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.