Introduction
Oriented carbon nanotube arrays (OCNTAs) are a unique microstructure consisting of carbon nanotubes (CNTs) oriented along their longitudinal axes normal to a substrate surface. These OCNTAs effectively preserve and often accentuate the unique anisotropic properties of individual carbon nanotubes, which possess a morphology that may be precisely controlled.
Fig 1 SEM images of the typical arrays of CNTs
There are a handful of experimental technologies available to align a single or an array of CNTs along a pre-determined orientation. These techniques are classified into in-situ techniques and ex-situ techniques. By in-situ techniques, the alignments are achieved during the CNT growth processes, while by ex-situ techniques, CNTs are originally grown in random orientations and alignment is achieved afterwards such as during the device integration process.
OCNTAs have extraordinary properties over individual CNTs and are consequently widely useful in a range of current and potential device applications, including field emission, flat screens, ultracapacitors, nanoelectrode arrays, fuel cells, solar cells, transistors, and biological sensors, due to the ordering of their aligned structures.
Applications
Field-emission devices: CNTs have high aspect ratios (length divided by diameter) and induce very high local electric field intensities around the tips. Field emission in solids occurs in intense electric fields and is strongly dependent on the work function of the emitting material. Typical field-enhancement factors ranging from 30,000 to 50,000 can be obtained from individual CNTs, therefore making OCNTAs one of the best electron-emitting materials.
Blackbody absorber: OCNTAs offer a unique light absorbing surface due to their extremely low index of refraction and the nanoscale surface roughness of the aligned CNTs. OCNTAs blackbody absorbers are useful as stray light absorbers to improve the resolution of sensitive spectroscopes, telescopes, microscopes, and optical sensing devices.
Thermal interface materials: OCNTAs interfaces are more thermally conductive than conventional thermal interface materials at the same temperatures because phonons propagate easily along the highly thermally conductive CNTs and thus heat is transported in one direction along the alignment of the CNTs.
Electronic devices: OCNTAs have great potential in low-cost, large-scale processing of high-performance electronic devices based on highdensity oriented CNT films with record electrical characteristics such as high conductance, low resistivity, and high career mobility.
