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Abstract
In this paper, we experimentally investigate optical and spin properties of NV centers in aggregates of detonation nanodiamonds. We show that despite the small size of nanodiamonds forming the aggregate, the NV centers in these aggregates exhibit spin properties comparable to similar size nanodiamonds grown by other methods, but with brightness enhanced by a factor of 2.
© 2017 Optical Society of America
1. Introduction
Detonation nanodiamonds (DND) are the smallest known diamond structures [1]. DNDs fill first “zone of stability” in diamond; in this zone, surface tension is an influential factor in determining the crystal structure of the diamond. As a result of this, their crystal structure is fundamentally different from that of bulk diamond. Because the crystal structure affects the formation and physical properties of embedded color center defects within the diamond, it is expected that these color centers would behave differently from their counterparts embedded within bulk diamond crystals. However, recent studies of ultra-small (below 7 nm) nanodiamonds [2–5] have shown that even the smallest possible diamonds can still contain nitrogen-vacancy (NV) or silicon vacancy (SiV) color centers with properties similar those found in bulk diamond.
The kind of application that a nanodiamond is suitable for depends on the type of color centers contained within it [6,7]. NV color centers, for example, have been shown to be excellent biocompatible temperature sensors [8–10], highly sensitive magnetometers capable of extreme spatial resolutions [11–15], sensitive probes of electric field [16], non-toxic fluorescent biomarkers [6], nano-scale nuclear magnetic resonance imaging (MRI) probes [17], and robust mechanical strain detectors [18–20]. NV color centers are also actively being developed as a key element of quantum information processing [21].
The exact process by which NV centers form in DNDs is still a matter of some debate in the literature. The presence of nitrogen defects and lattice vacancies in DNDs has been proven by various groups [1,22–24]. Nevertheless, observation of NV centers in isolated DNDs has proven to be challenging [25], with only one to our knowledge group claiming observation of an NV center in a single ultra-small (below 5 nm) detonation nanocrystal [5].
One of the interesting features of ultra-small nanodiamonds is their ability to form stable aggregates [26,27]. Interactions between DNDs depend on surface functionalization and the solvent used. In many cases, these interactions may be stronger than the interaction between the DND and the solvent [28–31]. This interaction can lead to formation of stable bonds between particles. In particular, the use of water as a solvent is known to assist in the formation of aggregates [28,29,31]. Thus, the properties of DND aggregates differ for both ultra-small nanodiamonds and diamonds of a size similar to the aggregate. DND aggregates have been shown to have stable non-quenched NV centers [32,33]. Given their ease of production [26], their large surface area to volume ratio and their porous structure [27], DND aggregates have immediate uses in bio imaging and sensing applications [17,34,35] as well as drug-delivery [36] and other biomedical applications [37,38].
In this work, we investigate both optical and spin properties of DND aggregates containing one or more NV color centers per aggregate. We show that despite their huge surface area, spin properties of DND aggregates are similar to or even better than those of individual nanodiamonds of similar size grown by high pressure high temperature [4], chemical vapor deposition [39] or crashing of bulk diamond methods [40], while possessing a brightness that is 2 times higher. This discovery has quite interesting implications for bio-sensing applications with DND aggregates [34,37].
2. Experimental setup and sample preparation
The DNDs used in this study were obtained from a water slurry of ultra-dispersed ozone-purified nanodiamonds produced by SKN Ltd Snezhinsk. In order to clean surface of the nanodiamonds, we performed commonly used acid boiling cleaning procedure [4,5,41–45] to remove graphite and other impurities from nanodiamond surface. This procedure is also known to reduce possible blinking behavior [4,42] and improve coherent properties of color centers inside small nanodiamonds [4] as well as in shallow implanted color centers in bulk diamond [46].
The cleaning procedure consisted of the following steps. First, the nanodimond suspension was centrifuged and mixed with concentrated solution of and in a ratio 1 to 9 respectively, and kept at for 3 days. Then the solution was centrifuged again and the acid solution was replaced with water. After that the solution was mixed with 0.1 mL of 0.4% aqueous and kept for 2 hours at . The same procedure was then performed with a 0.4% solution. The solution was centrifuged once more, replacing the remaining acid solution with water. The nanodiamonds were separated from solution by centrifugation for 10 minutes at 7000 rpm. A 0.1 mL portion of the solution rich in nanodiamonds was collected from the bottom of the vial. After that, the collected solution was sonicated for 1 hour.
This cleaning and cluster enrichment step is of paramount importance, as evidenced by the fact that no NV centers were found in samples from the same solution prepared without this process. After cleaning procedure, the nanodiamonds were spin-coated at 2000 rpm using Laurell CZ-650 spin-coater onto glass coverslip with a pre-existing gold mask.
To analyze optical and electron spin properties of NV centers within the resulting DND aggregates, we used home built confocal microscope (see Fig. 1(a)) employing high numerical aperture immersion oil objective (NIKON Apo TIRF 100X NA 1.49) with working distance of 120 micrometers. A continuous wave laser (Coherent Compass 300, wavelength 532 nm) served as the source of NV excitation and galvo mirrors (Cambridge Technologies) were used as a scanning element, enabling a 100 × 100 micrometer field of view for our microscope design (see Fig. 1(b)). The collected fluorescence was coupled into a Hunbury-Brown-Twiss (HBT) interferometer that consisted of two avalanche photodiodes (PerkinElmer SPCM-AQRH-14-FC) and a 45:55 beam splitter. We used the combination of an optical notch filter with a stop-band centered at 532 nm and longpass optical filter with cut-off at 600 nm to remove the residual green excitation light and Raman signal from the collected emission. A time-correlated single photon counting module (Picoquant Picoharp 300) was used to obtain second-order photon correlation functions from NV centers (see Fig. 2(c)).
Fig. 1 a) Schematic of the confocal microscope. b) Image of sample with deposited detonation nanodiamonds taken with confocal microscope. Red dashed circles indicate aggregates for which second order correlation function was measured to be less then ½ (see below). c) AFM image of the area, marked with red square in confocal image. Blue line demonstrates direction along which 1D image was taken to determine size of the particle. d) 1D AFM image of nanodiamonds aiding in the visualization of DND aggregate sizes. e) ODMR spectra obtained from nanocrystal marked at figure b.
Fig. 2 a) TEM image of nanodiamond aggregates under investigation, b) Histogram of sizes of nanodiamonds found via TEM microscopy, c) autocorrelation function obtained from single NV center, d) Lifetime measurement for single color center. Fast decay during the first nanoseconds is due to background fluorescence. e) Histogram of lifetimes of NV centers in DNDs. Grey distribution corresponds to single NV centers only (selected by level of second order correlation function below 0.5), yellow distribution corresponds to the total distribution for all aggregates studied.
Atomic force microscopy (AFM) images (Figs. 1(c) and 1(d)) were obtained using NTEGRA Spectra AFM. In order to ensure that the aggregates being imaged by the confocal microscope were the same as those imaged by the AFM, the nanodiamonds were spin coated onto glass cover slip with unique pre-printed gold features (for more details see section 3.2), allowing us to correlate images taken with confocal microscope (see Figs. 1(b) and 1(c)) and AFM setup and measure surface profile of nanodiamonds under investigation (see Figs. 1(c) and 1(d)).
For lifetime measurements, a picosecond diode laser (Picoquant LDH-P-FA-530 XL) was used with the same time-correlation single photon counting module. Due to the fact that lifetime of NV center excited state is much longer than the length of the laser pulse, the fluorescence signal recorded after the end of the applied laser pulse should exhibit exponential decay (Fig. 2(d)). In Fig. 2(d), signal observed consists of two exponents – fast and slow one. The fast exponential component corresponds to fluorescence of the coverslip and other similar background fluorescence signatures that do not have a slowly decaying exponential component. This was verified by focusing the microscope on a region of the glass cover slip that contained no diamonds and measuring the fluorescence decay time collected from this region. We therefore associate long-lived exponential tail in Fig. 2(d) with the NV center luminescence.
3. Results and discussion
3.1 Identification of color centers
The DND aggregates under investigation was confirmed to possess an NV center by measuring following things : an ODMR spectra in the absence of an external magnetic field (Fig. 1(e)), the second order correlation function (Fig. 2(c)) lifetime (Figs. 2(d) and 2(e)), and the optical spectra of object under study. The optical spectrum of these nanocrystal aggregates has few distinguishing features [47], but the ODMR spectrum of NV center does have a sharp line near 2.87 GHz which is mostly independent of the orientation of NV center axes in the crystal lattice in the absence of a magnetic field. This measurement allows for immediate identification of an NV center. Among nanocrystal aggregates that have been found to contain NV centers via the ODMR technique, we first focus on aggregates that demonstrate statistical antibunching with a threshold. This threshold in the second order correlation function corresponds to evidence of single photon statistics. The lifetime measurement taken for these nanocrystal aggregates shows that the average excited state lifetime of NV centers within these nanocrystals is approximately 26 ns long for single color center (see Fig. 2(e)). Diamond nanocrystals of similar size (average size 50 nm, Microdiamond AG (MSY 0.05, GAF)) have average exited state lifetime of about 18 ns [48]. As mentioned above, this longer lifetime is expected if the nanodiamonds within the aggregates are ultra-small, and may also be explained by porous structure of the aggregate. Indeed, lower average refractive index of the aggregate compared to the nanocrystal should reduce the density of states around the emitter which is not completely compensated by an effective change in susceptibility, and will eventually increase color center lifetime [49,50]. In general, nanocrystal aggregates containing fewer NV centers have a higher average lifetime (see Fig. 2(e)). We believe this is due to the fact that aggregates containing several color centers are on average of a larger size (see details below) and therefore the lifetime of their NV centers is closer to that of the lifetime of NV centers in bulk diamond.
It is important to note, that in small nanocrystals observed lifetime is often lower then one in bulk diamond, which is may be associated with high nitrogen concentration [25] or surface defects [51]. In some cases complete quenching of fluorescence for ultarasmall nanodiamonds was observed [25], nevertheless short-lifetime fluorescence was observed at 5 nm diamonds [5]. But, as it was demonstrated previously surface passivation [51] to high degree illuminate effect of surface defects on lifetime and restore reasonable for given size lifetime. In aggregates, surface passivation is also assisted by formation of aggregate, which effectively reduces surface of each nanocrystal in aggregates therefore surface associated quenching is strongly suppressed. Effect of nitrogen concentration may depend on exact diamond production method but there is no evidence of it in our nanocrystals or nanocrystals studied at [51].
In order to better understand the nature of the DND aggregates under study, we performed measurements with a transmission electron microscope (TEM) and an AFM. The TEM images (Fig. 2(a)) were taken on a separately prepared sample from the same solution of nanodiamonds. From the TEM images we were able to discern the typical diameter of individual nanocrystals in the DND aggregates to be 2-5 nm with average diameters of 3.3 nm ± 0.7 nm (Fig. 2(b)).
AFM studies were performed on the same samples that were studied optically. Using the pre-printed gold features, we were able to correlate positions of 25 DND aggregates containing NV centers in the AFM and confocal images. Each of these 25 nanocrystals was larger than 40 nm in diameter. Approximately 22% of aggregates studied (out of approximately 140 aggregates) contain at least one color center. On the other hand, no NV centers were found in areas of the sample having no aggregates but still coated with individual DND nanocrystals. From these observations we concluded that NV centers were only observed in DND aggregates. This conclusion nevertheless does not exclude existence of NV color center in DND itself. In fact, such existence was already proven experimentally [5]. But, very low probability (0.0015%, see Appendix) to have color center per DND and possibly worse charge stability of NV center in single DND make it very difficult to directly use NV center in DND.
3.2 Optical properties of NV centers in DND aggregates
For each aggregate we were able to measure its optical and geometrical properties, including effective number of NV centers inside the aggregate. The NV number measurement was performed by measuring the amplitude of the second order correlation function . To exclude the effect of background noise on these measurements, we used a noise compensation technique described in [52,53]. The second order correlation function is described by the formula:
In Eq. (1), is the photon number. Since a nanocrystal can have more than one color center, may differ from 0. In a nanocrystal possessing NV centers, and assuming each NV center is an identical and independent emitter, the photon number state emitted by such an ensemble will be:Here is probability of photon emission in given time interval. This expression leads to a threshold of the number of NV centers for known value of :
Here are binomial coefficients. These thresholds are not exact for a number of reasons. First, the NV center could have a different orientation and therefore have different excitation and emission probabilities in given mode. Since NV nanocrystals have different orientations, ratio between two emitters in one aggregate are random and could be anywhere between 0 and 1, and thus the actual level of will be lower than thresholds indicated above. The lifetime of each NV center in a given nanocrystal also varies depending on its positions in the crystal, creating a difference in excitation and emission probabilities. The background subtraction method that we used to remove noise is not perfect and some residual level of may be left from the substrate emission, masking actual level of second order correlation function. Finally, finite time resolution and non-zero excitation power also can contribute to the measured value [52]. Therefore, we use solution of Eq. (3) as a continuously varying parameter to estimate the actual number of color centers in aggregate:
Using Eq. (4), we found a correlation between the effective number of NV centers with the aggregate size (see Fig. 3(a)). Despite a relatively broad distribution of second order correlation functions for any given size of the aggregates there is a clear trend demonstrating a larger number of NV centers in larger aggregates. Figure 3(a) shows the fit of the observed number of NV centers with volume of the aggregate (calculated by taking the cube of the measured radius), which is in agreement with experimental data. This observation confirms our conclusion that only aggregates contain NV centers. Furthermore, we investigated dependence of NV center fluorescence lifetime on the effective number of NV centers in the cluster (see Fig. 3(b)). Again we found that large clusters containing more NV centers demonstrate smaller lifetime, as expected [49,50]. This measurement also confirms our conclusion that only clusters of DNDs contain NV centers, since the lifetime in individual nanodiamonds should not have any correlation with the aggregate size.
Fig. 3 a) Dependence of effective number of NV centers per aggregate from aggregate radius. Red curve represents fit with cubic dependence. Effective number of NV center was deduced from (see section 3.2). b) Dependence of lifetime of NV centers on nanocrystal size, Red line represent fit curve . c) Saturation curve for a single nanocrystal with . Solid curve represents fit to saturation curve (Eq. (5)), dashed orange line is the background contribution to the counts detected. d) Distribution of saturated counts for NV centers with for DND aggregates (yellow) and 50 nm nanocrystals (grey).
As a next step in our research, we measured saturation curves for single color centers in our aggregates. Since DNDs have a porous structure, it is expected that emission from the NV center is less affected by total internal reflection in diamond. To verify this, we analyzed the emission of 32 aggregates at the point of saturation and compared this saturated emission to the saturated emission of 30 individual nanocrystals with 50 nm average size on the same substrate. To find maximum counts possible for given NV center we fitted dependence of the number of counts registered from NV centers to excitation power (Fig. 3(c)) using the formula:
Here is coefficient characterizing background counts, is the power of the excitation light, is saturation intensity, and is extrapolated maximum number of counts for given NV center. Figure 3(d) demonstrate the statistics of the maximum possible brightness for an NV centers. From this it was found that the aggregates have average fluorescence count rate of and are approximately 2.2 times brighter than NV centers in individual diamond nanocrystals with diameter about 50 nm size (average brightness , see Appendix for sample details).
3.3 Electron spin properties of NV centers in DND
We began measurements of the spin properties of NV centers in DND aggregates by employing an optically detected magnetic resonance (ODMR) regime. We used a permanent magnet in front of the glass cover slip to separate magnetic sublevels. This was enough to create magnetic field of about 20 G (see Fig. 4(a)). Each ODMR resonance has a width of a few MHz, determined by hyperfine splitting of and states that results in the triplet spectral feature (Fig. 4(b)). The hyperfine splitting due to the presence of the nitrogen nuclear spin is clearly resolved, which is remarkable for a nanocrystal sample. The ODMR resonances were fitted with Lorentzian model, and the half width of each of the hyperfine structure spectral lines was around 1 MHz.
Fig. 4 a) Splitting of ODMR resonances in the magnetic field b). ODMR resonance detected in single NV contained in DND. c) Rabi oscillations of NV center electron spin under MW excitation (2822 MHz). d) Measurement of coherence time using echo sequence. e) Time sequence used to measure Rabi oscillations, f) Time sequence used to measure Hahn echo time.
Once the magnetic sublevel was isolated, we performed coherent manipulation of the electron spin. First, we measured Rabi oscillations of the spin under excitation by a microwave field. To measure Rabi oscillations, we vary the pulse duration and measured number of counts at two different times: after spin polarization with green light but before the application of the microwave pulse and right after the microwave pulse (see Figs. 4(e) and 4(f)). The observed spin contrast [54] was calculated using Eq. (6) and plotted (Fig. 4(c)):
The Rabi oscillation were found to have a frequency of 10.4 MHz, corresponding to pulse of 96 ns in duration and a coherence time of . Measurement of using a Ramsey sequence gave the same result (see Fig. 5); implementation of a Hahn echo sequence allow us to extend coherence time of about (Fig. 4(d)). This coherence time is typical for nanodiamonds [4,39]. Despite the fact that high quality NV centers implanted in isotopically pure diamond [55,56] demonstrate much better coherence properties, cheap and easily produced DND aggregates are strong candidates for biomedical application, especially for in-cell studies.
Fig. 5 Comparison of measurement of measured via envelop of Rabi oscillation (a) and Ramsey sequence (b). For this aggregate fitted time with Rabi sequence was 873 ns and with Ramsey sequence 863 ns. Red line indicates the fit, dashed lines correspond to the exponential decay of the Rabi oscillations.
4. Summary
We experimentally studied aggregates of DNDs containing single or few NV centers. We observed the presence of NV centers inside DND aggregates, but not in separate nanocrystals. We also found that DND aggregates with NV centers possess superior coherence properties (with Hahn Echo coherence times up to ) at room temperature compared to commercially available nanocrystals of similar size. These DND aggregates were also found to be at least twice as bright as individual nanocrystal of similar size grown by other methods. Although isotopically pure diamonds still demonstrate much better coherence properties, the coherent properties of aggregates are comparable to those of shallow implanted NVs in natural or non-purified synthetic diamond. The high brightness of DND aggregates and their porous structure make them quite promising candidates for bio-imaging and medical applications.
5 Appendix
Calculation of the probability of finding a NV in a single DND
To find out probability of finding NV center in single DND crystal we analyze 6 AFM maps. All AFM maps contain 178 aggregates with sizes larger 40 nm in diameter (see Fig. 3(a)), 25 among them totally contain 39 NV (see Fig. 3(a)). Therefore, we can calculate probability to find NV in aggregate as . Mean measured height of aggregate from AFM 1D profiles is 80.4 nm and mean size of single DND crystal obtained from analysis of TEM images is 3.3 nm (see Fig. 2(b)). Therefore, mean amount DND crystals in aggregate can be compute as . Using this number, we can calculate probability to find NV center in single DND crystal as .
Measurement of coherence time
For fitting time we used simple model with exponential coherence decay [40,57,58]. In case of Raby oscillation we used as fitting model:
where is Rabi frequency, is technical delay, is time, is amplitude and is level of completely decayed Rabi oscillations. Ramsey sequence was fitted by simple model [59]which take into account hyperfine splitting of the ODMR spectral line. Here amplitudes of 3 hyperfine components, is the detuning from exact resonance with central hyperfine component in MHz, is units conversion constant, is time in nanoseconds. We note, that Ramsey signal is easier to analyze at relatively low powers, at which one hyperfine component is dominating the others.Sample with 50 nm nanocrystals
The sample with 50 nm nanocrystals used for brightness comparison in Fig. 3(d) was prepared in following way. The surface of nanodiamonds (Microdiamond AG MSY 0.05, GAF) was treated with COOH groups following similar procedure as one used for DND. Detailed recipe could be found at [45]. Then solution of nanodiamonds was spin coated onto microscope coverslips (MENZEL-GLÄSER) at 2000 RPM for 2 minutes.
Funding
Russian Foundation for Basic Research (4–29–07127); Ministry of Education and Science of the Russian Federation (14.B25.31.0007).
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