Does Progressive Nitrogen Doping Intensify Negatively Charged

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Does Progressive Nitrogen Doping Intensify NV- Emission from e-Beam Irradiated Ib Type HPHT Diamonds? Alexander I. Shames, Vladimir Yu. Osipov, Kirill V. Bogdanov, Alexander V. Baranov, Marianna V. Zhukovskaya, Adam Dalis, Suresh S. Vagarali, and Arfaan A. Rampersaud J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12827 • Publication Date (Web): 14 Feb 2017 Downloaded from http://pubs.acs.org on February 19, 2017

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The Journal of Physical Chemistry

Does Progressive Nitrogen Doping Intensify NV- Emission from e-Beam Irradiated Ib Type HPHT Diamonds? Alexander I. Shames,1* Vladimir Yu. Osipov,2† Kirill V. Bogdanov3, Alexander V. Baranov3, Marianna V. Zhukovskaya3, Adam Dalis,4 Suresh S. Vagarali,4 Arfaan Rampersaud5 1

Department of Physics, Ben-Gurion University of the Negev, Be’er-Sheva, P.O. Box 653, 8410501, Israel

2

Ioffe Physical-Technical Institute, Polytechnicheskaya 26, St. Petersburg, 194021, Russia

3

ITMO University, Kronverksky pr. 49, St. Petersburg, 197101, Russia

4

Sandvik Hyperion, 6325 Huntley Road, P.O. Box 568, Worthington, Ohio 43085, USA

5

Columbus Nanowork Inc., 1507 Chambers Road, Columbus, Ohio 43212, USA

*

Corresponding author. Fax: +97286428529, e-mail: [email protected] Corresponding author. Fax: +78122971017, e-mail: [email protected],

†

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Abstract

Micron sized samples of Ib type HPHT diamonds synthesized with low and high substitutional nitrogen content and high energy e-beam irradiated to form luminescent negatively charged nitrogen-vacancy (NV-) centers are studied by X-band electron paramagnetic resonance (EPR), photoluminescence (PL) and Raman techniques. High nitrogen doping leads to the appearance of paramagnetic centers characterized by strong interactions between unpaired spins of substitutional nitrogen defects. Actual concentrations of paramagnetic substitutional nitrogen and NV- centers were obtained by EPR. Intensity of the PL emission from NV- centers was analyzed as a function of the content of NV- centers. We report that the NV- PL intensity is controlled by both the content of NV- centers and the presence of nitrogen related crystal defects/imperfections. Increasing nitrogen content increases structural imperfections which are responsible for the appearance of additional nonradiative recombination centers and significant intensification of PL quenching. It is suggested that PL intensity may be optimized by appropriate choice of nitrogen doping and irradiation fluence.

Keywords HPHT, diamonds, irradiation defects, EPR, FTIR, fluorescence


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1. Introduction Diamonds containing NV- color centers are within the scope of recent scientific and technological

interests

as

prospective

molecular

photon

emitters

for

quantum

telecommunication or qubits in quantum computing.1,2 The fairly long spin coherence time of their electron spin-triplet ground state at room temperature, makes NV- centers attractive objects for both research and technological applications.3,4 In addition, NV- centers may be effectively used as precise Optically Detected Magnetic Resonance (ODMR) sensors in magnetometry of super weak fields, single molecular Nuclear Magnetic Resonance (NMR) and super-high resolution NMR imaging (MRI) as well as single spin Electron Paramagnetic Resonance techniques. Submicron- and nano-sized NV- centers containing diamonds are under development as sensors for monitoring disease, as drug delivery vehicles for the treatment of cancer, and, at the cellular and subcellular levels, as unique biomarkers for the early detection of cancer, and cardiovascular and neurological diseases.5,6,7,8 The features of NV- center diamonds that are important for their use in biological systems include their remarkable photostability during persistent and intense optical pumping, absence of blinking and degradation, large Stokes shift, and their biocompatibility.9,10 Typical NV- emission within the spectral band over 600 nm lies close to the window of transparency for biological tissues and, thus, may be effectively used for optical bio-labeling of living objects including subcutaneous viscera visualization. The nitrogen-vacancy (NV) center is an impurity within the diamond lattice in which a nitrogen atom is adjacent to a vacancy.

Nitrogen is the most common inherent,

substitutional impurity in diamond crystals of both natural and artificial origins, whereas single, (non-aggregated), vacancies are induced purposefully. Usually this involves bombarding pristine diamond crystallites with high energy (up to 10 MeV) electron or proton beams to create vacancies within the diamond lattice. Irradiated diamonds are then annealed 3 ACS Paragon Plus Environment


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at 850 – 900 oC for several hours to mobilize the vacancies which are captured by substitutional nitrogen centers resulting in stable NV- centers. Additional details of NVfabrication for both micron-size and nanometer-sized diamond crystals have been described in Ref. [11,12]. The electron paramagnetic resonance (EPR) properties of NV- centers in diamonds, from various origins, were first described by van Wyk and co-workers13 and since have been designated as W15 paramagnetic centers. Such a center has electronic spin S = 1 and its EPR spectra are characterized by a set of multiple lines which correspond to so-called “allowed” (∆MS = 1) and “forbidden” (∆MS = 2) microwave transitions between Zeeman levels of triplet paramagnetic centers. Practical applications of NV- center diamonds, especially for biological imaging, demand optimization of the intensity of NV- related photoluminescence (PL). The simplest way of creating very bright fluorescent diamonds is just increasing the substitutional nitrogen content within a diamond to increase NV- density. Such an increase in the concentration of NV- centers within a diamond should lead to significant increases in the intensity of its optical fluorescent emission. However, increasing the substitutional nitrogen content in diamond crystal also causes crystal imperfectness due to formation of undesirable (for the aforementioned application) defects of A-type (close N-N pairs), B-type (4N-V complexes), H3 centers (N-V-N in a neutral charge state), clusterized nitrogen (tens of nitrogen atoms accumulated within nano-areas of ~3-4 nm sizes) etc.14,15,16 All these defects, together with vacancy clusters, which form from the accumulation of single vacancies during irradiation, milling and other treatments,14 may work as effective quenchers of useful PL emission from NV- centers. Fabricating fluorescent diamonds that are exceptionally bright due to high NV- center densities is not trivial, and simply increasing substitutional nitrogen density, might not, a priori, lead to desirable increases in NV- emission. Indeed, one opposing process that might 4 ACS Paragon Plus Environment

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accompany any increase in NV- density is an increase in the types of fluorescence quenching processes. As a rule, defects located a few tens of nanometers from the emitting center, (NVin this case), effectively quench PL by several mechanisms. Quenching mechanisms could include capture of photoexcited electrons by different defects in the crystal lattice and other nonradiative channels of energy dissipation such as Foster resonant energy transfer (FRET).17,18 Defects within the diamond crystal lattice which

quench PL can include

intergrain boundaries, dislocations, multi-vacancies complexes, micro-hollows, crystallite surface. All these defects increase local inhomogeneities and degradation of the crystal’s optical properties. Quenching mechanisms that modulate NV- emission intensity still have not been reliably proven. The observation of a plateau-like maximum in NV- emission on reaching the nitrogen content around 150-200 ppm has been reported.19 However, there are still no reliable data on the dependence of the emission intensity on the content of NV- centers responsible for such an emission in micron-sized crystals. Synthetic diamond crystals manufactured by high pressure high temperature (HPHT) techniques usually contain ppm levels of nitrogen which are unintentionally adsorbed from the atmosphere by the micrographite precursors and metal catalysts used for HPHT synthesis. Nitrogen concentrations higher than atmospheric levels can be achieved by adding additional inorganic nitrogen-containing additives (like sodium azide NaN3 or other azides and nitrides P3N5, BaN3, Ba(N3)2, Fe3N) or even additives of organic type (like C3N6H6) to the precursors.20,21,22 Thus, it is fairly simple to manufacture synthetic microcrystals containing up to 2000 ppm of nitrogen or higher.23,24 However usually these crystals are morphologically defective, having opaque, black or dark green colors, characterized by strong optical absorption and, thus, are useless for optical applications, although fabrication of transparent green and dark green crystals of acceptable quality is also possible in principle.22 Nitrogen 5 ACS Paragon Plus Environment


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content in diamond crystals may be checked by means of EPR (“isolated” neutral Ccenters,)25 and IR (A, B and C-centers) spectroscopies.23,24 The EPR technique is especially useful for studying crystallites of high structural and optical quality with low (up to 300 ppm) C-center content, regardless of the actual crystallite size (from tens of nanometers to millimeters).2525,26 This is because the EPR pattern of substitutional nitrogen in the diamond lattice has a unique triplet structure related with hyperfine interaction of magnetic moment of 14

N nuclei with spin 1/2 of unpaired electron orbital of nitrogen. In this work, we present our EPR and FTIR analysis of NV- center containing

diamonds characterized by different levels of nitrogen within the microcrystals. Microcrystals were prepared by HPHT synthesis and irradiated with high energy e-beams for different times. We show that these samples vary by NV- emission intensity, defectiveness of the crystal structure, and actual content of NV- center estimated by EPR. Our results show that optimization of nitrogen content for creating higher NV- density and PL emission needs to take into account increasing density of surrounding cluster defects affecting the integral PL intensity. 2. Experimental section 2.1 Samples’ preparation Micron sized diamonds were created from carbon precursors by a tightly controlled, high pressure high temperature (HPHT) process. While the detailed procedure is proprietary, the overall HPHT process is published in several reports – see, for instance, Ref. [15] and references therein. Briefly, an HPHT capsule is filled with commercial graphite as the carbon precursor, diamond “seeds,” and a catalyst that lowers the temperature and pressure conditions needed for diamond growth. The HPHT process begins when pressure and temperature within the capsule are increased to between 5–6 GPa and 1300–1600°C, respectively, so that the catalyst melts. The graphite dissolves in the catalyst, and crystallizes 6 ACS Paragon Plus Environment

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on the diamond seeds thus forming new diamond. The catalyst and unconverted graphite are removed by means of concentrated acid treatment, leaving behind diamond crystals containing a nominal level of nitrogen. The amount of nitrogen in the diamond crystals is controlled by doping the catalyst with inorganic nitrogen containing substances in low and high concentrations. Following the synthesis, two microcrystalline diamond batches, differing by doped nitrogen content, were characterized using optical microscopy image analysis and FTIR spectra. Diamond crystals, from the HPHT press, varied in size between about 200 to 400 µm, as measured by a commercially available optical image analyzer.27 The diamond crystals were mostly cubo-octahedral in shape with low levels of catalyst inclusions. Infrared absorption (IR) spectra of the low nitrogen content sample (designated as LN) and the high nitrogen content sample (designated as HN) are shown in Fig. 1. Bands at 1130 cm-1 and 1280 cm-1 are related to absorption due to C- and A-centers respectively.14 The small sharp peak at 1344 cm-1 is directly related to isolated C-defects in a neutral charge state (substitutional nitrogen N0) additionally indicating the presence of C-centers in these diamond samples. At the same time the characteristic absorption line at 1332 cm-1 related to substitutional nitrogen in the positively charged state (N+) is not observed in both FTIR spectra. It means that recharging of centers according to the scheme 2N0 → N+ + N- does not practically occur in the pristine HN sample.14,19,20 A wide absorption band in the range from 1700 cm-1 to 2340 cm-1 is due to twophonon absorption of IR in the diamond lattice. The band comprises two peaks at ~2033 cm-1 and ~2160 cm-1 and the dip at 2120 cm-1. The depth of that dip characterizes two-phonon absorption in bulk diamond.19,20 This spectral feature may be considered as a universal characteristic in various types of bulk diamonds. The integral intensities and other general 7 ACS Paragon Plus Environment


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features of this broad band are independent of starting nitrogen content. In contrast to the two-phonon absorption band, the intensities of the 1130 cm-1 and 1280 cm-1 bands in LN and HN strongly differ. Based on these data, the content of C-defects in HN is at least twice higher than that in LN. Using an empirical formula proposed for fine (particle size >5 µm) diamond crystals,19 we estimate the concentrations of С-centers as 340 ± 50 ppm in LN and 650 ± 50 ppm in HN. Details of the estimation technique may be found in Supporting Information (SI), Eq. (1) and Fig. S1. It is notable that the intensities of the sharp absorption line at 1344 cm-1 (due to N0) differ about two times. Based on the analysis of the ~1280 cm-1 band intensity,16 we estimate the A-center content in LN is below 0.8% whereas in HN it is ~3.5%.

Fig. 1. FTIR spectra of two microcrystalline diamond batches (5% diamond crystals in KBr): black trace – LN, red trace – HN.

Following their initial FTIR characterization, diamonds were milled to between 5 to 20 microns, and then extensively cleaned in concentrated acid. The LN and HN diamond


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samples were spread across a 774 cm2 stainless steel plate, and irradiated with 5 MeV electron beam (25 mA) applied for 16 hours. Based on capturing at least 60% of the total electrons within the area, we estimated the fluence to be approximately 7.0×1018 e-/cm2. In some cases, the irradiation was for a shorter period (5 hours) and the estimated fluence was calculated at 2.2×1018 e-/cm2. Following irradiation, the diamonds were annealed in an inert gas atmosphere at 800 ºC for 6 hours, and then air oxidized at 450 ºC for 1 hour. At this point the diamond sample appeared as a light purple powder. These irradiated and annealed samples are designated as follows: Low Nitrogen content/Low Fluence irradiation - LNLF; High Nitrogen content /Low Fluence irradiation – HNLF; Low Nitrogen content / High Fluence irradiation - LNHF and High Nitrogen content /High Fluence irradiation - HNHF.

Fig. 2. Fluorescence from NV- center diamonds prepared at low nitrogen (LN) and high nitrogen (HN) conditions then irradiated with electrons to achieve low electro fluences (LF) and high electron fluences (HF). Images were collected on a Nikon TiS epifluorescence microscope using a Texas Red filter cube. The isolated particles of micron size are clearly seen on some low-intensity images. All images were collected under identical conditions, 10 msec exposure using a 10x objective.



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Figure 2 shows optical images of fabricated fluorescent microcrystals after milling. The fluorescence images were taken on a Nikon Eclipse Ti S epifluorescence inverted microscope using a standard Texas Red filter cube (excitation 540-580 nm, dichroic 595 nm, emission 600 – 660 nm). This is a standard filter cube that is able to capture most of the emission from NV-center. The LN and HN microcrystalline powder samples, which were irradiated with different e-beam fluences, demonstrate very different NV- center emission intensities. Some fluorescence speckling was observed in the dark images (for example, HNHF image in Fig.2) representing the emission of a few individual diamond crystallites of micron sizes. The brightest emission is seen in the diamonds that contained low levels of nitrogen and received the largest e-beam fluences. The diamonds having the dimmest fluorescence were those that contained high levels of nitrogen and received a low e-beam fluence. It is notable that diamonds that contained low nitrogen levels and that received a low e-beam fluence showed brighter fluorescence than those that had high nitrogen levels and received large e-beam fluence. Based on the results shown in Fig. 2, increasing electron fluence appears to increase fluorescence intensity presumably by increasing the actual number of NV- color centers. However, a high nitrogen concentration does not increase the fluorescence.

2.2 EPR measurements Continuous wave X-band (ν = 9.4 GHz) EPR measurements on quasi-polycrystalline samples were carried out using a Bruker EMX-220 spectrometer equipped with Agilent 53150A frequency counter at room temperature (RT, T ~295 K). Precise determination of gfactors (for spin S = 1/2 species) and densities of paramagnetic centers Ns were done by comparison with the reference sample - well purified detonation ND powder with g = 2.0028(2) and Ns = 6.3×1019 spins/g.28 Evaluation of the electron spin-lattice and spin-spin 10 ACS Paragon Plus Environment

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relaxation times (TSLe and TSSe, correspondingly) was done by analysis of the saturation dependencies of the peak-to-peak intensities of the multicomponent central g = 2.00 EPR line following the technique described in Ref. [29]. Spectra processing and simulation were done using Bruker’s WIN-EPR/SimFonia and OriginLab software.

2.3 PL and Raman measurements PL and Raman characterization of nitrogen-doped and irradiated diamond samples were performed using "inVia" Renishaw micro-Raman spectrometer. The spectrometer was equipped by a thermoelecrically-cooled CCD detector and 3000 mm-1 diffraction grating allowing 0.3 cm-1 spectral resolution of Raman and PL spectra excited by 457.9 nm and 488 nm laser wavelengths. For quantitative comparison of emission of different diamond samples, both diamond Raman and NV--center PL spectra were normalized to the spectral sensitivity of the spectrometer and to the 1332 cm-1 diamond Raman band intensity. The Raman band measurement normalizes the PL response and provides assurance that the measured PL emission comes from the equal volume of samples. Thus, any observed difference in the PL intensity can be related solely to the change in concentration of the NV centers in the diamond crystal lattice. Since the PL intensities are significantly higher than the Raman bands, two acquisition modes were used. First, an overall Raman/PL spectrum was recorded with a relatively short accumulation time (30 s). Second, the part of the spectrum comprising Raman bands were recorded with an accumulation time of 3000 s to provide a more reliable measurement of band intensities. The width of the 1332 cm-1 Raman band for different diamond crystals was used to estimate the degree of crystal lattice disorder and was measured with 457.9 nm excitation line where the profile of the Raman band can be measured with more accuracy. All measurements were done at room temperature.



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3. Results and discussion The general features of room temperature (RT) EPR spectra obtained from diamonds irradiated at low- and high-electron fluences are consistent with those reported previously by us, Ref. [30] (see comparison of samples MD and FMD). Each general view spectrum (SI, Fig. S2) contains several groups of EPR signals which have been successfully recognized and reliably assigned:30 (i) intense signals in the region of g=2.00 due to P1 (also called C-centers or N0, substitutional nitrogen in a neutral charge state) and other paramagnetic defects with S = 1/2; (ii) intense broad lines in the g~4 region attributed to technologically induced unwanted ferro-and paramagnetic impurities; (iii) well distinguishable narrow signals within the half-field (HF) region (g = 4.00); and (iv) weak satellite signals symmetrically located at distances ~50 and ~100 mT at low- and high field regions of the g = 2.00 signals. Signals of both (iii) and (iv) groups were attributed to so-called “forbidden” ∆MS = 2 and “allowed”

∆MS = 1 microwave transitions in polycrystalline patterns of triplet (S = 1) paramagnetic defects, induced by the e-beam irradiation/annealing technique (see Ref. [30] and references therein). Figures 3a and 3b shows representative RT EPR spectra within the g = 2.00 region for all samples under study. Spectra of LNLF and LNHF in Fig. 3a demonstrate prevailing contribution of “isolated” (or, better, - weakly interacting distant) P1 centers with S = 1/2, nuclear spin of

14

N I = 1 and Spin-Hamiltonian (SH) parameters determined by SimFonia

simulations: giso = 2.0024(2), Azz = 4.07(2) mT, Axx = Ayy = 2.93(2) mT, individual line widths

∆HppLorentz = 0.10(2) mT (LNLF) and 0.27(2) mT (LNHF). Deconvolution of the experimental spectra in Fig. 3(a) (SI, Fig. S3) reveals presence of another weak singlet Lorentzian-like signal with g = 2.0024 and ∆Hpp ~0.5 mT (LNLF) and ~0.8 mT (LNHF).


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In contrast to the LN samples, EPR spectra of the HN samples show clearly observed superposition of two distinguishable types of signals: (I) broadened P1 patterns which may be described by the aforementioned SH parameters except ∆HppLorentz = 0.43(2) mT (HNLF) and 0.73(2) mT (HNHF) and (II) intense singlet Lorentzian-like line with the same g-factor and ∆Hpp ~2.46(5) mT. Deconvolutions of HNLF and HNHF EPR spectra are presented in Figs. 3c,d.

Fig. 3. (a,b) g = 2.00 region RT EPR spectra of the micron-sized diamond samples; black traces –e-beam irradiated by 2.2×1018 e-/cm2, red traces – e-beam irradiated by 7.0×1018 e-/cm2: low N content samples (LNLF, LNHF), ν = 9.466 GHz (a); high N content samples (HNLF, HNHF), ν = 9.465 GHz, (b). Spectra were recorded in the same experimental conditions: non-saturating microwave power level PMW = 20 µW, 100 kHz magnetic field modulation with the amplitude Amod = 0.02 mT, receiver gain RG = 2×104; (c,d) deconvolution of EPR spectra in Fig. 3(b); black trace – experimental spectrum; red trace - spectrum of so-called “isolated” P1 obtained by subtraction of broad Lorentzian line; green trace – simulated spectrum of broad Lorentzian line: LNLF (c) and LNHF (d); (e,f) half-field (g = 4.00) RT EPR spectra, black traces –e-beam irradiated by 2.2×1018 e-/cm2, red traces – e-beam irradiated by 7.0×1018 e-/cm2: (e) LNLF, LNHF, RG = 2×105, ν = 9.466 GHz; (f) HNLF, HNHF, RG = 2×106, ν = 9.465 GHz. Spectra were recorded in the same experimental conditions (except RG): PMW = 100 µW, Amod = 0.1 mT, number of coherent acquisitions naq = 25. All intensities are normalized per unit mass; spectra are shifted vertically for better presentation in (e) and (f).



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The most prominent features observed in the EPR spectra of all irradiated/annealed samples is the appearance of new, fluence-dependent EPR signals. Thus, the EPR spectra of e-beam irradiated fluorescent micron-sized diamonds reveal clear characteristic half-filed signals with g = 4.26(1) attributed to triplet (S = 1) NV- centers3030,31 - see Fig. 3e,f. The HF EPR spectrum of LNLF shows weak but detectable signal with experimental g =4.27(1) and ∆Hpp ~0.25(2) mT (Fig. 3e, black trace). The threefold increase of the irradiation fluence causes a significant growth of the g = 4.274 line peak intensity as well as almost twofold broadening of the g = 4.27 line - ∆Hpp

~0.44(2) mT. Simulations of experimental

polycrystalline patterns of “allowed” transitions (SI, Fig. S4) provide SH parameters giso = 2.003(1), D = 958(5)×10-4 cm-1 and E ~0 which are in a good agreement with data previously reported for NV- (or W15) triplet centers – see, for instance, Ref. [13] and references therein. Table 1 shows the changes in the content and composition of S = 1/2 defects and magnetic impurities as a function of both initial nitrogen content and applied high energy ebeam fluence. Here the total spin-half defects’ concentrations are obtained by double integration of g = 2.00 signals’, whereas concentrations of “isolated” P1 centers were determined as the differences between the aforementioned values and double integrated singlet signals obtained by subtraction of SymFonia simulated polycrystalline P1 patterns from the corresponding experimental spectra. It is clearly shown that for the LNLF sample 80% of S = 1/2 defects are of the conventional P1 origin. After a high fluence irradiation, (LNHF), both the total content of S = 1/2 defects as well as the P1 content decrease whereas the ratio between P1 and other S = 1/2 defects remains practically the same. Sample HNLF contains much higher content of paramagnetic species, with only 20% of them due to non- (or weakly) interacting P1 centers. Similar to low nitrogen samples, the high fluence irradiation sample, HNHF, causes reduction of total content of paramagnetic species but surprisingly increases P1 content almost twice. 14 ACS Paragon Plus Environment

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Previously30 we reported that integral intensity of the so-called “characteristic g = 4.26 line” is proportional to the content of NV- centers measured by double integration of the entire polycrystalline triplet pattern. This allows straightforward quantification of the fluence dependent content of NV- triplets. The quantification was done by comparison of the integral intensity of g = 4.26(1 ) line in the FMD sample with the known NV- content (5.4×1017 spin/g, see Table 2 in Ref. [3030]) with the intensities of the corresponding lines in EPR spectra of samples under study. Low-fluence irradiation induces in both LNLF and HNLF samples very few NV- centers – below 1 ppm. Higher irradiation fluence creates ~4 ppm of NV- centers, independently off the initial nitrogen content. Estimation of electron spin-lattice relaxation time for S = 1/2 paramagnetic species was done on the central g = 2.0024 line by analyzing microwave saturation dependences measured on all samples (SI, Fig. S5). Following aforementioned spectra’ deconvolutions, best fittings were obtained suggesting two groups of S = 1/2 spins contributing to this central line: slow relaxing spins and fast relaxing spins. We found that TSLe for the slow relaxing spins, which is associated with “isolated” P1, significantly depends on the initial nitrogen content: 20 µs in LNLF and LNHF vs. ~4 µs in HNLF and HNHF (SI, Fig. S6). Irradiation hardly affects TSLe for these spins. Spin-lattice relaxation for the fast relaxing spins (associated with non-P1 centers providing broader singlet lines) appears to be more sensitive to both nitrogen content and electron fluence. Thus, TSLe values for LNLF, LNHF, HNLF and HNHF are 0.6 µs, 0.3 µs, 0.24 µs and 0.18 µs, respectively (SI, Fig. S6). The relative contributions of slow and fast relaxing spins, obtained from saturation curves, agrees well with the corresponding values obtained by EPR spectra’ deconvolution. Independent data on the effects of initial nitrogen content and irradiation fluences on NV- center amount was obtained from the comparative analysis of PL and Raman spectra of



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the samples under study. Fig. 4a presents the diamond Raman bands and NV0 and NVcenters’ PL bands of the samples with different nitrogen content and e-beam fluences, excited at 488 nm radiation. The spectra are dominated by zero-phonon line (ZPL) due to NV- centers (637 nm) and their phonon sideband (PSB) at 640-800 nm.32 In our analysis the integrated PL intensity of NV- centers was used. It was found that both nitrogen content and e-beam fluence affect the NV--center PL. Integrated intensities of ZPL and their PSB are presented in Table 1.

Fig. 4. (a) The diamond Raman bands and NV0 and NV- centers’ PL bands of the samples with different nitrogen content and e-beam fluences. The rectangle in the down left corner points out the region of the Raman scattering shown as zoom in the panel (b); (b) Normalized Raman spectra of the same samples, wavenumbers of the Raman bands are shown. Excitation wavelength 488 nm.

The rectangle in the down left corner in Fig. 4a points out the region of the Raman scattering while Fig. 4b shows normalized Raman spectra of the samples in more details. Appearance of the 1475 cm-1 and 1545 cm-1 broad bands in Raman spectra of LNHF and HNHF samples together with 1332 cm-1 diamond Raman band indicates formation of certain amount of


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amorphous sp2-hybridized carbons at irradiation of samples.33 The widths of the 1332 cm-1 Raman band of samples with different treatments, measured with 457.9 nm excitation line, are presented in Table 1. Correct analysis of the evolution of EPR spectral patterns, occurring due to initial nitrogen content and fluences, must take into account careful, distinguishing between external and intrinsic factors which may affect shape and widths of EPR lines observed. Thus, a first look at the EPR spectra in Fig. 3a,b and the line widths’ values in Table 1 indicates that both starting nitrogen content and increasing irradiation fluence cause P1 line broadening. However, such a conclusion seems to be misleading. Indeed, one can easily see that line broadening in each pair of samples perfectly correlates with the actual level of magnetic impurities which is 4 times higher in high fluence irradiated samples (Table 1). Most likely, irradiation induces additional contamination of diamond samples which is proportional to the fluence. This amount of magnetic contamination is responsible for line broadening and corresponding reduction of peak intensities of P1-related signals, observed in each pair of samples (Figs. 3a,b). Thus, irradiated samples require additional thorough purification to remove these contaminants. Considering technologically induced magnetic impurities as an extrinsic factor, it is possible that increasing electron beam fluence (within the fluence range under study) might not significantly affect actual intrinsic SH parameters of P1 centers. On the other hand, when comparing pair of samples irradiated by the same fluence (LNLF vs. HNLF and LNHF vs. HNHF), one can find that the initial nitrogen content does affect the EPR line width independently of impurities. This broadening cannot be explained by strengthening exchange and dipole-dipole interactions between the lone P1 centers. For instance, P1 contents in LNLF and HDLD are practically the same, whereas ∆Hpp increases of a factor 4. Insignificant difference in impurities level (which is low in both samples) also cannot explain this broadening. The main intrinsic factor which may be responsible for the broadening effect is 17 ACS Paragon Plus Environment


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formation in the high N samples of another S = 1/2 centers, manifested in the appearance of an intense singlet broad line. Just these paramagnetic species are the main factor responsible for the broadening of P1 EPR lines in high N samples. The observed EPR signals could be interpreted more accurately if extrinsic factors are set aside. Main S = 1/2 paramagnetic centers in LNLF sample consists of ~80% P1 and ~20% of other defects induced by both sample’s micronization and irradiation. High fluence irradiation not significantly change both content and distribution of S = 1/2 defects. In contrast, EPR spectra of the HN samples suggest at least two well-distinguished types of Nrelated paramagnetic defects. Thus, along with lone, weakly interacting nitrogen substitution defects (P1), other defects clusters provide structureless, broad Lorentzian-like EPR line. There is also high inhomogeneous distribution of N-related defects in both HNLF and HNHF samples. HNLF sample contains only 20% of P1, the most of S = 1/2 defects seems to be distributed in the diamond lattice as magnetic entities with strongly interacting individual spins. The EPR spectra of HNHF (see Fig. 3b and data in Table 1) reveals a surprising effect: that is, a high irradiation fluence causes not only reduction of total amount of S=1/2 defects (which is observed for low N samples as well) but changes the actual distribution of these defects: upon irradiation, the P1 content increased of a factor 2, whereas content of defects responsible for the broad component decreased correspondingly. It is possible that, at higher irradiation fluences, magnetically concentrated N-related entities are destroyed and subsequently enrich the diamond with “isolated” P1. Half-field g = 4.27(1) EPR lines observed in all investigated samples reliably indicate presence of luminescent NV- centers. Since all these samples underwent irradiation and annealing processing, the appearance of these defects (NV- color centers) is a result of the


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corresponding treatment. Since the actual amount of NV- centers are obtained by double integration of the g = 4.27(1) EPR lines well observed changes in their line widths (Figs. 3c,d) may be neglected. Most likely, broadening of this line in HNHF has the same origin as broadening of P1 EPR lines in the same sample, i.e. the magnetic interaction of NV- defects with impurities. Low irradiation fluence 2.2×1018 e-/cm2 leads to relatively low densities of NV- centers: ~1 ppm in LNLF and ~0.4 ppm in HNLF. Increasing the electron beam fluence causes significant increases in the number of NV- centers. Thus, the LNHF sample shows almost fourfold increase of the NV- content (3.4 ppm) whereas HNHF sample provides tenfold increase (4 ppm). Comparison of PL intensities for different samples (see Table 1) indicates a fairly complicated dependence of NV- emission characteristics on nitrogen content and irradiation fluence. It may be that PL intensity of these color centers in the diamond lattice is controlled by both the number of NV- centers within a crystal and non-radiative dissipation of opticallyexcited electron states due to neighboring crystal defects.34,35 The combined use of PL and Raman data may provide valuable information on the formation of crystal defects. We have shown that additional defects may be induced by HPHT synthesis with nitrogen containing inorganic additive and treatment, and that these defects have an impact on the NV- luminescence intensity. On increasing the nitrogen content in low-fluence irradiated samples LNLF and HNLF the essential decrease in PL intensity is observed. Here it is worth mentioning that this decrease is more pronounced that just a simple reduction of the content of NV- centers detected by EPR - 5.8 vs 2.3. Such an effect may be attributed to the PL quenching through the point-like crystal defects (A-, B-centers etc.) and extended imperfections arising due to the excessive nitrogen doping.



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The presence of defects in the samples is clearly seen from the analysis of the 1332 cm-1 Raman bandwidth (FWHM). Even at low nitrogen content substitutional nitrogen impurities disturb the crystalline lattice and induce in the diamond crystal structure some additional disorder. This disorder causes broadening of the diamond Raman band (3.4 cm-1) compared to that in defect free bulk diamond crystals (1.6 cm-1).36 Further growth of the substitutional nitrogen content in HNLF sample (above 500 ppm) increases density of diamond lattice point-like defects and extended imperfections which is accompanied by additional broadening of the diamond Raman band (4.2 cm-1) and pronounced reduction of PL intensity. As expected, higher e-beam irradiation fluence causes formation of increasing density of vacancies and, correspondently, increasing number of NV- centers, which is manifested in increase of NV- PL intensities in both LNHF and HNHF samples. The fact that PL intensity in LNHF is substantially stronger than that in HNHF shows that NV- emission characteristics in high fluence irradiated samples are also controlled by vacancies despite the efficiency of luminescence quenchers enhances with growth the nitrogen content. On the other hand the real increase of the PL intensity on increasing irradiation fluence in HN series is ~1.4 times stronger than in LN one. Since the actual amounts of substitutional nitrogen in both LN and HN samples are much higher than resulting amounts of NV- centers, it is reasonable to propose that the vacancies’ densities in LNLF and HNLF are different. High fluence irradiation of the samples does not lead to noticeable broadening of the 1332 cm-1 diamond Raman band. This indicates that appearance of additional vacancies (as evidenced by the increase in both PL and g = 4.27 EPR signal intensities) does not lead to formation of additional defects in the diamond lattice (the latter are responsible for quenching of luminescence). At the same time high fluence irradiation of the samples under study results in formation of some amount of sp2-hybridized amorphous carbon which is evidenced 20 ACS Paragon Plus Environment

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by the appearance of broad bands at 1475 cm-1 and 1545 cm-1 in Raman spectra – see Fig. 4(b).33 However, we found no evidence that amorphous carbon species lead to noticeable luminescence quenching in contrast with defects introduced by substitutional nitrogen. Therefore, the observed luminescence intensity due to NV- centers created by nitrogen doping and e-beam irradiation in Ib type HPHT diamonds is actually determined by the competition between the NV- centers content (number of elementary light emitters) and the presence of crystal defects/imperfections responsible for the PL quenching. Thus, the PL intensity may be optimized by appropriate choice of nitrogen doping and irradiation fluence. Decrease of the nitrogen content is preferable due to the fact that it reduces the density of structural defects and lattice imperfections which are responsible for the non-radiative relaxation of optically excited electron state of NV- centers. Increases of PL intensity is also expected at higher e-beam fluence. Although such an increase is accompanied by formation of the sp2-hybridized amorphous carbon, the latter does not strongly affect the diamond crystal structure and, correspondingly, the NV- emission intensity.

4. Conclusions Samples of Ib type HPHT diamonds synthesized with low and high substitutional nitrogen content and e-beam irradiated to form luminescent NV- centers were studied by EPR and optical (PL and Raman) techniques. Intensity of the PL emission from NV- centers is analyzed as a function of the content of NV- centers directly obtained by EPR in samples with different content of substitutional nitrogen ions and irradiated by low and high e-beam fluence. High nitrogen doping leads to appearance of paramagnetic entities characterized by strong interactions between substitutional nitrogen defects. It is found that the NV- PL intensity is controlled not solely by the content of NV- centers but, to a large extent, by the presence of nitrogen related crystal defects. Increasing nitrogen content during HPHT



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synthesis correspondingly increases this structural imperfection and, thus, is responsible for appearance of additional non-radiative recombination channel via the defects contributing to intensification of PL quenching. It is suggested that PL intensity may be optimized by appropriate choice of nitrogen doping and irradiation fluence.

Supporting Information - estimation of N0 content in starting LN and HN samples from IR spectra: text, IR spectrum (figure); - continuous wave EPR: general view spectra (figure), deconvolution of EPR spectra of LN samples (figure), EPR spectra of “allowed” transitions (figure); - estimations of electron spin-lattice relaxation by microwave power saturation technique: text, saturation curves (figure), electron spin-lattice relaxation times (figure).


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Acknowledgements The ITMO University group gratefully acknowledges the financial support from the Ministry of Education and Science of the Russian Federation (Grant no. 14.B25.31.0002). Contribution of V.Yu.O. was supported by the Russian Science Foundation (Project No. 1413-00795 “Synthesis of Optically Active Materials Based on Nanodiamonds Modified with Ions of 3d−4f Elements”). Production of high and low nitrogen containing diamonds was supported with Federal funds from the National Heart, Lung and Blood Institute, National Institutes of Health, Department of Health and Human Services under contract No. HHSN268201500011C to A.R.



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Table 1. EPR, PL and Raman spectroscopy data

Sample

Irradiation

Total S = ½

“Isolated” P1

Magnetic

∆Hpp

NV-

ZPL and PSB

FWHM of 1332

fluence

content

content

impurities content

(mT)

content

integrated intensity

cm-1 Raman band

(e-/cm2)

(ppm) a

(ppm) a

(arb. units

(ppm) a

(×10-2)

(cm-1)

per mg) b LNLF

2.2×1018

135

108

621

0.10 ± 0.02

0.9

460 ± 50

3.4 ± 0.3

LNHF

7.0×1018

103

86

2523

0.27 ± 0.02

3.4

3540 ± 350

3.6 ± 0.3

HNLF

2.2×1018

580

116

1038

0.43 ± 0.02

0.4

80 ± 10

4.2 ± 0.3

HNHF

7.0×1018

538

215

4025

0.73 ± 0.02

4.0

865 ± 85

4.2 ± 0.3

a

Error in spin concentration determination does not exceed ±15%

b

Error in spin concentration determination does not exceed ±30%

24

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2011, 13, 035024/1-035024/27. 3. Balasubramanian, G.; Neumann, P.; Twitchen, D.; Markham, M.; Kolesov, R.; Mizuochi, N.; Isoya, J.; Achard, J.; Beck, J.; Tissler, J.; et al. Ultralong spin coherence time in isotopically engineered diamond. Nat. Mater. 2009, 8, 383-387. 4. Quantum Information Processing with Diamond - 1st Edition (Eds. C. Prawer, I. Aharonovich), Woodhead Publishing, UK/ USA, 2014, 5. Balasubramanian, G.; Lazariev, A.; Arumugam, S. R.; Duan, D.-W. Nitrogen-Vacancy color center in diamond - emerging nanoscale applications in bioimaging and biosensing. Curr. Opin. Chem. Biol. 2014, 20, 69-77. 6. Chang, H.-C. Development and Use of Fluorescent Nanodiamonds as Cellular Markers, in: Nanodiamonds: Applications in Biology and Nanoscale Medicine (Ed. D. Ho), Springer, 2010. 7. Mohan, N.; Chang, H.-C. Fluorescent Nanodiamonds and Their Prospects in Bioimaging, in: Optical Engineering of Diamond (Eds. R. P. Mildren and J. R. Rabeau), Wiley-VCH Verlag GmbH & Co. KGaA, 2013. 8. Suarez-Kelly, L. P. I.; Rampersaud, V.; Moritz, C. E.; Campbell, A. R.; Hu, Z.; Alkahtani, M. H.; Alghannam, F. S.; Hemmer, P.; Carson, W. E.; Rampersaud A. R. Fluorescent nanodiamonds and their use in biomedical research. Proc. SPIE 9762 Advances in Photonics of Quantum Computing, Memory, and Communication IX 2016, 976205.



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18. Tisler, J.; Reuter, R.; Lämmle, A.; Jelezko, F.; Balasubramanian, G.; Hemmer, P. R.; Reinhard, F.; Wrachtrup, J. Highly efficient FRET from a single nitrogen-vacancy center in nanodiamonds to a single organic molecule. ACS Nano. 2011, 5, 7893-7898. 19. Su, L.-J.; Fang, C.-Y.; Chang, Y.-T.; Chen, K.-M.; Yu, Y.-C.; Hsu, J.-H.; Chang, H.-C. Creation of high density ensembles of nitrogen-vacancy centers in nitrogen-rich type Ib nanodiamonds. Nanotechnology 2013, 24, 315702/1-315702/9. 20. Liang, Z. Z.; Kanda, H.; Jia, X.; Ma, H. A.; Zhu, P. W.; Guan, Q.-F.; Zang, C. Y. Synthesis of diamond with high nitrogen concentration from powder catalyst-C-additive NaN3 by HPHT. Carbon 2006, 44, 913-917. 21. Yu, R. Z.; Ma, H. A.; Liang, Z. Z.; Liu, W. Q.; Zheng, Y. J.; Jia, X. HPHT synthesis of diamond with high concentration nitrogen using powder catalyst with additive Ba(N3)2. Diamond Relat. Mater. 2008, 17, 180–184. 22. Sun, S.; Jia, X.; Yan, B.; Wang, F.; Li, Y.; Chen, N.; Ma, H.-A. Synergistic effect of nitrogen and hydrogen on diamond crystal growth at high pressure and high temperature. Diamond Relat. Mat. 2014, 42, 21-27. 23. Fang, C.; Jia, X.-P.; Yan, B.-M.; Chen, N.; Li, Y.-D.; Chen, L.-C.; Guo, L.-S.; Ma, H.-A. Effects of nitrogen and hydrogen co-doped on {100}-oriented single diamond under high temperature and high pressure. Acta Physica Sinica 2015, 64, 228101/1-228101/6. 24. Liang, Z. Z.; Jia, X.; Ma, H.-A.; Zang, C. Y.; Zhu, P. W.; Guan, Q. F.; Kanda, H. Synthesis of HPHT diamond containing high concentrations of nitrogen impurities using NaN3 as dopant in metal-carbon system. Diamond Relat. Mater. 2005, 14, 1932-1935. 25. van Wyk, J. A.; Reynhardt, E. C.; High, G. L.; Kiflawi, I. The dependences of ESR line widths and spin - spin relaxation times of single nitrogen defects on the concentration of nitrogen defects in diamond. J. Phys. D: Appl. Phys. 1997, 30, 1790-1793.



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26. Yavkin, B. V.; Mamin, G. V.; Gafurov, M. R.; Orlinskii, S. B.; Size-dependent concentration of N0 paramagnetic centres in HPHT nanodiamonds. Magn. Res. Solids. eJourn. 2015, 17, 15101/1-15101/7. 27. Schmid, H. G. Rapid quantitative image analysis for particle quality inspection. Powder Metall. 2004, 47, 121-123. 28. Osipov, V. Yu.; Shames, A. I.; Enoki, T.; Takai, K.; Baidakova, M. V.; Vul’, A. Ya. Paramagnetic defects and exchange coupled spins in pristine ultrananocrystalline diamonds. Diamond Relat. Mater. 2007, 16, 2035-2038. 29. Casabianca, L. B.; Shames, A. I.; Panich, A. M.; Shenderova, O.; Frydman, L. Factors affecting DNP NMR in polycrystalline diamond samples. J. Phys. Chem. C 2011, 115 1904119048. 30. Shames, A. I.; Osipov, V. Yu.; Boudou, J.-P.; Panich, A. M.; von Bardeleben, H.-J.; Treussart, F.; Vul’, A. Ya. Magnetic resonance tracking of fluorescent nanodiamond fabrication. J. Phys. D: Appl. Phys. 2015, 48, 155302/1- 155302/13. 31. Shames, A. I.; Osipov, V. Yu.; von Bardeleben, H.-J.; Boudou, J.-P.; Treussart, F.; Vul’ A. Ya. Native and induced triplet nitrogen-vacancy centers in nano- and micro-diamonds: Half-field electron paramagnetic resonance fingerprint. Appl. Phys. Lett. 2014, 104, 06310710631075. 32. Doherty, M. W.; Manson, N. B.; Delaney, P.; Jelezko, F.; Wrachtrup. J.; Hollenberg, L. C. L. The nitrogen-vacancy colour centre in diamond. Phys. Rep. 2013, 528, 1-45. 33. Shroder, R. E.; Nemanich, R. J.; Glass, J. T. Analysis of the composite structures in diamond thin films by Raman spectroscopy. Phys. Rev. B 1990, 41, 3738-3745., 34. Grudinkin, S. A.; Feoktistov, N. A.; Medvedev, A. V.; Bogdanov, K. V.; Baranov, A. V.; Vul`, A. Ya.; Golubev, V. G. Luminescent isolated diamond particles with controllably


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embedded silicon-vacancy colour centres. J. Phys. D: Appl. Phys. 2012, 45, 062001/1062001/4. 35. Bogdanov, K. V.; Zhukovskaya, M. A.; Osipov, V. Yu.; Jentgens, C.; Treussart, F.; Hayashi, T.; Takai, K.; Fedorov, A. V.; Baranov, A. V. Size-dependent Raman and SiVcenter luminescence in polycrystalline nanodiamonds produced by shock wave synthesis. RSC Adv. 2016, 6, 51783-51790. 36. Surovtsev, N. V.; Kupriyanov, I. N.; Malinovsky, V. K.; Gusev, V. A.; Pal'yanov, Yu. N Effect of nitrogen impurities on the Raman line width in diamonds. J. Phys.: Condens. Matter

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TOC Graphic


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Fig. 1. FTIR spectra of two microcrystalline diamond batches (5% diamond crystals in KBr): black trace – LN, red trace – HN. 1155x812mm (150 x 150 DPI)



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Fig. 2. Fluorescence from NV- center diamonds prepared at low nitrogen (LN) and high nitrogen (HN) conditions then irradiated with electrons to achieve low electro fluences (LF) and high electron fluences (HF). Images were collected on a Nikon TiS epifluorescence microscope using a Texas Red filter cube. The isolated particles of micron size are clearly seen on some low-intensity images. All images were collected under identical conditions, 10 msec exposure using a 10x objective. 1155x812mm (150 x 150 DPI)


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Fig. 3. (a,b) g = 2.00 region RT EPR spectra of the micron-sized diamond samples; black traces – e-beam irradiated by 2.2×1018 e-/cm2, red traces – e-beam irradiated by 7.0×1018 e-/cm2: low N content samples (LNLF, LNHF), ν = 9.466 GHz (a); high N content samples (HNLF, HNHF), ν = 9.465 GHz, (b). Spectra were recorded in the same experimental conditions: non-saturating microwave power level PMW = 20 µW, 100 kHz magnetic field modulation with the amplitude Amod = 0.02 mT, receiver gain RG = 2×104; (c,d) deconvolution of EPR spectra in Fig. 3(b); black trace – experimental spectrum; red trace - spectrum of socalled “isolated” P1 obtained by subtraction of broad Lorentzian line; green trace – simulated spectrum of broad Lorentzian line: LNLF (c) and LNHF (d); (e,f) half-field (g = 4.00) RT EPR spectra, black traces –ebeam irradiated by 2.2×1018 e-/cm2, red traces – e-beam irradiated by 7.0×1018 e-/cm2: (e) LNLF, LNHF, RG = 2×105, ν = 9.466 GHz; (f) HNLF, HNHF, RG = 2×106, ν = 9.465 GHz. Spectra were recorded in the same experimental conditions (except RG): PMW = 100 µW, Amod = 0.1 mT, number of coherent acquisitions naq = 25. All intensities are normalized per unit mass; spectra are shifted vertically for better presentation in (e) and (f). 1155x812mm (150 x 150 DPI)



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Fig. 4. (a) The diamond Raman bands and NV0 and NV- centers’ PL bands of the samples with different nitrogen content and e-beam fluences. The rectangle in the down left corner points out the region of the Raman scattering shown as zoom in the panel (b); (b) Normalized Raman spectra of the same samples, wavenumbers of the Raman bands are shown. Excitation wavelength 488 nm. 1155x812mm (150 x 150 DPI)


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TOC 254x190mm (96 x 96 DPI)


Does Progressive Nitrogen Doping Intensify Negatively Charged

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