Removing Non-Size-Dependent Electron Spin Decoherence of

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Removing Non-Size-Dependent Electron Spin Decoherence of Nanodiamond Quantum Sensors by Aerobic Oxidation Ryuta Tsukahara, Masazumi Fujiwara, Yoshihiko Sera, Yushi Nishimura, Yuko Sugai, Christian Jentgens, Yoshio Teki, Hideki Hashimoto, and Shinichi Shikata ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00614 • Publication Date (Web): 03 May 2019 Downloaded from http://pubs.acs.org on May 6, 2019

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ACS Applied Nano Materials

Removing Non-Size-Dependent Electron Spin Decoherence of Nanodiamond Quantum Sensors by Aerobic Oxidation Ryuta Tsukahara,†,§ Masazumi Fujiwara,∗,‡,†,§ Yoshihiko Sera,† Yushi Nishimura,‡ Yuko Sugai,† Christian Jentgens,¶ Yoshio Teki,‡ Hideki Hashimoto,† and Shinichi Shikata∗,† †School of Science and Technology, Kwansei Gakuin University, Sanda-shi, Hyogo, 669-1337, Japan ‡Department of Chemistry, Osaka City University, Sumiyoshi-ku, Osaka, 558-8585, Japan ¶Microdiamant AG, Kreuzlingerstrasse 1, CH-8574 Lengwil, Switzerland §Contributed equally to this work E-mail: [email protected]; [email protected]

Abstract

Keywords

Surface oxidation of nanodiamonds (NDs) is a primary step of their surface functionalization that is a key to the success of their recent emerging applications in nanoscale quantum sensors in biological samples. Here we investigate how the electron spin coherence of single nitrogen-vacancy (NV) centers in NDs is extended by two major oxidizing techniques, i.e. aerobic oxidation and anaerobic tri-acid oxidation with various processing parameters. Aerobic oxidation at 550 ◦C most effectively oxidizes the surface and extends T2 by a factor of 1.44 ± 0.33 to the original NDs. The ND-size dependence of this T2 extension shows that aerobic oxidation removes a constant decoherence contribution irrespective of the ND size, which clearly separates its origin from the surfacederived decoherence sources. The present results highlight the presence of the ND-specific decoherence sources other than surface termination spin noise and spin-active impurities, thereby improving the spin coherence of ND quantum sensors.

Nanodiamond, NV center, Coherence, Oxidation, Surface functionalization

Introduction Nanodiamonds (NDs) incorporating lightemitting color-defect centers have a great potential for applications in quantum information devices and quantum nanoscale sensors. Of particular interest are nitrogen vacancy (NV) centers, which allow optical access to coherently controllable electron spin systems, leading to attractive applications, such as spin-photon entanglement devices, 1–6 MRI contrast agents, 7,8 and nanoscale sensors. 9–14 The electron spin properties of NV centers are the key to achieving better performance of these applications. In particular, the transverse spin-coherence time (T2 ) limits the spin memory time in quantum devices and the sensitivity of quantum sensing. There has been a significant effort to extend T2 in both bulk diamonds and NDs. 15–23 However, T2 in NDs is still one or two orders of magnitude shorter than those in bulk diamonds because of the complicated surface effects, whose

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Results and Discussion

exact decoherence mechanism has not been clarified. 18,21,24,25 Surface oxidation is a primary step to realize most of the quantum applications of diamonds. For example, surface oxidation effectively extends T2 of NV centers; Ohashi et al. reported that oxygen termination by an anaerobic oxidation with a tri-acid mixture significantly extends T2 of near-surface NV centers of high-purity bulk diamonds. 19 Recently, aerobic oxidation has been used as an efficient oxidizing technique for high-nitrogen content bulk diamonds. For NDs, both types of the oxidation techniques have been confirmed to extend T2 . 18,26–29 However, the detailed effect of these oxidation methods are still not clear owing to their complicated chemical process where modification of the surface termination and removal of the surface-covering carbon soot take place simultaneously. 27,30,31 Furthermore, in recent studies, these two techniques have come to be used sequentially, or in combination with other techniques to facilitate further chemical processes of surface functionalization. 26,32–36 However, the effect of the differences in the oxidation techniques on T2 of NV centers in NDs has not been clarified. It is therefore mandatory to clarify the differences in the oxidation techniques when building up the more sophisticated surface termination protocols. In this study, we investigate how T2 of single NV centers in NDs of various sizes is affected by different oxidation methods and processing parameters. With surface-sensitive spectroscopies including Raman, XPS and FTIR, the surface properties are quantitatively characterized. Aerobic and anaerobic-acid oxidations are substantially different in their effect on both T2 and the surface properties. We find that aerobic oxidation most effectively oxidizes the surface and extends T2 . The ND-size dependence of T2 shows that aerobic oxidation removes a constant source of decoherence irrespective of the ND size; this decoherence source is clearly different from the surface-derived decoherence sources.

Surface oxidation and etching of NDs We prepare multiple sets of NDs oxidized by aerobic or anaerobic acidic oxidation under different temperatures and with different processing times (Scheme 1 and see Methods). For the aerobic oxidation step, NDs are heated in an oven in the presence of air at 450 or 550 ◦C. For the acidic oxidation step, a tri-acid mixture of sulfuric, nitric, and perchloric acids (volume ratio 1:1:1) is used at 90 ◦C. These NDs are suspended in water with polyvinyl alcohol (PVA) of 1 wt% and spin-coated on cleaned coverslips for the size characterization using an atomic force microscope (AFM) (see Fig. S1). Figures 1a–f show the particle size distributions; pre-cleaned (hereafter designated as 1), tri-acid oxidation for 6, 24 h (2a, 2b), aerobic oxidation at 450 ◦C for 2 h (3a), and aerobic oxidation at 550 ◦C for 1, 2 h (3b, 3c), respectively. The lognormal fitting gives the mean particle sizes of 28.3, 25.3, 22.3, 25.0, 24.7, and 22.5 nm, respectively, for Figs. 1a–f (the particle sizing error is ∼ ± 1.0 nm (see Table S1 for the detail). The etching rates of the aerobic oxidation step at 450 and 550 ◦C are 2.30 nm/h and 3.04 nm/h (Fig. 1g), respectively, and are constant over the observed size range as reported previously. 16,27,28,37 Tri-acid oxidation also etches the NDs with a constant etching rate of 0.25 nm/h (Fig. 1h). We confirmed that an excessively long processing time of oxidation, such as aerobic oxidation at 550 ◦C for 6 h or triacid oxidation for 72 h, significantly reduced the number of NV centers in the following optically detected magnetic resonance (ODMR) experiments owing to over-etching; 16,28 the lower boundary to obtain enough number of NV centers for the ODMR measurements is estimated to be 15–20 nm (see Fig. 1g,h). Therefore, in this study, we focus on the oxidation range in which we can find enough number of NV centers for obtaining the statistical values of T2 . Note that the incorporation of PVA for the ND spin-coating may affect the determination of the absolute sizes of the NDs. However, such


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Scheme 1: Procedures of pre-cleaning and surface oxidation of NDs. Upper panel: As-received NDs with a particle-size range of 0–50 nm are subjected to pre-cleaning to give 1, which is followed by the steps of tri-acid oxidation (2a, 2b) and aerobic oxidation (3a, 3b, 3c). Lower panel: As-received NDs with a particle-size range of 0–100 and 0–200 nm are subjected to pre-cleaning to give 4a and 4b, which is followed by aerobic oxidation (5a, 5b).

an effect should not be different for each of the present NDs and the relative differences in size determined between the ND samples presented in Fig. 1 are significant.

The intensity ratio of the diamond line and G band (ID /IG ) is shown in Fig. 2b. The aerobic oxidation at 550 ◦C decreases the G band and intensifies the diamond line (3b, 3c), resulting in an increase in ID /IG . At 550 ◦C, the G band has already disappeared after an oxidation duration of 1 h (3b), and no significant change is observed during further oxidation lasting for up to 2 h (3c). The temperature of 450 ◦C, to which the sample was heated, has a moderate effect (see Fig. S3 for longer duration of the etching), and most of the G band remains after a duration of 2 h of processing time(3a). In contrast, the tri-acid oxidation does not seem to remove the sp2 -like materials with the temperature set at 90 ◦C (2a, 2b), while the ND size is decreased: with processing times of 6 h and 24 h, the sample does not show significant differences in the Raman spectra, while the ND size is substantially decreased for these durations. Further extension of the processing time etches the NDs (see Fig. 1h), which however, results in a further size reduction, thus eliminating the presence of NV centers. These results clearly show the effectiveness of aerobic

Surface characterization We next characterize the surface properties of these ND samples using Raman spectroscopy, X-ray photo-emission spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR), which can provide detailed information of the surface. For this surface characterization, we use the ND samples without including PVA (see Methods). Figure 2a shows the Raman spectra of the ND samples excited at 325 nm, which is resonant with a band-to-band transition. A sharp peak observed at 1333 cm−1 is the symmetric stretching mode of the C-C bond of the diamond lattice structure (the so-called diamond line) and a broad peak observed at around 1600 cm−1 is the so-called “G” band of the surface-covering sp2 -like materials that includes sp2 carbon, sp2 clusters and mixed sp2 /sp3 as explained elsewhere. 26,27,38–42


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c Mean = 25.3 nm

Mean = 28.3 nm

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e Mean = 25.0 nm

Mean = 22.3 nm

f Mean = 24.7 nm

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Figure 1: Particle-size-distribution histograms of (a) pre-cleaned (1, N = 450), (b) tri-acid oxidation for 6 h (2a, N = 888), (c) tri-acid oxidation for 24 h (2b, N = 948), (d) aerobic oxidation at 450 ◦C for 2 h (3a, N = 236), (e) aerobic oxidation at 550 ◦C for 1 h (3b, N = 254), and (f) aerobic oxidation at 550 ◦C for 2 h (3c, N = 468). Here, N is the number of analyzed particles. The solid lines indicate the log-normal fitting to the data. The mean particle sizes based on the fitting are shown in each figure. (g) The ND size as a function of processing time for the aerobic oxidation and (h) tri-acid oxidation. The solid lines are linear fitting to the data. oxidation in removing the surface sp2 -like materials of NDs. Figure 2c shows XPS spectra of the ND samples. The prominent peak observed at 284.8 eV is ascribed to 1s of sp3 C-C bond and observed in all NDs. 43 The NDs with aerobic oxidation at 550 ◦C (3b, 3c) have peak tails in the higher energy side in the region of 286–288 eV. These tails are convolutions of the oxygen-related bonding peaks such as ester (C-O), carbonyl (C=O), and carboxyl (COOH); 44,45 however, the energy difference of these functional groups cannot be detected

owing to surface inhomogeneity. Further, we cannot obtain detailed information of surface termination from the present XPS data only, which is in striking contrast to the case with bulk diamonds, where even the surface coverage ratio of the functional groups is determined through XPS using AlKα spectral line. Note that one may resolve the peaks of the oxygen-related bonding of NDs with a more sophisticated method such as synchrotron-based wavelength-dependent XPS. 27 Figure 2d shows FTIR spectra of the present NDs (see Fig. S4 for the detailed assignment).


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b

a G

d

c

Figure 2: Surface characterization of ND samples using various spectroscopies. (a) Raman spectra (325 nm excitation) and (b) the peak ratio of the diamond line and the surface sp2 -originated band (ID /IG ), where the peak area is regarded as their intensities. (c) XPS spectra of the NDs near C1s transitions. (d) FTIR spectra of the NDs. the presence of carboxyl. 48 These results of the surface-sensitive spectroscopies show that aerobic oxidation effectively removes the sp2 -like materials that cover the ND surface and performs surface termination with oxygen, while the surface inhomogeneity of NDs prevents the formation of uniform and well-defined surfaces that one can obtain in bulk diamonds. 44 Note that, because of the aerobic oxidation at 450 ◦C, the G band is slightly decreased when the processing time is extended up to 6 h (see Fig. S3). It has been reported that around 450 ◦ C is the optimal temperature for aerobic oxidation as a trade-off between the removal of sp2 -like materials and ND size reduction. 37,49 It may be possible that the actual temperature in our electric oven is somewhat lower than the reported conditions because of the machine-

A strong peak is observed in the range of 1000– 1300 cm−1 , which is ascribed to the -C-OC- stretching vibrations of cyclic ethers. 46 The peaks at 1307 and 1451 cm−1 that appear very weakly in the anaerobically oxidized NDs (2a, 2b) and prominently in the aerobic ones (3a, 3b, 3c) are attributed to CO bending vibration and asymmetric -CH bending vibration, respectively. 47 A peak appearing at 1627 cm−1 observed in all of the NDs is owing to the -OH bending vibration, which either comes from the carboxyl group (-COOH) of the ND surface or water molecules adsorbed on the sample surface. 39 A broad peak of 3500–4000 cm−1 is similarly because of the -OH stretching mode. A peak at 1705 cm−1 is an acidic form of -COOH and is only observed in the tri-acid oxidation (2a, 2b). A peak at 1782 cm−1 results from the C=O stretching mode and is attributed to


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dependent difference of the real temperature from the initial preset value. However, the present condition of 450-◦C aerobic treatment already extends the T2 as described below. The extension of T2 in the presence of significant sp2 -like materials is an indication that the aerobic oxidation does not affect only the surfacetermination but also the bulk properties of NDs. While aerobic oxidation has already proven effective with NDs, the effect of tri-acid oxidation is not clear at present. Tri-acid oxidation significantly etches the ND particles (reduces the ND size), as seen in Fig. 1, however, the surface sp2 -like materials have not been removed thoroughly. The spin coherence of NV centers is also not prolonged by tri-acid oxidation, as we will see in the following section. Because the etching rate of tri-acid oxidation is significantly smaller than that of aerobic oxidation and the sp2 -like materials are not removed, the tri-acid oxidation in the present experiment does not seem to be simply etching the surface. The detailed chemical-reaction process of ND surface oxidation needs to be clarified for the further understanding of this phenomena, as Bradac et al recently reported the investigation of the detailed chemical process of aerobic oxidation and other types of anaerobic oxidations. 26 It is also required to take into account the difference in the chemical composites used for the anaerobic oxidation; the present study only analyzes the case of the so-called tri-acid oxidation. While the tri-acid oxidation is quite popular in the surface preparation of NDs, other oxidants should oxidize the ND surface in a different way, which may influence the results of the T2 extension.

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periments on the basis of antibunching detection in the second-order photon correlation histogram and the fluorescence spectrum (Fig. 3b– d), we identified single NV centers that are negatively charged. We determine T2 of multiple ND particles (N ≥ 20) by sequentially applying CW, Rabi, and spin-echo measurements to either of the spin transitions of |0i → |±1i (see Figs. 3e–g, Methods and Fig. S5 for the experimental details). A weak external magnetic field of ∼ 30 Gauss is applied to separate the two ODMR peaks sufficiently to prevent mutual interference between the split peaks in the pulsed ODMR measurements (Fig. 3 h). From the Rabi oscillations, we determine the duration of π-pulse for the spin-echo measurement (Fig. 3i). The spin-echo profile is fitted with a single exponential decay to determine T2 (Fig. 3j). The approximation of the decay profile as a single exponential is validated for a short coherence time, as described elsewhere. 17,18,24 Figures 4a–e show statistical histograms of T2 of the ND samples, 1, 2a, 3a, 3b, and 3c, respectively. Table 1 summarizes the statistical figures (mean and maximum). The pre-cleaned NDs (1) show the mean T2 of 1.8 ± 0.3 µs with a longest T2 of 4.7 µs. The aerobically oxidized NDs show the means of 2.1 ± 0.3 µs, 2.5 ± 0.3 µs, and 2.6 ± 0.4 µs for 3a, 3b, and 3c, respectively. The longest T2 for aerobic oxidation is found in 3c. The mean T2 is increased by factors of 1.17 ± 0.26 (3b), 1.39 ± 0.29 (3b), and 1.44 ± 0.33 (3c), compared to the pre-cleaned NDs (1). The results show that aerobic oxidation is almost complete in a processing time of an hour and at a temperature of 550◦C. In contrast to the aerobic oxidation, the tri-acid oxidation (2a) does not show any changes in T2 compared to 1. It is quite interesting to see that T2 is not extended in 2a despite the fact that its surface spectroscopic data (Raman, XPS) is very similar to 3a (450 ◦ C for 2 h) where T2 is extended. Note that the tri-acid oxidation at higher temperature of ∼ 200 ◦C is considered necessary to remove sp2 like materials. However, the present temperature of 90 ◦C was demonstrated to extend T2 of near-surface NV centers in pure bulk diamonds, which indicates that the T2 -extension effect of

Effect of surface oxidation on T2 The electron spin properties of these ND samples are measured with a home-built confocal fluorescence scanning microscope equipped with a microwave excitation system for spin characterization (Fig. 3a; see Methods and Supporting Information). 50,51 The NV centers we find in the NDs that are present are mostly single, owing to the low NV density. By choosing only single NV centers out for the ODMR ex-


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a

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Figure 3: (a) Close-up of the central part of the experimental setup and schematic of the NV-spin precession during Rabi and spin-echo sequences. The NDs are placed on a coverslip. A thin copper wire is used as a linear microwave antenna for spin excitation. Single NV centers hosted in NDs are excited by 532-nm laser light and observed with red-shifted fluorescence collected through the same microscope objective. (b) A representative confocal fluorescence scanning image. (c) The second-order photon correlation histogram and (d) fluorescence spectrum of the single NV center indicated by the circle in the scanning image are shown as insets. Schematic illustrations of the gated photon counting for (e) CW, (f) Rabi, and (g) spin-echo measurements. 532: 532-nm green laser pulse. MW: microwave pulse. Sig: photon counting while the microwave is ON. Ref: photon counting while the microwave is OFF. (h) The CW CW-ODMR spectrum of this NV center and its (i) Rabi and (j) spin-echo profiles.


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variation derived from the angle between the NV axis and the microwave magnetic field polarization, as has been reported using bulk diamonds. 52,53 However such an angle dependence is small for the case of very short T2 of the present study and also is likely to be buried under the effect of more striking variation of T2 coming from the particle inhomogeneity.

Mean = 1.8 ms

Mean = 1.4 ms

Detailed analysis of decoherence sources of NDs by size-dependence measurement of T2 To obtain further insight into the effect of surface oxidation to T2 , we measure the ND-size dependence of T2 for pre-cleaned and aerobic oxidation at 550 ◦C for 1 h. Specifically, the ND samples of larger sizes (50 and 90 nm in the original specification sheet) were pre-cleaned and then subjected to the aerobic oxidation at 550 ◦C for an hour. The ND samples were spin-coated on coverslips with PVA. The AFM size-measurement determined the mean particle size of 44.0 ± 1.2, and 67.3 ± 1.4 nm for the pre-cleaned NDs (designated as 4a, 4b, see Scheme 1) and 37.5 ± 0.5 and 58.8 ± 3.2 nm for the aerobically oxidized NDs (5a, 5b) as shown in Figs. 5a–d. These spin-coated ND samples are measured to obtain the mean T2 of single NVs in comparison with the pre-cleaned ND samples. These ND samples of 4b, 5b contained aggregated particles that have a size more than 200 nm and show very bright fluorescence without ODMR signals. These aggregated NDs are excluded from the measurements. Other isolated NDs still contain single NVs and can be used for the measurement (see Methods and Figs. S6 and S7 for the detail). Figure 5e(g) shows a dependence of T2 and Γ2 (T2 −1 ) on the ND size for these two different surface types of NDs. T2 (Γ2 ) is increased (decreased) for all sizes of ND samples. While surface oxidation would have more impact on smaller NDs owing to the small distance between the surface and NV centers, for the larger ND samples the aerobic oxidation seems to improve the spin coherence to an almost equal de-

Mean = 2.1 ms

Mean = 2.5 ms

Mean = 2.6 ms

Figure 4: Statistical data of the spin coherence time (T2 ) for different NDs: (a) pre-cleaned (1), (b) tri-acid oxidation for 6 h (2a), (c) aerobic oxidation at 550 ◦C for 1 h (3a), (d) aerobic oxidation at 550 ◦C for 2 h (3b), and (e) aerobic oxidation at 450 ◦C for 2 h (3c). The dashed lines indicate the mean values. The number of particles investigated is 22, 23, 20, 21, and 20 for 1–3c, respectively. the tri-acid oxidation at 90 ◦C is very small in the presence of sp2 -like materials. Note that the measured T2 may include a


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Table 1: T2 of each ND sample with the maximum and mean. ND sample Maximum [µs] Mean [µs]

1 4.7 1.8 ± 0.3

2a 4.2 1.4 ± 0.2

a

3a 5.4 2.1 ± 0.3

3b 5.5 2.5 ± 0.3

3c 6.1 2.6 ± 0.4

b 4a

4b

c

d 5a

5b

e

Ratio of G2

f

3b/1 4b/4a 5b/5a

g

G

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Lower bound

Figure 5: Particle size distributions of (a, b) pre-cleaned NDs (4a for N = 159, 4b for N = 239) and (c, d) aerobically oxidized NDs (5a for N = 319, 5b for N = 209). (e) The mean T2 of different sized NDs of pre-cleaned (black; 1, 4a, 4b) and aerobic oxidation at 550 ◦ C for 1 h (red; 3b, 5a, 5b). The error bars are 1σ of N = 22 for 1, 21 for 3b, and 5 for the others. (f) The ratio of Γ2 (T2 −1 ) of different ND-sized particles between the pre-cleaned and aerobic oxidation. (g) The ND-size dependence of Γ2 for the pre-cleaned and aerobic oxidation. ΓN , ΓOx , and ΓSurf are the decoherence contributions from nearby P1 centers, the sources removed by aerobic oxidation, and other sources derived from surface spin noise, respectively. Note that the lower bound of ΓN is 0.15 µs−1 taking from Ref. 54. gree as does for the small NDs (see Fig 5e). This tendency can be understood by analyz-

ing their decoherence sources. The observed decoherence rates are the sum of all possible de-


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face. 60,61 Applying hydrogen termination to the surface-oxidized ND surface can become a starting point of such noise spectroscopy, because the hydrogen termination gives a characteristic and simple FTIR spectrum, thereby defining the surface properties. 62 The hydrogenterminated surface can be then converted to oxygen termination (by either aerobic or triacid oxidation). The complicated signatures of the surface oxidation in FTIR spectra owing to the various functional groups (ester, carbonyl, carboxyl, and hydroxide) can be readily understood in comparison with hydrogen termination. Such a detailed study on the relation between NV decoherence and surface properties is necessary for the further extension of T2 in small NDs. It is now clear that ΓOx does not depend on ND size and has a significant influence even on the ND size of ∼ 60 nm, where the surfacerelated spin noise is no longer able to significantly influence the NV spin coherence. Assuming ND morphology to be a sphere and NV distribution random in the NDs, the NV-surface distance is half of the ND size at its longest. Surface termination, therefore, is not expected to significantly influence the large ND sample of ∼ 60 nm. The present observation of the constant influence of ΓOx irrespective of the ND size is a new piece of important knowledge toward realizing an extended T2 in NDs. Note that irregularity of the ND shape may affect the particle size measurement and ultimately the following T2 -measurements. 63 The aspect ratio of larger NDs having a size range of 1–3 µm is 1.3–1.6 (mostly 1.4–1.5) in the same product line (MSY series, Microdiamant) and seems similar to the smaller NDs used in the present work based on the TEM images of the same type of NDs reported in the literature. 27 However such sizing variations due to the irregularity is incorporated into the histograms of the particle size distribution and T2 measurements. Whereas the reason of the constant ΓOx is not clear at present, it can be probably ascribed to some changes in the spin-noise sources inside the ND particles. Recently, the importance of the charge state of NV centers and its

coherence contributions, which we describe here as Γ2 = ΓN + ΓOx + ΓSurf , (1) where ΓN , ΓOx , and ΓSurf are the decoherence contributions from nearby P1 centers, the contributions from sources removed by the aerobic oxidation, and contributions from other decoherence sources derived from the surface spin noise, respectively. First, we consider that ΓN only depends on the nitrogen concentration of the diamonds (red shaded area in Fig. 5g) and not on the size, which corresponds to a spin coherence time of NV centers deeply embedded in bulk diamonds. We set ΓN = 0.15 µs−1 or 0.2 µs−1 by referring to the previous reports using bulk diamonds with similar nitrogen impurity concentrations (see Table S2 for the summary of the previously reported T2 ). 54–56 The reported T2 values of ensemble NV centers in Type-Ib bulk diamonds (with similar nitrogen impurity of ∼ 100 ppm) in the literature show a broad range of 1–7 µs. Among them, we take 6.7 µs and 5 µs as the most probable limit for the measurement data presented in Fig. 5g. It can be seen that ΓSurf shows a strong size-dependence of T2 of NV centers in NDs. This size-dependence can be understood by considering the interactions between the NV spin and surface spins that provide d−4 0 dependence, 21,57,58 where d0 is the diameter of the NDs. The d−4 0 dependence shows a significant increase in the range d0 < 40 nm, which is consistent with the present observation. The remaining offset contribution of Γsurf comes from other surface decoherence sources, such as the carboxyl termination recently found as a decoherence source. 59 Further detailed analysis on the surface spin noise can help T2 to reach the ΓN limit, which may be the previous case for Type-IIa CVD-grown NDs with a size of 50– 150 nm, for example (Table S2). 20 A possible approach to investigate details of Γsurf is by using a noise spectroscopy of the surface-induced decoherence of NV centers in NDs that can characterize the noise species on the diamond surface, as demonstrated for shallow NV centers near the bulk diamond sur-


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in the form of NC/N@C species. 68 Variation of the bonding form is closely related to the charge state of P1 center and may be influenced by the present oxidation. These topics related to the charge-state stability of NV centers and the surrounding spin baths are at the forefront of the NV researches and further experimental studies are necessary to understand the mechanism of the present non-size dependent ΓOx . It is interesting to experimentally determine ΓN of the currently used ND samples, which is however very difficult at the present moment. The present commercially available ND samples are milled in a factory from capsules of HPHT crystals and sorted into each of the sizes of the commercial products (MSY 0-0.05, 0-0.1, 0-0.2). Such an industrial milling is applied for shaping of the particles, i.e. to break down needles, and prepare a defined aspect ratio of the particles. As well imperfect particles with fractures are crushed down; the milling is not applied to completely mill down the diamond particles from coarse micron to nano scale. Therefore, the largest size in the product family with a size range of 60–80 µm (MSY 60/80, Microdiamant) is expected to show ΓN -limited T2 . We therefore attempted to measure T2 of these sub-millimeter crystals (see Fig. S8). However, it was impossible to measure any pulsed ODMR signals. The NV distribution in these crystals was not uniform and was significantly concentrated in several spots. These NV spots exhibited very broad CW-ODMR spectra that are largely different from the data of the singleNV-level NDs. Surprisingly, the pulsed ODMR measurements (Rabi sequence) did not show Rabi oscillations (the signal rapidly decreases just after the time zero). Whereas it is not possible to clearly explain this observation at present, we speculate that it is probably due to a very short spin coherence time of NV centers in these spots: strong interactions between nearby spin decoherence sources such as adjacent NV centers and P1 centers causes a large amount of decoherence noise. By reducing the size of nanoparticles, NV centers may become free of these decoherence sources, allowing for the observation of pulsed ODMR at the singleNV level.

stability has been the focus of study in relation to NV spin-coherence measurements. 64–66 Bluvstein et al. has reported that the chargestate instability of NV centers compromises the spin measurements, thereby biasing the measured spin coherence time (both T1 and T2 ), causing them to vary from their real values. 66 The charge states of NV centers are influenced not only by the surface charges but also by the local electrostatic environments around the NV centers. The present aerobic oxidation of 400– 600 ◦C can change the local electrostatic environments of NV centers in case of NDs by annealing the inhomogeneous lattice structures near the surface. Such an annealing effect may also change the spin bath of the NV centers. For example, P1 centers that are the most influential decoherence sources of NV centers in the nitrogenrich HPHT diamonds, only cause decoherence in their charge neutral state. In this context, ΓN that we assumed independent of ND size may be size dependent. We also note that the “sp2 like materials” do not simply cover the ND surface; rather they are integrated into the NDs by forming chemical bonding with the internal sp3 structures. Such integrated sp2 -like structures do affect the electronic structures and energy levels of NV and P1 centers around the diamond surface. Indeed, how sp2 -like structures exist on the NDs is intensively studied in relation to ND catalytic behaviors for dehydrogenation of organic molecules. 67,68 In these studies, sp2 @sp3 core shell structure is considered to affect the activation barrier of chemical reactions of organic molecules on the ND surface. 67 Therefore, it may be possible that the actual density of the surface-integrated sp2 sites in the NDs oxidized at 550 ◦C is higher than those treated at 450 ◦C, which might lead to a depletion of nitrogen impurities in the NDs from the neutral to positive charge state, increasing T2 of NV centres. One can also consider a case where chemical bonding of nitrogen atoms (P1 center) in the sp3 and sp2 structures affect T2 . Kirmani et al. has reported that nitrogen in detonation nanodiamonds (DNDs) is present in the form of NO bonded species located at the surface region of sp2 shells, while in core of the DNDs it is


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Note that, because of the large variance of the T2 histograms as in Fig. 4 and of the uncertainty of ΓN , it is not possible to understand the present observation of non-size-dependency of decoherence in a straightforward manner. To get a clear picture of the present observation, it is important to begin with making NDs from well-characterized bulk diamonds whose ΓN is determined to be small (namely having longer T2 ). Because the experimental observable is T2 in time (inverse of Γ), one would be able to observe more drastic change caused by either ΓSurf or ΓOx . For example, a removal of ΓOx of ∼ 0.1 µs−1 appears only as T2 change of 2.27 µs in the presence of ΓN = 0.15 µs−1 (the lower bound currently depicted in Fig. 5) but can be seen as the change of 13.33 µs when it is reduced to one third, ΓN = 0.05 µs−1 . This clearly sets a future direction of the present research to understand the mechanism of the non-sizedependency of ΓOx in a way using NDs milled from well-characterized starting bulk diamonds that have small ΓN .

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sors and quantum information devices.

Methods Sample preparation of NDs We used commercially available NDs with a mean particle size of 25 nm (Microdiamant, MSY 0-0.05, Type Ib, HPHT) as a starting material. These NDs are a complex mixtures of diamond and sp2 -like carbons, and thus, are black. The NDs were purified five times by a centrifugation of 14,300 RCF for 30 min and the sedimentation was extracted and re-dispersed in distilled water each time in order to remove the sp2 -like materials that causes background fluorescence. The purified NDs were sonicated for an hour to obtain colloidal dispersion, which we denoted as pre-cleaned NDs. The sonication was performed in an ultrasonic bath (SND, US10KS, 38 kHz with 240 W). We oxidized the ND surface by aerobic annealing or solution-based tri-acid cleaning. For aerobic annealing, the pre-cleaned NDs were drop-casted on glass substrates and completely dried. They were heated in an electric oven (Nitto Kagaku, NHK-120BS-II) under a pressure of one atmosphere. The resultant ND powders were rinsed out from the substrate and redispersed in distilled water. For the tri-acid cleaning, the pre-cleaned NDs were processed in a mixture of sulfuric (98 %), nitric (97 %), and perchloric (60 %) tri-acids (volume ratio 1:1:1) at 90 ◦C, followed by a base-acid wash with 0.1-M NaOH and 0.1-M HCl for neutralization. 18 The NDs were rinsed five times with distilled water. The resultant ND dispersion turned dark gray, indicating the slight removal of the sp2 -like carbon material. Polyvinyl alcohol (Aldrich, Mw: 89,000) was added to ND dispersion at a concentration of 1 wt% to create a uniform film in the spin-coating process. 18 A small droplet of the dispersion was spin-coated onto cleaned coverslips with an initial rotation parameter of 500 rpm for 2 s, and a subsequent rotation parameter of 2000 rpm for 30 s. The topographies of the spin-coated samples were obtained using AFM in tapping mode (Bruker, Edge). These data were used

Conclusions In conclusion, we presented the detailed investigation of the effects of aerobic and anaerobic tri-acid ND oxidizing methods on T2 of single NV centers in Type-Ib HPHT NDs. By systematically analyzing the relation between T2 and the surface spectroscopic properties for aerobic and tri-acid oxidations using the statistical approach, we found that aerobic oxidation most effectively oxidized the surface and extended T2 by a factor of 1.44 ± 0.33 to the original NDs. The ND-size dependence of T2 clearly showed that surface oxidation removes a constant decoherence contribution irrespective of the ND size in the available size range of ≤∼ 60nm, which clearly separates its origin from the surface-derived decoherence sources. The observed results highlight the presence of the NDspecific decoherence sources other than surface termination and spin-active impurity contents, thereby leading to a new surface termination protocol of NDs toward a long spin coherence time of NV centers necessary for quantum sen-


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to obtain the particle height distributions by taking the particle height as the particle size because only the vertical information provides the correct particle size owing to the effect of the cantilever size.

camera. A time-correlated single-photon counting module (PicoQuant, TimeHarp-260) was used to obtain second-order photon correlation histograms to identify single NV centers by measuring the antibunching. Microwaves were generated from a source (Rohde&Schwarz, SMB100A) and amplified by 45 dB (Minicircuit, ZHL-16W-43+). They were then fed to a microwave linear antenna placed on the coverslips. The typical microwave excitation power for the continuous-wave ODMR spectral measurement was 35 dBm (3.2 W) under the impedance mismatching condition. The avalanche photodiode (APD) detection was gated by using a radio-frequency (RF) switch for the microwave ON and OFF triggered by a bit pattern generator (PBESR-PRO-300, Spincore). For the ODMR measurements, the optical excitation intensity was reduced to ∼ 10 kW/cm2 to avoid the optical decoherence of the NV spins. A small magnetic field (∼30 Gauss) was applied to split the magnetic sublevels to the extent that the peaks are well-separated in frequency. We employed Rabi and spin-echo sequences in the pulsed ODMR measurements. Rabi measurements determined the duration of π-pulse that was used for the subsequent spinecho measurements. The spin-echo sequence determined the spin coherence time (T2 ). We measured both π/2–π–π/2 and π/2–π–3π/2 sequences and subtracted these signals from each other to cancel the common mode noise in the spin echo measurements. 69

Surface characterizations We analyzed the ND surface through Raman spectroscopy, (XPS), and (FTIR). The ND dispersions were drop-casted and dried on silicon wafers for the Raman and XPS measurements. Raman spectra were measured using the excitation wavelength at 325 nm (HORIBA, LabRAM HR Evolution and Renishaw, InVia Reflex Spectrometer System). XPS was performed with an AlKα spectral line of the XPS spectrometer (JEOL, JPS-9200). Before the XPS measurements, electron gun neutralization was applied to discharge the samples. For the FTIR measurements, the dried powder samples were placed on top of the ZnSe prism of the FTIR-ATR spectrometer (JUSCO, FTIR, ATR) and measured. Note that PVA was not used for the surface characterization. ODMR We used a home-built confocal microscope system that integrates a microwave circuit for the spin control (Fig. S5). A continuous-wave 532nm laser was used for the excitation with a typical excitation intensity of ∼ 50–90 kW/cm2 to acquire the scanning image and second-order photon correlation histograms; the laser intensity was near the fluorescence saturation level. An oil-immersion objective (numerical aperture: 1.4) was used for both the excitation and fluorescence collection. NV fluorescence was filtered by a dichroic beam splitter and a long pass filter. The fluorescence was then coupled to an optical fiber having a core diameter of ∼ 10 µm. The fiber-coupled fluorescence was guided into a Hanbury-Brown-Twiss (HBT) setup, which consists of two avalanche photodiodes and a 50:50 beam splitter. For the spectral measurements, the microscope was connected to a fibercoupled spectrometer equipped with a liquidnitrogen-cooled charge-coupled device (CCD)

Size-dependence measurement of T2 We used commercially available NDs (Microdiamant, MSY 00.10 and MSY 00.20, Type Ib, HPHT). These samples were pre-cleaned by the method above and then subjected to the aerobic oxidation at 550 ◦C. The oxidized NDs were spin-coated onto coverslips with the 1wt%-PVA solution. The AFM measurements provided the mean particle sizes. In the optical measurements, most of the NV centers found in 4a, 4b were single and we proceeded to collect the subsequent ODMR measurements. The larger NDs of 5a, 5b contained aggregated ND


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particles and showed very bright fluorescence without ODMR signals. Because these aggregated particles have a size of more than 200 nm, which are beyond the size range of the majority of NDs (mean is ∼ 60 nm; see Fig. 5 and Fig. S6, S7), they could be omitted without giving bias of the ND size measurements.

SS, HH wrote the manuscript. All authors contributed to the discussion.

Acknowledgement We thank Prof. Noboru Ohtani for the AFM measurements, Mr. Keisuke Oshimi for the computational support of T2 measurements, and HORIBA Inc. for the Raman measurements. We also thank Dr. Akihiro Shimizu for the help with XPS measurements. A part of this study was conducted at Hokkaido University and Chitose Institute of Science and Technology, supported by “Nanotechnology Platform” Program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. MF acknowledges the financial support provided by JSPS-KAKENHI (Nos. 26706007, 26610077, 16K13646, and 17H02741), MEXT-LEADER program, and Osaka City University (OCUStrategic Research Grant 2017 & 2018 for young researchers and top-priority research). SS acknowledges the financial support provided by JSPS-KAKENHI (No. 26220602). HH thanks JSPS KAKENHI, Grant-in-Aids for Basic Research (B) (No. 16H04181) and Scientific Research on Innovative Areas “Innovations for Light-Energy Conversion (I4 LEC)” (Nos. 17H06433, 17H0637) for the financial support.

Supporting Information Available

Notes The authors declare no competing interest.

The following files are available free of charge. The Supporting Information is available free of charge on the ACS Publications website. Particle size determination using AFM, statistical figures of particle size distributions, Raman spectra of aerobic oxidation at 450 ◦C for longer durations, peak assignment of FTIR spectra, experimental setup for ODMR measurements, summary of the reported spin coherence times of various diamonds, brightness of NV centers in the larger ND samples, a statistical way to identify aggregated nanoparticles, and ODMR spectra of NV centers in MSY 60–80 submillimeter crystals (PDF).

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Graphical TOC Entry Mean 1.8 ms

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Removing Non-Size-Dependent Electron Spin Decoherence of

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