Less is more: the remarkable properties of the smallest possible one-dimensional materials

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A computer generated image of a nanotube

Scientists are developing the smallest possible one-dimensional (1D) materials called ‘extreme nanowires’. Less than a nanometre in diameter, they are 10,000 times smaller than a human hair. Now, an EPSRC-funded collaboration between the Universities of Birmingham and Warwick has reached the ultimate limit – making wires that are just one atom in diameter.

These extreme nanowires are constructed in Dr Jeremy Sloan’s laboratory at the University of Warwick by filling the inside of tiny tubes made from carbon, so-called carbon nanotubes (CNTs). Miniaturising materials, and reducing their dimensionality, can have miraculous effect on their properties.

‘It’s an understatement to say that a material’s properties depend critically on where its atoms are,’ says Dr Andrew J Morris, Senior Birmingham Fellow in the School of Metallurgy and Materials. ‘A great example is diamond and graphite. They’re the same atoms; it’s purely the arrangement that means you have either a clear, sparkly, hard diamond on your finger or grey, crumbly graphite at the end of your pencil.’

The success of the three-year project – which also involved researchers from the University of Cambridge – has predicted some remarkable new applications. The team has shown that some extreme nanowires enhance the current carrying capabilities of CNTs that could transform electronics by improving the conductivity of nano-connectors between microchips , making chips more efficient.

Other wires confined inside CNTs have their thermoelectric properties massively increased, enhancing their ability to convert waste heat into electricity.  All technologies, from cars to computers, generate waste heat: Collecting that waste and using it to recharge batteries would give them all huge boost in efficiency.

Andrew and Dr David Quigley, a physicist from Warwick, have also shown that extreme nanowires can behave as sub-nanoscale phase-change memory, for storing information more efficiently.

Using the UK’s super-computer, ARCHER, they employed a method called AIRSS (ab initio random structure searching) a quantum mechanical technique which allowed them to predict how atoms arranged themselves in a nanowire crystal inside a CNT.

Their article entitled "Electronic Structure Control of Sub-Nanometer 1D SnTe via Nanostructuring within Single-Walled Carbon Nanotubes”, published in the journal ACS Nano recently won the College of Engineering and Physics Sciences’ Paper of the Month award.

‘I’m a theoretical physicist who’s moved gradually into materials science,’ says Andrew. ‘Ab initio techniques use only quantum mechanics running algorithms on large computers to make their predictions – after all this time, it is still amazing to me that we can predict how matter behaves using only a computer and so know what Jeremy is going to make before he steps into the lab!’

Jeremy, Associate Professor/Reader in Electron Microscopy, who recently won an EPSRC advanced fellowship to continue his experimental investigations, adds: ‘Extreme nanowires represent the ultimate class of crystals: they are literally the smallest periodic materials possible with one to two atom-wide motifs repeating in 1D only. As such they offer a privileged perspective into the physics and chemistry of highly low-dimensional systems as well as giving a first look at the phase-change characteristics of these systems at the lowest conceivable dimension.’