Electron Beam Illumination Research Articles

Artificially Controlled Nanoscale Chemical Reduction via Electron Beam Illumination

Artificially Controlled Nanoscale Chemical Reduction via Electron Beam Illumination

In-situ imaging of heat-induced phase transition in a two-dimensional conjugated metal-organic framework

Atomically-resolved in-situ high-resolution transmission electron microscopy (HRTEM) imaging of the structural dynamics in organic materials remains a major challenge. This difficulty persists even with aberration-corrected instruments, as HRTEM images necessitate a high electron dose that is generally intolerable for organic materials. In this study, we report the in-situ HRTEM imaging of heat-induced structural dynamics in a benzenehexathiol-based two-dimensional conjugated metal-organic framework (2D c-MOF, i.e., Cu3(BHT)). Leveraging its hydrogen-free structure and high electrical conductivity, Cu3(BHT) exhibits high electron beam resistance. We demonstrate atomic resolution imaging at an 80 kV electron accelerating voltage using our Cc/Cs-corrected SALVE instrument. However, continuous electron irradiation eventually leads to its amorphization. Intriguingly, under heating in a MEMS holder, the Cu3(BHT) undergoes a phase transition to a new crystalline phase and its phase transition, occurring within the temperature range of 480 °C to 620 °C in dependence on the electron beam illumination. Using HRTEM and energy-dispersive X-ray mapping, we identify this new phase as CuS. Our findings provide insights into the mechanisms governing structural transitions in purposefully engineered structures, potentially pivotal for future endeavours involving the production of metal oxide/sulfide nanoparticles from MOF precursors.

Atomic-level polarization reversal in sliding ferroelectric semiconductors

Intriguing “slidetronics” has been reported in van der Waals (vdW) layered non-centrosymmetric materials and newly-emerging artificially-tuned twisted moiré superlattices, but correlative experiments that spatially track the interlayer sliding dynamics at atomic-level remain elusive. Here, we address the decisive challenge to in-situ trace the atomic-level interlayer sliding and the induced polarization reversal in vdW-layered yttrium-doped γ-InSe, step by step and atom by atom. We directly observe the real-time interlayer sliding by a 1/3-unit cell along the armchair direction, corresponding to vertical polarization reversal. The sliding driven only by low energetic electron-beam illumination suggests rather low switching barriers. Additionally, we propose a new sliding mechanism that supports the observed reversal pathway, i.e., two bilayer units slide towards each other simultaneously. Our insights into the polarization reversal via the atomic-scale interlayer sliding provide a momentous initial progress for the ongoing and future research on sliding ferroelectrics towards non-volatile storages or ferroelectric field-effect transistors.

Electron videography of a lipid-protein tango.

Biological phenomena, from enzymatic catalysis to synaptic transmission, originate in the structural transformations of biomolecules and biomolecular assemblies in liquid water. However, directly imaging these nanoscopic dynamics without probes or labels has been a fundamental methodological challenge. Here, we developed an approach for "electron videography"-combining liquid phase electron microscopy with molecular modeling-with which we filmed the nanoscale structural fluctuations of individual, suspended, and unlabeled membrane protein nanodiscs in liquid. Systematic comparisons with biochemical data and simulation indicate the graphene encapsulation involved can afford sufficiently reduced effects of the illuminating electron beam for these observations to yield quantitative fingerprints of nanoscale lipid-protein interactions. Our results suggest that lipid-protein interactions delineate dynamically modified membrane domains across unexpectedly long ranges. Moreover, they contribute to the molecular mechanics of the nanodisc as a whole in a manner specific to the protein within. Overall, this work illustrates an experimental approach to film, quantify, and understand biomolecular dynamics at the nanometer scale.

High-quality TEM specimen preparation for lithium-ion conducting solid electrolytes by low-energy ion milling

Focused ion beam (FIB) is a widely used method to prepare transmission electron microscopy (TEM) specimen from bulk materials. However, the surface amorphous layer induced by ion beam is an obstacle to obtain high-quality atomic-scale images for quantitative analysis, especially for the analysis of light elements such as lithium in lithium-ion conducting solid electrolytes. Here, taking lithium-ion conducting solid electrolyte materials as an example, the advantages and disadvantages of applying low-energy Ar+ ion for fine milling after FIB are investigated. Combining Monte-Carlo simulations with ion milling experiments, the milling parameters are evaluated and discussed in detail. With optimized parameters, TEM specimens with less beam damage and thinner amorphous layer were prepared, enabling the acquisition of high-quality atomic-scale images. Furthermore, low-energy Ar+ ion milling is also able to remove hydrocarbon contamination formed during the electron beam illumination inside the microscope, making the contaminated TEM specimens reusable.

Artificially controlled nanoscale chemical reduction in VO2 through electron beam illumination

Chemical reduction in oxides plays a crucial role in engineering the material properties through structural transformation and electron filling. Controlling the reduction at nanoscale forms a promising pathway to harvest functionalities, which however is of great challenge for conventional methods (e.g., thermal treatment and chemical reaction). Here, we demonstrate a convenient pathway to achieve nanoscale chemical reduction for vanadium dioxide through the electron-beam illumination. The electron beam induces both surface oxygen desorption through radiolytic process and positively charged background through secondary electrons, which contribute cooperatively to facilitate the vacancy migration from the surface toward the sample bulk. Consequently, the VO2 transforms into a reduced V2O3 phase, which is associated with a distinct insulator to metal transition at room temperature. Furthermore, this process shows an interesting facet-dependence with the pronounced transformation observed for the c-facet VO2 as compared with the a-facet, which is attributed to the intrinsically different oxygen vacancy formation energy between these facets. Remarkably, we readily achieve a lateral resolution of tens nanometer for the controlled structural transformation with a commercial scanning electron microscope. This work provides a feasible strategy to manipulate the nanoscale chemical reduction in complex oxides for exploiting functionalities.

An in-situ TEM characterization of electron beam induced dislocation motion in a single-crystalline gold thin film

Dislocation motions driven by electron-beam illumination were in-situ observed in a single-crystalline gold thin-foil specimen, under a medium-voltage transmission electron microscope operated at 200 kV. The dislocation motion characteristics were in-situ observed under the bright-field and weak-beam dark-field conditions. The structures formed through the dislocation motion were also characterized using bright−/dark-field imaging and high-resolution techniques. Two types of dislocation motions are concluded, where one type of motion left no apparent traces, and the other type left a clear trace of passage. The dislocation motions have jerky behaviors, and the motions are accelerated when the beam intensity is increased. The immobile trace structures left by the dislocation motions are identified to be twin structures oriented along the 〈111〉 crystal directions. The dislocation motion mechanism is discussed and the motion might be caused by the electron beam heating effect or the electron beam induced phonon effects. The dislocation disassociation, planar defect formation in the Au crystal, and the beam heating impacts are also discussed.

Fast versus conventional HAADF-STEM tomography of nanoparticles: advantages and challenges

HAADF-STEM tomography is a widely used experimental technique for analyzing nanometer-scale structures of a large variety of materials in three dimensions. It is especially useful for studying crystalline nanoparticles, where conventional TEM tomography suffers from diffraction-related artefacts. Unfortunately, the acquisition of a HAADF-STEM tilt series can easily take up one hour or more, depending on the complexity of the experiment. It is therefore challenging to investigate samples that do not withstand long electron beam illumination or to acquire a large number of tilt series during a single TEM experiment. The latter would facilitate obtaining more statistically representative 3D data, and enable performing dynamic in situ 3D characterizations with a finer time resolution. Various HAADF-STEM acquisition strategies have been proposed to accelerate the tomographic acquisition and reduce the required electron dose. These methods include tilting the holder continuously while acquiring a projection “movie” and a hybrid, incremental, methodology which combines the benefits of the conventional and continuous technique. However, until now an experimental evaluation of these techniques has been lacking. In this paper, the different acquisition strategies will be experimentally compared in terms of speed, resolution and electron dose. This evaluation will be performed based on experimental tilt series, acquired for various metallic nanoparticles with different shapes and sizes. We discuss the necessary data processing and provide a general guideline that can be used to determine the most optimal acquisition strategy for specific electron tomography experiments.

Structural Instability in Electrically Stressed, Oxygen Deficient BaTiO3 Nanocrystals

AbstractThe dynamics of oxygen vacancies under external stimuli dominates the performance of many solid‐state devices, including capacitors, oxide memristors, anionic conductors, etc. By means of in situ transmission electron microscopy, it is found in BaTiO3 perovskite nanocrystals that formation of oxygen vacancies due to electrical stressing renders the oxide amorphizable under electron beam illumination, suggesting the presence of a threshold concentration of oxygen vacancy affecting the structural stability of BaTiO3 crystals upon high energy radiation. In contrast to the structural change, the resistivity of the nanocrystal seems not liable to the amorphization prior to dielectric breakdown at higher voltage bias. It is proposed that an increase in oxygen vacancy content promotes oxygen mobility in the perovskite structure allowing electron beam induced electric field to modify the local structure and composition. The in situ observations reveal the central role of oxygen vacancies in the structural stability of perovskites which is of paramount importance to their applications in extreme environments and suggest a potential new route to micro‐processing perovskite oxides using the electron beam via oxygen vacancy management without severely compromising the electric property.

Challenges, myths, and opportunities of electron microscopy on halide perovskites

Organic–inorganic hybrid perovskites (OIHPs) have attracted extensive research interest as promising candidates for optoelectronic applications such as solar cells. Transmission electron microscopy (TEM)-based characterizations hold the key to revealing the morphological, microstructural, physical, and chemical information of OIHPs. However, their extreme sensitivity to the electron beam illumination usually inhibits us from obtaining the intrinsic information or even leads to significant artifacts. In this perspective, recent TEM studies on OIHPs are reviewed, wherein the discussions focus on how the electron beam destabilizes the structure of OIHPs and how to mitigate such effects as well as avoid misinterpretations. This perspective aims to catch researchers' attention on the beam sensitivity of OIHPs, guide the TEM characterization, and inspire electron microscopy development to reveal the working principle and failure mechanism of OIHPs.

Reconstruction of the interatomic forces from dynamic scanning transmission electron microscopy data

We explore the possibility for reconstruction of the generative physical models describing interactions between atomic units in solids from observational electron microscopy data. Here, scanning transmission electron microscopy (STEM) is used to observe the dynamic motion of Si atoms at the edge of monolayer graphene under continuous electron beam illumination. The resulting time-lapsed STEM images represent the snapshots of observed chemical states of the system. We use two approaches: potential of mean force calculation using a radial distribution function and a direct fitting of the graphene–Si interatomic pairwise potentials with force matching, to reconstruct the force fields in the materials. These studies lay the foundation for quantitative analysis of materials energetics from STEM data through the sampling of the metastable states in the chemical space of the system.

Transmission electron microscopy of organic-inorganic hybrid perovskites: myths and truths

Organic-inorganic hybrid perovskites (OIHPs) have attracted extensive research interest as a promising candidate for efficient and inexpensive solar cells. Transmission electron microscopy (TEM) characterizations that can benefit the fundamental understanding and the degradation mechanism are widely used for these materials. However, their sensitivity to the electron beam illumination and hence structural instabilities usually prevent us from obtaining the intrinsic information or even lead to significant artifacts. Here, we systematically investigate the structural degradation behaviors under different experimental factors to reveal the optimized conditions for TEM characterizations of OIHPs by using low-dose electron diffraction and imaging techniques. We find that a low temperature (−180 °C) does not slow down the beam damage but instead induces a rapid amorphization for OIHPs. Moreover, a less severe damage is observed at a higher accelerating voltage. The beam-sensitivity is found to be facet-dependent that a (1 0 0) exposed CH3NH3PbI3 (MAPbI3) surface is more stable than a (0 0 1) surface. With these guidance, we successfully acquire the atomic structure of pristine MAPbI3 and identify the characterization window that is very narrow. These findings are helpful to guide future electron microscopy characterizations of these beam-sensitive materials, which are also useful for finding strategies to improve the stability and performance of the perovskite solar cells.

Real-time observation of phase coexistence and a1/a2 to flux-closure domain transformation in ferroelectric films

Phase coexistence in ferroelectric oxide films displays complex phase competitions and transformation which suggest new multiple-coupled properties. Here we have successfully engineered the coexistence of flux-closure and a1/a2 phases in tensile-strained PbTiO3 films sandwiched between GdScO3 substrate and a SrTiO3 layer. Moreover, by using in-situ electron beam illumination, the unusual transformation from a1/a2 phase to the flux-closure phase was directly observed. In detail, there are two types of transformations: One is the nucleation, growth, and expansion of the flux-closure phase from the internal region of the a1/a2 phase. The other feature is a “dislocation gliding” like behavior: a thin lamella of a1/a2 shrinks like “partial dislocation pairs” and finally the gradually disappeared a1/a2 lamella forms a “perfect dislocation” in the flux-closure domain matrix. Phase-field simulations suggest that the a1/a2 to flux-closure transition is induced by the decrease of the depolarization field, which is screened by the injected electrons from electron beam irradiation. These results directly confirm the phase interconversion under an external stimulus at the nanometer scale, which shed new light on the fabrication of new polar textures.

An Unconventional Transient Phase with Cycloidal Order of Polarization in Energy‐Storage Antiferroelectric PbZrO3

Antiferroelectric-based dielectric capacitors are receiving tremendous attention for their outstanding energy-storage performance and extraordinary flexibility in collecting pulsed powers. Nevertheless, the in situ atomic-scale structural-evolution pathway, inherently coupling to the energy storage process, has not been elucidated for the ultimate mechanistic understanding so far. Here, time- and atomic-resolution structural phase evolution in antiferroelectric PbZrO3 during storage of energy from the electron-beam illumination is reported. By employing state-of-the-art negative-spherical-aberration imaging technique, the quantitative transmission electron microscopy study presented herein clarifies that the hierarchical evolution of polar oxygen octahedra associated with the unit-cell volume change and polarization rotation accounts for the stepwise antiferroelectric-to-ferroelectric phase transition. In particular, an unconventional ferroelectric category-the ferrodistortive phase characteristic of a unique cycloidal polarization order-is established during the dynamic structure investigation. Through clarifying the atomic-scale phase transformation pathway, findings of this work unveil a new territory to explore novel ferrodistortive phases in energy-storage materials with the nonpolar-to-polar phase transitions.

Smectic and nematic phase modulations and transitions under electron beam in Tb2Cu0.83Pd0.17O4

Understanding the structural origin of the functionality in cuprates has attracted tremendous attention over several decades. In particular, probing distortions in the Cu-O bonding is of great importance for exploring the coupling between the charge and the lattice, a key mechanism for superconductivity and other functionality in correlated materials. Here we study a superlattice modulation in the ${\mathrm{Tb}}_{2}{\mathrm{Cu}}_{0.83}{\mathrm{Pd}}_{0.17}{\mathrm{O}}_{4}$ ``214'' material, which possesses no superconductivity itself but has the parent structure of the ${R}_{2}{\mathrm{CuO}}_{4}$ (R = a rare-earth element) superconducting cuprate group. Using transmission electron microscopy (TEM), we find that this superlattice modulation is formed by Cu ion displacements in a direction perpendicular to the Cu-O planes. The superlattice modulation undergoes a reversible electronic smectic-nematic phase transition under electron-beam illumination. With the help of in situ TEM results, our findings imply that the superlattice modulation in this material arises from spatially modulated charge ordering at the Cu sites.

Dissociation path competition of radiolysis ionization-induced molecule damage under electron beam illumination.

Radiolysis ionization under electron beam illumination induces dissociation and damage of organic and biological molecules; thus, it is impossible to image the related materials by transmission electron microscopy (TEM). To understand the atomistic mechanism of radiolysis damage, we developed a systematical procedure based on real-time time-dependent density functional theory (rt-TDDFT) for simulating the radiolysis damage processes of molecules; this procedure can describe the ionization cross sections of the electronic states and the fast dissociation processes caused by hot carrier cooling and the Auger decay on deep levels. For the radiolysis damage of C2H6O2, our simulation unexpectedly showed that there is strong competition among three different dissociation paths, including fast dissociation caused by nonadiabatic cooling of the hot carrier; fast dissociation caused by Auger decay, which induces double ionization and Coulomb explosion; and slow dissociation caused by increased kinetic energy. As the energy of the incident electron beam changes, the time scales of these dissociation paths and their relative contributions to the molecule damage change significantly. These simulation results explain the measured mass spectra of the C2H6O2 dissociation fragments and also provide clear competition mechanisms for blocking these dissociation paths in the TEM imaging of organic and biological materials.

Charging ain't all bad: Complex physics in DyScO3

Although charging is ubiquitous in electron microscopy, its effects are typically avoided or ignored. However, avoiding charging is not possible in some materials, e.g. lanthanide scandates with well-ordered surfaces positively charge immensely under electron beam illumination because of their electronic structure, and ignoring charging can leave new science undiscovered. In this work, a combination of rapidly acquired electron energy loss spectra and cross-correlation were used to understand and overcome charging effects in DyScO3. A 5.4 eV band gap was extracted from the charging-corrected loss spectrum, in good agreement with previously reported band gaps, and a 3.8 eV in-gap peak was attributed to surface states via comparison with density functional theory calculations. Additionally, ultraviolet photoelectron spectroscopy measurements indicated that under some conditions well-annealed DyScO3 surfaces charge negatively causing upward band bending associated with occupied surface states in the gap. As was previously found in the case of positive charging under electron beam illumination with in-situ flexoelectric bending observations, the magnitude of negative charging under ultraviolet illumination is Zener tunneling limited in well-annealed DyScO3.

Atomic scale insights into structure instability and decomposition pathway of methylammonium lead iodide perovskite

Organic–inorganic hybrid perovskites are promising candidates for the next-generation solar cells. Many efforts have been made to study their structures in the search for a better mechanistic understanding to guide the materials optimization. Here, we investigate the structure instability of the single-crystalline CH3NH3PbI3 (MAPbI3) film by using transmission electron microscopy. We find that MAPbI3 is very sensitive to the electron beam illumination and rapidly decomposes into the hexagonal PbI2. We propose a decomposition pathway, initiated with the loss of iodine ions, resulting in eventual collapse of perovskite structure and its decomposition into PbI2. These findings impose important question on the interpretation of experimental data based on electron diffraction and highlight the need to circumvent material decomposition in future electron microscopy studies. The structural evolution during decomposition process also sheds light on the structure instability of organic–inorganic hybrid perovskites in solar cell applications.

Spontaneous Reshaping and Splitting of AgCl Nanocrystals under Electron Beam Illumination.

AgCl is photosensitive and thus often used as micromotors. However, the dynamics of individual AgCl nanoparticle motion in liquids upon illumination remains elusive. Here, using liquid cell transmission electron microscope (TEM), AgCl nanocrystals reshaping and splitting spontaneously in an aqueous solution under electron beam illumination are observed. It is found that the AgCl nanocrystals are negatively charged in the liquid environment, where the charge induces a repulsive Coulomb force that reshapes and stretches those nanocrystals. Upon extensive stretching, the AgCl nanocrystal splits into small nanocrystals and each nanocrystal retracts back into cuboid shapes due to the cohesive surface. This analysis shows that each nanocrystal maintains a single crystal rocksalt structure during splitting. The splitting of AgCl nanocrystals is analogous to the electrified liquid droplets or other reported the Coulomb fission phenomenon, but with distinctive structural properties. Revealing of the dynamic behavior of AgCl nanocrystals opens the opportunity to explore their potential applications as actuators for nanodevices.

Nanoporous Copper Pattern Fabricated by Electron Beam Irradiation on Cu3N Film for SERS Application

Copper nanostructures have aroused extensive research interests due to their wide applications in electronics, catalysis, and photonics. Here, a novel strategy to fabricate copper nanoporous structures by illuminating electron beam on as‐prepared copper nitride thin films is reported. This electron beam‐based direct writing method provides a precise control of the nanoporous sites on substrates. Experimental results reveal significant surface‐enhanced Raman scattering (SERS) for copper porous nanostructures compared with flat copper nitride thin films due to localized surface plasmon resonances supported at the nanopores. This work provides a maskless method to prepare copper nanoporous structures from copper nitride films and implies an efficient SERS platform for sensitive molecular diagnostics.