High-quality Electron Research Articles

Buffer-Less Gallium Nitride High Electron Mobility Heterostructures on Silicon.

Thick metamorphic buffers are considered indispensable for III-V semiconductor heteroepitaxy on large lattice and thermal-expansion mismatched silicon substrates. However, III-nitride buffers in conventional GaN-on-Si high electron mobility transistors (HEMT) impose a substantial thermal resistance, deteriorating device efficiency and lifetime by throttling heat extraction. To circumvent this, a systematic methodology for the direct growth of GaN after the AlN nucleation layer on six-inch silicon substrates is demonstrated using metal-organic vapor phase epitaxy (MOVPE). Crucial growth-stress modulation to prevent epilayer cracking is achieved even without buffers, and threading dislocation densities comparable to those in buffered structures are realized. The buffer-less design yields a GaN-to-substrate thermal resistance of (11 ± 4) m2KGW-1, an order of magnitude reduction over conventional GaN-on-Si and one of the lowest on any non-native substrate. As-grown AlGaN/AlN/GaN heterojunctions on this template show a high-quality 2D electron gas (2DEG) whose room-temperature Hall-effect mobility exceeds 2000 cm2V-1s-1, rivaling the best-reported values. As further validation, the low-temperature magnetoresistance of this 2DEG shows clear Shubnikov-de-Haas oscillations, a quantum lifetime > 0.180 ps, and tell-tale signatures of spin-splitting. These results could establish a new platform for III-nitrides, potentially enhancing the energy efficiency of power transistors and enabling fundamental investigations into electron dynamics in quasi-2D wide-bandgap systems.

Investigating the transverse jitter in ultrashort electron beam generation with optical compression

Optical compression scheme is a highly potential candidate for the generation of ultrashort electron beam. This scheme utilizes a tightly focused radially polarized laser to compress the electron beam from the photocathode radio frequency (RF) gun. However, the unavoidable relative transverse jitter between the electron beam and the modulation laser leads to a performance degradation of the compressed pulse duration. To address this issue, we propose a method for correcting this transverse jitter through a specific design of optical system and an optimized decoupled solenoid. This method concentrates on the transverse jitter caused by the pointing instability of the laser. Its objective is to align both the transverse jitter direction and magnitude of the modulation laser with that of the electron beam. The simulation results demonstrate that the deleterious impact of transverse jitter can be effectively mitigated by this method, thereby ensuring the sustained generation of high-quality ultrashort electron beams.

Simulation of laser plasma wakefield acceleration with external injection based on Bayesian optimization

Abstract In laser wakefield acceleration (LWFA), injecting an external electron beam at certain energy is a promising approach to achieving high-quality electron beam with low energy spread and low emittance. In this paper, the process of laser wakefield acceleration with an external injection at 10 pC has been studied in simulations. A Bayesian optimization method is used to optimize the key laser and plasma parameters, so that the electron beam is accelerated to the expected energy with a small emittance and energy spread growth. The effect of rising edge of the plasma on the transverse properties of the electron beam is simulated and optimized in order to ensure that the external electron beam is injected into the plasma without significant emittance growth. Finally, a high-quality electron beam with an energy of 1.5 GeV, the normalized transverse emittance of 0.5 mm·mrad and the relative energy spread of 0.5% at 10 pC is obtained.

A refined plan-view specimen preparation technique for high-quality electron microscopy studies of epitaxially grown atomically thin 2D layers

The structural studies of two-dimensional (2D) van der Waals heterostructures and understanding of their relationship with the orientation of crystalline substrates using transmission electron microscopy (TEM) presents a challenge in developing an easy-to-use plan-view specimen preparation technique. In this report, we introduce a simple approach for high-quality plan-view specimen preparation utilizing a dual beam system comprising focused ion beam and scanning electron microscopy.To protect the atomically thin 2D heterostructure during the preparation process, we employ an epoxy layer. This layer serves as a protective barrier and enables the creation of a TEM specimen comprising a thin substrate fragment with an overgrown 2D structure covered by a thin, electron-transparent epoxy layer. The coexistence of both 2D layers and substrate is essential for investigating the relative crystallographic orientations between the grown 2D structures and the substrates. The thickness of the specimen is monitored using low-voltage scanning electron microscopy.We apply this technique to prepare plan-view specimens of 2D germanium-antimony-telluride (GST) on Si and hexagonal boron nitride (h-BN)/epitaxial graphene (EG) heterostructures grown on 6H-SiC substrates. The grain-like atomic structure observed in the 2.2 nm thick GST layer on Si substrate provides evidence of the mosaicity of GST during the early stages of epitaxial growth. H-BN/EG on 6H-SiC structural studies indicate a rotation of h-BN/EG around the 6H-SiC[0001] axis by an angle of 30°. The observed BN particles with sizes in the nanometer range on top of the sample have the wurtzite lattice type and random orientation.The developed specimen preparation technique offers a powerful tool for TEM studies of atomically thin layers on crystals. Its simplicity and ability to provide valuable insights into the in-plane relationships between 2D structures and crystalline substrates make it a promising complement to grazing incident X-ray diffraction.

Technical Design Report for the LUXE experiment

AbstractThis Technical Design Report presents a detailed description of all aspects of the LUXE (Laser Und XFEL Experiment), an experiment that will combine the high-quality and high-energy electron beam of the European XFEL with a high-intensity laser, to explore the uncharted terrain of strong-field quantum electrodynamics characterised by both high energy and high intensity, reaching the Schwinger field and beyond. The further implications for the search of physics beyond the Standard Model are also discussed.

Effect of lanthanum doped SnO2 on the performance of mixed-cation mixed-halide perovskite layer

Perovskite solar cells (PSCs) have attracted significant attention due to their higher efficiencies and lower fabrication costs. But for the better performance of PSCs, a high-quality electron transport layer (ETL) is crucial. Various ETLs have been employed and among them Tin (IV) oxide (SnO2) has emerged as a promising candidate for electron selective layer in PSCs due to its superior optical and electrical characteristics. However, there is still improvement needed in terms of poor surface morphology and conductivities of SnO2. When SnO2 is used in conjunction with absorber layer in ambient conditions, stability, and charge carrier recombinations at SnO2/perovskite interface remains a serious challenge as well. This study presents the doping of lanthanum (La III), a rare earth element, into SnO2 ETLs to improve the quality and performance of the perovskite layer deposited on top of ETL in ambient condition. With the optimized 4% La (III) doping, SnO2 ETLs become more crystalline with lower parasitic light absorption and surface morphology improves significantly. The improvement in morphology due to doping facilitates larger crystal growth of perovskite in ambient environment. Moreover, Photoluminescence reveals that with optimized level of doping, interfacial charge recombinations are significantly mitigated ensuring smooth injection of electrons into ETL because of superior perovskite film quality. The mixed-cation mixed-halide perovskite film deposited on 4% La:SnO2 ETL show better resistance towards moisture ingress and will substantially contribute to develop long-life of planar PSCs.

Enhancing the Efficiency and Stability of Perovskite Solar Cells Using Chemical Bath Deposition of SnO2 Electron Transport Layers and 3D/2D Heterojunctions.

Chemical bath deposition (CBD) is an effective technique used to produce high-quality SnO2 electron transport layers (ETLs) employed in perovskite solar cells (PSCs). By optimizing the CBD process, high-quality SnO2 films are obtained with minimal oxygen vacancies and close energy level alignment with the perovskite layer. In addition, the 3D perovskite layers are passivated with n-butylammonium iodide (BAI), iso-pentylammonium iodide (PNAI), or 2-methoxyethylammonium iodide (MOAI) to form 3D/2D heterojunctions, resulting in defect passivation, suppressing ion migration and improving charge carrier extraction. As a result of these heterojunctions, the power conversion efficiency (PCE) of the PSCs increased from 21.39% for the reference device to 23.70% for the device containing the MOAI-passivated film. The 2D perovskite layer also provides a hydrophobic barrier, thus enhancing stability to humidity. Notably, the PNAI-based device exhibited remarkable stability, retaining approximately 95% of its initial efficiency after undergoing 1000-h testing in an N2 environment at room temperature.

Screen-Assisted Self-Spreading of TiO2 Precursor Solution on FTO Substrates for High-Quality Electron Transport Layers in Perovskite Solar Cells.

The electron transport layer (ETL) plays a critical role in efficient and stable perovskite solar cells (PSCs). The current effective method for the large-scale preparation of metal oxide ETLs is mainly based on expensive sputtering processes. Here, a screen-assisted self-spreading method is proposed as a novel approach to prepare uniformly thin and conformal TiO2 films on a rough fluorine-doped tin oxide (FTO) substrate as an ETL in planar PSCs. The TiO2 ETL deposited by this method exhibited good coverage and homogeneity on the rough FTO substrate, thereby minimizing interfacial recombination. The photovoltaic performance of the PSCs fabricated by this method is superior to that of the cells fabricated by spin coating, especially in terms of the fill factor. The performance enhancement can be attributed to the complete coverage of the FTO substrate by the conformal TiO2 film, confirming the effectiveness and reliability of the proposed method for the preparation of the TiO2 ETL. The advantages of this method lie in its scalability to prepare oxide films with a large area, eliminating the requirement of complex equipment, such as spinners, sputters, or physical vapor deposition equipment.

All-dielectric meta-surface transmission-mode ultra-thin GaAs negative electron affinity photocathode

Transmission-mode (t-mode) GaAs negative electron affinity photocathodes (NEA-PCs) can be integrated with the optical focusing lenses and microchannel plates to produce high-quality electron beams and high-sensitive detectors. Quantum efficiency (QE) of ∼40% has been reported for the t-mode thick (>1000 nm) GaAs NEA-PCs. Nevertheless, practical applications of these devices have been seriously restricted by their long response time (tens of picoseconds). In this work, the all-dielectric meta-surfaces (ADMS) were designed as the light managers for the t-mode ultra-thin GaAs NEA-PCs. For the 500–850 nm waveband, high light absorption (>80%) can be obtained through coupling the electromagnetic dipole moments of ADMS into the leaky optical modes in 100 nm ultra-thin GaAs NEA-PC layer, which leads to enhanced QE higher than that of the thick ones, the response time less than 5 ps, and the mean transverse energy less than 60 meV, respectively. Given these properties, ADMS t-model ultra-thin NEA-PCs represent a promising photocathode to provide the high-brightness short-pulse spin-polarized electron beams and high-sensitive fast-response detectors for the electron accelerator and low-light-level photodetection applications, respectively.

Printed circuit board and printed circuit board assembly methods for testing and visual inspection: a review

Testing and visual inspection of printed circuit boards (PCBs) and printed circuit board assemblies (PCBAs) are important procedures in the manufacturing process of electronic modules and devices related to locating and identifying possible defects and failures. Earlier defects detection leads to decreasing expenses, time and used resources to produce high quality electronics. In this paper an exploration and analysis about the current research regarding methods for PCB and PCBA testing, techniques for defects detection and vusial inspection is performed. The impact of machine and deep learning for testing and visual inspection procedures is also investigated. The used methodology comprises bibliometric approach and content analysis of papers, indexed in scientific database Scopus, considering the queries: “PCB and testing” and “PCB and testing”, “printed circuit board assembly and testing” and “PCBA and testing”, “PCB defect detection” and “PCBA defect detection”, “PCB and visual inspection”, and “PCBA and visual inspection”. The findings are presented in the form of a framework, which summarizes the contemporary landscape of methods for PCBs and PCBAs testing and visual inspection.

High-quality Scanning Electron Microscopy Incorporated with a Freeze-drying and Gaseous Nitrogen-based Approach for Cell-extracellular Vesicle Interactions

High-quality Scanning Electron Microscopy Incorporated with a Freeze-drying and Gaseous Nitrogen-based Approach for Cell-extracellular Vesicle Interactions

Symmetrical dual-D and dual-core single-mode fiber surface plasmon resonance liquid sensor.

A symmetrical dual-D and dual-core single-mode fiber surface plasmon resonance (SPR) liquid sensor is designed for biological detection. The dual-core design optimizes the transmission path, improves the momentum matching between free electrons and photons, and facilitates bidirectional coupling, consequently amplifying the SPR effect and enabling sensitive monitoring of the refractive index changes of biological solutions. In this structure, a gold wire is placed in the middle of the polished surface of the double-D-shaped single-mode fiber (SMF) to produce high-quality free electrons and promote the mode-coupling excitation of the SPR effect. The characteristics of the sensor are analyzed by the finite element method, and the important structural parameters are optimized systematically. The sensor can be operated in the near-infrared region for a refractive index (RI) range of 1.31-1.40 with a maximum wavelength sensitivity (WS) of 21,000nm/RIU, amplitude sensitivity of 586.62R I U -1, as well as resolution of 4.76×10-6 R I U. Small changes in the refractive index can be detected by the sensor and it can be produced easily by conventional manufacturing techniques. The sensor thus has wide application prospects in biomedical and chemical analysis and related applications.

Nanoscale heterojunctions of InGaN/GaN photocathodes for electron sources

The design of the photocathode is crucial for generating high-quality electron beam in electron sources. In this study, InGaN/GaN heterojunction nanowire photocathodes were proposed, and the photoemission theoretical model was developed. The results demonstrate that the built-in electric field along the axis enables the heterojunction nanowire photocathode to achieve higher collection efficiency across the response spectrum compared to In0.5Ga0.5N nanowire photocathodes. The variation of incidence angle results in distinct peaks in quantum efficiency and collection efficiency for the heterojunction nanowire photocathode. Meanwhile, the “additional electric field” has the potential to decrease the number of laterally emitted electrons from the nanowires, consequently reducing the shielding effect of adjacent nanowires. With an incidence angle of 15° and an “additional electric field” of 2 V/μm, the collection efficiency of photoelectrons at the collection end can be maximized to 47.6 %. This indicates that the heterojunction nanowire array photocathode has potential for use in high-performance vacuum electron sources.

Emission surface improvement of In0.5Ga0.5N tilted nanowire array photocathode for vacuum electron sources

Photocathode has received increasing attention in scientific devices that require high-quality electron beams. A theoretical photoemission model for enhancing the performance of In0.5Ga0.5N tilted nanowire array photocathodes with negative electron affinity surface is proposed. Finite Element and finite difference methods are employed in simulation experiments to study the In0.5Ga0.5N tilted nanowire array photocathode. The results demonstrate that the photoelectric conversion performance of the In0.5Ga0.5N tilted nanowire array photocathode can be significantly enhanced through the combined impact of oblique incident light and an additional electric field. Specifically, when θ = 20°, β = −40°, and Eout = 4 V/μm, the total quantum efficiency and collection efficiency are 2.79 times and 4.27 times higher than compared to vertical nanowires without external assistance, respectively.

Rediscovering Globigerina bollii Cita and Premoli Silva 1960

Abstract. Globigerina bollii Cita and Premoli Silva was described from the historical Langhian-type section in Langhe, Piedmont (Italy). Due to its peculiar compact morphology, it was set apart from all the other globigerinids typical of the coeval Mediterranean fauna, and it was only reported for a short and limited stratigraphic range. The taxon became a first-order marker for the local biostratigraphy with its own Globigerina bollii Zone within the Langhian stage. However, the species was later synonymised with Globigerina falconensis Blow, ending its use in biostratigraphic schemes, and it was no longer utilised by authors working in the Mediterranean area and Paratethys. We present a reassessment of Globigerina bollii, showing for the first time a full collection of high-quality scanning electron and optical microscope images of the type series of specimens and a comparative study with Mediterranean individuals from the Langhian of the Cretaccio Section (Italy) and extra-Mediterranean individuals from Ocean Drilling Program Site 747 in the Kerguelen Plateau (Indian Ocean). The stratigraphic ranges of all the occurrences cited in the scientific literature from 1960 to the present day and all the references including images of the taxon are compiled. We compare G. bollii to other four-chambered morphospecies inhabiting the oceans during the Miocene, providing a detailed discussion of their morphological differences, which allows us to retain G. bollii as a valid taxon and to disclaim its synonymy with Globigerina falconensis. Our taxonomical observations also allow us to reassign Globigerina bollii to the genus Globoturborotalita, due to its strong affinities with other members of that genus, such as G. eolabiacrassata Spezzaferri and Coxall, and G. ouachitaensis (Howe and Wallace). We present a direct visual comparison with the other representatives of middle Miocene globoturborotaliids. An additional comparison is also discussed with Globigerina bollii lentiana Rögl, a species endemic in the Paratethys. We conclude that the presence of G. bollii in the Mediterranean Basin during such a confined stratigraphic interval (Mediterranean Subzone MMi4c–MMi4d), might be a palaeogeographical indicator of the intermittent opening of the eastern gateway with the Paratethys, affecting the Mediterranean faunas during the Langhian and their migration from oceanic realms into the Paratethys and Mediterranean.

Structural snapshots of phenuivirus cap-snatching and transcription.

Severe fever with thrombocytopenia syndrome virus (SFTSV) is a human pathogen that is now endemic to several East Asian countries. The viral large (L) protein catalyzes viral transcription by stealing host mRNA caps via a process known as cap-snatching. Here, we establish an in vitro cap-snatching assay and present three high-quality electron cryo-microscopy (cryo-EM) structures of the SFTSV L protein in biologically relevant, transcription-specific states. In a priming-state structure, we show capped RNA bound to the L protein cap-binding domain (CBD). The L protein conformation in this priming structure is significantly different from published replication-state structures, in particular the N- and C-terminal domains. The capped-RNA is positioned in a way that it can feed directly into the RNA-dependent RNA polymerase (RdRp) ready for elongation. We also captured the L protein in an early-elongation state following primer-incorporation demonstrating that this priming conformation is retained at least in the very early stages of primer extension. This structural data is complemented by in vitro biochemical and cell-based assays. Together, these insights further our mechanistic understanding of how SFTSV and other bunyaviruses incorporate stolen host mRNA fragments into their viral transcripts thereby allowing the virus to hijack host cell translation machinery.

Evaluating the Quality of Serial EM Sections with Deep Learning.

Automated image acquisition can significantly improve the throughput of serial section scanning electron microscopy (ssSEM). However, image quality can vary from image to image depending on autofocusing and beam stigmation. Automatically evaluating the quality of images is, therefore, important for efficiently generating high-quality serial section scanning electron microscopy (ssSEM) datasets. We tested several convolutional neural networks for their ability to reproduce user-generated evaluations of ssSEM image quality. We found that a modification of ResNet-50 that we term quality evaluation Network (QEN) reliably predicts user-generated quality scores. Running QEN in parallel to ssSEM image acquisition therefore allows users to quickly identify imaging problems and flag images for retaking. We have publicly shared the Python code for evaluating images with QEN, the code for training QEN, and the training dataset.

Generation of attosecond electron bunches of tunable duration and density by relativistic vortex lasers in near-critical density plasma

Attosecond electron bunches have wide application prospects in free-electron laser injection, attosecond X/γ-ray generation, ultrafast physics, etc. Nowadays, there is one notable challenge in the generation of high-quality attosecond electron bunch, i.e., how to enhance the electron bunch density. Using theoretical analysis and three-dimensional particle-in-cell simulations, we discovered that a relativistic vortex laser pulse interacting with near-critical density plasma can not only effectively concentrate the attosecond electron bunches to over critical density, but also control the duration and density of the electron bunches by tuning the intensity and carrier-envelope phase of the drive laser. It is demonstrated that this method can efficiently produce attosecond electron bunches with a density up to 300 times of the original plasma density, peak divergence angle of less than 0.5∘, and duration of less than 67 attoseconds. Furthermore, by using near-critical density plasma instead of solid targets, our scheme is potential for the generation of high-repetition-frequency attosecond electron bunches, thus reducing the requirements for experiments, such as the beam alignment or target supporter.

4D-STEM Mapping of Nanocrystal Reaction Dynamics and Heterogeneity in a Graphene Liquid Cell.

Chemical reaction kinetics at the nanoscale are intertwined with heterogeneity in structure and composition. However, mapping such heterogeneity in a liquid environment is extremely challenging. Here we integrate graphene liquid cell (GLC) transmission electron microscopy and four-dimensional scanning transmission electron microscopy to image the etching dynamics of gold nanorods in the reaction media. Critical to our experiment is the small liquid thickness in a GLC that allows the collection of high-quality electron diffraction patterns at low dose conditions. Machine learning-based data-mining of the diffraction patterns maps the three-dimensional nanocrystal orientation, groups spatial domains of various species in the GLC, and identifies newly generated nanocrystallites during reaction, offering a comprehensive understanding on the reaction mechanism inside a nanoenvironment. This work opens opportunities in probing the interplay of structural properties such as phase and strain with solution-phase reaction dynamics, which is important for applications in catalysis, energy storage, and self-assembly.

Divergence angle consideration in energy spread measurement for high-quality relativistic electron beam in laser wakefield acceleration

The thorough exploration of the transverse quality represented by divergence angle has been lacking yet in the energy spread measurement of the relativistic electron beam for laser wakefield acceleration (LWFA). In this work, we fill this gap by numerical simulations based on the experimental data, which indicate that in a C-shape magnet, magnetic field possesses the beam focusing effect, considering that the divergence angle will result in an increase in the full width at half maxima (FWHM) of the electron density distribution in a uniformly isotropic manner, while the length-to-width ratio decreases. This indicates that the energy spread obtained from the electron deflection distance is smaller than the actual value, regardless of the divergence angle. A promising and efficient way to accurately correct the value is presented by considering the divergence angle (for instance, for an electron beam with a length-to-width ratio of 1.12, the energy spread correct from 1.2% to 1.5%), providing a reference for developing the high-quality electron beam source.