Hybrid Materials of Carbon Nanotubes and Nanowires
As scientists look for new materials to replace silicon for faster computers, carbon nanotubes and nanowires of various kinds have appeared to be the most promising candidates. Since these two kinds of materials offer different strengths, the question has been which one will make a better building block. Recently, Prof. Ajayan?s team at Rensselaer Polytechnic Institute has offered an alternate solution to the problem. We have demonstrated the ability to make hybrid materials by combining these two materials. Such hybrid materials suggest that perhaps nanotubes and nanowires can be used to complement each other and make better materials or even devices than either material by itself.
Moore’s law, which states that the number of transistors on a chip will double every two years, has been credited to be the main driver behind the amazing progress of computer technology. It has caused the size of silicon-based transistors to shrink over the decades while their performances have only improved. However, the size of existing transistors and their interconnects have become so small that they will soon reach their physical limits. In order to extend the evolutionary path described by Moore, engineers have started looking at other possible ingredients for their products. One class of materials that appears attractive to the scientists are nanomaterials such as carbon nanotubes and nanowires that became available in the 1990s. Both of these materials possess new and exciting physical properties that humble silicon. For example, carbon nanotubes are ballistic conductors, and therefore they are free of scattering. They have been shown to carry much more current densities than copper wires. Further, the availability of the two different types of carbon nanotubes--semiconducting or metallic--makes it possible to use them for both interconnects as well as transistors. On the other hand, metal and semiconducting nanowires are also of great interests. For example, gold nanowires possess interesting optical properties, and they are also promising for various biological applications due to their inert properties. Silicon nanowires can be synthesized with greater control in comparison to carbon nanotubes and they also allow easy incorporation into current processes since they are made of silicon. Since different strengths are offered by these two categories of materials, the question has been which of them will be a better building block for the next generation transistors.
In a paper published recently in Applied Physics Letters, we have offered a new alternative to the question by fabricating a new kind of hybrid materials consisting of carbon nanotubes and gold nanowires. Such hybrid materials will certainly make the shopping between nanotubes and nanowires easier, as one can have a single material contains the properties of both. Such hybrid materials not only possess the strengths of both materials, they will also offer new solutions to the existing problems faced by nanotubes or nanowires. For example, one of the key hurdles prohibiting the use of carbon nanotubes in interconnects is the inability to apply individual metal contacts to carbon nanotubes. All the current methods only allowed macroscopic metal contacts to be attached to bundles of carbon nanotubes, and there are no existing techniques that allow the application of nanosize contacts to carbon nanotubes. However, by using the hybrid materials of metal nanowires and carbon nanotubes, individual metal pins can be “glued” to single carbon nanotube with ease. This new approach allows engineers to assemble nanowire and nanotube like LEGO bricks and empower them to have precise control in the nanoscopic scale.
The making of the hybrid materials is based on very simple materials and techniques. In our method, a piece of ceramic filter, which contains tens of thousands of nanopores inside, is used as the starting material. These nanosize pores act as molds for the sequenced deposition of various nanomaterials. First, one side of the filter is coated with a thin layer of silver. We then immerse the alumina with silver-side down into a container fill with a solution containing ions of gold or copper. A voltage is then applied across the filter so that the ions are driven through the nanopores and metal atoms begin to deposit inside the pores to grow into metal nanowires. Once the nanowires with desire lengths are obtained, the template with the pores partially filled with nanowires is taken to a furnace where the growth of carbon nanotubes is carried out. Inside the furnace, the template is exposed to gas vapor of carbon compound and inert gas. When the furnace is heated up, carbon atoms arrange themselves along the empty part of the nanopores. Well-adhered junctions are then formed between the two materials. Since our method relies on processes that are used regularly for fabrication of microprocessors, this method can certainly be scaled up easily.
In summary, the hybrid materials will allow the making of more sophisticated nanomaterials or even devices than what is available today. Such hybrid materials also combine the strengths of the different novel materials such as carbon nanotubes and nanowires, they will offer new possibilities to current applications of nanomaterials or even new applications that we have not foreseen. Besides, the approach described introduces a precise LEGO brick like control to the nanoscopic scale, such ability will allow the bridging of different materials available in the nano tool box and brings us a step closer to the realization of nanotube or nanowire computers.
The research was performed under the guidance of Pulickel Ajayan, the Henry Burlage Professor of Materials Science and Engineering at Rensselaer Polytechnic Institute.
Fung Suong Ou et al. Appl. Phys. Lett. 89, 243122 (2006).