Chemical Bonding of Carbon Nanotubes Doug Wylie 04/20/04

2 Types of Bonding related to Nanotubes Covalent Bonding – Strong bond between individual neighbouring carbon atoms in a single tube Bonding found along the 2-D plane in a hexagonally close packed sheet of graphite Van der Waal Forces – Weak bond between the individual nanotubes themselves Bonding between separate layers of graphite (Reason why graphite is an excellent lubricant, and why talcum powder feel so smooth!)

Nanotubes are composed entirely of sp2 bonds, similar to graphite Nanotubes are composed entirely of sp2 bonds, similar to graphite. Stronger than the sp3 bonds found in diamond. This bonding structure provides them with their unique strength. Nanotubes naturally align themselves into "ropes" held together by Van Der Waal Force. Under high pressure, nanotubes can merge together, trading some sp2 bonds for sp3 bonds, giving great possibility for producing strong, unlimited-length wires through high-pressure nanotube linking.

Typical Values of a Single Walled Carbon Nanotube Equilibrium Structure Average Diameter of SWNT's 1.2-1.4 nm Distance from opposite Carbon Atoms 2.83 Å Analogous Carbon Atom Separation 2.456 Å Parallel Carbon Bond Separation 2.45 Å Carbon Bond Length 1.42 Å C - C Tight Bonding Overlap Energy ~ 2.5 eV Lattice Constant 17 Å

Comparison between Diamond and Graphite

Comparison showing Diamond and Graphite bonding along the cube body-diagonal direction ie: perpendicular to the planes of packing

Conclusions Covalent bonding is a very strong bond (can often be stronger than Ionic bonding) Weak VdW forces allow rolling up of indivitual graphite sheets into tubes. Electrons are shared between Carbon atoms so as to fill outer electron shells. Covalent bonds are highly directional (unlike Ionic) therefore only a few characteristic shapes are found. Carbon Nanotubes exhibit quite different properties related to size and width of tube but the bonding type remains the same.