Symposium F.MT01—Advanced In Situ Characterization of Materials Kinetics

Oil-water interactions play a critical role in wettability alteration and enhanced oil recovery (EOR). In this work, we study the dielectric response of oil-water mixtures in calcite and analyze the observed behavior in terms of structural ordering and fluctuation of charge carriers. In the presence of water, the dielectric response gives the rise to anomalous low-frequency dispersion (ALFD). Which follows two power-law regions, separated by a transition frequency (ω_c). The response is rationalized in terms of hopping transport. Additionally, we measured the dynamic dielectric response on untreated and aged calcite pressed pellets in partially saturated conditions using brines with different salinities and compositions. In the presence of oil, the dielectric dispersion and their corresponding transition frequency were shifted to lower frequencies and lower susceptibility values. This suggests less connectivity between water clusters in rocks, where the trapped oil is preventing direct contact between water and calcite grains. We also demonstrated that the power-law exponents can be used to understand the fractal structure of the inclusions. Whereas untreated calcite followed the predicted behavior of single fractal clusters, aged calcite followed that of percolation of many-clusters. Interestingly, aged calcite mixtures in various electrolytes fall between the two distinct behaviors, indicating a mixed contribution.

Understanding of ZnO nanocrystal morphology evolution during wet chemical etching process is crucial for constructing nanoscale structures with desired physical and chemical properties [1,2]. However, the dynamic shape transformation in etching process is less revealed and understood so far, although it holds a key to control the surface exposure regularity of the etched ZnO nanostructures [3-5]. Here, we directly observe the reaction of ZnO rod-like structures in HCl solution using a combination of in situ liquid cell transmission electron microscopy (LCTEM) and in situ optical microscopy. A crown-capped nanorod is captured as the intermediate structure. Microscopy images and calculations show that HCl attacks ZnO slowly to form a concave region of the crown base at each radial corner initially and exposes high index surfaces such as {11-2k}. The high atomic density and their semipolar nature of the high index planes of {11-2k} are suggested to be responsible for the stabilization in the concave crystal regions. Furthermore, when the new high index facets exposed outside during the etching, the surface energy and defects populate and lead to accelerating the etching rate of the crown cap. By using different acids with varied pH or temperature, the ZnO morphology can be readily controlled with different electronic and optical properties. These findings shed light on the understanding of wet chemical etching of ZnO and the future design of novel functional materials.

References
[1] X. Ye et al., Science 354, 874 (2016)
[2] P. Pal et al., Micro and Nano Systems Lett. 3, 6 (2015)
[3] J. Wu et al., ACS Nano 11, 1696 (2017)
[4] M. R. Hauwiller et al., Nano Lett. 18, 5731 (2018)
[5] M. Sun et al., Small 1906435 (2020)

Self-assembled BiFeO₃-CoFe₂O₄ vertically aligned nanocomposites (VAN) are promising for their multiferroic functionality. In this work, we directly investigate interfacial-strain-controlled ferroelectric switching in the fully epitaxial BFO nanostructures within a CFO matrix. By applying Tomographic Atomic Force Microscopy and its unique nanomachining capability, we achieved high fidelity PFM imaging sequences while minimizing imaging artifacts. This uniquely reveals interfacial strain effects on ferroelectric switching as a function of distance from the strained interface. Even a relatively low interfacial strain of 0.6% can significantly alter ferroelectric switching fields and especially domain nucleation behavior. Notably, ferroelectric domains along the BFO-CFO interfaces exhibit up to two times higher coercive voltages as well as diminished nucleation site densities. Such results from our new approach can provide critical insight into optimizing the properties of such laterally heterogeneous strain engineered functional materials systems.

Anion exchange membranes (AEMs) are membranes with positively charged functional groups that enable the transport of anions and have been utilized for numerous purposes. AEMs may be used as membranes for hydrogen fuel cells, which have the potential for providing access to energy storage in hydrogen in an alkaline environment at lower operating temperatures with non-precious metals, which would dissolve in proton membranes and their solution. AEMs are also used in water filtration, including in the process of desalination whereby relatively fresh water is obtained from sea or wastewater by letting the water flow through the membrane. As more purposes are explored for AEMs it is important to determine and record their physical properties, such as shear modulus, that indicate qualities such as durability, strength, and ion conductivity for various uses and environments, such as changes in pH. A. Kisliuk et. al. determined that shear modulus specifically is correlated with increased barrier to ion conduction¹. In this study, MTPN1-TMA membranes were soaked in potassium hydroxide solution of pH in the range of 8 to 14 for 1 hour before measuring shear modulus on an atomic force microscope. There were 6 trials conducted for each pH value and preliminary results reveal that the highest average shear modulus, 8.07, occurred at pH 9 and lowest average shear modulus, 1.84, occurred at pH 10. Changes in the measured shear modulus in response to the different pH environments could be due to changes in AEM surface roughness and the tendency for membranes to be hygroscopic when they are soaked in aqueous solution with KOH. Future characterizations of AEMs include zeta potential measurements, SEM, contact angle, and finally, shear modulus and durability measurements under other environmental conditions such as temperature and gas flow.

¹Kisliuk, A et al. (2019). “Fundamental parameters governing ion conductivity in polymer electrolytes.” Electrochimica Acta, Volume 299, Pages 191-196. https://doi.org/10.1016/j.electacta.2018.12.143

We gratefully acknowledge support from the Louis Morin Charitable Trust

Graphene liquid cell was used for in situ electron microscopy because the single atom thickness, extraordinary mechanical strength and high conductivity of graphene allows the study of material in pristine stages with atomic resolution[1]. Using high-resolution transmission electron microscopy and electron energy loss spectroscopy, we show that BeO and NaCl follows unclassical crystallization process in graphene liquid cell. BeO in nature has a wurtzite structure and crystallizes in the sp²-coordinated, layered structure in liquid cells formed by sheets of graphene. We further reveal that the layered crystals can be thicker than the thermodynamically determined ultra-thin limit, beyond which the layered phase is energetically unfavored. The discovery counters a long-held dismissal and calls for a reevaluation of the possible existence of sp²-coordinated, layered polymorphs of octet compounds beyond the ultra-thin limit[2]. Sodium chloride has long been a classical crystallization model. Here we used time-resolved transmission electron microscopy to monitor crystallization of sodium chloride nanoparticles with atomic resolution in and out of graphene cell. The in situ observations, combined with large-scale reactive molecular dynamics simulations, reveal the details of the non-classical crystallization of sodium chloride nanoparticles via a nanoscale Wulff construction i. e. hexagon shape selection as they grow in a two dimensional confined system. Visualization of hexagon crystal growth strongly suggests the prominence of 2D confinement effect and allows us to quantify the effect on surface energy. Understanding the growth mechanism inside 2D confined system provides new revenue to rational design of nanomaterials with controlled properties.
References:
[1] J. M. Yuk, J. Park, P. Ercius, K. Kim, D. J. Hellebusch, M. F. Crommie, J. Yong Lee, A. Zettl, A. P Alivisatos, Science 61-64 (2012): 336.
[2] C. L. Freeman, F. Claeyssens, N. L. Allan, Phys. Rev. Lett. 066102 (2006): 96.

The technique of liquid cell transmission electron microscopy (LC-TEM) provides exciting information by applying the unique capabilities of electron microscopy to imaging and controlling nanoscale phenomena in liquid media. The liquid cell itself is a microfabricated enclosure that is hermetically sealed from the vacuum of the microscope and is filled with a liquid sandwiched between two electron-transparent membranes. LC-TEM provides a unique combination of spatial and temporal resolution for nanoscale liquid phase processes. However, a general limitation of LC-TEM has been the challenge of controlling and quantifying key aspects of the experimental conditions.
Here, we are particularly interested in processes that require simultaneous temperature and electrochemical control. These are important for designing batteries that must operate reliably over a range of environmental conditions or for understanding corrosion processes at elevated temperatures. We observe temperature-dependent electrochemistry using custom-fabricated microfabricated liquid cells that combine electrodes with an electrically isolated heater strip. Here we will discuss, using simulation tools, three key questions that must be understood in order to obtain quantitative information from these experiments.
We first calculate the spatial distribution of temperature in the liquid cell chip. Our setup controls temperature by Joule heating a Pt strip heater embedded within one of the electron transparent windows. We model the heater as an electrothermal system and solve for the temperature profile for a given potential difference, incorporating the temperature-dependent resistance change of the heater material. We also model two common experimental situations that can change the temperature and hence reaction kinetics within the imaged region: misalignment of the top and bottom chips and window bulging due to pressure differences. We show that the strip heater creates temperature gradients in three dimensions, discuss the dependence on experimental conditions, and propose changes in heater chip geometry to allow more precise temperature control. We next calculate the effect of the electron beam on the liquid (radiolysis) as a function of temperature. In TEM, radiolysis is modeled by including the rate at which incident electrons transfer energy to water and create radiolysis species and the subsequent diffusion and interactions of these species with each other and with other molecules present. All steps of this process are temperature-dependent, and we highlight the effects most relevant to our electrochemical experiments. We finally model the temperature-dependent kinetics of an electrochemical reaction in the case where only part of the electrode is heated, as in our experiments. We focus in particular on Cu deposition under galvanostatic conditions, aiming to understand the role of spatially-dependent diffusion in determining how the deposited morphology evolves under simultaneous control of heat and electrochemical conditions. We are excited by the opportunities of LC-TEM in quantifying temperature-dependent electrochemical processes for a range of practical applications in energy storage, catalysis and corrosion.

Organic capping ligands play accompanying roles aside from controlling the size and shape of nanocrystals. For instance, heteronuclear ligand-metal complexes have been shown to facilitate alloying of colloidal bimetallic nanoparticles; however, the mechanism by which this occurs remains unclear. Here we utilize liquid-phase transmission electron microscopy (LP-TEM) to demonstrate the roles of metal thiolate complexes and thiol ligands in controlling the formation pathways of bimetallic AuCu nanoparticles. Direct visualization of AuCu nanocrystal under dose-rate controlled LP-TEM conditions shows that polyethylene glycol thiol (PEG-SH) capping ligands modulate the rates of inter-metal electron transfer and metal atom aggregation reactions by forming metal thiolate complexes and kinetically trapping random alloy structures via strong, rapid surface capping. Systematic LP-TEM experiments varying dose rate established imaging conditions that minimized electron beam induced nanoparticle aggregation and phase separation and produced alloyed bimetallic nanoparticles of similar structure to benchtop flask-based synthesis. Kinetic simulations of the reactions between ligands and electron beam produced radicals revealed that thiol functional groups were oxidized to disulfide groups and organic radicals. At sufficiently high levels of ligand oxidation (high electron beam fluxes), weak ligand capping by disulfide groups leads to nanoparticle aggregation and metal phase separation, while at low levels of capping ligand oxidation spherical alloyed nanocrystals are formed. Taken together, our results indicate that thiol ligands play a dual role in facilitating alloyed nanoparticle formation by confining metal species within metal-ligand precursor complexes, which promotes simultaneous chemical reduction of metal ions and minimizes inter-metal electron transfer reactions that cause phase separation, and strongly and rapidly capping nanoparticles to prevent phase separation during crystallization.

Two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs) are potential candidate materials for electrodes due to their exceptional intrinsic properties including their high specific surface area, high electrical charge mobility and good thermal conductivity and mechanical strength. Graphene in particular shows tuneable structures and electrocatalytic activity, excellent conductivity, good chemical resistance and mechanical flexibility, as well as the ease and low cost of production. Graphene-based composites have been explored as next generation electrode materials for electrical and optical devices, supercapacitors, organic electronics and dye-sensitized solar cells and graphene itself is considered a promising electrode material for applications ranging from photocatalysis and solar cell to optoelectronic devices. In electron microscopy, graphene is used in a different way: its remarkable mechanical and scavenging properties make it ideal for graphene liquid cells¹ where liquid is encapsulated between two graphene sheets for high-resolution imaging. However, the transfer method of graphene to fabricate high quality graphene liquid cells and the reproducibility of making liquid pockets with sufficient volume, exact chemical content and well-defined thickness are key experimental challenges with graphene liquid cells. Recent studies circumvent leakage issues by using a liquid cell that is a hybrid of SiN_x and graphene to form leak-proof liquid confinement.² Furthermore, separating top and bottom graphene windows with a thin layer of hexagonal boron nitride (hBN) enables more control over liquid thickness and can provide a two-fold improvement in resolution³ compared to conventional graphene liquid cells. Despite these and other advances in engineering 2D materials into liquid cell design, the full functionality associated with microfabricated liquid cells, such as addition of electrochemical, heating or flowing and mixing capabilities, are generally still absent.
Here we focus on the opportunities arising from the use of 2D materials in liquid cell electrochemistry. To establish whether 2D materials can indeed evolve into a reliable electrode material for both electroanalytical chemistry and electron microscopy, we need to develop a reproducible protocol in transferring 2D materials onto microfabricated liquid cell chips to achieve reliable electrical contact and mechanical robustness. We describe liquid cells equipped with 2D material electrodes and show results aimed at understanding the interplay of kinetics and thermodynamics during electrochemical deposition, measuring parameters such as the shape, size and epitaxy of electrochemically deposited nanoparticles. We discuss the effects of external parameters such as the thickness and nature of the 2D materials and the deposition potential and we consider beam damage of these 2D materials. Exploring the possibility of 2D materials as electrode materials potentially opens up new opportunities for investigating a wide range of problems pertaining to energy storage and electrocatalysis.

1. Yuk, J. M.; Park, J.; Ercius, P.; Kim, K.; Hellebusch, D. J.; Crommie, M. F.; Lee, J. Y.; Zettl, A.; Alivisatos, A. P., High-Resolution EM of Colloidal Nanocrystal Growth Using Graphene Liquid Cells. Science 2012, 336 (6077), 61-64.
2. Rasool, H.; Dunn, G.; Fathalizadeh, A.; Zettl, A., Graphene-sealed Si/SiN cavities for high-resolution in situ electron microscopy of nano-confined solutions (Phys. Status Solidi B 12/2016). physica status solidi (b) 2016, 253 (12), 2544-2544.
3. Kelly, D. J.; Zhou, M.; Clark, N.; Hamer, M. J.; Lewis, E. A.; Rakowski, A. M.; Haigh, S. J.; Gorbachev, R. V., Nanometer Resolution Elemental Mapping in Graphene-Based TEM Liquid Cells. Nano Letters 2018, 18 (2), 1168-1174.


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Understanding the chemical and the physical processes of nanomaterials (NMs) in liquid media is a crucial challenge and of fundamental interest to modern materials science, chemistry and biology [1]. For example, observing the initial steps of nucleation and the growth of solids is of highest importance for the development of novel materials. This requires techniques allowing the study of NMs in their liquid environments with the corresponding resolution necessary to observe the formation and evolution of clusters at atomic-scale. The most attractive and unique capabilities of a scanning/transmission electron microscope (S/TEM) is its high-resolution, enabling to study materials at the atomic-scale. Nevertheless, a key limitation of this technique is the need of high vacuum conditions to analyse samples. In recent years, a new frontier in the field of transmission electron microscopy has emerged to observe specimens in liquid environments instead of high-vacuum conditions, by separating the liquid sample from the vacuum column using thin impermeable/electron transparent membranes [2]. Recent advances in microfabrication technology has led to a number of commercially available liquid cell TEM holders, where the liquid cells are made of two micro-fabricated silicon nitride (Si_xN_1-x) electron transparent thin films. The cell thickness can be controlled by spacers ranging from 50 nm to 5000 nm. Since the thickness of the encapsulating material is one aspect that negatively affects the resolution, the commercial cells are not optimized around achieving the highest spatial resolution including atomic resolution. Graphene-based liquid cell electron microscopy (GLC-EM) have demonstrated a better imaging resolution [3].
In this study, we optimized graphene-based liquid cells to realize in real time atomic-scale observations of nucleation and growth of gold (Au) nanoparticles in liquid mode, using high-spatial resolution S/TEM. The cell shows excellent imaging capabilities compared to conventional Si_xN_1-x liquid cells especially at higher-magnification. This work demonstrates that, using graphene to encapsulate small pockets of liquid, we were able to image and track in real time the very early stage of material growth at an interpretable atomic-scale resolution.

References:
[1] Raymon R. Unoci, Microsc. Microanal 2015, 21.
[2] Damien Alloyeau, Walid Dachraoui, Yasir Javed, Hannen Belkahla, Guillaume Wang, Hélène Lecoq, Souad Ammar, Ovidiu Ersen, Andreas Wisnet, Florence Gazeau, Christian Ricolleau, Nano Letters 2015, 5, 4, 2574-2581.
[3] Astrid De Clercq, Walid Dachraoui, Olivier Margeat, Katrin Pelzer, Claude R. Henry, and Suzanne Giorgio, J.Phys.Chem.Lett 2014, 5, 2126−2130.
[4] This project has received funding from the European Research Council (ERC) under the EU's Horizon 2020 research and innovation program (grant No. 681312).

The formation of solid matter as modelled by the classical nucleation theory (CNT), assumes to be triggered by the generation of spherical nuclei. However, initially at sub-nm scale, non-spherical atomic clusters form with structures deviating from a bulk crystal. Understanding the atom-by-atom growth dynamics at this size, requires considering the individual atoms. Conventional liquid cell holders have enabled studying liquid systems in transmission electron microscopes, but are limited in spatial resolution by the thickness of the liquid and the encapsulating windows (in total 100-1000 nm), making imaging at sub-nm resolution challenging. To further progress to atomic resolution, we use low vapour-pressure liquids, (glycerol or ionic liquids, such as 1-butyl-3-methyl imidazolium chloride).^[1] The negligible vapour pressure makes such liquids directly vacuum compatible without windows, and enables atomic resolution in suspended nanofilms and supported nanodroplets (5-50 nm thickness). Furthermore, a melting point above room temperature enables adjusting the viscosity of the liquid, and thus reaction kinetics, by heating. In our liquid phase experiments, we can observe dispersed Pt atoms to nucleate into few-atom clusters, which further coalesce and grow into nanoparticles or disordered cluster agglomerates, or redissolve into the liquid. CNT tells us that the presence of a surface lowers the energy barrier of heterogeneous nucleation, compared to homogeneous nucleation in a liquid phase. In our liquid experiments, there is indeed a corresponding effect, where nucleation is rarely observed in the suspended liquid nanofilms, while easily observable at a high nucleation rate in the nanodroplets on a carbon surface. As the highly unstable sub-nm clusters nucleate and dissolve, increased stability is observed in clusters of ca 8-13 atoms, compared to smaller clusters. Indeed, nucleating clusters typically form directly into this size range, where various close-packed structures (such as fcc and icosahedral^[2]) often can be observed. This indicates that the formation of close-packed, crystalline structures, already at the few-atomic scale, is an essential step of the nucleation process.


References:
[1] D. Keller, T. R. Henninen, R. Erni, Micron 2019, 117, 16.
[2] T. R. Henninen, M. Bon, F. Wang, D. Passerone, R. Erni, Angew. Chemie Int. Ed. 2019, 2.
[3] This project has received funding from the European Research Council (ERC) under the EU's Horizon 2020 research and innovation program (grant No. 681312)

In situ transmission electron microscopy (TEM) is now routinely used to measure the effect of external stimuli, such as temperature, gas or liquid environment, mechanical stress, etc., on the structure, chemistry, morphology and functioning of nanostructured materials. Here we employ in situ TEM to show that local surface plasmon resonance (LSP) energies can be used to initiate chemicals reactions that mimics photocatalysis.
The energy barrier for most of the chemical reactions is overcome by heating the reactants in the presence of a suitable catalyst, where the thermal energy provided by heat helps in surpassing the barrier that has been reduced by the adsorption of the reactant molecules. In recent years, the localized surface plasmon (LSP) resonance energy has been used to replace the thermal energy to initiate reaction at low temperatures, even at room temperature.^1,2 As most of the plasmonic materials, Au, Ag or Al, are not generally catalytically active, i.e. do not adsorb most of the gas molecules, but have been successfully combined with active catalysts such as Pt, Ni, Ru, etc., for harvesting their LSP resonance energies for low temperature reactions. So far optical methods have been used to generate and measure the LSP resonance energies. However, the low spatial resolution, of the order of 100 nm, achievable by optical methods, does not provide sub-particle level distribution of coupling efficiency of different modes. On the other hand, high energy electrons not only excite all LSP modes simultaneously, but also provide high spatial resolution in the nanometer range to resolve their energy loss probability that indicates the location-specific efficiency of coupling to the LSP modes at certain resonance energies on the plasmonic nanoparticle. Therefore, we use a monochromated electron source in an environmental scanning-transmission electron microscope (ESTEM), combined with metallic nanoparticle boundary element method (MNPBEM) and density functional theory (DFT) simulations, to characterize various LSP modes and their coupling efficiency distribution within the shape- and size-controlled nanoparticles of Au and Al. We demonstrate that the knowledge gained from low-loss and core-loss EELS measurements can be used to design catalyst-plasmonic particle combination for selected chemical reactions.

1 Mukherjee, S. et al. Hot electrons do the impossible: plasmon-induced dissociation of H₂ on Au. Nano letters 13, 240-247 (2012).
2 Yang, W.-C. D. W., Canhui; Fredin, Lisa A.; Lin, Pin Ann; Shimomoto, Lisa;, Lezec, Henri J. and Sharma Renu Site-selective CO disproportionation mediated by localized surface plasmon resonance excited by electron beam. Nature materials 18, 614-619 (2019).

Interfaces in catalysts are inherently dynamic - they often form under vigorous synthesis conditions and dynamically adapt their structures down to atomic scale to the specific reaction conditions. Depending on the synthesis conditions and the phase of the reactant, the active interfaces include those between solid-gas, solid-liquid, and triple-phase interfaces of solid-gas-liquid. Hence, in order to understand the catalytic reaction mechanisms, not only their initial structural and chemical configurations matter, but also their evolution under reaction conditions have to be clearly elucidated. In this talk, I will present our recent studies on understanding the synthesis and reaction mechanisms of precious metal nanoparticle based heterogenous catalysts, by using scanning transmission electron microscopy. Specifically, three examples will be given, including the understanding of the deposition and growth mechanisms of metal core-shell nanoparticles using in situ liquid cell electron microscopy, the probing of the dynamic evolutions of surface atomic structures of core-shell nanocubes via environmental cell STEM, and at last, the mapping of strong interactions between support and metal as a function of post-synthesis treatments using four-dimensional STEM.

Acknowledgements: research supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, and was performed at the Center for Nanophase Materials Sciences at Oak Ridge National Laboratory (ORNL), which is a DOE Office of Science User Facility.

In situ electron microscopy allows us to reveal the correlation between local atomic structure and properties with high spatial resolution. The effect of electric fields, light, mechanical strain and temperature on both structure and properties can be imaged and studied using spectroscopy. The obtained knowledge is used to tune the properties of advanced materials and devices. Catalytic activity of metal nanoparticles and electrical properties of semiconducting nanowires are examples where the strain induced effects have a strong influence on the properties and performances. High resolution annular dark field (ADF) scanning transmission electron microscopy (STEM) imaging can provide high resolution (better than 1 Å) and high precision (better than 1 pm) information about the local atomic structure [1]. In situ and correlative microscopy can be used to perform spatially resolved electrical, mechanical and optical studies [2]. In addition, dynamic electric field induced changes on the atomic scale can be studied using in situ microscopy [3]. The precision of the measurement and the quantitative information enable important and theoretical modelling on the same material systems. New aspects of material properties and mechanisms, not obvious from measurements on the macro scale are revealed using in-situ electron microscopy where interfaces, surfaces, geometries and defects affect the material properties on the macro, micro, nano and atomic scale. The knowledge is crucial for the understanding of the mechanisms active on the nano- and atomic scale, the effect of atomic structure on the properties and also for the design of materials and devices with tailored properties.


References
[1] T. Nilsson Pingel, M. Jørgensen, A.B. Yankovich, H. Grönbeck and E. Olsson, “Influence of atomic site-specific strain on catalytic activity of supported nanoparticles”,Nature Communications 9 (2018) 149.

[2] J. Holmér, L.J. Zeng, T. Kanne Nordqvist, J. Nygård, P. Krogstrup, L. De Knoop and E. Olsson, ”An STM-SEM setup for characterizing photon and electron induced effects in single photovoltaic nanowires”, Nano Energy 53 (2018) 175; Y.B. Yankovich, B. Munkhbat, D.G. Baranov, J. Cuadra, E. Olsén, H. Lourenco-Martins, H. Tizei, M. Kociak, E. Olsson and T. Shegai, “Visualising spatial variations of plasmon-exciton polaritons at the nanoscale using electron microscopy”, Nano Letters 19 (2019) 8171.

[3] L. de Knoop, M.J. Kuisma, J. Löfgren, K. Lodewijks, M. Thuvander, P. Erhart, A. Dmitriev and E. Olsson, Electric field controlled reversible order-disorder switching of a metal tip surface”, Phys. Rev. Mat. 2 (2018) 085006.

Two-dimensional materials provide exciting opportunities as substrates for the growth of three-dimensional nanocrystals. The difference in the nature of the bonding at the 3D nanocrystal / 2D material interface, compared to the situation for conventional epitaxy, is expected to generate interesting and potentially useful new growth modes and interfacial structures. It is well known that deposition of gold and other metals onto certain van der Waals-bonded materials produces faceted nanocrystals that can be aligned with the substrate lattice. Here we describe the use of several in situ microscopy techniques to explore the principles governing epitaxial growth in a broader range of nanocrystal / van der Waals substrate combinations. An unusual feature of the in situ experiments is that substrate preparation and materials deposition can be carried out under ultra high vacuum conditions in chambers that are connected to the microscope, while materials deposition is also possible while the sample is under observation within the microscope. These multiple capabilities are helpful in allowing flexibility and experimental robustness. Furthermore, for these particular experiments, a UHV environment (low 10^-10 Torr base pressure range) allows reactive materials such as niobium or germanium to be deposited and examined without oxidation. We will describe nanocrystal growth on suspended 2D materials using a UHV-TEM that has integrated capabilities for heating, laser cleaning, metal evaporation, and semiconductor chemical vapor deposition. Post-growth observation using scanning transmission electron microscopy yields atomic level compositional information as well as movies showing nanoparticle motion and annealing phenomena. We will also describe complementary measurements performed in a multi-probe, UHV in situ scanning tunneling microscope. This instrument provides opportunities to examine the atomic and electronic structure as well as to measure the transport properties of the nanocrystal / van der Waals material interface. We apply these techniques to evaluate nanocrystal morphology, orientation relation and interfacial structure and properties as a function of deposition parameters. We discuss strategies for morphological control and describe nanocrystal coalescence events and the patterning of the 2D substrate to control nucleation sites. We finally consider the formation of epitaxial alloy and heterostructure islands via sequential deposition. The eventual aim of the in situ experiments is to understand the relationship between the interfacial structure and morphology in a nanocrystal / van der Waals-bonded material combination and the electronic properties that are required for its applications.

Due to a unique combination of spatial and temporal resolution, liquid phase (LP) TEM has provided new insights into poorly understood processes of particle nucleation and assembly. Here we describe the results of studies on metals, oxides, and carbonates aimed at understanding the effects of additives, both as surface-bound ligands and dopants, on nucleation pathways and particle assembly. Investigations into hematite (Hm) mesocrystal formation from a ferrihydrite (Fh) precursor show that, in the presence of oxalate, isolated Hm particles rarely form. However, when they do, new Hm particles then repeatedly nucleate about 2 nm away from the new surfaces and immediately undergo oriented attachment to the growing aggregates of coaligned particles. Because nucleation rates are statistically deterministic and direction-specific, the resulting mesocrystals exhibit a constant shape irrespective of size. Results on Au nanoparticle formation in the presence of citrate reveal a similar outcome and comparison to natural and synthetic systems suggests such interface-driven pathways are widespread. Classical DFT analyses indicate that, for both Hm and Au, the surface-bound organics create interfacial chemical gradients that result in enrichment of cation species about a nm from the surface, though in the case of Au the species and degree of enhancement is pH dependent. Investigations of calcite formation from amorphous calcium carbonate (ACC) precursors in the presence of additives reveal a very different outcome. When carboxyl-rich organics are added, ACC transforms to crystalline phases via dissolution-reprecipitation, but at slower rates than in pure solution. However, when Mg is added, ACC initially exhibits low electron contrast, which increases over time absent any change in particle morphology. Diffraction and spectroscopic analysis show that the ACC transforms to Mg-calcite accompanied by loss of structural water. In combination with molecular dynamics simulations, the results imply Mg brings excess water into ACC, inducing structural fluctuations that enable transformation without the need to nucleate a separate crystal. These results highlight the opportunity that LP-TEM provides for deciphering underlying mechanisms of particle nucleation and assembly.

Two-dimensional (2D) materials have attracted significant attention due to their unique structure and properties. However, our understanding of their growth mechanisms, especially those in solution, is still limited. Using liquid cell transmission electron microscopy (TEM), we study the growth of 2D nanostructures and defects evolution with high spatial resolution in-situ. First, as an example, I will show the growth of 2D cobalt oxide and cobalt nickel oxide nanosheets. Our direct observation reveals that 3D nanoparticles are initially formed from the molecular precursor solution and they transform into 2D nanosheets. Ab initio calculations show that a small nanocrystal is dominated by positive edge energy, but when it grows to a certain size, the negative surface energy becomes dominant, driving the 3D nanocrystal transformation into a 2D structure. Uncovering these growth pathways, including the 3D-to-2D transition, provides opportunities for the future design and solution synthesis of novel materials. Second, I will also show the growth of 2D indium chloride nanostructures. The nucleation and growth of 2D nanosheets in solution are found through both classical and non-classical pathways. The growth kinetics at the low temperature is reduced compared to that at the room temperature. The fast growth induces high density of defects (e.g., nanotwins). In addition, our atomic structural analysis of the nanosheets and chemical mapping of the reaction front provides useful complementary information. These systematic studies allow us to develop an understanding of the growth mechanisms of InCl₃ 2D nanosheets and defect evolution. In conclusion, our development of high resolution liquid phase TEM opens opportunities for future study of many other nanomaterials growth and transformations.

In this talk, I will present my group’s recent in using our customized analysis framework based on a machine learning model based on convolutional neural network in extracting quantitative information from liquid-phase TEM videos. We apply this framework to three typical systems of colloidal nanoparticles, concerning their diffusion and interaction, reaction kinetics, and assembly dynamics, all resolved in real-time and real-space by liquid-phase TEM. A diversity of properties for differently shaped anisotropic nanoparticles are mapped, including the anisotropic interaction landscape of nanoprisms, curvature-dependent and staged etching profiles of nanorods, and an unexpected kinetic law of first-order chaining assembly of concave nanocubes. These systems represent properties at the nanoscale are otherwise experimentally inaccessible. Compared to the prevalent image segmentation methods, our method shows a superior capability to predict the position and shape boundary of nanoparticles from highly noisy and fluctuating background—a challenge common and sometimes inevitable in liquid-phase TEM videos. We expect our framework to push the potency of liquid-phase TEM to its full quantitative level, and to shed insights, in high throughput and statistically significant fashion, on the nanoscale dynamics of synthetic and biological nanomaterials.

Metallic nanoparticles such as nano-aluminum (nAl) are promising candidates as energetic additives for improved combustion and detonation effects in explosive/propellant formulations because of the high heat of combustion for aluminum and potentially rapid burning of nAl enabled by the high surface area to volume ratio. Various carbon-coated core-shell nAl were produced via atmospheric dielectric barrier discharge plasmas using commercial nAl (40-60 nm avg. diameter). Preliminary ex-situ characterization via transmission electron microscopy (TEM) has discovered the formation of γ-alumina (γ-Al₂O₃) crystallites that are both present in the organic coating and in the matrix of amorphous shell surrounding the Al core, presumably due to the phase transformation of the originally amorphous Al₂O₃ (a-Al₂O₃) induced by the plasma process. This work aims to investigate in-situ the coupled effects of transformed Al₂O₃ shell and organic coating on the dynamic behavior of nAl during heating experiments induced by a low-energy focused laser beam in TEM. The laser energy varied from ~1.5 W to 3 W with an incremental increase of 0.1 W every few minutes. The dynamic morphological and phase transformation during heating were investigated and captured through increasing numbers of hexagon-shaped γ-Al₂O₃ crystals floating on molten Al cores at a lower threshold laser energy than the as-received commercial nAl. The results from this paper directly revealed the real-time interactions among Al, a-Al₂O₃, γ-Al₂O₃, and carbon-coating for enhancing fundamental understanding of nAl at high temperatures. This paper also underscores the significance of exploiting advanced in-situ materials characterization to enhance in-depth understanding of reactive nAl composites possessing plasma-altered surface modifications for optimal controls of synthesis conditions and resultant structural and chemical properties.

In nanocrystalline materials, smaller crystallite size and increasing grain boundary (GB) volume may result in different physical properties from those of the bulk. Thermal expansion coefficient (CTE), as one of the important factors for materials selection, has been investigated extensively in several nanocrystalline materials [1-3]. Conventional approaches include the measurement of bulk CTE with varied grain size as well as the determination of lattice constants by scattering methods. However, the inconsistency of experimental findings has left the question on whether GBs contribute to the enhancement of CTE. Direct determination of CTE near GBs may require the dedicated measurement of temperature and volume in nano-meter scale and is still challenging issue for conventional methods due to the lack of spatial resolution.

In recent years, a nanoscale thermometry has been realized using valence electron energy loss spectroscopy (EELS) in scanning transmission electron microscope (STEM) [4]. Local temperature can be estimated by mapping the variation of plasmon energy. With this method, CTE of various 2-D materials has been successfully determined in nanoscale region with the support of theoretical calculation [5]. However, GBs contribution has not yet been discussed in the previous reports.

In this work, we attempt to probe the CTE of individual GBs by using in-situ valence EELS. Bicrystals of Σ5 and Σ29 strontium titanate (SrTiO₃) GBs were investigated in a temperature range from 300 to 873K. The GB structure was analyzed by high angle annular dark field (HAADF) image. The plasmon energy was mapped near GBs in each temperature and compared with the density functional theory (DFT) calculation. By carefully monitoring the shift of plasmon energy, we achieved to obtain local CTE near selected GBs. The relationship between CTE and GB structure was further analyzed by molecular dynamics (MD) simulation. The details will be shown in my presentation.


[1] Klam, H. J., H. Hahn, and H. Gleiter. Acta Metallurgica 35.8 (1987).
[2] Szpunar, Barbara, et al. Physical Review B 60.14 (1999).
[3] Kuru, Y., et al. Applied physics letters 90.24 (2007).
[4] Mecklenburg, Matthew, et al. Science 347.6222 (2015).
[5] Hu, Xuan, et al. Physical review letters 120.5 (2018).

Observing atomic-friction and revealing the friction mechanism at the atomic-scale are significant to understand macro friction behaviors and control wastage induced by the friction between contacts in microdevices. Here, by designing the atomic-scale contacts under transmission electron microscopy (TEM) observation, the real-time atomic friction process is captured. Friction between single tungsten asperities shows a discrete stick-slip behavior and in-situ TEM experiment revealed that the accumulation and release of the strain energy during friction is an asynchronized process. This work paves a possible way to realize in-situ atomic-friction research and enriches the understanding of the atomic-friction.

Metal additive manufacturing (AM) refers to a group of disruptive technologies that build metallic three-dimensional objects by adding feedstock materials layer by layer based on computer models. AM not only unleashes the design freedom of engineers by allowing the build of geometrically complex parts, but also opens up tremendous opportunities for material scientists to synthesize materials far-from-equilibrium and fabricate functionally graded architectures. While AM holds the promise for completely revolutionizing the way we make thing, building defect-free metal products with precisely controlled material microstructures and part performance remains challenging. Indeed, substantial fundamental issues associated with metal AM still need to be addressed before it can reach its full potential, and many new research areas need to be explored.

At the Advanced Photon Source, we have been applying operando high-speed x-ray imaging and diffraction techniques to probe a variety of metal AM processes since 2016. The superior penetration power of high-energy x-rays and the extremely high photo fluxes afforded by the 3rd-generation synchrotron facility allows the quantitative characterization of dynamic structural evolution in bulk metallic materials with unprecedented spatial and temporal resolutions. Many highly transient phenomena that take place during the energy-matter interaction in metal AM were investigated, and the mechanisms responsible for different types of defects were identified.

In the presentation, I will give a brief overview of the operando synchrotron studies of metal AM performed by our team and other scientists around the world. These metal AM processes include laser powder bed fusion, directed energy deposition, and binder jetting. The strength and limitations of different beamlines (i.e. source, detector) and operando AM systems will be elaborated. I will then introduce the new understanding gained from our synchrotron x-ray experiments, as well as their broad impact on the development of AM materials, technologies, and numerical models.

Interactions between x-ray and matters carry the matters’ property information, e.g. chemical species, chemical states, electronic structures, morphological structures, etc. In a complex heterogeneous sample system that has a hierarchical structure, multi-modal and multi-scale characterizations are necessary to reveal the system’s properties. Various x-ray microscopy and imaging techniques are valuable tools in such applications.
X-ray micro-imaging with synchrotron sources has been successfully applied to study dynamic systems of large volumes at high spatial and temporal resolutions. Compared to radiography, tomography is capable to provide structural information in three dimensions that is crucial in studying heterogeneous systems. However, with tomography, it is challenging to pursue temporal resolution as high as radiography. On the one hand, a tomography scan needs to take hundreds to thousands of radiography images of a sample. On the other hand, the sample needs to rotate during the image acquisition that may induce disturbances to the sample system due to unavoidable centrifugal force under rapid spinning. In this presentation, a recent development aiming to overcome these limitations will be presented.
At higher spatial resolution, it is possible to integrate multimodal techniques onto the same imaging/microscopy platform. Transmission X-ray Microscopes (TXM) have been widely used in energy storage material e.g. battery researches. One uniqueness of TXM is fast spectro-imaging capability at tens of nanometer spatial resolution that provides complex chemo-mechanical interaction information at a single electrode particle level in battery systems. Such information helps not only to understand the electrode materials’ behaviors but also to improve battery electrode engineering designs to mitigate the potential damages under charge/discharge conditions.
Although TXM is capable to provide correlative spectro-morphological imaging capabilities, other scanning nano-probe techniques are needed to pursuit even more modalities at higher sensitivity. At a state-of-art nanoprobe beamline, nano-diffraction, nano-fluorescence, and scanning imaging techniques can be integrated onto the same platform. Multimodal measurements with these techniques provide correlative structural information of the samples from different aspects. However, such measurements are slow, so it is not suitable for in situ and high throughput experiments. Combining TXM and nano-probe techniques overcomes each technique’s limitations and provide deep insights into the measured system. Few examples of this approach will be discussed in the presentation.

High flux radiation sources where we have active research programs (APS, ALS, and NIST) enable the study of materials and material properties in non-ambient environments characteristic of their synthesis, processing conditions, and/or use in the field. The variety of sources allows for enhanced contrast mechanisms for the analysis of structure, composition, and phase behavior of a variety of systems across R&D from the atomic to near visible light scale. In-situ experiments, including thermal and mechanical, allow for in-depth understanding of mechanisms of energy transfer present in materials. This presentation will give an overview of the problem solving capabilities available to Dow as well as some case studies on in-situ tensile/X-ray to examine energy absorbing mechanisms and in-situ DSC/X-ray to examine crystallization dynamics.
Deformation and failure mechanisms of polyolefin blends are investigated by collecting X-ray scattering data during in-situ tensile experiments. By simultaneously collecting both WAXS and SAXS patterns during the tensile experiments various energy transfer mechanisms can be tracked/observed including changes in orientation, crystal breakup, microvoiding, and lamellar size and shape. It is the sequence, type, and magnitude of these mechanisms that dictate mechanical performance in a material. WAXS will reveal differences in orientation and crystallite size while SAXS will give insight into the presence of microvoiding as well as lamellar size and shape. By combining the X-ray data with the tensile data, we are able to track how the addition of propylene-based olefin block copolymer (P-OBC) compatibilizers of different composition to PE/PP blends affects the deformation mechanism present during strain.
Additionally, in-situ DSC/X-ray experiments were conducted in order to investigate the effect of P-OBC compatibilizers on the crystallization behavior of polyethylene. A change in temperature is observed in a static DSC experiment, however this test does not determine whether this change is caused by crystallization/nucleation of the P-OBC compatibilizer or microphase segregation of the P-OBC compatibilizer. In-situ experiments allows for this distinction and WAXS is used to track the crystallization and microphase segregation of these blends as a function of temperature.

The development and deployment of technologies for CO₂ capture and storage (CCS) is key to limit the global warming below the 2 °C target (Paris Agreement). One of the CCS techniques considered is the post-combustion capture of carbon dioxide with solid sorbents. In particular, MgO-based sorbents are highly promising due to their abundance and low cost, as well as their intermediate operational temperature range of 200–450 °C and high theoretical uptake of 1.09 g_CO2/g_MgO. However, an important disadvantage of MgO-based sorbents is their slow carbonation kinetics. To tackle this, promotion of MgO with molten salts, such as alkali metal nitrates, can drastically accelerate the rate of MgO carbonation. Nonetheless, it has been observed that over cyclic operation, the rate of CO₂ uptake in MgO-based sorbents can decrease considerably, leading to deactivation and representing a major issue for their industrial implementation. Thus, understanding the structural changes that occur during cyclic operation is crucial to advance sorbent design.
In this work, we use time-resolved (1 s) in situ synchrotron XRD to shed light onto the structural dynamics occurring during cyclic operation of MgO nanoparticles coated with NaNO₃ (MgO-NaNO₃). The in situ XRD study allowed us to determine the structural evolution of the material in the kinetically controlled regime over ten consecutive cycles of carbonation (315 °C, CO₂) and regeneration (450 °C, N₂). The high signal to noise ratio and the time resolution of the collected data allowed us to obtain the detailed information of the crystalline phases and their transformation with time and cycle number. To quantitatively describe the phase composition of the sorbent parametric Rietveld analysis was performed, allowing to quantify the phase composition and its dynamics with time on CO₂ stream. This information enabled us to perform a kinetic analysis of the rate of MgCO₃ formation (via MgO+CO₂→MgCO₃). The NaNO₃ promoter is molten during the entire cyclic operation, and was probed using Pair Distribution Function analysis.
Furthermore, the data recorded revealed the formation of small quantities of a crystalline Na₂CO₃ phase after the sorbent regeneration steps. We hypothesize that a partial decomposition of molten NaNO₃ into Na₂O occurs during sorbent regeneration in N₂ (likely through NaNO₃→NaNO₂+0.5O₂ and subsequently 2NaNO₂→ Na₂O+N₂+1.5O₂), followed by the reaction of Na₂O with CO₂ in the carbonation step to form Na₂CO₃. Na₂O is in an amorphous state or present as ionic species dissolved in the molten NaNO₃. In the beginning of each carbonation cycle, Na₂CO₃ rapidly (ca. 120 s) transformed into Na₂Mg(CO₃)₂, while MgCO₃ starts to form only after ca. 15 min on CO₂ stream. The rate of MgCO₃ formation was described with an Avrami-Erofeev model. The presence of Na₂Mg(CO₃)₂ drastically enhanced the carbonation rate in each cycle, possibly by increasing the number of nucleation sites. Both the formation of Na₂Mg(CO₃)₂ and MgCO₃ has been found to be reversible during the regeneration step, but Na₂CO₃ was not decomposed at any stage and its amount increased after each regeneration.
To conclude, we were able to show that the formation of Na₂Mg(CO₃)₂ via the decomposition of the promoter NaNO₃ enhances the carbonation rate and reduces the induction period for MgCO₃ formation. However, during cyclic operation the decomposition of the NaNO₃ promotor was found to be detrimental for the CO₂ capture performance of the sorbent.

Real-time coherent Grazing-Incidence Small-Angle X-ray Scattering was used to investigate the average kinetics and the fluctuation dynamics during self-organized nano-patterning of Silicon. Flat Silicon samples were bombarded by 1keV Ar⁺ and Kr⁺ at 65° polar angle, leading to the spontaneous formation of nanoscale ripples with 30 - 50 nm wavelengths. The temporal evolution of the average intensity shows the evolution of average kinetics, while the fluctuation dynamics is investigated by correlating speckles. At early times, the surface behavior can be understood within the framework of linear theory. The transition away from the linear theory behavior is observed in the dynamics through the intensity correlation function, which quickly evolves into a compressed exponential decay on length scales corresponding to the peak wavelength. On shorter length scales, the dynamics becomes stretched exponential in behavior. The correlation times are maximum at the ripple wavelengths while they are smaller at other wavelengths. This has notable similarities and differences with the phenomenon of de Gennes narrowing. Overall, this dynamics behavior is found to be consistent with the simulations of a nonlinear growth model by Harrison et al. (Phys. Rev. E, 96, 032804). Following the formation of self-organized nano-ripples, they move across the surface. Homodyne X-ray alone cannot detect the motion, but because of the gradient of ion flux throughout the sample, there is a corresponding ripple velocity gradient that can be measured by cross correlating speckles and tracking their movements.
This work was supported by NSF grant DMR-1709380 (BU) and DOE grant DE- SC0017802 (UVM).

Nanoporous metal-organic frameworks are promising materials for gas capture, separation, and storage due to their high surface area and tunable chemistry, allowing these frameworks to selectively adsorb small molecules. The metal-organic framework Cu^I-MFU-4l, which contains coordinatively unsaturated Cu^I centers, can engage in backbonding interactions with various small molecule guests, enabling distinct binding energies for different adsorbates. Understanding these fundamental electronic interactions is needed to design tailored frameworks with targeted adsorption properties. We use in situ near edge X-ray absorption fine structure spectroscopy coupled with theory to examine several gases expected to bind to the open Cu^I sites in Cu^I-MFU-4l via different electronic interactions, including σ-donation, π-backbonding, and formal electron transfer. We show that in situ Cu L-edge X-ray absorption spectroscopy of Cu^I-MFU-4l in the presence of a gas can elucidate the π-backbonding contributions by directly probing excitations to unoccupied backbonding orbitals with Cu d-character, even for gases where other interactions, such as ligand-to-metal σ-donation, dominate. First-principles calculations based on time-dependent density functional theory reveal the backbonding molecular orbitals associated with these spectroscopic transitions. The energies of the transitions correlate with the energy levels of the isolated small molecule adsorbate, and the transition intensities are proportional to the binding energies of the guest molecules with Cu^I-MFU-4l. This definitive probe of the molecular and electronic structure origins of backbonding between metal-organic frameworks with electron rich metal centers and small molecule guests provides guidelines for molecular-level design of solid-state sorbents for energy-efficient industrially relevant separations processes.

We experimentally demonstrate that the application of external electromagnetic (EM) fields influence ceramic nanoparticle synthesis via distortions introduced in the local atomic structure. Such distortions do not occur during conventional hydrothermal synthesis. Prior studies show that EM fields can promote rapid, low-temperature synthesis and atomic structures not replicable using conventional hydrothermal routes. However, the mechanisms promoting these changes are not well understood, despite decades of scientific investigations. A critical limitation of prior studies was the inability to monitor structural changes while the EM field was applied, resulting in a lack of information about dynamic, field-driven changes in local atomic structure. We address this knowledge gap through a custom-designed microwave waveguide reactor (Gerling Applied Engineering) enabling in-situ synchrotron x-ray pair distribution function (PDF) analysis. This configuration (implemented in beamline 28-ID-2 at National Synchrotron Light Source II (NSLS-II), Brookhaven National Laboratory, USA) allows us to monitor EM field-induced changes in atomic structure in real-time during synthesis.

PDF analysis is an ideal tool for such in situ structural studies, as it can quantitatively characterize changes in the local atomic structure which serve as precursors to phase transitions and crystalline phase formation. PDF thus contrasts with x-ray diffraction (XRD), which is applicable to well ordered crystalline materials and is limited in characterizing systems with structural disorder or nanoscale features. Specifically, we find that the enhanced growth of crystalline rutile SnO₂ nanoparticles during EM field exposure is predated by structural reordering of the oxygen sub-lattice. In contrast, nanoparticles synthesized under conventional hydrothermal conditions exhibit no such structural changes and experience limited particle growth compared with EM field-assisted conditions. Our studies thus establish a clear difference in the phase formation mechanisms under EM fields, which would not be observable using conventionally used XRD techniques and ex-situ studies.

The lack of mechanistic insight into EM field-assisted processes has limited their widespread application. Our demonstration of a change in the synthetic pathway under EM field exposure holds important implications for materials design and inspires future work towards manipulating external fields for atomic-scale design of local structure in nanomaterials. Additionally, by revealing that applied fields influence individual sub-lattices before and during crystallization, we contribute significant knowledge towards fully understanding the mechanisms surrounding EM field-assisted phase formation. We believe this work will thus help guide the design of novel nanoparticles with altered local atomic structure, which will have a significant impact on materials for energy storage, catalysis, and advanced sensors.

The capability to tailor the grain and defect structure of the final product is critical in realizing the full potential of metal additive manufacturing (MAM) technologies. With regard to fusion-based MAM techniques, a fundamental knowledge of the ensuing phase transformations (vapor-liquid, liquid-solid and solid-solid) is critical to establish a correlation between MAM process parameters and the key physical variables governing the build microstructure. A wealth of fundamental solidification and applied welding literature points to the thermal gradient across the liquid-solid interface (G) and the velocity of the liquid-solid interface (R) as being the key physical variables in the liquid-to-solid phase transformation.

Ex-situ microstructural characterization of AM Ti-6Al-4V (Ti-64) builds produced using different melt strategies (and therefore, different G and R) was performed using high-energy synchrotron x-ray diffraction (S-XRD). The analysis of the S-XRD patterns using Rietveld refinement revealed a distinct change in crystallographic texture type of the solidified β-BCC phase, preferred variant selection of daughter α-HCP and a higher phase fraction of retained β-BCC compared to the Ti-64 powder used for the build. The crystallographic texture of β-BCC phase was observed to go from a cube texture in raster/line melt to a rotated cube/fiber-type texture in spot melts. Ex-situ synchrotron x-ray tomography on the Ti-64 builds revealed that the spot melt strategies result in higher porosity (both gas and lack of fusion defects) compared to the raster melts.

Motivated by the findings made in the ex-situ studies, the present in-situ studies used a laser-AM simulator (developed at beamline 32-ID-B, APS, Argonne National Laboratory) to mimic the unit processes of spot and line melt strategies on Ti-6Al-4V and IN-625 alloy plates, along with varying the relevant process parameters namely laser power, line scan speed, spot dwell time, and temporal and spatial intervals between two consecutive spot melts. In-situ dynamic synchrotron x-ray radiography was used in tandem with the laser-AM simulator to obtain the interface velocity (R) of the liquid-solid interface with a state-of-the-art spatial resolution of 2 μm and temporal resolution of a few μs. Simultaneous thermal imaging was performed on the top surface of the melts during the laser melting experiments to obtain the thermal gradient (G) at the liquid-solid interface. Post-mortem, ex-situ SEM-EBSD characterization of the melt pools were used to correlate the measured G and R evolutions to the resulting microstructure, as well as indirectly estimating the thermal gradient (G) for comparison to the in-situ experiments.

The in-situ x-ray radiographs were also used to observe the fluctuations in the vapor-liquid interface to study the conduction-to-keyhole transition and defect formation/mitigation mechanisms. Furthermore, the characteristics of the vapor-liquid interface, along with the liquid-solid interface, were used to validate/develop existing heat transfer models used to describe MAM.

State-of-the art high-resolution in situ imaging techniques are limited to probing the surface of the material. This is an important drawback for the characterization of twinned materials as the strain state changes from biaxial at the surface to triaxial in the bulk, dramatically influencing the functional properties. One way around this is Dark-field X-Ray Microscopy, which allows to access structural information from the bulk. Its resolution, however, is determined by the optics of the lenses, which prohibits investigations on nano-sized domains common in complex twinned materials.

In this work, we overcome this limitation by using the full reciprocal-space intensity distribution, allowing to map nanosized domains highly localized in the bulk. Calculation of the local elastic stresses from measured raw data for a ferroelectric/ferroelastic (Ba,Ca)(Zr,Ti)O₃ model system as a function of the externally applied electric field demonstrates the power of our technique. We find that, for all electric fields, the local domain size is spatially coupled to the elastic stress. The electric field induced crystallographic texture narrows the distribution of the elastic stresses and reduces the mechanical elastic energies by 60%, while simultaneously increasing the domain size by 35%. The results pave the way towards in situ imaging of elasto-morphological correlations inside the bulk crucial for many scientific disciplines, ranging from complex oxides to other twinned systems, such as martensitic materials or high entropy alloys.

Emerging organolead-halide-perovskite-based materials and devices have recently received tremendous attention due to their extraordinary photonic and optoelectronic performance and potential low production costs. Many physical and chemical properties beyond light harvests have erupted. Despite the rapid development of perovskite materials and devices, the impact of environmental factors on different stages of materials synthesis and device fabrication still remains unclear. Moisture or humidity is widely recognized as one of the major lethal factors in perovskite devices degrade or decompose after fabrication. However, recent reports have shown that moisture could be crucial to obtain high-performance perovskite films in a suitable moisture of ~35% relative humidity. Although a few attempts have been made to investigate the morphological evolution of organohalide perovskite films, a comprehensive understanding of the environmental influence in the process of thermal annealing is not achieved yet, as it is critically important to further improve the optoelectronic device performance.

By taking advantage of synchrotron-based grazing incident wide-angle x-ray scattering (GIWAXS), we in situ monitor the transformation of organohalide perovskites from precursors to final films upon thermal annealing at different relative humidity. In situ GIWAXS reveals the formation of crystalline perovskite materials over relevant time and moisture scales to decipher the effect of humidity on both phase transition and crystal structures during annealing. These in situ measurements demonstrate that moderate humidity accelerates the formation of organohalide perovskites and improves the orientation of the film, but more than 50% realtive humidity retards the formation of perovskites and destructs their crystal structures. Furthermore, the highly orientated films obtained at optimized relative humidity are observed which could be attributed to the hydration of precursors. The beneficial influence of moisture content in the crystalline structure formation process on the operation stability of the resulting devices will also be discussed.
These findings clearly elucidate the influence of moisture environment in annealing process of organohalide perovskites and, in turn, allow us to correlate the improved performance of organohalide perovskite matrials and devices to structural features in terms of environmental effects.

Understanding the behavior of nanoscale magnetic structures relies on being able to understand the complex interplay of the energy terms that control this behavior at a local scale. The relative contribution of these energy terms can be modified by parameters such as geometric confinement, interfacial effects and interactions between adjacent magnetic nanostructures. In addition, it is important to be able to explore how a magnetic nanostructure responds to an external driving force, such as temperature, and electric or magnetic fields. Our approach is to use Lorentz transmission electron microscopy (LTEM) or electron holography combined with in-situ magnetizing, heating and biasing experiments to elucidate the micromagnetic behavior at the sub-micron scale in nanomagnetic structures. Quantitative analysis of the LTEM data is carried out using the transport of intensity equation (TIE) approach, which we have extended to allow us to visualize the magnetic structure in three dimensions. By comparing the phase reconstructions of the magnetic induction obtained experimentally with the results of micromagnetic simulations, we are able to gain a detailed understanding of the way in which the various energy terms contribute to the behavior that we observe. We are interested in both geometrically-confined structures, for example arrays of nanomagnets patterned on lattices (artificial spin ice (ASI) arrays), in addition to nanomagnetic structures such as skyrmions and other topological spin structures that we create in thin films.

Our recent interest in artificial spin ices has focused on ASIs patterned on quasicrystalline lattices, in which the lack of translational symmetry and the varying coordination number at the vertices at which the magnetic bars meet leads to a heterogeneous energy landscape and to varying magnetic frustration across the lattice, even in the ground state. Our reconstructed magnetic induction maps allow us to determine the local magnetization configuration and we can then calculate the energy at each vertex motif for each possible magnetic configuration from simulations and thus determine which vertices play the largest role in influencing magnetic frustration and thus behavior in response to magnetic fields and to changes in temperature. Applied to magnetic skyrmions we used in situ LTEM to observe skyrmions in a Co/Pt multilayer film as a function of applied magnetic field, and by combining with simulations we were able to determine the strength of the Dzyaloshinskii-Moriya interaction.

Topological structures based on controllable ferroelectric or ferromagnetic domain configurations offer the opportunity to develop microelectronic devices such as high-density memories. Despite the increasing experimental and theoretical insights into various domain structures (such as polar spirals, polar wave, polar vortex) over the past decade, manipulating the topological transformations of polar structures and comprehensively understanding its underlying mechanism remains lacking. We successfully developed a technique to realize domain structures modulation and take an atomic resolution observation simultaneously. This atomistic observation of dynamic evolution reveals the potential of modulating ferroelectric domain patterns in ferroelectric nano-devices. Here we systematically investigate the real-time topological transformations of polar structures in PbTiO3/SrTiO3 multilayers at an atomic level. The procedure of vortex pair splitting and the transformation from polar vortex to polar wave and out-of-plane polarization are observed step by step. Furthermore, the redistribution of charge in various topological structures has been demonstrated under an external bias. These findings advance the current understanding of topological structures transformation manipulated by the external field and its origination in multilayers; it would ultimately enable the design of nanostructured materials to realize their potential applications in microelectronics.

References
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The ability to switch magnetism through application of an electric field holds great promise to launch a new era of energy-efficient memory and logic devices.^[1,2] Tremendous progress has been achieved with the integration of magnetoelectric BiFeO₃ thin films into oxide electronics architectures.^[3,4] The remaining energy gap towards the coveted attojoule switching can be bridged by the refinement of materials properties such as reduced electric coercivity and lower polarization value. Bringing BiFeO₃ close to the ferroelectric-paraelectric phase boundary via high La doping has emerged as a promising approach.^[5,6]
Here, we evaluate the prospects of highly-La-doped BiFeO₃ for technological applications. Using optical second harmonic generation microscopy, we directly access the polarization of the films once integrated in technology-relevant capacitor heterostructures. We find that La-doping results in a drastic deviation of the ferroelectric domain architecture from the prototypical BiFeO₃ domain configuration. This leads to the loss of the net in-plane polarization in the films in the pristine state. We show, however, that the application of out-of-plane electric fields leads to a spontaneous ordering of the La-doped BiFeO₃ domains. In particular, our results demonstrate the electric-field-induced emergence of the stripe-like domain pattern, key ingredient in the BiFeO₃-based magnetoelectric switching. This suggests La-doped BiFeO₃ as an ideal candidate for integration into a new class of reduced-energy-consumption oxide memory and logic devices.
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[5] Y.-L. Huang et al., Nat. Commun. 11, 2836 (2020).
[6] B. Prasad et al., Adv. Mater. 2001943 (2020).

Design and engineering of the size, shape, and chemistry of photoactive building blocks enable the fabrication of functional nanoparticles for applications in light harvesting, photocatalytic synthesis, water splitting, phototherapy, and photodegradation. Here, we report the synthesis of such nanoparticles through a surfactant-assisted interfacial self-assembly process using optically active porphyrin as a functional building block. The self-assembly process relies on specific interactions such as π–π stacking and ligand coordination between individual porphyrin building blocks. Depending on the kinetic conditions, resulting structures exhibit well-defined one- to three-dimensional morphologies such as nanowires, nanooctahedra, and hierarchically ordered internal architectures. At the molecular level, porphyrins with well-defined size and chemistry possess unique optical and photocatalytic properties for potential synthesis of metallic structures. On the nanoscale, controlled assembly of macrocyclic monomers leads to formation of ordered nanostructures with precisely defined size, shape, and spatial monomer arrangement so as to facilitate intermolecular mass and energy transfer or delocalization for photocatalysis. Due to the hierarchical ordering of the porphyrins, the nanoparticles exhibit collective optical properties resulted from coupling of molecular porphyrins and photocatalytic activities such as photodegradation of methyl orange (MO) pollutants and hydrogen production. The capability of exerting rational control over dimension and morphology provides new opportunities for applications in sensing, nanoelectronics, and photocatalysis.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

X-ray photon correlation spectroscopy (XPCS) provides kinetics and dynamics information about condensed matter by characterizing fluctuations at nanoscales. We have used the technique to study the growth of amorphous WSi₂ on SiO₂ templates by sputter deposition at room temperature in order to explore the average kinetics and real-time dynamics as a function of pressure. It's known that, as a consequence of kinetic behavior, there is a transition from smooth to rough growth surfaces with increasing pressure. It's observed that the speckle-averaged X-ray scattering intensity doesn't change after the film reaches a kinetic steady-state. Four timescale parameters were determined for each of the three pressure conditions (16mTorr, 10mTorr and 8mTorr). Peaks are found in intensity vs. time curves, and the corresponding timescale parameters at the intensity peak for each wavenumber q_// represent the time when the local roughness is maximum at the length scale 2π/q_//. Dynamic behavior is analyzed by means of two-time correlation functions at different q_// values. Most of the time parameters show q_// dependence. The time for local morphology to reach a steady state decreases most rapidly with q_//. Therefore, for high q_// regions (i.e. short length scales), it is found that a kinetic end point may be reached before the dynamic steady state is reached. In other words, in these regions, an average local structure is stabilized while processes of nanoscale deposition, relaxation and coarsening have not yet reached a dynamic equilibrium. The timescales for reaching dynamic and kinetic steady states, and the correlation time of surface structure in the final dynamic steady state all decrease as growth pressure and final surface roughness increase. This work was supported by the National Science Foundation (NSF) under Grant No. DMR-1709380 and by the U.S. Department of Energy (DOE) Office of Science under Grant No. DE-SC0017802.

The phenomenology of solid-state transformations and diffusion at short length scales (where the interaction of point defects with free surfaces and interfaces is significant) remains poorly understood, is increasingly important for nanostructured devices that utilize the “nano properties” (quantum confinement, high surface-area-to-volume ratios, and short transport lengths), for applications utilizing nanoscale reactivity, and for understanding interfaces and defects responsible for mass transport through them. In this work, via in-situ synchrotron x-ray diffraction (XRD), we directly interrogate the structure and reaction kinetics of lead sulfide (PbS) nanocrystals transforming into cadmium sulfide (CdS) through cation exchange. The epitaxial relationship of zincblende CdS to rocksalt PbS breaks the overall symmetry of the core-shell nanocrystal without requiring the loss of unit cell symmetry, leading to anomalous peak shifts in the diffraction pattern. Conversion occurs in three stages: (1) surface exchange to form a metastable rocksalt CdS shell, (2) crystallization of this shell to zincblende, and (3) diffusive transport of ions through the completed shell. The interdiffusion coefficient, D, for ions diffusing through the shell follows the Arrhenius relationship with an activation energy of 160-180 kJ mol⁻¹, which exceeds that observed in many other experiments in diffusion in nanoparticles and is similar to values measured in bulk solids, suggesting the barrier to exchange is dominated by the energies of point defect formation rather than surface-bound reactions. However, the magnitude of D is larger by four orders of magnitude or more compared to the slowest diffusing species in our system (self-diffusion of Cd in CdS). This surprising result suggests interdiffusion is enhanced in nanocrystals, and possible mechanisms include high concentrations of induced extrinsic defects and/or increase in the diffusive jump length through high-diffusivity paths. Cation exchange illustrates that the distinction between chemical diffusion in a potential gradient and diffusion at thermodynamic equilibrium has not been fully appreciated. Acceleration of interdiffusion in core-shell nanoparticles due to large chemical potential gradients will be important for understanding for nanoscale heterostructure formation and stability.

Solution-processed organic solar cells display inferior thermal stability largely because the nanostructure of the active layer blend changes upon heating. While photovoltaic blends based on non-fullerene acceptors such as indacenodithienothiophene-based ITIC derivatives are touted as more thermally stable than those based on fullerenes, they readily crystallize even far below their nominal glass transition temperature. We study two halogenated ITIC derivatives that readily co-crystallize upon mixing, which indicates that the use of an acceptor mixture alone does not guarantee the formation of a disordered mixture. The addition of the donor polymer to the acceptor mixture readily suppresses the crystallization which results in a fine-grained ternary blend with nanometer-sized domains that do not coarsen due to a high Tg ~ 200 C. As a result, annealing at temperatures of up to 170 C does not markedly affect the photovoltaic performance of ternary devices, in contrast to binary devices that suffer from acceptor crystallization in the active layer. Our results indicate that the ternary approach enables the use of high-temperature processing protocols, which are needed for upscaling and high-throughput fabrication of organic solar cells.

Nanoscale-engineered metal oxides have demonstrated improvement in device performance through enhancement in physical and chemical properties. Key factors in achieving the improved performances include high surface-to-volume ratios and high surface activities which are attributed to the decreased feature size by nanostructuring. Of the many variables that contribute to the performance enhancement, grain size is one feature, which plays an important role for improving the latent property of the material. For example, in catalyst and sensor applications, decrease in grain size increases the activity as a result of increased surface area. Furthermore, in sensors, decreasing the grain size to less than 10nm significantly increases sensitivity due to full depletion and accumulation of carriers in the sensing channels. Therefore, in terms of physical and chemical properties, nanostructures with nanosized grains can significantly improve the properties of high-performance devices. However, metal oxide devices commonly require an annealing process for crystallization and stabilization, which is usually accompanied by grain growth, particle agglomeration, and deformation of the nanostructure leading to degradation of device performance. Also, metal oxide nanostructures often operate at elevated temperatures and in chemically harsh environments, which induces structural change and gradual grain growth causing instability, sensor response and signal drift, and ultimately failure. Therefore, a strategy to maintain grain size and structure in metal oxide nanostructure during device operation is needed.
Here, we demonstrate a method to maintain grain size in crystalline metal oxide (i.e. tin oxide) nanowire through grain growth suppression by excess oxygen stoichiometry and nanoscale confinement control. Even at high annealing temperatures, grains with size below 10 nm retain their structural integrity and grain size. Grain growth dynamics and structural changes of the metal oxide nanowire during annealing were studied by in-situ and ex-situ TEM. Conditions such as temperature, nanowire diameter, annealing environment, and oxygen content were varied to understand the parameters needed for grain suppression. From the analysis, it was found that grain formation and growth are simultaneously affected by oxygen content in the nanowire and that suppression of grain growth occurs when the effect of oxygen inside the nanowire is maximized by controlling the diameter and stoichiometric oxygen ratio of the metal oxide nanowire. Grain growth suppression of nanosized grains in metal oxide nanowire provides an opportunity to fabricate structurally stable metal oxide nanostructures for high-performance gas sensor applications. By applying the parameters for grain suppression, a metal oxide nanostructure was fabricated by solvent-assisted nanotransfer printing. Consequently, the gas sensor built from the metal oxide nanowire accompanied by suppressed nanosized grains as the building block showed improved thermal and operation stability with high sensitivity toward ethanol gas. Our findings provide new insights into thermally stable nano-sized grains in metal oxide nanowire and suggest a novel material design for metal oxide based applications.

The calcium sulphate phases (anhydrite, bassanite and gypsum) are industrially important due to their widespread use in plaster and as a cement additive. The CaSO₄●xH₂O system displays easily reversible hydration/dehydration reactions between the different phases. In an aqueous environment it is known that bassanite (CaSO₄●½H₂O) is formed first, before the thermodynamically more stable gypsum (CaSO₄●2H₂O). The bassanite then converts to gypsum, either by assembly into elongated aggregates followed by transformation or by dissolution and reprecipitation¹.

This work uses molecular dynamics simulations performed using the LAMMPS² code and the potential model of Byrne et al.³ to explore the crystal growth of CaSO₄ phases. In our work we compute the aqueous surface energies for bassanite and gypsum which have been used to predict equilibrium morphologies which are in good agreement with previous studies⁴. Simulations of the assembly of bassanite nanoparticles in aqueous solution have been performed and compared with the recent work of Stawski et al.⁵ The modelling studies are complemented by experimental liquid cell TEM studies of the bassanite to gypsum transformation. The implications for a non-classical mechanism of the crystallisation of calcium sulphates are discussed.

[1] Van Driessche, A.E.S., Benning, L.G., Rodriguez-Blanco, J.D., Ossorio, M., Bots, P. and García-Ruiz, J.M., “The Role and Implications of Bassanite as a Stable Precursor Phase to Gypsum Precipitation” Science 336 (2012) 69-72
[2] Plimpton, S., "Fast Parallel Algorithms for Short-Range Molecular Dynamics" J. Comput. Phys 117 no. 1 (1995) 1-19
[3] Byrne, E.H., Raiteri, P. and Gale, J.D., "Computational insight into calcium–sulfate ion pair formation" J. Phys. Chem. C 121 (2017) 25956-25966
[4] Aquilano, D., et al. "Three study cases of growth morphology in minerals: Halite, calcite and gypsum" Prog. Cryst. Growth Ch. 62.2 (2016) 227-251
[5] Stawski, T.M., Freeman, H.M., Van Driessche, A.E.S., Hövelmann, J., Besselink, R., Wirth, R. and Benning, L.G., “Particle-mediated nucleation pathways are imprinted in the internal structure of calcium sulfate single crystals” Cryst. Growth Des. 19 (2019) 3714-3721

Low-dimensional materials and their hetero-stacked systems have shown potential for various applications. Due to their atomically thin nature, atomic scale modifications are possible under external stimuli, e.g., heating and particle irradiation leading to nanofabrication techniques. Here, we show through atomic-resolution in situ scanning transmission electron microscopy (STEM) experiments, that Joule-heated suspended graphene can be used as an atomically thin and electron transparent high temperature "hot plate". In these experiments, we apply continuous voltage pulses through graphene while observing dynamical changes in surface contaminants on top of graphene [1], as well as drastic structural changes in a mono-layer MoS₂ layer [2]. The highest temperatures of the graphene substrate were estimated to be above 2000 K. Specifically, we observe cleaning through evaporation of surface residues and hydrocarbon contaminants, as well as positional and rotational dynamics of residual particles on otherwise atomically clean graphene. In the MoS₂ structure, heating through the graphene substrate leads to increase in edge disorder and appearance of voids, accompanied by appearance of clusters on top of the edges. At the highest temperatures, the initially 2D MoS₂ structure breaks apart, and only independent 3D nanocrystals remain on graphene. The crystals have typically a hexagon-like shape with the 2H lattice. Our results show that graphene can be used as an efficient high-temperature substrate for in situ microscopy experiments and provide new insights into nanofabrication techniques for 2D materials for applications, for example, in catalysis.
[1] J.H. Choi, D.H. Shin, H. Inani, M.H. Kwon, K. Mustonen, C. Mangler, M. Park, H. Jeong, D.S. Lee, J. Kotakoski & S.W. Lee, ACS Appl. Mater. Interfaces (2020). DOI: 10.1021/acsami.0c02056
[2] H. Inani et al., to be submitted

Vanadium dioxide (VO₂) is a thermochromic material with thermally driven crystallographic transformation resulting in an insulator-to-metal transition [1]. The first-order monoclinic insulator phase to rutile metallic phase transition in VO₂ results in a fast reversible transition of
(i) electrical properties [1] - from electrically insulating properties at temperatures below the phase transition temperature (T_c) of 68 °C to conductive properties at temperatures above T_c,
(ii) optical properties [2]- from being transparent to infrared radiation (IR) below T_c to reflecting IR above T_c,
(iii) mechanical properties – with elastic anisotropy, significant increase in modulus along the transitioned rutile c-axis, and a pronounced phonon softening
(iv) magnetic properties [3] - from nonmagnetic below T_c to magnetic above T_c.

The electrical transition of VO₂ has potential applications in switching and sensing. VO₂ has also received renewed attention as a strongly correlated material for new ultrafast and microscopy techniques, ionic gating, and improved computational approaches. Our earlier work has exploited the magnetic and optical transitions for smart carbon fiber reinforced aerospace composites integrating them with micron-sized VO₂ particles [4]. In addition, mechanical actuation with large amplitude, fast response of ∼10 ms, and long lifetime has been demonstrated combining the temperature dependent thermal expansion and actuation of vanadium dioxide (VO₂) and the excellent electrical conductivity of carbon nanotube (CNT) thin films [5]. Practical mechanical actuation will be investigated via trade studies on the film thickness, displacement, and energy associated with two-dimensional shape change.

We present the dramatic multiphysical transformation of VO₂ resulting from the thermally driven crystallographic transformation. The challenging VO₂ film growth, structural characterization of the thin film, electrical transition, the in-situ thermo-mechanical response and theoretical underpinnings for VO2-based actuation for aerospace applications will be discussed.

References
[1] “Insulator–metal transition in substrate-independent VO2 thin film for phase-change devices,” Mohammad Taha et al, Sci Rep 7, 17899 (2017)
[2] “Optical properties of VO₂ films at the phase transition: Influence of substrate and electronic correlations,” Tobias Peterseim et al, Journal of Applied Physics 120, 075102 (2016)
[3] “Understanding of metal-insulator transition in VO2 based on experimental and theoretical investigations of magnetic features,” R. Zhang et al, Sci Rrp, 17093 (2018)
[4] “Vanadium Dioxide-Based Infrared Shielding for Aerospace Composites,” by Latha Nataraj et al, ARL-TR 8958, 2020
[5] “Flexible, All-Inorganic Actuators Based on Vanadium Dioxide and Carbon Nanotube Bimorphs,” He Ma et al, Nano Lett. 2017, 17, 421−428

Epoxy polymers are often used as a base for composites in aviation and aerospace applications, owing to their light weight, good affinity to the fillers, excellent mechanical properties, and stability under chemical corrosion and high temperatures. Many studies previously demonstrated that reducing materials scales, such as thin films or nanotubes, provides significant increase in the mechanical properties compared to the bulk material. However, few studies have dealt with these size effects in epoxy resins, and none was able to validate a mechanism. We present here an in-situ flexural bending experiments of thin epoxy cantilevers (bellow 200 nm). A Picoindenter is combined within TEM in order to investigate size effects on the mechanical properties of epoxy. A real-time video of the fracture process is gathered with the corresponding mechanical data from force and displacement. Experiments show crack propagation within the elastic region until fracture, without a yield point. Furthermore, the nano-scale samples demonstrate a significant increase in the flexural strength and are impressively close to the theoretical strength of the material. The combination of mechanical measurements and morphological data enables deeper understanding of the mechanisms behind the mechanical behavior. This work can be further expanded towards potential application as thin inter-layer of matrix in reinforced composites or in MEMS applications.

Solid-state synthesis from powder precursors is the primary processing route to multicomponent ceramic materials. The black-box nature of the reaction vessel typically precludes an understanding of the solid-state reaction mechanisms that govern phase evolution, leading to trial-and-error approaches to synthesis. Here, we combine ab initio thermodynamics with in situ synchrotron X-ray scattering and TEM to build predictive theories for which non-equilibrium phases form during solid-state ceramic synthesis, and why. I will present examples from the ceramic synthesis of NaxMO2 (M = Co, Mn) layered oxides [1] and also the classic high-temperature superconductor YBa2Cu3O6+x (YBCO) [2]. We derive an interfacial reaction model to predict the first-phase-to-form between two powder precursors, which can consume a large fraction of the reaction energy and can topotactically template structural transformations to other non-equilibrium intermediates. When there are three precursors, such as in Y2O3 + BaO2 + CuO --> YBCO, we show how sequential pairwise reactions dictate the multi-step phase evolution process towards the target multicomponent ceramic. Our insights can help guide the choice of precursors and parameters employed in the solid-state synthesis of ceramic materials, and constitutes a step forward in unraveling the complex interplay between thermodynamics and kinetics during materials synthesis.

[1] M. Bianchini et al., "The interplay between thermodynamics and kinetics in the solid-state synthesis of layered oxides" Nature Materials 2020
[2] A. Miura et al., "Sequential pairwise reactions dictate phase evolution in the solid-state synthesis of YBa₂Cu₃O_6+x" (2020)

The highest room temperature conductivity of any solid electrolyte, 0.34 S/cm, was reported for the superionic conductor Cu₁₆Rb₄Cl₁₃I₇ [1]. The crystal structure of this material has been determined using single-crystal X-ray diffraction [2] and neutron powder diffraction [3, 4], allowing models to be developed for the conduction pathways within the structure. Due to its high conductivity and insensitivity to air and humidity, Cu₁₆Rb₄Cl₁₃I₇ is particularly interesting as a solid electrolyte for synaptic cells targeted for neuromorphic computing applications [5] and three-terminal, fast-switching memory elements have been developed. The operating principle of these devices is that a voltage applied to a control electrode causes Cu⁺ to migrate through the solid electrolyte and deposit onto a channel, changing its resistivity as measured using two read terminals. It is important to understand the physical phenomena that underlie copper ion transport in this material in order to optimize properties of the devices such as the on/off ratio and the reliability. We have therefore developed an experimental design for transmission electron microscopy (TEM) to bias devices based on solid electrolytes in situ while imaging the structural changes that take place during operation.
We fabricated electron-transparent thin film samples on commercial TEM window chips with a copper control electrode and various materials for a channel placed several hundred nm to one side. The chips are accommodated in a Hummingbird TEM holder with biasing capability. The entire active area of the device is visible and the structural changes both within the electrolyte and at the electrodes can be correlated with current flow. Movies recorded during these experiments show a variety of interlinked, dynamic phenomena. At the channel, copper is deposited as whiskers or nuclei, with morphology depending on the electrode material. Within the electrolyte, an unexpected structural modulation appears when the current density exceeds a certain threshold. Diffraction analysis reveals a phase transformation initiated at the electrode, converting the electrolyte from a polycrystalline structure to a nanocrystalline phase with different atomic spacings. A distinctive bright stripe becomes visible at the phase boundary with contrast that is consistent with the local reduction in ion concentration. We suggest that depletion of copper ions, initiated at the electrode, drives the conversion to a CuRbClI phase of different stoichiometry. Furthermore, we suggest that the lower ion conductivity of the new phase is responsible for the formation of the bright contrast: finite element and Monte Carlo simulations show that a drop in conductivity cause local depletion. The interlinked nature of the phase change, conductivity change, and ion concentration then alter the voltage and current flow at the next current pulse, allowing the structural modulation to propagate across the electrolyte in a series of bright stripes. We suggest that the insights achieved by TEM in terms of the structural stability and phase changes in the superionic conductor Cu₁₆Rb₄Cl₁₃I₇ help to assess its capabilities as an electrolyte for neuromorphic devices and provide useful guidance for its other applications.
[1] Takahashi T., Journal of The Electrochemical Society 1979;126:1654.
[2] Geller S, Akridge JR, Wilber SA., Physical Review B 1979;19:5396-402.
[3] Kanno R, Ohno K, Kawamoto Y, Takeda Y, Yamamoto O, Kamiyama T, et al., Journal of Solid State Chemistry 1993;102:79-92.
[4] Oikawa K, Kamiyama T, Kanno R, Izumi F, Ikeda T, Chakoumakos BC., Materials Science Forum 2004;443-444:337-40.
[5] Teodor Todorov, Takashi Ando, Frances M Ross, John A. Ott, Jianshi Tang, Douglas Bishop, John Collins, Vijay Narayanan and John Rozen, ECS Meeting Abstracts 2019.

Despite their intrinsic instabilities, nanocrystalline metals have demonstrated improved properties relative to their coarse-grained counterparts. Targeted doping of grain boundaries to offset their energetic penalty has been pursued as a thermodynamic pathway for increasing thermal and mechanical stability. However, as recently demonstrated in a number of systems, notably Cu-Ta, dopant species thought to produce thermodynamically preferred nanocrystalline structures instead contain features such as small nanoprecipitates that act to inhibit grain boundary migration through kinetic pinning forces. In this study, we use thermodynamic nanostructure stability models to select a model system – Mo-Au – for exploring the interplay between thermodynamic and kinetic contributions to thermal stability in nanocrystalline alloys. A unique starting microstructure containing nano-metallic multilayers was employed and thermally aged through in situ electron microscopy experiments to access evolving segregation states. A combination of imaging and analytical electron microscopy was used to map solute segregation to different microstructural features and corresponding phase transitions as a function of temperature. Our results demonstrate the microstructure initially evolved through a phase transformation at lower homologous temperatures (< 600°C) where Au solute atoms cluster and segregate to the underlying grain structure, consistent with predictions from thermodynamic stability models. With increasing temperature, grain boundaries are shown to migrate and produce an increase in grain size, which was accompanied by intermittent pinning events from solute-rich regions. This regime transpired at temperatures where the microstructure thermodynamically phase separated, thus demonstrating kinetic contributions to stability at higher homologous temperatures.

Understanding the evolution of systems of supported nanoparticles is vital to the field of industrial catalysis - high reaction temperatures and harsh conditions drive the degradation of small, highly active catalytic nanoparticles, thereby reducing activity. While research aimed to describe coarsening and degradation processes has been undertaken for decades, the difficulty of in situ observation and the analysis of in situ experimental data has inhibited the development of a theory which comprehensively describes the evolution of supported nanoparticles.
Here, we present the results of in situ Transmission Electron Microscopy (TEM) experiments studying the development of a model catalyst system, gold nanoparticles on a silicon nitride support, at high temperatures. After heating the system and annealing in vacuum we observe global mass loss through sublimation of the solid nanoparticles. Using deep learning to aid image analysis, we are able to observe nanoscale changes in sets of hundreds of nanoparticles with high spatial and temporal resolution. After compiling data describing size and shape change of these nanoparticles, we are able to study the evolution of both the entire system, to understand the fundamental behavior with respect the environmental conditions, and small groups of particles, to recognize and study short-range interactions and stochastic mass-exchange processes. With these data in hand we develop a theoretical model based on thermodynamics and kinetics which closely describes the evaporation and surface diffusion of gold nanoparticles leading to a degradation of the model catalyst. By imposing physical constraints on our model, we can relate parameters to individual physical properties of a catalyst material. Based on this, we will demonstrate how materials and processing methods can be chosen to develop supported catalysts which resist degradation, and how environmental factors impact the mechanisms by which nanoparticles interact and evolve.

Piezoelectric actuators, sensors, dielectric capacitors and memory devices are examples of applications of ferroelectrics in modern life. In data storage applications, ferroelectricity presents a unique alternative to ferromagnetism as it allows for more energy efficient and faster devices. Commercially ferroelectrics have been hindered by major reliability issues [1,2] such as retention loss [3], imprint [4], and fatigue [5]. Overcoming these issues requires a deeper understanding of the microscopic mechanisms of domain wall kinetics, nucleation and the role of various types of defects, such as oxygen vacancies. Even so, at present, there is a lack of dynamical data on the ferroelectric switching process at the atomic scale, which impedes further development in this field.
In this work, we report on the fabrication by focused ion beam (FIB) of ferroelectric thin film specimens for in-situ electrical biasing experiments inside the transmission electron microscope (TEM). The specimens were fabricated on commercial micro-electro-mechanical system (MEMS)-chips that allow for ferroeletric switching over large areas of the film under a uniform electric field in the absence of external strains. A plate capacitor geometry was adopted to mimic the environment of industrial applications for ferroelectric capacitors. The bottom electrode of the capacitor consists of a conductive substrate, such as Nb-doped SrTiO₃, while Pt is used as the top electrode. The investigated ferroelectric thin films comprise Pb(Zr_0.2Ti_0.8)O₃ (PZT) and BiFeO₃ (BFO) grown by pulsed laser deposition.
The specimen geometry was optimized to prevent the mechanical failure of the electrodes caused by the strain induced by the piezoelectric response of the ferroelectric. This allowed for stable PZT thin film specimens, with a film thickness of 100 nm, up to a biasing voltage of 20 V. The investigation of the currents passing through the capacitor in response to the applied voltage revealed a Schottky-diode like behaviour. Typical currents in the reverse-bias direction were on the order of 100 nA at a biasing voltage of 5 V. A thermal breakdown of the lamella occurred once the leakage currents exceed 1 µA. The characterization of the ferroelectric thin films by dark-field (DF) TEM, differential-phase contrast scanning TEM (DPC-STEM) and high-resolution high-angle annular dark-field (HAADF) STEM imaging was performed using a FEI Titan Themis microscope equipped with a probe CEOS DCOR spherical aberration corrector operated at 300 kV. DF-TEM allows for tracking the in-plane or out-of-plane domain distribution in the ferroelectric film, while in DPC- and HAADF-STEM imaging, the ferroelectric polarization is mapped on the atomic scale by combining time series averaging [6], probe deconvolution and atomic column fitting [7].
Our results provide the path to prepare highly stable ferroelectric thin film capacitor specimens for in-situ S/TEM observations. This shall enable future studies on the dynamic processes in ferroelectric thin films under an applied bias voltage, which are crucial for the practical implementation of these materials in data storage devices [8].

[1] O. Auciello, J. F. Scott, and R. Ramesh, Physics Today 51 (1998), p. 22-27.
[2] W. L. Warren, D. Dimos, and R. M. Waser, MRS Bulletin 21 (1996), p. 40-45.
[3] A. Gruverman and M. Tanaka, Journal of Applied Physics 89 (2001), p. 1836.
[4] A. K. Tagantsev et al., Journal of Applied Physics 96 (2004), p. 6616-6623.
[5] A. K. Tagantsev et al., Journal of Applied Physics 90 (2001), p. 1387-1402.
[6] L. Jones et al., Advanced Structural and Chemical Imaging 1 (2015), p. 8.
[7] M. Nord et al., Advanced Structural and Chemical Imaging 3 (2017), p. 9.
[8] The authors gratefully acknowledge funding from the Swiss National Science Foundation under project number 175926.

The corrosion of uranium by means of oxygen or hydrogen is a material degradation process that is a significant concern for long-term storage and disposal. Hydrogen can locally attack the uranium to form uranium hydride (UH₃), which has been shown to increase brittleness of the uranium and to produce less desirable properties of tensile strength, hardness, and elongation. Corrosion of uranium by hydrogen has typically been characterized by an initiation time followed by pitting corrosion; however, variability in the UH₃ initiation time, thought to be correlated to thickness of the coherent uranium oxide (UO₂) film, have made understanding of early-stage UH₃ growth quite difficult. We will introduce a novel characterization tool that can rapidly collect in-situ time-dependent 3-D spatial data using while-light interferometry and provides growth data for both the UO₂ film and UH₃ precipitates.

White-light interferometry is a non-destructive and non-contact optical surface profiling technique for materials characterization that offers µm-scale lateral and nm-scale vertical resolution. This optical profiling technique is based on interpreting a series of interferograms, a pattern of dark and bright lines, resulting from constructive and destructive interference caused by the optical path length difference between a beam interacting with a reference material and the sample of interest. The interferograms are individually processed using a frequency-domain analysis algorithm, which converts the vertical measurements interferometrically for each pixel in field of view creating a three-dimensional image of the sample surface.

The white-light interferometer is equipped with an environmental sample cell with flowing oxygen or hydrogen gas permitting in-situ heating experiments. Several experiments have been conducted at different temperatures ranging from 40 °C to 90 °C under oxygen and hydrogen gas flow will be presented. Becsuse a single measurement requires less than ten seconds, the spatial data will display the effectiveness of this rapid novel characterization tool for early-stage time-dependent oxide thickness and UH₃ growth kinetics. Furthermore, activation energies of oxide film and UH₃ growth will be reported and compared to literature. The spatial data provides visual features that improve the current understanding of the early-stage growth mechanisms for UO₂ and UH₃.

Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy, National Nuclear Security Administration under Contract DE-AC52-07NA27344.

Recent hardware and software advances are ushering in a new paradigm in materials characterization, in which the direct observation of dynamics is revealing exciting new connections between material microstructures and properties. By directly observing the same specimen as it undergoes change or is exposed to service conditions, the in situ experimental approach helps remove the ambiguity and sparse statistics often associated with interpreting disparate samples extracted from different stages of the materials lifecycle. Doing so properly requires that in situ observations must be made with appropriate length scales, time scales, and modalities to observe the relevant phenomena of interest.
As a ubiquitous materials characterization tool, scanning electron microscopy (SEM) offers prime opportunities for such in situ experiments, impacting a broad range of research fields. But to date, achieving such results in a coordinated fashion is not a simple feat. Performing in situ SEM experiments is in no way a trivial task, often involving manual coordination of hardware and software distributed across numerous platforms and vendors (electron imaging, analytics acquisition, in situ stage control). Beyond coordinating an experiment, further complications arise in data interpretation, with the challenge of correlating results in space and time originating from multiple data streams. These technical barriers serve to restrict access to these techniques to the elite super-users who are able to dedicate substantial time and resources to developing their instrumentation, expertise, and workflows.
This contribution will present a fully integrated solution to mitigate these problems, combining a tensile-compression stage, heating unit, dedicated high temperature detectors, EDS/EBSD sub-units, and processing tools controlled from a unified software environment. The user can prescribe automated runs with multiple ROI’s, implement automated feature tracking and autofocus, and define different imaging conditions or EDS/EBSD acquisition per ROI and load. Furthermore, integrated digital image correlation (DIC) can be built into the workflows for rapid interpretation of localized strain evolution within samples. Examples will be presented including copper, steel, and nickel alloy systems. The implementation presented serves several purposes including: 1) facilitating difficult and complex in situ SEM experiments in a routine manner, 2) improving consistency and repeatability, allowing for more samples and increased statistics, and 3) doing so in such a way that it brings the power of this approach into the hands of the average researcher, where the focus can be shifted onto the materials science and engineering rather than heavily relying on microscopy expertise.

Microwave radiation (MWR) is known to lower the temperatures required for crystallization of ceramic oxides by introducing changes in the local atomic structure. This structural disorder serves as a precursor to phase transitions and crystalline phase formation. To better understand these field-driven phenomena, structural changes during MWR-assisted synthesis of titanium dioxide (TiO₂) were studied using in situ synchrotron X-ray diffraction (XRD) conducted at beamline 6-ID of the Advanced Photon Source at Argonne National Laboratory. We observed anisotropic expansion-contraction of the lattice parameters, which differs from the linear increase of lattice parameters with increasing temperature that is observed conventionally. Such anisotropy can be attributed to the structure becoming disordered under microwave exposure. Our Raman spectroscopy and X-ray photoemission spectroscopy experiments support such increased defect generation. We then applied molecular dynamics (MD) simulations to test the hypothesis that MWR-assisted synthesis leads to defect generation, such as the nucleation of vacancy-interstitial pairs. The second-moment tight-binding charge equilibrium (SMTB-Q) variable charge potential was used to model the TiO₂ system. The mean square displacements (MSD) of the ions were calculated for different concentrations of titanium Frenkel defects, oxygen Frenkel defects, or oxygen vacancies to probe the influence of defects on atomic thermal vibration. The ionic translational mobility as the structures move toward equilibrium is enhanced as the defect concentration increases.

To more precisely understand the local disorder and defect structure under MWR, we combined MD simulations with pair distribution function (PDF) analysis, a powerful tool for studying atomic structures capable of quantitatively characterizing both the long and short-range atomic order of a material. The X-ray PDF data of MWR-grown anatase TiO₂ and tetragonal zirconia dioxide (ZrO₂) was collected at the National Synchrotron Light Source II (NSLS-II) at Brookhaven National Laboratory (BNL). The local order of the samples was characterized by a real space Rietveld refinement using the defected phases as structure models. The increase of the atomic displacement parameters (ADP) obtained from the refinement were consistent with the MSD calculation. The results strongly suggest the presence of point defects within MWR-grown TiO₂ and ZrO₂. Utilizing in situ and ex situ characterization techniques and atomistic simulations, we provide evidence for the nonthermal field effect through defect generation. Our efforts to understand the fundamental mechanisms underlying electromagnetic field-assisted materials processing can help to realize low-temperature techniques to engineer ceramic and ceramic-polymer hybrid materials for energy storage, catalysis, and other electronics applications.

Most analyses of high-speed materials transformations and kinetics are limited to comparisons before and after synthesis, and thus often forced to infer the materials’ behavior during processing. Laser spike annealing, or LSA, is a powerful exploration technique that has been shown to form and trap a broad range of metastable phases in thin film oxide materials. With a small spatial footprint and wide temperature range within each anneal, lateral gradient LSA has been highly effective in high-throughput combinatorial studies. These transient thermally induced structural transformations can be characterized through a range of techniques including optical imaging, reflectance spectroscopy, and direct X-ray diffraction. Furthermore, the temporal evolution of materials phases can be probed during LSA scanning using spatially resolved techniques by displacing the probe with respect to the localized heat source. A single 2D image can capture the entire temporal evolution as well as, with lgLSA, the behavior as a function of peak temperature. Key events during the thermal cycle (nucleation, phase conversions) can thus be identified for further analysis.
We demonstrate the power of this technique to understand the formation and evolution of metastable phase transformations in thin-film Bi₂O₃ and Ga₂O₃. In Bi₂O₃, as a function of dwell and temperature, the δ, β and α phases can be formed; using the in-situ tracking, the evolution dynamics for these phases were observed. Similarly, in analyzing metastable phases of Ga₂O₃, we gain insight into the mechanisms that give rise to the nucleating phase γ and its subsequent evolution to the thermodynamically stable β phase. This approach can be adapted to take advantage of various analytic techniques, including synchrotron source X-ray diffraction.

Analytical and empirical solutions to engineering problems are usually preferred because of their convenience in applications, especially in materials characterization. However, they are not always accessible in complex problems. A new class of solutions, based on machine learning (ML) models such as regression trees and neural networks (NNs), are proposed and their feasibility and value are demonstrated through the analysis of fracture toughness measurements including single cantilever and pillar indentation splitting methods. It is found that both solutions based on regression trees and NNs can provide accurate results for the specific problem, but NN-based solutions outperform regression-tree-based solutions in terms of their simplicity. This example demonstrates that ML solutions are a major improvement over analytical and empirical solutions in terms of both reliable functionality and rapid deployment. When analytical solutions are not available, the use of ML solutions can overcome the limitations of empirical solutions and substantially change the way that engineering problems are solved. This ML solution has been employed in our fracture toughness measurements of rGO-reinforced ceramic nanocomposites.

Directly observing the evolution of material performance in situ under extreme conditions remains a great challenge. Environments including high temperatures, high pressures, radiation fields, and others promote defect formation and microstructure evolution which may rapidly and drastically change the performance characteristics of metals and alloys. Emergent or transient behaviors in these conditions are often difficult or impossible to capture using property and microstructure characterization after the fact. In an effort to fill a gap in the ability to capture material performance characteristics directly under extremes, transient grating spectroscopy (TGS), an all-optical photoacoustic methodology, has been implemented as an in situ diagnostic able to capture multi-property information while materials are exposed to the combined extremes of high temperature and ion beam bombardment. This non-destructive method returns elastic and thermal transport properties with second-scale time resolution. Here, recent work using this methodology to track the evolution of Ni-based solid-solution alloys using the in situ ion irradiation TGS (I³TGS) beamline will be described. Tracking material property evolution during long exposure times (hours) provides a clear indication of performance degradation due to irradiation-induced void swelling. On short timescales (seconds to minutes), observing rapid changes in thermoelastic properties as defect generation is initiated allows a unique window into bulk transient defect populations which are otherwise difficult to observe. This presentation will demonstrate the utility of this powerful new in situ technology as well as provide insight into the kinetic behavior of this interesting class of chemically-complex alloys under extremes.

The maximum amount of thermal energy available during atomic layer deposition (ALD) is generally determined by the decomposition temperature of the precursors and also sets the maximum temperature in the “ALD window”. This maximum temperature in some cases limits the structural perfection and extent of crystallization in resulting films. Intermittent annealing during the film growth in between ALD chemical exposures has been explored previously and shown to increase density and quality of ALD films.¹ However, without direct monitoring of one or more of the physical properties of the films, it can be difficult to determine the nuances of film transformation, such as crystallization temperature, surface roughening, and dependence on gas ambient.

In this work we integrate reflection high energy electron diffraction (RHEED) into a home-built ALD system to monitor structural and morphological transformations during ALD growth and thermally-induced structural transformations. RHEED is a surface sensitive diffraction technique that utilizes high energy (> 10 keV) electrons at a glancing angle, and is most commonly utilized in molecular beam epitaxy and pulsed laser deposition systems. The relatively high pressures associated with ALD are incompatible with RHEED due to filament instability and resulting short electron mean free path within the chamber, necessitating either 1) differential pumping on the electron gun and a short path length between the electron gun and phosphor screen or 2) a pump down to high vacuum conditions. In addition to describing the system design, flow, and thermal modelling; we will show initial results of the deposition and annealing of ultrathin films (1-20 ALD cycles) focusing on transformations of polymorphic Ga₂O₃. The integration of RHEED with ALD offers a slow-motion picture of traditional epitaxial growth techniques by decoupling the deposition and crystallization steps with simultaneous monitoring of the surface structure.

¹ J.F. Conley, Y. Ono, and D.J. Tweet, Appl. Phys. Lett. 84, 1913 (2004).

Spectroscopic ellipsometry (SE) is a powerful technique that is widely used to determine the thickness d of thin films as well as their dispersion law as a function of the wavelength l, composed of the refractive index n(λ), and of the extinction coefficient k(λ). SE data interpretation relies on an optical model built to fit the measured psi and delta values in order to extract n(λ), k(λ) and d. Given its sensitivity to small thickness variation, SE has also been applied in situ to monitor thin film growth in ALD reactors. However, in the case of very thin film (~1nm) the extraction of both optical properties and thickness is limited by the non-uniqueness of the solution (thickness, n) that prevents from determining both values with precision. Thus, in an in situ ALD measurement, a model with fixed optical properties is usually applied and only thickness changes are considered. This is problematic as the different steps during the ALD growth cycles will obviously lead to different optical properties of the thin film that will incorrectly result as thickness variations on the model.
In order to address this issue, an expansion of the three-phase equation (air, thin film, substrate), was applied in the thin film limit (d<<λ) [1]. This expansion leads to a disambiguation of n, k and d by measurements of the changes in the reflectance and in the complex reflectance ratio. By using this method, we were able to monitor the growth process of amorphous gallium oxide films and determine more reliably the thickness evolution than with classic interpretation of in situ ellipsometry. We were also able to evidence changes in the optical properties of the grown layer occurring during the different ALD cycles. We consequently evidenced the role of plasma exposure time in the minimization of absorbing defects of the gallium oxide thin films.

[1] I.K. Kim, D.E. Aspnes, Analytic determination of n, κ, and d of an absorbing film from polarimetric data in the thin-film limit, J. Appl. Phys. 101 (2007) 033109.

A widely extended application in semiconductor-based technology is the use of rectifier diodes in vehicles alternators to convert part of the mechanical energy into electrical energy. The most common failures in electric alternators are due to the electrochemical corrosion of the diodes or to damages as a consequence of the high current that circulates through diodes, which causes a significant temperature increase. This temperature increase is one of the system weakness, affecting directly to the device operation parameters. The diodes show a multicomponent nature since they are built up by different materials: epoxies to provide mechanical resistance, metals to evacuate the heat and a silicon semiconductor part. All of them have different physical characteristics and, therefore, different behavior, which is responsible of the appearance of diverse damaging phenomena such as the mechanical stress generation in the device.
The aim of this project is to understand what happens in the interfaces between the different materials of the device during its performance, mainly in the silicon interfaces, since it is the active part of the diode. For that, a study of several diodes currently used in automotive alternators has been carried out under certain working conditions which can lead to system failures, as well as to some phenomena allowing to understand their working mode. Some techniques such as confocal Raman microscopy and infrared thermography have allowed an in operando study of the diodes in forward biased and reverse biased conditions. The results show the device behavior at the interfaces, the generated stress in the different components and some system failure modes.

The bottom-up assembly of nanowires facilitates control of their dimensions, structure, orientation and physical properties. Surface-guided growth of planar vapor-liquid-solid (VLS) nanowires has been shown to enable their assembly and alignment on substrates during growth, thus eliminating the need for additional post-growth processes. However, accurate control and understanding of the growth mechanism of the VLS planar nanowires were achieved only recently (PNAS 2020,117, 152). The new growth model presents two limiting regimes - either the Gibbs-Thompson effect controlling the growth of the thinner nanowires or surface-diffusion controlling the growth of thicker ones. Although the comprehensive growth model supported by post-growth and ex-situ characterization help understand the planar nanowire growth mechanism, it can only resolve the static morphology of the nanowire and is unable to provide a complete picture about the growth kinetics of the VLS growth. Recently, fundamental in situ studies of semiconductor nanowire growth using a transmission electron microscope (TEM) presented significant findings of the VLS growth kinetics. Unlike these studies whose precursor were supplied in the gas phase and therefore could be fed into the TEM chamber with no contamination issues, studying binary semiconductor nanowire as ZnSe and ZnS require solid-phase precursors. Here, we demonstrate the in situ observation of guided ZnSe and ZnS planar nanowires on sapphire substrate by environmental scanning electron microscope (ESEM). The ESEM chamber is used as a CVD system using solid-phase precursors, enabling the study of the nanowire growth mechanism. The study provides new insights during the nanowire growth process, such as nucleation and incubation times, the real growth rates, interaction between different nanowires and the evaluation of the final nanowire morphology. The experimental findings, combined with the suggested theoretical model, is an essential step in understanding the guided growth of planar nanowire phenomenon.

MeV-UED has led to a new paradigm in ultrafast electron scattering: higher electron beam energy means stronger diffraction and significantly reduces space-charge effects hence leads to atomic spatial-temporal resolution [1-3]. Furthermore, MeV electrons experience less multiple-scattering,possess “real” flat Ewald-sphere and favourable sample enviroments; MeV-UED is a unique platform for in situ studies of material.MeV-UED has enabled broad scientific opportunities in material science and chemical dynamics, such as revealing the energy flow in superconductor [4] and atomic movie of light-induced structural distortion in the perovskites solar cell [5].

I will present two examples in capturing metastable states of functional material using MeV-UED: topological properties & charge density wave controlled by light[6-7]. Recently in situ charterization oftransient state during the electrically-triggered insulator-metal transition was sucessfuuly realized at SLAC MeV-UED [8]. Future development of nano-probe at SLAC MeV-UED will open new window to probe metastable state and dynamics in spatially heterogeneous systems and in in-situ device geometries.

X.J. Wang et al, Phys. Rev. E , 54, No.4, R3121 -3124 (1996).
P Zhu et al, New Journal of Physics 17 (6), 063004 (2014).
S. Weathersby et al, Rev. Sci. Instrum. 2015, 86, 073702−073707.
Tatiana Konstantinova et al, Sci. Adv. 2018;4:eaap7427, doi: 10.1126/sciadv.aap7427.
Xiaoxi Wu et al, Sci. Adv. 3 e1602388 (2017) doi: 10.1126/sciadv.1602388.
Edbert J. Sie et al, Nature 565,61–66(2019).
A. Kogar et al, Nat. Phys.16, 159 (2019) doi: 10.1038/s41567-019-0705-3.
SLAC MeV-UED: https://lcls.slac.stanford.edu/instruments/mev-ued.

With recent advances in in-situ microscopy, a new era in microscopy has arrived that allows for the dynamic imaging of materials under reaction conditions in the (scanning) transmission electron microscope. To better understand the structure/property relationships of materials, it is essential to get closer to the conditions in which the material will be used, such as high/low temperature, liquid environments, gas environments, or a combination thereof. Within these environments, moving beyond “real time” imaging to ultrafast imaging can provide yet another level of fundamental understanding of nanomaterials. Further, combining an in-situ or ex-situ experiment with electron tomography is a very powerful method for materials characterization as this provides the 3-D morphology of the (nano)materials under more relevant conditions. This talk will focus on newer developments such as in-situ heating in liquid, and the future of ultrafast transmission electron microscopy at the Center for Nanoscale Materials.

Phase change materials (PCMs) are semiconducting alloys that rapidly and reversibly switch between an amorphous (or glassy) state and a crystalline (or ordered) state via electrical and optical pulses. The two phases have optical and electrical properties that vary by orders of magnitude, which means that they can be used in rewritable optical data storage media and in fast, non-volatile memory devices. For these memory device applications, PCMs need to have fast (on the order of nanosecond) crystallization times in order to write data quickly, and they need to be able to quench into a room temperature stable glassy state, in order to enable long-term memory storage. These application specifications require a pronounced temperature dependence of the kinetic properties. Recent X-ray studies at LCLS on AIST and GeSb [1] have provided evidence for structural changes that occur in the super-cooled state, namely a Peierls-distortion-driven liquid-liquid phase transition (LLPT), which governs the kinetic properties. It has been argued that a liquid-liquid phase transition is also responsible for the anomalous liquid-phase properties of water, silicon, and germanium. To understand the role that the liquid-liquid phase transition plays in these systems and to understand its “universality”, the single-pulse capabilities of the MeV Ultrafast Electron Diffraction (MeV-UED) instrument at the SLAC National Accelerator Laboratory were used to explore the transient structure and pair distribution function of laser-melt-quenched, strongly super-cooled PCMs, such as Ge₂Sb₂Te₅ (GST), Ag₄In₃Sb₆₇Te₂₆ (AIST), Ge₁₅Sb₈₅, and pure Sb at various thicknesses, as well as diamond-cubic structured semiconductors such as Ge and Si. Given the large momentum-transfer range of MeV-UED, the time-resolved pair-distribution functions are obtained at a high spatial resolution, which we use to explore interesting material dependent aspects of the liquid-liquid phase transitions.

References
[1] Zalden et al., 364, 6445 Science 2019

Gold nanocrystals exhibit attractive electronic, thermal and catalytic functionalities in response to light governed by dynamic energy conversion processes among photons, electrons and phonons. Although such processes have been typically investigated by optical methods, an understanding of the response of atomic structure following photoexcitation in nanocrystals has remained elusive to date. Here, we monitor energy conversion processes in gold nanocrystals from lattice point of view by resolving dynamic structural responses via femtosecond electron diffraction [1]. We uncover the effects of size and surface chemistry on the electron – phonon coupling and thermal relaxation. We show that smaller size leads to stronger electron – phonon coupling in these nanocrystals, which is associated with the reduced dielectric screening at the nanocrystal surfaces. In addition, we elucidate that surface ligands provide a tuning knob for the electron – phonon coupling. Particularly, thiol-based ligands slow down the cooling of hot carriers that are generated by surface plasmon polaritons. Overall, our findings provide new design principles to exploit dynamic energy conversion processes in gold nanocrystals in applications such as photocatalysis and thermoelectrics.
[1] B. Guzelturk et al. ACS Nano 14, 4792 (2020)

Layered materials are characterized by strong covalent bonding within each layer, while the layers themselves are held together by weak van der Waals forces. These materials have received considerable attention owing to their compatibility with other materials, and exceptional electronic, optical, thermal, mechanical, and thermoelectric properties. The presence of a van der Waals gap between individual layers enables the intercalation of several species which can be used for tuning these properties. However, intercalation of species can also be an unintended consequence of diffusion from patterned contacts on the layered materials. In either scenario, the introduction of intercalant species into a layered material may lead to unanticipated structural and phase transformations. These transformations can be further exacerbated at elevated temperatures, yet little is understood about the high-temperature dynamics that occur in intercalated layered materials. Here, we use in situ transmission electron microscopy to investigate temperature-dependent structural evolution of Bi₂Te₃, a layered material with well-known application as a thermoelectric, when Cu is intercalated within the van der Waals gap. As temperature is increased, the initially disordered Cu undergoes local ordering within the van der Waals gap, which is followed by high-mobility boundary motion, polycrystal growth, and anisotropic sublimation behavior. As the temperature is raised further, layered Cu₂Te crystals nucleate and grow with a crystallographic relationship to the Bi₂Te₃ lattice. The presence of Cu substantially alters both the transformation temperature of various phenomena, as well as the dynamic evolution of the crystals. These findings are consequential for the design of layered nanoscale devices and nanostructured systems where intercalants may intentionally or unintentionally alter the properties and stability of these systems.

MeV-UED has led to a new paradigm in ultrafast electron scattering: higher electron beam energy means stronger diffraction and significantly reduces space-charge effects hence leads to atomic spatial-temporal resolution [1-3]. Furthermore, MeV electrons experience less multiple-scattering,possess “real” flat Ewald-sphere and favourable sample enviroments; MeV-UED is a unique platform for in situ studies of material.MeV-UED has enabled broad scientific opportunities in material science and chemical dynamics, such as revealing the energy flow in superconductor [4] and atomic movie of light-induced structural distortion in the perovskites solar cell [5].

I will present two examples in capturing metastable states of functional material using MeV-UED: topological properties & charge density wave controlled by light[6-7]. Recently in situ charterization oftransient state during the electrically-triggered insulator-metal transition was sucessfuuly realized at SLAC MeV-UED [8]. Future development of nano-probe at SLAC MeV-UED will open new window to probe metastable state and dynamics in spatially heterogeneous systems and in in-situ device geometries.

X.J. Wang et al, Phys. Rev. E , 54, No.4, R3121 -3124 (1996).
P Zhu et al, New Journal of Physics 17 (6), 063004 (2014).
S. Weathersby et al, Rev. Sci. Instrum. 2015, 86, 073702−073707.
Tatiana Konstantinova et al, Sci. Adv. 2018;4:eaap7427, doi: 10.1126/sciadv.aap7427.
Xiaoxi Wu et al, Sci. Adv. 3 e1602388 (2017) doi: 10.1126/sciadv.1602388.
Edbert J. Sie et al, Nature 565,61–66(2019).
A. Kogar et al, Nat. Phys.16, 159 (2019) doi: 10.1038/s41567-019-0705-3.
SLAC MeV-UED: https://lcls.slac.stanford.edu/instruments/mev-ued.

With recent advances in in-situ microscopy, a new era in microscopy has arrived that allows for the dynamic imaging of materials under reaction conditions in the (scanning) transmission electron microscope. To better understand the structure/property relationships of materials, it is essential to get closer to the conditions in which the material will be used, such as high/low temperature, liquid environments, gas environments, or a combination thereof. Within these environments, moving beyond “real time” imaging to ultrafast imaging can provide yet another level of fundamental understanding of nanomaterials. Further, combining an in-situ or ex-situ experiment with electron tomography is a very powerful method for materials characterization as this provides the 3-D morphology of the (nano)materials under more relevant conditions. This talk will focus on newer developments such as in-situ heating in liquid, and the future of ultrafast transmission electron microscopy at the Center for Nanoscale Materials.

Two-dimensional materials provide exciting opportunities as substrates for the growth of three-dimensional nanocrystals. The difference in the nature of the bonding at the 3D nanocrystal / 2D material interface, compared to the situation for conventional epitaxy, is expected to generate interesting and potentially useful new growth modes and interfacial structures. It is well known that deposition of gold and other metals onto certain van der Waals-bonded materials produces faceted nanocrystals that can be aligned with the substrate lattice. Here we describe the use of several in situ microscopy techniques to explore the principles governing epitaxial growth in a broader range of nanocrystal / van der Waals substrate combinations. An unusual feature of the in situ experiments is that substrate preparation and materials deposition can be carried out under ultra high vacuum conditions in chambers that are connected to the microscope, while materials deposition is also possible while the sample is under observation within the microscope. These multiple capabilities are helpful in allowing flexibility and experimental robustness. Furthermore, for these particular experiments, a UHV environment (low 10^-10 Torr base pressure range) allows reactive materials such as niobium or germanium to be deposited and examined without oxidation. We will describe nanocrystal growth on suspended 2D materials using a UHV-TEM that has integrated capabilities for heating, laser cleaning, metal evaporation, and semiconductor chemical vapor deposition. Post-growth observation using scanning transmission electron microscopy yields atomic level compositional information as well as movies showing nanoparticle motion and annealing phenomena. We will also describe complementary measurements performed in a multi-probe, UHV in situ scanning tunneling microscope. This instrument provides opportunities to examine the atomic and electronic structure as well as to measure the transport properties of the nanocrystal / van der Waals material interface. We apply these techniques to evaluate nanocrystal morphology, orientation relation and interfacial structure and properties as a function of deposition parameters. We discuss strategies for morphological control and describe nanocrystal coalescence events and the patterning of the 2D substrate to control nucleation sites. We finally consider the formation of epitaxial alloy and heterostructure islands via sequential deposition. The eventual aim of the in situ experiments is to understand the relationship between the interfacial structure and morphology in a nanocrystal / van der Waals-bonded material combination and the electronic properties that are required for its applications.

Topological structures based on controllable ferroelectric or ferromagnetic domain configurations offer the opportunity to develop microelectronic devices such as high-density memories. Despite the increasing experimental and theoretical insights into various domain structures (such as polar spirals, polar wave, polar vortex) over the past decade, manipulating the topological transformations of polar structures and comprehensively understanding its underlying mechanism remains lacking. We successfully developed a technique to realize domain structures modulation and take an atomic resolution observation simultaneously. This atomistic observation of dynamic evolution reveals the potential of modulating ferroelectric domain patterns in ferroelectric nano-devices. Here we systematically investigate the real-time topological transformations of polar structures in PbTiO3/SrTiO3 multilayers at an atomic level. The procedure of vortex pair splitting and the transformation from polar vortex to polar wave and out-of-plane polarization are observed step by step. Furthermore, the redistribution of charge in various topological structures has been demonstrated under an external bias. These findings advance the current understanding of topological structures transformation manipulated by the external field and its origination in multilayers; it would ultimately enable the design of nanostructured materials to realize their potential applications in microelectronics.

References
[1] Yadav, A. K. et al. Observation of polar Vortices in oxide superlattices. Nature 530, 198-201 (2016).
[2] Tang, Y. L. et al. Observation of a periodic array of flux-closure quadrants in strained ferroelectric PbTiO3 films. Science 348, 547-551 (2015).
[3] Hong, Z. et al. Stability of Polar Vortex Lattice in Ferroelectric Superlattices. Nano Lett. 17, 2246-2252 (2017).
[4] Li, S. et al. Periodic arrays of flux-closure domains in ferroelectric thin films with oxide electrodes. Appl. Phys. Lett. 111, 052901 (2017).
[5] Rodriguez, B. J. et al. Vortex Polarization States in Nanoscale Ferroelectric Arrays. Nano Lett. 9, 1127-1131 (2009).
[6] Wang, W. Y. et al. Large scale arrays of four-state vortex domains in BiFeO3 thin film. Appl. Phys. Lett. 109, 202904 (2016).
[7] Lu, L. et al. Topological Defects with Distinct Dipole Configurations in PbTiO3/SrTiO3 Multilayer Films. Phys. Rev. Lett. 120, 177601 (2018).
[8] Jia, C. L., Urban, K. W., Alexe, M., Hesse, D. & Vrejoiu, I. Direct Observation of Continuous Electric Dipole Rotation in Flux-Closure Domains in Ferroelectric Pb(Zr,Ti)O3. Science 331, 1420-1423 (2011).
[9] Chen, W. J., Zheng, Y. & Wang, B. Vortex Domain Structure in Ferroelectric Nanoplatelets and Control of its Transformation by Mechanical Load. Sci. Rep. 2, 796 (2012).
[10] Damodaran, A. R. et al. Phase coexistence and electric-field control of toroidal order in oxide superlattices. Nat. Mater. 16,1003-1009 (2017).

Design and engineering of the size, shape, and chemistry of photoactive building blocks enable the fabrication of functional nanoparticles for applications in light harvesting, photocatalytic synthesis, water splitting, phototherapy, and photodegradation. Here, we report the synthesis of such nanoparticles through a surfactant-assisted interfacial self-assembly process using optically active porphyrin as a functional building block. The self-assembly process relies on specific interactions such as π–π stacking and ligand coordination between individual porphyrin building blocks. Depending on the kinetic conditions, resulting structures exhibit well-defined one- to three-dimensional morphologies such as nanowires, nanooctahedra, and hierarchically ordered internal architectures. At the molecular level, porphyrins with well-defined size and chemistry possess unique optical and photocatalytic properties for potential synthesis of metallic structures. On the nanoscale, controlled assembly of macrocyclic monomers leads to formation of ordered nanostructures with precisely defined size, shape, and spatial monomer arrangement so as to facilitate intermolecular mass and energy transfer or delocalization for photocatalysis. Due to the hierarchical ordering of the porphyrins, the nanoparticles exhibit collective optical properties resulted from coupling of molecular porphyrins and photocatalytic activities such as photodegradation of methyl orange (MO) pollutants and hydrogen production. The capability of exerting rational control over dimension and morphology provides new opportunities for applications in sensing, nanoelectronics, and photocatalysis.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Emerging organolead-halide-perovskite-based materials and devices have recently received tremendous attention due to their extraordinary photonic and optoelectronic performance and potential low production costs. Many physical and chemical properties beyond light harvests have erupted. Despite the rapid development of perovskite materials and devices, the impact of environmental factors on different stages of materials synthesis and device fabrication still remains unclear. Moisture or humidity is widely recognized as one of the major lethal factors in perovskite devices degrade or decompose after fabrication. However, recent reports have shown that moisture could be crucial to obtain high-performance perovskite films in a suitable moisture of ~35% relative humidity. Although a few attempts have been made to investigate the morphological evolution of organohalide perovskite films, a comprehensive understanding of the environmental influence in the process of thermal annealing is not achieved yet, as it is critically important to further improve the optoelectronic device performance.

By taking advantage of synchrotron-based grazing incident wide-angle x-ray scattering (GIWAXS), we in situ monitor the transformation of organohalide perovskites from precursors to final films upon thermal annealing at different relative humidity. In situ GIWAXS reveals the formation of crystalline perovskite materials over relevant time and moisture scales to decipher the effect of humidity on both phase transition and crystal structures during annealing. These in situ measurements demonstrate that moderate humidity accelerates the formation of organohalide perovskites and improves the orientation of the film, but more than 50% realtive humidity retards the formation of perovskites and destructs their crystal structures. Furthermore, the highly orientated films obtained at optimized relative humidity are observed which could be attributed to the hydration of precursors. The beneficial influence of moisture content in the crystalline structure formation process on the operation stability of the resulting devices will also be discussed.
These findings clearly elucidate the influence of moisture environment in annealing process of organohalide perovskites and, in turn, allow us to correlate the improved performance of organohalide perovskite matrials and devices to structural features in terms of environmental effects.