Schematic illustration of aligned SWNTs electrically interconnected with chromium atoms. The magnified image shows the hexahapto bond formed between a Cr atom (red) and the benzenoid rings of two carbon nanotubes, which decreases the resistance (r) between them and leads to decreased transverse resistivity of the aligned SWNT films.Bridging the gaps between aligned single-walled carbon nanotubes (SWNTs) with metal atoms provides a new direction for electrical conductivity, say researchers [Chen et al., Materials Today (2018), https://doi.org/10.1016/j.mattod.2018.08.019].
SWNTs have exceptional electronic properties, including high conductivity along the length of the tubes, or ‘parallel’ conductivity. But in the perpendicular direction, ‘transverse’ conductivity is very low. While this anisotropy can be useful, researchers from the University of California, Riverside have found a way of bonding the surfaces of SWNTs together.
“There is no simple route to chemically interconnecting two graphitic surfaces and preserving their electronic properties, because this involves breaking existing and creating new bonds,” says Elena Bekyarova, who led the research. “Most importantly, new bonds act as defects or scattering centers reducing the conductivity.”
The researchers used organometallic chemistry to overcome these challenges. Chromium (Cr) atoms neatly fit into the spaces between densely aligned SWNTs, which are about 3.15 Å, forming hexagonal-shaped, covalent hexahapto bonds with the graphitic surfaces of the nanotubes without breaking any carbon-carbon bonds.
“This type of bonding provides a pathway for electrons to travel from one nanotube to another and decreases the electrical resistance between them,” explains Bekyarova. “We anticipated that electrically interconnecting aligned SWCNTs with Cr atoms would have a dramatic effect on the anisotropy of the films.”
The idea of electrically connecting conjugated carbon surfaces with hexahapto-bonded metal atoms was originally conceived by the late Robert C. Haddon and the results are as dramatic as he could have hoped. While Cr atoms boost parallel conductivity by a modest 10%, conductivity in the transverse direction increases by over 2000%.
“The key to our chemistry is to obtain aligned SWNTs that are sufficiently close to afford the formation of bonds between the Cr atoms and adjacent carbon nanotubes,” points out Bekyarova.
The process itself is simple, however. A drop of a Cr-containing solution is placed on the surface of a SWNT thin film, which has been deposited on a glass substrate with pre-patterned gold contacts, and exposed to ultraviolet light. The photoactivation process drives the formation of hexahapto bonds between Cr and the benzenoid rings of the nanotubes. Moreover, while the transverse conductivity can be turned on with light, it can be turned off with an applied potential.
“We are the first to demonstrate electrically interconnected aligned SWNTs,” says Bekyarova. “We believe that we have discovered a completely new class of carbon-based materials.”
The approach could have important implications for the design of intercalation compounds based on hexahapto-bonding, as well as high-performance SWNT-based devices.
“We plan to explore the preparation of novel organometallic compounds with other metals,” Bekyarova told Materials Today, “along with magnetic, electronic and optoelectronic devices.”