Researchers have now discovered how different physical alterations to graphene, like layering and twisting, affect its optical characteristics and electrical conductivity.
Graphene, known for being lighter than steel and more conductive than copper, is a top contender among electrically conductive nanomaterials and is being explored for numerous technological applications.
Scientists are digging deeper into this remarkable form of carbon, which consists of a single, flat layer of carbon atoms arranged in a repeating hexagonal pattern.
A team from Florida State University’s Department of Physics and the National High Magnetic Field Laboratory has published new research that reveals how different physical manipulations of graphene can impact its optical features and electrical conductivity. Their findings appear in the journal Nano Letters.
Under the guidance of Assistant Professor Guangxin Ni, along with Assistant Professor Cyprian Lewandowski and graduate research assistant Ty Wilson, the team discovered that the conductivity of twisted bilayer graphene remains relatively stable under physical or chemical changes. Instead, it relies significantly on the subtle geometric alterations caused by the twisting of the layers. This insight paves the way for further research on how temperature and frequencies influence graphene’s properties.
“Our research originated from an effort to explain the optical properties of twisted bilayer graphene, which has previously been viewed using scanning near-field optical microscopes. However, no one had examined it while comparing different twisting angles,” Wilson explained. “We aimed to analyze this material from that angle.”
In their study, the team captured images of plasmons—tiny energy waves generated by the collective movement of electrons within the material—across various sections of the twisted bilayer graphene.
“The scanning near-field optical microscope shines a specific wavelength of infrared light onto the sample, collecting scattered light to create a nanoscale image that surpasses diffraction limits,” Wilson said. “A crucial feature is a needle that significantly enhances light-matter interactions, allowing us to observe these plasmons with nano-light.”
The researchers examined grain boundaries—defects in the crystal lattice—revealing different areas within the twisted bilayer graphene. These areas containing plasmons intrigued the team since the two layers of carbon atoms twisted at defined angles, while also being twisted relative to a layer of hexagonal boron nitride—a transparent layered crystal—below it.
In physics, the geometric design that results from superimposing sets of straight or curved lines is called a “moiré pattern,” a term derived from a French word meaning “watered.” The twisting of bilayer graphene and boron nitride created a “double-moire” structure or a “superlattice,” consisting of two layers of these patterns.
“We aimed to compare the reflected near-field signals from each domain, unlike most previous studies on graphene, which focused on a single twist angle and never explored these ‘moiré of moiré’ configurations,” Wilson noted.
The researchers found that the optical conductivity of twisted bilayer graphene combined with boron nitride doesn’t change significantly with twist angles under two degrees, even when the graphene is doped and subjected to varying infrared frequencies.
“This indicates that the opto-electronic behaviors of this super-moire material are largely unaffected by chemical doping or the graphene twist angle, and instead rely more on the super-moire structure itself and its influence on the electronic bands in the material, leading to improved optical conductivity,” Wilson explained.
Lewandowski expressed excitement over these findings, emphasizing the potential of multilayer moiré systems to create materials with customizable optical properties.
“The measurement methods employed by Professor Ni’s team enable us to investigate the local optical response of 2D materials, complementing other measurement techniques typically applied to 2D structures,” he said. “Interestingly, with support from theoretical models, the reported measurements demonstrate how a 2D system can attain an almost uniform optical response across a broad range of light frequencies passively, without needing active electronic feedback.”
The results from this research highlight the crucial role that geometric relaxations play in double-moire lattices, assisting scientists in understanding how nanomaterials like graphene might react to various manipulations. This knowledge could help researchers develop materials with desirable optical properties, like increased conductivity, fostering innovative developments in moiré optoelectronics, which may include advances in thermal imaging technology and optical switching for computer processors.
“This sets the stage for our ongoing exploration of nano-optical and electronic phenomena that cannot be achieved with conventional diffraction-limited far-field optics,” Ni stated.
