Figure 1

(Color online) PL spectra (normalized to their respective maximum value) of single ${\text{NV}}^{-}$ [red (gray) curve] and ${\text{NV}}^0$ [blue (dark gray) curve] color centers in nanodiamonds. The zero-phonon line ($\star$ symbols) of ${\text{NV}}^{-}$ (respectively, ${\text{NV}}^0$) emission is located at 637 nm (respectively, 575 nm). The inset shows the atomic structure of the NV defect, consisting of a substitutional nitrogen atom (N) associated to a vacancy (V) in an adjacent lattice site of the diamond crystalline matrix.Reuse & Permissions

Figure 2

(Color online) (a) Sketch of the experimental setup combining an atomic force microscope (AFM, Asylum Research MFP-3D-BIO) with an inverted confocal microscope. NDs: nanodiamonds; O: oil immersion microscope objective (Olympus, $\times 60$, $\text{NA}=1.35$); DM: dichroic beamsplitter; F: 580 nm long-pass filter; PH: $50\text{−}\mu \text{m}$-diameter pinhole; FM: flip mirror directing the collected photoluminescence either to an imaging spectrograph equipped with a back-illuminated cooled charge-coupled-device array, or to a Hanbury Brown and Twiss interferometer consisting of two silicon avalanche photodiodes placed on the output ports of a 50/50 beamsplitter. [(b) and (c)] Photoluminescence raster scan of the sample (c) with the corresponding topography image (b). Red dotted circles indicate NDs hosting NV defects. The correspondence between the two images can be easily inferred. (d) Typical second-order autocorrelation function $g^{\left(2\right)}\left(\tau \right)$ recorded for a single NV defect in a diamond nanocrystal.Reuse & Permissions

Figure 3

(Color online) Proportion of negatively charged ${\text{NV}}^{-}$ defects ${\mathcal{P}}_{{\text{NV}}^{-}}$ as a function of the size of the photoluminescent ND. The statistics is determined from a set of 146 single NV defects and the size binning in the histogram was adapted in order to have an approximately equal number of investigated color centers per size slot. The red solid line is a fit of the data using Eq. (6).Reuse & Permissions

Figure 4

(Color online) (a) Raman spectra associated with NDs cleaned with a piranha solution after electron irradiation and annealing (upper red curve) and with subsequent air oxidation at $550°\text{C}$ during 1 h (bottom blue curve). The G band at $1600{\text{cm}}^{-1}$ and the D band between 1250 and $1450{\text{cm}}^{-1}$ are associated to $s{p}^2$-hybridized carbon atoms. Air oxidation leads to a higher purity of $s{p}^3$-bonded carbon since both the D band and the G band are suppressed. Insets show the pictures of the aqueous colloidal solutions before and after air oxidation. (b) Proportion of negatively charged ${\text{NV}}^{-}$ defects ${\mathcal{P}}_{{\text{NV}}^{-}}$ as a function of the ND size after air oxidation [blue (dark gray) points]. The statistics is built from a set of 183 single NV centers. The blue (dark gray) solid line is a fit obtained using Eq. (6). The red (gray) points are reproduced from Fig. 3 for comparison.Reuse & Permissions