Top1. Overview And Chapter Objectives
The interest in carbon nanotubes (CNTs) and their transport models is motivated by their many interesting electronic properties. The search for new high-speed devices has moved CNTs into the focus of current research. In fact, nanotubes have become at the heart of nanoelectronic devices. In this chapter we study the transport of charge carriers (electrons and holes) in nanotube structures. Nanotubes and nanowires with dimensions on the nanometer length scale cannot be treated as classical conductors because their dimensions are much smaller than the mean free path length (between successive collisions of electrons). The length of these structures is also large for the full quantum mechanical treatment as they consist of thousands of atoms. For these reasons, nanotubes are sometimes called quasi-one-dimensional (Q1D) structures or quantum wires.
As a starting point, we present the notion of carbon nanotubes, which are rolled sheets of graphene. We know that graphene is a two-dimensional crystalline form of carbon: a single layer of graphite carbon atoms arranged in hexagons. Graphene has unusual electronic properties, which arise from the fact that the carbon atom has four electrons, three of which are tied up in bonding with its neighbors (forming sp2 ‘σ-bonds’). But the fourth electrons are in orbitals (pZ -orbitals) extending vertically above and below the plane, and the hybridization of these electrons mix together forming delocalized electron states (‘π-bonds’). These states are responsible for the electrical conductivity of graphene.
In the following sections, we present the basic properties of nanotubes and nanowires, such as silicon nanowires (SiNW) and carbon nanotubes (CNT’s) and describe the physical transport mechanisms of charge carriers along them, with ample examples of real devices.
Figure 1.
Schematic of the transport of a quasi-free electrons in a graphite sheet
Upon completion of this chapter, students and readers will be able to
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Differentiate between nanotubes and nanowires.
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Understand the notion of carbon nanotubes (CNT’s) and define their main types and basic properties.
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Explain the concepts of a quantum wires and their transport mechanisms.
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Describe the nanotubes-based devices, their properties and applications.
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Calculate the I-V characteristics of carbon-nanotube FET transistor.
Top2. Nanotubes And Nanowires
Nanotubes are hollow one-dimensional form of carbon or other materials with nanometer diameter. Nanowires are solid materials in the form of wire with diameter smaller than 100 nm. Their name is derived from their size, since the diameter of a nanowire or nanotube is on the order of a few nanometers. Nanowires and nanotubes are the most confining electrical conductors. Nanotubes could be either single walled (SWNT) or multi-walled (MWNT) consisting of nested tubes with outer diameters ranging from 5 to 100 nm. A large percentage of researchers attribute the discovery of hollow, nanometer-size carbon tubes (CNT) to the Japanese Sumio Iijima of NEC in 1991. CNT’s can be formed from graphene sheets which are rolled up to form tubes.
Currently, nanotubes are synthesized by different techniques. CNT synthesis can be performed near the focus of a high-power laser, in between two arcing graphite electrodes, or in a hot furnace full of hydrocarbon gas.
However, there are three famous methods for the synthesis of SWNTs: pulsed laser vaporization (Laser Ablation), arc discharge growth, or chemical vapor deposition (CVD) on supported or gas phase catalysts. The basic prerequisites for the formation of SWCNTs are an active catalyst, a source of carbon, and adequate energy. Most of these methods take place in vacuum or with gases. For the matter of completeness of this Chapter, we review these techniques and their growth methods.
Figure 2.
SEM picture of grown nanotubes