Fluence-Dependent Evolution of Paramagnetic Triplet Centers in e

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Fluence Dependent Evolution of Paramagnetic Triplet Centers in e-Beam Irradiated Microcrystalline Ib Type HPHT Diamond Alexander I. Shames, Alex I Smirnov, Sergey Milikisiyants, Evgeny O. Danilov, Nicholas Nunn, Gary McGuire, Marco D Torelli, and Olga A. Shenderova J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06514 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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The Journal of Physical Chemistry

Fluence Dependent Evolution of Paramagnetic Triplet Centers in e-Beam Irradiated Microcrystalline Ib type HPHT Diamond Alexander I. Shames,1* Alex I. Smirnov,2 Sergey Milikisiyants,2 Evgeny O. Danilov, 2 Nicholas Nunn,3 Gary McGuire,3 Marco D. Torelli,3 Olga Shenderova3* 1

Department of Physics, Ben-Gurion University of the Negev, P.O. Box 653, 8410501 Beer-

Sheva, Israel 2

Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh,

NC, 27695-8204, USA 3

Adámas Nanotechnologies, Inc., 8100 Brownleigh Drive, Raleigh, NC 27617, USA

Corresponding Authors: *Alexander Shames, [email protected] *Olga Shenderova, [email protected]

ACS Paragon Plus Environment


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Abstract Paramagnetic triplet centers produced by e-beam irradiation of synthetic microcrystalline Ib-type high-pressure high-temperature (HPHT) diamonds were studied by X-band (9.4 GHz) continuous wave (CW) electron paramagnetic resonance (EPR) spectroscopy at X-band (9.4 GHz), pulsed EPR at X- and Q-bands (34 GHz), and fluorescence spectroscopies as a function of radiation fluences up to 5×1019 e-/cm2. EPR spectra of mostly “forbidden” ∆ms = 2 electronic spin transitions observed at g ≈ 4 (i.e., so-called half-field EPR spectra) reveal the presence of the main W15 triplet defects associated with the fluorescent negatively charged nitrogen-vacancy (NV-) centers as well as additional triplet spin centers identified as W16, W17, W18, and W33 that appear upon increasing the e-beam fluence. Consequent annealing at 1,400 oC significantly reduces the content of W17, W18, and W33 but not W15 and W16 defects. The efficacy of NVcenter fabrication as a function of fluence dependent e-beam irradiation is also reported.


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Introduction Irradiation of synthetic and natural diamonds with high energy particles produces optically active crystallographic defects that are useful for a variety of applications ranging from colorated specialty diamond gemstones1-2 to materials’ platforms for quantum information processing.3 Irradiation creates vacancies and self-interstitials that would consequently form complexes with impurity atoms, particularly nitrogen, which is the most common natural and of a special importance impurity in diamond (e.g., classification of diamonds is based upon Nrelated optical absorption). Within the N-related family of optical-active defects, the nitrogen-vacancy (NV) consisting of a substitutional nitrogen atom and an adjacent vacancy has received, perhaps, the most attention because of numerous applications in both emerging and mature technologies. Such applications include background-free and long-term cell imaging in the red/near infrared (NIR) spectral region,4-5 tracing of cell progeny,6 flow cytometry,7 super-resolution imaging,8 correlative microscopy,9 multimodal microscopy,10 labeling of low-abundance cellular components,11 and the use as fiducial markers.12 In addition to the characteristic fluorescence, the negatively charged nitrogen-vacancy color center (NV-) in a ground state possesses electronic spin S = 1, which brings in a plethora of additional physical properties that can be utilized for high precision measurements of environmental parameters such as electromagnetic field,13 temperature,14-15 and mechanical strain.15-16 NV centers are either naturally present in the diamond lattice or can be created artificially by an irradiation with high energy electrons (2-10 MeV),17-20 protons (2-3 MeV),20-21 low energy (40 keV) alpha-particles,22 gamma rays,23-24 and fast neutrons.25 There are several factors which



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must be considered regarding the choice of the type of the irradiation and the overall strategy of NV- centers production, such as production efficiency, uniformity of the vacancies formed along the radiation path, vacancy survival rate after the irradiation, the overall lattice damage during irradiation, the amount of diamond material treated in a single run, and availability/access to the irradiation source. Each irradiation method has its own pros and cons. Irradiation with He+ produces a large number of vacancies per ion, therefore, the fluencies required for the desired vacancy concentration are very low (~1013 ions/cm2).22 The fluences employed in proton irradiation of diamonds vary from 1015 to 1016 p/cm2.19 Irradiation with electrons is not as efficient in terms of yield - the number of the vacancies produced per particle (e.g., ~2 vacancies/e- for 5 MeV electron23 versus 13 vacancies created by 3 MeV H+ ion and 40 vacancies per He+ ion)22, this low yield can be partially compensated by the availability of e-beams with currents up to 3 orders of magnitude higher than those for the He+ ion beam. Typical electron irradiation fluencies currently employed for the production of NV- centers in various diamond samples are from 1018 to 5×1019 e-/cm2 18-20 with a few research groups reporting the fluence as high as 1020 e/cm2.26 While the heavier ions are much more efficient in terms of yeld, such ions often cause of an extended lattice damage. For example, the maximum achieved brightness in ND particles was almost twofold higher by using proton irradiation as compared to heavier He+ ions at approximately the same theoretical density of the vacancies generated,27 presumably due to a lower lattice damage caused by the protons. With just a few vacancies produced per electron one would expect the overall lattice damage to be one of the lowest. Furthermore, an e-beam based radiation procedure results in a more homogeneous distribution of the vacancies vs. those created


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along the shorter radiation tracks of larger particle. For example, recent studies have shown that the distribution of NV- centers along a proton path in a millimeter-sized diamond crystal is highly non-uniform.28 The distribution of vacancies is more uniform upon irradiation with He+ vs. H+ beam22 but is still less uniform than the one obtained by e-beam.23 The irradiation penetration depth for 40 keV He+ and 3MeV H+ ions is short: 0.20 µm and 50 µm respectively.22 Irradiation with 5 MeV electrons provides for a much longer - up to 7 mm – penetration into the diamond crystal,23 thus, increasing the amount of the material treated in a single run. However, at high e-beam fluences the lattice damage can also start to accrue and contribute to a decreased efficiency of the NV- formation. To summarize, currently e-beam irradiation is considered to be one of the most efficient approaches in terms of the quality of NV- centers produced and the sample quantity that can be treated in a single run – both are essential considerations for a commercial production as well as the experimental runs. Irradiation with fast neutrons (>2,000-fold heavier than electrons) is another efficient method for fabricating vacancies on a large scale at fluences lower than those required for the e-beam.29 However, a residual sample radioactivity that is inevitably induced by fast neutrons is an important concern that prevents a wider adaptation of the method. To the best of the authors’ knowledge no systematic studies of the effects of e-beam fluence on the efficiency of the NV- formation in diamond can be found in the literature. Initially it could be thought that the fluence should be as high as possible in order to achieve the maximally possible concentration of NV-, i.e. 25–30% of Ns.30 However, the fluence must also stay below a threshold value at which vacancy–vacancy aggregation becomes efficient (multivacancy clusters can act as electron acceptors) and below the critical density of the defects (ca. 1022 vacancies/cm3) where the onset of graphitization is observed.31 These considerations call for



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an optimization of the e-beam fluence. In practice, unjustifiably high fluences also increase the treatment time and the production cost dramatically. Thus, in order to address these issues, here we report on the efficiency of formation of NV- centers in micron-sized HPHT particles as a function of the e-beam fluence. Continuous wave and pulsed EPR spectroscopy have been employed for qualitative and quantitative analysis of the triplet type paramagnetic defects appearing in the diamond samples as a result of e-beam irradiation and a consequent thermal annealing. Magnetic resonance results were also complemented by fluorescence spectroscopy and microscopy measurements for the same set of samples.

Experimental Initial microcrystalline Ib-type diamond powder (sample #0) was synthesized by a highpressure high-temperature (HPHT) method.32 The powder has the mean particle size of 15-20 µm as reported by the commercial vendor, Diamond Innovations Inc, USA. Monocrystalline particles had irregular blocky shapes since they had been milled to the designated size from larger HPHT particulate at the vendor site. The total nitrogen content was 170 ppm as measured by an instrumental gas analysis (IGA) performed by EAG Laboratories. The diamond powder was purified from the residual metals by a repeated washing with a 6 M HCl solution before exposing the sample to 3 MeV e-beam. Irradiation was carried out at temperature below approximately 80oC and the samples were collected at the following fluences (see also Table 1): 5×1018 e-/cm2 (sample #1), 1×1019 e-/cm2 (sample #2), 1.7×1019 e-/cm2 (sample #3), 3×1019 e/cm2 (sample #4), 4×1019 e-/cm2 (sample #5), and 5×1019 e-/cm2 (sample #6). Irradiated samples were annealed in vacuum (10-4 Torr) at 850 oC and then acid-washed to purify from sp2 carbon


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and other possible contaminations even though the amounts of impurities were insignificant for this size of the diamond particles. The sample #6 was additionally vacuum-annealed at 1,400 oC (sample #7). Continuous wave (CW) X-band (9.4 GHz) EPR measurements of polycrystalline (nonoriented) diamond samples were carried out using a Bruker EMX - 220 spectrometer (Bruker Scientific Israel Ltd., Rehovot, Israel) installed at the Ben-Gurion University of the Negev (Israel) and equipped with a 53150A frequency counter (Agilent Technologies Inc., Santa Clara, CA) and an Oxford Instruments ESR900 variable temperature accessory (Oxford Instruments plc, Tubney Woods, UK). The spectra were acquired at room temperature (RT, T ~295 K) and 50 K. Accurate determination of electronic g-factors and densities Ns of paramagnetic S = 1/2 species was assisted by a reference sample of a well-purified detonation nanodiamond (ND) powder with g = 2.0028(2) and Ns = 6.3×1019 spins/g.33 Electronic spin-lattice (TSL) and spinspin (TSS) relaxation times were evaluated by analyzing peak-to-peak amplitudes of the central line of a multicomponent g ≈ 2.00 EPR pattern as a function of the incident microwave power, PMW, using the methods described elsewhere.34-35 EPR data processing and simulation of polycrystalline patterns were carried out using WIN-EPR/SimFonia (Bruker), EasySpin 5.1.11,36 and OriginLab (OriginLab Corporation, Northampton, MA) software packages. Additional CW X-band experiments with a calibrated magnetic field were carried out using a Bruker ELEXSYS E-500 spectrometer system equipped with ER 049SX SuperX bridge (all from Bruker BioSpin, Billerica, MA) and installed at NCSU (Raleigh, NC). Microwave frequency was measured with an EIP 548A frequency counter (EIP Microwave, Inc., San Jose, CA). Magnetic field was calibrated using a Metrolab PT2025 NMR precision teslameter (GMW Associates, San Carlos, CA) that has 5 ppm absolute accuracy and 0.1 µT resolution for



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measurement of uniform magnetic fields. For the calibration, magnetic field from 90 to 260 mT was stepped in 5 mT intervals using a Bruker field controller and the magnetic field was measured with the Metrolab teslameter and a 1062-2 probe. The calibration plot of the measured vs. dialed field was found to be linear with R2 = 0.9995 and reproducible from an experiment to an experiment. This calibration plot was used to correct the magnetic field scan for the EPR experimental spectra. The field corrections for g = 4.0 signals were found to be significant. For example, for the dialed field of B0 = 165 mT the correction factor was ∆B0 = -0.3389 ± 0.0008 mT. Pulsed EPR data were acquired using a custom-built Bruker ELEXSYS E-580 spectrometer equipped with SuperQ-FT bridge operating in X-band (9.4 GHz) as well as Q-band (33.69 GHz) modes. The spectrometer was installed at NCSU in Raleigh, NC and equipped with an ER 4118X-MD5W dielectric resonator, an E-580-1030 high power 1 kW traveling wave tube X-band amplifier, a solid state 10 W Q-band amplifier and an ER 4118CF helium flow cryostat for maintaining temperature from 3.8 to 300 K using ER 4112HV-1017 temperature controller (all from BrukerBiospin). Echo-detected field-sweep X-band EPR spectra were measured by using either hard or soft pulses. The hard pulses of 12 ns and 22 ns in length for the first and the second pulse, respectively, were formed using the full 1 kW incident power while for the soft pulses the power was attenuated to 15 W and the length of the pulses increased to 40 ns and 80 ns, respectively. Electron spin echo envelope modulation (ESEEM) X-band spectra were obtained using the hard pulses. The delay between the first and the second pulse was fixed at 180 ns while the delay between the second and the third pulses was incremented by 12 ns steps from the initial value of 370 ns.


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Three pulse ESEEM Q-band spectra were obtained using the full 10 W incident power. The length of all pulses was set to 40 ns with delay of 134 ns between the first and the second pulses. Delay between the second and the third pulses was incremented by 12 ns from the initial value of 108 ns. Numerical simulations of the ESEEM spectra were carried out by Easyspin software36 assuming random orientations of the individual diamond micron-sized crystals in the magnetic field resulting in a powder pattern average. Such an assumption was justified by absence of either additional sharp features in the experimental EPR spectra that could be attributed to the individual crystals and no changes in the EPR spectra upon rotating or shaking the sample tube. To simplify the calculations, the half field transition of the NV- center was approximated as a two level S = 1/2 spin system with an effective electron-nuclear hyperfine tensor Aeff equal to the double of the real A tensor, Aeff = 2A. The obtained values of the principal value components of the real electron-nuclear hyperfine tensor A = [-2.68 -2.68 -2.16] MHz as well as the nuclear quadrupole tensor e2Qq/h = - 6.4 MHz corresponding to the best agreement between experimental and simulated spectra were found to be are identical with the experimental errors to those obtained earlier from a single crystal study.48 Fluorescent measurements were recorded at room temperature using an Olympus IX71 inverted fluorescent microscope with a modular USB spectrometer (HR2000, Ocean Optics, Dunedin, FL). The microscope was also fitted with 5.0 MP CCD color camera (AmScope, MT5000-CCD-CK, United Scope LLC dba AmScope, Irvine, CA). Filtered broadband 100 mW short arc mercury lamp was used for the PL excitation. The excitation light was focused onto a sample using a 40× microscope objective (Olympus, LUCPlanFL N), a 532 nm dichroic mirror (Semrock®, LPD02-532RU), and a 532 nm notch filter (Semrock®, NF01-532U). For optical



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measurements the diamond powder samples were pressed into pellets of approximately 2 mm thick and 5 mm in diameter by hand at room temperature, using a cylindrical metallic holder to ensure that the PL signal is collected from similar volumes of the material. The pallets were placed on glass slides. Multiple measurements were taken from the same sample to ensure reproducibility.

Results Figure 1 shows photographs of representative micron-sized samples irradiated to different e-beam fluences. Sample #1, which was irradiated to the lowest fluence of 5×1018 e-/cm2, has a pinkish coloration. Upon increasing the radiation fluence, a reddish hue becomes more pronounced (sample #4). At the highest fluence of 5×1019 e-/cm2, a brownish hue appears (sample #6).

Fig. 1. A photograph of representative samples irradiated to different fluences: #1 (5×1018 e/cm2), #4 (3×1019 e-/cm2), #5 (4×1019 e-/cm2), and #6 (5×1019 e-/cm2). The irradiated type Ib diamonds are rich with NV- defects and are pink in color due to absorption by the single nitrogen and NV- defects.2 The two defect centers exhibit blue (~640 nm) transmission bands with the red being usually stronger than the blue one. A combination of these two bands yields pink color. Because NV- center concentration


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increases with the e-beam fluence, the contribution from the red transmitted light becomes more pronounced. A brownish hue appearing at the highest radiation fluence is attributed to a formation of additional structural defects that were further characterized by EPR. Continuous Wave EPR Continuous wave room temperature X-band (9.4 GHz) EPR spectra from non-irradiated #0 and irradiated samples ## 1-7 (Fig. 2) were found to be similar to those of HPHT microdiamonds (MD) and fluorescent microdiamonds (FMD) studied in detail earlier.37 Each spectrum revealed a number of well-resolved EPR lines. All the samples studied gave rise to intense EPR lines in the g = 2.00 region. According to ref.,37 the sharp intense lines (group A in Fig. 2) are readily attributed to P1 (N0, nitrogen substitution) and other paramagnetic defects with the electronic spin S = 1/2 while unwanted but unavoidable ferro-and paramagnetic impurities acquired during the technological process give rise to intense broad lines (group B). In addition to the aforementioned signals, the EPR spectra of all the irradiated samples exhibit relatively strong characteristic signals in the half-field g = 4.00 region (group C) as well as symmetric satellite lines, which appear at magnetic fields 30 – 100 mT below and above the intense g = 2.00 signals (group D). All the latter signals are attributed to a polycrystalline pattern of so-called “forbidden” ∆ms = 2 and “allowed” ∆ms = 1 electronic spin transitions of the triplet (S = 1) state paramagnetic defects. Such defects are readily induced by the e-beam irradiation and/or thermal annealing (see 37 and references therein).



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Fig. 2. Black line – a general view of a room temperature CW X-band EPR spectrum of sample #6 recorded at incident microwave power PMW = 20 mW, 100 kHz modulation of amplitude Amod = 1 mT, receiver gain RG = 1×104, and resonant frequency of ν = 9.417 GHz. Arrows indicate different groups of signals discussed in the text: Green line – a zoom for the group A signals, PMW = 200 µW, Amod = 0.02 mT, RG = 1×104; Red line – a zoom for the group C signals, PMW = 100 µW, Amod = 0.1 mT, RG = 2×105, number of spectra averages nac = 25; Blue line – a zoom for the group D signals, PMW = 100 µW, Amod = 0.2 mT, RG = 2×105, nac = 100.

At 50 K an additional narrow single line EPR signal (line width ∆Hpp = 0.13(1) mT; g = 2.0320(2) is detected in the low field wing of the P1 spectrum (Fig. 3). This signal was observed for all the samples and is attributed to the substitutional Ni- ions (Nis-, 3d8, S = 1/2). Nis- is the second (next to N) impurity in synthetic diamonds that originates from Ni-containing precursors.38

The concentration of Nis- in the initial sample #0 is estimated from double

integration of the corresponding EPR component as ~1.5 ppm. The Nis- EPR signals were found


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to be unaffected by the e-beam fluence in any significant degree and will not be discussed further in this report.

Fig. 3. The g = 2.00 region of CW X-band EPR spectrum of sample #0 recorded at T = 50 K, PMW = 200 µW, Amod = 0.02 mT, RG = 1×104 and ν = 9.470 GHz. See text for further details.

Figure 4 and Table 1 summarize changes in the concentration of S = 1/2 defects upon increasing fluence of the e-beam radiation. The total spin concentration was obtained by a double integration of the g = 2.00 EPR signals whereas P1 concentration was determined from a double integration of the low-field hyperfine pattern and by taking into account the contribution of this pattern to the overall double integral in the simulated polycrystalline EPR spectrum of P1 center. The data show clearly that in the non-irradiated sample #0 nearly all S = 1/2 defects are of P1 origin.

Indeed, the relative contributions of the defects originating from natural and

mechanically induced dangling bonds to EPR spectra of micron-sized diamond polycrystallines are considered to be negligible but become quite significant upon further nanonization.37, 39-40



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Upon e-beam irradiation both the total concentration of P1 and other S = 1/2 centers decrease progressively. Specifically, even at the lowest fluence of 5×1018 e-/cm2 the

Fig. 4. Dependence of the primary paramagnetic defects’ concentration on the e-beam irradiation fluence: black circles – all S = 1/2 defects (excluding Nis-), red open stars – P1 centers, green triangles – NV- centers (concentration scale is magnified tenfold). The inset shows zoom-in data for NV- centers. Lines are guides for eyes.


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Table 1. Concentration of EPR-detected primary paramagnetic defects characterized by S = 1/2 and S = 1 electronic spins in micron-sized Ib type HPHT diamond powders as a function of ebeam fluence. Sample #

Fluence

Concentration of primary paramagnetic defects (ppm)

(×1019 e-/cm2) Total defects with

P1 (N0) a

NV- a

S = 1/2 a 0

0

119

111

0

1

0.5

101

63

4

2

1

76

45

7

3

1.7

50

24

8

4

3

45

12

11

5

4

45

10

13

6

5

43

10

12

7b

5

20

7

9

a

Errors in spin concentration do not exceed ±15%

b

Sample # 6 was additionally annealed at 1,400 oC



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P1 concentration drops to almost a half (63 ppm) of the initial 111 ppm recorded for the sample #0. Upon further e-beam irradiation the concentration of P1 centers drops to one tenth of the initial value at the fluence of 3×1019 e-/cm2 and then remains approximately the same at higher fluences. Additional annealing at 1,400 oC (sample #7) did not change the residual P1 concentration while the concentration of non-P1 S = 1/2 defects was found to be less than half of the initial value (i.e., 20 ppm vs. 43 ppm).

Fig. 5. Fluence dependence of X-band half-field EPR spectra recorded at room temperature and

ν = 9.417 GHz. The fluence of e-beam radiation is increasing from bottom to top and is indicated next to each spectrum (samples #0 - 7). All spectra were recorded at the same experimental conditions: PMW = 100 µW, Amod = 0.1 mT, RG = 2×105, nac = 25 and are normalized per unit mass except the spectrum of the non-irradiated sample that was magnified 50-fold. The spectra are shifted vertically for clarity. The vertical red arrow points to the position of a weak signal with g = 4.266(5) while the dashed lines indicate magnetic field positions corresponding to g =


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4.274(5) and g = 4.200(5) (marked by dashed lines and the corresponding black horizontal arrows). While EPR signals in the g = 2.00 region exhibit significant changes upon increasing the fluence of the e-beam radiation that are related to an evolution of the S = 1/2 defects, the most prominent effect of the radiation is an appearance of new EPR signals in g = 4 region for all of the irradiated samples #1-7 (Fig. 5). The signals evolve with fluence and further thermal annealing. Specifically, X-band EPR spectra of the e-beam irradiated micron-sized diamonds reveal half-field signals with g = 4.26(1) that are characteristic of the triplet (S = 1) NV- centers.37, 41 Fig. 5 shows the fluence dependence of the half-field EPR spectra recorded at room temperature. We note that the appearance of the half-field signals is a characteristic signature of the e-beam irradiation while the EPR spectrum from the initial non-irradiated sample #0 has no detectable signals within g = 4.36 – 4.06 range (bottom black line in Fig. 5). In contrast, an EPR spectrum of the sample #1 obtained with the lowest radiation fluence reveals an intense narrow line with an effective g = 4.274(5) and much weaker lines with lower g-factors (red line in Fig. 5). The fluence increase causes a further growth of the g = 4.274 signal as well as a rise of an additional weak signal at g = 4.200(5). The samples #3 and 4 prepared using intermediate e-beam fluences reveal other weak signals with g-factors 4.266(5), 4.179(5) and 4.145(5). All of the aforementioned signals reach their maximal double-integrated intensities at the highest e-beam fluences achieved in this experiment. Notably, the widths of these individual EPR lines is not affected by the radiation fluence. A consequent thermal annealing of the sample #7 at 1,400 oC results in an insignificant decrease in intensities of the g = 4.274 and 4.200 EPR lines whereas the signals with g-factors 4.266, 4.179 and 4.145 disappear entirely (Fig. 5, upper spectrum).



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Recently, it was established that the double-integrated intensity of the “characteristic g = 4.26 line” is proportional to the content of NV- centers measured by the double integration of the entire polycrystalline triplet pattern.37 Such an observation simplifies determination of concentration of NV- triplets in samples #1 – 7. The quantification was carried out by comparing the double-integrated intensities of the g = 4.26(1) EPR lines in all the samples studied with that of a fluorescent microdiamond sample with a known NV- content (sample FMD, NVconcentration 5.4×1017 spin/g, see Table 2 in Ref.37) Results of such quantification are summarized in Table 2 and plotted in Fig. 4. The peak-to-peak amplitude of the g = 4.274 line in NV- EPR spectra of samples # 1-7 was measured as a function of the incident microwave power to estimate electronic spin-lattice and spin-spin relaxation times for this “forbidden” ∆ms = 2 transition. The best fits of the data (not shown) were obtained for a model assuming two types of triplet spins having different electronic relaxation properties. Since the equation for the evaluation of TSL and Tss from the saturation curve34-35 has been developed for the “allowed” ∆ms = 1 transitions, the precise estimation of the relaxation times for “forbidden” ∆ms = 2 transitions would require knowledge of the corresponding transition moments that appears to be beyond the scope of this work. Without knowing the transition moments, both contributions could be characterized by some effective relaxation parameters. The fluence dependence of the effective TSL for each type of the spins is plotted in Fig. 6. The data reveal that the effective fractions of the slow and the fast relaxing spins (ratio ~9/1) remain about the same upon the fluence increase. Note that the actual fractions of the slow and the fast relaxing species could be different because the intensities of the “forbidden” lines are also proportional to the transition moments that were not determined here.


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Fig. 6. Effective electronic spin-lattice relaxation time TSL obtained from an analysis of power saturation of CW X-band EPR spectra for the g = 4.274 “forbidden” ∆ms = 2 line of NV- centers as a function of e-beam radiation fluence: black open circles – slow relaxing spins, red open diamonds – fast relaxing spins. Data for the samples annealed at 1,400 oC are indicated by arrows. See text for details. While the effective TSL for both fast and slow relaxing electronic spin species remain about the same at the low irradiation fluences, an increase in fluence to 3×1019 e-/cm2 and above results in almost twofold reduction in the effective TSL. The consequent annealing of the high fluence irradiated sample #6 at 1,400 oC restores the effective TSL to the initial longer values characteristic for samples prepared using low e-beam fluences (Fig. 6). Pulsed EPR We note that several paramagnetic species other than S =1 centers in diamonds potentially could be responsible for X-band EPR signals in the g ≈ 4 region. For example, the g ≈ 4.3 EPR signals are very common for S = 5/2 paramagnetic ions such as Fe3+. One cannot



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exclude a priori the presence of Fe3+ impurities in synthetic diamonds that went through an additional grinding procedure even though the final particles were acid washed. Thus, in order to further confirm the assignment of the observed g = 4.26 feature to the “forbidden” ∆ms = 2 transition of the NV- centers rather than S = 5/2 impurity species, high resolution pulsed EPR spectroscopy was employed. Fig. 7a shows a field-swept electron spin echo (ESE) detected spectrum of the sample #1 obtained using short hard microwave pulses (1 kW incident power). For echo-detected spectra the signal from P1 centers that dominates the CW-EPR spectrum appears to be strongly distorted and suppressed due to an excessive excitation bandwidth of the microwave pulses as well as their higher spin rotation angles. This signal could be completely recovered using properly tuned long soft pulses (15 W incident microwave power) as shown in Fig. 7b. Impurities from the metal ions that are readily detected by CW EPR spectra are not observed in the ESE spectra presumably due to a fast electronic relaxation of such spin centers at room temperature. The signals from the S = 1 species dominate the measured ESE spectrum (Fig. 8). The signal at g = 4.26, shown separately in Fig. 7c, dominates the low field region of the spectrum. A small additional signal at g = 4.19 is also observed and is in an agreement with the CW EPR spectrum (Fig. 5). The latter signal is more intense in the ESE EPR spectrum of the sample #6 (Fig. 8c). Interestingly, for the sample #6 additional new features appear in the g = 2 region of the “allowed” transitions. These lines are attributed to S = 1 species with axial zero field splitting (ZFS) parameters slightly smaller than those of the NV- centers.


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Fig. 7. Electron spin echo (ESE) detected X-band EPR spectrum of the sample # 1 recorded at ν = 9.634 GHz and different pulse power levels: black lines – power attenuation 18 dB, soft π/2π pulses of 50 ns and 100 ns respectively; red lines - power attenuation 0 dB, hard π/2-π pulses of 12 ns and 20 ns. The top panel (a) shows a general view, (b) is a zoom of g = 2.00 region, (c) is a zoom of half-field g = 4.00 region. All spectra are amplitude normalized. Arrows indicate spectral features corresponding to P1 EPR signal (b) and characteristic g-factors (c).

.



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Fig. 8. ESE room-temperature X-band EPR spectra of the samples # 1 (black line, ν = 9.634 GHz) and #6 (red line, ν = 9.637 GHz) recorded using hard microwave pulses: (a) entire spectrum, (b) zoom-in of the “allowed” triplet transitions region, (c) zoom-in of the half-field “forbidden” g = 4.00 region. The spectra are amplitude normalized.

Figures 9a and 9c show 3-pulse time domain electron spin echo envelope modulation (ESEEM) spectra measured for g = 4.26 line at X-band and Q-band EPR frequencies, respectively. The corresponding Fourier transformed spectra after the base line subtraction are shown in Figures 9b and 9d, respectively, together with the spectral simulations by EasySpin software.

Fig. 9. Room temperature experimental 3-pulse ESEEM spectra measured at g = 4.26 for the sample #1: (a) and (c) are time domain spectra measured at X- and Q-band resonant frequencies, respectively. Fourier transformed spectra are shown in (b) for X-band and (d) for Q-band: black lines are experimental and red lines are simulated spectra. See text for further details.


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Electronic spin-lattice relaxation times (TSL) for the “forbidden” g = 4.26 transition were measured in an inversion recovery experiment. Fig. 10 shows an example of an experimental magnetization recovery curve for the sample #1 and the results of an analysis using a single- and double-exponential fitting.

Fig. 10. Magnetization recovery curve measured at X-band using an inversion recovery pulse sequence at a magnetic field corresponding to g = 4.26 in the EPR spectrum of sample #1: black open circles – experimental points, green line – single exponential recovery with TSL = 1.7 ± 0.1 ms and R2 = 0.933; red line - double exponential recovery with TSL1 = 1.7 ± 0.1 ms, TSL2 = 0.101 ± 0.005 ms and R2 = 0.994.

Photoluminescent properties Photoluminescent (PL) spectra of the samples irradiated to different fluences are illustrated in Fig. 11. For all the fluences up to 3×1019 e-/cm2 applied in this study the PL intensity exceeds that of the sample irradiated to the lowest fluence of 5×1018 e-/cm2. There is no



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significant difference in the PL intensity of the spectra for sample #1 (4 ppm of NV- centers by EPR) and sample #4 (11 ppm of NV- centers) presumably due to a self-quenching of the PL in the pressed powder. While the sample #5 irradiated to 4×1019 e-/cm2 fluences contains the highest density of NV- centers (~13 ppm), its peak intensity drops below that for the sample #1 irradiated to the lowest dose, 5×1018 e-/cm2. For the sample #6 (5×1019 e-/cm2 dose) the intensity decreased even further. After annealing at 1,400oC the brightness of the sample #6 is increased (spectra not shown). From Fig. 11 it can be also seen that the fraction of NV0 centers (zero phonon line (ZPL) peaks at 575nm in the spectra) for the samples #5 and #6 is significantly increased. Upon an increase in the e-beam fluence the ZPL peaks corresponding to NV- centers (637 nm) become less pronounced (Fig.11). Figure 12 shows representative microphotographs of the samples #1, #4 and #6 taken under identical conditions of a green excitation. Visually, the brightness of the sample #4 is higher than that for #1. While the high brightness of the sample #6 is also evident from the micrograph, it can be attributed to a significant luminescent contribution from NV0 centers. The presence of high density of NV0 centers in a high density is also responsible for an orange hue in the sample #6 (Fig. 12c). A higher magnification image reveals a non-uniformity of the emission from different particles within the same pallet (Fig. 12d, e). At least two types of particles can be readily identified: tentatively called “red” (prevalence of NV- centers) and “orange” (prevalence of NV0 centers) particles. At the lowest fluence (sample #1) the population of the red particles prevails. At an intermediate fluence (sample #4) the fraction of the orange particles increases while at the highest fluence (sample #6) the fraction of orange particles dominates.


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Fig. 11. Room temperature photoluminescence spectra of the pressed powder samples of a micron-sized HPHT diamond irradiated to different fluences (samples #1, and #4 - 6) and obtained using 175 ms integration time and a green excitation.

Fig. 12. Microphotographs of fluorescent micron-sized HPHT diamond particles irradiated to different fluences and then annealed (samples #1, #4 and #6). The microphotographs, taken at room temperate under identical conditions of a green excitation and 10 × (a ,b ,c) and 40 × (d, e)



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magnification, demonstrate differences in the brightness and color (d and e) of the individual particles.

Discussion The obtained EPR data provide a clear demonstration that an intense e-beam irradiation causes a significant fluence-dependent reduction of the content of P1 centers inherent to asmanufactured HPHT diamonds. The same observation has been reported for both neutron and ebeam irradiated and annealed type-I diamond crystals.42 Similarly, in our study this P1 reduction is accompanied by an appearance of a new structureless EPR line with essentially the same (within the experimental error) g-factor as previously attributed to negative vacancies.42 The capture of the electrons from the nitrogen substituted sites by these vacancies leads to a formation of positively charged non-paramagnetic P1+ centers. The same vacancies are expected to become mobile at irradiation/annealing temperatures and, therefore, are responsible for the formation and a further evolution of various paramagnetic species with S = 1. The lowest irradiation fluence induces mainly paramagnetic defect centers giving rise to the g = 4.274 EPR line. Upon a further fluence increase the g = 4.274 line continues to grow and new EPR signals become detectable. The signal characterized by g = 4.200 appears first and then, starting with the fluence of 3×1019 e-/cm2, other signals with g = 4.266, 4.179 and 4.145 become detectable by Xband EPR at room temperature. At the highest fluences (sample #6) all of the aforementioned signals are clearly observed (see Fig. 5). The most intense half-field EPR signals can be reliably assigned using available literature data:37 the g = 4.274 line originates from the “forbidden” ∆ms = 2 transition in polycrystalline


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patterns of the triplet (S = 1) NV- paramagnetic centers. It is well established that an irradiation of diamonds leads to formation of a large variety of triplet centers with different from the NVSpin-Hamiltonian (SH) parameters different from those observed for NV-.43 We have chosen the former centers as the main candidates for other half-field signals. EasySpin simulations of polycrystalline pattern EPR spectra in the half-field region have been carried out for several triplet centers using SH parameters listed in Refs.43-44 Table 2 summarizes the results for the effective half-field g-factors obtained from the simulated EPR spectra and g-factors measured from the characteristic features observed in the experiment. The two sets of g-factors are clearly identical within the experimental error. Such a close agreement allows for an unambiguous assignment of all the signals observed in the half-field EPR spectra of the sample #6 (highest ebeam fluence) to W15, W33, W16, W17 and W18 triplet centers (sorted in a descending order of g). It is important to note that the error in g-factor determination of half-field EPR signals reported above is as high as ± 0.005. Such a high error is primarily caused by the lack of proper samples that could serve as reliable g-references for the half-field EPR range. Indeed, magnetic field settings for the majority of modern commercial digital EPR spectrometers are based on readings of pre-calibrated Hall probes. The latter may be easily referenced and corrected for the g = 2.0 region using g-factor standards based on well-characterized organic free radical like DPPH, TCNQ, etc.. Unfortunately, to the best of the authors’ knowledge, there are no reliable gfactor standards for the g = 4.0 region where the half-field EPR spectra are observed. To correct for such a deficiency, we employed a precision Metrolab PT2025 teslameter that is based on measurements of the resonance frequency of a narrow NMR line. Such calibration measurements revealed that the actual magnetic field in the g = 4.0 region was 0.2 - 0.3 mT smaller than those



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indicated by the spectrometer Hall probe. Such a difference yields 0.004 – 0.008 reductions in the observed vs. actual g-factors and explains the systematic discrepancy between effective gfactors of the simulated and experimental spectra listed in Table 2. The use of the PT2025 NMR teslameter as an external field reference provides for a drastic reduction in the systematic difference between the simulated and experimental g-factors. For example, the measurements for

the sample #6 at ν = 9.872556 GHz with no magnetic field correction yield

(W15) =

4.2416. Linearization of magnetic field values in the half-field region using PT2025 NMR

teslameter provides

(W15) = 4.2502 which almost perfectly coincides with the value

obtained by EasySpin simulations at this frequency:

(W15) = 4.2517.

Figure 13 shows a comparison of the experimental room temperature half-field spectrum of sample #6 with the spectrum simulated using EasySpin solidstate/pepper package36 and SH parameters listed in the Table 2. The spectrum was simulated at the same microwave frequency ν = 9.416502 GHz as the experiment and assuming Lorentzian shape of the individual lines with the following widths: ∆Hpp = 0.28 mT (W15), 0.22 mT (W16), 0.24 mT (W17), 0.2 mT (W18). 0.12 mT (W33) and the weighting factors 1:0.18:0.03:0.02:0.02, respectively. Figure 13 shows a close agreement between the simulated and the experimental spectrum.

In support to the

attribution based on the EasySpin simulations, the entire set of the “allowed” ∆ms = 1 lines are observed in the experimental spectra (Fig. 14). The experimental splittings between the principal axis components in these spectra corresponding to the “allowed” transitions are found to be D(W15) = (960 ± 10) ×10-4 cm-1, D(W16) = (840 ± 10) ×10-4 cm-1, D(W17) = (800 ± 10) ×10-4 cm-1 and D(W18) = (720 ± 10) ×10-4 cm-1 well agree with the literature SH parameters listed in the Table 2 and used for the aforementioned simulation. Unlike the well-studied and, thus, the most reliably identified W15 center (NV- defect, C3v symmetry), the centers W16-18 are less


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abandoned. These centers were previously observed in neutron-irradiated and then annealed Ib diamonds and attributed to some defect-impurity complexes.43

Fig. 13. Experimental (black line) and simulated (red) half-field X-band EPR spectra of e-beam irradiated microcrystalline diamond sample #6. The spectra are vertically shifted for a better presentation. See text for details.

W33 center has been observed in X-ray irradiated/annealed synthetic diamonds contained Ni, ~200 ppm of P1 centers and 20-40 ppm of A-centers. W33 center was attributed to a negatively charged vacancy with one nitrogen atom in the first coordination sphere. The center has lower symmetry arising from the presence of a nearby second positively charged nitrogen atom.44 In our sample all extra (to W15) triplet centers arise at the same initial annealing conditions but upon increasing the e-beam fluence. Thus, the high e-beam fluences appear to produce



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sufficiently mobile vacancies for the formation of the triplet centers and also cause further distortions/damages to the impurity enriched diamond lattice. Additional annealing at 1,400 oC completely quenches lower abandoned triplet centers W17, W18 and W33, while retaining only higher abandoned W15 and W16 centers with the integral intensities ratio ~5:1 – compare the upper lines in Fig. 5. Experimental data for the fluence-dependent spin-lattice relaxation obtained from power saturation of the well resolved “forbidden” CW EPR lines of W15 centers reveal some complimentary information on the distribution of these centers in the bulk of the diamond microcrystals as well as on interactions of these centers with the surrounding diamond matrix. First of all, the two-component spin-lattice relaxation is indicative of a largely inhomogeneous distribution of the NV- centers. A slower relaxing component (~90% of all W15 centers) may be attributed to some lonely/sparsely distributed NV- centers whereas a faster relaxing one could arise from NV- clusters (~10%). The main mechanisms of an accelerated electronic relaxation are likely to be the dipole-dipole and spin-spin exchange interactions with other paramagnetic centers of either S = 1/2 or S = 1 spin origin that are formed upon e-beam irradiation and are in a close proximity to each other. Upon increasing the e-beam fluence above 1.7×1019 e-/cm2 both effective TSL components demonstrate about a twofold decrease, which could be explained by the increase in the total density of the paramagnetic centers of this type or, alternatively, by additional distortions of the diamond lattice caused by the radiation. A resumption of the initial (measured for the low fluence samples) effective TSL values after annealing at 1,400 oC votes for the lattice distortion hypothesis. Indeed, after the annealing the density of NV- centers remains practically unchanged but the damage-induced triplet centers clearly disappear (Fig. 5).


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Table 2. A list of triplet (S = 1) centers found in irradiated and annealed type Ib microparticulate HPHT diamonds: SH parameters and effective half-field g-factors of characteristic Xband EPR signals.

Triplet

g1 a

g2 a

g3 a

center

(10

E

D -4

-1 a

(10

-4

simulation b

experiment c

-1 a

cm )

cm )

W15 (NV-)

2.0028

2.0028 2.0028

958.5

0

4.279

4.274

W16

2.0029

2.0026 2.0022

826.5

7.5

4.203

4.200

W17

2.0033

2.0025 2.0018

784.5

22.5

4.182

4.179

W18

2.0033

2.0027 2.0023

711

14.5

4.149

4.145

W19

2.0028

2.0031 2.0029

471

46

4.067

Not observed

W33

2.0028

2.0028 2.0028

931.4

82.4

4.269

4.266

a)

SH parameters taken from Ref. 43(centers W15-W19) and Ref. 44(center W33);

b)

Effective g-factors of the “forbidden” ∆ms = 2 line (

) were obtained from the

minima of the derivative curve of the polycrystalline half-field X-band EPR pattern simulated as conventional EPR signal (i.e., first derivative of the EPR absorption spectrum) using EasySpin software and SH parameters listed in the Table 2 and the experimental resonant frequency of the corresponding X-band EPR spectra;



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c)

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were obtained from the minima of the first derivative curve of the

experimental polycrystalline half-field X-band EPR pattern recorded at non-saturating PMW = 0.1 mW, Amod = 0.1 mT, sweep range 30 mT, sweep points 2048. Error of

determination does not exceed ±0.005.

Fig. 14. Experimental room temperature X-band EPR spectrum of satellite “allowed” ∆ms= 1 lines around g = 2.00 for the sample #6 and ν = 9.417 GHz. An intense truncated signal in the center of the spectrum is due to S = 1/2 centers. A broad background and impurity signals are subtracted. Field swept ESE EPR spectra (Figs. 7 and 8) provide an alternative way for detecting characteristic signals of paramagnetic defects in the diamond lattice. At room temperature the electronic relaxation is sufficiently long even for detecting characteristic signals in both g = 2.0 and g = 4.0 regions of X-band EPR spectra. For CW EPR spectroscopy such long relaxation times could represent a problem for detecting undistorted absorption line shapes because of


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power saturation and rapid passage effects (e.g., see Ref.45). As discussed by Eaton et al.46 on an example of an irradiated quartz, for such samples ESE-detected field swept spectra could be the preferred way to measure pure absorption shapes that are free of passage distortions.47 However, the application of pulse EPR detection to irradiated diamond samples is not as straightforward as for quartz. Specifically, the heterogeneous distribution of the radiation defects results in a pronounced differential weighting of the detected shapes by the electronic relaxation time. Furthermore, when using hard pulses the triplet signals corresponding to the “forbidden” transitions are found to be detected in a positive phase whereas the “allowed” ones demonstrate an inverted phase (cf. Figs. 7 and 8). This may occur due to an over-rotation of the spin magnetization by hard microwave pulses that is further complicated by a powder pattern averaging of the resonance signals from S = 1 spins experiencing an axially symmetric zero field splitting (ZFS). Detailed study of this phenomenon will be reported elsewhere. Finally, the echodetected spectra become to be distorted by nuclear modulations that are particularly strong for NV- centers.

Thus, detection of the distortion-free ESE field-swept EPR spectra from the

irradiated diamond samples has proven to be difficult and required a separate optimization of the pulse parameters for the signals in g = 4.0 and g = 2.0 regions. Nevertheless, optimization of such parameters allowed, for example, for measurements of nearly distortion-free spectra of the P1 defect in the g = 2.0 region (sample #1) and the absorption spectra of three different defects in the sample #6 (Fig. 6c). In order to confirm the assignment of the g = 4.26 signal to the partially “forbidden” halffield transition of the NV- centers, we compare the experimental spectra with the numerical simulations carried out using EasySpin software package36 and hyperfine parameters of NVcenters measured by Orlinski and coworkers from high field ENDOR spectra of an oriented



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single diamond crystal (see simulation details in the experimental section).48 As can be seen from the Figs 9b and 9c, all the main features of the 3-pulse ESEEM spectra are well reproduced in the simulations, providing an unambiguous evidence of the origin of the g = 4.26 signal being due to NV- centers. Small discrepancies between the experimental and simulated spectra (more pronounced at X-band) could be due to a non-ideality of the microwave pulses as well as a small inaccuracy originating from an approximation of the “forbidden” half-field transition by an effective S = 1/2 spin system. Similar to the analysis of power saturation of CW EPR spectra, measurements of electronic spin-lattice relaxation in pulsed inversion recovery experiments also revealed a largely heterogeneous environment of paramagnetic centers characterized by at least two effective relaxation times in magnetic field corresponding to g = 4.0 region (Fig. 10). However, the observed relaxation times (for example, for sample #1 TLS1 = 1.7 ± 0.1 ms, TLS2 = 0.101 ± 0.005 ms) were found to be significantly longer than TLS1 ≈ 0.05 ms and TLS2 ≈ 0.0006 ms determined from the analysis of CW saturation curves. This should not be surprising as the signals detected by pulsed EPR are heavily weighted by the spin species with longer relaxation times and, for example, the signals from the species with TLS2 ≈ 0.0006 ms will be missed entirely. Nevertheless, the pulsed inversion recovery experiments provided yet another demonstration of a broad distribution of the electronic relaxation times in irradiated samples that could be even broader than the one determined from CW X-band EPR experiments. Photoluminescent characteristics of the samples are consistent with the EPR analysis. For irradiation fluences up to 3×1019 e-/cm2 the PL intensity of the samples exceeds the intensity of the sample irradiated to the lowest fluence 5×1018 e-/cm2. Sample #5 irradiated to 4×1019 e-/cm2 fluence contains the highest density of NV- centers (~13 ppm) however, its peak PL intensity


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drops below one for the sample #1 irradiated to the lowest fluence. A further decrease in intensity for for the sample #6 (fluence 5×1019 e-/cm2) can be attributed in part to a selfquenching of NV- centers at these high concentrations. Based on EPR data other possible reasons include a formation of additional triplet centers and structural imperfections caused by ebeam at the highest fluence. A noticeable increase in brightness observed for the sample #6 after the annealing at 1,400oC the brightness of the sample #6 (spectra not shown) is consistent with a restoration of the spin relaxation times and a disappearance of the EPR of W17, W18 and W33 defects. Nevertheless, the intensity of the annealed sample #6 was still lower than that for the sample #1. From Fig. 8 it can be also seen that the fraction of NV0 centers for the samples #5 and #6 is significantly increased. In order for the negatively charged NV- centers to be formed, the electrons should be donated by nearby substitutional N atoms; therefore, a decrease in the density of the substitutional N at higher e-beam fluences observed by EPR is consistent with a growth of the proportion of NV0 defects. An inspection of the samples by a fluorescent microscope reveals the presence of particles emitting red (prevalence of NV- centers) and orange (prevalence of NV0 centers) light with the proportion of the latter increasing with increased e-beam fluence. This phenomenon originates from N atoms being heterogeneously distributed in raw industrial HPHT synthetic diamonds. The nitrogen spatial distribution is known to be strongly dependent on the diamond crystal growth conditions. The impurity concentration in synthetic diamond crystals decreases with increasing the radius of a diamond crystal due to a depletion of nitrogen over time in the metallic catalyst melt.32 Further, the impurity concentration in the diamond crystal depends on the growth sector. Specifically, in the case of the (1 0 0) growth sectors the diamond are completely free of nitrogen.32 As a result, the powder obtained by crushing of HPHT



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microdiamond to smaller sizes exhibits a heterogeneous spatial distribution of nitrogen defects with different particles having different nitrogen content. The density of the centers and the fractions of NV0 and NV- centers are expected to be different in these particles.

Conclusions Formation of paramagnetic centers by e-beam irradiation of synthetic microcrystalline Ibtype high-pressure high-temperature diamonds has been studied by X-band (9 GHz) continuous wave (CW) and X- and Q-band pulsed electron paramagnetic resonance (EPR) as well as fluorescence spectroscopies over a wide range of fluences and upon thermal annealing. It was shown that the EPR spectra in g ≈ 4 region originating from “forbidden” ∆ms = 2 electronic spin transitions allow for a reliable tracking, attribution, and quantification of the triplet centers produced by e-beam fluences. At low fluences mainly W15 triplet centers are formed. An increase in the fluence causes a progressive growth of W15-W18 and W33 triplet centers. At the highest e-beam fluence of 5×1019 e-/cm2 the relative abundances of the triplet centers are 1(W15):0.18(W16):0.03(W17):0.02(W18):0.02(W33). Analysis of effective electron spin-relaxation times corresponding to the “forbidden” W15 signal using power saturation of CW EPR spectra and direct time-domain measurements revealed a significant spatial inhomogeneity of NV- centers. For example, from CW EPR data ~ 90% of the signal originates from sparsely distributed “lonely” NV- centers with remaining ~ 10% attributed to clustered NV- defects. Consequent annealing at 1,400 oC quenches the W17, W18 and W33 triplet centers beyond the EPR detection limit while leaving concentration of W15


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and W16 centers practically unchanged. Annealing also restores effective spin-lattice electronic relaxation time TSL values previously shortened by the intense e-beam irradiation. Room temperature electron spin echo field swept X-band EPR spectra in the g ≈ 4 region were consistent with CW EPR measurements. Electron spin echo envelope modulation (ESEEM) spectra confirmed the origin of the observed signals as NV- S = 1 species and ruled out any contributions from S = 5/2 paramagnetic impurities that could also give rise to EPR signals in the same magnetic field region. Effective spin lattice electronic relaxation time was also found to be heterogeneous but over a broader range than determined by CW EPR. Photoluminescence data are consistent with a change in relative contributions from NVand NV0 centers upon an increase in the e-beam fluence and an appearance of an orange hue in the samples that were initially pink in color. Examination of fluorescence of the individual microdiamond particles under a microscope revealed differences in the brightness and color that are likely related to different nitrogen contents. The latter is an unavoidable consequence of a heterogeneous nitrogen distribution in commercially grown diamond crystals. While samples irradiated to highest fluences contained the highest density of NV- centers, however, their photoluminescence intensity was below one for the sample irradiated to the lowest fluence due to possibly self-quenching of the NV centers and formation of accompanying defects.

Acknowledgements The work has been funded in part by the NHLBI, Department of Health and Human Services, under Contract No. HHSN268201500010C. Time-domain EPR measurements and data modelling were carried out with support of U.S. DOE Contract DE-FG02-02ER15354 to A.I.S.



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EPR instrumentation at NCSU was supported by grants from the National Institutes of Health (no. RR023614), the National Science Foundation (no. CHE-0840501), and NCBC (no. 2009IDG-1015). The authors are thankful to Dr. Mark J. Nilges (University of Illinois at UrbanaChampaign, IL) for a loan of a Metrolab 1062-2 probe.


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TOC Graphic



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Figure Captions Fig. 1. A photograph of representative samples irradiated to different fluences: #1 (5×1018 e/cm2), #4 (3×1019 e-/cm2), #5 (4×1019 e-/cm2), and #6 (5×1019 e-/cm2). Fig. 2. Black line – a general view of a room temperature CW EPR spectrum of sample #6 recorded at incident microwave power PMW = 20 mW, 100 kHz modulation of amplitude Amod = 1 mT, receiver gain RG = 1×104, and resonant frequency of ν = 9.417 GHz. Arrows indicate different groups of signals discussed in the text: Green line – a zoom for the group A signals, PMW = 200 µW, Amod = 0.02 mT, RG = 1×104; Red line – a zoom for the group C signals, PMW = 100 µW, Amod = 0.1 mT, RG = 2×105, number of spectra averages nac = 25; Blue line – a zoom for the group C signals, PMW = 100 µW, Amod = 0.2 mT, RG = 2×105, nac = 100. Fig. 3. The g = 2.00 region of CW X-band EPR spectrum of sample #0 recorded at T = 50 K, PMW = 200 µW, Amod = 0.02 mT, RG = 1×104 and ν = 9.470 GHz. See text for further details. Fig. 4. Dependence of the primary paramagnetic defects’ concentration on the e-beam irradiation fluence: black circles – all S = 1/2 defects (excluding Nis+), red open stars – P1 centers, green triangles – NV- centers (concentration scale is magnified tenfold). The inset shows zoom-in data for NV- centers. Lines are guides for eyes. Fig. 5. Fluence dependence of X-band half-field EPR spectra recorded at room temperature and

ν = 9.417 GHz. The fluence of e-beam radiation is increasing from bottom to top and is indicated next to each spectrum (samples #0 - 7). All spectra were recorded at the same experimental conditions: PMW = 100 µW, Amod = 0.1 mT, RG = 2×105, nac = 25 and are normalized per unit mass except the spectrum of the non-irradiated sample that was magnified 50-fold. The spectra are shifted vertically for clarity. The vertical red arrow points to the position of a weak signal


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with g = 4.266(5) while the dashed lines indicate magnetic field positions corresponding to g = 4.274(5) and g = 4.200(5) (marked by dashed lines and the corresponding black horizontal arrows). Fig. 6. Effective electronic spin-lattice relaxation time TSL obtained from an analysis of power saturation of CW X-band EPR spectra for the g = 4.274 “forbidden” ∆ms = 2 line of NV- centers as a function of e-beam radiation fluence: black open circles – slow relaxing spins, red open diamonds – fast relaxing spins. Data for the samples annealed at 1,400 oC are indicated by arrows. See text for details. Fig. 7. Electron spin echo (ESE) detected X-band EPR spectrum of the sample # 1 recorded at ν = 9.634 GHz and different pulse power levels: black lines – power attenuation 18 dB, soft π/2π pulses of 50 ns and 100 ns respectively; red lines - power attenuation 0 dB, hard π/2-π pulses of 12 ns and 20 ns. The top panel (a) shows a general view, (b) is a zoom of g = 2.00 region, (c) is a zoom of half-field g = 4.00 region. All spectra are amplitude normalized. Arrows indicate the spectral features corresponding to P1 EPR signal (b) and characteristic g-factors (c). Fig. 8. ESE room-temperature X-band EPR spectra of the samples # 1 (black line, ν = 9.634 GHz) and #6 (red line, ν = 9.637 GHz) recorded using hard microwave pulses: (a) entire spectrum, (b) zoom-in of the “allowed” triplet transitions region, (c) zoom-in of the half-field “forbidden” g = 4.00 region. The spectra are amplitude normalized. Fig. 9. Room temperature experimental 3-pulse ESEEM spectra measured at g = 4.26 for the sample #1: (a) and (c) are time domain spectra measured at X- and Q-band resonant frequencies, respectively. Fourier transformed spectra are shown in (b) for X-band and (d) for Q-band: black lines are experimental and red lines are simulated spectra. See text for further details.



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Fig. 10. Magnetization recovery curve measured at X-band using an inversion recovery pulse sequence at a magnetic field corresponding to g = 4.26 in the EPR spectrum of sample #1: black open circles – experimental points, green line – single exponential recovery with TSL = 1.7 ± 0.1 ms and R2 = 0.933; red line - double exponential recovery with TSL1 = 1.7 ± 0.1 ms, TSL2 = 0.101 ± 0.005 ms and R2 = 0.994. Fig. 11. Room temperature photoluminescence spectra of the pressed powder samples of micronsized HPHT diamond irradiated to different fluences (samples #1, and #4 - 6) and obtained using 175 ms integration time and a green excitation. Fig. 12. Microphotographs of fluorescent micron-sized HPHT diamond particles irradiated to different fluences and then annealed (samples #1, #4 and #6). The microphotographs, taken at room temperature under identical conditions of green excitation and 10 × (a ,b ,c) and 40 × (d, e) magnification, demonstrate differences in the brightness and color (d and e) of the individual particles. Fig. 13. Experimental (black line) and simulated (red) half-field X-band EPR spectra of e-beam irradiated microcrystalline diamond sample #6. The spectra are vertically shifted for a better presentation. See text for details. Fig. 14. Experimental room temperature X-band EPR spectrum of satellite “allowed” ∆ms = 1 lines around g = 2.00 for the sample #6 and ν = 9.417 GHz. An intense truncated signal in the center of the spectrum is due to S = 1/2 centers. A broad background and impurity signals are subtracted.


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Fluence-Dependent Evolution of Paramagnetic Triplet Centers in e

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