• PDF / 4,111,264 Bytes
• 6 Pages / 417.6 x 639 pts Page_size
• 81 Downloads / 266 Views

DOWNLOAD

REPORT

---

out in a JEOL 2010F field-emission-gun transmission electron microscope (FEG-TEM). During in-situ anneals, temperature was monitored continuously via thermocouples. Only a few minutes were required to ramp between temperatures due to the small thermal mass of the heater. A JEOL 4000EX high-resolution TEM (HRTEM) was used to investigate the response of the nanotubes to electron beams of energies 80, 100, 150, 200, 300 and 400 keV. Electron flux to the specimen was approximately 3.4(10)19 electrons cnf 2 s1. The structure of both in-situ and exsitu specimens was examined via TEM phase contrast imaging in either the FEG-TEM or the HRTEM. Magnification was determined using polyaromatic carbon shells present in the specimen, which originate from the decomposition of carbide crystals. It is known that the strong lattice fringes from these turbostratic shells have a well-defined spacing of 0.34 rnm. Microscopy specimens were prepared from nanotube paper by tearing away a small sliver and fixing it inside an oyster TEM grid, thereby forgoing additional chemical or thermal processing. Imaging of individual molecules of C6o was performed in the JEOL 2010F at 100 keV. RESULTS As-received SWNTs are coated with surfactant as a result of the purification process. In the TEM, they appear as shown in Figure 1. A single SWNT is extended between two bundles of SWNTs oriented approximately perpendicularly at the top and bottom of the figure. A thin coating of disordered material is present on all surfaces. In Figure 2, the same specimen as shown in Figure 3 is imaged after annealing in vacuum at 225°C for 63 hours. We have found that baking for at least 24 hours under these conditions removes the surfactant without modifying the nanotube material. In the figure, the nanotubes are seen to be free of most impurities. The walls of the nanotubes appear as dark parallel lines and are seen to have breaks and disclinations that are the result of acid attack (see arrow on the figure). SWNTs are stable in non-oxidizing and mildly oxidizing environments. However, oxidizing environments, as well as strong sonic energy, as produced in high intensity ultrasounds, are known to damage the walls of SWNTs and to cut SWNTs. Figure 1. A FEG-TEM image of a SWNT in as-purifiedPLV material showing the coating of surfactant. Vacuum anneals of damaged nanotube material has been found to repair most of the damage to the walls. The annealing induced recovery of defects begins to occur at temperatures above 400 'C. At higher temperatures, larger defects are seen to anneal out and the expected structure of SWNTs is recovered, that of a wrapped graphene sheet of hexagonally arranged carbon atoms, in the form of a cylindrically-wrapped chicken wire. An image of a SWNT that was annealed in vacuum at 1100*C is shown in Figure 3. Under phase imaging conditions, nanotubes can be considered as weak phase objects. Images of weak phase objects will be twodimensional projections along the electron beam direction of the three dimensional specimen potential convoluted with