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10B and 11B high-resolution NMR studies on boron-doped diamond
Physica C 470 (2010) S625–S626
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
10
B and
11
B high-resolution NMR studies on boron-doped diamond
M. Murakami a,*, T. Shimizu a, M. Tansho a,b, Y. Takano c, S. Ishii c, E.A. Ekimov d, V.A. Sidorov d, K. Takegoshi e,f a
National Institute for Materials Science, 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan World Premier International Research Center for Materials Nanoarchitectonics, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan c National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan d Russian Academy of Science, Troitsk 142190 Moscow Region, Russia e Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan f CREST, JST, Chiyoda-ku, Tokyo 102-0075, Japan b
a r t i c l e
i n f o
Article history: Accepted 3 December 2009 Available online 11 December 2009 Keywords: Boron-doped diamond 11 B NMR Two-dimensional NMR
a b s t r a c t 11
B magic-angle spinning (MAS) NMR experiments are applied to B-doped diamond samples prepared by high-pressure and high-temperature methods. From the spectrum, we show that there are at least four boron signal components and the one at 28.5 ppm is ascribed to the substitutional boron in the diamond structure providing the carriers responsible for conductivity. We further apply two-dimensional (2D) NMR to examine 1H dipolar broadening and 11B–11B boron spin diffusion, and candidates purported so far for the excess boron, that is, a boron + hydrogen complex and –B–B– and/or –B–C–B– clusters are negated. Furthermore, we show that 10B MAS NMR is useful to selectively observe the substitutional boron in the diamond structure appearing at 28.5 ppm, whose quadrupolar coupling is much smaller than that of the excess boron at 65.5 ppm. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction In 2004, Ekimov et al. discovered superconductivity in heavily boron-doped diamond [1]. So far, two major synthesis techniques of heavily boron-doped diamond are known: high-pressure hightemperature (HPHT) method and chemical vapor deposition (CVD). In contrast to the expectation of higher TC (the superconductivity transition temperature) for higher boron concentrations [B], Takano [2] showed that TC of homoepitaxial (1 0 0) thin films, synthesized by the microwave plasma assisted CVD, increases with increasing [B] and reaches a maximum at [B] 1 1022 cm 3. They also pointed out that TC ([B]) dependences are similar for diamonds grown by the HPHT method and (1 0 0) CVD films. On the other hand, TC for (1 1 1) films increases with increasing [B] in the same concentration range, and those TC values are more than twice higher than those of the (1 0 0) films [2]. Detailed studies have been reported on synthesis and characterization of heavily boron-doped diamond. Despite the gained insights, the relation between the local structure around boron dopants and physical properties of boron-doped diamond has not been fully clarified. This may be attributed to the difficulty in applying many spectroscopic techniques because the doped boron sites lack a long-range order. Solid-state nuclear magnetic resonance (NMR) is particularly useful in analysis of amorphous inor-
* Corresponding author. Tel.: +81 29 863 5484; fax: +81 29 863 5571. E-mail address: [email protected] (M. Murakami). 0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.12.002
ganic solids without such long-range order; besides, NMR can determine site distributions of each atomic species. Due to the low natural abundance of 13C (1.1%), 11B is expected to be the most accessible nucleus in boron-doped diamond. In this work, we have summarized solid-state 11B magic-angle spinning (MAS) NMR results on the disordered local structure in several boron-doped diamonds and, in particular, the structure of the excess boron which does not contribute to electrical conductivity. 2. Experimental The boron-doped diamond sample (N3) prepared by the HPHT method using naphthalene and amorphous boron with the initial boron concentration B/(B + C) = 5.0% [3], were examined. 11B and 10 B MAS NMR spectra were recorded at 21.8 T on a JEOL ECA 930 spectrometer with a JEOL 4 mm MAS probe and at 11.7 T on a JEOL ECA 500 spectrometer with a Chemagnetics 3.2 mm MAS probe. All experiments were done at room temperature. 3. Results and discussion Fig. 1 shows the 11B MAS NMR spectra of N3 observed at 21.8 T, which consist mainly of four signals (signal-a to signal-d as indicated in the figure) with a few spinning sidebands designated by asterisks. From its isotropic chemical shift, a small quadrupolar interaction, and a short spin–lattice relaxation time, we assigned signal-c to the substitutional boron in the diamond structure and
S626
M. Murakami et al. / Physica C 470 (2010) S625–S626
(a)
b d
*
a c
* 150
100
0
50
(b) -100
-50
Chemical Shift/ppm
0
40
80
Chemical Shift/ppm F1
Fig. 1. 11B MAS NMR spectrum of N3 observed at 21.8 T. The peaks marked with are spinning side bands.
120 120
80
40
Chemical Shift/ppm
0
F2
Fig. 2. 2D 1H on/off spectrum of N3 observed at 11.7 T.
signal-d to the excess boron [4] and fit the spectrum to a sum of four Gaussian lineshapes. To the authors’ knowledge, two candidates have been invoked so far for boron not contributing to conductivity. One is a boron + hydrogen complex [5] and the other is boron clusters [6] including –B–B– and/or –B–C–B–. In this work, we examine 1H dipolar broadening by using the modified 1H on/off experiment operative under MAS [7]. Owing to the higher resolution achieved by MAS, we are able to discriminate 1H dipolar broadening for each signal component. Fig. 2 shows the contour plot of the 2D 1H on/off spectrum of N3. A cross-section lineshape along the horizontal frequency axis (F2) corresponds to a 1H-decoupled lineshape and that along the vertical axis (F1) to a 1H-coupled one. The observed spectrum exhibits only sharp diagonal components; no dipolar broadening which should appear along the F1 axis if an appreciable amount of the 1H + 11B complex exists was discernible. Hence, boron-d is not the 1H + 11B complex. The boron–boron (–B–B–/–B–C– B–) clusters has also been discarded by examining 11B–11B 2D exchange NMR and observing only diagonal peaks in the 2D exchange spectrum of the N3 sample [7].
(c)
150
100
50
0
-50
-100
Chemical Shift/ppm Fig. 3. 11B (a) and 10B (b) MAS NMR spectra of N3. (c) 11B MAS NMR spectrum observed with a short repetition time of 100 ms. All spectra were taken at 11.7 T.
To appreciate the relative size of the quadrupolar coupling for boron-c and boron-d, we used 10B MAS NMR. Fig. 3 shows the observed 11B (Fig. 3a) and 10B (Fig. 3b) MAS NMR spectra of N3 at 11.7 T. Signal-d is reduced in 10B, showing the signal-d intensity is distributed over spinning side bands as its quadrupolar coupling constant (mq) is much larger than the spinning frequency of MAS (mMAS:10 kHz). On the other hand, signal-c and signal-b remain in the 10B spectrum, which is consistent to the assignment, namely, signal-b is boron cluster impurity and signal-c is the tetrahedrally coordinated boron with small mq. Fig. 3c shows the 11B MAS spectrum measured with a fast pulse-repetition time of 100 ms. As shown in our previous work [4], boron-c has a short spin–lattice relaxation time (T1) of 500 ms while that of boron-d is long 3.3– 3.6 s. Two spectra in Fig. 3b and c are similar, and this confirms that the 10B NMR spectrum is ascribed to boron-c. References [1] E.A. Ekimov, V.A. Sidorov, E.D. Bauer, N.N. Mel’nik, N.J. Curro, J.D. Thompson, S.M. Stishov, Nature 428 (2004) 542. [2] Y. Takano, J. Phys.: Condens. Matter 21 (2009) 253201. [3] E.A. Ekimov, V.A. Sidorov, A.V. Rakhmanina, N.N. Mel’nik, R.A. Sadykov, J.D. Thompson, Sci. Technol. Adv. Mater. 7 (2006) S2. [4] M. Murakami, T. Shimizu, M. Tansho, Y. Takano, S. Ishii, E.A. Ekimov, V.A. Sidorov, H. Sumiya, H. Kawarada, K. Takegoshi, Diamond Relat. Mater. 17 (2008) 1835. [5] H. Mukuda, T. Tsuchida, A. Harada, Y. Kitaoka, T. Takenouchi, Y. Takano, M. Nagao, I. Sakaguchi, T. Oguchi, H. Kawarada, Phys. Rev. B 75 (2007) 033301. [6] M. Bernard, C. Baron, A. Deneuville, Diamond Relat. Mater. 13 (2004) 896. [7] M. Murakami, T. Shimizu, M. Tansho, Y. Takano, S. Ishii, E.A. Ekimov, V.A. Sidorov, K. Takegoshi, Diamond Relat. Mater. 18 (2009) 1267.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
10
B and
11
B high-resolution NMR studies on boron-doped diamond
M. Murakami a,*, T. Shimizu a, M. Tansho a,b, Y. Takano c, S. Ishii c, E.A. Ekimov d, V.A. Sidorov d, K. Takegoshi e,f a
National Institute for Materials Science, 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan World Premier International Research Center for Materials Nanoarchitectonics, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan c National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan d Russian Academy of Science, Troitsk 142190 Moscow Region, Russia e Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan f CREST, JST, Chiyoda-ku, Tokyo 102-0075, Japan b
a r t i c l e
i n f o
Article history: Accepted 3 December 2009 Available online 11 December 2009 Keywords: Boron-doped diamond 11 B NMR Two-dimensional NMR
a b s t r a c t 11
B magic-angle spinning (MAS) NMR experiments are applied to B-doped diamond samples prepared by high-pressure and high-temperature methods. From the spectrum, we show that there are at least four boron signal components and the one at 28.5 ppm is ascribed to the substitutional boron in the diamond structure providing the carriers responsible for conductivity. We further apply two-dimensional (2D) NMR to examine 1H dipolar broadening and 11B–11B boron spin diffusion, and candidates purported so far for the excess boron, that is, a boron + hydrogen complex and –B–B– and/or –B–C–B– clusters are negated. Furthermore, we show that 10B MAS NMR is useful to selectively observe the substitutional boron in the diamond structure appearing at 28.5 ppm, whose quadrupolar coupling is much smaller than that of the excess boron at 65.5 ppm. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction In 2004, Ekimov et al. discovered superconductivity in heavily boron-doped diamond [1]. So far, two major synthesis techniques of heavily boron-doped diamond are known: high-pressure hightemperature (HPHT) method and chemical vapor deposition (CVD). In contrast to the expectation of higher TC (the superconductivity transition temperature) for higher boron concentrations [B], Takano [2] showed that TC of homoepitaxial (1 0 0) thin films, synthesized by the microwave plasma assisted CVD, increases with increasing [B] and reaches a maximum at [B] 1 1022 cm 3. They also pointed out that TC ([B]) dependences are similar for diamonds grown by the HPHT method and (1 0 0) CVD films. On the other hand, TC for (1 1 1) films increases with increasing [B] in the same concentration range, and those TC values are more than twice higher than those of the (1 0 0) films [2]. Detailed studies have been reported on synthesis and characterization of heavily boron-doped diamond. Despite the gained insights, the relation between the local structure around boron dopants and physical properties of boron-doped diamond has not been fully clarified. This may be attributed to the difficulty in applying many spectroscopic techniques because the doped boron sites lack a long-range order. Solid-state nuclear magnetic resonance (NMR) is particularly useful in analysis of amorphous inor-
* Corresponding author. Tel.: +81 29 863 5484; fax: +81 29 863 5571. E-mail address: [email protected] (M. Murakami). 0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.12.002
ganic solids without such long-range order; besides, NMR can determine site distributions of each atomic species. Due to the low natural abundance of 13C (1.1%), 11B is expected to be the most accessible nucleus in boron-doped diamond. In this work, we have summarized solid-state 11B magic-angle spinning (MAS) NMR results on the disordered local structure in several boron-doped diamonds and, in particular, the structure of the excess boron which does not contribute to electrical conductivity. 2. Experimental The boron-doped diamond sample (N3) prepared by the HPHT method using naphthalene and amorphous boron with the initial boron concentration B/(B + C) = 5.0% [3], were examined. 11B and 10 B MAS NMR spectra were recorded at 21.8 T on a JEOL ECA 930 spectrometer with a JEOL 4 mm MAS probe and at 11.7 T on a JEOL ECA 500 spectrometer with a Chemagnetics 3.2 mm MAS probe. All experiments were done at room temperature. 3. Results and discussion Fig. 1 shows the 11B MAS NMR spectra of N3 observed at 21.8 T, which consist mainly of four signals (signal-a to signal-d as indicated in the figure) with a few spinning sidebands designated by asterisks. From its isotropic chemical shift, a small quadrupolar interaction, and a short spin–lattice relaxation time, we assigned signal-c to the substitutional boron in the diamond structure and
S626
M. Murakami et al. / Physica C 470 (2010) S625–S626
(a)
b d
*
a c
* 150
100
0
50
(b) -100
-50
Chemical Shift/ppm
0
40
80
Chemical Shift/ppm F1
Fig. 1. 11B MAS NMR spectrum of N3 observed at 21.8 T. The peaks marked with are spinning side bands.
120 120
80
40
Chemical Shift/ppm
0
F2
Fig. 2. 2D 1H on/off spectrum of N3 observed at 11.7 T.
signal-d to the excess boron [4] and fit the spectrum to a sum of four Gaussian lineshapes. To the authors’ knowledge, two candidates have been invoked so far for boron not contributing to conductivity. One is a boron + hydrogen complex [5] and the other is boron clusters [6] including –B–B– and/or –B–C–B–. In this work, we examine 1H dipolar broadening by using the modified 1H on/off experiment operative under MAS [7]. Owing to the higher resolution achieved by MAS, we are able to discriminate 1H dipolar broadening for each signal component. Fig. 2 shows the contour plot of the 2D 1H on/off spectrum of N3. A cross-section lineshape along the horizontal frequency axis (F2) corresponds to a 1H-decoupled lineshape and that along the vertical axis (F1) to a 1H-coupled one. The observed spectrum exhibits only sharp diagonal components; no dipolar broadening which should appear along the F1 axis if an appreciable amount of the 1H + 11B complex exists was discernible. Hence, boron-d is not the 1H + 11B complex. The boron–boron (–B–B–/–B–C– B–) clusters has also been discarded by examining 11B–11B 2D exchange NMR and observing only diagonal peaks in the 2D exchange spectrum of the N3 sample [7].
(c)
150
100
50
0
-50
-100
Chemical Shift/ppm Fig. 3. 11B (a) and 10B (b) MAS NMR spectra of N3. (c) 11B MAS NMR spectrum observed with a short repetition time of 100 ms. All spectra were taken at 11.7 T.
To appreciate the relative size of the quadrupolar coupling for boron-c and boron-d, we used 10B MAS NMR. Fig. 3 shows the observed 11B (Fig. 3a) and 10B (Fig. 3b) MAS NMR spectra of N3 at 11.7 T. Signal-d is reduced in 10B, showing the signal-d intensity is distributed over spinning side bands as its quadrupolar coupling constant (mq) is much larger than the spinning frequency of MAS (mMAS:10 kHz). On the other hand, signal-c and signal-b remain in the 10B spectrum, which is consistent to the assignment, namely, signal-b is boron cluster impurity and signal-c is the tetrahedrally coordinated boron with small mq. Fig. 3c shows the 11B MAS spectrum measured with a fast pulse-repetition time of 100 ms. As shown in our previous work [4], boron-c has a short spin–lattice relaxation time (T1) of 500 ms while that of boron-d is long 3.3– 3.6 s. Two spectra in Fig. 3b and c are similar, and this confirms that the 10B NMR spectrum is ascribed to boron-c. References [1] E.A. Ekimov, V.A. Sidorov, E.D. Bauer, N.N. Mel’nik, N.J. Curro, J.D. Thompson, S.M. Stishov, Nature 428 (2004) 542. [2] Y. Takano, J. Phys.: Condens. Matter 21 (2009) 253201. [3] E.A. Ekimov, V.A. Sidorov, A.V. Rakhmanina, N.N. Mel’nik, R.A. Sadykov, J.D. Thompson, Sci. Technol. Adv. Mater. 7 (2006) S2. [4] M. Murakami, T. Shimizu, M. Tansho, Y. Takano, S. Ishii, E.A. Ekimov, V.A. Sidorov, H. Sumiya, H. Kawarada, K. Takegoshi, Diamond Relat. Mater. 17 (2008) 1835. [5] H. Mukuda, T. Tsuchida, A. Harada, Y. Kitaoka, T. Takenouchi, Y. Takano, M. Nagao, I. Sakaguchi, T. Oguchi, H. Kawarada, Phys. Rev. B 75 (2007) 033301. [6] M. Bernard, C. Baron, A. Deneuville, Diamond Relat. Mater. 13 (2004) 896. [7] M. Murakami, T. Shimizu, M. Tansho, Y. Takano, S. Ishii, E.A. Ekimov, V.A. Sidorov, K. Takegoshi, Diamond Relat. Mater. 18 (2009) 1267.