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DOE OSTI2018

Imaging Coherent Structural Dynamics with Ultrafast Electron Microscopy

Development of ultrafast electron and X-ray scattering methods has enabled direct routes to probing atomic-scale structural dynamics. This in turn has led to new insight into lattice responses to a variety of dynamic processes associated with phase transformations, electron-lattice correlations, and nanoscale mechanical motion, to name a few. Presently, the most wide-spread approaches involve using table-top, laser-driven electron-scattering chambers or femtosecond (fs) X-ray beamlines to probe dynamics in reciprocal space and, in many instances, over specimen regions that are large relative to unit-cell dimensions and discrete (atomic and nanoscale) structural and morphological defects (e.g., vacancies, line and screw dislocations, grain boundaries, interfaces, etc.). As a result, the transients monitored for relatively large-spot-size reciprocal-space measurements – typically some aspect of a Bragg reflection (intensity, position, width, shape) – may be comprised of a range of responses occurring within the probed region. To directly resolve the role of distinct morphologies and structural features on dynamics associated with the nucleation, emergence, evolution, and decay of coherent phenomena, one would ideally directly image the local, nanoscale behaviors and precisely correlate responses to the nature of the feature. Here, I will show how such effects can be directly probed with ultrafast electron microscopy (UEM). After a brief overview of the operating principles of UEM, I will discuss how we are using real-space imaging to directly visualize coherent structural dynamics in a variety of nanostructured and nanoscale materials.

47 OTHER INSTRUMENTATION↗

DOE OSTI2018

Imaging Coherent Acoustic Phonons in LaFeAsO with Ultrafast Electron Microscopy

Several iron pnictide materials, such as LaFeAsO and BaFe 2 As 2 , display high-temperature superconductivity when lightly doped. Such compounds often undergo temperature-dependent structural, magnetic, and electronic transitions, which are generally thought to be linked in some way to the superconducting phase. In order to resolve the interdependencies of the various degrees of freedom, ultrafast spectroscopic methods have frequently been used to determine timescales associated with charge-carrier and switching dynamics in the superconducting materials, and the more-recent advent of ultrafast structural probes (femtosecond X-ray and electron scattering) has led to insights into both the cuprates and the pnictides (both the doped and parent compounds). As with dedicated reciprocal-space methods, ultrafast transmission electron microscopy (UEM) provides a method for directly probing impulsively-excited structural dynamics, with the added benefit of real-space imaging with combined nanometer and sub-picosecond spatiotemporal resolutions. In this work we report the direct imaging of coherent acoustic-phonon dynamics in the 1111-type iron-pnictide parent compound LaFeAsO via UEM.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Directed Hypersonic Strain Waves Imaged with Ultrafast Electron Microscopy

Intense, ultrafast photoexcitation of semiconducting materials produces several discrete but inter-related energy relaxation processes that span large spatial and temporal scales. Initial charge-carrier excitation, and generation of a dense electron-hole plasma, produces hypersonic plasma waves that propagate outward from the excitation zone. Related to this, myriad scattering processes (electron-electron, electron-phonon, etc.) occur within the first few picoseconds following photoexcitation, ultimately leading to the launch of coherent propagating strain waves. In thin membranes of semiconducting materials, propagating strain waves display Lamb-type symmetries and hypersonic velocities (depending on mode type and membrane thickness). In addition to often-used ultrafast spectroscopic methods, femtosecond X-ray and electron scattering has been used to study both hypersonic plasma waves and acoustic-phonon wavetrains. Observations from these experiments, combined with knowledge of the associated spatiotemporal scales, lead to questions about preferred nucleation points/regions, propagation directions, and time-varying phase-velocity dispersion behaviors. In this work, we discuss the results of real-space imaging, with ultrafast electron microscopy (UEM), of hypersonic coherent strain-wave dynamics in Ge.

47 OTHER INSTRUMENTATION↗

DOE OSTI2018

Understanding of a Novel Irradiation-induced Nanostructuring Process in SiC layer of TRISO fuel particles via Transmission Electron Microscopy

Metallic fission products are found to transport through the SiC layer of the tristructural isotropic (TRISO) nuclear fuel particle and various studies have been undertaken to understand the transport behavior [1, 2]. While the grain boundaries in SiC acts as a primary pathway to dissipate fission products in the form of multi-phased complex compounds, intragranular fission product precipitation has always been observed. Research on neutron irradiation damage tolerance of silicon carbide (SiC) and its composites, used in TRISO fuel, light water reactors (LWR) fuel cladding etc., has traditionally ignored the irradiation-induced “polymorphism” of SiC. In the post-irradiation advanced microscopy effort at Idaho National Laboratory, it has been recently discovered that the intragranular precipitation of fission products in SiC layer of TRISO coated fuels takes place via a dual step nucleation mechanism that involves cubic (?) ?hexagonal (?) SiC polymorphic transition, and subsequent metamorphosis of ?-SiC into fission product precipitates, as shown in Figure 1 [3]. This precipitation behavior cannot be explained by any reported nucleation mechanism. The extent of this precipitation has been observed to be influenced by the nature of the neutron irradiation induced damage structures. These damage structures (e.g., defect morphology and density) are also indicative of the irradiation temperature and neutron fluence levels experimented by the individual TRISO particles. The distinction of neutron irradiation damage structures as a function of the fuel kernel type, burn up level and fabrication histories of TRISO coated particles from the Advanced Gas reactor (AGR-1 and AGR-2) experiments has been examined and compared. This discovery not only adds new fundamental understanding of physical behavior of polymorphic ceramics under irradiation, but also provides the following beneficial traits for high temperature applications. 1) Since the extent of precipitation of the desired phase is directly dependent on the amount of precursor phase, the volume fraction of desired phase can be easily controlled by irradiation parameters. This eliminates the conventional limitation of volume fraction of a desired phase in the system as outlined by the phase diagram of the specific system. 2) Most of the precipitate-strengthened materials for high temperature applications suffer from degradation of size and shape of precipitate upon prolonged thermal exposures. But, precipitates are confined to the shape and size of the morphological template of surrogate phase (e.g. a Pd silicide precipitate is confined by the ?-SiC as shown in Figure 1(c)) in the present nucleation pathway. Hence, there is no possibility of desired phase to undergo any morphological degradation at high temperature that eventually will extend the lifetime of these materials at high temperature.

42 ENGINEERING↗

DOE OSTI2018

Compressed Sensing of Scanning Transmission Electron Microscopy (STEM) With Nonrectangular Scans

Scanning transmission electron microscopy (STEM) has become the main stay for materials characterization on atomic level, with applications ranging from visualization of localized and extended defects to mapping order parameter fields. In recent years, attention has focused on the potential of STEM to explore beam induced chemical processes and especially manipulating atomic motion, enabling atom-by-atom fabrication. These applications, as well as traditional imaging of beam sensitive materials, necessitate increasing the dynamic range of STEM in imaging and manipulation modes, and increasing the absolute scanning speed which can be achieved by combining sparse sensing methods with nonrectangular scanning trajectories. We have developed a general method for real-time reconstruction of sparsely sampled images from high-speed, noninvasive and diverse scanning pathways, including spiral scan and Lissajous scan. This approach is demonstrated on both the synthetic data and experimental STEM data on the beam sensitive material graphene. This work opens the door for comprehensive investigation and optimal design of dose efficient scanning strategies and real-time adaptive inference and control of e-beam induced atomic fabrication.

47 OTHER INSTRUMENTATION↗

DOE OSTI2021

Minimizing Crinkling of Soft Specimens Using Holey Gold Films on Molybdenum Grids for Cryogenic Electron Microscopy

We introduce a novel composite holey gold support that prevents cryo-crinkling and reduces beam-induced motion of soft specimens, building on the previously introduced all-gold support. The composite holey gold support for high-resolution cryogenic electron microscopy of soft crystalline membranes was fabricated in two steps. In the first step, a holey gold film was transferred on top of a molybdenum grid. In the second step, a continuous thin carbon film was transferred onto the holey gold film. This support (Au/Mo grid) was used to image crystalline synthetic polymer membranes. The low thermal expansion of Mo is not only expected to avoid cryo-crinkling of the membrane when the grids are cooled to cryogenic temperatures, but it may also act to reduce whatever crinkling existed even before cooling. Here, the Au/Mo grid exhibits excellent performance with specimens tilted to 45°. This is demonstrated by quantifying beam-induced motion and differences in local defocus values. In addition, images of specimens on the Au/Mo grids that are tilted at 45° show high-resolution information of the crystalline membranes that, after lattice-unbending, extends beyond 1.5 Å in the direction perpendicular to the tilt axis.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Development of liquid cells for high resolution imaging and chemical analysis in situ with Transmission Electron Microscopy

The development of liquid cells for transmission electron microscopy (TEM) have enabled breakthroughs in the study of nanoscale structural, morphological, or chemical changes during materials growth and applications [1]. The challenges of achieving high resolution imaging and chemical analysis through liquids often arise from the liquid cells being too thick. Making ultra-thin freestanding membranes and controlling the liquid thickness are some of the key attributes to the recent liquid cell developments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

Very-High Dynamic Range, 10,000 Frames/Second Pixel Array Detector for Electron Microscopy

Precision and accuracy of quantitative scanning transmission electron microscopy (STEM) methods such as ptychography, and the mapping of electric, magnetic, and strain fields depend on the dose. Reasonable acquisition time requires high beam current and the ability to quantitatively detect both large and minute changes in signal. A new hybrid pixel array detector (PAD), the second-generation Electron Microscope Pixel Array Detector (EMPAD-G2), addresses this challenge by advancing the technology of a previous generation PAD, the EMPAD. The EMPAD-G2 images continuously at a frame-rates up to 10 kHz with a dynamic range that spans from low-noise detection of single electrons to electron beam currents exceeding 180 pA per pixel, even at electron energies of 300 keV. The EMPAD-G2 enables rapid collection of high-quality STEM data that simultaneously contain full diffraction information from unsaturated bright-field disks to usable Kikuchi bands and higher-order Laue zones. Test results from 80 to 300 keV are presented, as are first experimental results demonstrating ptychographic reconstructions, strain and polarization maps. We introduce a new information metric, the maximum usable imaging speed (MUIS), to identify when a detector becomes electron-starved, saturated or its pixel count is mismatched with the beam current.

36 MATERIALS SCIENCE↗

DOE OSTI2017

Full Information Acquisition in Scanning Probe Microscopy

Scanning Probe Microscopy (SPM) has unlocked the nanoworld for exploration and control. While substantial effort has been dedicated towards the development of better instrumental platforms and probes, opportunities related to signal processing and data acquisition in SPMareoftenoverlooked. In this work, we discuss opportunities offered by capturing the full information of thedata stream from the detector, referred to as General Mode (G-Mode) SPM. This approach allows exploration of the complex tip-surface interactions, spatial mapping of multidimensional variability of material properties and their mutual interactions, and imaging at the information channel capacity limit, providing a new paradigm for SPM detection.

47 OTHER INSTRUMENTATION↗

DOE OSTI2013

Detecting Plasmon Resonance Energy Transfer with Differential Interference Contrast Microscopy

Gold nanoparticles are ideal probes for studying intracellular environments and energy transfer mechanisms due to their plasmonic properties. Plasmon resonance energy transfer (PRET) relies on a plasmonic nanoparticle to donate energy to a nearby resonant acceptor molecule, a process which can be observed due to the plasmonic quenching of the donor nanoparticle. In this study, a gold nanosphere was used as the plasmonic donor, while the metalloprotein cytochrome c was used as the acceptor molecule. Differential interference contrast (DIC) microscopy allows for simultaneous monitoring of complex environments and noble metal nanoparticles in real time. Using DIC and specially designed microfluidic channels, we were able to monitor PRET at the single gold particle level and observe the reversibility of PRET upon the introduction of phosphate-buffered saline to the channel. In an additional experiment, single gold particles were internalized by HeLa cells and were subsequently observed undergoing PRET as the cell hosts underwent morphological changes brought about by ethanol-induced apoptosis.

36 MATERIALS SCIENCE↗

DOE OSTI2014

Confocal Raman Microscopy for in Situ Detection of Solid-Phase Extraction of Pyrene into Single C 18 –Silica Particles

Solid-phase extraction (SPE) is a well known method for separating and pre concentrating analytes prior to ex-situ detection and quantification. Despite advances in miniaturization of solid-phase extraction, the technique has not evolved to meet the needs for detection in small scale microfluidic devices. While successful efforts have led to smaller scale extractors that allow preconcentration from small volumes, detection within correspondingly small volumes has remained out of reach because of analyte dilution during a post extraction transfer or “wash off” step prior to detection. In this work, successful extraction into sub-picoliter collection volumes is accomplished by using a single chromatographic particle as an extractor; wash off dilution is totally avoided by using confocal Raman microscopy for in-situ detection within the single-particle collection phase. Specifically, micromolar concentrations of pyrene in methanol water solution were equilibrated with an individual C 18 -functionalized silica particle, and Raman spectra were acquired from a small confocal sampling volume (~1 fL) within the particle interior. By comparing the Raman scattering intensity of the pyrene with that of the C 18 -chains in the stationary phase, it was possible to quantify the equilibrium coverage of pyrene relative to the C 18 chains. Spectroscopic isotherms were measured to determine the saturation surface coverage of pyrene relative to C 18 -chains and to study how the preconcentration equilibrium can be controlled by the source-phase solution composition. For extraction from aqueous solutions having the lowest methanol concentration (5% by volume), the preconcentration factor for pyrene into the particle was found to be 4.8 (±0.8) × 10 4 , which allowed detection of pyrene extracted from a 10 nM solution into a 0.52-pL particle volume.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

Photothermal Atomic Force Microscopy Coupled with Infrared Spectroscopy (AFM-IR) Analysis of High Extinction Coefficient Materials: A Case Study with Silica and Silicate Glasses

Photothermal atomic force microscopy coupled with infrared spectroscopy (AFM-IR) brings significant value as a spatially resolved surface analysis technique for disordered oxide materials such as glasses, but additional development and fundamental understanding of governing principles is needed to interpret AFM-IR spectra, since the existing theory described for organic materials does not work for materials with high extinction coefficients for infrared (IR) absorption. This paper describes theoretical calculation of a transient temperature profile inside the IR-absorbing material considering IR refraction at the interface as well as IR adsorption and heat transfer inside the sample. Furthermore, this calculation explains the differences in peak positions and amplitudes of AFM-IR spectra from those of specular reflectance and extinction coefficient spectra. It also addresses the information depth of the AFM-IR characterization of bulk materials. AFM-IR applied to silica and silicate glass surfaces has demonstrated novel capability of characterizing subsurface structural changes and surface heterogeneity due to mechanical stresses from physical contacts, as well as chemical alterations manifested in surface layers through aqueous corrosion.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗

DOE OSTI2015

Confocal Raman Microscopy for in Situ Measurement of Octanol–Water Partitioning within the Pores of Individual C 18 -Functionalized Chromatographic Particles

Octanol water partitioning is one of the most widely used predictors of hydrophobicity and lipophilicity. Traditional methods for measuring octanol water partition coefficients (K ow ), including shake flasks and generator columns, require hours for equilibration and milliliter quantities of sample solution. These challenges have led to development of smaller scale methods for measuring K ow . Recent advances in microfluidics have produced faster and smaller-volume approaches to measuring K ow . As flowing volumes are reduced, however, separation of water and octanol prior to measurement and detection in small volumes of octanol phase are especially challenging. In this work, we reduce the receiver volume of octanol water partitioning measurements from current practice by six-orders-of-magnitude, to the femtoliter scale, by using a single octanol filled reversed-phase, octadecylsilane-modified (C 18 -silica) chromatographic particle as a collector. The fluid handling challenges of working in such small volumes are circumvented by eliminating post equilibration phase separation. Partitioning is measured in situ within the pore confined octanol phase using confocal Raman microscopy, which is capable of detecting and quantifying a wide variety of molecular structures. Equilibration times are fast (less than a minute) because molecular diffusion is efficient over distance scales of micrometers. The demonstrated amount of analyte needed to carry out a measurement is very small, less than 50 fmol, which would be a useful attribute for drug screening applications or testing of small quantities of environmentally sensitive compounds. The method is tested for measurements of pH-dependent octanol-water partitioning of naphthoic acid, and the results are compared to both traditional shake-flask measurements and sorption onto C 18 -modified silica without octanol present within the pores.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2025

Scanning Electrochemical Microscopy for Kinetic Investigations in Viscous Deep Eutectic Solvents: Identifying Practical Approach Curves and Deviations from Electron Transfer Models

Determining heterogeneous electrochemical electron transfer (ET) kinetics in electrolytes with a wide range of physical properties is of great interest for achieving high-performance redox flow batteries. Among such electrolytes, concentrated hydrogen-bonded electrolytes (CoHBEs), including deep eutectic solvents (DESs), have recently garnered significant attention. Unfortunately, traditional Tafel analysis using macroelectrodes often encounters issues with mass transfer limitations in CoHBEs with high viscosities, thereby restricting kinetic analysis to a narrow potential window. Here, in this work, we introduce a methodology for evaluating ET kinetics in viscous DES using the scanning electrochemical microscopy (SECM). We first determined practical solutions to SECM tip positioning in ethaline DES, which yield pseudopositive feedback responses. Lattice Boltzmann method (LBM) simulations helped us rationalize the impact of the fluid and concentration fields, as well as tip geometry, tip approach velocity v, and the solvent viscosity ηs, on the shape of the approach curves. In addition to successfully recreating approach curves over a variety of conditions, we found that approaching a conductor ensured a practical point where the normalized tip response (Ni T = 2) converged at L = 0.7 within ∼10% error regardless of tip velocity. With positioning capabilities at hand, we investigated the kinetics of Fe 3+ /Fe 2+ redox couple in aqueous and the ethaline media. The experimental kinetic results were interpreted using the Butler–Volmer (BV) and Marcus–Hush–Chidsey (MHC) models. For ethaline, a nonideal kinetic behavior was observed, potentially attributed to solvent dynamics within DESs or to the interplay of chloride anions in the charge transfer process.

electrodes↗

DOE OSTI2017

Confocal Raman Microscopy Investigation of Molecular Transport into Individual Chromatographic Silica Particles

Porous silica is used as a support in a variety of separation processes, including chromatographic separation and solid-phase extraction. The resolution and efficiency of these applications is significantly impacted by the kinetics of partitioning and molecular transport into the interior of the porous particles. Molecular transport in porous silica has been explored previously by measuring chromatographic elution profiles, but such measurements are limited to relatively low retention conditions, where within-particle molecular transport must be inferred from elution profiles of solutes emerging from a packed column. In this work, a measurement of within-particle molecular transport is carried out using confocal Raman microscopy to probe the time-dependent accumulation of pyrene from an aqueous mobile phase into the center of individual C 18 -chromatographic particles. The measured time constants for pyrene accumulation were much slower than diffusion-limited transport of solute in solution to the particle surface. Furthermore, the accumulation into the center of the particle did not show a time-lag characteristic of slow-transport into the particle interior. The exponential rise of pyrene concentration is, however, consistent with first-order Langmuir adsorption kinetics at low surface coverages. The linear dependence of the time-constant on particle radius indicates an adsorption barrier near the outer boundary of the particle, where the accumulation rate depends on flux across the boundary (proportional to the particle area) to satisfy the within-particle capacity at equilibrium (proportional to the particle volume). The pyrene accumulation kinetics into the porous particle, expressed as a heterogeneous rate constant, were nearly 50-times faster than the pyrene adsorption rate at a planar C 18 -functionalized silica surface, which demonstrates the impact of multiple surface encounters within the porous structure leading to much greater capture efficiency compared to a planar surface. Monte Carlo simulations of within-particle pyrene diffusion, with the adsorption efficiency estimated from the planar-surface adsorption rate, predict a diffusion-to-capture distance within the porous particle that is within 40% of that observed in the radial dependence of the pyrene within-particle accumulation results.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2017

Helium Ion Microscopy for Imaging and Quantifying Porosity at the Nanoscale

Nanoporous materials are essential components in a vast number of applications from energy to drug delivery and to agriculture. Yet, the number of ways to analytically quantify the salient features of these materials, for example: surface structure, pore shape, and size, remain limited. The most common approach is gas absorption, where volumetric gas absorption and desorption are measured. This technique has some fundamental drawbacks such as low sample throughput and a lack of direct surface visualization. In this work, we demonstrate Helium Ion Microscopy (HIM) as a tool for imaging and quantification of pores in industrially relevant SiO 2 catalyst supports. We start with the fundamental principles of ion-sample interaction, and build on this knowledge to experimentally observe and quantify surface pores by using the HIM and image data analytics. We contrast our experimental results to gas absorption and demonstrate full statistical agreement between two techniques. The principles surrounding the theoretical, experimental, and analytical framework presented herein offer an automated framework for visualization and quantification of pore structures in a wide variety of materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2018

Confocal Raman Microscopy for In-situ Measurement of Phospholipid-Water Partitioning into Model Phospholipid Bilayers within Individual Chromatographic Particles

The phospholipid-water partition coefficient is a commonly measured parameter that correlates with drug efficacy, small-molecule toxicity, and accumulation of molecules in biological systems in the environment. Despite the utility of this parameter, methods for measuring phospholipid-water partition coefficients are limited. This is due to the difficulty of making quantitative measurements in vesicle membranes or supported phospholipid bilayers, both of which are small-volume phases that challenge the sensitivity of many analytical techniques. In this paper, we employ in-situ confocal Raman microscopy to probe the partitioning of a model membrane-active compound, 2-(4-isobutylphenyl) propionic acid or ibuprofen, into both hybrid- and supported-phospholipid bilayers deposited on the pore walls of individual chromatographic particles. The large surface-area-to-volume ratio of chromatographic silica allows interrogation of a significant lipid bilayer area within a very small volume. The local phospholipid concentration within a confocal probe volume inside the particle can be as high as 0.5 M, which overcomes the sensitivity limitations of making measurements in the limited membrane areas of single vesicles or planar supported bilayers. Quantitative determination of ibuprofen partitioning is achieved by using the phospholipid acyl-chains of the within-particle bilayer as an internal standard. This approach is tested for measurements of pH-dependent partitioning of ibuprofen into both hybrid-lipid and supported-lipid bilayers within silica particles, and the results are compared with octanol-water partitioning and with partitioning into individual optically-trapped phospholipid vesicle membranes. Finally and additionally, the impact of ibuprofen partitioning on bilayer structure is evaluated for both within-particle model membranes and compared with the structural impacts of partitioning into vesicle lipid bilayers.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2019

Photo-Scanning Electrochemical Microscopy on the Nanoscale with Through-Tip Illumination

Scanning electrochemical microscopy (SECM) has previously been employed in probing photoelectrochemical processes at semiconductor surfaces. Yet, the spatial resolution of these studies has not yet matched the nanoscale SECM resolution attained without substrate illumination. Herein, we introduce nanoscale photo-SECM with a glass-sealed, polished tip simultaneously serving as a nanoelectrode and a light guide to produce a microscopic light spot on the substrate surface. The advantages of this approach are demonstrated by comparing current transients obtained using through-tip and global illumination of the sample. The spot of light on the substrate surface facing the nanotip was sufficiently bright to measure the diffusion-controlled positive feedback current in good agreement with the theory. Here, we employed this approach for high-resolution photoelectrochemical mapping of ferrocenemethanol oxidation and oxygen evolution reactions at the Nb:TiO 2 rutile (110) single crystal surface. The images obtained using 40-50 nm radius tips showed only minor and random variations in photoelectrochemical reactivity for both processes, pointing to essentially uniform distribution of the Nb dopant over the TiO 2 surface and no measurable segregation on the ~50 nm scale.

14 SOLAR ENERGY↗