Hybrid Nanostructures Offer Best of Both Worlds

Carbon nanotubes (CNTs) and nanowires show great promise for future generations of semiconductors. Besides their obvious applicability at the nanoscale, CNTs have incredible mechanical strength, and are excellent conductors of electricity, offering the promise of interconnects that are many times faster than today’s leading-edge copper interconnects. Gold nanowires also show interesting optical and electrical properties. However, CNTs, for example, tend to grow in random locations with random orientations, making them difficult to apply in electronic applications. The International Technology Roadmap for Semiconductors (ITRS) notes this challenge, with a view to finding a way to make the materials more suitable for mass production.¹

An interdisciplinary group of researchers at Rensselaer Polytechnic Institute (RPI, Troy, N.Y.), led by Pulickel Ajayan, a professor of materials science and engineering at RPI, has come up with a way to take better control of the CNTs, creating hybrid structures that combine the best properties of CNTs and metal nanowires. The manufacturing method creates a more practical way of attaching individual nanotubes to metal contacts, which enables a solution for using CNTs as interconnects and devices in ICs.

“This technique allows us to bridge different pieces of the nanoelectronics puzzle, taking us a step closer to the realization of nanotube-based electronics,” said Fung Suong Ou, a graduate student in materials science and electrical engineering at RPI.

The hybrid structures, described in a recent issue of Applied Physics Letters ,² are multisegmented 1-D structures joining CNTs with either gold or copper nanowires. This brings the strengths of both materials together for practical application. The researchers have fabricated two-segmented (metal-CNT) and three-segmented (CNT-metal-CNT) hybrid structures.

To do so, first the metal nanowires are made using self-fabricated alumina templates with pore diameters of ~60 nm. Copper or gold wires are electrodeposited inside the template’s pores, with the length of the nanowire controlled by varying the deposition time. The multiwalled CNTs are then deposited on top of the metal nanowires through a chemical vapor deposition (CVD) process. The entire assembly is placed in a furnace, where a carbon-rich compound is present. When the furnace is heated to a high temperature (650°C), the carbon atoms arrange themselves along the channel wall of the template, and the CNTs grow directly on top of the metal wires. The three-segmented structures were made similarly, but used a sacrificial material to create areas both above and below the metal nanowires for the CNTs to grow in.

“It’s a really easy technique, and it could be applied to a lot of other materials,” Ou said. “The most exciting aspect is that it allows you to manipulate and control the junctions between nanotubes and nanowires over several hundred microns of length.” The alumina templates are already mass-produced for the filter industry, he noted.

The team has already created hybrid structures using both copper and gold nanowires. The Figure shows examples of arrays of each. As can be seen, the interface levels between the CNTs and the metal nanowires are not uniform, instead varying from wire to wire. In fact, the metal nanowires were of uniform height before CVD, but showed variation after CVD. With more details provided in the journal report, the explanation relates to varying melting temperatures of the metal nanowires. This difference is even more pronounced in the structures using copper, which has a lower melting point than gold.

Field-emission SEM images show the arrays of CNT-gold (left) and CNT-copper (right) hybrid structures. (Source: Rensselaer Polytechnic Institute)

Nonetheless, the technique offers a well-adhered junction between the metal and CNT. Also, importantly, the metal-contacted CNTs remain at the nanoscale, which is critical for nanoscale interconnects. The researchers are also working on combining the CNTs with a semiconductor material, making them suitable for use in diodes.

Along with Ajayan and Ou, researchers included Robert Vajtai of the Rensselaer Nanotechnology Center, Derek Benicewicz of RPI’s Department of Chemistry and Chemical Biology, and Lijie Ci and M.M. Shaijumon of RPI’s Department of Materials Science and Engineering.