Liquid Crystal Engineering of Carbon Nanofibers and Nanotubes

Much of the excitement surrounding new carbon nanomaterials can be traced to their directional properties, which arise through precise orientation of graphene layers that are the anisotropic building blocks of all sp-hybridized carbon forms. The high conductivity, strength, and stiffness of carbon nanotubes are directly related to the parallel orientation of graphene layers relative to the tube axis. Alternative crystal structures are seen in some cylindrical carbon nanoforms, including "platelet" nanofibers, whose graphene layers lie perpendicular to the fiber axis; and herringbone and cup-shaped nanofibers with tilted layer arrangements. These platelet, herringbone, and cup-shaped nanofibers are inferior to "conventional" nanotubes/fibers in mechanical strength and conductivity, but contain exposed graphene edge sites that make them attractive for complementary applications such as catalysis and Li intercalation electrochemistry. A long term goal in carbon synthesis is to develop techniques for systematic control of graphene layer arrangement in order to fabricate "designer" materials and nanomaterials with crystal structures preprogrammed for specific applications.