Reviewed by Lexie CornerNov 8 2024
A study published in Nano Letters by researchers from the Florida State University Department of Physics and FSU-headquartered National High Magnetic Field Laboratory explores how physical manipulations of graphene, such as layering and twisting, affect its optical properties and conductivity.
Graphene is known for its conductivity, which exceeds that of copper, as well as its strength and lightweight nature, making it suitable for various applications in electrically conductive nanomaterials. This form of naturally occurring elemental carbon consists of a single flat layer of carbon atoms arranged in a repeating hexagonal lattice, prompting ongoing research into its properties.
The research team, led by Assistant Professors Guangxin Ni and Cyprian Lewandowski, along with graduate research assistant Ty Wilson, found that the conductivity of twisted bilayer graphene is more influenced by small geometric structure changes caused by interlayer twisting than by physical or chemical manipulations. This discovery lays the groundwork for further investigation into the effects of lower temperatures and frequencies on graphene's characteristics.
This specific path of research began as an attempt to explain some of the optical properties of twisted bilayer graphene, as this material has been imaged with scanning near-field optical microscopes before, but not in a way that compared different twisting angles. We wanted to examine this material from that perspective.
Ty Wilson, Graduate Research Assistant, Florida State University
To conduct the study, the group captured images of plasmons—tiny energy waves generated when the electrons in a material move in unison—and observed their presence in various areas of the twisted bilayer graphene.
Wilson added, “The scanning near-field optical microscope essentially shines a certain wavelength of infrared light onto the sample, and the scattered light is collected back to form a nanoscale image that is way below the diffraction limit. The key here is that it involves a needle that substantially boosts the light-matter coupling, enabling us to see these plasmons using nano-light.”
To differentiate between various areas of the twisted bilayer graphene, the team examined the grain boundaries—flaws in the crystal structure—identified in the generated images. They noted that the two sheets of carbon atoms in these plasmon-containing regions were twisted at different angles relative to a layer of hexagonal boron nitride, a transparent layered crystal positioned beneath them.
Physicists describe the geometric pattern formed when a set of straight or curved lines is superimposed onto another set as a “moiré pattern,” derived from the French word for “watered.” When the bilayer graphene and boron nitride were twisted, they created a structure known as a “double-moiré”—two layers of patterns—also referred to as a “superlattice.”
Wilson stated, “The plan was to compare the reflected near-field signal we got for each domain, whereas most previous research on graphene looked only at a single twist angle, and never before with these ‘moiré of moiré’ systems.”
The team found that even when the graphene is electrically doped and subjected to varying infrared light frequencies, the optical conductivity of twisted bilayer graphene with boron nitride remains relatively unchanged for twist angles smaller than two degrees.
“What this tells us is the opto-electronic properties of this super-moiré material are independent of chemical doping or the twisted bilayer graphene’s twist angle, and instead depend more on the super-moiré structure itself and how it affects the electronic bands in the material, allowing for enhanced optical conductivity,” Wilson added.
Lewandowski stated that this result is exciting because it shows how multilayer moiré systems can be used to create materials with “on-demand” optical properties.
“The measurement technique used by Professor Ni’s group allows us to probe the local optical response of 2D systems, complementing other local measurement techniques commonly used for 2D materials. Interestingly, in conjunction with accompanying theoretical modeling, the reported measurement argues how a 2D system can achieve almost uniform optical response over a wide light frequency range passively, without the need for active electronic feedback,” Wilson further added.
The team's findings illustrate the important role of geometric relaxations in double-moiré lattices, improving researchers' understanding of how nanomaterials like graphene respond to different manipulations.
This knowledge can be used to aid in the development of specific optical characteristics, such as enhanced conductivity, in materials. Such advancements could contribute to practical applications in moiré optoelectronics, including thermal imaging technologies and optical switching in computer processors.
This paves the way for our continuous exploration of various nano-optical and electronic phenomena that are unattainable with alternative diffraction-limited far-field optics.
Guangxin Ni, Assistant Professor, Florida State University
Funding for this study at FSU comes from the Division of Materials Research at the National Science Foundation and the Basic Energy Sciences program of the US Department of Energy. The study also included contributions from Wuhan University in China, the Chinese Academy of Sciences, and the Shanghai Institute of Microsystem and Information Technology.
Journal Reference:
Cui, S. et. al. (2024) Nanoscale Optical Conductivity Imaging of Double-Moiré Twisted Bilayer Graphene. Nano Letters. doi.org/10.1021/acs.nanolett.4c02841