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Tubular Graphite Cones

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Science  18 Apr 2003:
Vol. 300, Issue 5618, pp. 472-474
DOI: 10.1126/science.1082264

Abstract

We report the synthesis of tubular graphite cones using a chemical vapor deposition method. The cones have nanometer-sized tips, micrometer-sized roots, and hollow interiors with a diameter ranging from about 2 to several tens of nanometers. The cones are composed of cylindrical graphite sheets; a continuous shortening of the graphite layers from the interior to the exterior makes them cone-shaped. All of the tubular graphite cones have a faceted morphology. The constituent graphite sheets have identical chiralities of a zigzag type across the entire diameter, imparting structural control to tubular-based carbon structures. The tubular graphite cones have potential for use as tips for scanning probe microscopy, but with greater rigidity and easier mounting than currently used carbon nanotubes.

Owing to their unique mechanical properties and atomic structures, carbon nanotubes (CNTs) have various potential applications, such as probes for scanning probe microscopy (13), field electron-emitting sources (46), and tiny tweezers for nanomanipulation (7). Their hollow interiors can also serve as nanochannels for different materials (810). On the basis of recent theoretical calculations, molecules such as water inside CNTs may exhibit unique features not seen in bulk materials (11, 12).

Although CNTs are axially robust (13), they are very flexible because of their high aspect ratios and are susceptible to lateral bending. The radial flexibility can be problematic in some applications. For example, mechanical or thermal vibration can cause a poor signal or noise when the CNTs are used as scanning probes and field emitters, which may prove fatal when serving as tips of nanodrills or nanoindenters. Their nanometer-sized diameters also make it difficult to mount and mechanically handle individual nanotubes, even when micromanipulation techniques are used. We report the synthesis of a related carbon morphology that we call “tubular graphite cones” (TGCs). The growth of TGCs is carried out using a microwave plasma assisted chemical vapor deposition (MPCVD) system with N2 and CH4 as the reaction gas (N2:CH4 = 200:3) and iron needles as substrates (see Materials and Methods in the supporting online material). The iron needles were made by electrochemical erosion of an iron wire (Φ= 0.25 mm) in a NaOH solution and have sharp tips with a radius of curvature of about 1 μm. The growth of the TGCs did not occur on the exact tip of the iron needle but at a lower region around the wire, where its diameter is about several tens of micrometers thick. Thus, the TGC structures with nanometer-sized tips are not merely conformal replicas of the iron needles. The graphite sample holder, on which the iron needles stand upward into the plasma, is heated to 600°C before growth. The total pressure and the microwave power are kept constant at 15 torr and 750 W, respectively.

Depending on the growth duration, TGCs of different sizes can be formed with their roots varying from nanometers to micrometers in size. Independent of their sizes, TGCs are always similar in shape and grow directly from the iron needle surface. Catalyst particles are not observed on their tips, suggesting a catalytic base-growth of the TGCs. Figure 1A shows a typical image of aligned TGCs after 30 min of growth. The roots reach a diameter of about 1 μm, and the tips vary from several to tens of nanometers. The average length is about 12 μm, and the average tip apex angle is 6° to 7°. The shape of the TGCs is determined by the ratio of the axial and radial growth rates (Ra and Rr), and cones are obtained when Ra/Rr is constant. Higher magnification images show that the TGCs are faceted and helical (Fig. 1B). A number of observations reveal that the TGCs are mostly octahedral on the surface, and the spiral direction can be either clockwise or counter-clockwise. The angle of the spiral is different for each TGC.

Fig. 1.

Scanning electron microscopy and HRTEM images. (A) Aligned TGCs grown on an iron needle surface. (B) A high-resolution view of one TGCshows the faceted and helical appearance. (C) TGC tip. The graphite sheet steps can be clearly seen on the surface of the cone in (C).

Electron energy loss spectroscopy (EELS) shows that all TGCs consist only of carbon atoms. The nature of the very sharp tips and the hollow interior are clearly demonstrated in a high-resolution transmission electron microscopy (HRTEM) image (Fig. 1C). The hollow interiors have a diameter ranging from about 2 to several tens of nanometers. The sharp tips usually consist of only a few layers of graphite sheets and have domed or conical caps. High-resolution micrographs reveal that all TGCs are made of coaxial tubular graphite sheets. The cone-shaped structure is caused by a gradual shortening of the length of the graphite sheets along the axial direction from the inner to the outer layers, and layer steps are present on the cone surface (Fig. 1C). Although micrometer-sized graphitic cones have been reported before (14), they were composed of cone-shaped graphite sheets, and therefore their microstructure is completely different from those of TGCs.

All TGCs have faceted surfaces (Fig. 1B), and one could imagine that the TGCs are made of polyhedral graphite sheets like those reported elsewhere (15, 16). However, HRTEM studies show that the wall layers of the TGC are cylindrical in shape, similar to CNTs, because the lattice fringes of the TGC walls do not change when the sample is rotated during imaging. The surface structure of the TGC is determined by the shape of the graphite sheet edges. If every graphite sheet of a TGC has a circular ending, the surface will be cone-shaped without ridges. The faceted TGC surface is thus due to an uneven ending of the graphite sheets, and the edges of adjacent sheets are similar in shape (zigzag). The ridges of the cones are formed by higher parts of the zigzag graphite sheet edges. The black spots found in the low-magnification TEM images (Fig. 2A) may provide a key clue to elucidate the nature of the cone shape. These alternative black dots appear left and right in the cone wall following a certain order along the axial direction. We conclude that these black spots are caused by stress inhomogeneities within the cone wall. At the ridge position of the faceted TGC, the additional layers of graphite sheets will produce locally increased lattice stress, and a black contrast will appear if the additional layers on the surface are quasi-parallel to the incident electron beam. Considering the spiral characters of the cone ridges, a regular shift of the spot locations from the left to the right side of the wall should occur, which is confirmed in Fig. 2A. Another possible explanation is that the black spots are strain contrast–induced by defects in the hexagonal graphite network.

Fig. 2.

TEM images of (A) TGCs and (B) the TGCtip.

To explore the atomic structure, we performed a detailed HRTEM study of the TGCs. Figure 3A shows a typical single TGC, and Figs. 3, B to E, represent those regions marked by B, C, D, and E, respectively. Unlike CNTs, lattice fringe images of all the layers of the TGC wall show fine structures of many sets of separated dots instead of continuous lines. Adjacent dot spacing d1 is ∼0.21 nm, and interlayer spacing d is ∼0.34 nm. Dots within one layer do not face directly those of the adjacent layer, but instead stagger roughly d1/2 along the axial direction, which shows that adjacent layers are ordered. This dotted-layer image feature can also be observed at the tips (Fig. 2B). Zigzag and armchair nanotubes (17) (Fig. 4A) are two typical types that give rise to separated dots when the incident electron beam is perpendicular to the tube axis. Given that the sp2 C-C bond length of hexagonal graphite is 0.1421 nm, the theoretical values of the distance between two adjacent projective spots of a zigzag tube and an armchair tube are d1 = 0.213 nm and d2 = 0.123 nm, respectively. The experimental value of about 0.21 nm coincides quite well with that of the zigzag tube, and the d1/2 stagger of adjacent layers coincides with ABAB... stacking of cylindrical graphite sheets. From Fig. 3, hexagonal patterns are observed at the central region with interstripe spacing d3 of about 0.21 nm. The hexagonal patterns usually cover the whole hollow center of the cone and overlap with these inner wall layers (Fig. 3, B and E). The patterns also coincide with those of graphite when the incident electron beam is along its c-zone axis. When an AB unit of the layers in a bulk graphite crystal is viewed along its c axis (Fig. 4A), all sites occupied by carbon atoms can be divided into two groups; one group of sites is occupied by one atom (marked with open circles) and another by two atoms (marked with solid circles with d3 = 0.213 nm). These sites with two atoms will contribute to the pattern of the hexagonal network in the images. In the present case, although the graphite sheets of the TGCs' wall are cylindrical, a slice of graphite sheets can be deemed flat (Fig. 4B). Thus, a hexagonal pattern can always be observed in the central region only if the cylindrical graphite sheets are ABAB... stacking and the incident electron beam is perpendicular to the cone axis, especially when the outer graphite sheets have small curvature. This is also why the hexagonal pattern in Fig. 3E is clearer than that in Fig. 3B and covers a larger area. Usually, carbon hexagons on the top and at the bottom of TGC walls do not face perfectly, and it is difficult to have the incident electron beam exactly perpendicular to the cone axis. In both cases, the obtained pattern in the central area is often of a set of parallel lines. The lattice image of wall layers is not always dotted but can be continuous lines, due to the electron beam not being perpendicular to the cone axis (Fig. 2, B and C). Figure 3C shows the outermost layers of the cone, and they are still graphitic. Selected area electron diffraction from the left- and right-half wall of the TGCs gives the same pattern. A typical diffraction pattern taken on the longitudinal edge, where the graphite layers are roughly quasi-parallel to the electron beam, is shown in Fig. 3F. The diffraction spots of (10 Math l) appear and reveal a typical graphitic stacking. The (hk0) diffraction spots form a ring that reveals a cylindrical tubular structure.

Fig. 3.

Microstructure of a TGC. (A) Low-magnification image of a TGC. Right (upper) and left (lower) insets are magnified images of those regions surrounded by bright lines. (B to E) High-resolution images of the corresponding position marked in (A). Labels d, d1, and d3 are defined in Fig. 4A. (F) Selected area electron diffraction pattern taken on the longitudinal edge of a TGC.

Fig. 4.

(A) Schematic representation of a monolayer of graphite. a1 and a2 are the unit vectors of the two-dimensional lattice, and R is a vector represented by (m, n). A(m, n) tube is formed by rolling the graphite sheet with position (m, n) superposing on the origin. (m,0) and (m, m) are the zigzag and armchair tubes, respectively. (B) A model for two adjacent graphite layers of a TGC. A small slice (shaded area) can be deemed flat.

The average inner interlayer spacing d measured from the HRTEM images was ∼0.34 nm, which is the same as that of CNT (18). For the identical zigzag-type graphite sheets of TGC, the circumference of the cone wall layer increases by 2πd from inner to outer layers; hence, each layer increases by about eight or nine hexagons as compared with the former layer. Therefore, to maintain the graphitic interlayer ABAB... arrangement, sp2 C-C bonds might be relaxed locally and defects or disorder might exist within one layer, especially for the innermost layers that have a small diameter and a small number of hexagons.

Supporting Online Material

www.sciencemag.org/cgi/content/full/300/5618/472/DC1

Materials and Methods

Figs. S1 to S3

Table S1

References and Notes

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