Yellampalli S. (ed.) Carbon Nanotubes - Polymer Nanocomposites
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About Grafting of Single-Walled Carbon Nanotubes on the
Oligo-N-Vinyl Carbazole and Copolymer Involving N-Vinylcarbazole and Hexylthiophene
327
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1. Introduction
There are reports of intrinsic weak ferromagnetism in graphite and carbon-based materials
well above room temperature Cervenka et al. (2009); Esquinazi et al. (2003); Kopelevich et al.
(2000); Mendoza et al. (1999); Moehlecke et al. (2002); Mombru et al. (2005), as well as a
theoretical prediction of a ferromagnetic instability in graphene sheets Bas & Jafari (2002).
On the other hand, Dzwilewski et al. Talyzin et al. (2007) show that the observed
high-temperature ferromagnetism in rhombohedral C
60
Makarova et al. (2001) is not intrinsic
but caused by contamination of magnetic impurities. In addition to the observation of
unusual high-temperature ferromagnetism in the carbon-based materials, there was a report
of extra magnetic moment induced in graphite due to a large magnetic proximity effect
between graphite and magnetic nanoparticles Coey et al. (2002). Similarly, high-temperature
magnetic data of multi-walled carbon nanotube (MWCNT) mat samples embedded with Fe
and Fe
3
O
4
nanoparticles Zhao et al. (2008) indicated that the room-temperature saturation
magnetizations of the magnetic nanoparticles embedded in the MWCNTs are enhanced by a
factor of about 3 as compared with what they would be expected to have for free magnetic
nanoparticles. Recently, the study has been extended to nickel nanoparticles embedded in
MWCNTs Wang et al. (2010) and shown a similar enhancement factor.
More intriguringly, there were reports of ultrahigh temperature superconducting behaviors
in carbon films Antonowicz (1974); Lebedev (2004), carbon nanotubes Zhao & Wang
(2001); Zhao (2004; 2006), graphite Kopelevich et al. (2000), and graphite-sulfur composites
Da Silva et al. (2001); Moehlecke et al (2004). Highly oriented pyrolithic graphite (HOPG)
was shown to display either a partial superconducting or a ferromagnetic-like response to
an applied magnetic field up to 400 K Kopelevich et al. (2000).
The existence of ultrahigh temperature superconductivity in the carbon-based materials is
not accidental. The unique electronic structures of the carbon-based materials make them
Giant Moment Enhancement of Magnetic
Nanoparticles Embedded in Multi-Walled
Carbon Nanotubes: Consistent with Ultrahigh
Temperature Superconductivity
Guo-meng Zhao
1,2
, JunWang
2
, Yang Ren
3
and Pieder Beeli
1
1
Department of Physics and Astronomy, California State University at Los Angeles, Los
Angeles
2
Department of Physics, Faculty of Science, Ningbo University, Ningbo
3
X-Ray Science Division, Advance Photon Source, Argonne National Laboratory, Argonne
1,3
USA
1,2
P. R. China
16
2 Will-be-set-by-IN-TECH
ideal for high-temperature superconductivity. Several theoretical models based on different
types of interactions predict high-temperature superconductivity in quasi-one-dimensional
(quasi-1D) and/or quasi-two-dimensional (quasi-2D) electronic systems. Alexandrov and
Mott Alexandrov & Mott (1995) demonstrated that strong electron-phonon coupling can
lead to the formation of intersite bipolarons and that the Bose-Einstein condensation of
the bipolarons can explain high-temperature superconductivity in cuprates. Little Little
(1964) proposed that high-temperature or room-temperature superconductivity could be
realized by exchanging high-energy excitons in quasi-1D systems. Lee and Mendoza showed
that superconductivity as high as 500 K can be achieved through a pairing interaction
mediated by undamped acoustic plasmon modes in quasi-1D systems Lee & Mendoza
(1989). High-temperature superconductivity can also occur in a multi-layer electronic
system due to an attraction of charge carriers in the same conducting layer via exchange of
virtual plasmons in neighboring layers Cui & Tsai (1991). If the plasmon-mediated pairing
mechanisms are relevant, one should be able to find high-temperature superconductivity
in quasi-one-dimensional and/or multi-layer systems such as cuprates, carbon nanotubes
(CNTs), and graphites. In contrast to the mechanisms based on the attractive interactions
between electrons by virtually exchanging phonons, excitons, and/or plasmons, an exotic
model based on resonating-valence-bond (RVB) theory originally proposed by Anderson
Anderson (1987) even predicts ultrahigh temperature d-wave superconductivity in heavily
doped graphene Black-Schaffer & Doniach (2007). Gonzalez et al. Gonzalez et al. (2001)
showed that both high-temperature ferromagnetic and p-wave superconducting instabilities
can occur in defective regions of g raphite, where topological disorder enhances the d ensity
of states. Schrieffer Schrieffer (2004) predicted ultrahigh temperature superconductivity at a
quantum critical point where ferromagnetic fluctuations are the strongest.
In this article, we will present the detailed magnetic properties of multi-walled carbon
nanotubes embedded with Ni Wang et al. (2010), Fe Zhao et al. (2008; 2011), Fe
3
O
4
Zhao et al.
(2008; 2011), and Fe
3
C magnetic nanoparticles. Magnetic measurements were carried out
using Quantum Design vibrating sample magnetometer (VSM). Inductively coupled plasma
mass spectrometer and high energy synchrotron x-ray diffractometer were used to accurately
determine the impurity concentrations. Scanning electron microscope and/or transmission
electron microscopy were used to characterize MWCNTs and magnetic nanoparticles
embedded. In sections 2,3, and 4, we will present the detailed experimental results
for multi-walled carbon nanotubes embedded with Fe
3
C, Ni, Fe, and Fe
3
O
4
magnetic
nanoparticles. We found that the saturation magnetizations of Fe, Fe
3
O
4
,andNimagnetic
nanoparticles are enhanced by a factor of about 3 as compared with what they would be
expected to have for free magnetic nanoparticles. In contrast, a smaller enhancement factor
(1.6) is found for Fe
3
C nanoparticles. In section 5, we will provide possible theoretical
interpretations to the giant moment enhancements. The results cannot be explained by a
magnetic-proximity model but can be naturally explained in terms of ultrahigh temperature
superconductivity in MWCNTs. In section 6, we will identify the diamagnetic Meissner effect
in the magnetic field parallel to the tube axis up to room temperature for aligned MWCNTs
that are physically separated and have negligibly small magnetic impurities. The magnitude
of the Meissner effect is in quantitative agreement with the predicted penetration depth
expected from the measured carrier density. In section 7, we will give concluding remarks and
discuss possible microscopic mechanisms for high-temperature superconductivity in carbon
nanotubes.
332
Carbon Nanotubes – Polymer Nanocomposites
Giant Moment Enhancement of Magnetic Nanoparticles Embedded in Multi-Walled Carbon Nanotubes: Consistent with Ultrahigh Temperature Superconductivity 3
2. Magnetic properties of Fe
3
C nanoparticles embedded in MWCNTs
Purified MWCNT mat samples (Catalog No. PD15L520) from Nanolab were synthesized by
chemical vapor deposition under catalyzation of Fe nanoparticles. The average outer diameter
is about 15 nm and the average inner diameter is about 10 nm. The morphology of the mat
sample can be seen from scanning electron microscopy (SEM) image shown in Fig. 1a. The
SEM image was taken by a field emission scanning electron microcopy (FE-SEM, Hitachi
S-4800) using an accelerating voltage of 2 kV. One can see that the outer diameters of these
MWCNTs are in the range of 10-20 nm and centered around 15 nm, in agreement with the
product specification from Nanolab. Fig. 1b shows a transmission electron microscopic (TEM)
image of the mat sample, which was provided by Nanolab. The image reveals the multiwall
nature of the carbon nanotubes with a mean inner diameter of about 10 nm.
Fig. 1. a) SEM image of a MWCNT mat sample of PD15L520. b) TEM image of the MWCNT
mat sample.
The total metal-based impurity concentrations of the mat sample can be determined from
the composition analysis of the residual of the sample, which was obtained by burning
off carbon-based materials in air. A Perkin-Elmer Elan-DRCe inductively coupled plasma
mass spectrometer (ICP-MS) was used to analyze the composition of the residual. From the
weight (3.6%) of the residual and the ICP-MS analysis, we obtain the metal-based magnetic
impurity concentrations in weight: Ni = 0.01936%, Fe = 1.001%, and Co = 0.00102%. The
Fe concentration determined from our ICP-MS is in excellent agreement with the product
specification (Fe = 0.94%) from Nanolab.
Since the magnetic impurity phases are so minor, it is impossible to identify the minor
phases from a normal low-energy x-ray diffraction (XRD) spectrum. But we can achieve
this goal by performing high-energy synchrotron x-ray diffraction experiment. Fig. 2 shows
synchrotron XRD spectrum for the mat sample along with the standard spectrum of Fe
3
C.
The XRD spectrum was taken on a high-energy synchrotron x-ray beam-line 11-ID-C at the
Advanced Photon Source, Argonne National Laboratory, using monochromatic radiation with
a wavelength of λ = 0.1078 Å. The major peaks in the spectrum of Fig. 2 correspond to
the diffraction peaks of the MWCNTs Reznik et al. (1995) and Fe
3
C. In particular, the (002)
diffraction peak of the MWCNTs is seen at 2 θ = 1.786
◦
.
Figure 3a shows XRD intensity as a function of the wave-vector transfer Q for the (002) peak
of a pure MWCNT sample. The data are digitized from Reznik et al. (1995). The solid red
333
Giant Moment Enhancement of Magnetic Nanoparticles Embedded
in Multi-Walled Carbon Nanotubes: Consistent with UltrahighTemperature Superconductivity
4 Will-be-set-by-IN-TECH
0
50
100
150
123456
PD15L520
Fe
3
C
Intensity (arb. units)
2θ (degree)
Fig. 2. High-energy synchrotron x-ray diffraction (XRD) spectrum for a virgin MWCNT
sample of PD15L520 and the standard spectrum of Fe
3
C.
line in Fig. 3a is the fitted curve by the sum of a Gaussian and a cut-off Lorentzian function,
which takes into account both domain size broadening and strain broadening Reznik et al.
(1995). The Lorentzian function is cut off to zero when
|Q − Q
◦
|≥3.65γ,whereQ
◦
is the
peak position and γ is the full width at half maximum (FWHM). The integrated intensity of
the cut-off Lorentzian is 91.4% of the intensity of the corresponding full Lorentzian. In Fig. 3b,
we display the expanded view of the (002) peak of our mat sample. We also fit the data with
the sum of a Gaussian and a cut-off Lorentzian function (solid red line). It is apparent that
the fit is excellent. In Fig. 3c and Fig 3d, we show the expanded views at 2θ around 2.6
◦
and
3.3
◦
, respectively. At 2θ around 2.6
◦
, there are closely spaced double peaks, corresponding
to the (121) and (210) diffraction peaks of the Fe
3
C phase. The intensity of the (121) peak is
higher than that of the (210) peak by a factor 1.52. The solid red line in Fig. 3c is the best
fitted curve by the sum of two Gaussian functions with the intensity ratio of 1.52 and peak
separation of 0.008
◦
, which are consistent with the standard spectrum of the Fe
3
C phase. The
Gaussian function is consistent with particle-size broadening Reznik et al. (1995). From the
best fit, we obtain γ = 0.0589
◦
for both peaks. The integrated intensity of the two Gaussian
peaks is calculated to be 0.501
±0.035% of the intensity of the MWCNT (002) peak. Using the
standard intensities of graphite’s (002) peak and Fe
3
C’s (121) and (210) peaks and assuming
that the intensity of MWCNT (002) peak is the same as that of graphite (002) peak, we find that
the Fe
3
C concentration is 0.935±0.065% (in weight). This corresponds to the Fe concentration
of 0.874
±0.061%, which is in good agreement the total Fe concentration (1.00%) determined
from the ICP-MS above and also close to the product specification for the Fe concentration
(0.94%). This implies that the dominant Fe-based phase is Fe
3
C and the minor phases may
also contain Fe and/or Fe oxides, which are not visible from the XRD spectrum.
Similarly, at 2θ around 3.3
◦
, there are also closely spaced double peaks, corresponding to the
(131) and (221) diffraction peaks of the FeC
3
phase. The intensity for the (131) peak is 2/3 of
that for the (221) peak. The solid red line in Fig. 3d is the best fitted curve by the sum of two
Gaussian functions with the intensity ratio of 2/3 and peak separation of 0.038
◦
.Thepeak
widths are kept the same as those of the (121) and (210) peaks. The integrated intensity of the
double Gaussian peaks is calculated to be 0.811
±0.024% of the intensity of the MWCNT (002)
334
Carbon Nanotubes – Polymer Nanocomposites
Giant Moment Enhancement of Magnetic Nanoparticles Embedded in Multi-Walled Carbon Nanotubes: Consistent with Ultrahigh Temperature Superconductivity 5
0
1
2
3
4
5
6
1.70 1.75 1.80 1.85 1.90 1.95
Intensity (arb. units)
Q (A
-1
)
o
a
MWCNT (002)
20
40
60
80
100
120
140
160
1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4
2θ (degree)
b
23
24
25
26
27
28
2.40 2.45 2.50 2.55 2.60 2.65 2.70
Intensity (arb. units)
2θ
(
de
g
ree
)
c
29
30
31
32
33
34
35
36
3.20 3.25 3.30 3.35 3.40
2θ (degree)
d
Fig. 3. a) The XRD intensity as a function of the wave-vector transfer Q for the (002) peak of a
pure MWCNT sample. The data are digitized from Reznik et al. (1995). b) The expanded
view of the MWCNT (002) peak of sample PD15L520. c) The expanded view at 2θ around
2.6
◦
of sample PD15L520. There are closely spaced double peaks, corresponding to the (121)
and (210) diffraction peaks of the Fe
3
C phase. d) The expanded view at 2θ around 3.3
◦
.There
are also closely spaced double peaks, corresponding to the (131) and (221) diffraction peaks
of the FeC
3
phase.
peak. The intensity ratio implies that the Fe
3
C concentration is 0.947±0.028% (in weight), in
excellent agreement with that (0.935
±0.065%) inferred from the (121) and (210) peaks above.
It is important to determine the average diameter d of the ferromagnetic Fe
3
C nanoparticles
embedded in MWCNTs. We can determine d from the peak width of the XRD spectrum. The
full width at half maximum has been found to be 0.0589
◦
. Using the Scherrer equation: d =
0.89λ/(γ
b
cos θ) and the width γ
b
= 0.0546
◦
(after correcting for the instrumental broadening),
we calculate d = 10.0 nm, in good agreement with the average inner diameter of the tubes (see
Fig. 1b).
Fig. 4a shows magnetization versus magnetic field for the MWCNT mat sample at 310 K. The
magnetization was measured using a Quantum Design vibrating sample magnetometer. The
linear field dependence of the magnetization with a negative slope at H
> 20 kOe is due to the
diamagnetic contribution. The linear extrapolation to H =0yieldsM
s
=1.53emu/g.InFig.4b,
we present temperature dependence of the saturation magnetization M
s
for the mat sample.
It is clear that the M
s
value is small (0.05 emu/g) above the Curie temperature (about 470 K)
of the Fe
3
C phase. Therefore, the saturation magnetization for the Fe
3
C phase is 1.47 emu/g.
335
Giant Moment Enhancement of Magnetic Nanoparticles Embedded
in Multi-Walled Carbon Nanotubes: Consistent with UltrahighTemperature Superconductivity
6 Will-be-set-by-IN-TECH
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
300 400 500 600 700 800 900
M
s
(emu/g)
T (K)
b
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
-60 -40 -20 0 20 40 60
M (emu/g)
H (kOe)
M
s
= 1.53 emu/g
a
Fig. 4. a) The magnetization versus magnetic field for sample PD15L520 at 310 K. b) The
temperature dependence of the saturation magnetization M
s
for the mat sample.
With the Fe
3
C concentration of 0.94±0.07%, we calculate the saturation magnetization to be
156
±11 emu per gram of Fe
3
C.
For 11-nm Fe
3
C nanoparticles embedded in carbon matrix and prepared at 900
◦
C, M
s
was found to be 89-97 emu per gram of Fe
3
C and the reduced remanence is 0.16-25 [see
Sajitha et al. (2007)]. The reduced remanence in our 10-nm Fe
3
C nanoparticles embedded in
MWCNTs is about 0.18, very close to those of the samples prepared at 900
◦
C. Therefore,
M
s
of our 10-nm Fe
3
C nanoparticles embedded in MWCNTs is enhanced by a factor of
1.6
±0.2, compared with that (93±4emu/g)offreeFe
3
C particles. This enhancement factor is
significantly lower than those for Ni, Fe, and Fe
3
O
4
nanoparticles (see below). It is interesting
to note that sample CFe05980 of Sajitha et al. (2007), prepared at 980
◦
C, has a large M
s
value
of 169 emu per gram of Fe
3
C. These authors tentatively attributed this large value to the
proximity of the nanoparticles to carbon nanotubes, which may have been formed at this
higher temperature. Our current results support this interpretation.
336
Carbon Nanotubes – Polymer Nanocomposites