Carbon Nanotubes (Part I)
Had anyone forgotten about them?
What are carbon nanotubes?
Carbon nanotubes (CNTs) are cylindrical molecules that consist of rolled-up sheets of single-layer carbon atoms (graphene). They can be single-walled (SWCNT) with a diameter of less than 1 nanometer (nm) or multi-walled (MWCNT), consisting of several concentrically interlinked nanotubes, with diameters reaching more than 100 nm. Their length can reach several micrometers or even millimeters.
Like their building block graphene, CNTs are chemically bonded with sp2 bonds, an extremely strong form of molecular interaction. This feature combined with carbon nanotubes’ natural inclination to rope together via van der Waals forces, provide the opportunity to develop ultra-high strength, low-weight materials that possess highly conductive electrical and thermal properties. This makes them highly attractive for numerous applications.
Who discovered carbon nanotubes?
Thousands of papers are being published every year on CNTs or related areas and most of these papers give credit for the discovery of CNTs to Sumio Iijima who, in 1991, published a ground-breaking paper in Nature ("Helical microtubules of graphitic carbon") reporting the discovery of multi-walled carbon nanotubes. I have had the opportunity to meet Prof. Iijima at several conferences and his talks are extremely interesting.
On taking a cursory look at the scientific literature, one might get the impression that Iijima is the de facto discoverer of carbon nanotubes. Of course, there is no doubt that he has made two seminal contributions to the field, however a careful analysis of the literature suggests that certainly he is not the first one who has reported the existence of CNTs.
An editorial in the journal Carbon ("Who should be given the credit for the discovery of carbon nanotubes?") tried to clear the air by describing the chronological events that led to the discovery of carbon nanotubes. By delving deeper into the history of carbon nanotubes, it becomes quite apparent that the origin of CNTs could be even pre-historic in nature.
Pre-historic Origin of CNTs
Recently, Ponomarchuk et al from Russia have reported the presence micro and nano carbon tubes in igneous rocks formed about 250 million years ago. They suggested the possibility of formation of carbon nanotubes during the magmatic processes. It is presumed that the migration of hydrocarbon fluids through the residual melt of the rock groundmass created gas-saturated areas (mostly CH4, CO2, CO) in which condensation and decomposition of hydrocarbon in presence of metal elements resulted in the formation of micro and sub-micron carbon tubes.
A wide range of elements (Fe, K, Ti, V, Cr, Mn, Ni, Pt, Zn, Ga, Ge, Br, Sr, Zr, and Pb) have been actually identified in the natural globules of graphite containing micro-and nanoscale structures of igneous rocks by X-ray fluorescence spectroscopy studies. Some of these elements were believed to act as natural catalysts in the formation of carbon nanostructures by process similar to chemical vapor phase deposition. The presence of carbon tubes was confirmed with the aid of electron microscopy.
Another most compelling evidence of pre-historic naturally occurring carbon nanotubes (MWCNTs) is based on the TEM studies carried out by Esquivel and Murr that analyzed 10,000-year-old Greenland ice core samples and it was suggested that probably they could have been formed during combustion of natural gas/methane during natural processes.
SEM image of aligned carbon nanotubes approximately 20 um long
Schematic representation of a single-walled nanotube (SWNT) and a multiwalled nanotube (MWNT)
Properties
Electrical Conductivity
There has been considerable practical interest in the conductivity of CNTs. CNTs with particular combinations of N and M (structural parameters indicating how much the nanotube is twisted) can be highly conducting, and hence can be said to be metallic. Their conductivity has been shown to be a function of their chirality (degree of twist), as well as their diameter. CNTs can be either metallic or semi-conducting in their electrical behavior.
Conductivity in MWNTs is quite complex. Some types of “armchair”-structured CNTs appear to conduct better than other metallic CNTs. Furthermore, interwall reactions within MWNTs have been found to redistribute the current over individual tubes non-uniformly. However, there is no change in current across different parts of metallic single-walled CNTs. However, the behavior of ropes of semi-conducting SWNTs is different, in that the transport current changes abruptly at various positions on the CNTs.
The conductivity and resistivity of ropes of SWNTs has been measured by placing electrodes at different parts of the CNTs. The resistivity of the SWNT ropes was in the order of 10–4 ohm-cm at 27°C. This means that SWNT ropes are the most conductive carbon fibers known. The current density that was possible to achieve was 107 A/cm2, however in theory the SWNT ropes should be able to sustain much higher stable current densities, as high as 1013 A/cm2.
It has been reported that individual SWNTs contain defects. Fortuitously, these defects allow the SWNTs to act as transistors. Likewise, joining CNTs together may form transistor-like devices. A nanotube with a natural junction (where a straight metallic section is joined to a chiral semiconducting section) behaves as a rectifying diode – that is, a half-transistor in a single molecule. It has also recently been reported that SWNTs can route electrical signals at high speeds (up to 10 GHz) when used as interconnects on semi-conducting devices
Strength And Elasticity
The carbon atoms of a single (graphene) sheet of graphite form a planar honeycomb lattice, in which each atom is connected via a strong chemical bond to three neighboring atoms. Because of these strong bonds, the basal-plane elastic modulus of graphite is one of the largest of any known material.
For this reason, CNTs are expected to be the ultimate high-strength fibers. SWNTs are stiffer than steel, and are very resistant to damage from physical forces. Pressing on the tip of a nanotube will cause it to bend, but without damage to the tip. When the force is removed, the tip returns to its original state. This property makes CNTs very useful as probe tips for very high-resolution scanning probe microscopy.
Thermal Conductivity And Expansion
Research from the University of Pennsylvania indicates that CNTs may be the best heat-conducting material man has ever known. Ultra-small SWNTs have even been shown to exhibit superconductivity below 20oK. Research suggests that these exotic strands, already heralded for their unparalleled strength and unique ability to adopt the electrical properties of either semiconductors or perfect metals, may someday also find applications as miniature heat conduits in a host of devices and materials.
The strong in-plane graphitic C-C bonds make them exceptionally strong and stiff against axial strains. The almost zero in-plane thermal expansion but large inter-plane expansion of SWNTs implies strong in-plane coupling and high flexibility against nonaxial strains. Many applications of CNTs, such as in nanoscale molecular electronics, sensing and actuating devices, or as reinforcing additive fibers in functional composite materials, have been proposed.
Electron Emission
Field emission results from the tunneling of electrons from a metal tip into vacuum, under application of a strong electric field. The small diameter and high aspect ratio of CNTs is very favorable for field emission. Even for moderate voltages, a strong electric field develops at the free end of supported CNTs because of their sharpness.
Title image: "Charge emission from the multiwalled carbon nanotube"
Image Description: Electrostatic Force Microscopy (EFM) image of multiwalled carbon nanotube after the electron injection experiment. The nanotube is 18 nm diameter. A bright halo is visible at the apex corresponding to charges emitted from the nanotube cap, while the discharged nanotube appears dark in the EFM image. The dark feature correspond to the uncharged nanotube. Courtesy: Dr Mariusz Zdrojek. Warsaw University of Technology (Poland)
This was observed by de Heer and co-workers at EPFL in 1995. He also immediately realized that these field emitters must be superior to conventional electron sources and might find their way into all kind of applications, most importantly flat-panel displays. It is remarkable that, after only five years, Samsung made a very bright color display, which was never marketed using this technology.
High Aspect Ratio
CNTs represent a very small, high aspect ratio conductive additive for plastics of all types. Their high aspect ratio means that a lower loading (concentration) of CNTs is needed compared to other conductive additives to achieve the same electrical conductivity. This low loading preserves more of the polymer resins’ toughness, especially at low temperatures, as well as maintaining other key performance properties of the matrix resin. CNTs have proven to be an excellent additive to impart electrical conductivity in plastics. Their high aspect ratio (about 1000:1) imparts electrical conductivity at lower loadings, compared to conventional additive materials such as carbon black, chopped carbon fiber, or stainless steel fiber.
In the next issue of this newsletter we will discover the applications of carbon nanotubes.
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