Enhanced magnetic hybridization of a spinterface

Interfaces between organic semiconductors and ferromagnetic metals offer intriguing opportunities in the rapidly developing field of organic spintronics. Understanding and controlling the spin-polarized electronic states at the interface is the key toward a reliable exploitation of this kind of systems. It is indeed important to master and reliably reproduce the chemical reactions responsible of the spin-polarization at the interface.

Here we propose an approach consisting in the insertion of an ultrathin, two-dimensional Cr4O5 magnetic oxide layer at the interface between a C60 fullerene organic semiconductor and a Fe(001) ferromagnetic metal to both maximize the spin polarization and to overcome the reproducibility issues usually present in case of direct interface between metallic layer and organic semiconductor.
C60 fullerene showed a greater surface diffusivity when growing on Cr4O5 compared to the Fe(001) case. From the first stages of surface coverage, C60 tends to form islands rather than isolated molecules, leading to a well-ordered growth at higher thicknesses (Figure 1, above).

>Read more on the Elettra website

Figure 1STM image 200 x 200 nm2 of the surface of a C60/ Cr4O5/Fe(001) sample with a fullerene coverage of about 0.5 ML. The image was taken at room temperature with ΔV= 1.7 V, I = 400 pA.

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.

Multi-orbital charge transfer at highly oriented organic/metal interfaces

Organic-based device performances have been rapidly improving in the last years, making them suitable for large-scale industrial applications, involving photo-voltaic cells, light emission systems and building of larger flexible electronics. In parallel, basic research has intensively been focused on the chemical and physical properties of semiconducting π-conjugated organic molecules, as they appear to be promising for organic-based device construction. In particular, in controlling the charge injection on such devices, a predominant role is played by the molecule-substrate interaction. Charge transfer at the molecule-metal interface strongly affects the overall physical and magnetic properties of the system, and ultimately, the device performance.
Read more on the Elettra website.

Image: (a) STM image including two Ni-TPP domains, labeled with A and B, respectively. STM image parameters: Vb = −1.5 V, It = 0.2 nA, image size 15 × 20 nm2, measured at 4.3 K. (b) . Proposed adsorption model for Ni-TPP/Cu(100), side view. (c) Valence band photoemission spectra of clean Cu(100) and Ni-TPP/Cu(100) acquired at 26 eV photon energy. (d) PDOS onto molecular orbitals for the Ni-TPP/Cu(100) system. The energy position of the corresponding gas-phase molecular orbitals, aligned with respect to the vacuum level, is indicated with colored bars on the top axis. (e) Comparison between μ-ARPES measured patterns (left) and the correspondent calculated |FT|2 of the molecular orbitals (right).

Ubiquitous formation of type-I and type-II bulk Dirac cones

… and topological surface states from a single orbital manifold in transition-metal dichalcogenides

Transition-metal dichalcogenides (TMDs) are renowned for their rich and varied properties. They range from metals and superconductorsto strongly spin-orbit-coupled semiconductors and charge-density-wave systems with their single-layer variants one of the most prominent current examples of two-dimensional materials beyond graphene.Their varied ground states largely depend on the transition metal d-electron-derived electronic states, on which the vast majority of attention has been concentrated to date.

>Read more on the Elettra website.

Image: Chalcogen-derived topological ladder in PdTe2.(a) Orbitally-resolved bulk electronic structure of PdTe2, indicating dominantly chalcogen-derived orbital character for the states in the vicinity of the Fermi level. (b) The measured out-of-plane dispersion together with the calculated band structure. Measured (c) and calculated (d) in-plane dispersion. (e,f) Spin-resolved energy distribution curves along the lines shown in (c).

Quantum spectroscopy, for the measurements of dynamical current current thermalization

Nearly all spectroscopic measurements deals with the measurements of the average properties of the material. As an example, the reflectivity of a material is simply defined by the ratio between the number of photons which are reflected by the sample divided by the number of those arriving on it. The interest in measuring mostly average properties is the main drive of the standard scientific practice of repeating the measurements a lot of times so that the error made in one single measurements is averaged out by the repetition of the measurements. In this context the noise which determines fluctuation of the repeated measurements have always been considered as an impediment to a good quality measurements which needs to be mitigated by careful experimentalists.
The approach of repeated measurements is employed conspicuously in pump-probe experiments which are the prime way to study condensed matter out of its equilibrium state. In standards optical pump-probe experiments, ultrashort pulses are always used in pairs. The pump triggers the dynamical response and the probe is used to detect changes in the optical properties of the sample.

Read more on the Elettra website.

Image: Schematic view of the pump-probe set-up used for the experiments. The intensity of every single probe pulse was separately acquired with low-electronic-noise detectors for every pump-probe delay.

Maximal Rashba-like spin splitting via kinetic-energy-coupled inversion-symmetry breaking

Research collaboration led by Professor Philip King from University of St. Andrews, and comprising the researchers from Max Planck Institute for Chemical Physics of Solids in Dresden, Institute for Theoretical Physics of the University of Heidelberg and researchers from I05 beamline at Diamond Light Source and APE beamline at Elettra, described a new route to maximise the spin-splitting of surface states.
The electronic properties of surfaces are often different from those of the bulk. In particular, the intrinsically broken symmetries of the surface compared with the bulk of the material allow for appearance of the new electronic surface states. For the systems in which spin-orbit interaction is strong, a non-negligible separation of these states according to their spin takes place. The spin splitting of surface- or interface-localized two-dimensional electron gases is characterized by a locking of the electron spin perpendicular to its momentum.

Read more on the Elettra website.

(a) Bulk and surface Fermi surfaces of PtCoO2 measured by ARPES; (b) Expected spin texture of the surface states; (c) Spin-resolved ARPES measurements of an in-plane spin polarization (〈Sy〉) of the Fermi surface for the cut along kx.

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.

>Read More

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).

>Read More

Chemistry at the protein-mineral interface

The nucleation site of iron mineral in human L ferritin revealed by anomalous -ray diffraction

Iron ions have crucial functions in every living organism being essential for cellular respiration, DNA synthesis, detoxification of exogenous compounds, just to provide a few examples. However, the redox properties of iron ions can also cause the occurrence of deleterious free-radicals. For these reasons, when unnecessary, iron must be kept in appropriate forms unable to cause damage. Nature evolved a special protein cage, called ferritin, consisting of 24 subunits arranged to form a hollow sphere with an internal diameter of about 80 Å where mineralized iron is stored, generally under the form of insoluble ferric oxides.

In mammals, two types of subunits build-up the 24-mer ferritins: the ‘heavy’ (H) and the ‘light’ (L). These subunits differ not only in molecular weight (21.2 kDa for H and 20.0 kDa for L) but, mainly, in function. The H subunit is able to catalyze the rapid oxidation of Fe2+ to Fe3+ followed by transfer in the storage cavity. On the contrary, the L-chain does not possess catalytic activity, but it is still able to mineralize ferric ions upon spontaneous oxidation by dioxygen of captured Fe2+. Despite the intensive research on ferritin chemistry, the mechanisms of iron oxidation and storage to form mineral nanoparticles inside the ferritin cavity are still to be fully established.

>Read More