Two years after physicists predicted that tin should be able to form a mesh just one atom thick, researchers say that they have made it. The thin film, called stanene, was detailed this month in Nature Materials. But researchers have not been able to confirm whether the material has the predicted exotic electronic properties that have excited theorists, such as being able to conduct electricity without generating any waste heat.
Stanene (from the Latin “stannum,” meaning tin, which also gives the element its chemical symbol, Sn), is the latest cousin of graphene, the honeycomb lattice of carbon atoms that has spurred thousands of studies into related 2D materials. Those include sheets of silicene, made from silicon atoms; phosphorene, made from phosphorus; germanene, from germanium; and thin stacks of sheets that combine different kinds of chemical elements.
Many of these sheets are excellent conductors of electricity, but stanene is – in theory – extra special. At room temperature, electrons should be able to travel along the edges of the mesh without colliding with other electrons and atoms as they do in most materials. This should allow the film to conduct electricity without losing energy as waste heat, according to predictions made in 2013 by Shou-Cheng Zhang, a physicist at Stanford
University, who is a co-author of the latest study. That means that a thin film of stanene might be the perfect highway along which to ferry current in electric circuits, says Peide Ye, a physicist and electrical engineer at Purdue University in West Lafayette, Indiana. “I’m always looking for something not only scientifically interesting, but that has potential for applications in a device,” he says. “It’s very interesting work.”
Stanene is predicted to be an example of a topological insulator, in which charge carriers (such as electrons) cannot travel through a material’s centre, but can move freely along its edge, with their direction of travel dependent on whether their spin – a quantum property – points “up” or “down.” Electric current is not dissipated because most impurities do not affect the spin and cannot slow the electrons, says Shou-Cheng.
But even after making stanene, Shou-Cheng and his colleagues at four universities in China have not been able to confirm that it is a topological insulator. Experimentalists at Shanghai Jiao Tong University created the mesh by vaporising tin in a vacuum and allowing the atoms to waft onto a supporting surface made of bismuth telluride. Although this surface allows 2D stanene crystals to form, it also interacts with them, creating the wrong conditions for a topological insulator, says Shou-Cheng. He has already co-authored another paper examining which surfaces would work better.
Ralph Claessen, a physicist at the University of Würzburg in Germany, says that it is not completely clear that the researchers have made stanene. Theory predicts that the 2D tin lattice should form a buckled honeycomb structure, with alternate atoms folding upward to form corrugated ridges; Shou-Cheng and his team mostly saw only the upper ridge of atoms with their scanning tunnelling microscope, except in a small spot where that ridge disappeared and a lower layer of tin atoms was exposed. However, they are confident that they have created a buckled honeycomb, partly because the distance between upper and lower layers matches predictions.
Ralph says that he would need to see direct measurements of the lattice’s structure – from X-ray diffraction – to be confident that the team has made stanene, and not some other arrangement of tin. This would require larger amounts of the material than Shou-Cheng and his co-authors have grown.
Yuanbo Zhang, a physicist at Fudan University in Shanghai, who was not involved in the study, is more convinced. “I think the work is a significant breakthrough that once again expands the 2D-material universe,” he says. “It’ll be exciting to see how the material lives up to its expectations.”
And Guy Le Lay, a physicist at Aix-Marseille University in France who was among the first to produce both silicene and germanene, preaches optimism in the attempt to verify stanene’s electronic properties. “It’s like going to the moon,” he says. “The first step is the crucial step.”