Elettra – Page 2 – Lightsources.org

The power of surfaces: getting meta-polyaniline from para-aminophenol

2021/02/052021/02/08

Within the relatively young research field of the on-surface Chemistry the catalytic properties of surfaces are used to synthesize new materials that are not accessible with other techniques; in particular, by wet chemistry conventional routes. Following this bottom-up strategy, surfaces can be used as suitable platforms for enabling the emergence of novel low-dimensional molecular networks. In synergistic contribution,para-aminophenol (p-AP) molecules are taken as precursors (building block) for the surface reaction. Within a multi-technique approach that includes STM, nc-AFM, STS, XPS, and DFT calculations, we have found that these molecules, when adsorbed on Pt(111) and upon a thermal stimulus, covalently react each other forming oligomers coupled in an unprecedented metaconfiguration.

STM and nc-AFM microscopies showed that starting from individual molecules and by annealing the surface, results in the formation of oligomer chains , while monitoring the thermal behavior of the p-AP/Pt(111) system by means of high-resolution XPS at the SuperESCA beamline of Elettra shed light on the chemical nature of the species present on the crystal surface in the steps which lead to the synthesis of meta-polyaniline oligomers.

Disclosing the time evolution of magnetic chirality after an optical excitation

2021/02/032021/02/08

Chiral magnetic structures, such as spin spirals, chiral domain walls and skyrmions, are intensively investigated due to their fascinating properties such as potentially enhanced stability and efficient spin-orbit torque driven dynamics. These structures are stabilized by the Dzyaloshinskii-Moriya interaction (DMI) that favours a chiral winding of the magnetisation.Cir

In a recent work, circularly polarized light pulses of the FERMI free-electron laser (FEL) has been used to disclose the dynamics of chiral order on ps time scale. After an optical excitation, the researchers observe a faster recovery of the chirality within the domain walls compared to the ferromagnetic order in the domains. The study paves the way for future investigations of fundamental aspects such as, e.g., the dependence of the timescales of the chiral order build-up on the absolute strength of the DMI. The control of the DMI can finally allow the manipulation at ultrafast timescale of chiral topological objects such as skyrmions and pave the path to applications in the field of ultrafast chiral spintronics.

Rad more on the Elettra website

Direct X-ray and electron-beam lithography of halogenated zeolitic imidazolate framework

2021/01/292021/01/29

Metal-organic frameworks (MOFs) offer disruptive potential in micro- and optoelectronics because of their chemical versatility and high porosity. For instance, the low dielectric constant (low-k) resulting from their porosity makes MOFs competitive candidates for high-performance insulators in future microchips. Both the MOF and microelectronics communities have been striving towards integrating MOFs in microchips, which requires two key engineering steps: thin film deposition and lithographic patterning. However, conventional lithography techniques use a sacrificial layer, so-called photoresist, to transfer a pattern into the desired material. The use of photoresist complicates the process, and might induce contamination of the highly porous MOF films.

A group of researchers from KU Leuven (Belgium) coordinated by Rob Ameloot has used the deep X-ray lithography (DXRL) beamline at Elettra to demonstrate that MOFs can be patterned by X-ray lithography without the use of resist layer. The method is based on selective X-ray exposure of the MOF film, which induces chemical changes that enable its removal by a common solvent. This process completely avoids the resist layer, thus significantly simplifying patterning while maintaining the physicochemical properties of patterned MOFs intact.

Titanium defective sites in TS-1: structural insights by combining spectroscopy and simulation

2020/12/212020/12/21

Titanium Silicalite-1 (TS-1) is a titanium zeolite, whose peculiarity is the presence of Ti atoms isomorphously substituting the Si ones at tetrahedral framework positions. However, real TS-1 samples are characterized by the co-presence of other Ti sites, ranging from extended TiO₂phases down to defective Ti sites. The “defective Ti” label covers a broad range of possible Ti moieties, whose structural description is in most of the cases barely qualitative in the literature. In this work, we combined experimental and theoretical approaches, aiming to unravel the exact structure of defective Ti sites.

A new tool in attosecond science

2020/11/272020/11/30

Measuring Angle-Resolved Phases in Photoemission

Photoionization is one of the earliest observations whose explanation led to the establishment of quantum mechanics. The process is fully described by few mathematical quantities—the probability amplitudes—that are of central interest in understanding the electronic structure of matter and its theoretical foundations. Probability amplitudes are complex numbers, which are described by a magnitude and a phase. Phase information (which can be equivalently expressed as a time, i.e., a fraction of the period of the light wave causing ionization) is lost in most measurements.

An international research team from Japan, Germany, Russia, Austria, Hungary, and the local team at the FERMI free-electron laser, combined two-color XUV photoelectron spectroscopy with real-time ab initio simulations to measure phase differences with a precision of few attoseconds. The measurements, in excellent agreement with calculations, revealed a significant anisotropy with the angle of observation of the outgoing photoelectron, particularly when the frequency of the light is nearly resonant with a transition in the atom.

Image: Scheme of the experiment: Bichromatic, linearly polarized light (red and blue waves), with momentum k_g and electric vector E_g, ionizes neon in the reaction volume. The electron wave packets (yellow and magenta waves) are emitted with electron momentum k. The averaged phase difference between wave packets created by one- and two-photon ionization depends on the emission angle. The photoelectron angular distribution depends on the relative (optical) w‑2w phase f. Lower figures: Polar plots of photoelectron intensity at E_k=16.6 eV for coherent harmonics (asymmetric, colored plot) and incoherent harmonics (symmetric, gray plot).

Credit: Reproduced from You et al., Phys. Rev. X, 10, 031070 (2020) doi: 10.1103/PhysRevX.10.031070; copyright 2020 by the Authors. The original figure has been published under a Creative Commons Attribution 4.0 International license (CC BY 4.0) http://creativecommons.org/licenses/by/4.0/

Electron and X‑ray Focused Beam-Induced Cross-Linking in Liquids:

2020/10/262020/10/27

Toward Rapid Continuous 3D Nanoprinting and Interfacing using Soft Materials

Modern additive fabrication of three-dimensional (3D) micron to centimeter size constructs made of polymers and soft materials has immensely benefited from the development of photocurable formulations suitable for optical photolithography,holographic,and stereolithographymethods. Recent implementation of multiphoton laser polymerization and its coupling with advanced irradiation schemes has drastically improved the writing rates and resolution, which now approaches the 100 nm range. Alternatively, traditional electron beam lithography and its variations such as electron-beam chemical lithography, etc. rely on tightly focused electron beams and a high interaction cross-section of 0.1−10 keV electrons with the matter and have been routinely used for complex patterning of polymer resists, self-assembled monolayers, and dried gel films with up to a few nanometers accuracy.

Similarly, a significant progress has been made in deep X-ray lithography, direct writing with zone plate focused X-ray beams for precise, and chemically selective fabrication of high aspect ratio microstructures. Reduced radiation damage within the so-called “water window” has spurred wide biomedical X-ray spectroscopy, microscopy, and tomography research including material processing, for example, gels related controlled swelling and polymerization inside live systems, particles encapsulations,and high aspect ratio structures fabrication.The potential of focused X-rays for additive fabrication through the deposition from gas-phase precursors or from liquid solutions is now well recognized and is becoming an active area of research.

Image: The electron/X-ray beam gelation in liquid polymer solution through a SiN ultrathin membrane. Varying the energy and focus of the soft X-rays smaller or larger excitation volumes and therefore finer or wider feature sizes and patterns can be generated.

Orbital angular momentum carried by an optical field can be imprinted onto a propagating electron wave

2020/10/232020/10/23

Photons have fixed spin and unbounded orbital angular momentum (OAM). While the former is manifested in the polarization of light, the latter corresponds to the spatial phase distribution of its wavefront. The distinctive way in which the photon spin dictates the electron motion upon light–matter interaction is the basis for numerous well-established spectroscopies. By contrast, imprinting OAM on a matter wave, specifically on a propagating electron, is generally considered very challenging and the anticipated effect undetectable.

We carried out an experiment at the LDM beam line at the FERMI free-electron laser, with the aim of inducing an OAM-dependent dichroic photoelectric effect on photo-electrons emitted by a sample of He atoms. The experiment involved a large international collaboration and surprisingly confirmed that the spatial distribution of an optical field with vortex phase profile can be imprinted coherently on a photoelectron wave packet that recedes from an atom. Our results explore new aspects of light–matter interaction and point to qualitatively novel analytical tools, which can be used to study, for example, the electronic structure of intrinsic chiral organic molecules. The results have been published in Nature Photonics.

Image: A VUV free-electron laser (violet) is used to ionize a sample of He atoms, and an infrared beam (red) to imprint orbital angular momentum on photo-emitted electrons. Credit: J. Wätzel (Halle university)

Transition-metal dichalcogenide NiTe2: an ambient-stable material for catalysis and nanoelectronics

2020/10/152020/10/27

Recently, transition-metal dichalcogenides hosting topological states have attracted considerable attention for their potential implications for catalysis and nanoelectronics. The investigation of their chemical reactivity and ambient stability of these materials is crucial in order to assess the suitability of technology transfer. With this aim, an international team of researchers from Italy, Russia, China, USA, India, and Taiwan has studied physicochemical properties of NiTe₂ by means of several experimental techniques and density functional theory. Surface chemical reactivity and ambient stability were followed by x-ray photoemission spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) experiments at the BACH beamline, while the electronic band structure was probed by spin- and angle-resolved photoelectron spectroscopy (spin-ARPES) at the APE-LE beamline.

Image: a) Ni-3p core-level spectra collected from as-cleaved NiTe2 (black curves) and from the same surface exposed to 2·10⁴L of CO (red curves), H₂O (green curves) and O₂ (blue curves). Credit: Adapted from “S. Nappini et al., Adv. Funct. Mater. 30, 2000915 (2020); DOI: 10.1002/adfm.202000915” with permission from Wiley (Copyright 2020) with license 4873681106527

Observation of flat bands in twisted bilayer graphene

2020/10/142020/10/28

Magic-angle materials represent a surprising recent physics discovery in double layers of graphene, the two-dimensional material made of carbon atoms in a hexagonal pattern.

When the upper layer of two stacked layers of graphene is rotated by about 1 degree, the material suddenly turns into a superconductor. At a temperature of 3 Kelvin, this so-called twisted bilayer graphene (tbg) conducts electricity without resistance.

Now, an international team of scientists from Geneva, Barcelona, and Leiden have finally confirmed the mechanism behind this new type of superconductors. In Nature Physics, they show that the slight twist causes the electrons in the material to slow down enough to sense each other. This enables them to form the electron pairs which are necessary for superconductivity.

How can such a small twist make such a big difference? This is connected with moiré patterns, a phenomenon also seen in the everyday world. When two patterned fences are in front of another, one observes additional dark and bright spots, caused by the varying overlap between the patterns. Such moiré patterns (derived from the the French name of textile patterns made in a similar way) generally appear where periodical structures overlap imperfectly.

Who stole the light?

2020/10/072020/10/07

Self-induced ultrafast demagnetization limits the amount of light diffracted from magnetic samples at soft x-ray energies.

Free electron X-ray lasers deliver intense ultrashort pulses of x-rays, which can be used to image nanometer-scale objects in a single shot. When the x-ray wavelength is tuned to an electronic resonance, magnetization patterns can be made visible. However, using increasingly intense pulses, the magnetization image fades away. The mechanism responsible for this loss in resonant magnetic scattering intensity has now been clarified.

A team of researchers from Max Born Institute Berlin (Germany), Helmholtz-Zentrum Berlin (Germany), Elettra Sincrotrone Trieste (Italy) and Sorbonne Université (France), has now precisely recorded the dependence of the resonant magnetic scattering intensity as a function of the x-ray intensity incident per unit area (the “fluence”) on a ferromagnetic domain sample. Via integration of a device to detect the intensity of every single shot hitting the actual sample area, they were able record the scattering intensity over three orders of magnitude in fluence with unprecedented precision, in spite of the intrinsic shot-to-shot variations of the x-ray beam hitting the tiny samples. The experiments with soft x-rays were carried out at the FERMI free-electron x-ray laser in Trieste, Italy.

In the results presented in the journal Physical Review Letters, the researchers show that while the loss in magnetic scattering in resonance with the Co 2p core levels has been attributed to stimulated emission in the past, for scattering in resonance with the shallower Co 3p core levels this process is not significant. The experimental data over the entire fluence range are well described by simply considering the actual demagnetization occurring within each magnetic domain, which the experimental team had previously characterized with laser-based experiments. Given the short lifetime of the Co 3p core, dominated by Auger decay, it is likely that the hot electrons generated by the Auger cascade, in concert with subsequent electron scattering events, lead to a reshuffling of spin up and spin down electrons transiently quenching the magnetization.

Image: Schematic sketch of the scattering experiment with two competing processes. The soft x-ray beam (blue line) hits the magnetic sample where it scatters from the microscopic, labyrinth-like magnetization pattern. In this process, an x-ray photon is first absorbed by a Co 3p core level (1). The resulting excited state can then relax either spontaneously (2), emitting a photon in a new direction (purple arrow), or by means the interaction with a second photon via stimulated emission (3). In this last case, the photons are emitted in the direction of the incident beam (blue arrow towards right).

Liquid carbon can be disclosed if one is ultrafast enough

2020/10/012020/10/07

At the FERMI FEL, beamline EIS-TIMEX, a novel approach combining FEL and fs-laser radiation has been developed for generating liquid carbon under controlled conditions and monitoring its properties of at the atomic scale. The method has been put to the test depositing a huge amount (5 eV/atom, 40 MJ/kg) of optical energy delivered by an ultrashort laser pulse (less than 100 fs, 10^-13 s) into a self-standing amorphous carbon foil (a-C, thickness about 80 nm) and subsequently probing the excited sample volume with the FEL pulse varying both the FEL photon energy across the C K-edge (~ 283 eV) and delay between FEL and laser. A time-resolved x-ray absorption spectroscopy (tr-XAS, Fig. 2a) has been obtained of l-C with a record time resolution of less than 100 fs.

This method allowed researchers to monitor the formation of the liquid carbon phase at a temperature of 14200 K and pressure of 0.5 Mbar occurring in about 300 fs after absorption of the laser pump pulse as an effect of the constant volume (isochoric) heating of the carbon sample.

Image: Artistic image illustrating the ultrafast laser-heating process used to generate liquid carbon in the laboratory. Illustration: Emiliano Principi.

Tracking attosecond wave packets with extreme ultraviolet pulses

2020/09/242020/09/24

The fastest dynamical process in atoms, molecules and complexes is the electronic motion. It occurs on time scales reaching down to the attosecond regime (1 as = 10^-18s). The advent of novel light sources, providing extreme ultraviolet (XUV) or even X-ray pulses with as pulse duration paves the way to study these dynamics in real-time. Therefore, researchers around the world are currently developing new spectroscopic techniques using pulses of XUV or X-ray radiation.

An international research collaboration from Germany, Italy, Sweden, Switzerland, Denmark and the local team at the FERMI free-electron laser, has succeeded in observing the ultrafast electronic wave-packet evolution induced by the coherent excitation of an electron out of an inner shell in argon atoms. The measured quantum interference pattern exhibits oscillations that have a period of only ≈ 150 as. In order to achieve this, the collaboration extended a spectroscopy technique known from the visible spectral range – coherent wave-packet interferometry – to the XUV regime. This required a so far unprecedented level of control over the phase and timing properties of free-electron laser pulse pairs, which was achieved by exploiting the coherence of the high-gain harmonic generation process at FERMI. This novel spectroscopy technique will provide substantial insights and real-time information about intra and inter particle decay mechanisms in the XUV range.

Image: Artistic rendering of the electronic motion in the electronic shell of an atom, induced and probed by a double pulse sequence.

Captured in the act: Free Electron Laser sheds light on ultrafast relaxation of superfluid helium nanodroplets

2020/09/082020/09/08

Superfluid He nanodroplets are ideal model systems for studying the photodynamics of weakly-bound nanostructures, both experimentally and theoretically; in most cases, superfluidity results in slow relaxation of energy and angular momentum. Using ultrashort tunable XUV pulses, it is now possible to follow the relaxation dynamics of excited helium nanodroplets in great detail.

The relaxation of photoexcited nanosystems is a fundamental process of light-matter interaction. Depending on the couplings of the internal degrees of freedom, relaxation can be ultrafast, converting electronic energy into atomic motion within a few fs, or slow, if the energy is trapped in a metastable state that decouples from its environment. An international research team from Germany, Spain, Italy, the USA, and the local team at the FERMI free-electron laser (FEL), studied helium nanodroplets resonantly excited by femtosecond extreme-ultraviolet (XUV) pulses from FERMI. The researchers found that, despite their superfluid nature, helium nanodroplets in their lower electronically excited states undergo ultrafast relaxation by forming a void bubble, which eventually bursts at the droplet surface thereby ejecting a single metastable helium atom. These results help understanding how nanoparticles interact with energetic radiation, as happens when single nanoparticles are directly imaged at hard-x-ray FEL facilities.

Image: Figure left: Simulated density distribution of a helium nanodroplet shorty after it is excited by an XUV laser pulse (Courtesy by M. Barranco). Figure right: Measured photoelectron spectra showing ultrafast energy relaxation within less than a picosecond.

Laser, camera, action: Ultrafast ring opening of thiophenone tracked by time-resolved XUV photoelectron spectroscopy

2020/08/212020/08/24

Light-induced ring opening reactions form the basis of important biological processes such as vitamin D synthesis, and are also touted as promising candidates for the development of molecular switches. In recent years, new time-resolved techniques have emerged to investigate these processes with unprecedented temporal and spatial resolution.

An international research team from the USA, UK, Germany, Sweden, Australia, and the local team at the FERMI free-electron laser, combined time-resolved photoelectron spectroscopy with high-level electronic structure and molecular dynamics calculations to unravel the dynamics of a prototypical reaction along the full photochemical cycle of a ring molecule (thiophenone) – from photoexcitation, ring opening, all the way through to the subsequent ground state dynamics, and spanning a range of tens of femtoseconds to hundreds of picoseconds. “These processes have intrigued the photochemistry community for decades” says Prof. Daniel Rolles from Kansas State University “and it is now routinely possible to visualize electronic changes and the movement of atoms in the molecule at each step of a chemical reaction”.

Image: Artistic rendering of the photo-induced ring opening of thiophenone (left) into several open-ring products (right). The thin white lines show smoothed paths of actual trajectories. Illustration: KSU, Daniel Roles.

Transition-metal dichalcogenide NiTe2: an ambient-stable material for catalysis and nanoelectronics

2020/08/122020/08/13

Image: a) Ni-3p and b) Te-4d XPS core-level spectra collected from as-cleaved NiTe2 (black curves) and from the same surface exposed to 2·10⁴L of CO (red curves), H₂O (green curves) and O₂ (blue curves). Adapted from “S. Nappini et al., Adv. Funct. Mater. 30, 2000915 (2020); DOI: 10.1002/adfm.202000915” with permission from Wiley (Copyright 2020) with license 4873681106527

Determination of interatomic coupling between two-dimensional crystals

2020/07/272020/08/26

Following the isolation of graphene, many other atomically thin two-dimensional crystals have been produced and can even be stacked on top of each other in a desired order to form so called van der Waals heterostructures.

Subtle changes in the stacking, especially the angle between the crystallographic axes of two adjacent layers, can have big impact on the properties of the whole heterostructure. We use angle-resolved photoemission spectroscopy measurements carried out at the Spectromicroscopy beamline at Elettra to obtain interatomic coupling for carbon atoms by studying a three-layer stack of graphene. The coupling between atoms in two two-dimensional crystals, knowledge of which is necessary to describe the properties of the stack, can be determined by studying a structure made of three layers with two similar interfaces but one with crystallographic axes aligned and one twisted. This is because each of the interfaces provides complementary information and together they enable self-consistent determination of the coupling.

Image: Angle resolved photoemission spectrum revealing the electronic bands of a microscopic three layer device having aligned and twisted graphene-graphene interfaces. Measurable band gaps are used to self-consistently determine fundamental parameters of interatomic coupling.