Ferromagnetic and antiferromagnetic coupling of spin molecular interfaces

Researchers from the physics department of the Università “La Sapienza” in Rome, Centro S3 of Modena and ALBA, have demonstrated that magnetic coupling of metal-organic molecules to a magnetic substrate mediated by a graphene layer can be tuned in strength and direction by choosing the symmetry of the molecular orbitals that is largely preserved thanks to the graphene layer. The results have been published in the journal Nano Letters.
Paramagnetic molecules become potential building blocks in spintronics when their magnetic moments are stabilized against thermal fluctuations, for example, by a controlled interaction with a magnetic substrate. Spin molecular interfaces with preserved magnetic activity and exhibiting magnetic remanence at room temperature (RT) can open the route to engineer highly spin-polarized, nanoscale current sources. The need to fully control the organic spin interface and the tuning of ferromagnetic (FM) or antiferromagnetic (AFM) coupling to achieve a stable conductance has motivated a vast experimental interest.

Image: Figure 1: a,b) Antiferromagnetic/Ferromagnetic coupling as deduced by element-specific hysteresis loops of  a FePc and CuPc (respectively) to a Cobalt layer with perpendicular magnetic anisotropy intercalated below graphene. c,d) orbital-porjection of the spin-density for the FePc and CoPc interface reflecting the different symmetry of the molecular orbitals involved in the ferromagnetic and antiferromagnetic interaction.

X-ray laser opens new view on Alzheimer proteins

Graphene enables structural analysis of naturally occurring amyloids

A new experimental method permits the X-ray analysis of amyloids, a class of large, filamentous biomolecules which are an important hallmark of diseases such as Alzheimer’s and Parkinson’s. An international team of researchers headed by DESY scientists has used a powerful X-ray laser to gain insights into the structure of different amyloid samples. The X-ray scattering from amyloid fibrils give patterns somewhat similar to those obtained by Rosalind Franklin from DNA in 1952, which led to the discovery of the well-known structure, the double helix. The X-ray laser, trillions of times more intense than Franklin’s X-ray tube, opens up the ability to examine individual amyloid fibrils, the constituents of amyloid filaments. With such powerful X-ray beams any extraneous material can overwhelm the signal from the invisibly small fibril sample. Ultrathin carbon film – graphene – solved this problem to allow extremely sensitive patterns to be recorded. This marks an important step towards studying individual molecules using X-ray lasers, a goal that structural biologists have long been pursuing. The scientists present their new technique in the journal Nature Communications.

Amyloids are long, ordered strands of proteins which consist of thousands of identical subunits. While amyloids are believed to play a major role in the development of neurodegenerative diseases, recently more and more functional amyloid forms have been identified. “The ‘feel-good hormone’ endorphin, for example, can form amyloid fibrils in the pituitary gland. They dissolve into individual molecules when the acidity of their surroundings changes, after which these molecules can fulfil their purpose in the body,” explains DESY’s Carolin Seuring, a scientist at the Center for Free-Electron Laser Science (CFEL) and the principal author of the paper. “Other amyloid proteins, such as those found in post-mortem brains of patients suffering from Alzheimer’s, accumulate as amyloid fibrils in the brain, and cannot be broken down and therefore impair brain function in the long term.”

Image: On the ultra-thin, extremely regular layer of graphene, the fibrils align themselves in parallel in large domains. The intense X-ray light from the X-rax free-electron laser LCLS at the SLAC National Accelerator Center enabled the researchers to gain partial information about the fibril structure from ensembles of just a few fibrils.
Credit: Greg Stewart/SLAC National Accelerator Laboratory

Subfilamentary Networks in Memristive Devices

Redox-based memristive devices are one of the most attractive emerging memory technologies.

…in terms of scaling, power consumption and speed. In these devices, external electrical stimuli cause changes of the resistance of an oxide layer sandwiched between two metal electrodes. In the simplest application, the device can be set into a low resistance state (LRS) and reset into a high resistance state (HRS), which may encode a logical one and zero, respectively. The major obstacle delaying large-scale application, however, is the large cycle-to-cycle (C2C) and device-to-device (D2D) variability of both LRS and HRS resistance values. These variabilities describe the stochastic nature of the switching process within one cell, resulting in different resistances obtained for each switching cycle and different resistances obtained for different cells on the same chip.

Read more on the Elettra website.

Image:(a) Schematic of the device geometry. A SrTiO3 layer (blue) is sandwiched between a Nb:SrTiO3 bottom electrode (dark grey) and graphene top electrode (grey honeycomb lattice). The graphene electrode is contacted through a metal lead, which is electrically separated from the continuous bottom electrode, allowing for biasing inside PEEM instruments. (b) Quasistatic I-V curve of a representative graphene/SrTiO3/Nb:SrTiO3 device. The bottom electrode serves as virtual ground, while the bias is applied to the graphene top electrode. (c) PEEM image of a graphene/Al2O3/SrTiO3 device in the LRS at an electron energy E – EF of 3.4 eV. Scale bar, 5 µm. (d) PEEM image of the same device after Reset. (e) and (f) PEEM images after one additional Set and Reset operation, respectively. Insets: magnified photoemission threshold map of the area around the conductive filament. The maps were obtained by fitting the threshold spectrum for each pixel.

Topological insulator gap in graphene contacted with Pb

Up to now the proposed modifications do not allow to introduce graphene to existing electronic devices.

Graphene is the most promising two dimensional material for nanoelectronic applications featuring the relativistic-like electronic spectrum. Contact of graphene with various materials and its functionalization allows to manipulate the electronic structure, e.g. to change the conductivity type and band gap creation. The latter is of great interest due to the requirements for graphene transistor realisation. Furthermore, graphene contact with heavy/magnetic metals results in the lifting of the spin degeneracy of the Dirac cone, opening the spintronics field for its applications. However, up to now the proposed modifications do not allow to introduce graphene to existing electronic devices.

>Read more on the Elettra website.

Image: a) Sketch of the studied system, the Pb atoms presented by yellow spheres; b) ARPES image of graphene/Pb/Pt(111) in the region of K point, taken as a sum of two spectra with p-and s-polarization of light; c) schematic spin structure of the graphene states in the case of large “intrinsic” spin-orbit interaction d) ARPES mapping of the system in two orthogonal k-directions near the K point of graphene.

The miracle material graphene: convex as a chesterfield

Graphene possesses extreme properties and can be utilised in many ways.

Even the spins of graphene can be controlled through use of a trick. This had already been demonstrated by a HZB team some time ago: the physicists applied a layer of graphene onto a nickel substrate and introduced atoms of gold in between (intercalation).

The scientists now show why this has such a dramatic influence on the spins in a paper published in 2D Materials. As a result, graphene can also be considered as a material for future information technologies that are based on processing spins as units of information.

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Fluorination of suspended graphene

Functionalization is a well-established method to manipulate the electronic properties of graphenes

It consists in the substitution of carbon atoms in the hexagonal network by other elements such as heteroatoms (nitrogen or boron, the most common) or in the introduction of more complex functional groups.

The customization of the graphene exceptional electronic properties by the functionalization opens different avenues for future applications including bio and chemical-sensors. Among various functionalization methods, plasma process and ion irradiation have been widely employed for the modification of surface chemical composition and properties. These techniques have attracted the attention of a vast scientific audience because they can be used to tailor the surface reactivity in different materials making them suitable for various applications ranging from chemical sensing to medical implants. In particular, the fluorination of graphene allows the tuning of the optical bandgap, introducing a progressive semiconducting behaviour for increasing fluorine content ending in insulating properties for fully fluorinated graphene.

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Liquid-phase chemistry: Graphene nanobubbles

X-Ray Photoelectron Spectroscopy (XPS) and X-Ray Absorption Spectroscopy (XAS) provide unique knowledge on the electronic structure and chemical properties of materials.

Unfortunately this information is scarce when investigating solid/liquid interfaces, chemical or photochemical reactions in ambient conditions because of the short electron inelastic mean free path (IMFP) that requires a vacuum environment, which poses serious limitation on the application of XPS and XAS to samples operating in atmosphere or in the presence of a solvent. One promising approach to enable the use of conventional electron spectroscopies is the use of thin membrane, such as graphene (Gr), which is transparent to both X-ray photons and photoelectrons. For these purposes, this work proposes an innovative system based on sealed Gr nanobubbles (GNBs) on a titanium dioxide TiO2 (100) rutile single crystal filled with the solution of interest during the fabrication stage (Figure 1a).

The formation of irregularly shaped vesicles with an average height of 6 nm and lateral size of a few hundreds of nanometers was proved by using a multi-technique approach involving Atomic Force Microscopy (AFM, see Figure 1b,c,d), Raman (Figure 1e) and synchrotron radiation spectroscopies (Figure 2), which have unequivocally demonstrated the presence of water inside the GNBs and the transition to a flat Gr layer after water evaporation by thermal heating up to 350 °C in ultra high vacuum (UHV).

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