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.
Electric mineral: Molybdenite (bottom) was separated into atom-thick sheets and used to make experimental digital transistors (top).
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.
But this is not enough for a material to show promise for digital logic, cautions Avouris. Molybdenite's properties must be further demonstrated before people in the electronics industry can get excited about it, he says. Researchers will also have to show, for example, that molybdenite has the properties necessary to significantly amplify electrical signals. "It's too early to say how promising this is," Avouris cautions.
Even before molybdenite's promise for high-performance microprocessors is proven—or isn't—researchers expect to find other uses for it. "You can buy metric tons of this stuff," says Hone. He points to work on making liquid suspensions of molybdenite sheets, which might be practical for making flexible solar cells and other electronics—the manual peeling method Kis used isn't practical for making large volumes of devices. "Typically, flexible electronics use polymers, but molybdenite would be more stable," he predicts.
Kis hopes that his results will encourage chemists to work on the problem of producing molybdenite sheets, as Geim and Novoselov's work encouraged people to work on methods for making large amounts of graphene. "To be promising for industry, you need to have some large-scale synthesis method for making a material," says Avouris. "It's the same problem there was in the beginning with graphene."
Tour says that Kis's results will indeed encourage chemical engineers to jump in and work with molybdenite. He says that the first experiments with graphene did not fully demonstrate its promise—researchers didn't know how to work with it. After years of working with graphene, chemists should be better able to work with molybdenite. "You already have a sense of how to handle it. This will be greatly benefited by the work we've been doing with graphene," Tour says.
By Katherine Bourzac
From Technology Review