Power/Performance Bits: April 5

Graphene: wafer-scale, straintronics, and kagome lattice.


Wafer-scale graphene
In an attempt to make graphene more useful for photonic devices, researchers from CNIT, Istituto Italiano di Tecnologia (IIT), Tecip Institute, University of Cambridge, and Graphene Flagship Associated Member and spin-off CamGraphIC developed a wafer-scale graphene fabrication technology that uses predetermined graphene single-crystal templates, allowing for integration into silicon wafers.

The new fabrication technique is enabled by the adoption of single-crystal graphene arrays. “Traditionally, when aiming at wafer-scale integration, one grows a wafer-sized layer of graphene and then transfer it onto silicon,” explained Camilla Coletti, coordinator of IIT’s Graphene Labs. “Transferring an atom-thick layer of graphene over wafers while maintaining its integrity and quality is challenging. The crystal seeding, growth and transfer technique adopted in this work ensures wafer-scale high-mobility graphene exactly where is needed: a great advantage for the scalable fabrication of photonic devices like modulators.”

“Silicon and germanium alone have limitations; however, graphene provides many advantages,” added Marco Romagnoli from CNIT, INPHOTEC, and CamGraphiC. “This methodology allows us to obtain over 12,000 graphene crystals in one wafer, matching the exact configuration and disposition we need for graphene-enabled photonic devices.”

Additionally, the process is compatible with existing automated fabrication systems, which the team says will accelerate industrial uptake and implementation.

To put the method to the test, the researchers used it to design high-speed graphene photodetectors. Graphene photodetectors have the ability to absorb light from ultraviolet to the far-infrared for ultra-broadband communications. Thanks to the ultra-high mobility of carriers in graphene devices, the team said data transmission could exceed 100 gigabits per second.

Romagnoli noted that graphene could reduce energy usage, as well. “In graphene, almost all the energy of light can be converted into electric signals, which massively reduces power consumption and maximizes efficiency.”

“This is the first time that high-quality graphene has been integrated on the wafer-scale,” said Frank Koppens, Graphene Flagship Leader for Photonics and Optoelectronics. “The work shows direct relevance by revealing high-yield and high-speed absorption modulators. These impressive achievements bring commercialization of graphene devices into 5G communications very close.”

Graphene straintronics
Physicists at the University of Sussex, Foundation for Research and Technology- Hellas (FORTH/ICE-HT), Loughborough University, Rice University, and University of Trento used origami-like techniques to make graphene behave like a transistor, adding kinks and crumples to the material’s structure.

“We’re mechanically creating kinks in a layer of graphene. It’s a bit like nano-origami. Using these nanomaterials will make our computer chips smaller and faster,” said Alan Dalton, a professor in the School of Mathematical and Physics Sciences at the University of Sussex. “This kind of technology – “straintronics” using nanomaterials as opposed to electronics – allows space for more chips inside any device. Everything we want to do with computers – to speed them up – can be done by crinkling graphene like this.”

The researchers used atomic force microscopy and Raman spectroscopic mapping to study the properties that resulted from introducing structural defects into both graphene and molybdenum disulfide, another 2D material.

“Instead of having to add foreign materials into a device, we’ve shown we can create structures from graphene and other 2D materials simply by adding deliberate kinks into the structure,” added Dr Manoj Tripathi, Research Fellow in Nano-structured Materials at the University of Sussex. “By making this sort of corrugation we can create a smart electronic component, like a transistor, or a logic gate.”

Kagome graphene
Physicists from the University of Basel and University of Bern produced a new graphene compound that behaves as a semiconductor and may have other interesting properties.

Made up of carbon atoms and a small number of nitrogen atoms in a regular grid of hexagons and triangles, the researchers call it “kagome lattice” graphene, after the Japanese weaving technique that uses the same pattern.

To produce the kagome graphene, the team applied a precursor to a silver substrate by vapor deposition and then heated it to form an organometallic intermediate on the metal surface. Further heating produced kagome graphene, which is made up exclusively of carbon and nitrogen atoms.

“We used scanning tunneling and atomic force microscopes to study the structural and electronic properties of the kagome lattice,” said Dr. Rémy Pawlak,

With the use of an atomic force microscope, the researchers found that electrons of a defined energy, which is selected by applying an electrical voltage, are “trapped” between the triangles that appear in the crystal lattice of kagome graphene. In conventional graphene, electrons are delocalized, or distributed across various energy states in the lattice.

“The localization observed in kagome graphene is desirable and precisely what we were looking for,” explained Professor Ernst Meyer of the University of Basel. “It causes strong interactions between the electrons — and, in turn, these interactions provide the basis for unusual phenomena, such as conduction without resistance.”

Additionally, the kagome graphene showed semiconductor properties. The team plans to detach the kagome lattice from its metallic substrate and study its electronic properties further.

“The flat band structure identified in the experiments supports the theoretical calculations, which predict that exciting electronic and magnetic phenomena could occur in kagome lattices. In the future, kagome graphene could act as a key building block in sustainable and efficient electronic components,” added Meyer.

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