Swift Electrons Research Articles

Theory and simulations of angular momentum transfer from swift electrons to spherical nanoparticles in scanning transmission electron microscopy

Electron beams in scanning transmission electron microscopy (STEM) exert forces and torques on study samples, with magnitudes that allow the controlled manipulation of nanoparticles (a technique called electron tweezers). Related theoretical research has mostly focused on the study of forces and linear momentum transfers from swift electrons (like those used in STEM) to nanoparticles. However, theoretical research on the rotational aspects of the interaction would benefit not only the development of electron tweezers, but also other fields within electron microscopy such as electron vortices. Starting from a classical-electrodynamics description, we present a theoretical model, alongside an efficient numerical methodology, to calculate the angular momentum transfer from a STEM swift electron to a spherical nanoparticle. We show simulations of angular momentum transfers to aluminum, gold, and bismuth nanoparticles of different sizes. We found that the transferred angular momentum is always perpendicular to the system's plane of symmetry, displaying a constant direction for all the cases considered. In the simulations, the angular momentum transfer increased with the radius of the nanoparticle, but decreased as the speed of the electron or the impact parameter increased. Also, the electric contribution to the angular momentum transfer dominated over the magnetic one, being comparable only for high electron's speeds (greater than 90% of the speed of light). Additionally, for nanoparticles with 1 nm radius of the studied materials, we found validity criteria for the small-particle approximation (in which the nanoparticle is modeled as an electric point dipole). We believe that these findings contribute to the understanding of rotational aspects present in STEM experiments, and might be useful for further developments in electron tweezers and other electron microscopy related techniques.

Determination of the Primary Excitation Spectra in XPS and AES

This paper reviews a procedure that allows for extracting primary photoelectron or Auger electron emissions from homogeneous isotropic samples. It is based on a quantitative dielectric description of the energy losses of swift electrons travelling nearby surfaces in presence of stationary positive charges. The theory behind the modeling of the electron energy losses, implemented in a freely available QUEELS-XPS software package, takes into account intrinsic and extrinsic effects affecting the electron transport. The procedure allows for interpretation of shake-up and multiplet structures on a quantitative basis. We outline the basic theory behind it and illustrate its capabilities with several case examples. Thus, we report on the angular dependence of the intrinsic and extrinsic Al 2s photoelectron emission from aluminum, the shake-up structure of the Ag 3d, Cu 2p, and Ce 3d photoelectron emission from silver, CuO and CeO2, respectively, and the quantification of the two-hole final states contributing to the L3M45M45 Auger electron emission of copper. These examples illustrate the procedure, that can be applied to any homogeneous isotropic material.

Chiral Plasmons Enable Coherent Vortex Smith–Purcell Radiation

AbstractRecent advances in vortex Smith–Purcell radiation (V‐SPR) have spurred many breakthroughs in the generation of vortex beams for their potential applications in chiral detection, particle acceleration, communication, and imaging. However, advanced multi‐mode applications of the V‐SPR suffer from the incoherent spectrum and poor purity in producing high‐order topological charges. Here, by exploiting the rotational symmetry and Brillouin‐folding phenomenon, it is proved that the coherent V‐SPR with independently steerable high‐order topological charge is directly emitted from chiral spoof surface plasmons. Moreover, both topological charges and operation frequencies can be readily tuned, thus enabling flexible adaptability over a broad bandwidth. A proof‐of‐concept prototype is designed and fabricated in the microwave regime, in which the method is demonstrated from the perspective of symmetry and tunability. The proposed methodology can be exploited as a platform to investigate the interaction between chiral plasmons and swift electrons, promising a feasible scheme for generating coherent and tunable vortex beams from the microwave to the terahertz band.

Platform for probing radiation transport properties of hydrogen at conditions found in the deep interiors of red dwarfs

We describe an experimental concept at the National Ignition Facility for specifically tailored spherical implosions to compress hydrogen to extreme densities (up to ∼800× solid density, electron number density ne∼4×1025 cm−3) at moderate temperatures (T∼200 eV), i.e., to conditions, which are relevant to the interiors of red dwarf stars. The dense plasma will be probed by laser-generated x-ray radiation of different photon energy to determine the plasma opacity due to collisional (free–free) absorption and Thomson scattering. The obtained results will benchmark radiation transport models, which in the case for free–free absorption show strong deviations at conditions relevant to red dwarfs. This very first experimental test of free–free opacity models at these extreme states will help to constrain where inside those celestial objects energy transport is dominated by radiation or convection. Moreover, our study will inform models for other important processes in dense plasmas, which are based on electron–ion collisions, e.g., stopping of swift ions or electron–ion temperature relaxation.

Review of Radiation-Induced Effects on β-Ga2O3 Materials and Devices

β-Ga2O3 has become an ultimate choice of emerging new-generation material for its wide range of compelling applications in power electronics. In this review, we have explored the available radiations in the atmosphere and the effects of radiation on the β-Ga2O3 material and devices. The focus in this review summarizes various studies covering different radiation such as swift heavy ions, protons, neutrons, electrons, Gamma, and X-rays to understand the radiation-induced effects on the structure and their reliable performance in harsh environments. In addition, we focused on the various pre-existing defects in β-Ga2O3 and the emergence of radiation-induced defects that provoke a severe concern, especially from the device performance point of view. This review presents the irradiation-induced effects on the devices such as high-power devices such as Schottky barrier diodes (SBDs), field-effect transistors (FETs), metal-oxide-semiconductor (MOS) devices, and photodetectors. Some key studies including the changes in carrier concentration with a removal rate, Schottky barrier height (SBH), ideality factor, defect dynamics dielectric damage, interface charge trapping, a thermally activated recovery mechanism for charge carriers at elevated temperature, and diffusion length of minority charge carriers. These reports show that β-Ga2O3-based devices could be deployable for space or high-radiation terrestrial applications. These results provide/suggest a better device design based on the radiation degradation studies in the state-of-the-art β-Ga2O3 devices.

Energy-filtered dark-field imaging of nanoparticles by PINEM in 4D electron microscopy

Time-resolved dark-field imaging of alpha Fe2O3 nanoparticle and Ag nanowires using scattered electrons by selected crystal planes are realized by photon-induced near-field electron microscopy (PINEM) selecting probing electrons which absorb energy from a transient laser field during their passage through the target particles in a four-dimensional transmission electron microscope (4D-TEM). The high laser fluence illuminated on the particles causes significant part of probing swift electrons exchange energy with the laser light, creating enough PINEM electrons required for the dark-field imaging of particles with high spatiotemporal resolution at nanometer and femtosecond scale. Different from the bright-field PINEM imaging of particles where the outerspace with a close distance to the particle are illuminated by transmitted PINEM electrons, illumination is confined on the particles by selected scattered PINEM electrons, leading to a much more defined and sharp imaging of particles compared with a bright-field PINEM image. In combination with PINEM temporal gating and dark-field selective imaging, the PINEM dark-field imaging technique in the 4D-TEM enables the studies of structural dynamics of selective crystal planes or elements with high spatiotemporal resolution.

QUEELS: Software to calculate the energy loss processes in TEELS, REELS, XPS and AES including effects of the core hole

We present the user‐friendly and freely available software package QUEELS (QUantitative analysis of Electron Energy Losses at Surfaces) that allows to calculate effective inelastic scattering cross sections within the dielectric response description, for swift electrons travelling nearby surfaces in several environments. We briefly describe the underlying theoretical models and illustrate its use to evaluate the distribution of energy losses taking place in electron spectroscopies like transmission electron energy loss spectroscopy (TEELS), X‐ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and reflection electron energy loss spectroscopy (REELS), which are widely used for material analysis. This includes the intrinsic excitations due to the core hole in XPS and AES.

Sub-optical-cycle electron pulse trains from metal nanotips

The coherent modulation of swift electron beams with strong laser fields has enabled the generation of attosecond electron pulses, opening up new research avenues in ultrafast science. Here we study a comparatively simple alternative, the production of electron pulse trains directly at the source. In our theory work, we show that sub-optical-cycle electron bursts induced by tunneling photoemission from a metal nanotip can retain the temporal fingerprint of their emission dynamics in a typical low-energy point-projection microscope setup. We find that strong acceleration by a static field, a short propagation distance and a sufficiently large optical cycle duration mitigate temporal smearing due to matter-wave dispersion. Our approach enables studies of coherent interactions of slow electrons with matter on sub-femtosecond and nanometer scales, a regime which has hitherto remained inaccessible.

Microwave-assisted synthesis of RuTe2/black TiO2 photocatalyst for enhanced diclofenac degradation: Performance, mechanistic investigation and intermediates analysis

• The crystallized RuTe 2 acted as a cocatalyst for photocatalysis was reported for the first time. • 0.5% RuTe 2 /B-TiO 2 achieved 95.2% DCF degradation efficiency, which was 1.2 times higher than that of pure B-TiO 2 . • The intermediates for the photodegradation of DCF were detected and removal pathways was rationalized. Seeking active and low-cost cocatalysts is a vital task for solving the sluggish kinetics of photocatalysis process in environmental management. Herein, we report a novel and high-efficiency crystallized RuTe 2 cocatalyst, and loaded it onto black TiO 2 to form photocatalytic heterostructure (denoted as RuTe 2 /B-TiO 2 ) for diclofenac (DCF) degradation. To the best of our knowledge, the RuTe 2 as a cocatalyst for photocatalysis based on swift electron capture has not been documented yet. The degradation efficiency of DCF over 0.5% RuTe 2 /B-TiO 2 reached 95.2% for 120 min, which is 1.2 times higher than that of pure B-TiO 2 . In addition, the effects of different calcination temperatures, catalyst amounts, DCF concentrations and solution pH of the as-prepared catalysts were also investigated in depth. Spectroscopy/electrochemistry showed that 0.5% RuTe 2 /B-TiO 2 exhibited a narrower band gap, more active sites and better electrical conductivity than pure B-TiO 2 . Experiments with trapping agents have shown that O 2 – was the main active substance under irradiation. Furthermore, intermediates for the photodegradation of DCF were detected by liquid chromatography mass system (LC-MS) and density functional theory (DFT) calculations. Finally, the degradation pathway was rationalized and a possible photocatalytic mechanism for 0.5% RuTe 2 /B-TiO 2 was proposed.

Effects of a noncausal electromagnetic response on the linear momentum transfer from a swift electron to a metallic nanoparticle

Electron beams in Scanning Transmission Electron Microscopes (STEMs) can be used as a tool to induce movement on nanoparticles. Employing a classical-electrodynamics approach, it has been reported that the linear momentum transfer from a STEM-beam electron to a metallic spherical nanoparticle can be either repulsive or attractive towards the swift electron trajectory. This is in qualitative agreement with experimental observations. The interaction time between a swift electron and a nanoparticle is typically on the order of attoseconds. Hence, the electromagnetic response of the nanoparticle at short times is of utmost importance. However, it has been reported that the dielectric function employed in previous studies presented a non-causal pre-echo at the attosecond timescale, which might have lead to incorrect unphysical results. Therefore, the validity of these linear momentum transfer results should be revisited. In this theoretical work, we study the non-causality effects on the linear momentum transferred from a swift electron to a metallic nanoparticle, made of either aluminum or gold. Using an efficient numerical methodology, we found that non-causality, as well as deficient numerical convergence, may lead to incorrect repulsive linear momentum transfer results. Contrary to what previous theoretical studies have reported, our results show that the linear momentum transfer from a swift electron to spherical aluminum and gold nanoparticles, with radius 1 nm, is always attractive.

Nanoscale Visualization of a Photoinduced Plasmonic Near-Field in a Single Nanowire by Free Electrons.

Swift electrons can undergo inelastic interactions not only with electrons but also with near-fields, which may result in an energy loss or gain. Developments in photon-induced near-field electron microscopy (PINEM) enable direct imaging of the plasmon near-field distribution with nanometer resolution. Here, we report an analysis of the surface plasmonic near-field structure based on PINEM observations of silver nanowires. Single-photon order-selected electron images revealed the wavelike and banded structure of electric equipotential regions for a confined near-field integral associated with typical absorption of photon quanta (nℏω). Multimodal plasmon oscillations and second-harmonic generation were simultaneously observed, and the polarization dependence of plasmon wavelength and symmetry properties were analyzed. Based on advanced imaging techniques, our work has implications for future studies of the localized-field structures at interfaces and visualization of novel phenomena in nanostructures, nanosensors, and plasmonic devices.

Mirror Optical Activity: Nanophotonic Chiral Sensing from Parity Indefiniteness

Mirror symmetry is among the most fundamental concepts of physics, and its spontaneous breaking at the molecular level allows chiral molecules to exist in two enantiomers that are mirror images of each other. The majority of the chiro-optical effects routinely used to detect enantiomers in mixtures, such as circular dichroism, rely on chiral sensitivity to photon circular polarization, and thus do not harness the full potential of mirror-symmetry breaking, which also involves the spatial profile of the radiation. Here we show that the parity indefiniteness of an electromagnetic field interacting with chiral matter supports mirror optical activity, a chiro-optical effect where a chiral film, when it is probed by the mirror-symmetric field of a nanoemitter, produces a near field whose spatial profile has broken mirror symmetry. The detection of near-field dissymmetry can provide an efficient chiral sensing technique. We specialize the discussion to nanofilms with infrared chirality by using a swift electron in an aloof configuration as the nanoemitter and an off-axis transparent conducting nanoparticle as the near-field probe; the spatial dissymmetry factor of the nanoparticle cathodoluminescence is 1 order of magnitude larger than the dimensionless circular dichroism, and is further enhanced to 2 orders of magnitude if an additional graphene sheet is deposited on the film interface.

Electrochemical and photoluminescence response of laser-induced graphene/electrodeposited ZnO composites

The inherent scalability, low production cost and mechanical flexibility of laser-induced graphene (LIG) combined with its high electrical conductivity, hierarchical porosity and large surface area are appealing characteristics for many applications. Still, other materials can be combined with LIG to provide added functionalities and enhanced performance. This work exploits the most adequate electrodeposition parameters to produce LIG/ZnO nanocomposites. Low-temperature pulsed electrodeposition allowed the conformal and controlled deposition of ZnO rods deep inside the LIG pores whilst maintaining its inherent porosity, which constitute fundamental advances regarding other methods for LIG/ZnO composite production. Compared to bare LIG, the composites more than doubled electrode capacitance up to 1.41 mF cm−2 in 1 M KCl, while maintaining long-term cycle stability, low ohmic losses and swift electron transfer. The composites also display a luminescence band peaked at the orange/red spectral region, with the main excitation maxima at ~ 3.33 eV matching the expected for the ZnO bandgap at room temperature. A pronounced sub-bandgap tail of states with an onset absorption near 3.07 eV indicates a high amount of defect states, namely surface-related defects. This work shows that these environmentally sustainable multifunctional nanocomposites are valid alternatives for supercapacitors, electrochemical/optical biosensors and photocatalytic/photoelectrochemical devices.

Monte Carlo transport of swift protons and light ions in water: The influence of excitation cross sections, relativistic effects, and Auger electron emission in w-values.

To develop a particle transport code to compute w-values and stopping power of swift ions in liquid water and gases of interest for reference dosimetry in hadrontherapy. To analyze the relevance of inelastic and post-collisional processes considered. The Monte Carlo code MDM was extended to the case of swift ion impact on liquid water (MDM-Ion). Relativistic corrections in the inelastic cross sections and the post-collisional Auger emission were considered. The effects of introducing different electronic excitation cross sections were also studied. The stopping power of swift ions on liquid water, calculated with MDM-Ion, are in excellent agreement with recommended data. The w-values show a strong dependence on the electronic excitation cross sections and on the Auger electron emission. Comparisons with other Monte Carlo codes show the relevance of both the processes considered and of the cross sections employed. W and w-values for swift electron, proton, and carbon ions calculated with the MDM and MDM-Ion codes are in very close agreement with each other and with the 20.8 eV experimental value. We found that w-values in liquid water are independent of ion charge and energy, as assumed in reference dosimetry for hadrontherapy from sparse experimental results for electron and ion impact on gases. Excitation cross sections and Auger emission included in Monte Carlo codes are critical in w-values calculations. The computation of this physical parameter should be used as a benchmark for micro-dosimetry investigations, to assess the reliability of the cross sections employed.

SnO2/SnS2 heterojunctions anchored on biochar through the −C[dbnd]Sn− bonds: ∙CO2– cooperation with e− to effectively convert Cr(VI)

Controlling the migration behavior of photogenerated carriers by designing the microstructure of materials remains a significant challenge. In this work, the solvothermal method was used to fabricate a SnO2/SnS2 heterojunction anchored on biochar through −CSn− bonds between biochar and SnS2, as demonstrated by X-ray photoelectron spectroscopy characterization. DFT calculations further confirmed −CSn− bonds can shorten theoretical band gap of SnS2@biochar from 1.01 eV to 0.66 eV and provide a pathway for swift electron migration, which can effectively modulate the separation of photo-induced carriers and promote the generation of e− and h+. Additionally, the oxalate activated by −CSn− bonds can effectively capture h+ and produce abundant ∙CO2– free radicals. When the obtained composites were used as photocatalysts, Cr(VI) was thoroughly reduced by the SnO2/SnS2@biochar composite with a molar ratio of SnO2 and SnS2 of 2 within 20 min under visible light irradiation when oxalate was 1.0 mM (CSO-2/H2C2O4-1). Furthermore, CSO-2/H2C2O4-1 exhibited excellent reusability, and the degradation efficiency of Cr(VI) remained above 91.71% after five cycles. The remarkable photocatalytic properties can be ascribed to the synergistic effects of e− and ∙CO2–. This work proposes a new strategy for the preparation of an applied photocatalyst with a high separation rate of photoinduced carriers.

Nonreciprocal guided waves in the presence of swift electron beams

Breaking the reciprocity of electromagnetic interactions is of paramount importance in photonic and microwave technologies, as it enables unidirectional power flows and other unique electromagnetic phenomena. Here we explore a method to break the reciprocity of electromagnetic guided waves utilizing an electron beam with a constant velocity. By introducing an effective dynamic conductivity for the beam, we theoretically demonstrate how nonreciprocal guided waves and a one-way propagating regime can be achieved through the interaction of swift electrons with electromagnetic waves in two-dimensional (2D) parallel-plate and three-dimensional (3D) circular-cylindrical waveguides. Unlike the conventional electron beam structures such as traveling wave tubes and electron accelerators, here the goal is neither to generate and/or amplify the wave nor to accelerate electrons. Instead, we study the salient features of nonreciprocity and unidirectionality of guided waves in such structures. The relevant electromagnetic properties such as the modal dispersion, the field distributions, the operating frequency range, and the nonreciprocity strength and its dependence on the electron velocity and number density are presented and discussed. Moreover, we compare the dispersion characteristics of waves in such structures with some electric-current-based scenarios in materials reported earlier. This broadband tunable magnet-free method offers a unique opportunity to have a switchable strong nonreciprocal response in optoelectronics, nanophotonics, and THz systems.

Angular dynamics of small nanoparticles induced by non-vortex electron beams

In this work, we present a theoretical study of the angular dynamics of small nanoparticles induced by fast non-vortex electron beams. General expressions for the torque and the angular momentum transferred from an electron to an arbitrary—but small—nanoparticle are obtained using a full-retarded classical electrodynamics approach, within the small particle limit. We applied this methodology to study a particular case of interest: the angular dynamics of spherical nanoparticles with homogeneous and isotropic electromagnetic responses. We analytically calculate the total angular momentum transferred from a swift electron to such nanoparticles, finding that it is electric in nature and it is always in a direction determined by the electron trajectory relative to the center of the nanoparticle. We realize that it is possible to represent the angular momentum transferred as the product of two functions: the extinction cross-section of the nanoparticle and a function that only contains information about the swift electron. We present numerical results for the total angular momentum transferred from a swift electron to an aluminum and a gold nanoparticle. We also present an analysis of the temporal behavior of the torque and the electric dipole moment induced within the nanoparticle by the swift electron. We compare the angular momentum transfer calculated in this work with a previously reported case of vortex beams, finding that, for both aluminum and gold nanoparticles, our results are two orders of magnitude smaller. Finally, we consider a particular case of a frictionless gold spherical nanoparticle of radius a=5nm, obtaining that it can spin with an angular frequency up to 29.3Hz.

IR and UV Laser‐Induced Graphene: Application as Dopamine Electrochemical Sensors

AbstractLaser‐induced graphene (LIG) is inexpensive, fast, and easy to produce when compared to many other forms of graphene. Within the biosensing field, LIG electrodes are most often produced via infrared (IR) laser irradiation of polyimide sheets. Nevertheless, the usage of ultraviolet (UV) laser to produce LIG provides advantages in terms of sensor miniaturization because of its inherently higher scribing resolution. Yet, studies on the electrochemical performance of UV LIG, its relation with morphological and structural aspects as well as its comparison with IR LIG are still lacking. This work shows that both LIGs present swift electron transfer kinetics constituting excellent electrodes for electroanalysis. Extreme sensitivities of 93 and 58 µA µm−1 cm−2 at physiologically relevant dopamine (DA) concentrations are found for IR and UV LIG, respectively. Such sensitivities and good selectivity are achieved in the presence of physiologically relevant concentrations of ascorbic and uric acids, contrasting to the related literature employing IR LIG where such interferents are below the physiological range. Despite providing lower sensitivity, UV LIG is still an excellent material for DA biosensors, with the above‐mentioned advantages in terms of miniaturization. To our knowledge, these are among the highest sensitivities reported for voltammetric measurement of DA using carbon‐based materials.

Time-dependent forces between a swift electron and a small nanoparticle within the dipole approximation

In this paper we calculate the time-dependent forces between a swift electron traveling at constant velocity and a metallic nanoparticle made of either aluminum or gold. We consider that the nanoparticle responds as an electric point dipole and we use classical electrodynamics to calculate the force on both the nanoparticle and the electron. The values for the velocity of the electron and the radius of the nanoparticle were chosen in accordance with electron microscopy observations, and the impact parameter was selected to fulfill the constraints imposed by the dipole approximation. We found that there are times when the force on the nanoparticle is attractive and others when it is repulsive, and show that this is due to the delayed electromagnetic response of the nanoparticle. To establish the limits of validity of our approach, we calculate the total linear momentum transfer to the nanoparticle, and compare it with results obtained, in frequency space, using the full multipole expansion of the fields induced on the nanoparticle, considering the effects of electromagnetic radiation.

Exploring the Spatial Features of Electronic Transitions in Molecular and Biomolecular Systems by Swift Electrons

We devise a new kind of experiment that extends the technology of electron energy loss spectroscopy to probe (supra-)molecular systems: by using an electron beam in a configuration that avoids molecular damage and a very recently introduced electron optics setup for the analysis of the outcoming electrons, one can obtain information on the spatial features of the investigated excitations. Physical insight into the proposed experiment is provided by means of a simple but rigorous model to obtain the transition rate and selection rule. Numerical simulations of DNA G-quadruplexes and other biomolecular systems, based on time dependent density functional theory calculations, point out that the conceived new technique can probe the multipolar components and even the chirality of molecular transitions, superseding the usual optical spectroscopies for those cases that are problematic, such as dipole-forbidden transitions, at a very high spatial resolution.