Time-resolved Scanning Electron Microscopy Research Articles

New Insights into Internal Oxidation of Alloy 690 and Model Alloys in Hydrogenated Steam

The oxidation behavior of Alloy 690 and Ni-xCr-10Fe model alloys exposed to 480°C hydrogenated steam, in the nickel metal stability region, was studied using a variety of techniques, including time-resolved scanning electron microscopy, transmission electron microscopy, time-of-flight secondary ion mass spectrometry, and electron energy loss spectroscopy. The alloy underwent internal oxidation intragranularly, resulting in the expulsion of metallic Ni nodules to the surface. A compact Cr-rich “healing” layer developed over time at the intragranular internal oxidation front near grain boundaries and significantly retards further internal oxidation. Protective external Cr-rich oxide formed at and very near the grain boundaries, hindering intergranular oxidation. In view of the fact that the protective oxide forms micrometers away from the grain boundaries, one cannot simply say that Cr diffuses up the boundary and forms an oxide that spreads laterally—internal stress relief must play a role, just as it does within the grains, only with different mechanisms at play.

Dynamic transformation of cubic copper catalysts during CO2 electroreduction and its impact on catalytic selectivity

To rationally design effective and stable catalysts for energy conversion applications, we need to understand how they transform under reaction conditions and reveal their underlying structure-property relationships. This is especially important for catalysts used in the electroreduction of carbon dioxide where product selectivity is sensitive to catalyst structure. Here, we present real-time electrochemical liquid cell transmission electron microscopy studies showing the restructuring of copper(I) oxide cubes during reaction. Fragmentation of the solid cubes, re-deposition of new nanoparticles, catalyst detachment and catalyst aggregation are observed as a function of the applied potential and time. Using cubes with different initial sizes and loading, we further correlate this dynamic morphology with the catalytic selectivity through time-resolved scanning electron microscopy measurements and product analysis. These comparative studies reveal the impact of nanoparticle re-deposition and detachment on the catalyst reactivity, and how the increased surface metal loading created by re-deposited nanoparticles can lead to enhanced C2+ selectivity and stability.

Quantification of Competing Magnetic States and Switching Pathways in Curved Nanowires by Direct Dynamic Imaging.

For viable applications, spintronic devices based, for example, on domain wall motion need to be highly reliable with stable magnetization states and highly reproducible switching pathways transforming one state to another. The existence of multiple stable states and switching pathways in a system is a definitive barrier for device operation, yet rare and stochastic events are difficult to detect and understand. We demonstrate an approach to quantify competing magnetic states and stochastic switching pathways based on time-resolved scanning electron microscopy with polarization analysis, applied to the technologically relevant control of vortex domain wall chirality via field and curvature in curved wires. As a pump-probe technique, our analysis scheme nonetheless allows for the disentanglement of different occurring dynamic pathways, and we can even identify the rare events leading to changes from one magnetization switching pathway to another pathway via temperature- and geometry-dependent measurements. The experimental imaging is supported by micromagnetic simulations to reveal the mechanisms responsible for the change of the pathway. Together the results allow us to explain the origin and details of the domain wall chirality control and to quantify the frequency and the associated energy barriers of thermally activated changes of the states and switching pathways.

Evaluation of Mechanism for EGR Deposit Formation Based on Spatially- and Time-Resolved Scanning-Electron-Microscope Observation

<div class="section abstract"><div class="htmlview paragraph">Exhaust gas recirculation (EGR) is widely used in diesel engines to reduce nitrogen oxide emissions. To meet the strict emission regulations, e.g., Real Driving Emissions, the EGR system is required to be used at temperatures lower than the present ones. However, under cool conditions, an adhesive deposit forms on the EGR valve or cooler because of the particulate matter and other components present in the diesel exhaust. This causes sticking of the EGR valve or degradation of the heat-exchange performance, which are serious problems. In this study, the EGR deposit formation mechanism was investigated based on spatially- and time-resolved scanning electron microscopy (SEM) observation. The deposit was formed in a custom-made sample line using real exhaust emitted from a diesel engine. The exhaust including soot was introduced into the sample line for 24 h (maximum duration), and the formed deposit was observed using SEM. The deposit was formed on the surface of the sample line below 300 °C (wall temperature) and had two layers: the deposit formed near the surface of the sample line had hard and dense structures, such as a lacquer (called a hard deposit), and the deposit formed on the hard deposit was powdery and exhibited a “grape-like” shape similar to particulate matter (called a powdery deposit). The amounts of both types of deposits increased in proportion to the duration of the exhaust supply. According to the SEM observations of the hard deposit, powdery deposit, and boundary layer between the positions where the hard and powdery deposits were formed, the hard deposit was formed by taking in the powdery deposit, when the exhaust including soot was supplied.</div></div>

Magnetic State Control via Field-Angle-Selective Switching in Asymmetric Rings

Switching the chirality of the vortex state in asymmetric ferromagnetic rings is interesting for multistate memory devices, logic elements, and stray-field-based rotation sensors. This study shows that different magnetic states can be configured by carefully tuning the magnetic field angle. Using time-resolved scanning electron microscopy with polarization analysis to image the magnetization dynamics of these rings, the authors detect competing switching pathways for certain field angles. These different pathways do not change the resulting magnetic states, though, which is advantageous for engineering reliable devices for a range of potential spintronic applications.

Time-dependent characteristics of secondary electron emission

The recent development of the time-resolving capability for scanning electron microscopy (SEM) enables it to be a real 4D space-time imaging technique, which is extremely suitable for investigating the ultrafast dynamic processes concerned with secondary electron emission (SEE). This paper attempts to investigate the dynamic SEE process with the aid of a Monte Carlo method; the understanding of the mechanism will surely benefit the construction and application of various kinds of time-resolved SEMs. Our simulation modeling is based on the use of the Mott cross section and a dielectric function approach for the respective description of the electron elastic and inelastic scattering. One secondary electron is assumed to be produced in an inelastic scattering event, and the owned kinetic energy enables it to transport and produce other secondary electrons, forming the cascade production process. From the simulation, not only the time delay of SEE from the incidence instant of primary electrons but also the time dependences of the involved physical quantities, including the energy-, depth-, direction-, emission site-, and production site-distributions can be theoretically derived. The calculations provide useful knowledge on the time dependence of SEE from the theoretical perspective for the applications to the available time-resolved SEMs.

Coupling between vortices and antivortices in a cross-tie wall studied by time-resolved SEMPA

Using time-resolved scanning electron microscopy with polarization analysis (TR-SEMPA) the dynamics of magnetic vortices and antivortices in a cross-tie wall is investigated. Under quasistatic external field drive both quasiparticles oscillate at opposite phase to each other (in analogy to an optical mode). An additional common motion, which stems from the finite boundary conditions in a patterned FeCoSiB rectangle, is observed (in analogy to an acoustic mode), leading in total to a stronger field-dependent displacement for the vortices compared to the antivortices. Both types of magnetic solitons are mutually coupled via the minimization of the total magnetic energy of the structure. It is shown that the TR-SEMPA results can be explained via coupling in the framework of micromagnetic simulations and that the coupling constants can be extracted. The motion is composed from the contributions of the two terminating vortices and a mutual vortex-antivortex coupling along the chain.

Time-resolved scanning electron microscopy with polarization analysis

We demonstrate the feasibility of investigating periodically driven magnetization dynamics in a scanning electron microscope with polarization analysis based on spin-polarized low-energy electron diffraction. With the present setup, analyzing the time structure of the scattering events, we obtain a temporal resolution of 700 ps, which is demonstrated by means of imaging the field-driven 100 MHz gyration of the vortex in a soft-magnetic FeCoSiB square. Owing to the efficient intrinsic timing scheme, high-quality movies, giving two components of the magnetization simultaneously, can be recorded on the time scale of hours.

Silver disposition and dynamics during electroless metal thin film synthesis

Morphological and physicochemical disposition of silver (Ag) during redox-driven self-assembly of metal films on silica surfaces under equilibrium hydraulic conditions has been examined in real time in a novel electroless (EL) metal deposition cell by transmission UV-vis (T-UV) spectroscopy. Optical features due to localized surface plasmon resonance, surface plasmon polaritons, and photoluminescence from Ag and gold (Au) nanoarchitectures such as particles, clusters, and films were attributed by correlating T-UV with time-resolved scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Silver that deposited onto tin-sensitized surfaces in thin films nucleated nanoparticles when exposed to reductant or broadband light. Kinetic changes in plasmon features suggested four previously unrecognized time-dependent physicochemical regimes occur during consecutive EL deposition of Ag and Au onto tin-sensitized silica surfaces: self-limiting Ag activation; transitory Ag nanoparticle formation; transitional Au–Ag alloy formation during galvanic replacement of Ag by Au; and uniform metal deposition under controlled hydraulic conditions. Growth mechanisms at the surface, interior, and interface of the resulting thin metal films inferred from real time T-UV spectra were characterized by depth profile XPS analysis.

Application of time-resolved scanning electron microscopy to the analysis of the motion of micromechanical structures

We applied a time-resolved scanning electron microscope to the analysis of the movement of micromechanical structures. A detector signal modulation method is used in our experiment. The major advantage of this method is that no hardware modifications are required to a commercially available scanning electron microscope. The test devices are simple cantilever beams fabricated from silicon dioxide with and without an aluminum coating. These cantilever beams are electrostatically excited at resonance under several conditions. Both time-resolved images and precise resonant frequency measurements of these microcantilever beams are obtained. We found that electrically floating silicon dioxide beams without an aluminum coating can also be excited stably by an ac driving voltage at resonance under scanning electron microscopy observation. Accumulated charge on the cantilever beam caused by the electron beam irradiation is found to be positive and this charge results in a driving force between the cantilever beam and the driving electrode. A time-resolved scanning electron microscopy method is proved to be a useful technique to determine the resonant frequencies and resonant mode shapes of micromechanical systems.

Time-Resolved Scanning Electron Microscopy and Its Application to Bulk-Effect Oscillators

The application of the scanning electron microscope to the examination of time-varying phenomena is discussed. The limitations of response time are mentioned, and methods for increasing response speed are considered. These include the use of electrostatic deflection plates to chop the primary electron beam, the use of solid-state semiconductor diodes as electron detectors, and the use of sampling and the storage and processing of data prior to display. Time-resolved techniques are then applied to a study of the motion of domains of high electric field in CdS ultrasonic oscillator diodes and in GaAs Gunn effect diodes. In both photoconducting and semiconducting CdS, the domain formation and propagation is correlated to the current waveform of the oscillator. Nonuniform domain propagation in two dimensions is examined in a GaAs oscillator.