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Apr 2, 2019 - Fluorescent emitters in diamond have far-reaching potential applications in areas like quantum information, advanced biosensing, and ...
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Article Cite This: ACS Photonics 2019, 6, 1266−1271

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Growth of High-Purity Low-Strain Fluorescent Nanodiamonds Masfer Alkahtani,*,†,‡ Johannes Lang,¶ Boris Naydenov,¶,§ Fedor Jelezko,¶ and Philip Hemmer†,∥,⊥ †

Institute for Quantum Science and Engineering, Texas A&M University, College Station, Texas 77843, United States Center for Quantum Optics and Quantum Informatics, KACST, Riyadh 11442, Saudi Arabia ¶ Institute for Quantum Optics, Ulm University, Ulm, Germany § Helmholtz-Zentrum Berlin fuer Materialien und Energie (HZB), Kekulestrasse 5, Berlin 12489, Germany ∥ Department of Electrical and Computer Engineering, Texas A&M University, College Station, Texas 77843, United States ⊥ Zavoisky Physical-Technical Institute, Federal Research Center Kazan Scientific Center of RAS, Kazan 420111, Tatarstan, Russian Federation

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ABSTRACT: Fluorescent emitters in diamond have farreaching potential applications in areas like quantum information, advanced biosensing, and materials research (especially magnetic and superconductor materials). However, many of these applications are limited by imperfections in commercially available fluorescent nanodiamonds (FNDs) due to paramagnetic impurities and crystal lattice strains. These limitations are a direct consequence of the way fluorescent nanodiamonds are produced. Here, we show that for high pressure growth, at a relatively low temperature of 400 °C, we can produce high-purity and low-strain FNDs after standard irradiation and annealing treatments. This work is a milestone toward the engineering of high-quality ultrasmall fluorescent nanodiamonds. KEYWORDS: High-purity FNDs, Low-strain FNDs, Low-temperature growth, HPHT diamond growth, NV color center

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INTRODUCTION The negatively charged nitrogen-vacancy (NV) fluorescent color center in nanodiamonds (FNDs) has a wide variety of potential applications. These include qubits for quantum information that can be optically initialized and read out,1−4 advanced fluorescent markers for biological systems,5−7 very precise sensors for magnetometry,8−10 and thermometry.11−14 The NV center combines exceptional optical and spin properties at both low and room temperature, which make it the leading candidate for the above-mentioned applications. Despite all the unique applications demonstrated so far, the NV centers in small nanodiamonds still have some critical limitations. One of the major constraints is that commercial FNDs have a high concentration of paramagnetic impurities, as well as a highly strained lattice,15 which is a direct consequence of their fabrication methods, for example, crushing larger diamonds or ultrarapid growth as in detonation techniques. This typically results in a nonuniform behavior of the NV centers, which degrades their sensitivity to temperature, electric, and magnetic fields,15 especially for ensemble-based sensing. There are several approaches to overcome this problem. One is to produce high quality NDs from high quality bulk diamonds, starting with a shallow layer of strain-free NV centers, then patterning and breaking off nanodiamonds.15,16 Such strain-free NVs are incorporated into ultrapure bulk diamonds using implantation15 or delta-doping growth techniques.16 However, this method is very expensive and hard to scale to large quantities. Alternatively, high temperature annealing can be used to anneal out unwanted defects © 2019 American Chemical Society

and alleviate strain in existing NDs. This method has shown improvement in the morphology and lattice strain of diamond nanocrystals but strongly depends on the initial strain state.17 A better approach is to directly grow higher quality nanodiamonds using slow-growth techniques, such as those that are known to give high-quality bulk diamonds.18−20 Toward this goal, the direct growth of nanodiamonds from organic molecules has been reported, using high-pressure hightemperature (HPHT) and a relatively low growth temperature 900 °C,21,22 thereby improving on a growth technique reported in 1965.23 The result was NDs with higher quality NV centers than for commercial FNDs. Here, it should be noted that this trend is opposite to chemical vapor deposition (CVD) diamond growth, where crystal quality degrades rapidly at lower growth temperatures.24 Recently, HPHT-NDs were produced at much lower growth temperatures.25,26 Pushing this approach to even lower growth temperatures, at high pressure, promises even slower growth and therefore higher quality diamond. Here, we report the cold growth (below 400 °C) of low-strain FNDs with optical properties much superior to commercially available NDs.

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RESULTS AND DISCUSSION

Following the earlier work,25,26 a commercially available diamond-like organic molecule containing a single nitrogen atom (2-azaadamantane hydrochloride, purity 98%, Aurum Received: February 8, 2019 Published: April 2, 2019 1266

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Figure 1. (a) Illustration of organic nanodiamond growth at low temperature; a diamond-like organic molecule (2-azaadamantane hydrochloride) with a nitrogen atom needed to form a nitrogen-vacancy (NV) color center mixed with a carbon source, like the heptamethylnonane and tetracosane, and then heated to moderate temperature at high pressure. (b) Home-built diamond anvil cell with open windows. (b, inset) Illustration of the growth chamber placed between two diamond anvils where the sample will be exposed to high pressure. (c,d) Low and high magnification TEM images of grown nanodiamonds with size 10−150 nm. (c, right-bottom inset) Single 10 nm roundly shape nanodiamond crystal. (e) TEM diffraction patterns show single-crystal cubic nanodiamonds. (f) Sketch of a confocal microscope used to observe the NVs. It is equipped with a 100× microscope objective, CW and pulsed 518 nm lasers, a microwave wire, a photon counter, and a spectrometer.

to ambient and the samples were extracted with a 2 μm tip needle (American Probe Technologies, Inc.). To ensure a successful transfer of nanodiamonds from the growth chamber, we dip the tip of the extraction needle into 200 μL of an isopropanol/ethanol solution, repeating several times until the growth chamber is emptied. The presence of nanodiamonds in the growth mix was then confirmed by transmission electron microscope (TEM) characterizations. A few drops of the nanodiamond solution were dropped on a lacy carbon TEM grid and then placed on a vacuum TEM heating stage. As shown Figure 1c, the TEM image shows many nanodiamonds with size range varying from 10 to 150 nm. Furthermore, a single 10 nm roundly shape nanodiamond crystal is presented in Figure 1c, inset. High magnification, presented in Figure 1d, shows well-crystallized and roundly shape nanodiamonds. As shown in Figure 1e these nanodiamonds showed a crystal lattice spacing (2.06 and 1.27 Å) which matches the diamond (111 and 022) lattice planes spacing reported in ref 28. Note that it was necessary to heat the TEM grid to 800 °C for 10 min to remove residual growth material before the diamonds could be seen. To create vacancies in the nanodiamonds, electron irradiation followed by high temperature annealing are needed.

company, USA) was used to serve as a precursor for NV centers. It is worth mentioning that other chemically modified diamondoids can be used to produce desired color centers in NDs. This nanodiamond template was then introduced into a growth mixture consisting of reactive carbon sources, as described in Figure1a, and then exposed to high-pressure and moderate-temperature growth conditions. Furthermore, we also introduced tetramethylhydrazine into the growth mixture to prevent the formation of graphite and diamond-like carbon. At the growth temperature, tetramethylhydrazine was expected to decompose and give liquid nitrogen (at high pressure), which shields the growth mix from the chamber walls.27 Finally, we added chlorinated hydrocarbons like dichloromethane to assist dissolving of the polar seed molecule into the nonpolar carbon sources. The growth mixture was then transferred into a 100 μm (gasket hole) diameter chamber placed between two diamond anvils in a home-built diamond anvil cell (DAC) as shown in Figure1b. The growth was carried out for 24 h in a custombuilt furnace at a pressure around 10 GPa and moderate temperature (380−400 °C), well below the seed molecule decomposition temperature.25,26 After the growth was completed, the temperature and pressure were then returned 1267

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Figure 2. Nanodiamonds grown from organic molecule containing one nitrogen atom, 2-azaadamantane hydrochloride, gives the fluorescence spectrum (after irradiation and annealing) shown in (a). This spectrum shows the characteristic NV center spectrum with NV0 and NV− zerophonon lines (ZPLs) centered at 575 and 637 nm, respectively. (b) Optically detected magnetic resonance (ODMR) of the nanodiamonds described in part (a) under a green (518 nm) excitation laser. The high contrast and narrow line width (at low microwave power) agrees with good quality NVs in a bulk crystal. (c) ODMR spectrum of a nanodiamond sample containing at least two NV centers with different splittings in the presence of a magnetic field of 10.7 G. (d) Low-contrast ODMR spectrum with clear strain splitting of NV center in nanodiamonds grown at high temperature (650 °C).

For this purpose, a few drops of the nanodiamond solution were placed on a chip of silicon cut from a wafer. A silicon substrate was chosen due its minimal fluorescence background and high melting point. The chip was then heated to 800 °C for 10 min to drive off the volatile components prior to irradiation. The irradiation was conducted in a commercial ion irradiation facility at an energy of 200 keV at room temperature. After irradiation, postannealing at 800 °C for 2 h was necessary to mobilize the vacancies and create NVs. Annealing at higher temperature is recommended as it is known to further improve the diamond crystal quality.29 Next, to study optical properties of NV centers in the nanodiamonds, we put the silicon chip with nanodiamonds into a custom-built confocal microscope as illustrated in Figure 1f equipped with 100× (N.A. 0.95) objective, a 518 nm laser (both continuous wave (CW) and pulsed), photon counter, spectrometer, and a complete microwave system to study spin transitions of NV centers. After scanning the sample, we found many spots uniformly distributed on the silicon substrate. The optical fluorescence spectrum collected from each spot shows a clear spectrum of the NV center emission with NV0 and NV−

zero-phonon lines peaked at 575 and 638 nm, respectively, as illustrated in Figure 2a. NV centers can serve as a sensitive local probe of the diamond crystal quality. In particular, optically detected magnetic resonance (ODMR) in the NV can serve as a quantum sensor to read out important physical quantities of a nanoscale system such as temperature, electric field, and magnetic field. Therefore, we use the NV in our nanodiamonds to probe crystal quality. ODMR in the NV is performed by first optically pumping the NV into the ms = 0 spin sublevel of the triplet ground state. When a resonant microwave field induces a magnetic transition between the ms = 0 spin sublevel and the ms = ±1 levels, a significant decrease of NV fluorescence results.30,31 Figure2b demonstrates a high-contrast (12.5%) ODMR spectrum of the NV center in our fluorescence nanodiamonds (FNDs). A narrow line width (20 MHz) is observed at both low microwave and green excitation power (200 μW), which is two times narrower than ODMR line width reported in commercially available nanodiamonds.32−35 This narrow, highcontrast ODMR spectrum is typical for NV ensembles in highquality bulk diamond. To roughly estimate how many color 1268

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Figure 3. (a) Rabi oscillations between ms = 0 and ms = ±1 states. (a, inset) NV center is first spin polarized by a green laser pulse, and then a microwave pulse is applied at fixed frequency (corresponding to the transition frequency between the ms = 0 and one of the ms = ± 1 sublevels) with varying time duration τ. A final green laser pulse reads out the NV centers spin state. (b) Longitudinal relaxation time T1 of the NV center. (b, inset) Corresponding pulse sequence. (c) Hahn-echo measurement to determine the spin coherence time (T2). (c, inset) Corresponding pulse sequence.

centers per illuminated spot, we applied a small magnetic field (10.7 G) to the sample and recorded the ODMR spectrum. Figure 2c shows a clear double splitting in the NV center ODMR spectrum with a relatively narrow line width (20 MHz), which indicates the presence of at least two NV centers. To observe a single NV, we need to disperse the NDs, but we were not able to do so with the limited quantities produced by our DACs. Importantly, the large zero-field ODMR splitting characteristic of NVs in nanodiamonds is absent here, which is a good indicator of high quality FNDs. To verify that low temperature growth is responsible for the low strain, we repeated the same diamond growth, except at higher temperature (650−700 °C) where the other growth conditions remain unchanged. Figure 2d illustrates the resulting lower-contrast (6%) ODMR spectrum with clear zero-field splitting, more like commercially available FNDs produced from either detonation or crushed HPHT diamonds. Recently, it has been shown that the zerofield ODMR splitting is likely caused by local electric fields, rather than strain.36 Our data, plus NV charge stability data taken for both growth temperatures (not shown), is consistent with this prior work since we find evidence of more electron donors (presumably charged P1 centers) in the high temperature growth material. We next performed Rabi oscillations measurements to determine our ability to coherently manipulate the low temperature grown NV centers electronic ground spin state. For this, we polarize the NV center by a green laser pulse (about 1 μs) and then apply microwave pulses, with varying time duration τ, at fixed frequency (corresponding to the

transition frequency between the ms = 0 and one of the ms = ±1 sublevels). After that, a final green laser pulse will be applied to read out the NV centers state. The result are clear Rabi oscillations as shown in Figure3a. It is also important to study the relaxation dynamics of the NV center electron-spin polarization. In particular, the NV spin longitudinal relaxation time T1 (the decay lifetime for NVs spin population to the ground-state magnetic sublevel) is of special interest for quantum decoherence spectroscopy in biological applications.37−40 For our nanodiamonds the T1 measurements used a 1 μs laser pulse to first optically pump and polarize the NV center into the ms = 0 ground spin sublevel (3A2 state). The NV defect is then kept in the dark for a time τ, causing the system to relax toward a mixture of states ms = 0, ±1. The final electron spin population is read out by applying a second laser pulse. After such measurements, the T1 of the NV center in our FNDs is evaluated to be 150 μs as shown in Figure 3b. Compared to previously reported values in commercially available nanodiamonds,40,41 our nanodiamonds demonstrated a longer relaxation time T1 but still not comparable to those reported in high quality bulk diamonds. However, we expect that the relaxation time could be longer if we clean the diamond surface from low frequency impurities, as these are known to shorten longitudinal relaxation time (T1) especially in nanodiamonds.40−42 Unfortunately, cleaning the small quantities of nanodiamonds we get from the DAC is a challenge. Nonetheless, it is expected that this can be overcome in the future by producing larger quantities of these FNDs in large presses. 1269

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(4) Robledo, L.; Childress, L.; Bernien, H.; Hensen, B.; Alkemade, P. F. A.; Hanson, R. High-fidelity projective read-out of a solid-state spin quantum register. Nature 2011, 477, 574. (5) Fu, C.-C.; Lee, H.-Y.; Chen, K.; Lim, T.-S.; Wu, H.-Y.; Lin, P.-K.; Wei, P.-K.; Tsao, P.-H.; Chang, H.-C.; Fann, W. Characterization and application of single fluorescent nanodiamonds as cellular biomarkers. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 727−732. (6) Faklaris, O.; Joshi, V.; Irinopoulou, T.; Tauc, P.; Sennour, M.; Girard, H.; Gesset, C.; Arnault, J.-C.; Thorel, A.; Boudou, J.-P.; Curmi, P. A.; Treussart, F. Photoluminescent Diamond Nanoparticles for Cell Labeling: Study of the Uptake Mechanism in Mammalian Cells. ACS Nano 2009, 3, 3955−3962. (7) McGuinness, L. P.; Yan, Y.; Stacey, A.; Simpson, D. A.; Hall, L. T.; Maclaurin, D.; Prawer, S.; Mulvaney, P.; Wrachtrup, J.; Caruso, F.; Scholten, R. E.; Hollenberg, L. C. L. Quantum measurement and orientation tracking of fluorescent nanodiamonds inside living cells. Nat. Nanotechnol. 2011, 6, 358. (8) Taylor, J. M.; Cappellaro, P.; Childress, L.; Jiang, L.; Budker, D.; Hemmer, P. R.; Yacoby, A.; Walsworth, R.; Lukin, M. D. Highsensitivity diamond magnetometer with nanoscale resolution. Nat. Phys. 2008, 4, 810. (9) Balasubramanian, G.; Chan, I. Y.; Kolesov, R.; Al-Hmoud, M.; Tisler, J.; Shin, C.; Kim, C.; Wojcik, A.; Hemmer, P. R.; Krueger, A.; Hanke, T.; Leitenstorfer, A.; Bratschitsch, R.; Jelezko, F.; Wrachtrup, J. Nanoscale imaging magnetometry with diamond spins under ambient conditions. Nature 2008, 455, 648. (10) Maze, J. R.; Stanwix, P. L.; Hodges, J. S.; Hong, S.; Taylor, J. M.; Cappellaro, P.; Jiang, L.; Dutt, M. V. G.; Togan, E.; Zibrov, A. S.; Yacoby, A.; Walsworth, R. L.; Lukin, M. D. Nanoscale magnetic sensing with an individual electronic spin in diamond. Nature 2008, 455, 644. (11) Plakhotnik, T.; Doherty, M. W.; Cole, J. H.; Chapman, R.; Manson, N. B. All-Optical Thermometry and Thermal Properties of the Optically Detected Spin Resonances of the NV Center in Nanodiamond. Nano Lett. 2014, 14, 4989−4996. (12) Tzeng, Y.-K.; Tsai, P.-C.; Liu, H.-Y.; Chen, O. Y.; Hsu, H.; Yee, F.-G.; Chang, M.-S.; Chang, H.-C. Time-Resolved Luminescence Nanothermometry with Nitrogen-Vacancy Centers in Nanodiamonds. Nano Lett. 2015, 15, 3945−3952. (13) Alkahtani, M.; Jiang, L.; Brick, R.; Hemmer, P.; Scully, M. Nanometer-scale luminescent thermometry in bovine embryos. Opt. Lett. 2017, 42, 4812−4815. (14) Kucsko, G.; Maurer, P. C.; Yao, N. Y.; Kubo, M.; Noh, H. J.; Lo, P. K.; Park, H.; Lukin, M. D. Nanometer scale thermometry in a living cell. Nature 2013, 500, 54−58. (15) Trusheim, M. E.; Li, L.; Laraoui, A.; Chen, E. H.; Bakhru, H.; Schrder, T.; Gaathon, O.; Meriles, C. A.; Englund, D. Scalable Fabrication of High Purity Diamond Nanocrystals with Long-SpinCoherence Nitrogen Vacancy Centers. Nano Lett. 2014, 14, 32−36. (16) Andrich, P.; Alemn, B. J.; Lee, J. C.; Ohno, K.; de las Casas, C. F.; Heremans, F. J.; Hu, E. L.; Awschalom, D. D. Engineered Microand Nanoscale Diamonds as Mobile Probes for High-Resolution Sensing in Fluid. Nano Lett. 2014, 14, 4959−4964. (17) Hruszkewycz, S. O.; Cha, W.; Andrich, P.; Anderson, C. P.; Ulvestad, A.; Harder, R.; Fuoss, P. H.; Awschalom, D. D.; Heremans, F. J. In situ study of annealing-induced strain relaxation in diamond nanoparticles using Bragg coherent diffraction imaging. APL Mater. 2017, 5, 026105. (18) Lesik, M.; Tetienne, J.-P.; Tallaire, A.; Achard, J.; Mille, V.; Gicquel, A.; Roch, J.-F.; Jacques, V. Perfect preferential orientation of nitrogen-vacancy defects in a synthetic diamond sample. Appl. Phys. Lett. 2014, 104, 113107. (19) Fukui, T.; et al. Perfect selective alignment of nitrogen-vacancy centers in diamond. Appl. Phys. Express 2014, 7, 055201. (20) Pinault-Thaury, M.-A.; Tillocher, T.; Habka, N.; Kobor, D.; Jomard, F.; Chevallier, J.; Barjon, J. n-Type CVD diamond: Epitaxy and doping. Mater. Sci. Eng., B 2011, 176, 1401−1408. (21) Davydov, V. A.; Rakhmanina, A. V.; Agafonov, V. N.; Khabashesku, V. N. Synergistic Effect of Fluorine and Hydrogen on

Finally, we performed Hahn-echo measurements to determine the spin coherence time (T2). From the Rabi oscillations shown in Figure 3a, we determined the pulse durations of π/2 and π pulses for the subsequent Hahn-echo measurements. Then, following a green initialization laser pulse, three resonant microwave pulses π/2−π−π/2 are applied as illustrated in Figure 3c, inset. Between these pulses the NV center electron spin will accumulate a phase proportional to the amplitude of oscillating magnetic field acting along the NV center defect axis. A second 518 nm laser pulse is then applied to readout the final spin state of the NV center at the end of the measurement. The resulting spin coherence time (T2) of 1.22 μs was seen, as illustrated in Figure 3c.

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CONCLUSION We successfully grew high purity fluorescent nanodiamonds (FNDs) at relatively low temperature and high pressure. The NV color centers in these FNDs show a high-contrast ODMR spectra with a narrow line width (at both low microwave and laser powers). Furthermore, compared to commercially available FNDs, our low growth-temperature material demonstrated low strain and electric field broadening of the NV ODMR, and good longitudinal and transversal relaxation times, considering our limited ability to remove external contaminations. Finally, in related experiments, we have grown nanodiamonds at pressures down to 5 GPa, which is accessible by large-volume commercial presses. This work opens the door to the fabrication of ultrasmall and photostable fluorescent nanodiamonds for future quantum information and biological applications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. ORCID

Masfer Alkahtani: 0000-0002-5783-0836 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support of King Adulaziz City for Science and Technology (KACST), Saudi Arabia. P.H. acknowledges financial support from the Government of the Russian Federation (Mega-grant No. 14.W03.31.0028). Texas A&M University (T3 program) Grant 101 is also acknowledged. F.J. is supported by VW Stiftung and BW Stiftung.

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REFERENCES

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