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13C NMR spectroscopy in diamonds using dynamic nuclear polarization
CHEMICAL PHYSICS LETTERS
Volume 102, number 1
11 November 1983
l3 C NMR SPECTROSCOPY IN DIAMONDS USING DYNAMIC NUCLEAR POLARIZATION M-J. DUIJVESTIJN,
C. VAN DER LUGT, J. SMIDT, R-A. WIND
Department of Applied Physics, Lklft University of Technology. P-0. Box 5046. 2600 GA Delft, 77zeNetherlands
K-W_ ZILM Department of CEemiwy.
Yale University ,225 -Prospect St.. New Haven Connecticut 06511. USA
and D.C. STAPLIN Christensen Im.. Diamond Technology Center. _7532 South 32 70 West. Salt Luke City, Utah 84119, USA
Received 5 August 1983
In diamonds unpaired electrons associated with nitrogen impurities can be used to enhance the 13C NMR signal via the solid-state effect. 13C spectra of three natural and two synthetic diamonds are shown that were obrained in this manner in lo-30 min.
l_ Introduction
[ 121 up to 8 Oe [IO] are found.
The use of the dynamic nuclear polarization (DNP) effect [l-4] greatly facilitates the measurement of 1 3C spectra of solids containing free electrons. By irradiat-
It is difficult to obtain carbon-13 magnetic resonance spectra of diamonds because of the low natural abundance (1.1%) of 13C and the long spin-lattice relaxation times. Retcofsky et al_ [ 131 measured a
ing near the Larmor frequency of these electrons the 13C NMR signal can be enhanced appreciably_ Recently we showed the utility of the DNP effect in coal research [S-7] _This letter presents the first results obtained with diamonds. It is well known that most types of diamonds contain nitrogen impurities. The amount of nitrogen can vary between large limits (1 014-1 OZo atoms cmm3) and depends upon the environment in which the crystals are grown. Considerable attention has been devoted to the electron spin resonance (ESR) spectrum of diamonds [8,9]_ The unpaired electrons are provided by the substitutional nitrogen atoms. The main feature of the spectrum is a triplet caused by interaction of the electrons with the nitrogen nuclei which have a spin of 1. Industrial diamonds also contain ferromagnetic impurities which may cause broadening of the ESR lines [ 10,l l] _Linewidths from 0.1 Oe 0 009-2614/83/0000-0000/$03.00
0 1983 North-Holland
13C spectrum of a gem-quality diamond, consisting of one line with a width smaller than 400 Hz, while
Wilkie et al. [ 141 found two broader lines (l-1.5 kHz) in gem quality and industrial diamonds_ The measuring time is a few days and even then the spectra have a poor signal-to-noise ratio_ Abragam et al. [ 151 observed a l3 C Overhauser effect in graphite. In diamonds, where the electrons are localized, we found a solid-state effect which makes it possible to obtain 13C spectra in a few minutes.
2. Experimental The 13C NMR spectra were obtained using a Bruker CXP 4-100 pulse spectrometer operating at 15 MHz. A Varian klystron was used, providing a continuous microwave power of 8 W at 39.49 GHz. The microwave 25
\‘oIums
102,
CHEIWCAL PHYSICS LETTERS
number 1
equipment and probes for combining DNP with 13C NhlR experiments are described elsewhere [7]. ESR spectra were obtained with a home-built spectrometer operating at 9.07 GHz. Five different samples have been examined. which we shall call A-E for simplicity: A: de Beer’s SDA100 S -10/45 US mesh size: B: GE-MSD 30/40 US mesh size; C: 16000 series_ - 16 t 20 size range: D: congo rounds. untreated: and E: congo rounds. furnace treated. Samples A and B are synthetic commercially produced diamonds containing inclusions of Fe-Co and Fe-M catalyst respectively. Both are type 1 B diamonds and the crystallites xe typical cubo-octahedrd. C-E are naturally occurring type 1.A diamonds. In sample C the most paramagnetic particles were magnct~cally removed and a portion of the remdining diamond visually judged to contain the mosf grapbitic inclusions was used. D and E are congo rounds of 4 and 5 mm diameter respectively and sample E was furnace-treated to enhmce graphite formation on the surf-dce.
3. Results and discussion 3_I. ESR
Two of rhe live avaiLMe diamonds (A and B) have wry broad ESR lines. presumably due to the presence oi ferromagnetic inclusions (Fe. Xi. Co). Their spectra are very sm~ilsr to hose obtained by Dyer et al. [lo]. OUI ESK specfruni of dmnond A is shown in Iig. 1. This specrrunl consisrs of three lines separated 230 0s and the ccntlal line has a peak-to-peak
_
256 Oe
by
distance
34
FIG. 1. ESR specrrurn or diamond A (tisr derivative absorption). blicrowac frcquenc!’ 9.07 GHz: modulation amplitude 50 mOe: werp 1, idrh 256 Or. s\reep time4 min: RC time 1 s-
26
11 November 19 83
&Y of ~6.5 Oe. The linewidth is not likely to be due to interactions among the electrons themselves since diamond A contains almost twice as many free electrons (5 X 10ly cm-s) as B (3 X 10lg cm-3) and B has almost exactly the same spectrum as A. Diamonds C-E have much weaker ESR signals and the three main lines in their spectra are well separated in contrast to the results on A and B. The linewidth M was a few tenths of an oersted and the number of free electrons 1015--1017 cmm3_ These spectra are comparable with the ones Smith et al. [S] show. 3.2. IWR By varying the magnetic field (and 13C frequency accordingly) it was proven that it is the presence of the unpaired electrons supplied by the nitrogen that causes a solid-state-effect enhancement. The mrtuimmil obtairlable enhancement is estimated to be about one hundred for diamonds C-E and even a few times more for diamonds A and B. The *3 C spectra in figs. 2A--2E were measured by applying a 90” pulse to the 13C spins. while irradiating near the Larmor frequency of the free electrons (DNP FID experiment). Diamonds A and B. which have the broad ESR lines, also appear to have a very broad 13C NMR line due to the dipolar interaction with the ferromagnetic impurities_ Because of the breadth of the ESR line the solid-state effect is extremely unresolved for these diamonds. As the 13C NhlR line broadening is of the same origin. we have a positive as well as a negative enhancement within one and the same spectrum which gives rise to dispersion-like spectra as show~l in figs. 2A and 2B. Furthermore we note that the NMR linewidth is comparable to the ESR linewidth. Since the ESR linewidth of the other three d’iamonds is much smaller the solid-state effect is resolved. These samples exhibit one 13C NhlR line with a relatively small width at half height of =200 Hz (13 ppm) or 0.2 Oe. which is again comparable to the ESR linewidth. The lines of diamonds C-E are at the same position, 33 + 3 ppm below TMS, which is comparable to the value of 36 * 3 ppm found by Retcofsky et al. [ 131. There is no sign of a second line in the spectra of figs. 2C--2E. Wilkie et al. [ 143 suggested their downfield resonance, 150 ppm below TMS, to be due to an aromatic type of impurity which relaxes fast. After putting the sample in the magnet under microwave
Volume 102, number 1
CHEMICAL PHYSICS LE-l-l-ERS
140 ‘kHz
I
-100
-50 PPM
1
I
0
50
FROM
TMS
2. t3C DNP FID spectra of diamonds A-E. 13C 90” pulse 3.5 KG;13C frequency 15.095 hlHz. (A, B) 10 scans. recycle de&: 60 s; acquisition 2k points/6 ms, 300 kHz LF fiber; lorentzian line broadening 500 Hz applied. (C-E) 1 scan. waiting time before scan: 30 min; acquisition 2k points/SO ms, 10 kHz LF filter; lorentzian line broadening 20 Hz applied. Fig.
irradiation we waited for 30 rnin before applying the 90’ pulse, which is touch longer than the recycle delay of a few seconds he used.
We measured the 13C Zeeman relaxation behaviour of diamond B. The ‘SC! magnetization was found ultimately to approach its equilibrium value as exp(---r/T1c)1’2, which is common in systems where spin diffusion is absent or slow [ 161. TIC was found to be 220 2 20 s. As the relaxation time T1, of the electrons supplied by the nitrogen is a few milliseconds [ 173, these electrons are not likely to be the cause of TIC_ It is probably the presence of the transition-metal ions with a short relaxation time (a1O-8 s) which causes the 13C spins to relax relatively fast. We also measured the 13C relaxation behaviour of dia-
11 November 1983
mend C. Due to the low concentration of impurities, T,, appeared to be more than half an hour. The 13C NMR signal of diamonds A and B is larger than that of diamonds C-E. This is due to the fact that, in a DNP FID experiment, one only detects the 13C spins in the vicinity of the electrons, because their enhancement is much larger than the enhancement of spins further away. Since A and B have many more free electrons their NMR signal is thus larger_ Spin difusion is very weak among 13C spins and is not able to transfer the polarization completely to further-removed 13C nuclei. Spin diffusion does play its role however, since magic-angle spinning (MAS) [ 151, which inhibits spin diffusion, made the signal decrease by a factor 2 or 3; even at very low spinning frequencies (few hundred Hz). MAS spectra of diamonds A and B were measured with a spinning frequency of * kHz, which made sidebands occur on each side of the central peak. The width of the central peak was x700 Hz. MAS experiments with samples C-E were not done. Summarizing, we may state that we are able to measure 13C NMR spectra of diamonds with a good S/N ratio in a few minutes. At present we observe only one line in the l3 C spectrum even in samples which are believed to contain appreciable amounts of graphite-like material_ The magnitude of the signal provides information about the number of paramagnetic impurities and the width of the lines gives information about the number of ferromagnetic impurities_ In fact, the width is closely related to that of the ESR lines. In the case that the ferromagnetic impurities severely broaden the ESR spectrum an unresolved solid-state effect is observed which gives rise to a characteristic dispersion-like lineshape in the DNP FID experiment_ This is likely to be the case in all synthetically produced diamonds. Finally it is also possible to measure T,, in an acceptable time which can be used to detemrine the number and the relaxation time of the ions responsible for TIC_ This will be further investigated in the near future, as well as the possible uses of this technique for characterizing and grading diamonds and diamond products_
27
Volume
103, number
1
CHEMICAL
Acknowledgement One of us (MD) was supported by the Netherlands Foundation for Chemical Research (S-0-N.) with financial aid from the Netherlands Organization for the Advancement of Pure Research (Z.W.O.).
References [ I] A. Abragam. The principles of nuclear ma_enetism (Oxford
Univ. Press. London, 1961) ch. 9. Spin temperature and nuclear mqgetic resonance in solids (Osford Univ. Press. London. 1970) ch. 7. [3] A. Abragam and hl. Goldman. Rept. Progr. Phys. 41 (197s) 395. ]-I] A. Abrqam and M. Goldman, Nuclear magnetism: order and disorder (Clarendon Press. Olford, 19SZ) ch. 6. [S] R.A. IVind. J. Trammel and J. Smidt. FUEL 56 (1979) 900.
[ 2 1 hl. Goldman.
PHYSICS LETTERS
[61 R.A.
11 November
1983
Wind. J. Trammel and J. Smidt, FUEL 61 (1982) 39s. 171 R-4. Wind, F-E Antonio. J&J_ Duijvestijn, J_ Smidt, J. Trommel and G.M.C. de Vette. J. Magn. Reson. 52 (1983) 424. ISI IV-V. Smith, P.P. Sorokin, 1-L. Gelles and G.J. Lasher, Phys. Rev. 115 (1959) 1546. PI J.H.N. Loubser and J.A. van Wijk, Rept. Progr. Phys. 41 (1978) 1201. H.B. Dyer. F.A. Raal. L. du Preez and J.H.N. Loubser, Phil. hfag. 11 (1965) 763. M.J.A. Smith and B.R. Angel, Phil. hiag. 15 (1967) 783. R.C. Barklie and J. Guven, J. Phys C!4 (1981) 3621. H.L. Retcofsky and RA. Friedel. J. Phys. Chem. 77 (1973) 6s. CA. Wilkie. T-C. Ehlert and D-T_ Haworth. J. Inorp. Nucl. Chem. 40 (1978) 1893. 1151A. Abragam, A. Landesman and J.M. Winter. Compt. Rend. Acad. Sci (Paris) 247 (1958) 1852. I161 D. Tse and S.R. Hartmann, Phys. Rev. Letters 21 (1968) 511. iI71 E.R. Andrew, Phil. Trans. Roy. Sot. (London) A299 (1961) 505.
Volume 102, number 1
11 November 1983
l3 C NMR SPECTROSCOPY IN DIAMONDS USING DYNAMIC NUCLEAR POLARIZATION M-J. DUIJVESTIJN,
C. VAN DER LUGT, J. SMIDT, R-A. WIND
Department of Applied Physics, Lklft University of Technology. P-0. Box 5046. 2600 GA Delft, 77zeNetherlands
K-W_ ZILM Department of CEemiwy.
Yale University ,225 -Prospect St.. New Haven Connecticut 06511. USA
and D.C. STAPLIN Christensen Im.. Diamond Technology Center. _7532 South 32 70 West. Salt Luke City, Utah 84119, USA
Received 5 August 1983
In diamonds unpaired electrons associated with nitrogen impurities can be used to enhance the 13C NMR signal via the solid-state effect. 13C spectra of three natural and two synthetic diamonds are shown that were obrained in this manner in lo-30 min.
l_ Introduction
[ 121 up to 8 Oe [IO] are found.
The use of the dynamic nuclear polarization (DNP) effect [l-4] greatly facilitates the measurement of 1 3C spectra of solids containing free electrons. By irradiat-
It is difficult to obtain carbon-13 magnetic resonance spectra of diamonds because of the low natural abundance (1.1%) of 13C and the long spin-lattice relaxation times. Retcofsky et al_ [ 131 measured a
ing near the Larmor frequency of these electrons the 13C NMR signal can be enhanced appreciably_ Recently we showed the utility of the DNP effect in coal research [S-7] _This letter presents the first results obtained with diamonds. It is well known that most types of diamonds contain nitrogen impurities. The amount of nitrogen can vary between large limits (1 014-1 OZo atoms cmm3) and depends upon the environment in which the crystals are grown. Considerable attention has been devoted to the electron spin resonance (ESR) spectrum of diamonds [8,9]_ The unpaired electrons are provided by the substitutional nitrogen atoms. The main feature of the spectrum is a triplet caused by interaction of the electrons with the nitrogen nuclei which have a spin of 1. Industrial diamonds also contain ferromagnetic impurities which may cause broadening of the ESR lines [ 10,l l] _Linewidths from 0.1 Oe 0 009-2614/83/0000-0000/$03.00
0 1983 North-Holland
13C spectrum of a gem-quality diamond, consisting of one line with a width smaller than 400 Hz, while
Wilkie et al. [ 141 found two broader lines (l-1.5 kHz) in gem quality and industrial diamonds_ The measuring time is a few days and even then the spectra have a poor signal-to-noise ratio_ Abragam et al. [ 151 observed a l3 C Overhauser effect in graphite. In diamonds, where the electrons are localized, we found a solid-state effect which makes it possible to obtain 13C spectra in a few minutes.
2. Experimental The 13C NMR spectra were obtained using a Bruker CXP 4-100 pulse spectrometer operating at 15 MHz. A Varian klystron was used, providing a continuous microwave power of 8 W at 39.49 GHz. The microwave 25
\‘oIums
102,
CHEIWCAL PHYSICS LETTERS
number 1
equipment and probes for combining DNP with 13C NhlR experiments are described elsewhere [7]. ESR spectra were obtained with a home-built spectrometer operating at 9.07 GHz. Five different samples have been examined. which we shall call A-E for simplicity: A: de Beer’s SDA100 S -10/45 US mesh size: B: GE-MSD 30/40 US mesh size; C: 16000 series_ - 16 t 20 size range: D: congo rounds. untreated: and E: congo rounds. furnace treated. Samples A and B are synthetic commercially produced diamonds containing inclusions of Fe-Co and Fe-M catalyst respectively. Both are type 1 B diamonds and the crystallites xe typical cubo-octahedrd. C-E are naturally occurring type 1.A diamonds. In sample C the most paramagnetic particles were magnct~cally removed and a portion of the remdining diamond visually judged to contain the mosf grapbitic inclusions was used. D and E are congo rounds of 4 and 5 mm diameter respectively and sample E was furnace-treated to enhmce graphite formation on the surf-dce.
3. Results and discussion 3_I. ESR
Two of rhe live avaiLMe diamonds (A and B) have wry broad ESR lines. presumably due to the presence oi ferromagnetic inclusions (Fe. Xi. Co). Their spectra are very sm~ilsr to hose obtained by Dyer et al. [lo]. OUI ESK specfruni of dmnond A is shown in Iig. 1. This specrrunl consisrs of three lines separated 230 0s and the ccntlal line has a peak-to-peak
_
256 Oe
by
distance
34
FIG. 1. ESR specrrurn or diamond A (tisr derivative absorption). blicrowac frcquenc!’ 9.07 GHz: modulation amplitude 50 mOe: werp 1, idrh 256 Or. s\reep time4 min: RC time 1 s-
26
11 November 19 83
&Y of ~6.5 Oe. The linewidth is not likely to be due to interactions among the electrons themselves since diamond A contains almost twice as many free electrons (5 X 10ly cm-s) as B (3 X 10lg cm-3) and B has almost exactly the same spectrum as A. Diamonds C-E have much weaker ESR signals and the three main lines in their spectra are well separated in contrast to the results on A and B. The linewidth M was a few tenths of an oersted and the number of free electrons 1015--1017 cmm3_ These spectra are comparable with the ones Smith et al. [S] show. 3.2. IWR By varying the magnetic field (and 13C frequency accordingly) it was proven that it is the presence of the unpaired electrons supplied by the nitrogen that causes a solid-state-effect enhancement. The mrtuimmil obtairlable enhancement is estimated to be about one hundred for diamonds C-E and even a few times more for diamonds A and B. The *3 C spectra in figs. 2A--2E were measured by applying a 90” pulse to the 13C spins. while irradiating near the Larmor frequency of the free electrons (DNP FID experiment). Diamonds A and B. which have the broad ESR lines, also appear to have a very broad 13C NMR line due to the dipolar interaction with the ferromagnetic impurities_ Because of the breadth of the ESR line the solid-state effect is extremely unresolved for these diamonds. As the 13C NhlR line broadening is of the same origin. we have a positive as well as a negative enhancement within one and the same spectrum which gives rise to dispersion-like spectra as show~l in figs. 2A and 2B. Furthermore we note that the NMR linewidth is comparable to the ESR linewidth. Since the ESR linewidth of the other three d’iamonds is much smaller the solid-state effect is resolved. These samples exhibit one 13C NhlR line with a relatively small width at half height of =200 Hz (13 ppm) or 0.2 Oe. which is again comparable to the ESR linewidth. The lines of diamonds C-E are at the same position, 33 + 3 ppm below TMS, which is comparable to the value of 36 * 3 ppm found by Retcofsky et al. [ 131. There is no sign of a second line in the spectra of figs. 2C--2E. Wilkie et al. [ 143 suggested their downfield resonance, 150 ppm below TMS, to be due to an aromatic type of impurity which relaxes fast. After putting the sample in the magnet under microwave
Volume 102, number 1
CHEMICAL PHYSICS LE-l-l-ERS
140 ‘kHz
I
-100
-50 PPM
1
I
0
50
FROM
TMS
2. t3C DNP FID spectra of diamonds A-E. 13C 90” pulse 3.5 KG;13C frequency 15.095 hlHz. (A, B) 10 scans. recycle de&: 60 s; acquisition 2k points/6 ms, 300 kHz LF fiber; lorentzian line broadening 500 Hz applied. (C-E) 1 scan. waiting time before scan: 30 min; acquisition 2k points/SO ms, 10 kHz LF filter; lorentzian line broadening 20 Hz applied. Fig.
irradiation we waited for 30 rnin before applying the 90’ pulse, which is touch longer than the recycle delay of a few seconds he used.
We measured the 13C Zeeman relaxation behaviour of diamond B. The ‘SC! magnetization was found ultimately to approach its equilibrium value as exp(---r/T1c)1’2, which is common in systems where spin diffusion is absent or slow [ 161. TIC was found to be 220 2 20 s. As the relaxation time T1, of the electrons supplied by the nitrogen is a few milliseconds [ 173, these electrons are not likely to be the cause of TIC_ It is probably the presence of the transition-metal ions with a short relaxation time (a1O-8 s) which causes the 13C spins to relax relatively fast. We also measured the 13C relaxation behaviour of dia-
11 November 1983
mend C. Due to the low concentration of impurities, T,, appeared to be more than half an hour. The 13C NMR signal of diamonds A and B is larger than that of diamonds C-E. This is due to the fact that, in a DNP FID experiment, one only detects the 13C spins in the vicinity of the electrons, because their enhancement is much larger than the enhancement of spins further away. Since A and B have many more free electrons their NMR signal is thus larger_ Spin difusion is very weak among 13C spins and is not able to transfer the polarization completely to further-removed 13C nuclei. Spin diffusion does play its role however, since magic-angle spinning (MAS) [ 151, which inhibits spin diffusion, made the signal decrease by a factor 2 or 3; even at very low spinning frequencies (few hundred Hz). MAS spectra of diamonds A and B were measured with a spinning frequency of * kHz, which made sidebands occur on each side of the central peak. The width of the central peak was x700 Hz. MAS experiments with samples C-E were not done. Summarizing, we may state that we are able to measure 13C NMR spectra of diamonds with a good S/N ratio in a few minutes. At present we observe only one line in the l3 C spectrum even in samples which are believed to contain appreciable amounts of graphite-like material_ The magnitude of the signal provides information about the number of paramagnetic impurities and the width of the lines gives information about the number of ferromagnetic impurities_ In fact, the width is closely related to that of the ESR lines. In the case that the ferromagnetic impurities severely broaden the ESR spectrum an unresolved solid-state effect is observed which gives rise to a characteristic dispersion-like lineshape in the DNP FID experiment_ This is likely to be the case in all synthetically produced diamonds. Finally it is also possible to measure T,, in an acceptable time which can be used to detemrine the number and the relaxation time of the ions responsible for TIC_ This will be further investigated in the near future, as well as the possible uses of this technique for characterizing and grading diamonds and diamond products_
27
Volume
103, number
1
CHEMICAL
Acknowledgement One of us (MD) was supported by the Netherlands Foundation for Chemical Research (S-0-N.) with financial aid from the Netherlands Organization for the Advancement of Pure Research (Z.W.O.).
References [ I] A. Abragam. The principles of nuclear ma_enetism (Oxford
Univ. Press. London, 1961) ch. 9. Spin temperature and nuclear mqgetic resonance in solids (Osford Univ. Press. London. 1970) ch. 7. [3] A. Abragam and hl. Goldman. Rept. Progr. Phys. 41 (197s) 395. ]-I] A. Abrqam and M. Goldman, Nuclear magnetism: order and disorder (Clarendon Press. Olford, 19SZ) ch. 6. [S] R.A. IVind. J. Trammel and J. Smidt. FUEL 56 (1979) 900.
[ 2 1 hl. Goldman.
PHYSICS LETTERS
[61 R.A.
11 November
1983
Wind. J. Trammel and J. Smidt, FUEL 61 (1982) 39s. 171 R-4. Wind, F-E Antonio. J&J_ Duijvestijn, J_ Smidt, J. Trommel and G.M.C. de Vette. J. Magn. Reson. 52 (1983) 424. ISI IV-V. Smith, P.P. Sorokin, 1-L. Gelles and G.J. Lasher, Phys. Rev. 115 (1959) 1546. PI J.H.N. Loubser and J.A. van Wijk, Rept. Progr. Phys. 41 (1978) 1201. H.B. Dyer. F.A. Raal. L. du Preez and J.H.N. Loubser, Phil. hfag. 11 (1965) 763. M.J.A. Smith and B.R. Angel, Phil. hiag. 15 (1967) 783. R.C. Barklie and J. Guven, J. Phys C!4 (1981) 3621. H.L. Retcofsky and RA. Friedel. J. Phys. Chem. 77 (1973) 6s. CA. Wilkie. T-C. Ehlert and D-T_ Haworth. J. Inorp. Nucl. Chem. 40 (1978) 1893. 1151A. Abragam, A. Landesman and J.M. Winter. Compt. Rend. Acad. Sci (Paris) 247 (1958) 1852. I161 D. Tse and S.R. Hartmann, Phys. Rev. Letters 21 (1968) 511. iI71 E.R. Andrew, Phil. Trans. Roy. Sot. (London) A299 (1961) 505.