Ultra-low Electron Research Articles

Molecular Design of n-Type Organic Semiconductors with Ultralow Electron Reorganization Energies.

Bottom-up design of n-type organic semiconductors with salient electron-transport properties is of fundamental importance. Here, with the aid of density functional theory, we demonstrate that condensing odd-membered carbon rings (5MR/7MR) to small polycyclic aromatic hydrocarbons (PAHs) can endow molecules with ultralow electron reorganization energies (λ < 100 meV). Studying 60 molecules, we find that introducing polycyclic fragments with built-in 5MR and 7MR to linear PAHs at the Cα,β positions or to nonlinear PAHs in D2h symmetry constitutes molecules with λ as low as 65 meV. A joint ACID and NICS analysis proves that the stronger the molecular aromaticity, the lower the λ will be. This is contrary to what has been previously found for p-type molecules. Furthermore, we propose "acupoints" on molecules for N-doping and cyanation, which can be used to precisely locate the substitution sites to reduce the LUMO energies (in favor of electron injection and air stability) of low-λ molecules while it does not elevate λ. These findings would help to reduce the synthetic blindness/cost and contribute to the bottom-up design of n-type small-molecule organic semiconductors.

All-optical source size and emittance measurements of laser-accelerated electron beams

Novel schemes for generating ultralow emittance electron beams have been developed in past years and promise compact particle sources with excellent beam quality suitable for future high-energy physics experiments and free-electron lasers. Recent theoretical work has proposed a laser-based method capable of resolving emittances in the sub 0.1 mm mrad regime by modulating the electron phase space ponderomotively. Here we present the first experimental demonstration of this scheme using a laser wakefield accelerator. The observed emittance and source size are consistent with published values. We also show calculations demonstrating that tight bounds on the upper limit for emittance and source size can be derived from the “laser-grating” method even in the presence of low signal to noise and uncertainty in laser-grating parameters. Published by the American Physical Society 2024

Optimize Electron Beam Energy toward In Situ Imaging of Thick Frozen Bio-Samples with Nanometer Resolution Using MeV-STEM.

To optimize electron energy for in situ imaging of large biological samples up to 10 μm in thickness with nanoscale resolutions, we implemented an analytical model based on elastic and inelastic characteristic angles. This model has been benchmarked by Monte Carlo simulations and can be used to predict the transverse beam size broadening as a function of electron energy while the probe beam traverses through the sample. As a result, the optimal choice of the electron beam energy can be realized. In addition, the impact of the dose-limited resolution was analysed. While the sample thickness is less than 10 μm, there exists an optimal electron beam energy below 10 MeV regarding a specific sample thickness. However, for samples thicker than 10 μm, the optimal beam energy is 10 MeV or higher depending on the sample thickness, and the ultimate resolution could become worse with the increase in the sample thickness. Moreover, a MeV-STEM column based on a two-stage lens system can be applied to reduce the beam size from one micron at aperture to one nanometre at the sample with the energy tuning range from 3 to 10 MeV. In conjunction with the state-of-the-art ultralow emittance electron source that we recently implemented, the maximum size of an electron beam when it traverses through an up to 10 μm thick bio-sample can be kept less than 10 nm. This is a critical step toward the in situ imaging of large, thick biological samples with nanometer resolution.

Pinhole camera for electron beam size diagnostic at storage ring with an ultralow emittance

In this work, we propose to use a pinhole camera at high photon energies, specifically 200–300 keV, to measure ultra-small electron beam size by means of bending magnet radiation. We show that there is a sufficient photon flux at the detector position. Our theoretical analysis includes an examination of the applicability of the van Cittert-Zernike theorem for the bending magnet radiation generated by an ultralow emittance electron beam and a detailed analysis of the imaging properties of rectangular pinhole cameras. This led us to practical, universal formulas. We identify the optimal aperture size and resolution of the camera in the given geometry. The theoretical findings are further substantiated by wavefront propagation numerical simulations of partially coherent radiation. This study serves both as a practical guide for optical engineering and an educational resource for explaining the imaging properties of pinhole cameras. Published by the American Physical Society 2024

Imaging built-in electric fields and light matter by Fourier-precession TEM

We report the precise measurement of electric fields in nanostructures, and high-contrast imaging of soft matter at ultralow electron doses by transmission electron microscopy (TEM). In particular, a versatile method based on the theorem of reciprocity is introduced to enable differential phase contrast imaging and ptychography in conventional, plane-wave illumination TEM. This is realised by a series of TEM images acquired under different tilts, thereby introducing the sampling rate in reciprocal space as a tuneable parameter, in contrast to momentum-resolved scanning techniques. First, the electric field of a p–n junction in GaAs is imaged. Second, low-dose, in-focus ptychographic and DPC characterisation of Kagome pores in weakly scattering covalent organic frameworks is demonstrated by using a precessing electron beam in combination with a direct electron detector. The approach offers utmost flexibility to record relevant spatial frequencies selectively, while acquisition times and dose requirements are significantly reduced compared to the 4D-STEM counterpart.

Elettra 2.0: The Vacuum System Design for a New Generation Storage Ring

Next-generation synchrotron facilities currently under development and construction are promising and delivering great advances in terms of machine design and research physics. At the same time, they are pushing the limits of technological solutions for various subsystems: to achieve the desired ultra-low electron beam emittance required in the next generation machines, the magnetic lattice has to adopt a compact design with reduced magnet apertures and other constraints like high heat load from synchrotron radiation, dynamic pressure inside vacuum chambers and chamber wall impedance are presenting unique challenges for the vacuum and engineering teams. At Elettra-Sincrotrone Trieste (Italy), the Elettra 2.0 project aims to develop a new-generation storage ring. Taking into consideration the above-mentioned constraints, we decided to adopt a new design of a vacuum chamber, while utilizing novel pumping solutions to overcome hugely reduced conductance compared to the current machine. Large sputter ion pumps (SIP) will be in the majority replaced by distributed non-evaporable getter (NEG) coatings and small NEG cartridges and SIP pumps. For the synchrotron radiation handling, due to the tight space constraints imposed by the compact lattice, the photon absorption will be managed jointly with discrete and distributed solutions: photon absorbers have been carefully studied for combining compact form and high power density loads, while key sections of the new storage ring will be water-cooled. Throughout the whole development phase, we were using Monte-Carlo simulation codes like SynRad and MolFlow+ as effective tools supporting the design of new vacuum chambers and photon absorbers. The current state of development for the Elettra 2.0 vacuum system, the challenges we faced and the solutions we adopted are presented here.

Nano-fusiform Li2FeSiO4 with highly exposed (001) facets for Lithium-Ion Batteries

Li2FeSiO4 is a promising cathode material with high energy density and low cost for lithium-ion batteries. However, it is suffering from ultralow ion and electron kinetics, which limit its future application. To overcome the above drawbacks, crystal habits engineering and nanoscale effect strategies are adopted to improve the kinetics process. A nano-scale fusiform Li2FeSiO4 with highly exposed (001) facets is successfully synthesized by using cetyl trimethyl ammonium bromide (CTAB) as a structure directing agent, and the specific formation mechanism is also revealed. Benefiting from this fusiform face-exposed crystal structure, the fusiform-Li2FeSiO4 nanocrystal delivers a reversible capacity of 225.4 mAh g−1 at 0.1C, and 90 % capacity retention for 800 cycles. Such ion/electron kinetic-enhanced engineered nanocrystals offer an exemplification in developing next-generation cathodes for high-energy-density Lithium-ion batteries.

Real-Space Imaging of the Molecular Changes in Metal-Organic Frameworks under Electron Irradiation.

Electron-induced structural changes influence the characterizations of the local structure of various materials by electron microscope. However, for beam-sensitive materials, it is still challenging to detect such changes by electron microscopy, which may help us quantitatively reveal how electrons interact with materials under electron irradiation. Here, we use an emergent phase contrast technique in electron microscopy to clearly image a metal-organic framework, UiO-66 (Zr), at an ultralow electron dose and dose rate. The effects of both the dose and dose rate on the UiO-66 (Zr) structure are visualized, which induce obvious missing organic linkers. The kinetics of the missing linker based on the radiolysis mechanism are semiquantitatively expressed by the different intensities of the imaged organic linkers. Deformation of the UiO-66 (Zr) lattice after the missing linker is also observed. These observations make it possible to visually investigate the electron-induced chemistry in various beam-sensitive materials and avoid electron damage to them.

Ultralow electron emission yield achieved on alumina ceramic surfaces and its application in multipactor suppression

Alumina ceramics used in microwave systems are susceptible to the multiplication of secondary electron emission on the surface due to the influence of resonance between electrons and the radiofrequency electric field, and a detrimental multipactor effect may therefore be triggered. For the alumina-loaded microwave components, it is essential to achieve low secondary electron yield (SEY) on the inserted alumina surfaces to mitigate multipactor. In this work, to achieve an ultralow SEY surface of alumina, two recognized low-SEY treatments were combined. For the primary SEY suppression, a series of microstructures were fabricated on the alumina surfaces with varied porosity and aspect ratio at the hundred-micrometer scale by infrared laser etching. The microstructure with 52.14% porosity and 1.78 aspect ratio showed an excellent low-SEY property, which could suppress the SEY peak value ( δ m ) of alumina from 2.46 to 1.00. For the secondary SEY suppression, the SEY dependence of TiN coating on sputtering parameters was studied, and the lowest δ m of 1.19 was achieved when the gas flow ratio of Ar:N2 was 15:7.5. Thereafter, by depositing TiN ceramic coating onto the laser-etched porous samples, an ultralow SEY, with δ m of 0.69, was achieved on the alumina surfaces. The simulation work revealed the impact of dielectric surface charge on electron multiplication and revealed a mechanism of using low-SEY surfaces to inhibit multipactor. Some coaxial filters filled with alumina were fabricated for verification; the results revealed that the multipactor threshold increased from 125 W to 425 W after applying the TiN-coated porous alumina, and to 650 W after treating another multipactor-sensitive area with the same low-SEY process. This work developed an advisable method to sharply reduce SEY, which is of great significance for the multipactor mitigation of alumina-loaded microwave components.

Efficient vibrational excitation of molecular nitrogen in low-pressure plasma with ultralow electron temperature

We investigated the vibrational temperature of molecular nitrogen in the downstream of helicon-wave excited helium and argon-based plasmas. It was confirmed by optical emission spectroscopy that the major part of the helium plasma was at a recombining state and it had an ultralow electron temperature of approximately 0.1 eV. In spite of the ultralow electron temperature, the vibrational temperature of molecular nitrogen, which was added into the helium plasma, was higher than that in the argon-based plasma at an ionizing state with an electron temperature of 1.7 eV. According to the relationship between the rate coefficient of electron impact vibrational excitation and the electron temperature, the higher vibrational temperature in the helium plasma is not attributable to the more efficient vibrational excitation. Therefore, the higher vibrational temperature is owing to the less efficient destruction of vibrational excited states in the helium plasma with the ultralow electron temperature.

Direct Location of Organic Molecules in Framework Materials by Three-Dimensional Electron Diffraction.

In the study of framework materials, probing interactions between frameworks and organic molecules is one of the most important tasks, which offers us a fundamental understanding of host-guest interactions in gas sorption, separation, catalysis, and framework structure formation. Single-crystal X-ray diffraction (SCXRD) is a conventional method to locate organic species and study such interactions. However, SCXRD demands large crystals whose quality is often vulnerable to, e.g., cracking on the crystals by introducing organic molecules, and this is a major challenge to use SCXRD for structural analysis. With the development of three-dimensional electron diffraction (3D ED), single-crystal structural analysis can be performed on very tiny crystals with sizes on the nanometer scale. Here, we analyze two framework materials, SU-8 and SU-68, with organic molecules inside their inorganic crystal structures. By applying 3D ED, with fast data collection and an ultralow electron dose (0.8-2.6 e- Å-2), we demonstrate for the first time that each nonhydrogen atom from the organic molecules can be ab initio located from structure solution, and they are shown as distinct and well-separated peaks in the difference electrostatic potential maps showing high accuracy and reliability. As a result, two different spatial configurations are identified for the same guest molecule in SU-8. We find that the organic molecules interact with the framework through strong hydrogen bonding, which is the key to immobilizing them at well-defined positions. In addition, we demonstrate that host-guest systems can be studied at room temperature. Providing high accuracy and reliability, we believe that 3D ED can be used as a powerful tool to study host-guest interactions, especially for nanocrystals.

Strategy for optimizing experimental settings for studying low atomic number colloidal assemblies using liquid phase scanning transmission electron microscopy

Observing processes of nanoscale materials of low atomic number is possible using liquid phase electron microscopy (LP-EM). However, the achievable spatial resolution (d) is limited by radiation damage. Here, we examine a strategy for optimizing LP-EM experiments based on an analytical model and experimental measurements, and develop a method for quantifying image quality at ultra low electron dose De using scanning transmission electron microscopy (STEM). As experimental test case we study the formation of a colloidal binary system containing 30 nm diameter SiO2 nanoparticles (SiONPs), and 100 nm diameter polystyrene microspheres (PMs). We show that annular dark field (DF) STEM is preferred over bright field (BF) STEM for practical reasons. Precise knowledge of the material's density is crucial for the calculations in order to match experimental data. To calculate the detectability of nano-objects in an image, the Rose criterion for single pixels is expanded to a model of the signal to noise ratio obtained for multiple pixels spanning the image of an object. Using optimized settings, it is possible to visualize the radiation-sensitive, hierarchical low-Z binary structures, and identify both components.

Reducing Contact Resistance and Boosting Device Performance of Monolayer MoS2 by In Situ Fe Doping.

2D semiconductors are emerging as plausible candidates for next-generation "More-than-Moore" nanoelectronics to tackle the scaling challenge of transistors. Wafer-scale 2D semiconductors, such as MoS2 and WS2 , have been successfully synthesized recently; nevertheless, the absence of effective doping technology fundamentally results in energy barriers and high contact resistances at the metal-semiconductor interfaces, and thus restrict their practical applications. Herein, a controllable doping strategy in centimeter-sized monolayer MoS2 films is developed to address this critical issue and boost the device performance. The ultralow contact resistance and perfect Ohmic contact with metal electrodes are uncovered in monolayer Fe-doped MoS2 , which deliver excellent device performance featured with ultrahigh electron mobility and outstanding on/off current ratio. Impurity scattering is suppressed significantly thanks to the ultralow electron effective mass and appropriate doping site. Particularly, unidirectionally aligned monolayer Fe-doped MoS2 domains are prepared on 2 in. commercial c-plane sapphire, suggesting the feasibility of synthesizing wafer-scale 2D single-crystal semiconductors with outstanding device performance. This work presents the potential of high-performance monolayer transistors and enables further device downscaling and extension of Moore's law.

Ultralow secondary electron emission and improved vacuum surface insulation of polyimide with scalable nanocomposite coating

Polymers with high surface electric insulation capability are greatly demanded in various electrical devices operating in vacuum ambient. Low secondary electron emission (SEE) is favorable to mitigate vacuum flashover by inhibiting the SEE avalanche across the surface. Although numerous approaches, esp. the surface modifications, promote the flashover strength of the insulator, a facile yet cost-effective modification strategy of a high-quality and scalable flexible insulator remains a formidable challenge. Herein, a facile ion-exchange technique is implemented to fabricate a nanoscale polyimide/chromic oxide (PI/Cr2O3) nanocomposite coating comprising hundreds of organic/inorganic interfaces securely adhered to a flexible PI substrate, which features ultralow secondary electron emission yields (SEEY). The low emissivity is achieved by introducing weakly emissive Cr2O3 nanoparticles as well as interfacial traps within the nanocomposite layer, which outperforms the existing and modified polymers. The PI/Cr2O3 shows a remarkable increase in flashover strength while keeping sufficient mechanical durability. The proposed surface engineering strategy is convenient and scalable, bringing up new insight into industrial applications.

Ytterbium oxide electron injection interface in organic light-emitting diode

The ytterbium oxide (Yb3+) is found to have an extremely low work function of 2.42 eV, which is even lower than that of its metallic form Yb0 (2.64 eV). The stability of oxides makes Yb3+ an ideal electron injection material for both top-emitting and bottom-emitting organic light-emitting diodes (TOLED and BOLED). The device test data indeed show that at 1000 nit luminance, the TOLED has a 94 cd/A current efficiency and 70 lm/W power efficiency, and BOLED has a 76 cd/A and 60 lm/W efficiency, respectively. X-ray and ultraviolet photoemission spectroscopical studies indicate that the Fermi level of the metal oxide is pinned to the lowest unoccupied molecular orbital of the electron transport layer, leading to the formation of a cathode interface with an ultra-low electron injection barrier.

Improvement of ultra-low dose electron image of TEM by deep learning

Improvement of ultra-low dose electron image of TEM by deep learning

Effect of co-dopants on the spectral property of Yb3+ doped silica glasses at 1018 nm

To enhance the laser performance of Yb3+-doped silica fiber for 1018-nm tandem-pumped technology, three series of Yb–Al, Yb-Al-P, and Yb–P co-doped silica glasses with a large size, high homogeneity, and precise composition control were prepared using a sol-gel method combined with high-temperature sintering technology. In addition, the effects of Al3+ and P5+ co-dopants on the absorption and emission properties of Yb3+-doped silica glasses at 1018 nm were investigated systematically. The normalized spectral characteristics of the glass samples with different doping series at 1018 nm were compared horizontally, and the results showed that the normalized emission intensity of the Yb-P-doped silica glass series samples at 1018 nm was significantly better than that of the other series. The emission subpeak was located near 1018 nm when the content of doped P was 5 mol%, and the position of the subpeak gradually blue-shifted as the content of the doped P increased. Using Raman spectroscopy combined with ultra-low temperature electron paramagnetic resonance (4K), the coordination environment of Yb3+ was accurately resolved at the atomic scale. In Yb3+-doped silica glasses, the coordination environment of Yb3+ was extremely different from the introduction of Al3+ and P5+, which is consistent with the influence of Al3+ and P5+ on the spectral properties of Yb3+ at 1018 nm.

Atomic-scale imaging of CH3NH3PbI3 structure and its decomposition pathway

Understanding the atomic structure and structural instability of organic-inorganic hybrid perovskites is the key to appreciate their remarkable photoelectric properties and understand failure mechanism. Here, using low-dose imaging technique by direct-detection electron-counting camera in a transmission electron microscope, we investigate the atomic structure and decomposition pathway of CH3NH3PbI3 (MAPbI3) at the atomic scale. We successfully image the atomic structure of perovskite in real space under ultra-low electron dose condition, and observe a two-step decomposition process, i.e., initial loss of MA+ followed by the collapse of perovskite structure into 6H-PbI2 with their critical threshold doses also determined. Interestingly, an intermediate phase (MA0.5PbI3) with locally ordered vacancies can robustly exist before perovskite collapses, enlightening strategies for prevention and recovery of perovskite structure during the degradation. Associated with the structure evolution, the bandgap gradually increases from ~1.6 eV to ~2.1 eV. In addition, it is found that C-N bonds can be readily destroyed under irradiation, releasing NH3 and HI and leaving hydrocarbons. These findings enhance our understanding of the photoelectric properties and failure mechanism of MAPbI3, providing potential strategies into material optimization.

Characterizing ultralow emittance electron beams using structured light fields

Novel schemes for generating ultra-low emittance electron beams have been developed in the last years and promise compact particle sources with excellent beam quality suitable for future high-energy physics experiments and free-electron lasers. Current methods for the characterization of low emittance electron beams such as pepperpot measurements or beam focus scanning are limited in their capability to resolve emittances in the sub $0.1$ mm mrad regime. Here we propose a novel, highly sensitive method for the single shot characterization of the beam waist and emittance using interfering laser beams. In this scheme, two laser pulses are focused under an angle creating a grating-like interference pattern. When the electron beam interacts with the structured laser field, the phase space of the electron beam becomes modulated by the laser ponderomotive force and results in a modulated beam profile after further electron beam phase advance, which allows for the characterization of ultra-low emittance beams. 2D PIC simulations show the effectiveness of the technique for normalized emittances in the range of $\epsilon_n=[0.01,1]$ mm mrad.

Unraveling the electron transfer rates of highly crystalline carbon nanowires with surface oxides.

In the development of glassy carbon fiber toward graphene fiber, highly crystalline carbon wires have attracted attention. More importantly, a charge cannot be accommodated at the surface of highly oriented pyrolytic graphite as it would be in a metal. In this work, we demonstrate that enhancing the decyanation reaction rate and reducing the nanowire diameter to below the crystallite size (≲50 nm) greatly contribute to the microstructure transformation of carbon from low crystalline glassy carbon to crystalline micro-structure. Using silica surfaces to limit the shrinkage of electrospun nanofibers during oxidation and carbonization, enhances the conversion of alcohol groups to normal carbonyl groups on the surface of the carbon wires derived from PAN fibers deposited with near field electrospinning (NFES). Cyclic voltammograms (CVs) on the carbon nanowires reveal that the enhancement of alcohol groups to normal carbonyl groups slows down the rapid electron transfer on glassy carbon electrodes. Using electrochemical impedance spectroscopy (EIS), we also establish that the electron transfer on the surface of highly crystalline carbon nanowires almost completely depends on the presence of oxygen groups. The highly crystalline structure of nanoscale carbon wires with a large amount of normal carbonyl groups exhibits an ultra-low electron transfer rate (less than 1.2 μm s-1), showing the ability to make the charges reside on the highly crystalline carbon nanowires. The straight line in CV allows for EIS measurements at high alternating current voltages, improving upon the non-linearity of traditional electrochemical cells by overcoming the stochastic errors and the lower signal-to-noise ratio for ultra-sensitive biomolecule detection (≤25 mV). The latter could spur the development of a new generation of electrochemical cells and biomedical signal measurements.