Technical Comments

Response to Comment on "Tubular Graphite Cones"

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Science  06 Feb 2004:
Vol. 303, Issue 5659, pp. 766
DOI: 10.1126/science.1091130

Synthesized tubular graphite cones (TGCs) represent a new member in the family of carbon-based nanomaterials (1). Luo et al. (2) question our characterization of the microstructure of these cones and assert that the cones do not have identical zigzag chiralities. We disagree with their assessment.

The high-resolution transmission electron microscopy (HRTEM) images presented in (1) are lattice images. As indicated at the beginning of our paper, a lattice image results from the interference of diffraction waves, although we did not demonstrate a Fourier transformation to show what type of diffraction waves the image came from. Therefore, there is no way to reasonably interpret the HRTEM image as a simple atomic projection. Our purpose for constructing a model of a single-layer, zigzag-type graphite sheet was to measure the repeating period along the axial direction and to compare it with experimental data [the wall lattice image comes mainly from the interference of the first order {0002} and {101̄1} diffractions]. The measured stagger distance of the dots between adjacent layers is indeed d1/3, as pointed out by Luo et al. (where d1 is the adjacent dot spacing). The distance was miswritten as d1/2 in (1), but this mistake does not alter our discussion and conclusion.

The growth process of carbon filaments in chemical vapor deposition (CVD) is complex (3, 4) and controlling the structure of the final product remains a major challenge. We did not claim that all TGCs have the same chirality, but that we observed numerous TGCs with identical zigzag chiralities experimentally. The selected area electron diffraction (SAED) pattern shown in figure 3F in (1) was not taken from the cone shown in figure 3A in (1), which has identical zigzag chiralities, but from another cone with multi-chiralities. Thus, the HRTEM images [figure 3, B to E in (1)] and the SAED pattern are from two different samples, which Luo et al. may not have realized. The SAED pattern was only used to study the ABAB.. .graphitic stacking of wall-layers, and we did not draw any conclusions about TGC chirality from it.

Our assertion of identical zigzag chiralities was based both on the dotted walls of the layer lattice image and on the hexagonal lattice image of the core region of the TGC. More extensive characterizations of the cone structure have been done, and compared with previous chirality studies (57). Figure 1 shows the SAED pattern from a TGC, which has similar HRTEM features as shown in our previous study (1). The SAED patterns for different regions ranging from tip to root exhibit essentially the same features: the six {101̄0} diffraction spots are not split and form a hexagonal ring, while the {101̄l} diffraction spots are very clear. These results unambiguously reveal that the TGC studied here has identical zigzag chiralities.

Fig. 1.

(A) Image of a TGC. The SAED patterns(B to D) were taken from the region smarked a, b, and c, respectively, in (A). The SAED patterns are similar despite differences in intensity: {101̄0} diffraction spots form a hexagonal ring, and {101̄0} and {1̄010} diffraction spots stay in the cone axis. These features reveal identical zigzag chiralities of the wall layers. Diffraction spots of {101̄l}, and where l = 1, 3, and 5, also appear and reveal a typical graphitic stacking.

Luo et al. (2) argue that the dotted wall lattice images should not be used to conclude the presence of identical zigzag chirality. Again, we emphasize that our conclusion regarding TGC chirality was based on results obtained from both the dotted wall layer lattice image across the entire tube diameter, and the hexagonal lattice image in the core region. The SAED pattern taken from the entire diameter region [figure 2D in (2)] indeed reveals a CNT with various chiralities, but Luo et al. presented no corresponding HRTEM lattice images for the same tube region. Comparison of the SAED pattern to a local HRTEM image is misleading because the data depict different regions within the material. In conclusion, we do not believe that comments of Luo et al. add any new informaton to our results in (1).


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