Carbon nanotubes
Carbon nanotubes can be prepared by arc-evaporation, pyrolysis method, laser ablation of graphite, electrochemical and templating (using porous alumina membrane) techniques. The arc evaporation and laser ablation of graphite employ processes which are far from equilibrium where as the chemical routes are generally closer to equilibrium conditions.
Inorganic nanotubes
Several methods exist for the synthesis of inorganic nanotubes. Each method will result in different atomic structure, chirality, diameter, length of the tube and purity of the product. Nanotubes of metal chalcogenides and boron nitride are also prepared by employing techniques similar to those of CNTs, although there is an inherent difference in that the nanotubes of inorganic materials such as MoS2 or BN would require reactions involving the component elements or compounds containing the elements. Decomposition of precursor compounds containing the elements is another possible route.
Two of the best understood inorganic nanotubes, Boron Nitride and Tungsten Disulfide, are described in more detail below.
Boron Nitride nanotubes
Boron Nitride (BN) nanotubes can by synthesized by several methods, depending on the desired properties and yields. For instance, tubes as long as 40 microns can be fabricated by continuous laser heating. Ball milling and thermal annealing, a discharge using a tungsten electrode hollowed and filled with boron nitride powder or a substitution reaction with carbon nanotubes are a few other methods.
BN nanotubes, however, should all be insulating with a band gap of about 5 volts. Unlike carbon nanotubes, the conductive properties of boron nitride nanotubes are more or less independent of chirality, diameter and even the number of walls in the tube. The mechanical properties are related to the chiral vector. Like the semiconductors, BN Nanotubes could be doped in order to tune their conductivity for use in molecular electronic devices. It is inert in nature and can endure more stress than carbon nanotubes. Such inert nanotubes may be a critical component for molecular devices.
In general inorganic nanotubes are heavier than carbon nanotubes, weak under tensile stress, strong under compression, with promissing use in impact resistant applications such as bullet proof vests. They are isoelectronic with benzene and can form sheets, fullerene analogs and nanotube analogs.
WS2 nanotubes
WS2 fullerenes which are structurally similar to nanotubes, but spherical in shape have important lubrication and coating applications. By impregnating fullerene-like WS2 and MoS2 to the amount of 5% into real machine bearings, their lifetime can be extended by a factor of 10-100. Layered 2-D sheets of WS2 are extremely inert and durable when sheets are rolled into tubes.
Tip destruction is a major problem limiting the cost, reproducibility and time required to carry out scanning probe measurements. A tip with a radius of only a few nanometers requires only a very small force to make it dull. A standard AFM tip will rarely remain very sharp for more than a few hours under normal operation conditions. WS2 nanotube synthesis has helped in the development of novel SPM tips to have extremely durable and sharp scanning probe tips. Their high aspect ratio makes them able to accurately probe deep nanoscale features. Furthermore, their rigid and inert nature allows forces to be transferred from the tip to the image without incurring the dreaded tip damage without much wear.