Molybdenite, a mineral that’s currently used as a lubricant, turns out to have extraordinary electronic properties when deposited in two-dimensional strips. Researchers in Switzerland have now made high-performance transistors out of this form of molybdenite. Used in this way, the mineral could hold promise for more efficient flexible solar cells, electronics, or high-performance digital microprocessors.
Like graphene, an atom-thick form of carbon, “two-dimensional” molybdenite has electrical and optical properties that are much better than those found in three-dimensional forms of the material.
Researchers led by Andras Kis at the École Polytechnique Fédérale de Lausanne (EPFL) made molybdenite transistors using methods used in the early days of graphene research. Molybdenite, a relatively inexpensive mineral of molybdenum disulfide, has a layered structure similar to that of raw graphite. Kis’s group crushed crystals of molybdenite between folded pieces of tape, peeling back layer after layer until all that remained were three-atom-thick sheets. They then deposited the molybdenite sheets onto a substrate, added a layer of insulating material, and used standard lithography to add source and drain electrodes and a gate to make a transistor. Other researchers had done this before but didn’t get good performance. Kis says the molybdenite transistors have a comparable electrical mobility to similar ones made from graphene nanoribbons.
After Andre Geim and Kostya Novoselov demonstrated the promise of graphene in 2004—a feat that won them the Nobel Prize in 2010—there was a burst of interest in making and testing other two-dimensional materials. But graphene was considered more promising than anything else, and other materials came to be seen as curiosities, says James Hone, professor of mechanical engineering at Columbia University. Hone was part of a group that demonstrated that graphene is the strongest material ever tested. Hone, who is not affiliated with the EPFL researchers, expects their results to generate new interest in other two-dimensional materials, and molybdenite in particular. “This is a very promising result that will make us look at this material more carefully and see how we can squeeze better performance out of it,” he says.
Importantly, molybdenite is a semiconductor, which means it provides discrete energy levels for electrons to jump through—a property known as its bandgap. This is key for any material used in a digital transistor. Graphene does not have a bandgap, and to give it one, researchers must layer it or cut it into ribbons, which is complex and can lead to the degradation of graphene’s other properties. “You have to work very hard to open up a bandgap in graphene,” says James Tour, professor of chemistry and computer science at Rice University.
Graphene was originally seen as a material that could replace silicon in digital logic circuits, the type at the heart of today’s microprocessors. But because it’s so hard to make it into a semiconductor, it’s becoming clear that graphene’s promise lies elsewhere, for example in superfast analog circuits, the type used for telecommunications and radar, says Phaedon Avouris, who leads the IBM group developing graphene electronics. Molybdenite’s bandgap is particularly promising for solar cells, LEDs, and other electro-optical devices.