Columns, pipes, bearings and springs are a few common ways that engineers have made use of the geometric shape known as a 'cylinder.' The utility of this shape is apparent in architecture, plumbing and mechanical devices. Carbon nanotubes are molecular cylinders that are rapidly extending our ability to fabricate nanoscale devices by providing molecular probes, pipes, wires, bearings and springs.Their strength as structural supports comes from their sturdy molecular structure, which looks like what one would get if one could roll a two dimensional sheet of graphite into a three dimensional cylinder.
The limit to how long they can be is unknown, thus aerospace scientists are seriously considering using them as cables extending into space, an idea that is not possible with traditional ropes since they would break under their own weight. Furthermore, carbon nanotubes can easily be cut into sections as small as a few nanometers . One of the first important applications of carbon nanotubes has been in the fabrication of sharp, strong and functionalized AFM probe tips.
The hollow nature of nanotubes allow them to function as pipes for transporting and molding atoms and molecules. Furthermore, the tubes come in insulating, semiconducting and conducting form, meaning that they can also be used as molecular wires and circuits . Whats more, capillary induced filling of the nanotubes with other materials further extends the diversity of nanowires that can be fabricated.
The electronic properties of carbon nanotubes are directly related to their shape, making them an important Nano-Electromechanical System (NEMS). For example, the feasibility of a nanotube-based random access memory device with a memory density around 100 gigabytes/cm2 and an operation frequency around 100 gigahertz has recently been developed at Harvard University.In addition to their high aspect ratio (meaning long and thin) and particle transport capabilities, carbon nanotubes can also function as durable bearings and springs. Nanotubes can be fabricated in two forms: single-wall nanotubes (SWNT) or multi-wall nanotubes (MWNT). While a SWNT consists of only a single cylinder, a MWNT consists of several (between 2 and 30) concentric tubes, each with a specific diameter. Physicists at the University of California, Berkeley have recently demonstrated that a MWNT can act as a molecular bearing when one of the inner tubes rotates, or as a molecular spring when an inner tube is pulled out, causing the MWNT to stretch in a way similar to a telescope .