Electron Microscopy of Nanotubes

A single-walled carbon nanotube is made by rolling up a graphene to form a seamless cylindrical tubule of only one atomic layer with diameter on the nanometer scale. The uniqueness in structure and the unusual characteristics in properties of single-walled carbon nanotubes were predicted soon after carbon nanotubes were discovered by Iijima [1], who reported the identification of multiwalled carbon nanotubes and also established the facts that the nanotubes were cylindrical, concentric, and helical in atomic arrangement. For example, electronically, depending on its diameter and helicity, a single-walled carbon nanotube can be either metallic or semiconducting [2‐9]; mechanically, a single-walled carbon nanotube is expected to have the highest tensile strength known to man due to its high tensile modulus (estimated to be about 2 TPa) and large stretchability (estimated as high as 10%) [10‐15]. Given the single dimensionality and the structural simplicity, single-walled carbon nanotubes are an ideal nanomaterial for studying the structure-property relationships.

Since their discovery [1] in 1991, carbon nanotubes have been the subject of intensive research because of their extraordinary mechanical [2] and electronic [3, 4] properties. Moreover, the intrinsic simplicity of the single-walled carbon nanotubes makes them ideal objects for the investigation of reduced-dimensionality effects. Indeed, a single- walled carbon nanotube can be built by rolling up a single graphene sheet and is uniquely defined by its chiral vector \( {C_{n,m}} = na + mb \), where a and b are the unit vectors of the honeycomb network, and n and m are integers [5]. Depending on the wrapping indices (n, m), different types of nanotubes are obtained. The first extreme is zigzag nanotubes, corresponding to (n, 0) and having a chiral angle of 0°, and the second extreme is (n, n), armchair nanotubes having a chiral angle of 30°. Actually, all chiral angles ranging from 0° to 30° can be imagined and are observed as well. The properties of a SWNT are determined by both parameters—the diameter and the chiral angle (helicity). Depending on these parameters, a SWNT can behave either as a metal or a semiconductor [6].

The technique of electron nanodiffraction (END), in which diffraction patterns are obtained from regions 1 nm in diameter or smaller, would seem to be an ideal tool for the study of nanotubes and related structures. Electron beams with energies in the range of from 50 to 1000 kV may readily be focused into small probes having diameters of 1 nm or less by means of the same strong electromagnetic lenses that are used for high-resolution electron microscopy. With the high-brightness electron sources of cold-field-emission electron guns, the intensity of the 1 nm diameter electron beams is sufficient to allow nanodiffractiion patterns to be recorded in a small fraction of a second if a suitable detection system is used. Thus a very effective and versatile arrangement is possible for the detailed study of structures on a nanometer scale.

An ideal carbon nanotube can be constructed by rolling up a single sheet of graphite to make a seamless hollow cylinder. It can be thought of as a tubular nanocrystal of graphite. Different from bulk-quantity graphite material, however, carbon nanotubes have demonstrated remarkable electronic, optical, and mechanical properties sensitive to the diameter and helicity of the tube lattice. The tremendous recent interest in carbon nanotubes stems not only from their novel properties but also from their potential use in a variety of technological applications.

In electron energy-loss spectroscopy (EELS), the kinetic energy of a beam of initially monokinetic electrons after interaction with a sample is analyzed. This can be in transmission geometry, where electrons pass through a sufficiently thin film, or in reflection geometry, combination with a transmission electron microscope (TEM), where one precisely has the transmission geometry described above. This configuration has the advantage that high-resolution spatial information can readily be combined with spectroscopic data. In principle, there are two variations of the technique—either (1) the electron beam where electrons incident at a gracing angle are reflected back into an energy analyzer. Most often, EELS is carried out in is focused to a subnanometer spot that is scanned across the sample while at each point the spectroscopic information is recorded or (2) electrons within given energy windows are used to form a series of energy filtered images of the sample [1].

Transmission Electron Microscopy (TEM) has been demonstrated to be a well-suited technique for characterizing and studying in-situ phenomena in materials at the nanometer scale, as proved by several demonstrations involving carbon nanotubes [1]. For example, the feasibility of confining different elements in the cavity of the nanotubes, like C₆₀, gases and fluid systems, provides the opportunity of studying dynamically at the nanometer scale specific in-situ induced-processes like fullerene coalescence, local gas pressure increase and liquid/gas movement inside the nanotubes. In this chapter we will focus our attention on exploring recent results of TEM use, either in its High-Resolution mode (HRTEM) or in its scanning mode (STEM), as an active, dynamic probe station. The first part of the chapter reviews the electron irradiation studies carried out on carbon materials, like graphite and nanotubes. Then, several in-situ experiments like thermal annealing, electron beam irradiation and time resolved electron energy-loss spectroscopy show the capacity of the TEM to create new carbon based nanostructures (for example, onion-like structures, CN_x nano-islands etc. within C nanotubes) and nanowires, while simultaneously monitoring the processes. Finally, the efficiency of HRTEM dynamical studies for understanding the formation process of novel carbon nanostructures, for investigating dynamic phenomena such as the coalescence of SWNTs and for studying the dynamic aspect of fluid transport in the nanotubes will be discussed.

Due to the highly size and structure selectivity of nanomaterials, their physical properties can be quite diverse, depending on their atomic-scale structure, size, and chemistry. To maintain and utilize the basic and technological advantages offered by the size specificity and selectivity of the nanomaterials, there are three key challenges that we need to overcome for the future technological applications of nanomaterials. First, synthesis of size, morphology, and structurally controlled nanomaterials, which are likely to have the precisely designed and controlled properties. Second, novel techniques for characterizing the properties of individual nanostructures and their collective properties. This is essential for understanding the characteristics of the nanostructures. Finally, integration of nanomaterials with the existing technology is the most important step for their applications, especially in nano-scale electronics and optoelectronics.

Carbon nanotubes possess various superior properties for use as field emitters, such as sharp tips with a nanometer-scale radius of curvature [1], high mechanical stiffness [2‐4], high chemical stability [5], and unique electrical properties [6, 7]. Due to the unique tip geometry of the carbon nanotubes, their field emission property is one of the most attractive application [8], which has been extensively studied using the classical technique. In this chapter, we introduce a few novel applications of TEM in characterizing the field emission properties of carbon nanotubes, with a focus on the characteristics of individual carbon nanotubes.

Electrical transport in single-walled nanotubes (SWNTs) and multiwalled nanotubes (MWNTs) is of great importance for their applications in electronics [1]. The electronic band structure of SWNTs is well known: depending on the helicity and statistically in a third of the cases, a tube has two one-dimensional subbands (channels) that intercept the Fermi level, giving rise to metallic conduction. More precisely, only armchair tubes are gapless: all others are often referred to as metallic, although small gaps that are introduced by curvature effects of the order of 10 meV for 1.4 nm diameter SWNTs effect transport at low temperatures. The gap diminishes with increasing tube diameter. Measurements of nanotube conductance mainly use two techniques. Using lithographically made gold electrodes, a carbon nanotube is laid down across two or four electrodes, and the I-V characteristic is measured [2, 3]. The other technique takes the advantage of using liquid mercury as a soft contacting electrode; a nanotube is inserted into the mercury, and the conductance is monitored as a function of the depth that the nanotube is inserted into the mercury [4]. The latter has been carried out in-situ in TEM. This chapter is intended to review the progress in applying the second technique in electrical property characterization of nanotubes. A comprehensive review about all of the existing literature and the comparison of data in electrical characterization can be found from [5].

The boron nitride (BN) nanotube (NT) [1] is a structural analog of carbon (C) NT [2], in which alternating B and N atoms substitute for C atoms in a graphitic sheet. They were first predicted to exist theoretically [3, 4] and were subsequently synthesized in 1995 [1]. The growing interest to the BN NT is primarily governed by the fact that in contrast to the metallic or semiconducting C NT [5], the BN NT was predicted to be insulating with a -5.5 eV band-gap independent of its chirality and morphology [4]. In addition, layered BN is known to be more thermally and chemically stable than graphite [6]; therefore, there have been intuitive expectations that the BN NTs may inherit these advantageous technological properties. Surprisingly, the amount of successful research work performed to date on BN NTs has been negligibly low compared to that on C NTs.

Graphitic nanoparticles that are not allowed to grow to a macroscopic size spontaneously form polyhedral structures—fullerenes [1] and nanotubes [2]. Thus, fullerenes and nanotubes are the thermodynamically favorable form of carbon if the number of atoms in the particle is not allowed to grow indefinitely (beyond, say, 0.1 micron). The driving force for the formation of such closed-cage nanostructures stems from the abundance of peripheral atoms in the graphitic lattice, which are only twofold bonded. In order to annihilate these edge atoms, pentagons are produced and inserted into the otherwise honeycomb lattice, which leads to the folding of the planar nanostructure. When 12 such pentagons occur in the nanoparticle, a closed cage nanostructure is obtained. The bending of the graphitic sheet imposes elastic strain into the nanoparticle. High temperatures or other sources of energetic excitation are needed in order to overcome the strain energy, which is more than compensated by the annihilation of the dangling bonds, once the nanoparticle is fully closed and is therefore seamless. It was hypothesized that this virtue is not limited to graphite but is common to highly anisotropic layered materials, like MoS₂. Therefore, the formation of closed polyhedra and nanotubes is believed to be a generic property of materials with anisotropic (2D) layered structures [3‐5]. In analogy to carbon fullerenes, other related structures, like multilayer polyhedra (onions) and nanotubes, could be anticipated. These new structures received the generic name inorganic fullerene-like structures (IF)

In his famous 1959 address “There’s Plenty of Room at the Bottom” [1], Richard Feynman stated: “What could we do with layered structures with just the right layers? What would the properties of materials be if we could really arrange the atoms the way we want them? They would be very interesting to investigate theoretically. I can’t see exactly what would happen, but I can hardly doubt that when we have some control of the arrangement of things on a small scale we will get an enormously greater range of possible properties that substances can have, and of different things that we can do.” Although advances in materials fabrication technology since 1959 have made it possible to manipulate the formation of matter on an atomic scale, there are relatively few methodologies for the production of discrete atomically regulated or Feynman-type solids on a large (i.e., bulk) scale. To date, some of the most successful strategies have involved either local atomically regulated deposition of materials on solid surfaces (as in, e.g., the formation of quantum dot type structures [2]) or, alternatively, manipulation of discrete molecules and atoms via atomic force or scanning tunneling microscopy (AFM or STM) [3‐9].