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Excitons spotted in bilayer graphene

By Kendra Redmond January 12, 2018
Excitons spotted
A photocurrent spectrum showing two sharp peaks (P1 and P2) observed as the lowest energy spectrum features in bilayer graphene. These peaks are signs of the optical transitions of exciton states. The inset illustrates the photocurrent generation process. Excitons excited by incident light are dissociated into free electrons and holes and contribute to electrical current under an external electric field. Reprinted with permission from Long Ju et al., Science 358, 907-910 (2017), DOI: 10.1126/science.aam9175.

Scientists have observed electron-hole pairs (excitons) in bilayer graphene for the first time. As reported in a recent issue of Science, these findings provide new avenues for studying exciton physics and could lead to tunable infrared detectors and other next-generation optoelectronic instruments for thermal imaging and astronomy.

Like single layer graphene, bilayer graphene has zero bandgap. Researchers have shown that an external electric field can induce a large, tunable bandgap in bilayer graphene. This bandgap lies in the terahertz (THz) to mid infrared (IR) range and enables the material to be tuned from a metal to a semiconductor.

“Bilayer graphene is a unique material system for both semiconductor applications and fundamental physics,” according to Long Ju from Cornell University, the first author of this report and a member of Paul McEuen’s laboratory at Cornell University. “There are very few semiconductors that have bandgaps in this range, let alone their electrical tunability,” he says.

Excitons play an important role in determining the optical properties of semiconductors, and signs of their existence generally appear in optical absorption and emission spectra. Detecting excitons with optical transition energies in the THz-mid IR range could lead to significant advances in fundamental science as well as technological applications such as imaging. Although theoretical work has suggested that excitons tunable in this range could be supported by bilayer graphene, they have gone undetected until now.

The exciton observations in bilayer graphene were carried out by a team of researchers from the United States and Japan. They performed photocurrent spectroscopy on a high-quality sample of bilayer graphene encapsulated in boron nitride (BN), a dielectric material with a hexagonal structure similar to graphene. Encapsulation in BN provides a clean environment for observing the intrinsic excitons in bilayer graphene. Previous experiments relied on oxide dielectrics, but researchers found that oxides introduced disorders that masked the exciton signals.

The BN-bilayer graphene-BN stack was placed on a graphite back gate and a gate of a nickel/chrome alloy was deposited on top of the stack. The researchers applied a voltage bias using both gate electrodes to open up the bandgap. 

When the device was illuminated by infrared light, photons absorbed by the graphene bilayer excited valence electrons into the conduction band, leaving behind holes that remained paired to the electrons by the Coulomb force. When the excitons dissociated into free electrons and holes, they produced a photocurrent proportional to the amount of light originally absorbed.

By measuring this photocurrent using modified Fourier transform infrared spectroscopy (FTIR), the researchers determined the optical absorption spectra of bilayer graphene. The results showed two sharp absorption peaks corresponding to the optical transitions of exciton states. Further research showed that the resonant frequencies of the transitions could be tuned by electric fields of different magnitudes.

“The strong optical resonance of excitons could enable various optical and optoelectronic applications in this very important, yet underdeveloped spectrum range,” Ju says. This work may also have interesting applications in the area of “valleytronics.” In momentum space, electrons occupy different local minimums called valleys. Just as electron charges and spins can be used to process, transport, and store information, researchers expect that the valley degree of freedom of electrons can be exploited in analogous valleytronic devices. This new research suggests that excitons have a valley-dependent structure that can be manipulated by light polarization and magnetic fields in such devices.

In addition to these applications, the discovery paves the way for fundamental research on electron spin. Excitons in bilayer graphene display a property called pseudospin that mimics real electron spin in many ways, and differentiates them from the excitons in conventional semiconductors.

Ju says, “Bilayer graphene provides a system where you can make a 'spin' from purely the orbital degree of freedom. And by tuning the bandgap of bilayer graphene, you can study the evolution of this pseudospin with the electron mass continuously and test its effects on material properties. To some extent you can use a table-top experiment to test and study some of the relativistic physics of electrons that [are currently] only possible in high energy physics.” 

The tunability of bilayer graphene makes it a very unique two-dimensional material says Philip Kim, a Harvard University professor whose work focuses on transport phenomena in low-dimensional nanoscale materials. “Ju and his collaborators’ work presents a substantial progress in our understanding of this material by demonstrating the existence of excitons whose energy can be tunable. Together with the valley structure the authors discovered in them, these excitons can serve as an important material platform to realize novel optoelectronic devices,” he says.

Read the abstract in Science.