ISSN 00213640, JETP Letters, 2010, Vol. 92, No. 2, pp. 102–106. © Pleiades Publishing, Inc., 2010. Original Russian Text © A.A. Soltamova, I.V. Il’in, F.M. Shakhov, S.V. Kidalov, A.Ya. Vul’, B.V. Yavkin, G.V. Mamin, S.B. Orlinskii, P.G. Baranov, 2010, published in Pis’ma v Zhurnal Éksperimental’noі i Teoreticheskoі Fiziki, 2010, Vol. 92, No. 2, pp. 106–110.
Electron Paramagnetic Resonance Detection of the Giant Concentration of Nitrogen Vacancy Defects in Sintered Detonation Nanodiamonds A. A. Soltamovaa, I. V. Il’ina, F. M. Shakhova, S. V. Kidalova, A. Ya. Vul’a, B. V. Yavkinb, G. V. Maminb, S. B. Orlinskiib, and P. G. Baranova, c a
Ioffe Physicotechnical Institute, Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg, 194021 Russia email:
[email protected] b Federal Center of Shared Usage for Physicochemical Measurements, Kazan State University, ul. Kremlevskaya 18, Kazan, 420008 Russia c St. Petersburg State Polytechnic University, St. Petersburg, 194021 Russia Received May 31, 2010
A giant concentration of nitrogen vacancy defects has been revealed by the electron paramagnetic resonance (EPR) method in a detonation nanodiamond sintered at high pressure and temperature. A high coherence of the electron spins at room temperature has been observed and the angular dependences of the EPR spectra indicate the complete orientation of the diamond system. DOI: 10.1134/S0021364010140067
The extreme object of the miniaturization of the elemental basis of micro and optoelectronics is a device based on a single atom, a single molecule, or a single defect. This fantastic scenario is beginning to be implemented at present after the discovery of unique properties of nitrogen vacancy defects (nitrogen vacancy defects) in a diamond, which make it possible to detect the magnetic resonance on individual spins at room temperature [1, 2]. Intense and stable lumi nescence of these centers with a characteristic zero phonon line at a wavelength of 637 nm [3] open numerous possibilities for applying nitrogen vacancy defects in promising fields such as magnetometry [4– 6], quantum optics [7], and biomedicine [8], as well as for developing new information technologies based on the quantum properties of the spins [9]. The basic method of the creation of nitrogen vacancy defects, which are carbon vacancies in a dia mond in whose nearest neighborhood one of four car bon atoms is replaced by a nitrogen atom, in bulk dia mond crystals, as well as in micro and nanodiamonds is their irradiation by neutrons, protons, or electrons with energies above the thresholds [1, 3]; as a result, carbon vacancies are formed. The subsequent anneal ing at temperatures of 800–900°C leads to the diffu sion of vacancies and their capture on deep donors in the form of isolated nitrogen atoms, thus creating nitrogen vacancy defects. Another method for creating nitrogen vacancy defects is the method of the ionic implantation of nitrogen into an ultrapure diamond (with a low nitrogen content) with subsequent anneal
ing, at which the vacancies created in the process of implantation are captured by the implanted nitrogen atoms [10]. The aforementioned processes of the formation of nitrogen vacancy defects are statistical and the proba bility of their creation in nanodiamonds depends strongly on the sizes of the nanoparticles. Nitrogen vacancy defects in nanoparticles with sizes smaller than 20 nm are almost not formed [11] in agreement with theoretical calculations [12, 13]. Several works have recently appeared where it was shown that nitrogen vacancy defects can be created in detonation nanodiamonds without irradiation [14, 15], but the efficiency of the formation of nitrogen vacancy defects is very low. Moreover, the properties of these defects are significantly different from those obtained in bulk diamond crystals due to the graphite like shell of the nanocrystal and internal stresses and the zerophonon luminescence lines were not observed in such samples. The chemical vapor deposi tion growth of diamonds is considered as one of the promising methods [12]. However, the probability of the creation of nitrogen vacancy defects in nanodia monds obtained by the chemical vapor deposition method is very low: only one defect with a probability of 100% is created for crystals with a diameter of 110 nm; a decrease in the sizes of the crystal is accom panied by a strong decrease in the probability of the creation of at least one defect and this probability for nanodiamonds with sizes of 60–70 nm decreases to about 2%.
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The analysis of the technological possibility of directionally creating nitrogen vacancy defects in sig nificant concentrations in micro and nanodiamonds [5, 8] without the use of ionization radiation is of cur rent interest, because this is necessary for the creation of highly sensitive magnetometers and for the investi gation of biological systems. In this work, we report the results of the electron paramagnetic resonance (EPR) investigation of high concentrations of nitrogen vacancy defects in dia mond nanostructures without the use of ionization radiation. The samples used for sintering have the size of 4.5 to 5nm diamond purified by the technology described in [16]. Sintering was performed at a temperature of 800°C and a pressure of 6 GPa for 11 s in a toroidal highpressure chamber by the technology described in [17]. The mean size of the diamond array in a sintered detonation nanodiamond powder was determined from the total halfwidth of the reflections of the Xray diffraction and was 5.8 nm. The dependence of the Xray coherent scattering region in detonation nano diamonds from the sintering temperature to 1800°C was presented in [18]. After the sintering, the powder was individual arrays with a linear size of about 10 μm, where a cer tain order with planes and steps was observed in a microscope. Figure 1a shows the electronspin echo (ESE) detected EPR spectra at a frequency of 94 GHz at room temperature for several orientations of the magnetic field in a single 10 × 10 × 10μm array. The spectra were recorded at a microwave power of about 100 mW with a π/2 pulse of 48 ns, an interval between pulses of 260 ns, and a pulse repetition time of 2 ms. Despite the very small size of the sample, intense anisotropic EPR signals are observed. Two character istic groups of the signals can be identified. The first group consists of three lines in the range of 3345– 3360 mT with a relatively small anisotropy. The sec ond group consists of numerous highly anisotropic lines covering a wide magnetic field range of 3240– 3460 mT. The presence of anisotropic EPR spectra certainly indicates that the diamond system is ori ented. The central group of lines characterized by the anisotropic hyperfine interaction belongs to the nitro gen donors in the form of individual atoms N0 (P1 centers) and is shown in the inset for one of the orien tations. The angular dependences of the EPR spectra detected in the range of 0°–180° with a step of 10° are shown by the closed circles in Fig. 1b. The ESE spec tra are also shown for several angles for illustration. To reveal the nature of these lines, we calculated the angular dependences of the EPR spectra with the spin Hamiltonian 2
H = gμ B BS + D [ S z – 1/3S ( S + 1 ) ], where the first term presents the Zeeman interaction in the magnetic field B for a center with the electron JETP LETTERS
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Fig. 1. (a) The ESEdetected EPR signal at a frequency of 94 GHz and room temperature in the sample obtained by sintering a detonation nanodiamond at T = 800°C and P = 6 GPa for several orientations of the magnetic field with respect to the 〈111〉 axis. The inset shows the central part of the EPR spectrum for B || 〈111〉 corresponding to individ ual nitrogen centers N0. (b) The (points) measured and (lines) calculated dependences of the EPR spectra on the direction of the magnetic field B with respect to the 〈111〉 axis (ϕ = 45°) and the ESE spectra for several angles.
spin S and isotropic electron g factor, μB is the Bohr magneton, D is the parameter characterizing the split ting of the fine structure in the axial crystal field, and z is the defect axis. The observed angular dependences are characteristic of the system with the electron spin
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S = 1 in the axial field with the C3v symmetry. The ini tial point for the calculation of the EPR spectra is the presence of a signal in which two pairs of lines are observed: one pair with the maximum splitting near 200 mT and the other pair with a large integral inten sity and a smaller splitting. Assuming that this orienta tion is close to the orientation of the magnetic field along the 〈111〉 axis of the diamond (this spectrum in Figs. 1a and 1b is marked as θ = 0°), for the calculation we took the following Euler angles for the four orien tations of the 〈111〉 axis, which exist in the diamond crystal: α = 0°, β = 0°, γ = 0°; α = 0°, β = 110°, γ = 0°; α = 120°, β = 110°, γ = 0°; and α = 240°, β = 110°, γ = 0°. The calculation was performed at the azi muth angle ϕ = 45°. The calculation results are shown by the solid lines in Fig. 1b. It is seen in Fig. 1b that not all of the experimental points can be described with the above Euler angles and that the number of these points is twice as large. Nevertheless, since all of the lines converge in the ori entation B || 〈111〉 (θ = 0°), a natural explanation of the existence of additional lines is the formation of twin regions in the diamond system. To calculate the angular dependences of twins, we added four centers with the Euler angles α = 0°, β = 0°, γ = 0°; α = 0°, β = 250°, γ = 0°; α = 120°, β = 250°, γ = 0°; and α = 240°, β = 250°, γ = 0°. The calculated angular depen dences for twins are shown by dashed lines. The com plete coincidence of the measured and calculated angular dependences is seen and indicates the correct ness of the chosen model. The used parameters of the Hamiltonian, g = 2.0028 and D = 0.0958 cm–1, almost coincide with the known parameters for nitrogen vacancy defects in bulk diamond crystals [19]. Thus, a giant concentration of nitrogen vacancy defects is observed in a microarray obtained by only sintering without irradiation. The concentration is estimated from the EPR signals taking into account that the sensitivity of the experimental setup is 5 × 109 spins at a linewidth of 1 G. Taking into account the width of individual lines and the signaltonoise ratio for each line referring to the center under consider ation, we conclude that the detected number of spins of the nitrogen vacancy defects is about 2.8 × 1012 spins. The sizes of the sample are 10 × 10 × 10 μm. Thus, the concentration of nitrogen vacancy defects is about 2.8 × 1021 cm–3. A similar estimate for individual nitrogen donors provides a value of about 3 × 1021 cm–3 for the concentration of the N0 centers, which is close to the concentration of the nitrogen vacancy defects. The concentration of carbon atoms in the crystalline diamond is 1.76 × 1023 atm/cm3. This means that an oriented diamond system is obtained, where about 1% of carbon atoms are replaced by nitrogen vacancy defects and about 1% of carbon atoms are replaced by individual nitrogen donors. To obtain information regarding the relaxation spin processes in the diamond structure containing ultra
high concentrations of nitrogen vacancy defects and deep nitrogen donors N0, the spin–lattice and spin– spin relaxation times T1 and T2, respectively, for nitro gen vacancy and N0 centers were measured at room temperature using the ESE method. The spin–lattice and spin–spin relaxation times for nitrogen vacancy defects appear to be T1 = 1050 ± 100 μs and T2 = 1.2 ± 0.2 μs, respectively. For the N0 nitrogen centers, T1 = 570 ± 10 μs and T2 = 0.55 ± 0.1 μs (the measurements were carried out for the lowfield hyperfine compo nent of nitrogen; these times for the central line are about 15% shorter because of the partial overlapping with the EPR signal from the surface of the nanoparti cle). Comparatively long times T2 indicate that the system is highly coherent even at room temperature despite the giant concentration of nitrogen vacancy and N0 centers. For the formation of nitrogen vacancy defects, sig nificant concentrations of individual nitrogen atoms (P1 centers) should be initially present in the detona tion nanodiamond. However, according to the theo retical calculations, individual nitrogen atoms should be located on the surface of the nanodiamond particle rather than in the crystalline core [20]; this circum stance is also evidence of the small probability of the observation of nitrogen vacancy defects in nanodia monds with a core smaller than 10 nm. It is also shown that nitrogen vacancy defects can be present in bulk diamond crystals before the irradiation, but with insig nificant concentrations of about 1010 cm–3, and the probability of the creation of at least one nitrogen vacancy defect in nanodiamond particles and arrays of particles with sizes less than 10 nm is almost zero, which cannot explain the observed giant concentra tions of nitrogen vacancy defects. Thus, no existing models can explain the detection of such giant con centrations of nitrogen vacancy defects in aggregated nanodiamonds. In detonation nanodiamonds used for sintering, we previously revealed both individual nitrogen atoms and vacancy complexes [21]. The ESEdetected EPR signal at a frequency of 94 GHz in the detonation nan odiamond sample before the sintering is shown in Fig. 2. Since the detonation nanodiamond has a com plex structure in the form of a diamond crystalline core and a shell consisting of sp2 hybridized carbon atoms [22], the EPR signals should present all of the features of the structure of the detonation nanodia mond. To separate the ESE signals from the diamond core and surface shell, we used different time sequences of microwave pulses (the pulse sequence is shown in the inset in Fig. 2), because the relaxation characteristics of two objects are significantly different from each other. Spectrum 1 recorded with a short interval τ = 230 ns between the first and second pulses exhibits an intense double line with the g factors of 2.0030 and 2.0004, which belongs to broken bonds on the surface shell of the detonation nanodiamond [21, JETP LETTERS
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23] and is characterized by short relaxation times. The fivefold enhancement of spectrum 1 makes it possible to identify four additional lines on both sides of the central line, which become more pronounced with an increase in the time interval between the pulses. Spec trum 2 in Fig. 2 was recorded at a temperature of 10 K with the interval τ = 800 ns between the pulses. Owing to the suppression of the intense central lines associ ated with the shell, the spectrum consisting of five lines marked by the vertical dashes becomes observable (g = 2.0037 for the central line). Moreover, additional lines are seen in the fields of 3350–3360 mT. These lines are apparently due to the interaction with individual nitrogen atoms (N0) whose simulated EPR spectrum is shown by the dashed line. The EPR spectrum consist ing of five lines (spectrum 2 in Fig. 2) likely belongs to the multivacancy complex. A defect with the spin S = 3/2 consisting of three unpaired electrons attached to broken bonds, i.e., the socalled R8 center [24], is the most probable candidate for such a complex whose EPR spectra are in satisfactory agreement with the observed signals (spectrum 2 in Fig. 2). It is assumed that the R8 center is a threevacancy complex. Thus, the initial detonation nanodiamond has defects necessary for the formation of nitrogen vacancy defects. Since the temperature at which sin tering was performed (800°C) corresponds to the tem perature of the mobility of vacancies, the conditions very favorable for the creation of nitrogen vacancy defects assumingly appear in the process of sintering. It is also worth noting that nitrogen vacancy defects were not observed in detonation nanodiamonds sin tered at higher temperatures of, e.g., 1500–1700°C [25]; in this case, intense EPR signals of nitrogen cen ters N0 were detected. Such a spectrum is shown by line 3 in Fig. 2. The absence of EPR signals of nitrogen vacancy defects in this structure is natural, because nitrogen vacancy defects are destroyed at high temper atures. To conclude, the EPR investigations have demon strated that very high concentrations of nitrogen vacancy defects reaching 1% are formed in detonation nanodiamonds sintered at a temperature of 800°C and a pressure of 6 GPa. The striking result is that such high concentrations are obtained without the prelimi nary irradiation of the samples. This irradiationfree process of the creation of nitrogen vacancy defects is particularly interesting for the investigation of the mechanisms responsible for the creation of such cen ters and for technologies, especially taking into account that the sintered detonation nanodiamond contains individual nitrogen atoms N0 with almost the same concentration as nitrogen vacancy defects. Moreover, the investigations show that microarrays are oriented systems with the properties of bulk crystals in which twins are revealed. The preliminary experi ments indicate the presence of intense luminescence characteristic of nitrogen vacancy defects; the experi JETP LETTERS
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Fig. 2. (a) The ESEdetected EPR signal at a frequency of 94 GHz in a detonation nanodiamond and before sintering at a temperature of 10 K at the interval between the pulses τ = (1) 230 and (2) 800 ns. The dashed line corresponds to the simulation of individual nitrogen donors N0 in the nanodiamond. Line 3 is the EPR spectrum detected at T = 50 K in the sample obtained by sintering the detonation nanodiamond at T = 1500°C and P = 6 GPa. The inset shows the sequence of the pulses in the experiment regard ing the detection of the ESE.
mental opticalinvestigation results will be published separately. This work was supported by the Ministry of Educa tion and Science of the Russian Federation (state con tract no. 02.740.11.0108), by the Russian Academy of Sciences (programs “Spin Effects in Solids and Spin tronics,” “Support of Innovations and Develop ments,” “Basic Research in Nanotechnologies and Nanomaterials,” and “Quantum Physics of Con densed Matter”), and by the Russian Foundation for Basic Research (project nos. 090201409 and 0902 00730). REFERENCES 1. A. Gruber, A. Drabenstedt, C. Tietz, et al., Science 47, 2012 (1997). 2. J. Wrachtrup and F. Jelezko, J. Phys.: Condens. Matter 18, S807 (2006). 3. G. Davies and M. F. Hamer, Proc. R. Soc. A 384, 285 (1976). 4. J. R. Maze, P. L. Stanwix, J. S. Hodges, et al., Nature 455, 644 (2008). 5. J. M. Taylor, P. Cappellaro, L. Childress, et al., Nature Phys. 4, 810 (2008). 6. S. Steinert, F. Dolde, P. Neumann, et al., Rev. Sci. Instrum. 81, 043705 (2010).
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Translated by R. Tyapaev
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