74                            P. W. de Bent et al. /Applied Surface Science 74 (1994) 73-80

    The atomic structure of ...
P. W. de Bent et al. /Applied   Surface Science 74 (1994) 73-80                               75

Fig. 1. (a) 153 X 153...
76                                   P.U? de Bont et al. /Applied   Surface Science 74 (1994) 73-80

P. W. de Bont et al. /Applied   Surface Science 74 (1994) 73-80                                17

shows a hexaEona1 supe...
78                                  P. W de Bont et al. /Applied   Surface Science 74 (1994) 73-80

Six peaks from the he...
P. W de Bont et al. /Applied Surface Science 74 (1994) 73-80                       79

observed in Fig. 3. The (fi x fi)R...
80                                    P. W. de Bont et al. /Applied   Surface Science 74 (1994) 73-80

References        ...
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1994 atomic structure of longitudinal sections of a pitch based carbon fiber studied by stm


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1994 atomic structure of longitudinal sections of a pitch based carbon fiber studied by stm

  1. 1. applied surface science 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) Abstract 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 [2]. 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 SSDI 0169-4332(93IE0221-7
  2. 2. 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 [5]. 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 data. 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-’ [2]. The structural pa- rameters of this fiber were determined by 3.1. Superstructures on HOPG Northolt et al. [2] 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. [6] and Kuwabara et al. [7]. 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.
  3. 3. 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 as: 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 if -6O”IeI60”. 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.
  4. 4. 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.
  5. 5. 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 power spectrum. 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.
  6. 6. 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. [2]. 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
  7. 7. 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 [2]. 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 [8]. 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.
  8. 8. 80 P. W. de Bont et al. /Applied Surface Science 74 (1994) 73-80 References [5] K. Besocke, Surf. Sci. 181 (1987) 145. [6] P.I. Oden, T. Thundat, L.A. Nagahara, S.M. Lindsay, G.B. Adams and O.F. Sankey, Surf. sci. L&t. 254 (1991) L454. ill J.-B. Donnet and R.C. Bansal, Carbon Fibers (Dekker, [7] M. Kuwabara, D.R. Clarke and D.A. Smith, Appl. Phys. New York, 1984) chs. 1 and 2. Lett. 56 (1990) 2396. PI M.G. Northolt, L.H. Veldhuizen and H. Jansen, Carbon [S] J. Xhie, K. Sattler, U. Mueller, N. Venkateswaran and G. 29 (1991) 1267. Raina, Phys. Rev. B 43 (1991) 8917. [31W.P. Hoffman, W.C. Hurley, T.W. Owens and H.T. Phan, [9] G.M. Shedd and P.E. Russell, Surf. Sci. 266 (1992) 259. J. Mater. Res. 26 (1991) 4545. [41S.N. Magonov, H.-J. Contow and J.-B. Donnet, Polym. Bull. 23 (1990) 555.