What are Single-Walled Carbon Nanotubes (SWCNT)?

Single-walled carbon nanotubes (SWCNTs) are sheets of graphene that have been rolled up to form a long hollow tube, with wall thickness of a single atom. Their one-dimensional structure gives them extraordinary mechanical, electrical and thermal properties. These properties have been exploited for applications in flexible electronics, sensors and energy storage.

Nomenclature:

• Single-walled carbon nanotubes
• Single-wall carbon nanotubes
• SWCNT
• SWNT

Single-Walled Carbon Nanotube Structure and Properties

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Much like graphene, SWCNTs have properties that differ considerably to those of bulk carbon (e.g. graphite). SWCNT are described as one graphene layer rolled up as a hollow tube with a typical diameter of < 2 nm and lengths > 5 μm. The aspect ratio (ratio of length/diameter) is well into the thousands.

It is this one-dimensional structure of hexagonal carbon lattice, as shown in the video, that give SWCNTs their unique mechanical, electronic and thermal properties.

Mechanical Properties

The mechanical properties of single-walled carbon nanotubes vary significantly depending upon the axis you are measuring with nanotubes having extremely high Youngs Moduli (up to 1TPa) and tensile strength (up to 100 GPa) along the longitudinal axis. Along the radial axis, these values are a few orders of magnitude lower.

Electrical Properties

The electrical properties of single-walled carbon nanotubes are dependent upon the circumference and the orientation of the lattice. The lattice orientation is given by two parameters (n, m) known as the chiral index. The image shows how the n and m orientations relate to the longitudinal axis of the nanotube and the rotational axis. There are typically three types of nanotubes that can form:

• Armchair (where n = m)
• Zig-zag (n=x, m=0)
• Chiral (n=x, m=y)

Metallic and Semiconducting properties

Single-walled carbon nanotubes can exhibit either metallic properties or semiconducting properties, depending upon the orientation of the lattice:

Lattice Orientation	Property	Comment
Armchair	Metallic	No gap between the valance band and conduction band
Chiral	Either metallic or semiconducting	Depends upon the difference between the n and m units.
Zig-zag	Semiconducting	Tiny gap between the valence band and conduction band

If the difference between n and m equals a multiple of three then the SWCNT is metallic, if not then it is semiconducting. These electronic properties result from the energy band dispersion of graphene, specifically at the K point where these energy band meets (no bandgap). For a cylindrical SWCNT, only certain wave vectors are allowed, and they exist as lines in the reciprocal space of graphene. If these lines intersect the K points of graphene then the nanotube is metallic as they inherit the metallic properties of graphene. If the lines do not interect the K points then the SWCNT semiconducting.

In addition to this ability to exhibit both metallic and semiconducting electronic structures carbon nanotubes offer exceptional charge carrier mobilities. This is due to the combination of the delocalization of electrons across the lattice and the small dimensions in the radial axis constraining movement of charge carriers along the longitudinal axis of the tubes.

Optical Properties

The optical properties of SWCNT are also dependent on their chirality. The way they interact with electromagnetic radiation also depends on whether individual or bundles of SWCNTs are investigated due to interactions between the nanotubes.

	Metallic SWCNT	Semiconducting SWCNT
Raman	G-Band broadening asymmetric peak	G-Band sharp, symmetric peak
Absorption	Different energies May have plasmonic absorption in the far-infrared	NIR, visible, UV
Photoluminescence	Not usually observed	Emission in NIR

Raman Spectroscopy

Dependence on chirality means that Raman spectroscopy can be used to determine whether SWCNTs are metallic or semiconducting. SWCNTs exhibit strong Raman signals due to resonant effects. Important Raman features include:

• Radial Breathing Mode (RBM): A peak in the low-frequency range (100–400 cm⁻¹), where the entire nanotube contracts and expands radially. The RBM frequency is inversely proportional to the nanotube diameter, providing a way to estimate its size.
• G-band: A high-frequency mode (around 1500–1600 cm⁻¹) related to in-plane vibrations of carbon atoms. The G-band's shape and intensity can differentiate between metallic and semiconducting SWCNTs. Peaks broaden and become asymmetric for metallic SWCNTs attributed to the presence of free electrons in SWCNTs
• D-band: A defect-related mode around 1300 cm⁻¹, which indicates the presence of structural defects in the nanotube.

Absorption Spectroscopy

The absorption spectrum for a given SWCNT is also dependent on chirality. It is related to the energy difference between the valence and conduction bands. SWCNTs exhibit distinct absorption peaks corresponding to electronic transitions between different energy levels, called van Hove singularities, in their density of states.

• Semiconducting SWCNTs: Absorption peaks in the near-infrared (NIR), visible, and ultraviolet (UV) regions due to transitions between their first and second van Hove singularities.
• Metallic SWCNTs: Absorption peaks but generally occur at different energies compared to semiconducting ones. Metallic SWCNTs may also exhibit plasmonic absorption in the far-infrared.

Photoluminescence Spectroscopy

Photoluminescence (PL) in single-walled carbon nanotubes (SWCNTs) is an important optical property, especially for semiconducting SWCNTs. Photoluminescence occurs when a material absorbs light (photons), excites electrons to a higher energy state, and then emits light as the electrons return to their lower energy state. This process is highly dependent on the electronic structure of the material.

• Semiconducting SWCNTs: Emit light when excited, making them useful in optical sensing and bioimaging applications. The emitted light corresponds to the energy difference between the first excited state and the ground state (E11 transition). This emission occurs in the near-infrared (NIR) range, typically between 900 nm and 1600 nm, depending on the SWCNT’s chirality and diameter.
• Metallic SWCNTs: Do not show significant photoluminescence because their electronic structure allows rapid non-radiative recombination of excited electrons.

Thermal properties

The thermal properties of SWCNTs exhibit extreme anisotropy. Along the length of the tube, thermal conductivity can be up to 9 times higher than materials such as copper. However - across the radial axis, the thermal conductivity can be 250 times lower than that of copper. Much like its electrical and mechanical properties, SWNT's thermal properties can be severely affected by the presence of defects along the nanotube length. The presence of these defects lead to phonon scattering. When these defects interact with low frequency phonons, scattering can occur - reducing the thermal conductivity.

Single-Walled Carbon Nanotubes Applications

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SWCNTs are used in composite materials as a method of improving mechanical strength and electrical properties. SWCNTs can be assembled with a variety of chirality and architectures, from an individual SWCNT to vertically aligned mats and fibres. As a result, SWCNT have been investigated in a variety of applications including:

• Flexible electronics
• Transparent Conductors
• Thermal Interfaces
• Advanced/Reinforced Composites
• Energy Storage/ Li-ion batteries
• Membrane
• Sensors: optical, biological, chemical
• Electronics: transistors, interconnect, memory
• Quantum Wires

One of the current limiting factors in improving the range of applications of carbon nanotubes is the ordering of nanotube structure. Current commercial applications use disordered bundles of nanotubes, and these bundles have a significantly lower performance than that of individual nanotubes. Potential future uses for carbon nanotubes could be seen in areas such as:

• Carbon nanotube yarns for ultra-strong fabrics
• Thermal management systems
• Advanced drug delivery systems

Single-Walled Carbon Nanotubes Preparation

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The first observations of SWCNTs were not until 1976 when M. Endo synthesized a series of hollow carbon tubes via chemical vapour-growth. Wider interest in these low-dimensional materials did not occur until the early 90’s, when two articles were independently published by:

• Iijima on the fabrication of multi-walled carbon nanotubes via arc discharge
• W. Mintire, B. I. Dunlap, and C. T. White on the predicted properties of SWNTs

The combination of a simple method for producing SWNTs and the potentially extraordinary properties they exhibit kick-started the growth of a wider research community into carbon nanotubes. Production of SWCNT is now carried out through a range of techniques including:

• Catalytic chemical vapor deposition (CVD) growth
• Vapor phase epitaxy (VPE)

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