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Unraveling the magnetism of a graphene triangular flake

Posted By Graphene Council, Tuesday, May 12, 2020
Graphene is a diamagnetic material, this is, unable of becoming magnetic. However, a triangular piece of graphene is predicted to be magnetic. This apparent contradiction is a consequence of “magic” shapes in the structure of graphene flakes, which force electrons to “spin” easier in one direction.

Triangulene is a triangular graphene flake, which possesses a net magnetic moment: it is a graphene nanometer-size magnet. This magnetic state opens fascinating perspectives on the use of these pure-carbon magnets in technology.

However, the robust predictions of triangulene magnetism stumbled with the absence of clear experimental proofs, because the production of triangulene by organic synthesis methods in solution was difficult. The bi-radical character of this molecule caused it to be very reactive and difficult to fabricate, and the magnetism appears to be very elusive in those few successful cases.

In a new study, published in Physical Review Letters ("Uncovering the Triplet Ground State of Triangular Graphene Nanoflakes Engineered with Atomic Precision on a Metal Surface"), this challenge was revisited using a scanning tunneling microscope (STM).

After assembling a triangular-like piece of graphene on a clean gold surface, high-resolution scanning tunneling spectroscopy measurements revealed that this compound has a net magnetic state characterized by a spin S=1 ground state and, therefore, that this molecule is a small, pure carbon paramagnet. These results are the first experimental demonstration of a high-spin graphene flake.

The findings were further complemented with atomic manipulation steps of hydrogen-passivated triangulene side-products occasionally found in the experiment. By controlled removal of these additional hydrogen atoms in the experiments, the spin state of the flake could be modified from a closed-shell, doubly hydrogenated structure, to an intermediate S=1/2 spin state, and finally to the high-spin S=1 state of the ideal molecular structure.

The experimental proof of a spin-state in the absence of a magnetic quantization axis (detectable by spin-polarized STM) or magnetic anisotropy (detectable by spin-flip inelastic tunneling spectroscopy) is not simple. In this work, the spin signature was obtained from the underscreened Kondo effect – an exotic version of the standard Kondo effect described in the 1960s – that can arise in high-spin systems. Its observation in a graphene flake on a metal has not been reported before and brings here novel insights to understanding spins interacting with surfaces.

The work was the result of a fruitful collaboration between theoretical and experimental groups at the Donostia International Physics Center (DIPC) and CIC nanoGUNE, both research institutes in San Sebastian, as well as an organic synthesis group at CiQUS, in Santiago de Compostela.

Tags:  CIC nanoGUNE  Donostia International Physics Center  Graphene  Graphene Nanoflakes 

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Posted By Graphene Council, Tuesday, November 26, 2019
Updated: Tuesday, November 26, 2019
The meeting marks the starting point of a 4-year research project that is coordinated by CIC nanoGUNE and integrates IBM Research, Donostia International Physics Center, and University of Santiago de Compostela, Technical University of Delft and the University of Oxford. The consortium of these 6 leading European research institutions has been granted a total of €3.5 million from the European Commission under the highly competitive Horizon 2020 FET-Open call, which funds cutting-edge high-risk / high-impact interdisciplinary research projects that must lay the foundations for radically new future technologies.

The SPRING project combines recent scientific breakthroughs from the consortium members to fabricate custom-crafted magnetic graphene nanostructures and test their potential as basic elements in quantum spintronic devices. The targeted long-term vision is the development of an all-graphene – environmentally friendly – platform where spins can be used for transporting, storing and processing information.

As the name suggests, spin can be loosely understood as the rotation of a fundamental particle of matter around itself. For instance, every electron in any material carries both a charge and a spin, the latter playing a key role in magnetism.

Within the scientific community there is consensus that spin is the ideal property of matter to expand the performance of current charge-based nanoelectronics into a class of faster and more power-efficient components, being the basis for the emerging technology called quantum spintronics. The SPRING project will investigate the fundamental laws for creating and detecting spins in graphene, this is to read and write spins, and using them to transmit information.

Jose Ignacio Pascual, Ikerbasque Research Professor at CIC nanoGUNE and scientific coordinator of the project, explains that “graphene is ideal to host spins and to transport them. This atomically thin material can now be fabricated with atomic precision, opening the door to fabrication of designer structures with precise shape, composition, spin arrangement, and interconnected by graphene electrodes for electrostatic or quantum gates. The potential is a platform for the second quantum revolution as qubit elements for quantum computation.”

Tags:  CIC nanoGUNE  Donostia International Physics Center  Graphene  IBM Research  Jose Ignacio Pascual  nanoelectronics  Technical University of Delft  University of Oxford  University of Santiago de Compostela 

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Graphene gives a tremendous boost to future terahertz cameras

Posted By Graphene Council, Tuesday, April 23, 2019
Updated: Saturday, April 20, 2019
Scientists have developed a novel graphene-enabled photodetector that operates at room temperature, is highly sensitive, very fast, has a wide dynamic range and covers a broad range of THz frequencies.

Detecting terahertz (THz) light is extremely useful for two main reasons:

First, THz technology is becoming a key element in applications regarding security (such as airport scanners), wireless data communication, and quality control, to mention just a few. However, current THz detectors have shown strong limitations in terms of simultaneously meeting the requirements for sensitivity, speed, spectral range, being able to operate at room temperature, etc.

Second, it is a very safe type of radiation due to its low-energy photons, with more than a hundred times less energy than that of photons in the visible light range.

Many graphene-based applications are expected to emerge from its use as material for detecting light. Graphene has the particularity of not having a bandgap, as compared to standard materials used for photodetection, such as silicon. The bandgap in silicon causes incident light with wavelengths longer than one micron to not be absorbed and thus not detected. In contrast, for graphene, even terahertz light with a wavelength of hundreds of microns can be absorbed and detected. Whereas THz detectors based on graphene have shown promising results so far, none of the detectors so far could beat commercially available detectors in terms of speed and sensitivity.

In a recent study, ICFO researchers Sebastian Castilla and Dr. Bernat Terres, led by ICREA Prof. at ICFO Frank Koppens and former ICFO scientist Dr. Klaas-Jan Tielrooij (now Junior Group Leader at ICN2), in collaboration with scientists from CIC NanoGUNE, NEST (CNR), Nanjing University, Donostia International Physics Center, University of Ioannina and the National Institute for Material Sciences, have been able to overcome these challenges. They have developed a novel graphene-enabled photodetector that operates at room temperature, and is highly sensitive, very fast, has a wide dynamic range and covers a broad range of THz frequencies.

In their experiment, the scientists were able to optimize the photoresponse mechanism of a THz photodetector using the following approach. They integrated a dipole antenna into the detector to concentrate the incident THz light around the antenna gap region. By fabricating a very small (100 nm, about one thousand times smaller than the thickness of a hair) antenna gap, they were able to obtain a great intensity concentration of THz incident light in the photoactive region of the graphene channel. They observed that the light absorbed by the graphene creates hot carriers at a pn-junction in graphene; subsequently, the unequal Seebeck coefficients in the p- and n-regions produce a local voltage and a current through the device generating a very large photoresponse and, thus, leading to a very high sensitivity, high speed response detector, with a wide dynamic range and a broad spectral coverage.

The results of this study open a pathway towards the development a fully digital low-cost camera system. This could be as cheap as the camera inside the smartphone, since such a detector has proven to have a very low power consumption and is fully compatible with CMOS technology.

Tags:  Bernat Terres  CIC NanoGUNE  Donostia International Physics Center  Frank Koppens  Graphene  ICERA  ICFO  Klaas-Jan Tielrooij  Nanjing University  National Institute for Material Sciences  photodetectors  Sebastian Castilla  University of Ioannina 

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