San Francisco

San Francisco State Researchers Use Lasers, Gold to Test Super-Thin Materials



​​As technologies become more compact—with the room-sized computers of the past shrinking to smartphones as just one example—a limit will eventually be reached. In a new study, San Francisco State University scientists push against that limit, probing the properties of a futuristic material made up of just a single layer of molecules. The team used lasers, vaporized gold and even some strategic sticky tape to gain a better understanding of the materials that one day may allow for extreme miniaturization of technologies, including lasers and LED lights.

AKM Newaz, San Francisco State assistant professor of physics and astronomy, started digging into the material molybdenum disulfide (or MoS2) in 2016. He was intrigued by its ability to create a single molecule thick layer—nearly 100,000 times thinner than a human hair. It is also a semiconductor, a class of materials that play a crucial role in computers due to their ability to alter electric currents.

The same trait that gives MoS2 its interesting properties also makes it difficult to work with. To test the material in the way that it will eventually be used in next-generation technologies, it needs to be sandwiched between metals. However, the typical process for doing so involves placing it on a gold surface that has hills and valleys bending the material out of shape. “You’re putting a blanket on a mountain,” Newaz said. The resulting curvature makes it impossible to accurately measure the material’s properties.

Newaz and his lab members found a way around that. They developed a technique that involved evaporating gold onto a flat surface with super-hot temperatures, sticking a flat sheet of silicon wafer on the condensed molten metal and then peeling it off. The resulting, far smoother gold surface allowed the team to mount the MoS2 on its surface with sticky tape—a must-have in any lab studying ultrathin materials—while keeping its properties intact. “That was one of the most important things in this work: We found a way to make this ultraflat gold surface,” he said.

The next step was scanning a tiny metal tip paired with a laser attached to an instrument, known as an Atomic Force Microscope (AFM), over the material to map out its physical peaks and valleys. “It senses the ups and downs, like reading Braille,” Newaz said. Combining that with measurements of electric currents let the team measure MoS2’s electric characteristics and how those characteristics change when layers of the material are stacked on top of one another.

Using a similar setup with a transparent metal, his team then measured the material’s “opto-electric” behavior—how light influences the electric current. That led to a surprising discovery: shining a laser on the material decreases, not increases, the electric current running through MoS2, a rare property called negative photoconductivity. The results of this work were published in the journal ACS Applied Materials and Interfaces in August 2019 with graduate student Hao Lee (M.S., 2019), now an alumnus, as lead author.

It is not yet clear how these properties will translate into the material’s usefulness, but the study is an important fundamental step toward identifying uses for ultraflat materials, Newaz said. The team’s next step will be figuring out why the material behaves the way it does, performing similar tests with different colors of light and under varying temperatures down to just a few degrees above absolute zero.