Berkeley engineers give graphene a new twist to boost optoelectronics

By stacking two single-atom-thick sheets of graphene on top of each other and twisting them, researchers at UC Berkeley have converted a common linear material into one with nonlinear optical capabilities crucial to everyday technology — from spectroscopy and material analysis to communications and computing.

The discovery, reported in the October issue of the journal Matter, might one day be used to enhance the performance of the silicon microchips found in nearly all laptops, tablets and phones, as faster photonic circuitry is increasingly combined with traditional electronic circuitry.

In the study of optics, scientists distinguish between linear and nonlinear materials. Most materials, including sheets of graphene, are linear. If you shine red light at a sheet of graphene, the photons will either be absorbed or scattered. But they will still be red.

Nonlinear materials can combine multiple photons into one. The frequency of the resulting photon — which scientists call a “second harmonic generation” — is double that of the original, so it has twice the energy. If you shine red light at a nonlinear material, two red photons combine to create an ultraviolet photon. Scientists use this process to stimulate photo-chemical reactions that need high-energy photons, such as photocatalysis and photosynthesis. More common are the laser pointers that rely on nonlinear crystals to convert invisible infrared photons into green photons.

Nonlinear materials are rare, and their ability to alter photons is usually well-defined and can’t be changed. But the Berkeley team showed that by twisting two layers of graphene in opposite directions, it’s possible to fine-tune their ability to combine photons.

“Just by introducing this twisting mechanism, electrons in the graphene layers have very different behaviors,” said Jie Yao, associate professor of materials science and engineering and the paper’s senior author. “The results are better than we thought. The nonlinear capability generated by twisting graphene layers is surprisingly strong.”

That such a simple action has such a strong effect is due to the fact that a material’s properties are defined at the atomic level.

A graphite crystal is formed of atomically thin sheets of graphene. While the carbon atoms in a sheet of graphene are chemically bonded, the sheets themselves are loosely stacked on top of each other and held together by a relatively weak attractive force, called a Van der Waals force.

Atoms in a sheet of graphene are arranged in a hexagonal honeycomb pattern. A graphite crystal found in nature will be made up of sheets of graphene stacked in one of two highly ordered ways: The atoms in one layer will either sit directly on top of the atoms in another, or they will position themselves above the hexagonal void.

By twisting the sheets of graphene in opposite directions, the symmetry of their atomic arrangement is altered and the atoms’ valence electrons are thrown out of whack. The graphene’s properties change.

The weak Van der Waals force holding sheets of graphene together is what makes it possible to separate and twist them. When physicists in the United Kingdom first peeled layers of graphene off of graphite crystals — research that led to the 2010 Nobel Prize in physics — they used a piece of Scotch tape.

The Berkeley team relied on an approach developed from the same low-tech process. Fuyi Yang, a Ph.D. student in Yao’s lab and the paper’s lead author, used tape to peel layers of graphene off a large graphite crystal. She pressed that graphene sheet onto a substrate of silicon and silicon dioxide and peeled the tape away.

Yang then used a piece of glass outfitted with a small cube of sticky transparent polymer to pick up a piece of hexagonal boron nitride, which can attract sheets of graphene like a magnet. By raising and lowering the nitride with a vertical stage, Yang could lift a second piece of graphene off the silicon substrate and, like a crane loading shipping containers onto a truck, place it onto the first layer at an angle. The whole process takes more than two hours.

In the end, Yang produced 30 samples of bilayer graphene at varying twist angles. With increasing twist angles, the team was able to combine higher energy photons into one.

While the research showed that turning graphene into a nonlinear material is possible, Yao said it’s just a milestone toward the ultimate goal: To make two layers of graphene twist on command.

“That’s one potential direction for future exploration,” Yao said. “If we can change the twisting angle between two graphene sheets in real time, then changing the nonlinear property of that bilayered graphene would be as simple as tuning a radio.”

Other co-authors of the paper include Fenhao Meng, Fuchuan Luo, Shuai Lou, Shuren Lin, Zilun Gong and Jinhua Cao of the Department of Materials Science and Engineering; Wenshen Song and Li Yang of the Department of Physics at Washington University in St. Louis; and Edward Bernard and Emory Chan of the Molecular Foundry at the Lawrence Berkeley National Laboratory.

Work at the Molecular Foundry was supported by the U.S. Department of Energy’s Office of Basic Energy Sciences. The research was also supported by the Bakar Fellowship and grants from the Air Force Office of Scientific Research and the National Science Foundation.