Electron Beam Sensitivity Research Articles

Low-dose electron microscopy imaging for beam-sensitive metal-organic frameworks.

Metal-organic frameworks (MOFs) have garnered significant attention in recent years owing to their exceptional properties. Understanding the intricate relationship between the structure of a material and its properties is crucial for guiding the synthesis and application of these materials. (Scanning) Transmission electron microscopy (S)TEM imaging stands out as a powerful tool for structural characterization at the nanoscale, capable of detailing both periodic and aperiodic local structures. However, the high electron-beam sensitivity of MOFs presents substantial challenges in their structural characterization using (S)TEM. This paper summarizes the latest advancements in low-dose high-resolution (S)TEM imaging technology and its application in MOF material characterization. It covers aspects such as framework structure, defects, and surface and interface analysis, along with the distribution of guest molecules within MOFs. This review also discusses emerging technologies like electron ptychography and outlines several prospective research directions in this field.

Investigation of the Octahedral Network Structure in Formamidinium Lead Bromide Nanocrystals by Low-Dose Scanning Transmission Electron Microscopy.

Metal halide perovskites (MHP) are highly promising semiconductors. In this study, we focus on FAPbBr3 nanocrystals, which are of great interest for green light-emitting diodes. Structural parameters significantly impact the properties of MHPs and are linked to phase instability, which hampers long-term applications. Clearly, there is a need for local and precise characterization techniques at the atomic scale, such as transmission electron microscopy. Because of the high electron beam sensitivity of MHPs, these investigations are extremely challenging. Here, we applied a low-dose method based on four-dimensional scanning transmission electron microscopy. We quantified the observed elongation of the projections of the Br atomic columns, suggesting an alternation in the position of the Br atoms perpendicular to the Pb-Br-Pb bonds. Together with molecular dynamics simulations, these results remarkably reveal local distortions in an on-average cubic structure. Additionally, this study provides an approach to prospectively investigating the fundamental degradation mechanisms of MHPs.

Direct visualization of ligands on gold nanoparticles in a liquid environment.

The interactions between gold nanoparticles, their surface ligands and the solvent critically influence the properties of these nanoparticles. Although spectroscopic and scattering techniques have been used to investigate their ensemble structure, a comprehensive understanding of these processes at the nanoscale remains challenging. Electron microscopy makes it possible to characterize the local structure and composition but is limited by insufficient contrast, electron beam sensitivity and the requirement for ultrahigh-vacuum conditions, which prevent the investigation of dynamic aspects. Here we show that, by exploiting high-quality graphene liquid cells, we can overcome these limitations and investigate the structure of the ligand shell around gold nanoparticles and at the ligand-gold interface in a liquid environment. Using this graphene liquid cell, we visualize the anisotropy, composition and dynamics of ligand distribution on gold nanorod surfaces. Our results indicate a micellar model for surfactant organization. This work provides a reliable and direct visualization of ligand distribution around colloidal nanoparticles.

Electron beam analysis and sensitivity studies for the EuPRAXIA@SPARC_LAB RF injector

The interest in plasma-based accelerators as drivers of user facilities is growing worldwide thanks to their compactness and reduced costs. The EuPRAXIA@SPARC_LAB collaboration is preparing a technical design report for a multi-GeV plasma-based accelerator with outstanding electron beam quality to pilot an X-ray FEL, the most demanding in terms of beam brightness. The beam dynamics has been studied aiming to a reliable operation of the RF injector to generate a so-called comb-beam with 500 MeV energy suitable as driver of the Beam-driven Plasma Wakefield Accelerator. A case of interest is the generation of a trailing bunch with 1 GeV energy, less than 1 mm-mrad transverse emittance and up to 2 kA peak current at the undulator entrance. The comb-beam is generated through the velocity bunching technique, an RF compression tool that enables high brightness beams within relatively compact machine. Since it is based on a rotation of the beam phase space inside the external RF fields, it could be particularly sensitive to amplitude and phase jitters in the RF injector. The electron beam dynamics and the machine sensitivity to the possible jitters are presented in terms of effect on the beam quality so to provide the basis for the jitter tolerances.

Wet etching of gold on graphene for high-quality resist-free graphene surfaces

Wet etching of gold on graphene is challenging due to the weak adhesion of the resist mask to graphene. We report an operating procedure for alkali ion-free wet etching of gold on graphene using a mixture of hydrochloric and nitric acids (aqua regia) with a high lateral resolution down to 100 nm. We investigate the role of positive and negative resists, electron beam lithography (EBL) dose, hard-bake, oxygen etching, aging, and sensitivity to the etch parameters, such as the freshness of dilute aqua regia, etch time, and the order of etched samples. The negative-tone resist provides the best results. The over-dosed EBL exposure can enhance the resist adhesion, as hard-bake below the glass-transition temperature and well-defined wet etch of the resist-residua-free gold surface. We also present a cleaning procedure to avoid bubble formation after the hard bake. Our results demonstrate that wet etching of gold on graphene using aqua regia is a viable method for achieving high-quality resist-free graphene surfaces. This method has potential applications in graphene nanoelectronics and nanophotonics, where high-quality graphene surfaces are essential for device performance.

Three-dimensional inhomogeneity of zeolite structure and composition revealed by electron ptychography.

Structural and compositional inhomogeneity is common in zeolites and considerably affects their properties. Thickness-limited lateral resolution, lack of depth resolution, and electron dose-constrained focusing limit local structural studies of zeolites in conventional transmission electron microscopy (TEM). We demonstrate that a multislice ptychography method based on four-dimensional scanning TEM (4D-STEM) data can overcome these limitations. Images obtained from a ~40-nanometer-thick MFI zeolite exhibited a lateral resolution of ~0.85 angstrom that enabled the identification of individual framework oxygen (O) atoms and the precise determination of the orientations of adsorbed molecules. Furthermore, a depth resolution of ~6.6 nanometers allowed probing of the three-dimensional distribution of O vacancies, as well as the phase boundaries in intergrown MFI and MEL zeolites. The 4D-STEM ptychography can be generally applied to other materials with similar high electron-beam sensitivity.

Atomic Insights into the Cu Species Supported on Zeolite for Direct Oxidation of Methane to Methanol via Low-Damage HAADF-STEM.

Precise determination of the structure-property relationship of zeolite-based metal catalysts is critical for the development toward practical applications. However, the scarcity of real-space imaging of zeolite-based low-atomic-number (LAN) metal materials due to the electron-beam sensitivity of zeolites has led to continuous debates regarding the exact LAN metal configurations. Here, a low-damage high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging technique is employed for direct visualization and determination of LAN metal (Cu) species in ZSM-5 zeolite frameworks. The structures of the Cu species are revealed based on the microscopy evidence and also proved by the complementary spectroscopy results. The correlation between the characteristic Cu size in Cu/ZSM-5 catalysts and their direct oxidation of methane to methanol reaction properties is unveiled. As a result, the mono-Cu species stably anchored by Al pairs inside the zeolite channels are identified as the key structure for higher C1 oxygenates yield and methanol selectivity for direct oxidation of methane. Meanwhile, the local topological flexibility of the rigid zeolite frameworks induced by the Cu agglomeration in the channels is also revealed. This work exemplifies the combination of microscopy imaging and spectroscopy characterization serves as a complete arsenal for revealing structure-property relationships of the supported metal-zeolite catalysts.

透射电子显微镜空间分辨率综述

Transmission electron microscope (TEM) is one of the most important research tools in a wide range of research fields such as chemistry, materials science, physics, and biological science. There are many factors affect the spatial resolution of TEM, including not only the construction of electron microscope and imaging mechanism, but also the nature of the sample and other reasons. In order to systematically and comprehensively understand the meaning, principle and application of TEM spatial resolution, this paper firstly briefly describes the history of TEM spatial resolution advancement, and then theoretically clarifies the concept, physical meaning, influencing factors and application scope of TEM spatial resolution, then outlines the influence of TEM electron gun, magnetic lens, image detector and internal and external environment on the resolution, the development of monochromator, aberration corrector and new image detector from the perspective of electron microscope device. Finally, from the perspective of practical applications, the mechanism of resolution degradation due to thickness, electron beam sensitivity, carbon accumulation and atomic vibrations and the corresponding solutions are highlighted.

Electron Microscopy of Nanoporous Crystals

ConspectusNanoporous crystals, such as silica mesoporous crystals (SMCs), zeolites, metal–organic frameworks (MOFs) and covalent organic frameworks (COFs), usually have unique geometrical features of periodically arranged pores from micro-, meso- to macro-scale. They have great potential to be structurally designed toward novel materials for various applications, such as storage, separation, and catalysis. In addition to the synthesis and applications of these materials, the structural analysis of nanoporous crystals is also critical for obtaining novel materials and unravelling their structure–property relationships. The crystal structure determines the pore size and connectivity of cages/channels, which are closely related to the performance of these materials in adsorption and catalysis. However, there are significant challenges in structural analysis due to small crystal size, electron-beam sensitivity, and local defects. Due to the strong interaction between electrons and matter, electron microscopy (EM) can provide rich structural information, which at the same time can also cause structural damage to samples. As early as 1960s, EM had been used to directly observe the lattice fringes of zeolites. Subsequently, the structures of many nanoporous crystals were solved and structural details such as defects, intergrowths, and surface structures were revealed by exploiting EM during the past decades. With the development of three-dimensional electron diffraction (3D ED) and low-dose high-resolution electron microscopy imaging, these approaches become more routine for solving the structure of nanocrystals and for revealing the local structural details. In this Account, we present our previous work on EM studies of several typical nanoporous materials, including SMCs, zeolites, MOFs, the TiO2@MIL-101 composite, and COFs. Various methods and strategies were developed and applied to different materials based on the characteristics of their structures. For example, 3D electrostatic potential maps of SMCs can be reconstructed by Fourier synthesis of crystal structure factors obtained from high-resolution TEM images, leading to the discovery of many 3D mesostructures. New zeolites/MOFs/COFs structures, which were not previously solved due to their structural complexity and small crystal size, have been determined using 3D ED data from nanosized crystals or using a combination of electron diffraction and high-resolution imaging. In addition, EM methods for determining the handedness of nanocrystals have also been developed and successfully applied to SMCs and STW zeolite based on high-resolution EM imaging or dynamical electron diffraction. Recently, a new strategy involving a combination of 3D ED and cryogenic protocol was proposed to study the structure, dynamics, and the host–guest interactions in a COF material. The direct-space strategy for structure solution, implemented using a genetic algorithm, was also demonstrated to be a successful approach for solving structures from low-resolution 3D ED data. By now, EM has become one of the most widely used methods in the study of nanoporous materials. We hope this Account will promote further developments of EM and stimulate the design and synthesis of new functional nanoporous materials.

Resolving atomic SAPO-34/18 intergrowth architectures for methanol conversion by identifying light atoms and bonds

The micro-structures of catalyst materials basically affect their macro-architectures and catalytic performances. Atomically resolving the micro-structures of zeolite catalysts, which have been widely used in the methanol conversion, will bring us a deeper insight into their structure-property correlations. However, it is still challenging for the atomic imaging of silicoaluminophosphate zeolites by electron microscopy due to the limits of their electron beam sensitivity. Here, we achieve the real-space imaging of the atomic lattices in SAPO-34 and SAPO-18 zeolites, including the Al–O–P atoms and bonds, by the integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM). The spatial distribution of SAPO-34 and SAPO-18 domains in SAPO-34/18 intergrowths can be clearly resolved. By changing the Si contents and templates in feed, we obtain two SAPO-34/18 catalysts, hierarchical and sandwich catalysts, with highly-mixed and separated SAPO-34 and SAPO-18 lattices respectively. The reduced diffusion distances of inside products greatly improve the catalytic performances of two catalysts in methanol conversion. Based on the observed distributions of lattices and elements in these catalysts, we can have a preliminary understanding on the correlation between the synthesis conditions and structures of SAPO-34/18 intergrowth catalysts to further modify their performances based on unique architectures.

Medusa 82—Hydrogen silsesquioxane based high sensitivity negative-tone resist with long shelf-life and grayscale lithography capability

The high suitability of hydrogen silsesquioxane (HSQ) as e-beam resist has long been known. Despite its undoubtedly good and reliable properties, HSQ nevertheless proves to be problematic in certain aspects due to its relatively short shelf-life and the small processing window between coating preparation and exposure. We thus intended to optimize the silsesquioxane with respect to a prolonged shelf-life and larger processing window while retaining all advantages like the high silicon content for high etch resistance and high pattern resolution. Our combined knowledge resulted in the development of the hydrogen silsesquioxane-based e-beam resist Medusa 82 with improved characteristics. Medusa 82 can be processed with HSQ standard procedures but allows for a delay of several weeks between layer preparation and exposure under standard conditions. Medusa 82 resist compositions tolerate storage periods of several weeks at room temperature. In addition, we generated and investigated variants of Medusa 82, which offer the possibility for exposure with less energy to cross-link the resist. Furthermore, weaker alkaline developers can be applied. A postexposure bake of these new Medusa 82 variants provides a significant enhancement of sensitivity and contrast. In this context, applications of Medusa 82 in deep to extreme ultraviolet and grayscale lithography are described. The use of glasslike resists with moderate electron beam sensitivity has the potential to reduce the effort and to simplify the manufacturing process of micro-optical devices that traditionally have to be structured in glass surfaces. The transformation process of Medusa 82 into a glasslike material involves an e-beam exposure, a thermal treatment, or a combination of both. Moreover, the adjustable contrast and sensitivity enable grayscale lithography. Different e-beam exposures trigger a different cross-linking degree within the layer, resulting in height variations after development. A postexposure bake step induces further cross-linking and a complete conversion into silicon oxide.

Imaging the node-linker coordination in the bulk and local structures of metal-organic frameworks

Porous metal-organic frameworks (MOFs) have shown wide applications in catalysis, gas storage and separation due to their highly tunable porosity, connectivity and local structures. However, the electron-beam sensitivity of MOFs makes it difficult to achieve the atomic imaging of their bulk and local structures under (scanning) transmission electron microscopy ((S)TEM) to study their structure-property relations. Here, we report the low-dose imaging of a beam-sensitive MOF, MIL-101, under a Cs-corrected STEM based on the integrated differential phase contrast (iDPC) technique. The images resolve the coordination of Cr nodes and organic linkers inside the frameworks with an information transfer of ~1.8Å. The local structures in MIL-101 are also revealed under iDPC-STEM, including the surfaces, interfaces and defects. These results provide an extensible method to image various beam-sensitive materials with ultrahigh resolution, and unravel the whole framework architectures for further defect and surface engineering of MOFs towards tailored functions.

Direct atomic scale characterization of the surface structure and planar defects in the organic-inorganic hybrid CH3NH3PbI3 by Cryo-TEM

Abstract The performance of halide perovskite solar cells is often dominated by structural defects. However, atomic scale characterization of the crystalline defects in organic–inorganic hybrid perovskites is hindered by the electron-beam sensitivity of the organic components in the structure. Here we reported the atomic scale characterization of CH3NH3PbI3 (MAPbI3) single crystal using the state-of-the-art cryo-transmission electron microscopy. We confirm that the MAPbI3 structure is intact during high resolution cryo-TEM analysis by probing the content of carbon, nitrogen from the [CH3NH3]+ component using electron energy loss spectroscopy. Atomic steps of {200}T surfaces were observed which shed light on the crystal growth details and low-energy surfaces. Surprisingly, high density of stacking faults are observed in the hybrid perovskite materials. We believe these fault structures serves the role of micro-interface between otherwise perfect lattices which facilitates charge separation and reduces the photon-generated carrier recombination within the crystal solids. First-principles calculations show that the presence of such stacking faults significantly changes the electronic structures of the materials, which may play a critical role in further optimizing the properties such as charge-carrier mobility, the carrier diffusion length and energy conversion efficiency of organic-inorganic hybrid perovskite-based energy harvesting devices.

Analyzing Energy Materials by Cryogenic Electron Microscopy.

Safe and high-energy-density rechargeable batteries are increasingly indispensable in the pursuit of a wireless and fossil-free society. Advancements in present battery technologies and the investigation of next-generation batteries highly depend on the ever-deepening fundamental understanding and the rational designs of working electrodes, electrolytes, and interfaces. However, accurately analyzing energy materials and interfaces is severely hindered by their intrinsic limitations of air and electron-beam sensitivity, which restrains the research of energy materials in a low-efficiency trial-and-error paradigm. The emergence of cryogenic electron microscopy (cryo-EM) has enabled the nondestructive characterization of air- and electron-beam sensitive energy materials in the microscale and nanoscale, and even at atomic resolutions, affording closer insights into the primary chemistry and physics of working batteries. Herein, the development of cryo-EM and the applications in detecting energy materials are reviewed and analyzed from its overwhelming advantages in disclosing the underlying mystery of energy materials. Critical sample preparation methods as the precondition for cryo-EM are compared, which strongly affect the characterization accuracy. Furthermore, new developments in the analysis of energy materials, especially bulk electrodes and interfaces in lithium metal batteries, are presented according to different functions of cryo-EM. Finally, future directions of cryo-EM for analyzing energy materials are prospected.

Atomic-Scale Imaging of a Free-Standing Monolayer Clay Mineral Nanosheet Using Scanning Transmission Electron Microscopy.

Although aberration-corrected scanning transmission electron microscope (STEM) enables the atomic-scale visualization of ultrathin 2D materials such as graphene, imaging of electron-beam sensitive 2D materials with structural complexity is an intricate problem. We here report the first atomic-scale imaging of a free-standing monolayer clay mineral nanosheet via the annular dark field (ADF) STEM. The monolayer clay nanosheet was stably observed under optimal conditions, and we confirmed that the hexagonal contrast pattern with a pore of ∼4 Å corresponds to the atomic structure of clay mineral that consisted of adjacent Si, Al, Mg, and O atoms by comparison with simulations. The findings offer the usefulness of ADF-STEM techniques for the atomic scale imaging of clay minerals and various 2D materials having electron-beam sensitivity and structural complexity than few-atom-thick graphene analogues.

Low dose scanning transmission electron microscopy of organic crystals by scanning moiré fringes

In the pharmaceutical industry, it is important to determine the effects of crystallisation and processes, such as milling, on the generation of crystalline defects in formulated products. Conventional transmission electron microscopy and scanning transmission electron microscopy (STEM) can be used to obtain information on length scales unobtainable by other techniques, however, organic crystals are extremely susceptible to electron beam damage. This work demonstrates a bright field (BF) STEM method that can increase the information content per unit specimen damage by the use of scanning moiré fringes (SMFs). SMF imaging essentially provides a magnification of the crystal lattice through the interference between closely aligned lattice fringes and a scanning lattice of similar spacing. The generation of SMFs is shown for three different organic crystals with varying electron beam sensitivity, theophylline, furosemide and felodipine. The electron fluence used to acquire the BF-STEM for the most sensitive material, felodipine was approximately 3.5 e−/Å2. After one additional scan of felodipine (total fluence of approximately 7.0 e−/Å2), the SMFs were no longer visible due to extensive damage caused to the crystal. Irregularity in the SMFs suggested the presence of defects in all the organic crystals. Further effort is required to improve the data analysis and interpretation of the resulting SMF images, allowing more information regarding the crystal structure and defects to be extracted.

Electron-beam radiation induced degradation of silicon nitride and its impact to semiconductor failure analysis by TEM

By in-situ transmission electron microscopy (TEM), we performed a detailed study on the electron-beam radiation damage to nanostructured silicon nitride thin-film process layers in a typical semiconductor NVM device. It was found that high-dose electron-beam radiation at 200 kV led to rapid degradation of silicon nitride process layers, i.e. thin-downing of nanostructured silicon nitride, inter-diffusion of O and N, the formation of bubble-like defects and segregation of N at neighbouring interfaces. Further detailed analysis revealed that radiation-induced modification in the microstructure and chemical composition of silicon nitride layers could be ascribed to the electron radiation induced knock-on damage and ionization damage. The radiation enhanced diffusion (RED) accounted for the continuous thin-down of the nitride process layer and the formation of bubble-like defects in thick nitride spacer process layers. The work well demonstrated the electron-beam sensitivity of nanostructured silicon nitride materials in the semiconductor devices, and thus may give useful information about electron-dose control during TEM failure analysis of the semiconductor devices containing nanostructured silicon nitride process layers.

Electron radiation-induced material diffusion and nanocrystallization in nanostructured amorphous CoFeB thin film

Transmission electron microscopy (TEM) is widely used for physical characterization of CoFeB based magnetic tunneling junctions (MTJ) with its atomic-scale resolution. However, highly energetic electron radiation during TEM analysis may cause phase and microstructure modification of CoFeB and its associated MTJ layers. It is the intention of this work to address the issues of the electron-beam sensitivity of CoFeB material. With in-situ TEM, we investigated the electron beam radiation-induced material diffusion and the nanocrystallization behaviors in nanostructured amorphous CowFexByOz/Co60Fe20B20/SiO2 thin films. It was found that electron radiation with different electron dose led to massive diffusion of Co, Fe, B and O atoms across the whole thin film layers, which directly resulted in the modification of the phase and composition of the thin film layers, i.e. the oxidation of Co, Fe, B with O diffusion and the formation of pure Si phase from SiO2. Two stages of material diffusion were observed. While Stage-I material diffusion proceeded with a high diffusion speed, Stage-II had a relatively low diffusion rate accompanying with the nanocrystallization at the bottom of the CoFeB layer. A detailed kinetic study by in-situ TEM revealed the electron-beam radiation induced massive diffusion was a non-thermal process, and the underlying driving force arose from radiation-enhanced diffusion (RED) effects. Nanocrystallization during Stage-II electron-radiation experiment showed unique phase transformation phenomena, repeated nanocrystallization, amorphization, and nanocrystallization processes in the sequence before a stable grain growth could be achieved. A detailed TEM analysis revealed that RED-enhanced B diffusion was responsible for such unique repeated phase transformation processes. B diffusion and the associated structure distortion and the local short-range re-ordering may also account for the phase transformation from fcc-CoxFe23-xB6 to B-rich orthorhombic- CoxFe3-xB phase.

A Quantitative Evaluation of Electron Beam Sensitivity in Calcite Nanoparticles

We present analysis and quantification of electron beam sensitivity in calcite in order to identify damage thresholds under which bright field TEM imaging, selected area electron diffraction and scanning TEM should be performed. A large reduction in damage under TEM was found when operating at 300 kV rather than 200 kV, suggesting that the irradiation induced degradation of calcite to calcium oxide is dominated by radiolysis. At 300 kV, bright field STEM imaging was able to retain lattice information at higher fluences than was possible using TEM and, although damage was still seen to occur, there was no observation of the formation of crystalline calcium oxide.

Assessing electron beam sensitivity for SrTiO3 and La0.7Sr0.3MnO3 using electron energy loss spectroscopy

Thresholds for beam damage have been assessed for La0.7Sr0.3MnO3 and SrTiO3 as a function of electron probe current and exposure time at 80 and 200kV acceleration voltage. The materials were exposed to an intense electron probe by aberration corrected scanning transmission electron microscopy (STEM) with simultaneous acquisition of electron energy loss spectroscopy (EELS) data. Electron beam damage was identified by changes of the core loss fine structure after quantification by a refined and improved model based approach. At 200kV acceleration voltage, damage in SrTiO3 was identified by changes both in the EEL fine structure and by contrast changes in the STEM images. However, the changes in the STEM image contrast as introduced by minor damage can be difficult to detect under several common experimental conditions. No damage was observed in SrTiO3 at 80kV acceleration voltage, independent of probe current and exposure time. In La0.7Sr0.3MnO3, beam damage was observed at both 80 and 200kV acceleration voltages. This damage was observed by large changes in the EEL fine structure, but not by any detectable changes in the STEM images. The typical method to validate if damage has been introduced during acquisitions is to compare STEM images prior to and after spectroscopy. Quantifications in this work show that this method possibly can result in misinterpretation of beam damage as changes of material properties.