Pyrite — perhaps better known as “fool’s gold” for its yellowish metallic appearance — is a common, naturally occurring mineral. It holds promise as a high-tech material, with potential uses in solar cells, spintronic devices and catalysts, but is also a byproduct of corrosion of steel in deep-sea oil and gas wells. Both its potential usefulness in devices and its role in corrosion are largely influenced by the fundamental electronic properties of its surface — which have remained relatively unexplored.



But a team of MIT researchers has now found a way to probe these elusive surface properties for the first time. Their findings are reported in the journal Surface Science, in a paper by professors Bilge Yildiz and Krystyn Van Vliet and graduate students F. William Herbert and Aravind Krishnamoorthy.



“The surface of this material is very different from the bulk, something that is common for many materials,” Yildiz explains. “The bulk has been widely characterized, but when it comes to the surface, there is only a small amount of data, and it’s not consistent.”



Yet in reaping the benefits of pyrite, “it’s important to know about the surface,” she says.

Measuring the surface electronic characteristics of materials is technically much more challenging than measuring their bulk properties — difficulties that arise from “a combination of the available tools as well as the characteristics of the surfaces of materials,” says Yildiz, an associate professor of nuclear science and engineering.



The new work was made possible by combining scanning tunneling spectroscopy (STS), a tool developed in the 1980s, with modern computational methods to interpret its output, Herbert explains. “You need a tool that is measuring just to a shallow depth of one or two atomic layers” on a sample, he says, but also the computational ability to establish “how the surface behaves differently from the bulk, and how to model the experimental data.”



The new measurements reveal, among other things, that on pyrite’s surface, a property called an energy bandgap — essential for making solar cells or semiconductor devices — has a value less than half that of the bulk material. Previous studies had produced conflicting results for the bandgap at the surface. In order to uncover the true nature of this surface, Herbert performed tunneling spectroscopy measurements on pristine pyrite surfaces, and analyzed his results with theoretical modeling of the tunneling spectra. The model was adapted from semiconductor physics, and informed by Krishnamoorthy’s electronic structure calculations.



The reason for the discrepancy between surface and bulk properties, Yildiz explains: “The surface has dangling bonds, providing electronic states into the bulk bandgap.”