Physicists reveal how layers and twist affect graphene’s photoconductivity

New research led by FSU assistant professor of physics Guangxin Ni reveals the optoelectronic properties of twisted bilayer graphene. Credit: Guangxin Ni
When it comes to conductive nanomaterials, graphene, which is stronger and lighter than steel and more conductive than copper, has been shown to be an excellent choice for a wide range of technologies.
Physicists are working to learn more about this wonderful form of naturally occurring elemental carbon. This amazing morphology consists of a single flat layer of carbon atoms arranged in a repeating hexagonal lattice.
Now, researchers from the Florida State University Department of Physics and the FSU-based National High Magnetic Field Laboratory are investigating how various physical manipulations of graphene, such as layering and twisting, affect its optical properties and conductivity. We published the results of a study that revealed the The study was published in the journal Nano Letters.
A team led by Assistant Professor Guangxin Ni, along with Assistant Professor Cyprian Lewandowski and Graduate Research Assistant Ty Wilson, found that the electrical conductivity of twisted bilayer graphene is not significantly affected by physical or chemical manipulation, but rather relies heavily on the material’s microscopic geometry. I discovered that. Structural changes due to interlayer twisting. This is a new discovery that opens the door to further research into how decreasing temperature and frequency affect graphene’s properties.
“This particular research path began as an attempt to explain some of the optical properties of twisted bilayer graphene, as this material had previously been imaged with scanning near-field optical microscopy, but with different twists. It wasn’t a way to compare corners,” Wilson said. Said. “We wanted to look at this material from that perspective.”
To conduct the study, the researchers took images of plasmons (tiny waves of energy produced when electrons move together in the material) appearing in different regions of twisted bilayer graphene.
“Scanning near-field optical microscopy basically shines infrared light at a specific wavelength onto a sample, and the scattered light is collected to form nanoscale images well below the diffraction limit,” Wilson said. said. “The key here is that there is a needle involved that greatly enhances the coupling of light and matter, allowing us to see these plasmons using nanolight.”
The researchers analyzed grain boundaries, or defects in the crystal structure, in the resulting images and identified different regions of the twisted bilayer graphene. These plasmon-containing regions intrigued the researchers because, in addition to the fact that the two sheets of carbon atoms are twisted at different angles, the hexagonal boron nitride layer beneath them This is because the layer (transparent layered crystal) is twisted.
Physicists refer to the geometric pattern created when one set of straight lines or curves is superimposed on another set as a moiré pattern, which is French for “watering.” By twisting bilayer graphene and boron nitride, a so-called double moiré structure, or two-layer pattern, also known as a superlattice, was formed.
“The plan was to compare the reflected near-field signals obtained in each domain. On the other hand, most previous studies on graphene have only focused on a single twist angle, and this “I had never studied a ‘moiré’ system before,” Wilson said.
The research team found that the photoconductivity of twisted bilayer graphene containing boron nitride is high when the twist angle is less than 2 degrees, even when the graphene is electrically doped and exposed to infrared light of varying frequencies. I found that not much changed.
“What this tells us is that the optoelectronic properties of this supermoiré material are independent of chemical doping or the twist angle of the twisted bilayer graphene, but are instead dependent on the supermoiré structure itself and how it affects the electronic bands. “It depends on how it affects the material,” Wilson said.
Lewandowski added that the results are interesting because they highlight the potential of multilayer moiré systems in building materials with “on-demand” optical properties.
“The measurement technique used by Professor Ni’s group allows us to investigate the local optical response of 2D systems and complements other local measurement techniques commonly used for 2D materials.” said. “Interestingly, the reported measurements, together with accompanying theoretical modeling, demonstrate how a 2D system passively achieves near-uniform optical performance over a wide optical frequency range, without the need for active electronic feedback. We are discussing whether a response can be achieved.”
The research team’s findings demonstrate the significant influence of geometric relaxation in double Moiré lattices, which will help researchers better understand how nanomaterials like graphene respond to various manipulations. Helpful. Additionally, this information can help scientists create desirable optical properties, such as increased electrical conductivity in materials, and enable innovative advances in moiré optoelectronics, such as thermal imaging technology and optical switching in computer processors.
“This paves the way for continued exploration of various nano-optical and electronic phenomena that are unattainable with alternative diffraction-limited far-field optics,” Ni said.
Further information: Songbin Cui et al. Nanoscale photoconductivity imaging of double moire twisted bilayer graphene, Nano Letters (2024). DOI: 10.1021/acs.nanolett.4c02841
Provided by Florida State University
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