Nanoscale Imaging and Control of Volatile and Non-Volatile Resistive Switching in VO2.

Control of the metal-insulator phase transition is vital for emerging neuromorphic and memristive technologies. The ability to alter the electrically driven transition between volatile and non-volatile states is particularly important for quantum-materials-based emulation of neurons and synapses. The major challenge of this implementation is to understand and control the nanoscale mechanisms behind these two fundamental switching modalities. Here, in situ X-ray nanoimaging is used to follow the evolution of the nanostructure and disorder in the archetypal Mott insulator VO2 during an electrically driven transition. Our findings demonstrate selective and reversible stabilization of either the insulating or metallic phases achieved by manipulating the defect concentration. This mechanism enables us to alter the local switching response between volatile and persistent regimes and demonstrates a new possibility for nanoscale control of the resistive switching in Mott materials.

Extracting contrast in an X-ray speckle visibility spectroscopy experiment under imperfect conditions.

Pump-probe experiments at synchrotrons and free-electron lasers to study ultrafast dynamics in materials far from equilibrium have been well established, but techniques to investigate equilibrium dynamics on the nano- and pico-second timescales remain underdeveloped and experimentally challenging. A promising approach relies on a double-probe X-ray speckle visibility spectroscopy setup at split-and-delay beamlines of X-ray free-electron lasers. However, the logistics in consistently producing two collinear, perfectly overlapping pulses necessary to conduct a faithful experiment is difficult to achieve. In this paper, a method is introduced to extract contrast in the case where an angular misalignment and imperfect overlap exists between the two pulses. Numerical simulations of a dynamical system show that contrast can still be extracted for significant angular misalignments accompanied by partial overlap between the two pulses.

Synchrotron X-ray Analytical Techniques for Studying Materials Electrochemistry in Rechargeable Batteries.

Rechargeable battery technologies have ignited major breakthroughs in contemporary society, including but not limited to revolutions in transportation, electronics, and grid energy storage. The remarkable development of rechargeable batteries is largely attributed to in-depth efforts to improve battery electrode and electrolyte materials. There are, however, still intimidating challenges of lower cost, longer cycle and calendar life, higher energy density, and better safety for large scale energy storage and vehicular applications. Further progress with rechargeable batteries may require new chemistries (lithium ion batteries and beyond) and better understanding of materials electrochemistry in the various battery technologies. In the past decade, advancement of battery materials has been complemented by new analytical techniques that are capable of probing battery chemistries at various length and time scales. Synchrotron X-ray techniques stand out as one of the most effective methods that allow for nearly nondestructive probing of materials characteristics such as electronic and geometric structures with various depth sensitivities through spectroscopy, scattering, and imaging capabilities. This article begins with the discussion of various rechargeable batteries and associated important scientific questions in the field, followed by a review of synchrotron X-ray based analytical tools (scattering, spectroscopy, and imaging) and their successful applications (ex situ, in situ, and in operando) in gaining fundamental insights into these scientific questions. Furthermore, electron microscopy and spectroscopy complement the detection length scales of synchrotron X-ray tools and are also discussed toward the end. We highlight the importance of studying battery materials by combining analytical techniques with complementary length sensitivities, such as the combination of X-ray absorption spectroscopy and electron spectroscopy with spatial resolution, because a sole technique may lead to biased and inaccurate conclusions. We then discuss the current progress of experimental design for synchrotron experiments and methods to mitigate beam effects. Finally, a perspective is provided to elaborate how synchrotron techniques can impact the development of next-generation battery chemistries.

Orbital Domain Dynamics in Magnetite below the Verwey Transition.

The metal-insulator phase transition in magnetite, known as the Verwey transition, is characterized by a charge-orbital ordering and a lattice transformation from a cubic to monoclinic structure. We use x-ray photon correlation spectroscopy to investigate the dynamics of this charge-orbitally ordered insulating phase undergoing the insulator-to-metal transition. By tuning to the Fe L_{3} edge at the (001/2) superlattice peak, we probe the evolution of the Fe t_{2g} orbitally ordered domains present in the low temperature insulating phase and forbidden in the high temperature metallic phase. We observe two distinct regimes below the Verwey transition. In the first regime, magnetite follows an Arrhenius behavior and the characteristic timescale for orbital fluctuations decreases as the temperature increases. In the second regime, magnetite phase separates into metallic and insulating domains, and the kinetics of the phase transition is dictated by metallic-insulating interfacial boundary conditions.

Nonequilibrium Phase Precursors during a Photoexcited Insulator-to-Metal Transition in V_{2}O_{3}.

Here, we photoinduce and directly observe with x-ray scattering an ultrafast enhancement of the structural long-range order in the archetypal Mott system V_{2}O_{3}. Despite the ultrafast increase in crystal symmetry, the change of unit cell volume occurs an order of magnitude slower and coincides with the insulator-to-metal transition. The decoupling between the two structural responses in the time domain highlights the existence of a transient photoinduced precursor phase, which is distinct from the two structural phases present in equilibrium. X-ray nanoscopy reveals that acoustic phonons trapped in nanoscale twin domains govern the dynamics of the ultrafast transition into the precursor phase, while nucleation and growth of metallic domains dictate the duration of the slower transition into the metallic phase. The enhancement of the long-range order before completion of the electronic transition demonstrates the critical role the nonequilibrium structural phases play during electronic phase transitions in correlated electrons systems.

Bright circularly polarized soft X-ray high harmonics for X-ray magnetic circular dichroism.

We demonstrate, to our knowledge, the first bright circularly polarized high-harmonic beams in the soft X-ray region of the electromagnetic spectrum, and use them to implement X-ray magnetic circular dichroism measurements in a tabletop-scale setup. Using counterrotating circularly polarized laser fields at 1.3 and 0.79 µm, we generate circularly polarized harmonics with photon energies exceeding 160 eV. The harmonic spectra emerge as a sequence of closely spaced pairs of left and right circularly polarized peaks, with energies determined by conservation of energy and spin angular momentum. We explain the single-atom and macroscopic physics by identifying the dominant electron quantum trajectories and optimal phase-matching conditions. The first advanced phase-matched propagation simulations for circularly polarized harmonics reveal the influence of the finite phase-matching temporal window on the spectrum, as well as the unique polarization-shaped attosecond pulse train. Finally, we use, to our knowledge, the first tabletop X-ray magnetic circular dichroism measurements at the N4,5 absorption edges of Gd to validate the high degree of circularity, brightness, and stability of this light source. These results demonstrate the feasibility of manipulating the polarization, spectrum, and temporal shape of high harmonics in the soft X-ray region by manipulating the driving laser waveform.

3D Imaging of Twin Domain Defects in Gold Nanoparticles.

Topological defects are ubiquitous in physics and include crystallographic imperfections such as defects in condensed matter systems. Defects can determine many of the material's properties, thus providing novel opportunities for defect engineering. However, it is difficult to track buried defects and their interfaces in three dimensions with nanoscale resolution. Here, we report three-dimensional visualization of gold nanocrystal twin domains using Bragg coherent X-ray diffractive imaging in an aqueous environment. We capture the size and location of twin domains, which appear as voids in the Bragg electron density, in addition to a component of the strain field. Twin domains can interrupt the stacking order of the parent crystal, leading to a phase offset between the separated parent crystal pieces. We utilize this phase offset to estimate the roughness of the twin boundary. We measure the diffraction signal from the crystal twin and show its Bragg electron density fits into the parent crystal void. Defect imaging will likely facilitate improvement and rational design of nanostructured materials.

A new method for studying sub-pulse dynamics at synchrotron sources.

The possibility of studying dynamics at time scales on the order of the pulse duration at synchrotron X-ray sources with present avalanche photodiode point detection technology is investigated, without adopting pump-probe techniques. It is found that sample dynamics can be characterized by counting single and double photon events and an analytical approach is developed to estimate the time required for a statistically significant measurement to be made. The amount of scattering required to make such a measurement possible presently within a few days is indicated and it is shown that at next-generation synchrotron sources this time will be reduced dramatically, i.e. by more than three orders of magnitude. The analytical results are confirmed with simulations in the frame of Gaussian statistics. In the future, this approach could be extended to even shorter time scales with the implementation of ultrafast streak cameras.

In situ strain evolution during a disconnection event in a battery nanoparticle.

Lithium ion batteries are the dominant form of energy storage in mobile devices, increasingly employed in transportation, and likely candidates for renewable energy storage and integration into the electrical grid. To fulfil their powerful potential, electrodes with increased capacity, faster charge rates, and longer cycle life must be developed. Understanding the mechanics and chemistry of individual nanoparticles under in situ conditions is a crucial step to improving performance and mitigating damage. Here we reveal three-dimensional strain evolution within a single nanoparticle of a promising high voltage cathode material, LiNi0.5Mn1.5O4, under in situ conditions. The particle becomes disconnected during the second charging cycle. This is attributed to the formation of a cathode electrolyte interphase layer with slow ionic conduction. The three-dimensional strain pattern within the particle is independent of cell voltage after disconnection, indicating that the particle is unable to redistribute lithium within its volume or to its neighbours. Understanding the disconnection process at the single particle level and the equilibrium or non-equilibrium state of nanoparticles is essential to improving performance of current and future electrochemical energy storage systems.

Single particle nanomechanics in operando batteries via lensless strain mapping.

We reveal three-dimensional strain evolution in situ of a single LiNi0.5Mn1.5O4 nanoparticle in a coin cell battery under operando conditions during charge/discharge cycles with coherent X-ray diffractive imaging. We report direct observation of both stripe morphologies and coherency strain at the nanoscale. Our results suggest the critical size for stripe formation is 50 nm. Surprisingly, the single nanoparticle elastic energy landscape, which we map with femtojoule precision, depends on charge versus discharge, indicating hysteresis at the single particle level. This approach opens a powerful new avenue for studying battery nanomechanics, phase transformations, and capacity fade under operando conditions at the single particle level that will enable profound insight into the nanoscale mechanisms that govern electrochemical energy storage systems.

Nonequilibrium structural dynamics of nanoparticles in LiNi(1/2)Mn(3/2)O4 cathode under operando conditions.

We study nonequilibrium structural dynamics in LiNi1/2Mn3/2O4 spinel cathode material during fast charge/discharge under operando conditions using coherent X-rays. Our in situ measurements reveal a hysteretic behavior of the structure upon cycling and we directly observe the interplay between different transformation mechanisms: solid solution and two-phase reactions at the single nanoparticle level. For high lithium concentrations solid solution is observed upon both charge and discharge. For low lithium concentration, we find concurrent solid solution and two-phase reactions upon charge, while a pure two-phase reaction is found upon discharge. A delithiation model based on an ionic blockade layer on the particle surface is proposed to explain the distinct structural transformation mechanisms in nonequilibrium conditions. This study addresses the controversy of why two-phase materials show exemplary kinetics and opens new avenues to understand fundamental processes underlying charge transfer, which will be invaluable for developing the next generation battery materials.

X-ray photon correlation spectroscopy.

In recent years, X-ray photon correlation spectroscopy (XPCS) has emerged as one of the key probes of slow nanoscale fluctuations, applicable to a wide range of condensed matter and materials systems. This article briefly reviews the basic principles of XPCS as well as some of its recent applications, and discusses some novel approaches to XPCS analysis. It concludes with a discussion of the future impact of diffraction-limited storage rings on new types of XPCS experiments, pushing the temporal resolution to nanosecond and possibly even picosecond time scales.

Ptychographic overlap constraint errors and the limits of their numerical recovery using conjugate gradient descent methods.

Ptychographic coherent x-ray diffractive imaging is a form of scanning microscopy that does not require optics to image a sample. A series of scanned coherent diffraction patterns recorded from multiple overlapping illuminated regions on the sample are inverted numerically to retrieve its image. The technique recovers the phase lost by detecting the diffraction patterns by using experimentally known constraints, in this case the measured diffraction intensities and the assumed scan positions on the sample. The spatial resolution of the recovered image of the sample is limited by the angular extent over which the diffraction patterns are recorded and how well these constraints are known. Here, we explore how reconstruction quality degrades with uncertainties in the scan positions. We show experimentally that large errors in the assumed scan positions on the sample can be numerically determined and corrected using conjugate gradient descent methods. We also explore in simulations the limits, based on the signal to noise of the diffraction patterns and amount of overlap between adjacent scan positions, of just how large these errors can be and still be rendered tractable by this method.

Dichroic coherent diffractive imaging.

Understanding electronic structure at the nanoscale is crucial to untangling fundamental physics puzzles such as phase separation and emergent behavior in complex magnetic oxides. Probes with the ability to see beyond surfaces on nanometer length and subpicosecond time scales can greatly enhance our understanding of these systems and will undoubtedly impact development of future information technologies. Polarized X-rays are an appealing choice of probe due to their penetrating power, elemental and magnetic specificity, and high spatial resolution. The resolution of traditional X-ray microscopes is limited by the nanometer precision required to fabricate X-ray optics. Here we present a novel approach to lensless imaging of an extended magnetic nanostructure, in which a scanned series of dichroic coherent diffraction patterns is recorded and numerically inverted to map its magnetic domain configuration. Unlike holographic methods, it does not require a reference wave or precision optics. In addition, it enables the imaging of samples with arbitrarily large spatial dimensions, at a spatial resolution limited solely by the coherent X-ray flux, wavelength, and stability of the sample with respect to the beam. It can readily be extended to nonmagnetic systems that exhibit circular or linear dichroism. We demonstrate this approach by imaging ferrimagnetic labyrinthine domains in a Gd/Fe multilayer with perpendicular anisotropy and follow the evolution of the domain structure through part of its magnetization hysteresis loop. This approach is scalable to imaging with diffraction-limited resolution, a prospect rapidly becoming a reality in view of the new generation of phenomenally brilliant X-ray sources.

Synchrotron X-ray studies of rapidly evolving morphology of self-assembled nanoparticle films under lateral compression.

Interfacial nanostructures represent a class of systems that are highly relevant to studies of quasi-2D phases, chemical self-assembly, surfactant behavior, and biologically relevant membranes. Previous studies have shown that under lateral compression a Langmuir film of gold (Au) nanoparticles assembled at the liquid-air interface exhibits rich mechanical behavior: it undergoes a rapid structural and morphological evolution from a monolayer to a trilayer via an intermediate hash-like phase. We report the results of studying this structural evolution using grazing incidence X-ray off-specular scattering (GIXOS). We utilize GIXOS to obtain a quantitative mapping of electron density profile normal to the liquid surface with a subnanometer resolution and follow the structural evolution of the Au nanoparticle film under lateral compression with a subminute temporal resolution. As the surface pressure is increased, the self-assembled nanoparticle monolayer first crinkles into a double-layer phase before forming a trilayer. This study reveals the existence of a transient bilayer phase and provides a microscopic picture of the particle-level crinkling phenomena of ultrathin films. These studies were previously impossible due to the relatively short time scales involved in crinkling formation of these transient phases and their intrinsically inhomogeneous nature.

Multiwavelength anomalous X-ray diffraction for combined imaging of atomic displacement and strain.

The X-ray Bragg coherent diffractive imaging (CDI) technique assumes that the structure factor holds constant over the measured crystal. This approximation breaks down for materials exhibiting variations in the unit-cell configuration, such as piezo- and ferroelectrics. In that case, the strain field cannot be reliably determined from the reconstruction because the lattice deformation and the structure factor contribute concomitantly. Proposed here is a solution to this problem achieved by combining Bragg CDI and the multiwavelength anomalous diffraction approach that measures a Friedel pair of reflections at two different photon energies near an absorption edge. Comparing the obtained reconstructions with a parametric model that includes calculating the scattering amplitude as a function of wavelength and the unit-cell configuration, the contributions of the lattice deformation and the structure factor are separated. Simulations of the ferroelectric material BaTiO3 demonstrate the possibility of simultaneous probing of the strain and displacement of the Ti atoms. The proposed method opens up an opportunity to apply coherent X-ray diffraction for nanoscale-resolved 3D mapping of polarization domains in micro- and nanocrystals.

Perovskite neural trees.

Trees are used by animals, humans and machines to classify information and make decisions. Natural tree structures displayed by synapses of the brain involves potentiation and depression capable of branching and is essential for survival and learning. Demonstration of such features in synthetic matter is challenging due to the need to host a complex energy landscape capable of learning, memory and electrical interrogation. We report experimental realization of tree-like conductance states at room temperature in strongly correlated perovskite nickelates by modulating proton distribution under high speed electric pulses. This demonstration represents physical realization of ultrametric trees, a concept from number theory applied to the study of spin glasses in physics that inspired early neural network theory dating almost forty years ago. We apply the tree-like memory features in spiking neural networks to demonstrate high fidelity object recognition, and in future can open new directions for neuromorphic computing and artificial intelligence.

Stress relaxation in quasi-two-dimensional self-assembled nanoparticle monolayers.

We experimentally probed the stress relaxation of a monolayer of iron oxide nanoparticles at the water-air interface. Upon drop-casting onto a water surface, the nanoparticles self-assembled into islands of two-dimensional hexagonally close packed crystalline domains surrounded by large voids. When compressed laterally, the voids gradually disappeared as the surface pressure increased. After the compression was stopped, the surface pressure (as measured by a Wilhelmy plate) evolved as a function of the film aging time with three distinct timescales. These aging dynamics were intrinsic to the stressed state built up during the non-equilibrium compression of the film. Utilizing x-ray photon correlation spectroscopy, we measured the characteristic relaxation time (τ) of in-plane nanoparticle motion as a function of the aging time through both second-order and two-time autocorrelation analysis. Compressed and stretched exponential fitting of the intermediate scattering function yielded exponents (ß) indicating different relaxation mechanisms of the films under different compression stresses. For a monolayer compressed to a lower surface pressure (between 20 mN/m and 30 mN/m), the relaxation time (τ) decreased continuously as a function of the aging time, as did the fitted exponent, which transitioned from being compressed (>1) to stretched (<1), indicating that the monolayer underwent a stress release through crystalline domain reorganization. However, for a monolayer compressed to a higher surface pressure (around 40 mN/m), the relaxation time increased continuously and the compressed exponent varied very little from a value of 1.6, suggesting that the system may have been highly stressed and jammed. Despite the interesting stress relaxation signatures seen in these samples, the structural ordering of the monolayer remained the same over the sample lifetime, as revealed by grazing incidence x-ray diffraction.

3D Bragg coherent diffractive imaging of five-fold multiply twinned gold nanoparticle.

The formation mechanism of five-fold multiply twinned nanoparticles has been a long-term topic because of their geometrical incompatibility. So, various models have been proposed to explain how the internal structure of the multiply twinned nanoparticles accommodates the constraints of the solid-angle deficiency. We investigate the internal structure, strain field and strain energy density of 600 nm sized five-fold multiply twinned gold nanoparticles quantitatively using Bragg coherent diffractive imaging, which is suitable for the study of buried defects and three-dimensional strain distribution with great precision. Our study reveals that the strain energy density in five-fold multiply twinned gold nanoparticles is an order of magnitude higher than that of the single nanocrystals such as an octahedron and triangular plate synthesized under the same conditions. This result indicates that the strain developed while accommodating an angular misfit, although partially released through the introduction of structural defects, is still large throughout the crystal.