CARBON - THE WONDER ELEMENT:
THE MANY FACES OF CARBON
THE MANY FACES OF CARBON
It appears as though carbon is poised to become the wonder element of The Nano Age. The coming era of molecular nanotechnology is sometimes synonymously referred to as The Diamond Age in reference to the tremendous potential of carbon as an all-purpose building material.
"Carbon has this genius of making a chemically stable, two-dimensional, one-atom-thick membrane in a three-dimensional world. And that, I believe, is going to be very important in the future of chemistry and technology in general." - Richard Smalley
Graphene is a one-atom-thick planar sheet of carbon atoms, densely packed together into a honeycomb shaped crystal lattice. Graphene looks like atomic-scale chicken wire made up of carbon atoms and their bonds.
The carbon-carbon bond length in graphene is about 0.142 nm. Graphene is the basic structural element of several carbon allotropes including graphite, carbon nanotubes and fullerenes.
Many sheets of graphene stacked together are collectively called graphite.
Carbon nanotubes, (a.k.a. buckytubes), are cylindrical molecules of carbon with novel properties that are potentially useful in a wide variety of applications including nano-electronics, optics, materials applications, etc. They exhibit extraordinary tensile strength, a unique range of electrical properties, and are efficient thermal conductors.
Most single-walled nanotubes (SWNT) are close to 1 nanometer in diameter, with a tube length that can be many millions of times longer. The structure of a SWNT can be thought of as a one-atom-thick layer of graphite, called graphene, wrapped into a seamless cylinder. The way the graphene sheet is wrapped is represented by a pair of indices.
Single-walled nanotubes are still very expensive to produce, around $1500 per gram as of 2000. Several suppliers offer as-produced arc discharge SWNTs for $100 per gram as of 2007.
Multi-walled nanotubes (MWNT) consist of multiple rolled layers (concentric tubes) of graphene. The interlayer distance in multi-walled nanotubes is close to the distance between graphene layers in graphite, approximately 3.3 Å (330 pm).
Inorganic nanotubes have also been synthesized. A nanotube is a member of the fullerene structural family, which also includes buckyballs. Whereas buckyballs are spherical in shape, a nanotube is cylindrical, with at least one end typically capped with a hemisphere of the buckyball structure. Their name is derived from their size, since the diameter of a nanotube is on the order of a few nanometers or less (approximately 50,000 times smaller than the width of a human hair), while they can be up to several centimeters in length. There are two main types of nanotubes: single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs).
A macroscopic aggregate of carbon nanotubes is called "buckypaper." Buckypaper is extremely lightweight - only a tenth the weight of steel, yet potentially 500 times stronger when sheets are stacked to form a composite. Buckypaper is one of the most thermally conductive materials known. It is also very electrically conductive, flexible like regular paper, and fire retardant.
Touted as "harder than diamonds" and "stronger than steel at a fraction of the weight," buckypaper's potential applications include use as an aerospace material, fire protection, novel television screens, heat sinks, electromagnetic interference shielding, filtration, armor and even artificial muscles. Buckypaper will find a myriad uses in manufacturing as soon as production can be ramped up and cost is decreased enough to become viable.
Fullerenes are an allotrope (physical arrangement of atoms) of carbon distinct from both graphite and diamond. They are a cylindrical, spherical or ellipsoid arrangement consisting of dozens of carbon atoms. Discovered in 1985 byRobert F. Curl, Harold W. Kroto and Richard E. Smalley, fullerenes are a molecular form of carbon distinct from graphite and diamond. "Fullerenes" were named after the architectural designer, Richard Buckminster Fuller, for their resemblance to the geodesic dome. Fullerenes can be spherical, ellipsoid, or cylindrical arrangements of dozens to hundreds of carbon atoms. A spherical fullerene made of exactly sixty carbon atoms is called buckminsterfullerene (C60) looks similar to a soccer ball, and is often referred to as a "buckyball." Cylindrical fullerenes are known as "buckytubes" or most commonly "nanotubes." One hemisphere of a Bucky ball can be used to cap the ends of a nanotube.
Buckyballs could be used as diagnostic tools, or drug delivery vessels. They also have potential as superconductors, catalysts, Scanning Tunneling Microscope tips, or even as a nano ball-bearing lubricant.
The Diamond Age - Diamond will be one of the simplest materials to construct out of inexpensive and plentiful carbon using molecular nanotechnology (MNT.)
Diamond is a remarkably durable material. Known to the ancient Greeks as adamas ("proper", "unalterable", "unbreakable") and scoring a "perfect 10" on the Mohs mineral hardness scale, diamond also has the highest tensile strength of any material, the second highest compressive strength (after osmium - the densest element), the highest melting point (3820 Kelvin), greatest clarity over a wide range of wavelengths, the highest lattice density (carbon atoms in diamond are more closely packed together than the atoms of any other substance), and conducts heat fully five times better than silver.
Diamonds' extreme strength is due to the carbon atoms it is composed of. Each atom of carbon is joined to four others with strong covalent bonds. No matter its size, a diamond crystal can be considered to be a single carbon molecule. Surprisingly, at surface temperature and pressure, graphite is more stable than diamond, and all diamonds near the earth's surface are constantly transforming into graphite - albeit extremely slowly. Graphite is forever!
Aggregated diamond nanorods, or ADNRs (also called a hyperdiamond), are a nanocrystalline form of diamond believed to be the hardest and least compressible material known.
Lonsdaleite, also called Hexagonal diamond in reference to its crystal structure, is an allotrope of carbon with a hexagonal lattice. In nature, it forms from graphite present in meteorites upon their impact with Earth. The great heat and stress of the impact transforms the graphite into 'diamond', but retains graphite's hexagonal crystal lattice. Lonsdaleite was first identified from the Canyon Diablo meteorite, where it occurs as microscopic crystals associated with diamond. It was first discovered in nature in 1967. In naturally occurring lonsdaleite, impurities present weaken the material to a large extent. However, a simulated pure sample has been found to be 58% harder than diamond. Pure lonsdaleite will resist indentation pressures of 152 GPa, whereas diamond would break at 97 GPa.
d) C60 (Buckminsterfullerene)
e) C540 (see Fullerene)
f) C70 (see Fullerene)
g) Amorphous carbon
h) single-walled carbon nanotube