Fast growing doped nanotubes
US researchers have doped and grown carbon nanotubes in a controlled way by regulating the amount of active nitrogen-doping species. This finding makes possible to tune the electronic character of these nanostructures. The researchers have chosen the best precursor for an efficient yield and found that hydrogen cyanide (HCN) whose structure is similar to that of C2H2, is the active precursor responsible for doping the nanotubes with nitrogen. The team was able to measure dopant concentrations down to as low as 10–5 atomic percent nitrogen in the nanotube lattice which allowed seeing potential dopants at the levels employed when making practical doped devices.
Super growth method
The researchers made aligned single-walled carbon nanotubes using an ultra thin catalytic layer of 0.5 nm of iron using a "super growth" method. Here, the nanotubes usually self-assemble into vertically oriented structures upwards faster from the catalyst layer, which itself remains on the base of the growth substrate due to a small amount of water vapour present during the process.
The idea of conventional super growth was combined with a precursor that decomposed into an active molecule with an N-C triple bond, and as the carbon is incorporated into the lattice, the nitrogen gets scooped up into the growing nanotube as well. Nitrogen going into the tubes was controlled by simply regulating the amount of the precursor.
Applications
The N-doped nanotubes might be used to build more conductive structures for energy storage devices such as super capacitors or highly conducting lightweight wires of nanotubes, energy-harvesting or conductive armour applications and develop 1D nano templates using doping as a 'knob' for controlled functionalization of nanotubes. Such components will have wide applications in space science.
High conductivity
The N-doped nanotubes are more conducting than their undoped counterparts because the dopant shifts the Fermi level in these semiconductor materials into the conduction band. This property could be exploited to make tubes that have varying Fermi levels from one end to the other by modulating the amount of N along their lengths. This means that the nanotubes would then have different conductivities along their lengths. Such a concept is at the heart of bottom-up nanomaterials engineering.
High yield
A research team at the Chinese Academy of Sciences in Beijing has produced high yields of semi conducting CNTs by a method, where the CNTs were doped with boron and nitrogen. The doping introduced a semiconductor band gap, so that the top electrons in the nanotube had slightly lower energy than that required for conducting electricity. When boron and nitrogen were added together, they replaced neighboring pairs of carbon atoms in the lattice, producing samples in which over 97% of the nanotubes were semi conducting.
Electrodes
Researchers from the University of Dayton have found that aligned nitrogen-containing carbon nanotubes can act as an efficient metal-free electrode in a fuel cell. They can successfully replace existing platinum-based catalysts, having a much better electrocatalytic activity and long-term operation stability. CO impurities in the hydrogen do not affect electrodes, the crossover effect is reduced to a minimum and the cost of fuel cells is briought down.
Fluorescing nanotubes
Using a chemical vapor-deposition technique, a research team from Pennsylvania State University in University Park and the Center for Applied Energy Research in Lexington, Ky., has fabricated ruthenium-doped multiwall carbon nanotubes that photoluminesce at 515 nm.
The researchers believe that the method may be applied to other transition-metal and rare-earth elements to produce different frequencies of luminescence, potentially leading to display applications with pixel sizes down to 10 nm.
Field-effect transistors
The doped nanotubes might be used to build field-effect transistors with excellent switching ability, producing an ‘ON’ current of around a million times higher than the ‘OFF’ current. This could open the way to complex integrated circuits of assorted nanotube devices.
Hydrogen storage
By making doped carbon nanotubes with transition metals and alloys a weak covalent bond similar to cases of dihydrogen bond that is not restricted to pure physisorption or chemisorption bond can be produced. It is possible to enhance and tune the hydrogen storage capabilities of the nanotubes due to the introduction of transition metals and hydrogen bonding clusters into the nanotubes.
Doped nanotube cables
Iodine-doped, double-walled nanotube cables have very low electrical resistivity due to the low density and high specific conductivity. Such doped nanotube cables can replace metal wires in a household circuit and find a range of applications, from low dimensional interconnects to transmission lines.