Copper chains more than 4000 atoms long are the longest ever produced

The longest chains of single metal atoms ever have been made by researchers in China. The scientists believe these carbon-sheathed wires could have wide-ranging applications, although questions remain about their scalability.

Single-metal-atom chains are the ultimate miniature wires. As silicon chips become as small as physically possible, atom-thick wires could find use in new electronics platforms as well as in other areas of engineering and fundamental physics research.

However, their true potential remains relatively unexplored. ‘Research into the fundamental properties of single-metal-atom chains has remained remarkably scarce, largely due to the longstanding challenges in synthesising them reliably,’ explains Kuo Li at the Centre for High Pressure Science and Technology Advanced Research in Beijing. Researchers have tried techniques like self-assembly along grain boundaries and chemical vapour deposition, but these require conditions like high vacuum for stability. And while there have been reports of organic-ligand-stabilised chains, very few have been more than 10 atoms long.

Li’s team started from a copper phthalocyanine crystal, comprising regularly spaced copper (II) ions, each coordinated by four nitrogen atoms within a planar phthalocyanine ring. As the researchers increased the pressure to 21.5GPa, the crystal contracted by 46% along the axis parallel to the copper ions. On heating, this was rendered irreversible. Infrared spectroscopy showed that the ligands had polymerised, forming a sheath with the copper ions inside.

The researchers then exfoliated the bulk single crystals, leaving individual sheathed single-metal atom chains more than 4000 atoms long. Further analysis indicated that the metal-atom chains were actually antiferromagnetic owing to the alternating electron spins on adjacent copper ions. Electrical conductivity arose from the intermediate layer along the interior of the carbon chain.

The researchers are now working to achieve better single crystals and to characterise the properties of other phthalocyanine atom chains: ‘As long as a metal–phthalocyanine has this analogous crystal structure, we expect the high-pressure strategy to work.’ says Li ‘We demonstrated this principle with cobalt, nickel, zinc and hydogen. The hydrogen–phthlalocyanine result is important as it demonstrates that the metal is not the driving force for the reaction, proving the generality of the organic framework’s reactivity.’ Li hopes that ‘interesting and useful applications may very well be hiding in plain sight, waiting to be discovered by perceptive researchers from different fields’.

‘I think it’s significant, because they’ve made the longest single-metal-atom chains ever – and they’re very long,’ says condensed matter theoretician Kristen Fichthorn at Pennsylvania State University in the US. However, she questions the scalability of the process given the extreme conditions required. Fichthorn notes that the final product does not demonstrate theoretically predicted values of conductivity, and suspects that ‘even if this was a scalable process, they would have some kinks to work out.’

‘I’m not an experimentalist’ she says, ‘but I would look for a method like assembly or growth within a nanotube – something more elegant than just compressing the heck out of some metal-containing molecule.’