A chain of copper and carbon atoms can be the thinnest metal wire

Researchers from EPFL’s Laboratory for Theory and Simulation of Materials in Lausanne, part of the NCCR MARVEL, have used computational methods to identify what could be the thinnest possible metal wire, as well as several other one-dimensional materials with properties that could be of interest to many applications.

One-dimensional (or 1-D) materials are one of the most intriguing products of nanotechnology and are made of atoms aligned in the form of wires or tubes. Their electrical, magnetic and optical properties make them excellent candidates for applications ranging from microelectronics to biosensors and catalysis.

Although carbon nanotubes are the materials that have received the most attention to date, they have proven very difficult to manufacture and control. That’s why scientists are eager to find other compounds that can be used to make nanowires and nanotubes with equally interesting properties, but that are easier. to deal with.

So Chiara Cignarella, Davide Campi and Nicola Marzari thought they would use computer simulations to dissect known three-dimensional crystals, looking for crystals that – based on their structural and electronic properties – look like they could easily be ‘exfoliated’, essentially breaking them loose of the crystals. they have a stable 1D structure. The same method has been used successfully in the past to study 2D materials, but this is the first application to their 1D counterparts.

The researchers started from a collection of more than 780,000 crystals, taken from various databases in the literature and held together by van der Waals forces, the type of weak interactions that take place when atoms are so close that their electrons can overlap. They then applied an algorithm that took into account the spatial organization of their atoms, looking for atoms that contained thread-like structures, and calculating how much energy would be needed to separate that 1-D structure from the rest of the crystal.

“We were specifically looking for metal threads, which would be difficult to find because 1-D metals should in principle not be sufficiently stable to allow exfoliation,” says Cignarella, the paper’s first author.

Ultimately, they came up with a list of 800 1-D materials, from which they selected the 14 best candidates: compounds that have not yet been synthesized as real wires, but which simulations suggest are feasible. They then proceeded to calculate their properties in more detail, to verify how stable they would be and what electronic behavior you would expect from them.

Four materials – two metals and two semi-metals – stood out as the most interesting. Among them, the metal wire is CuC₂a straight chain consisting of two carbon atoms and one copper atom, the thinnest metal nanowire so far stable at 0 K.

“It’s really interesting because you wouldn’t expect a real string of atoms along a single line to be stable in the metallic phase,” says Cignarella. The scientists discovered that it could be exfoliated from three different parent crystals, all known from experiments (NaCuC₂KCuC₂ and RbCuC₂). It requires little energy to extract, and the chain can be bent while retaining its metallic properties, which would make it interesting for flexible electronics.

Other interesting materials found in the study, which was published in ACS Nanoinclude the semi-metallic Sb₂At₂, which due to its properties could make it possible to study an exotic state of matter predicted 50 years ago but never observed, called excitonic insulators, one of those rare cases where quantum phenomena become visible on a macroscopic scale. Then there are Ag₂Se₂another semimetal, and TaSe₃a well-known compound that has already been exfoliated as a nanowire in experiments, and which the scientists have used as a benchmark.

Looking ahead, Cignarella explains that the group wants to work with experimentalists to actually synthesize the materials, while continuing computational studies to see how they transport electrical charges and how they behave at different temperatures. Both of these things will be fundamental to understanding how they would perform in real applications.

Provided by MARVEL, the National Center of Competence in Research (NCCR).