Now, researchers at the Jet Propulsion Laboratory (JPL), California Institute of Technology, US, have combined top-down nanofabrication approaches with bottom-up tube synthesis techniques to form controlled structures comprising of just single tubes. In their paper, published recently in Nanotechnology, low-cost, wafer-scale approaches were used to form single, aligned tubes that were centred precisely and placed within a few hundred nanometres of high-aspect ratio 3D nano-electrodes.

In the past, e-beam lithography has typically been used to lithographically define catalyst islands below a few hundred nanometres in diameter. These suitably sized catalyst sites then go on to serve as a template for nucleating single tubes. However, e-beam lithography is slow and expensive, and as a result, limits the transition of nanoscale devices from the laboratory into commercial production. Instead, the JPL team has implemented deep-UV chemically amplified resists with step-and-repeat optical lithography, as a low-cost, top-down approach to define deep-submicron catalyst features. Such catalyst sites not only nucleated single tubes, but these tubes grew with unprecedented alignment accuracy within pre-existing nano-electrodes.

Although CNTs have exceptional material properties, it is still difficult to control their characteristics. This lack of control over properties is not uncommon with other bottom-up processes, where single atoms and molecules have to arrange themselves into structures that extend into the micro-scale and beyond. For example, conventional CNT growth techniques such as thermal chemical vapour deposition (CVD), generate randomly oriented tubes that form a "mat", making them undesirable for some applications. The JPL group employed plasma-enhanced (PE) CVD, where the tubes align to the electric field within the plasma as they emerge from the narrow gap between the pre-defined electrodes. The materials used for the electrodes were also compatible with the corrosive and high-temperature CNT growth environment. Such top-down and bottom-up wafer-scale approaches developed here should accelerate the integration of PECVD-grown nanotubes for a variety of applications in 3D-electronics.