1994 atomic structure of longitudinal sections of a pitch based carbon fiber studied by stmDocument Transcript
ELSEVIER Applied Surface Science 74 (1994) 73-80
Atomic structure of longitudinal sections of a pitch-based carbon
fiber studied by STM
P.W. de Bont a, P.M.L.O. Scholte a,*, M.H.J. Hottenhuis b, G.M.P. van Kempen a,
J.W. Kerssemakers a, F. Tuinstra a
n Solid State Group, Department of Applied Physics, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands
b Akzo Research Laboratories Arnhem, Corporate Research, P.O. Box 9300, 6800 SB Arnhem, The Netherlands
(Received 6 May 1993; accepted for publication 14 September 1993)
Longitudinal sections of pitch-based carbon fibers have been studied with scanning tunneling microscopy. A
hexagonal superstructure due to a rotation of 9.5” of the top graphitic plane with respect to the underlying bulk was
observed. Remarkably this superstructure was modulated near defects by a (6 x fi)R30” modulation. The same
modulation was found on the images with atomic resolution. It was concluded that the atomic structure of the fiber
resembles the hexagonal structure of graphite. But locally this structure is disturbed. From the modulation of the
superstructure it is deduced that this disturbance extends at least two layers into the bulk.
1. Introduction orientation distribution of the graphitic layers in
the fiber. In a frequently used model the fiber is
Carbon fibers form a class of carbon modifica- thought to consist of a disordered core that is
tions, with remarkable mechanical properties that surrounded by an ordered mantle. Both the core
make them attractive for applications in compos- and the mantle consist of graphitic layers that are
ite materials. The structure and morphology of preferentially oriented parallel to the fiber axis.
carbon fibers have been studied extensively [1,21. At the atomic level these graphitic layers are
It has been shown conclusively that they consist thought to be connected by interlinking, i.e.
of graphitic layers that are preferentially oriented merging of different layers 111, or by covalent
parallel to the fiber axis; however, the mechanical cross-linking . In the latter case the layers are
properties of the fibers do not resemble those of connected by sp3 bonds between some of the C
graphite as can be seen from the observed high atoms in adjacent layers. Through this connection
Young’s moduli up to 800 GPa. This difference in the weak van der Waals interaction, which is
mechanical properties can be ascribed to the present in ordinary graphite crystallites, is re-
placed by strong chemical bonds. This immobi-
lizes the layers with respect to each other and
consequently increases the shear modulus be-
* Corresponding author. tween the graphitic layers.
0169-4332/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved
74 P. W. de Bent et al. /Applied Surface Science 74 (1994) 73-80
The atomic structure of carbon fibers has been tome (LKB, 2128) using a diamond knife. Electri-
investigated before with scanning tunneling mi- cal contact between the filaments and the STM
croscopy @TM) [3,4]. In these studies either the sample holder was made at one end of the fiber
outer surface or sections perpendicular to the bundle, with a small drop of Eccobond 66C.
fiber axis were analyzed. However, longitudinal The scanning tunneling microscope used in
cuts should be studied in order to understand the this study was of the Beetle type . Constant-
relation between the high moduli and the atomic current images were taken in air using a Pt/Ir
structure of the graphitic layers. Because the tip. The tunnel current was set between 1 and 10
graphitic layers are oriented parallel to the fiber nA and the bias voltage of the tip was in between
axis, only a longitudinal section allows the atomic -0.33 and 0.33 V. No difference was observed
structure of the layers to be imaged, while per- between empty state and filled state images. Each
pendicular cuts do not. In this paper we present image consisted of 512 x 512 pixels. Scans were
the results of a STM study of longitudinal sec- made over areas from 40 x 40 A2 up to 4900 x
tions of a pitch-based carbon fiber. 4900 AZ.
The paper is organized as follows. After a The surfaces of the longitudinal sections ap-
short introduction into the experimental details, peared to be very rough. In order to be able to
first a superstructure that has been observed on obtain lateral atomic resolution, it was necessary
highly oriented pyrolytic graphite (HOPG) will be to apply a hardware high-pass filter. The filter
discussed. In the subsequent sections the results enhances features in the image with high fre-
on the longitudinal cuts are presented. In the last quencies, such as step edges, and removes low-
section the superstructure on HOPG is used to frequency features, such as a tilt. The effect is
draw conclusions about the atomic structure of similar to the effect of a derivative filter. As a
the fibers. result the images show the corrugation of the
gradient of the height, rather than the corruga-
tion of the height itself. Apart from this hardware
2. Experimental details filtering all images in this work represent raw
The samples used in this study were pitch-
based carbon fibers of the Carbonic HM70 type
produced by Kashima oil. This fiber has a Young’s 3. Results
modulus of 716 GN m-’ . The structural pa-
rameters of this fiber were determined by 3.1. Superstructures on HOPG
Northolt et al.  with X-ray diffraction. It was
concluded that the fiber consisted of small In Fig. la a superpericdic structure is shown
graphite crystallites with lattice parameters de- with periodic&y 40 f 3 A, which has been ob-
pendent on the crystallite size, but very close to served near a defect on HOPG. The resolution of
the values of the lattice parameters of the hexag- the image is sufficient to observe the atomic
onal graphite lattice. The size of the crystallites in periodicity of the graphite net, in addition to the
the HM70 fiber parallel and perpendicular to the superstructure. Similar superstructures on graph-
c-axis of the graphite lattice was found to be 17.0 ite (HOPG) have been observed earlier by Oden
and 59.2 nm, respectively. The structural p%rame- et al.  and Kuwabara et al. . These images
ters were determined as d(10) = 2.131 A and can be understood as an atomic moire pattern
d(002) = 3.411 A [21. due to a rotation of the top graphite plane with
For the STM experiments a bundle of fila- respect to the underlying bulk [71.
ments was embedded in a resin, each of the This can be seen most easily in the reciprocal
filaments with a diameter of approximately 10 space. The two-dimensional power spectrum of
pm. Subsequently a longitudinal section was made the hexagonal graphite net contains six wavevec-
through the resin and fibers with an ultramicro- tors. These wavevectors are related by symmetry.
P. W. de Bent et al. /Applied Surface Science 74 (1994) 73-80 75
Fig. 1. (a) 153 X 153 A* constant-current image of HOPG (I,,, = 2.0 nA, V,, = 0.50 V) near a defect on the surface. A modulation
with a periodicity of 40 f 3 w is observed of the atomic hexagonal graphite net. (b) Power spectrum calculated from (a). Two
groups of wavevectors are observed. The six peaks at small wavevector values are due to the superperiodic structure. The six broad
peaks at large wavevector values are due to the periodicity of the graphite net. Note that the relative contrast of the latter set of
peaks has been increased to make them visible.
In Fig. 2 three wavevectors of a graphite net are ing moire pattern is represented by the small
shown in the reciprocal space, together with the solid difference vectors in Fig. 2. From this figure
three wavevectors of the same graphite net that it is immediately obvious that the resulting super-
has been rotated over a small angle 6. The result- structure has hexagonal symmetry and from sim-
ple goniometry it follows that the periodicity P in
real space of the superstructure can be expressed
P = +p/sin( +8),
where p is the periodicity of the graphite net as
observed with an STM (p = 2.46 A) and 0 is the
rotation angle of the uppermost graphite layer.
For symmetry reasons this formula is correct only
In Fig. lb the power spectrum is shown of the
moire pattern in Fig. la. Six peaks are observed
at small wavevector values from the superperiod-
icity and six very broad peaks from the periodicity
of the graphite net. The broadening in the verti-
cal direction of the latter peaks is due to the
Fig. 2. Generation of a moire pattern in reciprocal space. Two limited correlation between subsequent scan lines.
hexagonal nets, each represented by three respectively dashed
and dashed-dotted wavevectors, are rotated with respect to
From the ratio between the superperiodicity and
each other over an angle 0. The resulting moire pattern is the periodicity of the graphite net a rotation
generated by the small solid difference wavevectors. angle of 0 = 3.5” k 0.3” can be calculated.
76 P.U? de Bont et al. /Applied Surface Science 74 (1994) 73-80
Because of the atomic resolution that has been
achieved in Fig. la, the rotation angle may be
determined also in a different, independent way.
From Fig. 2 it can be deduced that the angle
between the orientations of the wavevectors of
the super-periodic structure and those of the
atomic structure should be (90 - $9). From the
maxima in the power spectrum shown in Fig. lb
this angle was found to be 88.5”. From this value
a rotation angle 0 = 3” is calculated. This value is
in good agreement with the value 8 = 3.5” f. 0.3”
deduced from the relative length of the wavevec-
tors of the graphite lattice and the superstruc-
ture. Therefore, we conclude that the interpreta-
tion of the superstructure in terms of an atomic
moire pattern is justified.
Fig. 3. 610X610 A* constant-current image (ZrUn 9.1 nA,
= 3.2. Superstructures in fibers
V,, = - 0.31 V) of a Carbonic HM70 carbon fiber withOsuper-
structure. The period of the superstructure is 14.9 A. The
square regions have been used to calculate the power spectra In Fig. 3 a 610 x 610 A2 scan of Carbonic
from. HM70 carbon fiber is shown. The STM image
Fig. 4. (a) Power spectrum calculated from the region at the lower left corner of Fig. 3. (b) Power spectrum calculated from the
region in the lower right corner of Fig. 3. The inset identifies the mo st significant peaks. The symbols are explained in the text. The
lines are just to guide the eye.
P. W. de Bont et al. /Applied Surface Science 74 (1994) 73-80 17
shows a hexaEona1 superstructure with a periodic- 3.3. Atomic structure of fibers
ity of 14.9 A. This periodicity is too large to
represent the translation symmetry of the graphite Atomic resolution could be obtained on a small
net. It is a superstructure similar to the atomic fraction of the exposed longitudinal sections only.
moire pattern observed in HOPG. This is corrob- This is attributed at least partly to the roughness
orated by the large defect area that is visible in of the samples and to the contamination of the
the upper right corner. Usually on HOPG the surface during the cutting process. The region
moire superstructure is observed near defects over which atomic resolution could be obtained,
such as steps or grain boundaries [6,7]. is limited also by the random orientation of the
Immediately below the defect area a (6 graphitic layers. Although the graphitic layers are
X &)R30” modulation of the intensities of the aligned parallel to the fiber axis, they need not to
superstructure is visible in Fig. 3. The amplitude be parallel to the stanning ptane of the STM tip.
of this modulation decreases from the defect area. In Fig. 5 a 76 A by 76 A area is shown on
From the superperiodicity of 14.9 A it can be which atomic resolution was achieved. The upper
estimated that the top layer in this longitudinal left area again shows a defect area. On other
section of the fiber is rotated over 9.5” with parts of the image clearly the hexagonal pattern
respect to the underlying bulk. of the graphite net can be observed. The corruga-
To analyze the STM images in more detail, tion and the apparent periodicity change over the
regions of interest were defined of 128 x 128 displayed area. At some parts in this figure the
pixels in each image. The power spectra of these atomic structure is blurred due to contamination.
regions were calculated. On close inspection, however, it can be seen that
in Fig. 4a the power spectrum of the region in the atomic structure continues in registry with the
the lower left corner of Fig. 3 is shown. Only the parts with full atomic resolution.
six peaks of the hexagonal net of the superperiod- In Fig. 6 the power spectrum of the indicated
icity are visible, and two peaks in the center that area in the upper right corner of Fig. 5 is shown.
are artefacts of the FFT routine used to calculate
the power spectrum. The resolution of Fig. 3 is
not sufficient to resolve the underlying periodicity
of the graphite net, therefore the wavevectors of
the atomic graphite net are also missing in the
Fig. 4b shows the power spectrum of the re-
gion just below the large defect area in the upper
right corner of Fig. 3. This spectrum displays
many more peaks. The most significant peaks are
identified in the inset of Fig. 4b. The peaks near
the center of Fig. 4b (open circles in the inset)
are artefacts due to the FFT transformation. Six
peaks are at the same positions as in Fig. 4a and
are due to the hexagonal superperiodic net (indi-
cated by solid squares). Six peaks at small
wavevector values (marked by solid circles) origi-
nate from the (6 X fi)R30” modulation of the
corrugation in this part of Fig. 3. In addition six
higher-order peaks (open squares) are visible that
can be attributed as the sum of a wavevector of Fig. 5. 16.3 X 76.3 A2 constant-current image (It,, = 2.0 nA,
the hexagonal superperiodic net and a wavevec- VtiP= -0.33 V) of a Carbonic HM70 carbon fiber, showing
tor of the (fi X J?;)R30” modulation. the atomic structure of the graphitic layers.
78 P. W de Bont et al. /Applied Surface Science 74 (1994) 73-80
Six peaks from the hexagonal graphitic net are
observed. But additionally four extra peaks
emerge at smaller wavevector values. These peaks
are at the positions of a (6 X &)R30” modula-
tion, although for the fully symmetric modulation
six (fi x fi)R30” peaks should have been ob-
served. Also a number of peaks is visible that are
understood to be linear combinations of wavevec-
tors from the hexagonal graphitic net and the
(6 x &)R30” wavevectors.
In different regions in Fig. 5 additional peaks
were always found at the same positions in recip-
rocal space. Only the relative intensities of the
peaks changed from one region to the other. In
Fig. 7 the positions of the peaks corresponding to
the five regions indicated in Fig. 5 are given. The
solid circle represents the length of the wavevec-
tor of the ideal graphite net. The peaks related to Fig. 7. Superimposed power spectra calculated from the five
regions indicated in Fig. 5. The light squares represent the
the (fi x fi)R30” modulation on ideal graphite
wavevectors from the hexagonal graphitic net, the dark squares
should all lie on the dashed circle. From this are due to the (fixfi)R30” modulation. The circles are
figure we conclude that the atomic structure explained in the text.
shown in Fig. 5 is compatible with the translation
symmetry of the graphite net. This is in accor-
dance with the X-ray observations by Northolt et due to the changes in relative intensities of the
al. . The altered appearance of the atomic fourier components [8,9]. Its is not due to changes
structure in different regions of Fig. 5 is mainly in the atomic structure.
The (fi X 6)R30” modulation turned out to
be a general feature. A defect area was visible on
all the images with atomic resolution. In all those
image peaks (fi X &)R30” modulation could be
identified in the power spectrum. So we conclude
that the (fi X fi)R30” modulation is at least
partially present at the atomic level.
4. Discussion and conclusion
On HOPG the (6 x &)R30” reconstruction
is often observed close to defects or adatoms 181.
It is not a reconstruction in the sense that atoms
are displaced over or removed from the surface.
But rather the electronic charge density is modu-
lated due to the presence of an impurity or a
defect. This is similar to the Friedel oscillations
in a charge density around an impurity. The
charge density tries to screen the defect or
Fig. 6. Power spectrum calculated from the region in the adatom. So away from the impurity the amplitude
upper right corner of Fig. 5. of the density modulation will decrease, as can be
P. W de Bont et al. /Applied Surface Science 74 (1994) 73-80 79
observed in Fig. 3. The (fi x fi)R30” wavevec- the defect must extend at least two layers deep.
tors represent the first-order components of such An example of such a defect are cross-links be-
a modulation in reciprocal space. Higher compo- tween neighboring graphitic layers. Northolt et al.
nents do not appear in the power spectra shown concluded from their X-ray experiments that the
in Figs. 4 and 6, because of the limited resolution graphitic layers were cross-linked by covalent sp3
of the STM. bonds, near the edge of the layers . Such a
In the previous section we pointed out that the bond will deform the graphite net locally and may
(6 x fi)R30” modulation is present in the give rise to the (fi x fi)R30” modulation over
fibers, although the intensities of the Fourier the atomic structure. Also the second graphite
components in the power spectrum change be- plane that is connected to the top layer will
tween different parts of the surface. An STM contain a similar defect. If only one bond is
image is a convolution of the point-spread func- present the two involved graphitic layers are free
tion of the tip and the (electronic) structure of to rotate around the cross-link with respect to
the surface. Therefore, at first sight it is not clear each other. A similar construction as in Fig. 2
whether the changes in intensity are due to the shows that the superposition of two (6
surface structure or to random changes in the tip x &)R30”-modulated hexagonal nets results in a
state. But the intensities of the (fi X fi)R30” superstructure with a similar modulation. Such a
Fourier components do not change randomly over superstructure has been observed and is shown in
different parts of the image. If one considers Fig. 3. A superposition of two (6 x fi)R30”
individual scan lines, the Fourier intensities modulated graphitic nets is the simplest model to
change at the same point for a large number of explain the (6 X &)R30” modulation of the
consecutive scan lines. Therefore, we are confi- moire pattern. It can be seen in Fig. 3 that the
dent that the variations in intensity are due to a (6 x &)R30” modulation is constrained around
surface effect. Xhie showed that combinations of the large defect. This suggests that the cross-links
(6 x fi)R30” Fourier components with differ- are located in the neighborhood of the large
ent intensities, give rise to totally different real- defect.
space images . This explains the different ap- We conclude that in the pitch-based carbon
pearance of the atomic structure in the different fibers Carbonic HM70 the atomic structure is
regions in Fig. 5. The intensity changes of the similar to the atomic structure in graphite. How-
Fourier components can be understood to arise ever, the graphitic nets contain a large number of
from the differences between the defects that defects as can be deduced from the presence of
cause the (a X fi)R30” modulation in the re- the (6 x fi)R30” modulation of the STM im-
spective regions. As in the case of the electro- age on the atomic scale. But these defects are not
static screening of an impurity, the spatial distri- limited to the top layer. From the presence of the
bution of the screening charge density will adapt modulation on the superstructure it can be con-
itself to the symmetry of the defect. The Fourier cluded that at least also the second layer contains
components of the (6 x &)R30” modulation sufficient defects to modulate the electronic
will be affected especially, since they arise di- charge density.
rectly from the presence of the defect. This ex-
plains the presence of only four (6 x fi)R30
wavevectors in Figs. 6 and 7, while all six Acknowledgments
wavevectors from the graphitic net are present.
It should be noted that we cannot decide con- Mr. A.J. van de Berg is acknowledged for
clusively the nature of the defect that causes the enlightening discussions and assistance with the
(6 X &)R30” modulation. But since the (fi analysis of the images. One of us (P.d.B.1 grate-
X filR30” modulation has been observed on the fully acknowledges the financial support of Akzo
superperiodic structure also, we conclude that Research Laboratories.
80 P. W. de Bont et al. /Applied Surface Science 74 (1994) 73-80
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