Quantitative Analysis of Nitrogen Defect N4 in Diamond with

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Quantitative Analysis of Nitrogen Defect N4 in Diamond with Photoluminescence Excited in Region 170-240 nm Hsiao-Chi Lu, Meng-Yeh Lin, Yu-Chain Peng, Jen-Iu Lo, Sheng-Lung Chou, and Bing-Ming Cheng Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac503268q • Publication Date (Web): 26 Sep 2014 Downloaded from http://pubs.acs.org on September 29, 2014

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Analytical Chemistry

Table of contain:

Quantitative Analysis of Nitrogen Defect N4 in Diamond with Photoluminescence Excited in the 170-240 nm Region Hsiao-Chi Lu, Meng-Yeh Lin, Yu-Chain Peng, Jen-Iu Lo, Sheng-Lung Chou and Bing-Ming Cheng

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Concentration of nitrogen defect N4 in diamond can be quantitatively determined with the PLE line at 236.4 nm down to ppb level.

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Quantitative Analysis of Nitrogen Defect N4 in Diamond with Photoluminescence Excited in Region 170-240 nm

Hsiao-Chi Lu,* Meng-Yeh Lin, Yu-Chain Peng, Jen-Iu Lo, Sheng-Lung Chou and Bing-Ming Cheng*

National Synchrotron Radiation Research Center, No. 101, Hsin-Ann Road, Hsinchu Science Park, Hsinchu 30076, Taiwan * E-mail:[email protected]; [email protected]

Abstract Upon excitation at 170-240 nm, diamonds emit strong luminescence in wavelength range 300-700 nm. The spectral features observed in the photoluminescence excitation (PLE) spectra show two vibrational progressions, A and B, related to nitrogen defects N2 and N4, respectively. We used PLE spectra excited in region 170-240 nm to identify the type of diamond, and demonstrate quantitative analysis of the B center as a N4 nitrogen defect in diamonds; the least detectable concentration of the N4 nitrogen defect is about 13 ppb; the sensitivity of PLE is about 30 times than that practicable with infrared absorption spectra.

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INTRODUCTION Not only is diamond a precious gem but also it exhibits unique and outstanding physical and chemical properties such as extreme hardness, great thermal conductivity, minuscule electrical conductivity,1-2 chemical stability and excellent optical transmission of visible and infrared light.3 Diamond is thus an extremely attractive material worldwide. Perfect diamond is a purely covalent crystal comprising carbon atoms tetrahedrally disposed, and it is colorless. The structure of both natural and synthetic diamond is however, not ideal, but possesses defects, and diamond is typically impure, being contaminated with hydrogen, boron, nitrogen and other elements.4-5 These defects and impurities in diamond affect its optical properties and thermal conductivity, and thus its applications; those issues also impinge on its value in the gem market. The identification of impurities and defects in diamond has hence significant scientific ramifications. In diamond, nitrogen is a pervasive impurity with many forms, from an isolated single substitution (diamond type Ib) for a carbon atom through aggregates to platelets (type Ia). The most common aggregates are the A aggregate, comprising two adjacent nitrogen atoms (type IaA), and the B aggregate, possessing a cluster of four nitrogen atoms surrounding a vacancy (type IaB).4 The impurities in diamonds are routinely distinguished by their absorption spectra, which have hence served to characterize and to identify diamonds.6-17 With these absorption techniques, the sensitivities of analysis for these nitrogen impurities are about the level of part per million. A problem with absorption techniques arises from the sample preparation. As diamond is the hardest natural material and can be dissolved intact in no solvent, these properties hinder its preparation for the measurement of absorption spectra, for which purpose a sample must typically be treated as a film, pellet or parallel disc. To solve 2



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this problem, we applied photoluminescence (PL) for the analysis of various diamonds excited with radiation from a synchrotron source;18-19 one can thereby analyze and identify the nitrogen defects in a diamond from its luminescence with excitation in the wavelength range 170-240 nm. The greatest advantage of this analytical technique is that a diamond sample remains intact and entirely undamaged during this PL analysis. Photoluminescence is a sensitive technique to characterize optically the impurities, defects and absorption edges of diamonds. For a quantitative characterization of the nitrogen defects in diamond, infrared absorption has been advantageously developed to determine various centers at a level of ppm.20-25 With the PL technique, our previous work still had limitations to achieve quantitative information.18-19 In the present work, we investigated the quantitative analysis of nitrogen defect N4 in diamond detecting the PL excitation (PLE) at 236 nm, demonstrating that the sensitivity can attain a ppb level.

EXPERIMENTS The

experimental

end-station

for

measurements

of

absorption

and

photoluminescence spectra was described previously.26 The light for excitation was dispersed from beamline 03A with a monochromator (cylindrical grating, 450 lines /mm, focal length 6 m), attached to the 1.5-GeV storage ring in Taiwan’s National Synchrotron Radiation Research Center (NSRRC). The intensity of the synchrotron light is monitored with a gold mesh transmitting about 90 % and recorded with an electrometer (Keithley 6512). The light dispersed from the synchrotron and transmitted through the gold mesh irradiated the specimen on a rotatable holder. For the absorption experiments, the 3


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synchrotron light was incident perpendicular to the diamond samples; the transmitted light irradiated a glass window coated with sodium salicylate to convert to visible light measured with a photomultiplier (Hamamatsu R943-02) in a photon-counting mode. To measure the photoluminescence, we rotated the specimen holder arranged at angles near 45o with respect to both the incident synchrotron source and the entrance slit of the dispersing monochromator. The emission of the sample was analyzed with a monochromator (Jobin-Yvon HR320, focal length 0.32 m). The photoluminescent intensity of the sample was detected with a photomultiplier in a photon-counting mode. Emission spectra were recorded at resolution 0.5 nm with scanning step 0.5 nm, unless noted otherwise in particular figures. To measure PL excitation spectra, we monitored the dispersive emission at a wavelength selected with a grating. Infrared absorption spectra were recorded with an interferometric spectrometer Bomem DA8 or Nicolet Magna 860; a KBr beamsplitter and a HgCdTe or DTGS detector served to span the mid infrared range 4000-500 or 4000-400 cm-1, respectively. These spectra were typically measured with resolution 0.5 cm-1 and 500 scans. A diamond disc (optical quality, deposited as a chemical vapor, thickness 1.20±0.05 mm and diameter 15 mm, from Diamond Materials Advanced Diamond Technology Company) was polished to 20 nm Ra. Two natural diamonds (triangular shape and weight 0.497 and 0.445 carats, Well Expediting Ent. Co., Ltd.) were polished as parallel plates (thicknesses 1.02±0.01 and 0.96±0.01 mm, respectively).

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RESULTS AND DISCUSSION Figure 1 displays infrared spectra of the CVD diamond window and the plate of natural diamond at 300 K. For an ideal diamond, one-phonon transitions near 1330 cm-1 are forbidden by the symmetry of a perfect lattice, but the intrinsic absorption of diamonds can be observed in two-phonon regions. The prominent absorptions in both spectra at 2515, 2442, 2177, 2158, 2094, 2030 and1976cm-1 are associated with these two-phonon absorptions of diamonds.6,7 Other than two-phonon lines, Figure 1(a) contains no feature that can be attributed to an impurity or defect; this CVD diamond window is hence apparently free of structural defect and chemical impurity, so denoted as type IIa.

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(a)

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Figure 1. Infrared absorption spectra of disc diamonds at 300 K: (a) CVD window (thickness 1.20±0.05 mm); (b) natural plate (thickness 0.96±0.01 mm). The spectra were accumulated with 500 scans at resolution 0.5 cm-1.

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Apart from the intrinsic lines due to diamond itself, Figure 1(b) shows characteristic absorptions in region 1500-1000 cm-1. Those lines might be associated with one-phonon absorption due to lattice defects and impurities in this specimen. For the diamond to have an A center signifies the presence of two adjacent substitutional nitrogen atoms (N2), signaled by a prominent absorption line near 1280 cm-1;8 the diamond type is then classified as IaA. The B center implies four nitrogen atoms (N4) on substitutional sites symmetrically surrounding a vacancy; a characteristic line appears near 1175 cm-1 for this diamond denoted as type IaB.9 We accordingly characterize and identify the type of this diamond. For detailed analysis, we converted the ordinate scales of the absorption spectra of these two diamonds to absorption coefficient in cm-1, as shown in Figure 2; the baseline of the curve for the natural diamond plate was corrected with data from the CVD diamond window in region 1500-500 cm-1. The absorption lines of the natural diamond plate appear at the values shown in Figure 2(b). The weak line at 1405 cm-1 is associated with the angular-deformation mode of hydrogen relative to carbon in the >C=CH2 or −CH=CH− group;10-11 the corresponding line for the stretching CH-stretching mode is at 3107 cm-1,11 displayed in Figure 1(b). A prominent line at 1172 cm-1 is the feature of the B center, with accompanying lines at 1362, 1332 and 1011 cm-1. Weak lines at 1280 and 1096 cm-1 are due to an A center; this N2 defect evidently exists in a small proportion of the N4 defect (B center). We hence classified this natural diamond as type IaAB; the A center is much less abundant than the B center (A« B).

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Figure 2. Infrared absorption of diamonds at 300 K: (a) CVD window (thickness 1.20±0.05 mm); (b) natural plate (thickness 0.96±0.01 mm). The spectra were accumulated with 500 scans at resolution 0.5 cm-1.

Upon excitation at 223 nm at 300 K, the emission spectra of the CVD diamond window and of the natural diamond plate are displayed in Figures 3 (a) and (b), respectively. The emission signal from the CVD diamond window is weak and contains two broad lines with maxima about 428 and 470 nm, corresponding to photon energies 2.90 and 2.64 eV, respectively. The natural diamond plate emitted strong signals beginning near 300 nm and extending to 700 nm; the emission spectrum also features two broad lines with maxima about 420 and 470 nm, corresponding to energies 2.96 and 2.64 eV, respectively. The total intensity emitted by the natural diamond plate is about 65 times that of the CVD diamond window. The natural diamond plate contains nitrogen defects, A (N2) and B (N4) centers, which enhance the emissions upon excitation near 220 nm. Having confirmed that photoluminescence is a sensitive technique to characterize optically the impurities and defects of various diamonds, we apply this technique to quantify the nitrogen defects in diamond. Although displaying the shape 7


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and position of the emission features, a luminescence spectrum reveals neither the origin of the luminescence nor the position of the impurity or defect levels, but this information is derivable from excitation measurements.

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wavelength / nm Figure 3. Photoluminescence emission spectra (resolution 0.5 nm, scan step 0.5 nm) of diamonds upon excitation 223 nm at 300 K (a) CVD window (thickness 1.20±0.05 mm); (b) natural plate (thickness 0.96±0.01 mm). The spectra were accumulated for 5 s at each step.

We measured the PL excitation spectra of the CVD diamond window and the natural diamond plate at 300 K on detecting the emission at 428 and 420 nm, respectively, as shown in Figure 4. For the CVD diamond window, the signals recorded were small; no distinct feature was identified, as displayed in Figure 4(a). With careful examination, two PLE thresholds from the CVD diamond window were derived at 235.8 nm (photon energy 5.258 eV) and 226.3 nm (5.479 eV) at 300 K. On 8



comparison with the UV absorption spectrum as displayed in Figure 5(a), we found that the excitation and absorption spectra possess similar optical behavior in threshold region 223-240 nm.

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wavelength / nm Figure 4. Photoluminescence excitation spectra (resolution 0.2 nm, scan step 0.2 nm) at 300 K of (a) a CVD diamond window (thickness 1.20±0.05 mm) monitored at 428.0 nm, and (b) a natural plate (thickness 0.96±0.01 mm) monitored at 420 nm. The spectra were accumulated for 5 s at each step. Figure 4(b) displays the excitation spectrum from the photoluminescence of the natural diamond plate with emission monitored at 420 nm; this spectrum exhibits distinct features, which are separable into two vibrational progressions. According to our preceding works,18-19 progression A includes lines at 222.8, 216.7, 210.9, 205.4 and 200.2 nm with average interval 1267 ± 40 cm−1; this progression is associated with the N2 defect (A center). The other progression has only two members, at 236.4 and 230.0 nm, with interval 1177±40 cm-1; one additional split feature at 231.2 nm appears with the line at 230.0 nm. Based on previous works,18-19 this progression is related to the N4 9


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defect (B center). As the photoluminescence excitation of diamonds characterizes the distinct features of nitrogen defects, we may use this technique to identify sensitively the nitrogen defects in diamonds. The qualitative analysis of nitrogen defects in diamonds with this method is proven; for a quantitative analysis, we must consider the possibility of an effect of saturation on samples. We can learn such saturation information from the measurements of absorptions for the samples in the concerned wavelength region. Figure 5(b) shows the ultraviolet absorption spectrum of natural diamond plate at 300 K; two prominent absorption lines appear near 230.0 and 236.4 nm, which coincide with the characteristic lines of the B center in the PL excitation spectrum. As the absorption of this natural diamond tends to become saturated at wavelengths less than 231 nm, this natural diamond plate can yield no quantitative information below that wavelength. As the distinctive lines in the luminescence excitation of the A center locate below 224 nm, we cannot quantitatively analyze the N2 defect in this diamond; certainly, the distinguishing PLE lines still provide the qualitative information for identification of A center in this diamond. In the case of the B center, the photoluminescence excitation produces two characteristic lines at 230.0 and 236.4 nm; as, for the latter, saturation is no problem for this diamond, we can derive a quantitative knowledge about the N4 defect on monitoring the intensity of PLE at 236.4 nm from this diamond.

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wavelength / nm Figure 5. Absorption spectra (resolution 0.1 nm, scan step 0.05 nm) at 300 K (a) of a CVD diamond window (thickness 1.20±0.05 mm) and (b) natural plate (thickness 0.96±0.01 mm)

Infrared absorption spectra enable one to identify and to analyze quantitatively the nitrogen defects in diamond; we thus could obtain the concentration information for calibration. For a quantitative analysis of the B center in diamond type IaB, we could estimate the nitrogen content in the B-aggregates on measuring the absorption coefficient at 1282 cm-1 (plateau area); NB[ppm] = (79.4 to 103.8) xμ1282 [cm-1].20-24 In diamonds containing A- and B-aggregates, Davis obtained the concentration of the B-aggregates with NB[%] = t/85(1+r), in which t is the total measured absorption coefficient at 1282 cm-1, r equals (2.72 m-1)/(1-0.41m), and m is the ratio of the absorption at 1282 cm-1 to the absorption at the maximum near 1175 cm-1.25 As our natural diamond samples belong to type IaAB, we applied the method of Davis to derive the concentration NB to be 72.7 ppm in the natural diamond plate (data from Figure 2(b) ); similarly, NB of another natural diamond sample was 28.5 ppm. As the CVD 11


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diamond window contains no B center, the value of NB must be zero. Combining these three data, we plot in Figure 6 the intensity measured at 236.4 nm from luminescence excitation vs concentration of B center, NB, in diamond; the relation seems linear.

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To provide multiple samples of the same type from natural sources for analysis is difficult. Although only three data were applied to Figure 6, we believe that we can thereby derive valuable knowledge. From such a linear relation, one can estimate a sensitivity of this technique. The noise level of PLE measured in the CVD diamond window near 236.4 nm was 2~3 counts per 5 s; the least detection of NB is thus estimated to be about 0.013 ppm. The noise level of PLE can be improved on accumulating signals for a greater duration; the detection limit of NB can thereby decrease to the ppb level, whereas the noise level of the measured infrared absorbance was about 0.001; from Figure 2, the best corresponding detection limit is about 0.4 ppm. Although the PL technique was utilized synchrotron radiation source in this work, but the wavelength 12



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employed in the analysis for N4 defect is in the range 225-240 nm. Which is located in the UV region and could in principle be undertaken with conventional spectroscopic equipment. In summary, photoluminescence excitation has a sensitivity of detection at least about 30 times that of infrared spectra, which demonstrates its prospective utility in analytical applications for diamonds.

ACKNOWLEDGMENTS National Science Council of Taiwan (grant 102-2113-M213-005-MY3) and NSRRC provided support for this research.

KEYWORDS: diamond; infrared spectra; nitrogen defects; photoluminescence; synchrotron radiation

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Quantitative Analysis of Nitrogen Defect N4 in Diamond with

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