Observation and control of blinking nitrogen-vacancy centres in ...

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LETTERSNATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2010.56ab500 nm500 nmcZ (nm)7d655 nm 34322100 10 20 30 40X (nm)1Intensity (a.u.)g (2) (τ)1.51.00.50.0−75 0 75Delay, τ (s)100 nm0640 720 800 880Wavelength (nm)Figure 1 | Characterization of discrete 5-nm diamonds on a glass coverslip. a, AFM image of nanodiamonds. The brightness of the spots is proportional tothe height of the crystals (circles highlight the correspondence between the AFM image and the confocal image of b). b, Corresponding confocal scanningfluorescence microscopy image. Bright spots indicate NV emitters. c, Magnified AFM image and corresponding surface profile (inset) of a representativenanocrystal 5 nm in height. d, Emission spectrum of an NV centre in a 5-nm crystal host and corresponding second-order correlation function g (2) (inset).a2b 4.6Intensity (a.u.)1(i)(ii)Counts (×10 3 )4.44.24.00600 660 720 780 840Wavelength (nm)3.82,840 2,880 2,920Frequency (MHz)Figure 2 | Luminescence and magnetic resonance spectra from detonation nanodiamond. a, Representative NV centre spectra, as acquired innanodiamonds incorporated in aggregates and/or a PVA film (i) and in the free-space sample (ii). b, Optically detected magnetic resonance spectrumassociated with the luminescence spectrum (i), showing the two characteristic magnetic resonance dips indicating a strain-induced splitting of the m s ¼+1spin sublevels typical of NV centres in nanodiamonds.probably included a strong sp 2 -bonded carbon luminescence fromthe grain boundary of the nanodiamond agglomerates, which overshadowedthe NV luminescence (centred at 700 nm, compare withFig. 1d), but not the ODMR signal. ODMR was undetectable withour apparatus in the free-space nanodiamonds. We attribute thisto strong dephasing owing to the distorted crystal order and aweak signal exacerbated by the reduced count rate due to blinking.The observation of luminescence from single, isolated NVcentres in 5-nm detonation nanodiamonds, which has been thesubject of intense debate, is important because, until now, luminescencefrom ≏5-nm crystals has been observed only in detonationnanodiamond agglomerates 5 . Furthermore, and in contrast toagglomerates (where the diamond surface is largely encased), wealso observed a new property of NV centres: luminescence intermittencyor ‘blinking’. NV has been described as an extremely photostableemitter immune to bleaching 1 or blinking, as representedby a typical luminescence time trajectory (Fig. 3a) acquired in asample prepared using the polyvinyl alcohol (PVA) polymersample preparation method 6 .Figure 3c shows a representative blinking luminescence time trajectoryfrom a nanodiamond containing an NV centre, in which thediamonds were prepared as described in the Methods. Blinking was2NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology© 2010 Macmillan Publishers Limited. All rights reserved.
NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2010.56LETTERSabPhotocounts per4-ms bin400200Photocounts per4-ms bin4002000 1 2 0 400 800Time (s)OccurrencecdPhotocounts per4-ms bin400200Photocounts per4-ms bin400200eOccurrence01 2 0 400 80010 0 10 0Time (s)Occurrence10 3 f 10 39020601710 210 214300.4 0.6 0.80.4 0.6 0.8Laser intensityLaser intensity10 1(MW cm −2 )10 1(MW cm −2 )τ on (ms)Occurrenceτ off (ms)0 75 150 225 300075150‘On’ time interval (ms)‘Off’ time interval (ms)Figure 3 | Detailed analysis of blinking statistics from an NV centre in detonation nanodiamond. a–d, A 2-s fragment of a 200-s luminescence timetrajectory of the NV centres sampled into 4-ms bins from two separate diamond samples: NV luminescence from nanodiamonds embedded in a PVApolymer film (a), with corresponding histogram fitted with a single Gaussian (b), where the line in a is the mean count rate; a single isolated NV innanodiamond displaying intermittent luminescence (c), with corresponding histogram of the full time trajectory fitted with two Gaussians (d). Gaussians arefitted to the ‘on’ and ‘off’ state photon distributions and a line is drawn through c to indicate the on/off ‘threshold’ for analysing the emission statistics.e,f, Blinking statistics of the ‘on’ and ‘off’ states on a semi-logarithmic scale (symbols). A single exponential is fitted in both cases, with time constants 45and 18 ms, respectively. Insets to e and f plot blinking time constant versus the excitation laser intensities for ‘on’ and ‘off’ states, respectively.observed in ≏25% of the luminescent sites. The ‘off’ state was at thelevel of the background signal, and blinking due to photochromism16 was ruled out as the neutral NV luminescence signaturewas never observed in the spectra. Photobleaching was also observedon some of the centres; however, those analysed in this Lettershowed luminescence over several hours of illumination. We analysedblinking by sampling the luminescence time trajectories at4-ms time bins and analysed the data using accepted signal-processingalgorithms, in which the position of the signal relative to athreshold level determines the assignment of the states 17 . The blinkingphenomena were also characterized as a function of the excitationintensity (shown in the insets of Fig. 3e,f, and detailed inthe Supplementary Information).The 4-ms bin width was chosen because it provides the deepestminimum between the two Gaussian curves shown in Fig. 3d.Averaging data into 4-ms bins on the one hand ignores the rapidevents that may display complex dynamics 18 , but on the otherhand decreases the noise and emphasizes the two-state NV-nanodiamondblinking system, as shown in Fig. 3d. Changing the bintime to 2 ms did not change the result, and with 1-ms bins thestatistical noise was too pronounced for a meaningful result.These two states are identified as ‘on’ and ‘off’ 19 . Quantum dots 20 ,single molecules 21 , silicon nanocrystals in a porous siliconmatrix 22 and others 23 are well-known systems that show blinking.The validity of the two-state on/off model is intensely debated 24 ,especially in the context of quantum-dot studies, where, forexample, three states 25 and a continuous distribution of states 26have been reported. In our case, the validity of the two-statemodel was underpinned by the photon distribution in ‘on’ and‘off’ states based on the Gaussian distribution and expressed as⎡ ( ) 2⎤PN ( ) = p N − ¯N onon√⎢⎥exp⎣−⎦2p son2s 2 on⎡ ( ) 2⎤+ p N − ¯N offoff√⎢⎥exp⎣−⎦2p soff2s 2 offwhere ¯N represents the mean value of the number of photons ineach state, p is the population of the states, N the total number ofphotons and s 2 the variance of the distribution. For a stablephoton emission, the variance equals the mean value. The fit tothe distribution reveals s 2 on to be only about two times largerthan the mean ¯N on . This determined our choice of the thresholdlevel between the ‘on’ and ‘off’ states, which was set at the intersectionof the two Gaussians (Fig. 3c,d), where the probabilities for N tobe at the ‘on’ and ‘off’ levels were equal. The distribution of the ‘on’and ‘off’ times are plotted on a semi-logarithmic scale in Fig. 3e,f,NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology 3© 2010 Macmillan Publishers Limited. All rights reserved.
Page 1: LETTERSPUBLISHED ONLINE: 11 APRIL 2
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