First observation of 2D tin can't confirm whether material can conduct electricity without heat loss.
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, is reported on 3 August in Nature Materials1. 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 (see ‘The super materials that could trump graphene’).
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 predictions2 made in 2013 by Shou-Cheng Zhang, a physicist at Stanford University in California, 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 Zhang.
But even after making stanene, Zhang 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 vaporizing 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 Zhang. He has already co-authored another paper3 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 upwards to form corrugated ridges; Zhang 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.
Claessen 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 Zhang and his co-authors have grown.
Yuanbo Zhang, a physicist at Fudan University in Shanghai, China, 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.”
Zhu, F. F. et al. Nature Mater. http://dx.doi.org/10.1038/nmat4384 (2015).
Xu, Y. et al. Phys. Rev. Lett. 111, 136804 (2013).
Xu, Y., Tang, P. & Zhang, S.-C. Preprint at http://arxiv.org/abs/1507.00419 (2015).
Vogt, P. et al. Phys. Rev. Lett. 108, 155501 (2012).
Dávila, M. E., Xian, L., Cahangirov, S., Rubio, A. & Le Lay, G. New J. Phys. 16, 095002 (2014).
Related links in Nature Research
The super materials that could trump graphene 2015-Jun-17
Batches of ultra-thin transistors made from 2D materials 2015-Apr-29
Phosphorene excites materials scientists 2014-Feb-04
Sticky problem snares wonder material 2013-Mar-12
Topological insulators: Star material 2010-Jul-14
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Cesare, C. Physicists announce graphene’s latest cousin: stanene. Nature 524, 18 (2015). https://doi.org/10.1038/nature.2015.18113
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