Magnetism of amorphous carbon nanofibers

Article abstract of DOI:10.1063/1.3272940

Amorphous carbon nanofibers (ACNFs) have been synthesized by a thermal chemical vapor deposition technique. The ACNFs grow as two branches perpendicular to {111} facets of a catalytic copper nanoparticle. The carbon nanofibers are composed of disordered l

Full text of DOI:10.1063/1.3272940

Magnetism of amorphous carbon nanofibers
S. Ma, J. H. Xia, Vadali V. S. S. Srikanth, X. Sun, T. Staedler, X. Jiang, F. Yang, and Z. D. Zhang
Citation: Applied Physics Letters 95, 263105 (2009); doi: 10.1063/1.3272940
View online: http://dx.doi.org/10.1063/1.3272940
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APPLIED PHYSICS LETTERS 95, 263105 ͑2009͒
Magnetism of amorphous carbon nanofibers
S. Ma,1,2 J. H. Xia,1 Vadali V. S. S. Srikanth,1,a͒ X. Sun,1 T. Staedler,1 X. Jiang,1,b͒
F. Yang,2 and Z. D. Zhang2
1Institute of Materials Engineering, University of Siegen, Paul-Bonatz-Str. 9-11, 57076 Siegen, Germany
2Shenyang National Laboratory for Material Science, Institute of Metal Research, and International
Centre for Material Physics, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016,
People’s Republic of China
͑Received 27 August 2009; accepted 18 November 2009; published online 30 December 2009͒
Amorphous carbon nanofibers ͑ACNFs͒ have been synthesized by a thermal chemical vapor
deposition technique. The ACNFs grow as two branches perpendicular to ͕111͖ facets of a catalytic
copper nanoparticle. The carbon nanofibers are composed of disordered localized nanofragments
which in turn consist of several graphene layers. The ACNFs show a paramagnetic characteristics
at 2, 5, and 10 K. The magnetic moments are suggested to originate from a large amount of defects
in the graphene layers of the nanofragments. © 2009 American Institute of Physics.
͓doi:10.1063/1.3272940͔
The discovery of ferromagnetism in polymerized C60,1,2
has led to similar findings in other carbon nanomaterials,
namely, nanographite,3 nanofoam,4 nanospheres,5 and
nanodiamond6 which are zero-dimensional. However, when
it comes to applications, only one-dimensional ͑1D͒ materi-
als can facilitate carbon-nanostructures-based magnetic-
device assembling. In this context, a kind of pitch-based ac-
tivated carbon fibers ͑ACFs͒ with a diameter of 10 ␮m have
been synthesized.7 However, these ACFs only show antifer-
romagnetic behavior at 2 K,8 which has less possibility to be
applied in the design of nanodevices. For synthesizing useful
1D carbon materials, such as paramagnetic or ferromagnetic
carbon nanomaterials, carbon nanofibers with optimized
structure should be designed. In this letter, we present the
paramagnetic characteristics of a unique class of 1D carbon
nanostructures which we name amorphous carbon nanofibers
͑ACNFs͒. These ACNFs are expected to be promising can-
didates in nanodevice designing. It is also expected that they
will generate new research interest with regard to magnetism
in the carbon-based 1D nanomaterials and that further modi-
fied nanostructures may show ferromagnetic characteristics.
The ACNFs were synthesized by a thermal chemical-
vapor-deposition technique.9 Copper tartrate ͑dispersed on a
copper substrate͒ and acetylene were used as reaction precur-
sors. The reaction temperature, reaction-gas pressure, and an-
nealing temperature were 523 K, 500 mbar, and 1173 K,
respectively. Structural and phase analysis of the ACNFs was
carried out by scanning electron microscopy ͑SEM͒, trans-
mission electron microcopy ͑TEM͒, x-ray diffraction ͑XRD͒
with Cu-K␣ radiation, and micro Raman scattering with a
laser wavelength of 532 nm. The magnetic behavior of the
ACNFs was studied by using a superconducting quantum
interference device. Before the measurements, the ACNFs
were washed two to three times with 2M HNO3 for removing
ferromagnetic-metal impurities, if any.
of the individual ACNFs is at least 10 ␮m while the diam-
eter distribution ranges from 150 to 300 nm. The TEM image
͓Fig. 1͑b͔͒ presents the two-branches feature of the ACNFs.
It can be observed that two nanofibers have simultaneously
grown perpendicular to two facets of the catalyst nanopar-
ticle ͓darker-contrast part in Fig. 1͑b͔͒. It can also be seen
that the diameter of the fiber is about the same as the size of
the catalyst. The selected area electron diffraction ͑SEAD͒
pattern of one ACNF indicates the crystalline characteristic
of the two nanofibers and the catalytic particle ͓Fig. 1͑c͔͒. In
the SEAD pattern, the single-crystal-diffraction pattern of a
¯
Cu nanoparticle with ͓101͔ zone axis is seen. The two carbon
nanofibers grow on two ͕111͖ facets of the Cu particle and
the growth direction of the carbon fiber is along the ͗111͘
direction of the copper nanoparticles. On the other hand, no
typical diffraction ring for amorphous carbon is observed in
the SAED pattern, while two elongated diffraction spots in
the ͗111͘ direction of copper suggest that the carbon nanofi-
bers are crystalline along the ͗111͘ direction of the copper
nanoparticle.
The powder XRD pattern, shown in Fig. 2, presents the
phase composition of the ACNFs. The broad reflection at
24.1° indicates the amorphous nature of the ACNFs. The
other four peaks indicate the existence of fcc Cu and the
The SEM image ͓Fig. 1͑a͔͒ shows the morphology and
distribution of the ACNFs. It can be observed that the length
a͒
Present address. School of Engineering Sciences and Technology, Univer-
sity of Hyderabad, Central University, Hyderabad, India 500046.
Author to whom correspondence should be addressed. Electronic mail:
xin.jiang@uni-siegen.de.
FIG. 1. ͑a͒ Low-magnification SEM plane view image of ACNFs, ͑b͒ TEM
plane view image of a representative ACNF, and ͑c͒ the SAED of the ACNF
shown in ͑b͒.
b͒
0003-6951/2009/95͑26͒/263105/3/$25.00
95, 263105-1
© 2009 American Institute of Physics
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263105-2
Ma et al.
Appl. Phys. Lett. 95, 263105 ͑2009͒
FIG. 2. XRD pattern of the ACNFs.
absence of any other phases ͑for example, metallic impuri-
ties͒ in the sample.
The Raman spectrum ͑Fig. 3͒ reveals the structure of
the carbon nanofibers in ACNFs. Two distinct peaks of
1345 and 1580 cm−1 are assigned to the D and G band of
graphitic material, respectively. The G band has sp2 content
indicating the crystalline graphite, while the D band has sp3
content indicating the disorder modes induced by defects
͑vacancies͒.10 The broad G peak with full width at half maxi-
mum ͑FWHM͒ of 75.34 cm−1 indicates that the carbon
nanofibers are constructed by the graphitic fragments with
smaller grain sizes, because the value of FWHM is much
larger than 12 cm−1 for graphitic samples with a large grain
size.7,10,11 The average size of the graphitic fragments can be
further estimated from the intensity ratio of D and G bands.
The in-plane crystallite size ͑L␣͒ of the graphitic fragment
can be calculated by the function L␣͑nm͒=Cϫ͑IG/ID͒.12 The
coefficient C was calculated by the function C͑␭ ͒ϷC0
L
+␭LC1, where C0 and C1 were estimated to be Ϫ12.6 and
0.033 nm.12 In this way, values of C ͑␭L=532 nm͒ and
IG/ID were calculated equal to 1.37 and 4.956, and the aver-
age size L␣ of the graphitic fragments was estimated to be
6.79 nm. Therefore, according to the above Raman and XRD
analysis, it can be inferred that the structure of the ACNFs is
constituted by disordered localized nanofragments, while
each nanofragment in turn consists of several graphene lay-
ers with a large amount of defects ͑vacancies͒. The structural
schematic image of an amorphous carbon nanofiber is shown
in the inset of Fig. 3.
FIG. 4. ͑Color online͒ ͑a͒ FC M-T curve of ACNFs in a magnetic field of
200 Oe from 5 to 300 K and, in the inset, H/M vs T, ͑b͒ M-H of ACNFs at
5 K ͑triangles͒ and 10 K ͑circles͒ and, in the inset, M-H at 2 K, and ͑c͒
Arrott plots of ACNFS at 5 and 10 K and, in the inset, the Arrott plot at 2 K.
magnetic field of 200 Oe. The FC magnetization is positive
below 200 K, as shown in Fig. 4͑a͒. Having measured a
positive magnetization for the sample, it is now important to
determine the phase that is responsible for this. Only for
copper nanoparticles with size below 5 nm and at tempera-
tures below 560 pK, the magnetic behavior may change from
diamagnetic to paramagnetic.13,14 However, the copper nano-
particles in the present sample are larger than 150 nm as
shown by the SEM and TEM results. This means that the
copper nanoparticles in the present sample are diamagnetic at
all temperatures ͓Fig. 4͑a͔͒. Also, carbon ͑graphite and dia-
mond͒ and copper in bulk form are typically diamagnetic.
From this, it can be concluded that the ACNFs in the sample
are responsible for the positive magnetization. The H/M
Figure 4͑a͒ shows the field-cooled ͑FC͒ magnetization of
a sample ͑catalyst Cu nanoparticles+carbon branches͒ at a
−1
versus T, i.e., ␹ versus T, plot presented in the inset of
Fig. 4͑a͒ is linear at low temperatures and passes through
the origin, exhibiting Curie-type of behavior. Because, at
these low temperatures, the diamagnetism of Cu ͑−0.1
ϫ10−6 emu/g Oe͒ can be neglected compared to the large
paramagnetic susceptibility, the observed Curie behavior of
the susceptibility can entirely be attributed to the ACNFs.
The determination of the value of the initial susceptibil-
ity from the almost linear M-H curves at 5 and 10 K ͓Fig.
FIG. 3. ͑Color online͒ Raman spectrum of the ACNFs and schematic image
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of the carbon nanofiber in the inset.

263105-3
Ma et al.
Appl. Phys. Lett. 95, 263105 ͑2009͒
4͑b͔͒ is straightforward. Because of the stronger curvature of
the M-H curve at 2 K ͓inset of Fig. 4͑b͔͒, we have performed
an Arrott-plot analysis to determine the value of the initial
susceptibility at this temperature. Figure 4͑c͒ presents the
result ͑inset͒, together with the Arrott plots at 5 and 10 K.
The Arrott plots clearly prove the paramagnetic behavior of
the ACNFs at the temperatures investigated. In order to esti-
mate the size of the magnetic moments involved, we have
interpreted the nonlinear magnetization, measured at 2 K up
to 50 kOe, in terms of the paramagnetic Langevin curve.
After correcting for the diamagnetic contribution of the cop-
per, the average moment of the carbon branch can be esti-
ferromagnetism.16 Therefore, control of the quantity and the
distribution of defects in the nanofragments will be of prime
importance in preparing ferromagnetic ACNFs.
In summary, by using the thermal chemical-vapor depo-
sition technique, paramagnetic ACNFs have been synthe-
sized. The moments of the ACNFs may originate from the
defects ͑mainly vacant sites͒ in the graphene layer of the
nanofragments.
This work was supported by Deutsche Forschungsge-
meinschaft ͑Grant No. DFG JI22/3/JI22/7-2͒ and National
Basic Research Program of China, Ministry of Science and
Technology China ͑Grant No. 2010CB934603͒.
mated to be about 0.6 ␮ per 1000 C atoms. If the average
B
magnetic moment induced by one defect is around 2 ␮
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branches of the ACNFs, with their structural units being
nanofragments constructed by graphene layers, present para-
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