The Study On Carbon Nanotubes Using Positron Annihilation Techniques

The extraordinary mechanical properties and unique electrical properties of carbon nanotubes (CNTs) have stimulated extensive research activities across the world since their discovery by Sumio Iijima of the NEC Corporation in the early 1990s. Carbon nanotubes can usually be viewed as a hollow cylinder formed by rolling up graphite sheets, which are several nanometers in diameter and many microns in length. According to the sheet numbers of rolling, carbon nanotubes are classified as single-walled carbon nanotubes (SWNT), double-walled carbon nanotubes (DWNT) and multi-walled carbon nanotubes (MWNT). Carbon nanotubes can be metals or semiconductors with different size energy gaps, depending very sensitively on the diameter and helicity of the tubes, i.e., on the indices (n,m). Besides, carbon nanotubes display the highest Young's modulus of over 1TPa and tensile strength of 100 GPa, the highest thermal conductivity.Positron annihilation spectroscopy (PAS) is a powerful technique to study vacancy defects and electronic structure. The basic principle of Positron annihilation spectroscopy lies on the fact that gamma photons which derive from the annihilation of positron with electron carry information on the electronic environment at which the positron annihilates. Positron annihilation spectroscopy mainly includes positron annihilation lifetime spectroscopy (PAL), positron coincidence Doppler broadening spectroscopy (CDB) and angular correlation of annihilation radiation (ACAR).The thesis is focused on the research on carbon nanotubes by employing positron annihilation technology.In chapter 1 and chapter 2, the basic knowledge on carbon nanotubes and positron annihilation is respectively introduced.In chapter 3, positron annihilation lifetime spectroscopy for SWNT and MWNT powder sample, pressed CNTs sample under various pressure were respectively measured. In addition, positron lifetime experiments for carbon nanotubes in vacuum, nitrogen and air were performed respectively. These results display a single-component positron lifetime. We conclude that a positron annihilates with an electron on the external surface of carbon nanotubes or open space between carbon nanotubes.In chapter 4, coincidence Doppler broadening spectroscopy for SWNT, MWNT, short-SWNT, short-MWNT and graphite were measured. The Ratio curves of the Doppler broadening spectroscopy for these samples to silicon were obtained. It is shown that there are distinct peaks at the position of 10×10^-3 m0 c for both carbon nanotubes and graphite, however the amplitudes of the peaks are not the same. We present that these peaks mainly arise from the annihilation of positron with the 2p electron of carbon element.In chapter 5, electron momentum distribution for diamond, graphite, silicon carbide, lithium and beryllium were respectively calculated by using Doppler software package. The annihilation probability ratio of 2p to 2s electron as well as total electron, 2s +2p electron to silicon for these materials were obtained. These calculation results agrees well with the experimental conclusion on coincidence Doppler broadening measurement.