Electron emission and structural characterization of a rope of single-walled carbon nanotubes

Abstract

The electron emission and structural properties of an isolated ‘‘rope’’ comprised of ;70 individual singlewalled nanotubes ~SWNT’s! were investigated by measuring the field-emission energy distributions and by using field-ion microscopy~FIM!. Field-emission energy distributions, obtained under ultrahigh-vacuum conditions, revealed that the emitting nanotube has a large density of states near the Fermi energy, with an energy distribution of emitted electrons close to that predicted by the free-electron theory. Two small features located on the trailing edge of the energy distributions are attributed to localized features in the density of states of a SWNT. FIM studies were also performed on the same rope in an attempt to provide structural information about the emitting nanotube. Initial FIM micrographs showed an uneven distribution of atoms. Eventually, rings of atoms were imaged. The atom placement around an individual ring structure is analyzed and found to be consistent with that expected from a single ~19,13! SWNT. I. INTRODUCTION Much work has been done toward an understanding of the structural and electronic properties of carbon nanotubes. Many calculations of the electronic structure of single-walled nanotubes ~SWNT’s! have been reported. 1‐4 SWNT’s of the armchair ~n,n! variety are expected to be metallic; nanotubes of the zigzag ( n,0) family can be either metallic or semiconducting; and nanotubes with an inherent ~n,m! helicity are expected to be either semiconducting or metallic, depending on the exact values of n and m. Calculations illustrating how the density of states ~DOS! varies near the capped end of a nanotube have also been reported. 5‐7 Nanotubes can be imaged in the transmission electron microscope ~TEM! relatively easily which has provided an understanding of the structural properties and the growth mechanisms involved in nanotube formation. 3,8‐10 Scanning