In-situ electron microscopy mapping of an order-disorder transition in a superionic conductor

Ion transport induced room-temperature insulator-metal transition in single-crystalline Cu2Se

Cu2Se is a superionic conductor above 414 K, with ionic conductivities reaching that of molten salts. The superionic behavior results from hopping Cu ions between different crystallographic sites within the Se scaffold. However, the properties of Cu2Se below 414 K are far less known due to experimental limitations imposed by the bulk or polycrystalline samples that have been available so far. Here, we report the synthesis of ultra-thin, large-area single crystalline Cu2Se samples using a chemical vapor deposition method. The as-synthesized Cu2Se crystals exhibit optically and electrically detectable and controllable robust phases at room temperature and above. We demonstrate that Cu ion vacancies can be manipulated to induce an insulator-metal transition, which exhibits 6 orders of magnitude change in the electrical resistance of two terminal devices, accompanied by an optical change in the phase configuration. Our experiments show that the high mobility of the liquid-like Cu ion vacancies in Cu2Se causes macroscopic ordering in the Cu vacancies. Consequently, phase distribution over the crystals is not dictated by the diffusive motion of the ions but by the local energy minima formed due to the phase transition. As a result, long-range vacancy ordering of the crystal below 414 K becomes optically observable at a micrometer scale. This work demonstrates that Cu2Se could be a prototypical system where long-range ordering properties can be studied via electrical and optical methods.

Time-resolved transmission electron microscopy for nanoscale chemical dynamics

The ability of transmission electron microscopy (TEM) to image a structure ranging from millimetres to Ångströms has made it an indispensable component of the toolkit of modern chemists. TEM has enabled unprecedented understanding of the atomic structures of materials and how structure relates to properties and functions. Recent developments in TEM have advanced the technique beyond static material characterization to probing structural evolution on the nanoscale in real time. Accompanying advances in data collection have pushed the temporal resolution into the microsecond regime with the use of direct-electron detectors and down to the femtosecond regime with pump-probe microscopy. Consequently, studies have deftly applied TEM for understanding nanoscale dynamics, often in operando. In this Review, time-resolved in situ TEM techniques and their applications for probing chemical and physical processes are discussed, along with emerging directions in the TEM field.

Multistep nucleation visualized during solid-state crystallization

Mechanisms of nucleation have been debated for more than a century, despite successes of classical nucleation theory. The nucleation process has been recently argued as involving a nonclassical mechanism (the "two-step" mechanism) in which an intermediate step occurs before the formation of a nascent ordered phase. However, a thorough understanding of this mechanism, in terms of both microscopic kinetics and thermodynamics, remains experimentally challenging. Here, in situ observations using transmission electron microscopy on a solid-state nucleation case indicate that early-stage crystallization can follow the non-classical pathway, yet proceed via a more complex manner in which multiple metastable states precede the emergence of a stable nucleus. The intermediate steps were sequentially isolated as spinodal decomposition of amorphous precursor, mass transport and structural oscillations between crystalline and amorphous states. Our experimental and theoretical analyses support the idea that the energetic favorability is the driving force for the observed sequence of events. Due to the broad applicability of solid-state crystallization, the findings of this study offer new insights into modern nucleation theory and a potential avenue for materials design.

Exploring Blob Detection to Determine Atomic Column Positions and Intensities in Time-Resolved TEM Images with Ultra-Low Signal-to-Noise

Spatially resolved in situ transmission electron microscopy (TEM), equipped with direct electron detection systems, is a suitable technique to record information about the atom-scale dynamics with millisecond temporal resolution from materials. However, characterizing dynamics or fluxional behavior requires processing short time exposure images which usually have severely degraded signal-to-noise ratios. The poor signal-to-noise associated with high temporal resolution makes it challenging to determine the position and intensity of atomic columns in materials undergoing structural dynamics. To address this challenge, we propose a noise-robust, processing approach based on blob detection, which has been previously established for identifying objects in images in the community of computer vision. In particular, a blob detection algorithm has been tailored to deal with noisy TEM image series from nanoparticle systems. In the presence of high noise content, our blob detection approach is demonstrated to outperform the results of other algorithms, enabling the determination of atomic column position and its intensity with a higher degree of precision.

Atomistic Imaging of Competition between Surface Diffusion and Phase Transition during the Intermetallic Formation of Faceted Particles

To explore the ordering mechanism of facet alloy nanocrystals with randomly distributed atoms, we investigate kinetic and thermodynamic behaviors of the ordering phase transition from face-centered cubic Pt₃Co nanocrystals to L1₂-Pt₃Co intermetallic nanocrystals. It is observed that the ordering occurs from the surface and then gradually into the interior in a layer-by-layer mode, involving the competition between two kinds of phase transition modes: long-range surface diffusion-induced phase transition (SDIPT) and short-range reconstruction-induced body phase transition (RIBPT). The density functional theory calculations demonstrate that the surface status acts as a pivotal part in the thermodynamics and kinetics of the nanoscale ordering transition. With the development of the controllable heating process, both SDIPT and RIBPT modes can be manipulated as well as the morphology of the final product. This in situ work lays the foundations for potentially realizing shape-controlled intermetallic nanostructures by utilizing the thermal annealing method and makes preparations for the rational design of the surface and near-surface atomic configurations at the atomic scale.

Motion of Defects in Ion-Conducting Nanowires

Superionic conductors are prime candidates for the electrolytes of all-solid-state batteries. Our understanding of the mechanism and performance of superionic conductors is largely based on their idealized lattice structures. But how do defects in the lattice affect ionic structure and transport in these materials? This is a question answered here by in situ transmission electron microscopy of copper selenide, a classic superionic conductor. Nanowires of copper selenide exhibit antiphase boundaries which are a form of a planar defect. We examine the lattice structure around an antiphase boundary and monitor with atomic resolution how this structure evolves in an ordered-to-superionic phase transition. Antiphase boundaries are found to act as barriers to the propagation of the superionic phase. Antiphase boundaries also undergo spatial diffusion and shape changes resulting from thermally activated fluctuations of the neighboring ionic structure. These spatiotemporal insights highlight the importance of collective ionic transport and the role of defects in superionic conduction.

Revealing the Phase-Transition Dynamics and Mechanism in a Spinel Li4Ti5O12 Anode Material through in Situ Electron Microscopy

Spinel Li₄Ti₅O₁₂ is considered as a promising anode material for long-life lithium-ion batteries because of the negligible volumetric variation during the insertion and extraction of Li ions. Phase transition is an inevitable process during the migration of Li ions, and the transition process and mechanism need detailed investigation down to the atomic scale. In this study, we investigated the behavior and mechanism on the phase transition of Li₄Ti₅O₁₂ through in situ transmission electron microscopy (TEM). It has been found that the spinel-structured Li₄Ti₅O₁₂ was gradually transformed to a rock salt structure under electron beam irradiation. A sharp interface with an epitaxial relationship was observed between the transformed rock salt phase and the parent spinel phase. Furthermore, the heterostructure with different crystal structures of Li₄Ti₅O₁₂ has been precisely tailored with electron beam irradiation. Our detailed in situ TEM results and theoretical calculations led to unprecedented level on the understanding of phase-transition mechanism in Li₄Ti₅O₁₂. This study demonstrates a possible approach to precisely engineer the crystal structure of materials and to realize a well-designed heterostructure in electrode materials.

In Situ Investigation of Dynamic Silver Crystallization Driven by Chemical Reaction and Diffusion

Rational synthesis of materials is a long-term challenging issue due to the poor understanding on the formation mechanism of material structure and the limited capability in controlling nanoscale crystallization. The emergent in situ electron microscope provides an insight to this issue. By employing an in situ scanning electron microscope, silver crystallization is investigated in real time, in which a reversible crystallization is observed. To disclose this reversible crystallization, the radicals generated by the irradiation of electron beam are calculated. It is found that the concentrations of radicals are spatiotemporally variable in the liquid cell due to the diffusion and reaction of radicals. The fluctuation of the reductive hydrated electrons and the oxidative hydroxyl radicals in the cell leads to the alternative dominance of the reduction and oxidation reactions. The reduction leads to the growth of silver crystals while the oxidation leads to their dissolution, which results in the reversible silver crystallization. A regulation of radical distribution by electron dose rates leads to the formation of diverse silver structures, confirming the dominant role of local chemical concentration in the structure evolution of materials.

Size-Dependent Phase Transformation of Noble Metal Nanomaterials

As an important aspect of crystal phase engineering, controlled crystal phase transformation of noble metal nanomaterials has emerged as an effective strategy to explore novel crystal phases of nanomaterials. In particular, it is of significant importance to observe the transformation pathway and reveal the transformation mechanism in situ. Here, the phase transformation behavior of face-centered cubic (fcc) Au nanoparticles (fcc-AuNPs), adhering to the surface of 4H nanodomains in 4H/fcc Au nanorods, referred to as 4H-AuNDs, during in situ transmission electron microscopy imaging is systematically studied. It is found that the phase transformation is dependent on the ratio of the size of the monocrystalline nanoparticle (NP) to the diameter of 4H-AuND. Furthermore, molecular dynamics simulation and theoretical modeling are used to explain the experimental results, giving a size-dependent phase transformation diagram which provides a general guidance to predict the phase transformation pathway between fcc and 4H Au nanomaterials. Impressively, this method is general, which is used to study the phase transformation of other metal NPs, such as Pd, Ag, and PtPdAg, adhering to 4H-AuNDs. The work opens an avenue for selective phase engineering of nanomaterials which may possess unique physicochemical properties and promising applications.

Room-temperature superionic-phase nanocrystals synthesized with a twinned lattice

The engineering of nanoscale features enables the properties of solid-state materials to be tuned. Here, we show the tunable preparation of cuprous sulfide nanocrystals ranging in internal structures from single-domain to multi-domain. The synthetic method utilizes in-situ oxidation to grow nanocrystals with a controlled degree of copper deficiency. Copper-deficient nanocrystals spontaneously undergo twinning to a multi-domain structure. Nanocrystals with twinned domains exhibit markedly altered crystallographic phase and phase transition characteristics as compared to single-domain nanocrystals. In the presence of twin boundaries, the temperature for transition from the ordered phase to the high-copper-mobility superionic phase is depressed. Whereas the superionic phase is stable in the bulk only above ca. 100 °C, cuprous sulfide nanocrystals of ca. 7 nm diameter and a twinned structure are stable in the superionic phase well below ambient temperature. These findings demonstrate twinning to be a structural handle for nanoscale materials design and enable applications for an earth-abundant mineral in solid electrolytes for Li-S batteries.

The ability to control the internal domain structure of a nanocrystal represents a new direction in nanomaterials design. Here, the authors develop a method to controllably introduce twin boundaries in cuprous sulfide nanocrystals, and find that twinning stabilizes these nanocrystals in the superionic phase well below room temperature.