Letter pubs.acs.org/NanoLett
Proper Surface Termination for Luminescent Near-Surface NV Centers in Diamond Moloud Kaviani,† Peter Deák,*,† Bálint Aradi,† Thomas Frauenheim,† Jyh-Pin Chou,‡ and Adam Gali*,‡,§ †
Bremen Center for Computational Materials Science, University of Bremen, Am Fallturm 1, D-28359, Bremen, Germany Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, Budapest, POB 49, H-1525, Hungary § Department of Atomic Physics, Budapest University of Technology and Economics, Budafoki út 8, H-1111, Budapest, Hungary ‡
S Supporting Information *
ABSTRACT: By accurate quantum mechanical simulations, we show that typical diamond surfaces possess image states with sub-bandgap energies, and compromise the photostability of NV centers placed within a few nm of the surface. This occurs due to the mixture of the NV-related gap states and the surface image states, which is a novel and distinct process from the well-established band bending effect. We also find that certain types of coverages on the diamond surface may lead to blinking or bleaching due to the presence of acceptor surface states. We identify a combination of surface terminators that is perfect for NV-center based nanoscale sensing. is negative (χ = −1.3 ± 0.1 eV) for hydrogenated while positive for oxygenated diamonds, where its actual value depends on the oxygen coverage and the type of oxidation. The largest positive electron affinity (PEA), χ = +2.56 eV for diamond, was measured on the fluorinated (001) surface.24 As a consequence of the negative electron affinity (NEA), the conduction and valence bands of hydrogenated diamond bend upward at the surface at ambient conditions, so, except for heavy n-type doping, the Fermi-level will shift below the acceptor level of the NV-defect, and NV(0) will dominantly appear.1 Oxidation removes this bending, and luminescent NV(−) defects also appear beside NV(0).1,2 In fluorinated surface diamonds further enhancement of the luminescent NV(−) to NV(0) ratio was observed.25 This band bending model was applied to explain the enhanced NV(−) to NV(0) ratio in larger nanodiamonds after oxygenation,4 however, we note that it does not explain all the features of the NV center near the diamond surface. For instance, intermittency in fluorescence, i.e., blinking was detected in nanodiamonds of 5 nm in diameter15 where the concept of solid state bands is strictly not valid any more, so acceptor states should play a role in this process.26 In a recent study, 30% of the NV(−) centers engineered 5−2 nm beneath the F-terminated diamond surface showed permanent bleaching.27 Since F-termination of diamond leads to a PEA surface,24 a band bending model cannot account for such a bleaching behavior of the NV(−) defects. Blinking or bleaching is
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s the quantum-optics protocols of the nitrogen-vacancy (NV) center rely on its negative charge state in diamond, the control of the charge state of the NV center1−4 is a prerequisite to apply them in sensor applications5−13 where NV centers should be placed as close as possible to the surface of diamond for efficient sensing of the targeted nano-objects. Advanced technologies have made possible to implant NV centers
