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

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.

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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

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.

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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

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

Read more on the Elettra website

Image: a) Ni-3p core-level spectra collected from as-cleaved NiTe2 (black curves) and from the same surface exposed to 2·10L 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

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.

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Image: Angle resolved photoemission spectrum revealing flat non-dispersing electronic band filled with slow electrons separated by mini gaps from the rest of electronic structure in twisted bilayer graphene device.

Who stole the light?

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.

Read more on the ELETTRA website

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

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.

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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

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 = 10-18 s).  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.

Read more on the Elettra website

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

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.

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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

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”.

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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

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

Read more on the Elettra website

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 2·10L 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

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.

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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.

Terahertz tuning of Dirac plasmons in Bi2Se3 Topological Insulator

Light can be strongly confined in subwavelength spatial regions through the interaction with plasmons, the collective electronic modes appearing in metals and semiconductors. This confinement, which is particularly important in the terahertz spectral region, amplifies light-matter interaction and provides a powerful mechanism for efficiently generating nonlinear optical phenomena. These effects are particularly relevant in graphene and topological insulators, where massless Dirac fermions show a naturally nonlinear optical behaviour in the terahertz range. We have shown that the Dirac plasmon resonance in Bi2Se3 topological insulators can be tuned over one octave by employing intense broadband terahertz radiation delivered by the TeraFERMI beamline at FERMI@Elettra. This paves the way towards tunable terahertz nonlinear devices based on topological insulators, with potential applications in opto-electronics, communication, and sensing technologies.

>Read more on the Elettra website

Image: Plasmons are collective oscillations of electrons that can be directly excited by electromagnetic radiation in the presence of an extra momentum (red arrow). This is achieved in the present experiment, through ribbon arrays fabricated onto the surface of topological insulator Bi2Se3 films, excited after illumination with sub-ps, half-cycle THz pulses produced at the FERMI free-electron laser.

Synthesis of mesoscale ordered 2D π-conjugated polymers with semiconducting properties

Two-dimensional materials can exhibit intriguing electronic properties that stem from their geometry. The best-known example is graphene’s Dirac cone that gives rise to massless electrons, which originates from the all-carbon hexagonal lattice. Two-dimensional conjugated polymers (2DCPs) can be considered as analogues of graphene, yet offering greater potential to design geometry and properties by carefully selecting their building blocks. Strikingly, 2DCPs on a kagome lattice (i.e. a trihexagonal tiling) can show both Dirac cones and flat bands, with highly-massive charge carriers.

Despite experimental efforts spanning more than a decade, the poor crystallinity of the synthesized polymers made the study of the electronic properties of 2DCPs a scientific niche reserved to theorists. A collaboration between the “Istituto di Struttura della Materia” of the Italian CNR three Canadian universities (INRS, McGill and Lakehead) realized the milestone of the synthesis of a long-range ordered 2D polymeric network, enabling the measurement of their Dirac cone and flat band features by angle-resolved photoelectron spectroscopy (ARPES). This achievement paves the way to study the intriguing electronic properties of this new class of materials, which make them promising for applications in future electronic and optoelectronic technologies.

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Image :  a) Scanning tunneling microscope image of a highly-ordered polymeric network with the theoretical model superimposed b) second derivative of the ARPES map for the polymer on Au(111) along the ΓKM direction, where it is possible to observe the Dirac cone feature converging at a Dirac point (DP) around 0.55 eV; the theoretical calculated band structure is superimposed.

Titanium-based potassium-ion battery positive electrode

Small energy storage devices (like the ones used in cell phones, tablets, and laptops) based on the mature Lithium-ion technology have become a key element of our daily life. Facing the pressing challenges posed by Global Warming, the increasing demand of storage systems for the large-scale automotive industry will soon clash with the sparse provision of lithium in the Earth’s crust.
In this panorama, the development of economically feasible emerging battery technologies based on alternative, earth-abundant, elements, is thus highly desirable.
Potassium-ion batteries could represent a viable substitute to Lithium-ion technology in a large-scale green economy. However, the key problem preventing the success of the K-ion technology is linked to the low efficiency of cathode materials. 

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Image: Structural evolution of KTiPO4F. (a) Initial crystal structure (b) In operando SXPD: phase transformations. (c) Corresponding charge-discharge profile

Covalent Organic Framework (COF‐1) under pressure

Covalent Organic Frameworks (COF’s) form a family of polymeric materials composed only by light elements. The absence of metal atoms in their structure makes COF’s distinctly different compared to their relatives, Metal Organic Framework materials (MOF’s). Historically first COF structure (named COF-1) was reported back in 2005 by Cote et al., (Science 310 (2015) 1166).  It consists of benzene rings linked by B3Ointo hexagon-shaped 2D sheets which are stacked into a layered structure, resembling in this respect the structure of graphite composed by graphene layers. By analogy with graphene the single layer of COF material could be named as COFene since it represents a true 2D material composed by carbon, hydrogen, boron and oxygen. Unlike graphite, COF-1 is porous material with relatively high surface area which makes it promising for various applications, e.g. for energy storage devices, as a sorbents for gas storage or for membranes.  However, little was known about mechanical properties of COF’s or single layered COFenes except for few theoretical estimations. Unlike graphite or MOF’s, no high pressure studies were available for COFs. The study by A. Talyzin group from Umeå University (Sweden) performed at Elettra at the Xpress beamline and SOLEIL synchrotrons in collaboration with the Technical University of Dresden (Germany) and the Chalmers University (Sweden) is first to evaluate compressibility and pressure limits for stability of COF-1 structure.

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Picture: schematics of the high pressure experiments involving diamond anvil cell

Translucency of graphene to van der Waals forces

If in the infinitely large it is the gravitational force that determines the evolution in space and time of planets, stars and galaxies, when we focus our observation on the atomic scale other are the forces that allow materials to exist. These are forces that, like a “special glue”, allow atoms and molecules to aggregate to form living and non-living systems. Among them we find one that, although discovered 150 years ago by Johannes Diderik van der Waals (vdW), still carries with it some aspects of ambiguity. Van der Waals was the first to reveal its origin and to give a first and simple analytical description, even though it took more than a century, with the new discoveries of quantum field theory, to be able to fully understand its quantum character and its relation to the vacuum energy and Casimir force. And only in the last 30 years it has been realized how much this force pervades the natural world. One of the wonders is represented by the geckos, who use these forces to climb vertical and smooth walls thanks to the vdW forces, which are enhanced because of the multitude of hairs present in each finger of their legs. These forces are also known to affect the stability of the double helix of the DNA and are also responsible for the interactions between different groups of amino acids.
What makes the vdW force unique is the fact that it is the weakest of the inter-atomic and inter-molecular forces present in nature and therefore it remains extremely difficult to measure with great accuracy. At the same time, even the inclusion of these force in the most accurate methods of calculation has not yet found a universal solution and the different approaches used by theoretical physicists and chemists to take them into account can sometimes lead to conflicting results.

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Image:   CO desorption from Gr/Ir(111). (a) Selected spectra of the uptake corresponding to θCO=0.08 ML (bottom) and 0.30 ML (top). (b) TP-XPS C 1s core level spectra showing its evolution during thermal desorption of CO from Gr/Ir(111). (c) Comparison of CO coverage evolution as a function of temperature for selected CO initial coverages.