Swift Electrons Research Articles

Photonic and plasmonic transition radiation from graphene

We theoretically study the transition radiation in the framework of full Maxwell equations, when a swift electron crosses a monolayer graphene. Based on the Sommerfeld integration, we demonstrate in the frequency domain the spatial distribution of this free-electron radiation, which clearly shows the broadband excitation of both photons and graphene plasmons. Moreover, the radiation spectra for photons and graphene plasmons are analytically derived. We find that the excitation of photons and graphene plasmons favors different particle velocities. To be specific, a higher particle velocity gives rise to the excitation of photons with better directivity and higher intensity, while a lower particle velocity enables the efficient excitation of graphene plasmons in a broader frequency range. Our work indicates that the interaction between swift charged particles and various 2D materials or van der Waals heterostructures is promising for the design of terahertz on-chip radiation sources.

FREE-ELECTRON RADIATION ENGINEERING VIA STRUCTURED ENVIRONMENTS

Free-electron radiation results from the interaction between swift electrons and the local electromagnetic environment. Recent advances in material technologies provide powerful tools to control light emission from free electrons and may facilitate many intriguing applications of free-electron radiation in particle detections, lasers, quantum information processing, etc. Here, we provide a brief overview on the recent theoretical developments and experimental observations of spontaneous free-electron radiation in various structured environments, including two-dimensional materials, metasurfaces, metamaterials, and photonic crystals. We also report on research progresses on the stimulated free-electron radiation that results from the interaction between free electrons and photonic quasi-particles induced by the external field. Moreover, we provide an outlook of potential research directions for this vigorous realm of free-electron radiation.

Co3C as a promising cocatalyst for superior photocatalytic H2production based on swift electron transfer processes

The inexpensive Co3C proved itself as highly efficient cocatalyst to generate economical H2. An apparent quantum yield of 19% was achieved which is among the best reported carbide-based cocatalysts.

Broad-Spectral-Range Sustainability and Controllable Excitation of Hyperbolic Phonon Polaritons in α-MoO3.

Hyperbolic phonon polaritons (HPhPs) in orthorhombic-phase molybdenum trioxide (α-MoO3 ) show in-plane hyperbolicity, great wavelength compression, and ultralong lifetime, therefore holding great potential in nanophotonic applications. However, its polaritonic response in the far-infrared (FIR) range remains unexplored due to challenges in experimental characterization. Here, monochromated electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) is used to probe HPhPs in α-MoO3 in both mid-infrared (MIR) and FIR frequencies and correlate their behaviors with microstructures and orientations. It is found that low structural symmetry leads to various phonon modes and multiple Reststrahlen bands (RBs) over a broad spectral range (over 70 meV) and in different directions (55-63 meV and 119-125 meV along the b-axis, 68-106 meV along the c-axis, and 101-121 meV along the a-axis). These HPhPs can be selectively excited by controlling the direction of swift electrons. These findings provide new opportunities in nanophotonic and optoelectronic applications, such as directed light propagation, hyperlenses, and heat transfer.

Electron-driven photon sources for correlative electron-photon spectroscopy with electron microscopes

Abstract Electron beams in electron microscopes are efficient probes of optical near-fields, thanks to spectroscopy tools like electron energy-loss spectroscopy and cathodoluminescence spectroscopy. Nowadays, we can acquire multitudes of information about nanophotonic systems by applying space-resolved diffraction and time-resolved spectroscopy techniques. In addition, moving electrons interacting with metallic materials and optical gratings appear as coherent sources of radiation. A swift electron traversing metallic nanostructures induces polarization density waves in the form of electronic collective excitations, i.e., the so-called plasmon polariton. Propagating plasmon polariton waves normally do not contribute to the radiation; nevertheless, they diffract from natural and engineered defects and cause radiation. Additionally, electrons can emit coherent light waves due to transition radiation, diffraction radiation, and Smith-Purcell radiation. Some of the mechanisms of radiation from electron beams have so far been employed for designing tunable radiation sources, particularly in those energy ranges not easily accessible by the state-of-the-art laser technology, such as the THz regime. Here, we review various approaches for the design of coherent electron-driven photon sources. In particular, we introduce the theory and nanofabrication techniques and discuss the possibilities for designing and realizing electron-driven photon sources for on-demand radiation beam shaping in an ultrabroadband spectral range to be able to realize ultrafast few-photon sources. We also discuss our recent attempts for generating structured light from precisely fabricated nanostructures. Our outlook for the realization of a correlative electron-photon microscope/spectroscope, which utilizes the above-mentioned radiation sources, is also described.

Electrospun fibrous active bimetallic electrocatalyst for hydrogen evolution

Fabricating earth-abundant bifunctional water splitting electrocatalysts with high efficiencies to replace noble metal-based Pt and IrO2 catalysts is in great demand for the development of clean energy conversion technologies. Molybdenum disulfide (MoS2) nanostructures have attracted much attention as promising material for hydrogen evolution reaction (HER). The production of hydrogen gas by help of potential efficient earth abundant metal oxides, and stable electrolysis seems a promising for hydrogen evolution reaction pathway in 1 M potassium hydroxide electrolyte media is a hot research topic in the field for clean energy conversion, renewable energies and storage. Here we propose asystem composed NiO nanostructures and MoS2 deposited on (MoS2@NiO). Here, by hydrothermal method NiO prepared and MoS2@NiO by an electrospinning technique complex, can be used as catalyst to produce a large amount of hydrogen gas bubbles. The NiO nanostructures composite having highest synergistic behavior fully and covered by the MoS2. For the MoS2@NiO nano composite catalyst, experiment applied in 1 M KOH for the production of hydrogen evolution reaction which exhibits distinct properties from the bulk material. Overpotential values recorded low 406 mV and current density 10 mA cm−2 measured. Co-catalysts characterized by using different techniques for deep study as scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Owing to their unique structure, as-prepared nanocomposite exhibited enhanced catalytic performance for HER due to high electroactive surface area and swift electron transfer kinetics. Based on the HER polarization curves at low potential electrochemical to examine the effects of intercalants HER catalytic efficiency. Our findings establish low Tafel slope (44 mV/decade) and the catalyst stable for at least 13 h. This simple exploitation of MoS2@NiO composite catalysts depending on the intended application of their electrochemistry.

Strong bond rearrangement and hydrogen-pair emission in linear geometry following water ionization by electron impact

SynopsisThe H0 + O+ + H0 fragmentation of water by electron impact is studied using the delayed extraction time-of-flight technique to uncover kinematic attributes of suprathermal O+ ions. It is found that the H0 pair ejections occur under a large angular rearrangement, in an approximate linear geometry, and leaving the O+ near at rest, with ∼50% of the O+ produced in water fragmentation by swift electrons presenting this feature. It is shown that this behavior is closely associated to the excitation of satellites one-electron-two-vacancies states near the 2a1 orbital.

The impact of swift electrons on the segregation of Ni-Au nanoalloys

We report on the electron beam-induced segregation of alloyed Ni-Au clusters into a Ni and Au rich phase at temperatures above the miscibility gap of a binary system. The nanoparticles, with diameters less than 10 nm, are grown fully inert in superfluid helium droplets with a Ni-Au core-shell morphology. Upon heating, the clusters are alloyed and subsequently transformed to a Janus-type morphology under irradiation with swift electrons. The underlying mechanisms are studied experimentally via in situ scanning transmission electron microscopy and theoretically via atomistic simulation techniques under consideration of elastic electron interactions. We find that the segregation kinetics is highly temperature-dependent and attribute this behavior to diffusive relaxation processes. The presented results shed light on radiation induced phenomena using clusters as a model system and suggest new routes for the synthesis of structures in nonequilibrium configurations.

Interference between quantum paths in coherent Kapitza–Dirac effect

In the Kapitza–Dirac effect, atoms, molecules, or swift electrons are diffracted off a standing wave grating of the light intensity created by two counter-propagating laser fields. In ultrafast electron optics, such a coherent beam splitter offers interesting perspectives for ultrafast beam shaping. Here, we study, both analytically and numerically, the effect of the inclination angle between two laser fields on the diffraction of pulsed, low-energy electron beams. For sufficiently high light intensities, we observe a rich variety of complex diffraction patterns. These do not only reflect interferences between electrons scattered off intensity gratings that are formed by different vector components of the laser field. They may also result, for certain light intensities and electron velocities, from interferences between these ponderomotive scattering and direct light absorption and stimulated emission processes, usually forbidden for far-field light. Our findings may open up perspectives for the coherent manipulation and control of ultrafast electron beams by free-space light.

Co3O4/ NiO bifunctional electrocatalyst for water splitting

The development of noble metal free and active bifunctional catalysts for water splitting in alkaline media is highly demanded but very challenging. Herein, synergetic effects developed between two nonprecious metal oxides, Co3O4 and NiO, are reported, with the resulting composite showing promising properties as a catalyst for alkaline water electrolysis. The activity of the composite material towards both the HER and the OER was enhanced and the dynamic potential decreased, as compared with its counterparts. Importantly, low Tafel slopes of 101 and 61 mVdec−1are found for the composite catalyst for OER and HER respectively. EIS measurements revealed a decreased impedance response of the composite dominated by the intermediate frequency relaxation, related to the adsorption of intermediates. Moreover, based on the structural features the improved catalytic activity of the composite is also due to high electroactive surface area, swift electron transfer kinetics, and excellent electrical chemical coupling between Co3O4 and NiO.

Spiral Field Generation in Smith-Purcell Radiation by Helical Metagratings

Moving electrons interacting with media can give rise to electromagnetic radiations and has been emerged as a promising platform for particle detection, spectroscopies, and free-electron lasers. In this letter, we investigate the Smith-Purcell radiation from helical metagratings, chiral structures similar to deoxyribonucleic acid (DNA), in order to understand the interplay between electrons, photons, and object chirality. Spiral field patterns can be generated while introducing a gradient azimuthal phase distribution to the induced electric dipole array at the cylindrical interface. Experimental measurements show efficient control over angular momentum of the radiated field at microwave regime, utilizing a phased electromagnetic dipole array to mimic moving charged particles. The angular momentum of the radiated wave is determined solely by the handedness of the helical structure, and it thus serves as a potential candidate for the detection of chiral objects. Our findings not only pave a way for design of orbital angular momentum free-electron lasers but also provide a platform to study the interplay between swift electrons with chiral objects.

Electron energy loss spectroscopy simulation by a frequency domain surface integral equation solver

Plasmonic nanoparticles have been mostly studied using conventional light sources. Recently, electron energy loss spectroscopy (EELS) has started to be used to analyze plasmonic nanoparticles where incident electromagnetic fields are created by swift electrons. To accurately simulate EELS experiments, several numerical methods have been adapted. In this paper, a frequency domain surface integral equation (FDSIE) solver is modified to simulate EELS for plasmonic nanoparticles of gold and silver. Accuracy and versatility of the proposed FDSIE solver are shown by several numerical examples and compared to existing numerical, analytical, and experimental results.

Electromagnetic fields produced by a swift electron: A source of white light

In this work we calculate the electromagnetic fields generated by a swift electron in both time and frequency domains. Although the electromagnetic fields in the time domain produced by a bare fast electron are obtained in standard textbooks, their time Fourier transform is not satisfactorily discussed neither are the integrals involved in the computation of it. We show that care must be taken when calculating the electromagnetic fields in the frequency domain to preserve their real nature.

Electronic tweezers for magnesium oxide nanoparticles

Abstract In this work, we present recent findings regarding the linear momentum transferred from swift electrons, like those typically used in Scanning and Transmission Electron Microscopy, to a magnesium oxide nanoparticle. We performed full-retarded numerical calculations based on Classical Electrodynamics formalism considering the interaction of a single fast electron, which is a good approximation due to the small electric current (tens of pA) involved in modern electron microscopes, with a spherical magnesium oxide nanoparticle. As previously calculated for plasmonic nanoparticles, our results also show that the linear momentum transfer can be either attractive or repulsive for a magnesium oxide nanoparticle, depending on the relevant parameters of the problem: the distance between the electron and the nanoparticle and the electron’s speed. Interestingly, we found that the magnitude of the linear momentum transferred to the nanoparticle lies in the same range as for gold and aluminum nanoparticles. These theoretical results pave the way to consider the possibility of implementing electronic tweezers to manipulate not only metallic nanoparticles.

Spiral Field Generation in Smith-Purcell Radiation by Helical Metagratings.

Moving electrons interacting with media can give rise to electromagnetic radiations and has been emerged as a promising platform for particle detection, spectroscopies, and free-electron lasers. In this letter, we investigate the Smith-Purcell radiation from helical metagratings, chiral structures similar to deoxyribonucleic acid (DNA), in order to understand the interplay between electrons, photons, and object chirality. Spiral field patterns can be generated while introducing a gradient azimuthal phase distribution to the induced electric dipole array at the cylindrical interface. Experimental measurements show efficient control over angular momentum of the radiated field at microwave regime, utilizing a phased electromagnetic dipole array to mimic moving charged particles. The angular momentum of the radiated wave is determined solely by the handedness of the helical structure, and it thus serves as a potential candidate for the detection of chiral objects. Our findings not only pave a way for design of orbital angular momentum free-electron lasers but also provide a platform to study the interplay between swift electrons with chiral objects.

Ultrafast optics with slow electrons

We study the interaction of swift electrons with laser and solids using a numerical approach, and specifically show that low-energy electrons can be used to map the time-resolved response of a Fermi gas.

Inelastic mean free path of swift electrons and stopping power in Ta2O5

Energy Loss Function (ELF) of 2 5 Ta O derived from optical limitand extended to the total part of momentum and their energyexcitation region ELF plays an important function in calculatingenergy loss of electron in materials. The parameter Inelastic MeanFree Path (IMFP) is most important in quantitative surface sensitiveelectron spectroscopies, defined as the average distance that anelectron with a given energy travels between successive inelasticcollisions. The stopping cross section and single differential crosssectionSDCS are also calculated and gives good agreement withprevious work.

Site-selective Pt dewetting on WO3-coated TiO2 nanotube arrays: An electron transfer cascade-based H2 evolution photocatalyst

Among several parameters that affect the yield of a photocatalytic process mediated by a metal oxide semiconductor, key is the efficient separation and transfer of photo-generated charge carriers. To overcome kinetic limitations and enable charge transfer, an effective strategy is to decorate the photocatalyst surface with cocatalytic nanoparticles of either a second semiconductor metal oxide or a noble metal. Nevertheless, classical deposition techniques based on powder technology approaches lead to randomly placed cocatalytic nanoparticles at the photocatalytic surface. The poor control over cocatalyst placement can drastically hamper the photocatalytic efficiencies, and can also prevent a full understanding of the charge carrier dynamics and photocatalytic mechanism. Here we investigate a highly defined charge separation platform for photocatalytic H2 evolution based on a Pt-WO3-TiO2 “stacked” structure constructed on anodically grown TiO2 nanotube arrays. Key is the formation of a site-selective and sequential W and Pt metal sputter-decoration only at the mouth of highly-ordered TiO2 nanotubes. After placing the W-Pt bilayer at the nanotubes mouth, a suitable thermal treatment forms a WO3 layer atop the nanotubes while the Pt film undergoes solid state dewetting into 2–6 nm-sized Pt nanoparticles. These structures show strongly improved photocatalytic H2 evolution efficiency compared to any other single-cocatalyst system (Pt-TiO2 and WO3-TiO2) and pristine TiO2 nanotubes. The photocatalytic activity improvement is ascribed to an enhanced charge carrier separation mechanism enabled by the well-defined TiO2-WO3-Pt architecture that provides swift electron transfer through WO3 and towards Pt for H2 evolution.

Inelastic vibrational bulk and surface losses of swift electrons in ionic nanostructures

In a recent paper [Lagos et al., Nature (London) 543, 533 (2017)] we have used electron energy loss spectroscopy with sub-10 meV energy and atomic spatial resolution to map optical and acoustic, bulk and surface vibrational modes in magnesium oxide nanocubes. We found that a local dielectric description works well for the simulation of aloof geometries, similar to related work for surface plasmons and surface plasmon polaritons, while for intersecting geometries such a description fails to reproduce the rich spectral features associated with excitation of bulk acoustic and optical phonons. To account for scatterings with a finite momentum exchange, in this paper we investigate molecular and lattice dynamics simulations of bulk losses in magnesium-oxide nanocubes using a rigid-ion description and investigate the loss spectra for intersecting electron beams. From our analysis we can evaluate the capability of electron energy loss spectroscopy for the investigation of phonon modes at the nanoscale, and we discuss shortcomings of our simplified approach as well as directions for future investigations.

Production of plasmons in two layers of graphene with different doping densities traversed by swift electrons

We apply a fully relativistic formulation to calculate the average number of plasmons excited by an energetic electron traversing two parallel graphene layers separated by a given distance d. We use a Drude model in the non-dissipative limit to describe the conductivity of each layer in the THz frequency range, and address the general case in which the layers have different doping densities of charge carriers by defining an asymmetry parameter. With this, we obtain the probability density of exciting the hybridized Dirac Plasmon-Polariton modes that result from the coupling of the layers, and hence the number of plasmons excited by the passing electron along its trajectory. We analyze the effect of different parameters, such as the interlayer distance, the incident particle velocity, and the asymmetry in doping densities. In particular, we find a cusp-like behaviour for nearly equal conductivities, that indicates a high sensitivity of the system to slight variations in doping densities.