Carbon & Carbon Nanotubes

Carbon, this treasure!

There is a Greek popular expression about high expectations that are subsequently refuted. The prevailing English translation is “the treasure was coal”; however, in Greek the word used is carbons. The meaning is actually quite straightforward: one expects to find something of extremely high value (a treasure), but instead finds something cheap (carbon). However, viewing Carbon as something without value would be quite unjustifiable.

Indeed, let’s thing what we know about Carbon even from school (C):

  • Carbon is the chemical basis of all known life forms.
  • Carbon was the initial force for the industrial revolution, providing the required energy for the newly-founded factories.
  • Today, a large portion of the chemical industry (e.g. plastics, petrochemical industry etc) is based on organic substances, i.e. on Carbon chemistry.
  • One of Carbon’s more classic allotropes, diamond, is indeed very expensive.

Thus, it is maybe more just to say that “Carbon is a treasure”!

Carbon Allotropes

Carbon is a chemically very active element. Therefore, it does not remain free (in its atomic form) for a long period of time but, instead, it is stable in different multi-atom conformations, which are known as carbon allotropes. 

The three commonly found in nature carbon allotropes are the:

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  • amorphous carbon (Figure 1g): it is the allotropic form of coal.
  • graphite (Figure 1b): it is the common form of carbon organization at usual environmental conditions and the form that is present in pencils.
  • diamond (Figure 1a): This allotrope is formed due to the high pressure. In nature, the required conditions are found in subsoil.

Another carbon allotrope that can be found in nature is lonsdaleite or hexagonal diamond (Figure 1c), which is formed by quick reorganization of graphite found in meteorites, due to the high temperature rise during Earth’s atmosphere entrance.

After 1985 and entering in the Nanotechnology era, more carbon allotropes with characteristic dimension of 1 to several nanometers (1nm: 1 billionth of meter) were found. The more significant of these allotropes are the:

  • fullerenes (Figure 1d: C60, Figure 1e: C540, Figure 1f: C70): spherical or elliptical forms with many carbon atoms; their diameter is some nm.
  • graphene: a single graphite sheet with less than 1 nm thickness.
  • carbon nanotubes: on (Figure 1h) or several graphene sheets wrapped as a tube with diameters ranging from 1 to hundreds of nm.

Carbon Nanotubes

Carbon Nanotubes were found by Iijima in 1991. As already mentioned, one can imagine a carbon nanotube as a single or several graphene sheets wrapped together in a specific way (known as chirality) to form a tube. If a single sheet is wrapped then the nanotube will have a single-sheet wall thus the name Single-Wall Carbon Nanotube. (Figure 1h). In the opposite case, a Multi-Wall Carbon Nanotube will be formed.

To image this, one could attempt to wrap a single paper sheet (which will play the role of the graphene sheet). Although we shall always obtain a tube, the geometry is fundamentally different depending on the direction of wrapping. However, this is not merely a geometric game; chirality actually determines the physical properties of the nanotube (e.g. if the nanotube will be a semiconductor or present metallic behavior). Now, imagine that you are wrapping multiple paper sheets. Probably, this will result in multiple directions of wrapping for the different sheets. Similarly, in multi-wall carbon nanotubes physical multiple chiralities may be present to the different graphene sheets that form the nanotubes wall, resulting in a much more complex physical reality.

Carbon Nanotube Physical Properties

As stated, single-wall carbon nanotubes’ physical properties depended on their chirality. Moreover, they are also depended on the direction of measurement (in contrast with usual materials which are homogeneous). For instance, materials comprising many parallel to each other single-wall carbon nanotubes are stiff, with high tensile strength and thermal conductivity, and present semiconducting or metallic electrical properties (depending on chirality) in the direction of the nanotubes’ axis. In contrast, if examined on the radial direction of the nanotubes, the same material presents no thermal or electrical conductivity and is mechanically less resilient.

Optical absorption spectrum from dispersed single-wall carbon nanotubes
Materialscientist at English Wikipedia [CC BY-SA 3.0], via Wikimedia Commons

In the current project, crucial role play the opticla properties of carbon nanotubes and especially their ability to absorb electromagnetic radiation of a wide spectrum ranging from infrared (IR) to ultraviolet (UV) (Figure 2). Except for this, carbon nanotubes can also be a light source after excitation by electromagnetic radiation (photoluminescence) or application of an electric voltage (electroluminescence).

Carbon Nanotubes’ Applications

At present, carbon nanotubes are used on commercial applications as additive to other plastic or composite material for increasing durability or mechanical hardness. From the 38 commercial products in the Consumer Products Inventory of the Nanotechnology Project only 2 (4.8%) has a different application field:

However, a wide research is active both on scientific as on corporate level, with the goal of exploiting the promising properties of materials based on carbon nanotubes. Applications actively pursued are batteries (replacing the graphite electrode with carbon nanotubes), water purification, aerospace and defence, while Samsung is exploring the application of carbon nanotubes in the next-generation of environmentally-friendly computer and TV monitors.

References

  1. Carbon Wikipedia page
  2. Carbon Nanotubes Wikipedia page
  3. Carbon Nanotubes: A review Elsevier website
  4. Carbon for Electronics Webpage about Carbon Nanotubes’ properties and applications
  5. Carbon Nanotubes Science Daily
  6. Carbon Nanotube Applications and Uses
  7. Patents and IPs for Carbon Nanotubes