Nitrogen-vacancy colour centres in diamond can undergo strong, spin-sensitive optical transitions under ambient conditions, which makes them attractive for applications in quantum optics 1 , nanoscale magnetometry 2,3 and biolabelling 4 . Although nitrogen-vacancy centres have been observed in aggregated detonation nanodiamonds 5 and milled nanodiamonds 6 , they have not been observed in very small isolated nanodiamonds 7 . Here, we report the first direct observation of nitrogen-vacancy centres in discrete 5-nm nanodiamonds at room temperature, including evidence for intermittency in the luminescence (blinking) from the nanodiamonds. We also show that it is possible to control this blinking by modifying the surface of the nanodiamonds.Detonation nanodiamond is routinely produced on an industrial scale, and the raw material can be disintegrated into a stable 5-nm monodisperse colloid 8 . The combination of inert core and chemically reactive surface, which can host a variety of moieties, is appealing for chemists, biologists and material scientists 9,10 . Quantum magnetometry 2,3 is an example of an emerging technology that will directly benefit from the availability of nanocrystals with welldefined sizes in the 5-nm range, because the sensitivity to single spins is inversely proportional to the cube of the distance between the sensor (that is, the nitrogen-vacancy (NV) centre) and the spin being detected.Producing and detecting NV colour centres in isolated 5-nm detonation nanodiamond has been controversial, and there has been some scepticism regarding their stability as a useful emitter in a discrete crystal. For example, theoretical calculations of the crystal energy budget favour the location of nitrogen on the surface rather than in the core, which seems to explain the limited observation of NV centres in chemical vapour deposition and high-pressure high-temperature grains of less than 40 nm in size 11,12 , and favours the prediction that nanodiamonds smaller than 10 nm in size do not contain NV centres 7,13 . Although sub-10-nm nanodiamonds with NV centres have been produced using a top-down approach (milling luminescent high-pressure hightemperature microdiamonds into 7-nm particles 6,14 ), the question of NV stability in isolated detonation nanodiamonds persists.In aggregated detonation nanodiamonds (agglomerates and agglutinates 8 ), high-sensitivity, time-gated luminescence and electronic paramagnetic resonance spectroscopy have been used to extract a weak NV signal from a strong luminescence background 5 . The experiments highlight the eclipsing nature of the graphitic surface layers in nanodiamond aggregates-NV centres were simply not visible through the broadband luminescence from the surface and grain boundary material. To distinguish the NV spectral signature from the large grain boundary luminescence overhead, diamond synthesis yielding discrete sub-10-nm detonation nanodiamonds is vital. Here, we use a robust deaggregation and dispersion method, which diminishes the crystal-crystal interaction to...

The negatively-charged nitrogen-vacancy (NV − ) center in diamond is at the frontier of quantum nano-metrology and bio-sensing. Recent attention has focused on the application of high-sensitivity thermometry using the spin resonances of NV − centers in nano-diamond to sub-cellular biological and biomedical research. Here, we report a comprehensive investigation

Various types of luminescent color centers made in diamond by substitution of carbon with nitrogen, [1] nickel, [2] silicon, [3] and/or a vacancy have been of interest for applications in many fields. One of the most widely used ways for making diamond luminescent involves substitution of one carbon atom with nitrogen and creation of a vacancy at a location adjacent to the nitrogen atom, thus forming a nitrogen-vacancy (NV) color center. [1] NV-photoluminescent diamonds are extremely photostable, [4] biocompatible, [5] exhibit amiable surface chemistry, [6] and show optically detectable sensitivity to magnetic fields. [1] Although the production of 25-nm luminescent diamond based on high-temperature high-pressure (HTHP) synthesis [7,8] has already brought exciting results in quantum physics [1,9] and the life sciences, [4,7,10] crystals no larger than a few nanometers will break ground in these applications [3,6,11] and other fields. [12,13] Functionalized single-digit nanodiamonds (SNDs) may be used to track biomolecules with minimal steric and biochemical perturbations, are small enough to show detectable quantum interactions between NV centers located in different crystals, [14] and will facilitate the realization of high-resolution magnetic [12,15] and near-field optical microscopes. [13] SNDs have recently been produced by breaking detonation-synthesized nanodiamonds [16] into 5-nm primary crystals [17] but no progress has yet been reported towards embedding NV centers into SNDs. Furthermore, there is growing concern that NV centers in SNDs cannot form due to physical barriers, such as the proximity to surface traps and reduced stability of defects. [3,18,19] A study of NV centers in similar-scale diamond grains created with chemical vapor deposition found no NV centers in crystals smaller than 20 nm. [18] Furthermore, theoretical work suggests that nitrogen becomes less energetically stable in the core of nanodiamonds as they become smaller. [20] It has also been suggested that luminescence may be quenched by nearby surface defects, [18] and that high levels of oxygen and other impurities in detonation-synthesized diamond may affect the formation of NV centers. Intrinsic short-lived luminescence from surface defects in SNDs further confounds the issue. [21] Herein, we examine the properties of weakly bound clusters of SNDs by using spectrally and temporally resolved luminescence detection, electron paramagnetic resonance (EPR) spectroscopy, and transmission electron microscopy (TEM), and present the first report of the successful detection of NV centers in 5-nm diamond. Furthermore, we provide a simple physical argument on why the probability of creating a color center in a small crystal scales as the fifth power of the crystal size.NV centers in diamonds were created by high-energy proton irradiation followed by thermal annealing (see Experimental Section). For luminescence measurements, samples containing equal weights of 55-nm HTHP diamonds and SNDs were uniformly distributed on quartz substrates. Prist...

Recent experimental and theoretical studies concerning single-molecule spectroscopy in solids are discussed. Pure quantum effects--such as photon bunching, antibunching, and spectral jumps--and more classical phenomena--such as near-field excitation, saturation, ac/dc Stark shifts, spectral diffusion, two-photon excitation, and customary spectroscopic analysis--are considered. The emphasis of this review is on physical results and their interpretation. This is preceded by a general introduction, where fundamentals of single-molecule spectroscopy are explained.

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Nanoscale thermometry is of paramount importance to study primary processes of heat transfer in solids and is a subject of hot debate in cell biology. Here we report ultrafast temperature sensing using all-optical thermometry exploiting synthetic nanodiamonds with silicon-vacancy (SiV) centres embedded at a high concentration.Using multi-parametric analysis of photoluminescence (PL) of these centres, we have achieved an intrinsic noise floor of about 10 mK Hz −1/2 , which is a thousand-fold increase in the readout speed in comparison to the current record values demonstrated with all-optical methods of comparable spatial-resolution and precision. Our thermometers are smaller than 250-nm across but can detect a 0.4 • C change of tem-

We demonstrate a temperature noise floor of 0.3 K Hz(-1/2) and a long-term stability better than 0.6 K (peak-to-peak value) using a single crystal of diamond smaller than 50 nm across and containing about 100 nitrogen-vacancy centres as a temperature sensor. We compare the achieved characteristics to other single-particle sensors and show that it is one of the best ratiometric all-optical nano-probes of temperature to date.

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