Post on 30-Nov-2023
transcript
Electrical conductivity of thermally hydrogenated nanodiamond powdersTakeshi Kondo, Ioannis Neitzel, Vadym N. Mochalin, Junichi Urai, Makoto Yuasa et al. Citation: J. Appl. Phys. 113, 214307 (2013); doi: 10.1063/1.4809549 View online: http://dx.doi.org/10.1063/1.4809549 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v113/i21 Published by the American Institute of Physics. Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors
Downloaded 07 Jun 2013 to 144.118.118.249. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions
Electrical conductivity of thermally hydrogenated nanodiamond powders
Takeshi Kondo,1,2 Ioannis Neitzel,3 Vadym N. Mochalin,3 Junichi Urai,1 Makoto Yuasa,1,2
and Yury Gogotsi3,a)
1Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science,2641 Yamazaki, Noda, Chiba 278-8510, Japan2Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda,Chiba 278-8510, Japan3Department of Materials Science and Engineering and A.J. Drexel Nanotechnology Institute,Drexel University, 3141 Chestnut Street, Philadelphia, Pennsylvania 19104, USA
(Received 7 March 2013; accepted 20 May 2013; published online 7 June 2013)
Electrical properties of detonation diamond nanoparticles (NDs) with individual diameters of �5 nm
are important for many applications. Although diamond is an insulator, it is known that hydrogen-
terminated bulk diamond becomes conductive when exposed to water. We show that heating ND in
hydrogen gas at 600–900 �C resulted in a remarkable decrease in resistivity from 107 to 105 X cm,
while the resistivity was essentially unchanged after treatment at 400 �C and lower temperatures.
Fourier Transform Infrared Spectroscopy and X-ray photoelectron spectroscopy (XPS) studies
revealed that hydrogenation of ND occurs at 600–900 �C, suggesting that the decrease in resistivity is
based on transfer doping at the hydrogenated ND surface. Oxidation of the hydrogenated sample at
300 �C recovers resistivity to its original value. The resistivity of treated ND as a function of the O/C
atomic ratio showed a transition from resistive (O/C ratio > 0.033) to conductive (O/C ratio < 0.033)
state. This is consistent with the idea that the change in the resistivity is caused by the shift of the
valence band maximum to above the Fermi level due to the dipole of the C-H bonds leading to
transfer doping. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4809549]
I. INTRODUCTION
Diamond nanoparticles (NDs) fabricated by detonation
of explosives have superior chemical stability, hardness, and
biocompatibility.1 Based on these excellent properties, many
applications of NDs are being developed in the fields of
nanocomposites,2–5 electrochemical energy storage,6 biologi-
cal fluorescent probes,7–10 and drug delivery carriers.11–13
Nanoparticles have a very high surface-to-volume (surface-
to-weight) ratio; therefore, they exhibit surface-dependent
properties more prominently than their bulk counterparts.
Diamond surface can be modified by various treatments,
including dry (thermal and plasma) and wet (chemical, elec-
trochemical, and photochemical) techniques.14 Hydrogenated
diamond surfaces are known to exhibit unique properties
including hydrophobicity15,16 and, in the case of powder, a
high positive zeta potential.17 Negative electron affinity18,19
and surface conductivity20 were also reported for hydrogen-
ated diamond. Surface conductivity of hydrogenated diamond
has been explained via the transfer doping mechanism: elec-
tron transfer from the valence band of the hydrogenated dia-
mond to the redox species in the electrolyte (adsorbed water)
results in accumulation of holes on the diamond surface. This
allows two-dimensional electric conduction.20–22 Since ND
has a much higher surface-to-volume ratio compared to bulk
diamond, the effect of surface hydrogenation of ND on its
conductivity should be even more pronounced than for bulk
diamond. Recently, Su et al. reported that hydrogenation of
detonation ND by hydrogen plasma treatment increased the
electrical conductivity, as estimated by impedance spectros-
copy, by four orders of magnitude.23 In the present study, we
investigated the surface hydrogenation of NDs by high tem-
perature treatment in hydrogen gas and the resistivity of the
NDs as a function of treatment temperature. Based upon the
estimation of the surface oxygen content and the sp2/sp3 car-
bon ratio of the treated NDs, the decrease in the resistivity
was found to be consistent with the transfer doping caused by
the surface hydrogenation of ND.
II. EXPERIMENTAL
The ND used in this study (UD90, nominal particle di-
ameter 5 nm) was provided by Nanoblox, Inc., USA. The
UD90 was first oxidized in air in a muffle furnace at 430 �Cfor 5 h to remove the non-diamond carbon from the particle
surface.24,25 The oxidized ND was then refluxed in a mixture
of concentrated hydrochloric and nitric acids at 100 �C for
24 h to remove metallic impurities. The purified ND was
obtained by pipetting out the supernatant of the acidic aque-
ous dispersion and adding fresh deionized water in steps
until the supernatant became neutral, and drying the powder.
The purified ND (denoted here as UD90p) was heated in
hydrogen gas atmosphere using a quartz tube furnace at a
fixed temperature (200–900 �C) for 1 h. After the heat was
turned off, the sample was kept under hydrogen atmosphere
until the furnace temperature dropped to below 100 �C. The
UD90p after the heat treatment (denoted as H-ND) was char-
acterized by Fourier Transform Infrared Spectroscopy (FTIR
in KBr disks; Excalibur FTS-3000, Varian) and X-ray photo-
electron spectroscopy (XPS; Axis-nova, Kratos). The XP
spectra were calibrated by assigning the C 1 s peak position
a)Author to whom correspondence should be addressed. Electronic mail:
gogotsi@drexel.edu.
0021-8979/2013/113(21)/214307/5/$30.00 VC 2013 AIP Publishing LLC113, 214307-1
JOURNAL OF APPLIED PHYSICS 113, 214307 (2013)
Downloaded 07 Jun 2013 to 144.118.118.249. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions
to be 284.8 eV. The O/C atomic ratio of the ND samples was
estimated by quantitative XPS analysis. The sp2/sp3 carbon
ratio of the ND sample was estimated by peak deconvolution
of the XPS C 1 s spectra with Gaussian band profiles. The re-
sistivity of ND was calculated from the I-V curves of ND
packed in a glass tube (inner diameter 1 mm). Both ends of
the ND cylinder inside the tube were pressed with copper
wires which were connected to the power source. The
applied voltage oscillated between �5 and þ5 V at a rate of
5 V s�1, and the electric resistance was calculated from the
slope of the I-V curve in the range of �1 to þ1 V.
III. RESULTS AND DISCUSSION
A. Characterization of hydrogenated ND
The effect of hydrogenation temperature on the surface
chemical structure of H-NDs was investigated by FTIR and
XPS. Figure 1(a) shows FTIR spectra of the H-NDs treated
at 400–900 �C. The spectrum for the sample after hydrogena-
tion at 400 �C showed a band of the C¼O stretching mode at
1750 cm�1.26 The spectrum for the sample treated at 200 �Clooked similar. The presence of the C¼O peak and the ab-
sence of clear CH stretching modes at around
2800–3000 cm�1 indicate that heat treatment in hydrogen
gas at 400 �C is not sufficient for hydrogenation of the
UD90p surface. On the other hand, the samples treated in a
hydrogen atmosphere at 600–900 �C showed pronounced
C-H stretch vibrations of CH3 and CH2 groups at
2800–3000 cm�1. In the spectrum of the H-ND treated at
800 �C, bands of CH3 antisymmetric (2957 cm�1), CH2 anti-
symmetric (2932 cm�1), and CH2 symmetric (2876 cm�1)
vibrations are clearly observed (Fig. 1(b)).27 The relative
intensity of the C¼O band at 1750 cm�1 was found to
decrease as the treatment temperature increased. The band at
1630 cm�1, which remains in H-ND samples produced in the
temperature range of 400–600 �C, originates from O-H bend
vibrations and is likely due to covalently bonded surface
O-H groups or water adsorbed on the surface of H-ND.28
These changes in the spectra indicate that the oxidized ND
surface is reduced by heating in a hydrogen gas atmosphere
at 600–900 �C. The extent of reduction at fixed treatment
time correlates well with the temperature of hydrogenation.
The hydrogenation of UD90p was also investigated with
XPS by monitoring the content of oxygen, which is substi-
tuted with hydrogen as hydrogenation progresses. Figure
2(a) shows XP spectra of H-NDs treated at 400 and 900 �C.
The spectra for the H-ND sample treated at 400 �C showed
an O 1 s peak at 530 eV. The O 1 s peak was less prominent
for the H-ND (900 �C) than for the H-ND (400 �C). The O/C
atomic ratio estimated from the XPS quantitative analysis
plotted as a function of hydrogenation temperature in Fig.
2(b). The O/C ratio of 0.38 for H-ND (400 �C) indicates high
oxygen content on the surface. In comparison, the O/C ratio
of an oxygen plasma-treated diamond surface was reported
to be 0.18.15 The O/C ratio decreased as the hydrogenation
temperature increased from 400 �C to 600 �C, consistent
FIG. 1. FTIR spectra of NDs after treat-
ment in hydrogen. (a) The spectra for
various treatment temperatures: (i) 400,
(ii) 500, (iii) 600, (vi) 800, and (v)
900 �C. (b) (800 �C).
FIG. 2. (a) XPS of H-NDs. Hydrogenation temperatures are (i) 400 and (ii)
900 �C. (b) O/C atomic ratio of H-NDs as a function of hydrogenation
temperature.
214307-2 Kondo et al. J. Appl. Phys. 113, 214307 (2013)
Downloaded 07 Jun 2013 to 144.118.118.249. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions
with Williams et al.,17 and then remained constant (<0.02)
at 600–900 �C. Thus, the range of temperatures correspond-
ing to hydrogenation of ND is 600–900 �C, which is consist-
ent with the FTIR results (Fig. 1).
Figures 3(a) and 3(b) show XPS C 1 s spectra of H-ND
(400 �C) and H-ND (900 �C), respectively. The C1s peaks
were fit by four Gaussian profiles corresponding to C¼C
(283.5 eV), C-C and C-H (284.8 eV), C-O (286.5 V), and
C¼O (288.5 eV). The intensity of the C¼O peak (288.5 eV)
in H-ND (900 �C) is slightly lower compared to H-ND
(400 �C), while the intensity of C-O (286.5 eV) is much
higher in H-ND (900 �C) compared to H-ND (400 �C) due to
reduction of C¼O into C-OH, in agreement with FTIR data
(Fig. 1). The sp2/sp3 carbon ratio, estimated from the area of
the peaks at 283.5 eV (C¼C) and 284.8 eV (C-C and C-H),
decreased as the hydrogenation temperature increased (Fig.
3(c)). The trend was more pronounced for samples treated at
higher temperatures (800–900 �C). Heating at 700 �C or
above can cause partial graphitization of ND in both vacuum
and argon atmospheres.29,30 According to Fig. 3, hydrogen
suppresses the ND graphitization at temperatures up to
900 �C. At these temperatures, hydrogen can react with sp2
carbon atoms, converting them into sp3 hybridized atoms,
and eventually eliminating surface carbon in the form of vol-
atile hydrocarbons.31 Hence, we conclude that the heating of
ND in hydrogen at temperatures up to 900 �C does not
induce graphitization of ND.
B. Resistivity of hydrogenated ND
The resistivities of the H-NDs (Fig. 4) were calculated
from the I-V curves. The H-ND samples treated at or below
500 �C exhibited high resistivity on the order of 107 X cm,
similar to that of UD90 before purification (1.0� 107 X cm)
and UD90p (1.9� 107 X cm). On the other hand, the H-ND
samples treated at 600 �C or above showed almost two orders
of magnitude lower resistivity than those treated at tempera-
tures below 500 �C or untreated NDs. Further increase in hy-
drogenation temperature did not result in significant changes
of resistivity (Fig. 4). The relationship between the hydro-
genation temperature and the sp2/sp3 carbon ratio of ND
(Fig. 3(b)) indicates that the treatment at high temperature
did not cause graphitization of ND. Thus, it is obvious that
the decrease of resistivity is not related to surface graphitiza-
tion of ND. On the other hand, ND surface hydrogenation
was evidenced by FTIR and XP spectra at 600 �C and above.
Thus, the decrease of resistivity should be related to the ND
surface hydrogenation. It is well known that hydrogen-
terminated diamond films can exhibit significant electric
conductivity when in contact with water or ambient humid
air due to transfer doping.32–34 This mechanism was used to
explain significant two-dimensional surface conductivity of
hydrogen-terminated diamond.22,35,36 The sharp drop in the
resistivity of our ND samples after hydrogenation at 600 �Cand above can also be related to the transfer doping of the
hydrogenated ND surface exposed to ambient air.
Hydrogenated ND can be considered as a surface-conductive
ND, a material with many potential applications in electro-
chemistry, biomedical sensing, and nanocomposites.
C. Loss of conductivity by oxidation of H-ND
In order to further confirm that the increased electric
conductivity of the H-ND is related to hydrogen termination,
we measured the resistivity of H-NDs after oxidation. Figure 5
shows resistivity of the H-ND (600–900 �C) samples after
FIG. 5. Resistivity of H-NDs (600–900 �C) after oxidation in air at 200 and
300 �C.
FIG. 3. XPS C 1 s spectra of (a) H-ND (400 �C) and (b) H-ND (900 �C). (c)
Estimated sp2/sp3 carbon ratio as a function of hydrogenation temperature.
FIG. 4. Resistivity of ND as a function of hydrogenation temperature.
214307-3 Kondo et al. J. Appl. Phys. 113, 214307 (2013)
Downloaded 07 Jun 2013 to 144.118.118.249. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions
heating in air at 200 and 300 �C for 1 h. While the resistivity
of all H-ND samples showed almost no change after heating
in air at 200 �C, �50 times increased resistivities were meas-
ured for the samples after heating in air at 300 �C. Figure 6
shows FTIR spectra of H-ND (600 �C) before and after heat-
ing in air at 200 and 300 �C. Although the spectrum of a
sample treated at 200 �C in air is similar to the spectrum
before the treatment, the increase of C¼O stretching band in-
tensity at ca. 1720 cm�1 and the decrease of CH2 symmetric
and antisymmetric stretching band intensity at around
2900 cm�1 was found after the treatment at 300 �C. This
result indicates that the H-ND surface can be oxidized by
heating in air at 300 �C or above, while 200 �C is not suffi-
cient for oxygen chemisorption, which is consistent with our
previous results on ND oxidation.24 The O/C atomic ratio of
H-ND samples after heating in air was quantified by XPS
(Fig. 7(a)). For all the H-ND samples prepared at 600–900 �C,
the O/C ratio increased only slightly after the treatment at
200 �C. After the treatment at 300 �C, the increase in the O/C
ratio was greater than that after the treatment at 200 �C. This
result also confirms that the loss of electric conductivity of
H-ND after heating in air is due to surface oxidation. In con-
trast, the sp2/sp3 ratio was almost unchanged for all the H-ND
samples even after the treatment in air at 300 �C (Fig. 7(b))
likely because 300 �C is too low for oxidation of sp2 carbon in
air, which starts at temperatures above 375 �C.24 The O/C ra-
tio of the H-ND samples after heating in air at 300 �C was
around 0.04, much lower than before surface hydrogenation
(0.38). Thus, the heating in air at 300 �C for 1 h results only in
a mild oxidation of ND surface. However, even this mild oxi-
dation is sufficient to reduce the conductivity of ND by almost
two orders of magnitude (Fig. 5).
Figure 8(a) summarizes resistivity of the ND samples
treated under various conditions as a function of the O/C
FIG. 6. (a) FTIR spectra of H-ND
(600 �C) after oxidation in air. Before
oxidation (i) and oxidized at 200 (ii) and
300 �C (iii). (b) CH2/CH3 stretching
mode region of the spectra.
FIG. 7. (a) O/C atomic ratio and (b) sp2/sp3 carbon ratio estimated by XPS
analysis of H-NDs (600–900 �C) after oxidation in air at 200 and 300 �C.
FIG. 8. (a) Resistivity of H-NDs treated under various conditions as a func-
tion of O/C atomic ratio measured by XPS. (b) Schematic illustration of
band diagrams for H-NDs with various O/C ratios.
214307-4 Kondo et al. J. Appl. Phys. 113, 214307 (2013)
Downloaded 07 Jun 2013 to 144.118.118.249. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions
ratio. The resistivity shows a steep transition between low
(�105 X cm, region I) and high (�107 X cm, region II)
regions at a critical O/C ratio of ca. 0.033. This behavior is
consistent with the concept of transfer doping. The position
of the valence band maximum, which is below the Fermi
level in oxidized ND, shifts gradually to higher energy with
increasing density of the surface C-H groups due to the
dipoles with dþ charge on the H atom.37,38 However, in
order for the transfer doping to occur, it is critical that the va-
lence band maximum is above the Fermi level (redox poten-
tial derived from surface adsorbates, possibly oxygen and
water in this case21,36) (Fig. 8(b)), which can only be realized
at very low O/C ratios. The O/C atomic ratio of 0.033 is less
than 1/10 of that for the oxidized surface (0.38). Thus, the
highly hydrogenated surface is necessary for surface conduc-
tivity, which, again, is consistent with the transfer doping
mechanism.
IV. CONCLUSION
The electric conductivity of ND increased by almost 2
orders of magnitude after heat treatment in hydrogen gas at
600–900 �C. Characterization with FTIR and XPS revealed
that the surface hydrogenation of ND occurred at 600 �C and
higher temperatures. On the other hand, analysis of the XPS
C 1 s spectra showed no evidence of surface graphitization
after heating ND in hydrogen gas at temperatures up to
900 �C. Based on these results, we conclude that the increase
of the conductivity is due to the surface hydrogenation rather
than graphitization. From the relationship between the O/C
ratio and the resistivity of the NDs, we found a sharp transi-
tion of ND powder from electrically insulating to conductive
state at a critical O/C ratio of ca. 0.033. This stepwise transi-
tion is consistent with the transfer doping mediated by sur-
face hydrogenation of ND.
ACKNOWLEDGMENTS
This work was partly supported by the Overseas
Research Program of Tokyo University of Science. The
authors thank Ms. Amanda Pentecost, Drexel University, for
proofreading the manuscript.
1V. N. Mochalin, O. Shenderova, D. Ho, and Y. Gogotsi, Nat. Nanotechnol.
7, 11 (2012).2K. Hanada, K. Yamamoto, T. Taguchi, E. �Osawa, M. Inakuma, V.
Livramento, J. B. Correia, and N. Shohoji, Diamond Relat. Mater. 16,
2054 (2007).3K. D. Behler, A. Stravato, V. Mochalin, G. Korneva, G. Yushin, and Y.
Gogotsi, ACS Nano 3, 363 (2009).4O. Shenderova, C. Jones, V. Borjanovic, S. Hens, G. Cunningham, S.
Moseenkov, V. Kuznetsov, and G. McGuire, Phys. Status Solidi A 205,
2245 (2008).
5V. N. Mochalin, I. Neitzel, B. J. M. Etzold, A. Peterson, G. Palmese, and
Y. Gogotsi, ACS Nano 5, 7494 (2011).6I. Kovalenko, D. G. Bucknall, and G. Yushin, Adv. Funct. Mater. 20, 3979
(2010).7C.-C. Fu, H.-Y. Lee, K. Chen, T.-S. Lim, H.-Y. Wu, P.-K. Lin, P.-K. Wei,
P.-H. Tsao, H.-C. Chang, and W. Fann, Proc. Natl. Acad. Sci. U.S.A. 104,
727 (2007).8Y.-R. Chang, H.-Y. Lee, K. Chen, C.-C. Chang, D.-S. Tsai, C.-C. Fu, T.-S.
Lim, Y.-K. Tzeng, C.-Y. Fang, C.-C. Han, H.-C. Chang, and W. Fann,
Nat. Nanotechnol. 3, 284 (2008).9V. N. Mochalin and Y. Gogotsi, J. Am. Chem. Soc. 131, 4594 (2009).
10L. P. McGuinness, Y. Yan, A. Stacey, D. A. Simpson, L. T. Hall, D.
Maclaurin, S. Prawer, P. Mulvaney, J. Wrachtrup, F. Caruso, R. E.
Scholten, and L. C. L. Hollenberg, Nat. Nanotechnol. 6, 358 (2011).11H. Huang, E. Pierstorff, E. Osawa, and D. Ho, Nano Lett. 7, 3305 (2007).12X.-Q. Zhang, R. Lam, X. Xu, E. K. Chow, H.-J. Kim, and D. Ho, Adv.
Mater. 23, 4770 (2011).13M. Chen, E. D. Pierstorff, R. Lam, S.-Y. Li, H. Huang, E. Osawa, and D.
Ho, ACS Nano 3, 2016 (2009).14A. Krueger and D. Lang, Adv. Funct. Mater. 22, 890 (2012).15I. Yagi, H. Notsu, T. Kondo, D. A. Tryk, and A. Fujishima, J. Electroanal.
Chem. 473, 173 (1999).16L. Ostrovskaya, V. Perevertailo, V. Ralchenko, A. Dementjev, and O.
Loginova, Diamond Relat. Mater. 11, 845 (2002).17O. A. Williams, J. Hees, C. Dieker, W. J€ager, L. Kirste, and C. E. Nebel,
ACS Nano 4, 4824 (2010).18J. van der Weide, Z. Zhang, P. K. Baumann, M. G. Wensell, J. Bernholc,
and R. J. Nemanich, Phys. Rev. B 50, 5803 (1994).19D. Takeuchi, H. Kato, G. S. Ri, T. Yamada, P. R. Vinod, D. Hwang, C.
E. Nebel, H. Okushi, and S. Yamasaki, Appl. Phys. Lett. 86, 152103
(2005).20F. Maier, M. Riedel, B. Mantel, J. Ristein, and L. Ley, Phys. Rev. Lett. 85,
3472 (2000).21V. Chakrapani, J. C. Angus, A. B. Anderson, S. D. Wolter, B. R. Stoner,
and G. U. Sumanasekera, Science 318, 1424 (2007).22A. Bolker, C. Saguy, M. Tordjman, L. Gan, and R. Kalish, Phys. Rev. B
83, 155434 (2011).23S. Su, J. Li, V. Kundrat, A. M. Abbot, and H. Ye, J. Appl. Phys. 113,
023707 (2013).24S. Osswald, G. Yushin, V. Mochalin, S. O. Kucheyev, and Y. Gogotsi,
J. Am. Chem. Soc. 128, 11635 (2006).25S. Osswald, V. N. Mochalin, M. Havel, G. Yushin, and Y. Gogotsi, Phys.
Rev. B 80, 075419 (2009).26J. Mona, J. S. Tu, T. Y. Kang, C.-Y. Tsai, E. Perevedentseva, and C. L.
Cheng, Diamond Relat. Mater. 24, 134 (2012).27T. Jiang, K. Xu, and S. Ji, J. Chem. Soc., Faraday Trans. 92, 3401
(1996).28V. Mochalin, S. Osswald, and Y. Gogotsi, Chem. Mater. 21, 273 (2009).29T. Petit, J.-C. Arnault, H. A. Girard, M. Sennour, and P. Bergonzo, Phys.
Rev. B 84, 233407 (2011).30J. Chen, S. Z. Deng, J. Chen, Z. X. Yu, and N. S. Xu, Appl. Phys. Lett. 74,
3651 (1999).31M. Yeganeh, P. R. Coxon, A. C. Brieva, V. R. Dhanak, L. �Siller, and Y. V.
Butenko, Phys. Rev. B 75, 155404 (2007).32P. Strobel, M. Riedel, J. Ristein, and L. Ley, Nature 430, 439 (2004).33J. Ristein, Science 313, 1057 (2006).34C. E. Nebel, Science 318, 1391 (2007).35L. Ley, J. Ristein, F. Meier, M. Riedel, and P. Strobel, Physica B 376–377,
262 (2006).36C. E. Nebel, B. Rezek, D. Shin, and H. Watanabe, Phys. Status Solidi A
203, 3273 (2006).37F. Maier, J. Ristein, and L. Ley, Phys. Rev. B 64, 165411 (2001).38M. Riedel, J. Ristein, and L. Ley, Diamond Relat. Mater. 13, 746
(2004).
214307-5 Kondo et al. J. Appl. Phys. 113, 214307 (2013)
Downloaded 07 Jun 2013 to 144.118.118.249. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions