Tham khảo tài liệu 'carbon nanotubes episode 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 68 K. Sattler Fig. 3. STM images of fullerene tubes on a graphite substrate. formed further concentric shells can be added by graphitic cylindrical layer growth. The C60 Oj j 1 2 3 . . . tube has an outer diameter of A 18 . In its armchair configuration the hexagonal rings are arranged in a helical fashion with a chiral angle of 30 degrees. In this case the tube axis is the five-fold symmetric axis of the C60-cap. The single-shell tube can be treated as a rolled-up graphite sheet that matches perfectly at the closure line. Choosing the cylinder joint in different directions leads to different helicities. One single helicity gives a set of discrete diameters. To obtain the diameter that matches exactly the required interlayer spacing the tube layers need to adjust their helicities. Therefore in general different helicities for different layers in a multilayer tube are expected. In fact this is confirmed by our experiment. In Fig. 4 we observe in addition to the atomic lattice a zigzag superpattern along the axis at the surface of the tube. The zigzag angle is 120 and the period is about 16 Ả. Such superstructure giant lattice was found earlier for plane graphite 10 . It is a Moiré electron state density lattice produced by different helicities of top shell and second shell of a tube. The measured period of 16 Ả reveals that the second layer of the tube is rotated relative to the first layer by 9 . The first and the second cylindrical layers therefore have chiral angles of 5 and 4 respectively. This proves that the tubes are indeed composed of at least two coaxial graphitic cylinders with different helicities. 5. BUNDLES In some regions of the samples the tubes are found to be closely packed in bundles 20 . Fig. 5 shows a 200 Ả broad bundle of tubes. Its total length is 2000 Ả as determined from a larger scale image. The diameters of the individual tubes range from 20-40 A. They are perfectly aligned and closely packed over the whole length of the bundle. Our .
