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Stopping the unstoppable with atomic bricks

Posted By Graphene Council, Monday, June 29, 2020
Graphene's unique 2D structure means that electrons travel through it differently to most other materials. One consequence of this unique transport is that applying a voltage to them doesn't stop the electrons like it does in most other materials. This is a problem because to make useful applications out of graphene and its unique electrons like quantum computers, it is necessary to be able to stop and control graphene electrons.

An interdisciplinary team of scientists from the Universidad Autonoma de Madrid (Spain), Université Grenoble Alpes (France), International Iberian Nanotechnology Laboratory (Portugal) and Aalto University has managed to solve this long-standing problem. They combined experimental researchers including Eva Cortés del Río, Pierre Mallet, Héctor González‐Herrero, José María Gómez‐Rodríguez, Jean‐Yves Veuillen and Iván Brihuega with theorists, including Joaquín Fernández-Rossier and Jose Lado, assistant Professor in the department of Applied Physics at Aalto.

The experimental team used atomic bricks to build walls capable of stopping the graphene electrons. This was achieved by creating atomic walls that confined the electrons, leading to structures whose spectrum was then compared with theoretical predictions, demonstrating that electrons were confined. In particular, it was obtained that the engineered structures gave rise to nearly perfect confinement of electrons, as demonstrated from the emergence of sharp quantum well resonances with a remarkably long lifetime.

The work, published this week in Advanced Materials, demonstrates that impenetrable walls for graphene electrons can be created by collective manipulation of a large number of hydrogen atoms. In the experiments, a scanning tunnelling microscope was used to construct artificial walls with sub nanometric precision. This led to graphene nanostructures of arbitrarily complex shapes, with dimensions ranging from two nanometres to one micron.

Importantly, the developed method is non-destructive, allowing to erase and rebuild the nanostructures at will, providing an unprecedented degree of control to create artificial graphene devices. The experiments demonstrate that the engineered nanostructures are capable of perfectly confining the graphene electrons in these artificially designed structures, overcoming the critical challenge imposed by Klein tunnelling. Ultimately, this opens up a plethora of exciting new possibilities, as the created nanostructures realize graphene quantum dots that can be selectively coupled, opening ground-breaking possibilities for artificially designed quantum matter.

Tags:  2D materials  Aalto University  Graphene  Jose Lado  nanostructures  quantum materials  Universidad Autonoma de Madrid 

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Researchers discover new boron-lanthanide nanostructure

Posted By Graphene Council, Friday, June 26, 2020
The discovery of carbon nanostructures like two-dimensional graphene and soccer ball-shaped buckyballs helped to launch a nanotechnology revolution. In recent years, researchers from Brown University and elsewhere have shown that boron, carbon’s neighbor on the periodic table, can make interesting nanostructures too, including two-dimensional borophene and a buckyball-like hollow cage structure called borospherene.

Now, researchers from Brown and Tsinghua University have added another boron nanostructure to the list. In a paper published in Nature Communications, they show that clusters of 18 boron atoms and three atoms of lanthanide elements form a bizarre cage-like structure unlike anything they’ve ever seen. 

“This is just not a type of structure you expect to see in chemistry,” said Lai-Sheng Wang, a professor of chemistry at Brown and the study’s senior author. “When we wrote the paper we really struggled to describe it. It’s basically a spherical trihedron. Normally you can’t have a closed three-dimensional structure with only three sides, but since it’s spherical, it works.”

The researchers are hopeful that the nanostructure may shed light on the bulk structure and chemical bonding behavior of boron lanthanides, an important class of materials widely used in electronics and other applications. The nanostructure by itself may have interesting properties as well, the researchers say. 

“Lanthanide elements are important magnetic materials, each with very different magnetic moments,” Wang said. “We think any of the lanthanides will make this structure, so they could have very interesting magnetic properties.”

Wang and his students created the lanthanide-boron clusters by focusing a powerful laser onto a solid target made of a mixture of boron and a lanthanide element. The clusters are formed upon cooling of the vaporized atoms. Then they used a technique called photoelectron spectroscopy to study the electronic properties of the clusters. The technique involves zapping clusters of atoms with another high-powered laser. Each zap knocks an electron out of the cluster. By measuring the kinetic energies of those freed electrons, researchers can create a spectrum of binding energies for the electrons that bond the cluster together.

“When we see a simple, beautiful spectrum, we know there’s a beautiful structure behind it,” Wang said. 

To figure out what that structure looks like, Wang compared the photoelectron spectra with theoretical calculations done by Professor Jun Li and his students from Tsinghua. Once they find a theoretical structure with a binding spectrum that matches the experiment, they know they’ve found the right structure. 

“This structure was something we never would have predicted,” Wang said. “That’s the value of combining theoretical calculation with experimental data.”

Wang and his colleagues have dubbed the new structures metallo-borospherenes, and they’re hopeful that further research will reveal their properties.

Tags:  2D materials  boron nanostructure  Brown University  Graphene  Lai-Sheng Wang  nanostructures  Tsinghua University 

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