"We demonstrate that carbon nanotube crystallinity is not perfect - approximately one out of a trillion chemical bonds is out of place," said Phil Collins of the University of California at Irvine. "The presence of these defects, even in extremely small quantities, has enormous electrical consequences."
Collins and colleagues used selective electrochemical deposition in a custom electrochemical cell to grow nickel spots on defects on the nanotube surface. Particles of nickel nucleated at surface defect sites because of their greater chemical reactivity. Depending on the growth time, the particles were visible by wide-field scanning electron microscopy, atomic force microscopy, side-illuminated or dark-field optical microscopy, or even by the naked eye.
While Raman spectroscopy can detect high defect densities in carbon nanotubes, the technique is not sensitive enough to spot the lower defect densities that are significant for electronic applications.
Although the mean defect density was one for each 4 µm, individual nanotubes showed a wide variation in defect distribution. Many of the nanotubes had 1 or 2 µm long sections that were entirely defect-free, while others had many defects clustered together. Curved nanotube regions often showed higher defect densities than straight regions. The scientists say this may indicate that small fluctuations in the chemical vapour deposition (CVD) environment can directly influence the introduction of defects during growth.
"Carbon-nanotube purity is already as high as state-of-the-art silicon crystals, despite having benefited from only a tiny fraction of the investment," said Collins. "On one hand, the work suggests that nanotubes might need to be far, far purer than silicon to ever form reliably competitive transistors for logic and memory chips. On the other hand, the special importance of defect sites makes nanotube circuits perfect for chemical and biological sensors and may explain the sensitivity of these circuits."
The team also used selective electrochemical deposition to examine defects in circuits based on single-walled carbon nanotubes.
"Because carbon nanotubes are extremely narrow wires, there is no alternate way for current to flow 'around' a defect site," said Collins. "The presence of a single atomic defect will disrupt or dominate an entire circuit based on carbon nanotubes."
The researchers say that selective electrochemical deposition could become a useful quality and process-control technique for reproducible single-walled carbon nanotube electronics.
"We show that all of the electronic behaviour of a particular nanotube transistor is due to the presence of a defect, and is not a characteristic of the nanotube itself," said Collins. "Similar experiments on chemical sensitivity are underway. Because metal dots are placed directly on the electronically sensitive sites, they can be used to specifically anchor single sensor molecules to the circuits right where the circuits are most sensitive."
The researchers reported their work in Nature Materials.