EA). In the case of the hydrogenated surface, the system spin is quenched, yielding a negatively charged azafullerene (C$_{59}$N$^-$) and neutrally charged NV$^0$ as the most stable electronic configuration. In contrast, fluorinating the surface favours the negatively charged NV$^-$, and conserves the C$_{59}$N$^\bullet$, neutrality and stabilizes uncompensated free spins. This opposing behaviour is attributed to surface charge doping emerging from different band bending effects associated with the surface EA. This study is consistent with experimentally observed photoluminescence quenching, and shows that surface passivation by fluorination could efficiently tackle the problem of charge transfer between adsorbed molecules and shallow NV centers.

Figures (7)

• Figure 1: The energies in a) represent C$_{60}$ (left) and C$_{59}$N$^\bullet$ (right) eigenstates with symmetries and spin orientation. Zero is set to the vacuum level of each molecule. Dashed lines correspond to respective Fermi levels. Full and faded bars show occupied and unoccupied states respectively. With b) the C$_{59}$N$^\bullet$ radical state wave function is shown: the visualisation has been done using Jmol. Grey and blue spheres represent carbon and nitrogen atoms, respectively, while purple lobes show the wave function isosurface with a $7.5$ meV.$\mathring{\rm A}^{-3}$ cut-off.
• Figure 2: Hetero-system geometries after complete structural optimisation for a) hydrogenated and b) fluorinated surface. Relevant c-axis lengths (Å): fullerene diameter and distance to the surface, slab thickness, NV defect distance to surface, total system width and c-axis unit cell length (including vacuum).
• Figure 3: Electronic band structure of NV$^-$ within a 4$\times$4$\times$4 diamond supercell. Zero is set to the top of the diamond valence band. Solid/dashed lines represent spin-up/-down electrons, respectively, while dark/faded lines showing filled/empty states, respectively. Black and green colours indicate bulk diamond and NV defect related eigen states. The wavefunction isosurfaces of the $a_1$ and $e$ states are shown with a 20 meV$.\mathring{\rm A}^{-3}$ cut-off.
• Figure 4: Electronic band structure of (2$\times$1)-(001) hydrogenated diamond surface with a single NV$^-$ embedded in the centre of the diamond slab. Zero is set to the top of the valence band. Solid/dashed lines represent spin-up/-down electrons respectively, while dark/faded lines showing filled/empty states respectively. Black and green colours indicate diamond slab and NV defect related eigen states respectively.
• Figure 5: Electronic band structure of (2$\times$1)-(001) fluorinated diamond surface with a single NV$^-$ embedded in the centre of the diamond slab. Zero is set to the top of the valence band. Solid/dashed lines represent spin-up/-down electrons, respectively, while dark/faded lines showing filled/empty states, respectively. Black and green colours indicate diamond slab and NV defect related eigen states, respectively.