Dreidel-like dislocations lead to remarkable properties

A new material structure predicted at Rice University offers the tantalizing possibility of a signal path smaller than the nanowires for advanced electronics now under development at Rice and elsewhere.

Theoretical physicist Boris Yakobson and post-doctoral fellow Xiaolong Zou were investigating the atomic-scale properties of two-dimensional materials when they found to their surprise that a particular formation, a grain boundary in metal disulfides, creates a metallic – and therefore conducting – path only a fraction of a nanometer wide.

That's basically the width of a chain of atoms, Yakobson said.

The discovery reported this week in the American Chemical Society journal Nano Letters sprang from an investigation of how atoms energetically relate to each other and form topological defects in two-dimensional semiconductors. In recent work, Yakobson's group has analysed defects in graphene, the single-atom sheet of carbon that is under intense scrutiny by labs around the world.

But flat graphene has no band gap; electrons flow straight through. ''There is a lot of effort to open a gap in graphene, but this is not easy,'' said Yakobson, Rice's Karl F. Hasselmann Professor of Mechanical Engineering and Materials Science and professor of chemistry. ''People are trying different ways, but none of them are straightforward. This motivated the search for other two-dimensional materials.''

Molybdenum / sulphur (or tungsten / sulphur) materials are becoming interesting to scientists because they have a useful natural band gap, about two electron volts in the case of molybdenum / sulfur. And while they are technically two-dimensional materials, the energies at play force their atoms into a staggered arrangement.