Chemical control of the charge state of nitrogen-vacancy centers in diamond

We investigate the effect of surface termination on the charge state of nitrogen-vacancy (NV) centers, which have been ion-implanted a few nanometers below the surface of diamond. We find that, when changing the surface termination from oxygen to hydrogen, previously stable NV${}^{-}$ centers convert into NV${}^0$ and, subsequently, into an unknown nonfluorescent state. This effect is found to depend strongly on the implantation dose. Simulations of the electronic band structure confirm the disappearance of NV${}^{-}$ in the vicinity of the hydrogen-terminated surface. The band bending, which induces a $p$-type surface conductive layer, leads to a depletion of electrons in the nitrogen-vacancies close to the surface. Therefore, hydrogen surface termination provides a chemical way to control the charge state of nitrogen-vacancy centers in diamond. Furthermore, it opens the way to electrostatic control of the charge state with the use of an external gate electrode.

Figure 1

Energy-band schematic of diamond. (a) O-terminated diamond: The conduction band $E_C$ lies below the vacuum level $E_{\text{vac}}$ (electron affinity $\chi =+1.7$ eV). The NV${}^{-}$ level lies beneath the Fermi level $E_F$, where the position is mainly determined by the bulk concentration of nitrogen. (b) H-terminated diamond: The bands are shifted upward by the hydrogen termination ($\chi \approx -1.0$ eV). Electrons can transfer into acceptor states $\mu$ of an adsorbed water layer. (c) Equilibrium is established and a two-dimensional hole gas (2-DHG) is induced at the surface. Close to the surface, the strong band bending modifies the occupancy probability of the impurity levels.

Figure 2

(a) Schematic of the implantation profile at the diamond surface for energies of 10 and 30 keV. (b), (c) Confocal scans of diamond surfaces showing the fluorescence intensity of single NV${}^{-}$ [650 nm longpass filter (LP)] for implantation spots of different energies (10 and 30 keV). The darker regions correspond to the H-terminated part of the surface. Scale bar is 5 $\mu \mathrm{m}.$

Figure 3

(a)–(c) Confocal scans of a 10-keV implanted diamond surface showing the fluorescence intensity of NV${}^{-}$ (650 nm LP, 532 nm excitation at 240 µW) at different implantation doses (low, medium, high, respectively). In (c), dashed lines indicate the O- and H-terminated regions. Scale bar is 10 $\mu$m. (d) Relative contribution to the local fluorescence intensity of NV${}^{-}$ and NV${}^0$, obtained from fits to spectra of single NV${}^{-}$ or NV${}^0$.

Figure 4

Simulation results from nextnano${}^3$. The diamond surface is positioned at 0 nm. (a) Density profiles for 3 different implantation energies [3.6 keV (black), 7.1 keV (red), and 10.7 keV (blue)] at a nitrogen implantation dose of $1\times {10}^{13}$ cm${}^{-2}$ (dashed line). The solid line represents the density of NV${}^{-}$. (b) Normalized area density of NV${}^{-}$ versus implantation energy (Gaussian center position) in a range of 1 to 32 keV for different implantation doses. The solid symbols correspond to the data from (a).