Crystallization, Phase Stability, Microstructure, and Chemical Bonding in Ga2O3 Nanofibers Made by Electrospinning.

We report on the crystal structure, phase stability, surface morphology, microstructure, chemical bonding, and electronic properties of gallium oxide (Ga2O3) nanofibers made by a simple and economically viable electrospinning process. The effect of processing parameters on the properties of Ga2O3 nanofibers were evaluated by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Thermal treatments in the range of 700-900 °C induce crystallization of amorphous fibers and lead to phase stabilization of α-GaOOH, ß-Ga2O3, or mixtures of these phases. The electron diffraction analyses coupled with XPS indicate that the transformation sequence progresses by forming amorphous fibers, which then transform to crystalline fibers with a mixture of α-GaOOH and ß-Ga2O3 at intermediate temperatures and fully transforms to the ß-Ga2O3 phase at higher temperatures (800-900 °C). Raman spectroscopic analyses corroborate the structural evolution and confirm the high chemical quality of the ß-Ga2O3 nanofibers. The surface analysis by XPS studies indicates that the hydroxyl groups are present for the as-synthesized samples, while thermal treatment at higher temperatures fully removes those hydroxyl groups, resulting in the formation of ß-Ga2O3 nanofibers.

Orientation Gradients in Rapidly Solidified Pure Aluminum Thin Films: Comparison of Experiments and Phase-Field Crystal Simulations.

Rapid solidification experiments on thin film aluminum samples reveal the presence of lattice orientation gradients within crystallizing grains. To study this phenomenon, a single-component phase-field crystal (PFC) model that captures the properties of solid, liquid, and vapor phases is proposed to model pure aluminium quantitatively. A coarse-grained amplitude representation of this model is used to simulate solidification in samples approaching micrometer scales. The simulations reproduce the experimentally observed orientation gradients within crystallizing grains when grown at experimentally relevant rapid quenches. We propose a causal connection between defect formation and orientation gradients.

Simultaneous Decomposition and Dewetting of Nanoscale Alloys: A Comparison of Experiment and Theory.

We consider the coupled process of phase separation and dewetting of metal alloys of nanoscale thickness deposited on solid substrates. The experiments involve applying nanosecond laser pulses that melt the Ag40Ni60 alloy films in two setups: either on thin supporting membranes or on bulk substrates. These two setups allow for extracting both temporal and spatial scales on which the considered processes occur. The theoretical model involves a longwave version of the Cahn-Hilliard formulation used to describe spinodal decomposition, coupled with an asymptotically consistent longwave-based description of dewetting that occurs due to destabilizing interactions between the alloy and the substrate, modeled using the disjoining pressure approach. Careful modeling, combined with linear stability analysis and fully nonlinear simulations, leads to results consistent with the experiments. In particular, we find that the two instability mechanisms occur concurrently, with the phase separation occurring faster and on shorter temporal scales. The modeling results show a strong influence of the temperature dependence of relevant material properties, implying that such a dependence is crucial for the understanding of the experimental findings. The agreement between theory and experiment suggests the utility of the proposed theoretical approach in helping to develop further experiments directed toward formation of metallic alloy nanoparticles of desired properties.

Dynamic Observation of Dendritic Quasicrystal Growth upon Laser-Induced Solid-State Transformation.

We report the laser-induced solid-state transformation between a periodic "approximant" and quasicrystal in the Al-Cr system during rapid quenching. Dynamic transmission electron microscopy allows us to capture in situ the dendritic growth of the metastable quasicrystals. The formation of dendrites during solid-state transformation is a rare phenomenon, which we attribute to the structural similarity between the two intermetallics. Through ab initio molecular dynamics simulations, we identify the dominant structural motif to be a 13-atom icosahedral cluster transcending the phases of matter.

Comparison of Orientation Mapping in SEM and TEM.

Multiple experimental configurations for performing nanoscale orientation mapping are compared to determine their fidelity to the true microstructure of a sample. Transmission Kikuchi diffraction (TKD) experiments in a scanning electron microscope (SEM) and nanobeam diffraction (NBD) experiments in a transmission electron microscope (TEM) were performed on thin electrodeposited hard Au films with two different microstructures. The Au samples either had a grain size that is >50 or <20 nm. The same regions of the samples were measured with TKD apparatuses at 30 kV in an SEM with detectors in the horizontal and vertical configurations and in the TEM at 300 kV. Under the proper conditions, we demonstrate that all three configurations can produce data of equivalent quality. Each method has strengths and challenges associated with its application and representation of the true microstructure. The conditions needed to obtain high-quality data for each acquisition method and the challenges associated with each are discussed.

Comparing Scanning Electron Microscope and Transmission Electron Microscope Grain Mapping Techniques Applied to Well-Defined and Highly Irregular Nanoparticles.

Investigating how grain structure affects the functional properties of nanoparticles requires a robust method for nanoscale grain mapping. In this study, we directly compare the grain mapping ability of transmission Kikuchi diffraction (TKD) in a scanning electron microscope to automated crystal orientation mapping (ACOM) in a transmission electron microscope across multiple nanoparticle materials. Analysis of well-defined Au, ZnO, and ZnSe nanoparticles showed that the grain orientations and GB geometries obtained by TKD are accurate and match those obtained by ACOM. For more complex polycrystalline Cu nanostructures, TKD provided an interpretable grain map whereas ACOM, with or without precession electron diffraction, yielded speckled, uninterpretable maps with orientation errors. Acquisition times for TKD were generally shorter than those for ACOM. Our results validate the use of TKD for characterizing grain orientation and grain boundary distributions in nanoparticles, providing a framework for the broader exploration of how microstructure influences nanoparticle properties.

Scalable Light-Printing of Substrate-Engraved Free-Form Metasurfaces.

A key challenge for metasurface research is locally controlling at will the nanoscale geometric features on meter-scale apertures. Such a technology is expected to enable large aperture meta-optics and revolutionize fields such as long-range imaging, lasers, laser detection and ranging (LADAR), and optical communications. Furthermore, these applications are often more sensitive to light-induced and environmental degradation, which constrains the possible materials and fabrication process. Here, we present a relatively simple and scalable method to fabricate a substrate-engraved metasurface with locally printed index determined by induced illumination, which, therefore, addresses both the challenges of scalability and durability. In this process, a thin metal film is deposited onto a substrate and transformed into a mask via local laser-induced dewetting into nanoparticles. The substrate is then dry-etched through this mask, and selective mask removal finally reveals the metasurface. We show that masking by the local nanoparticle distribution, and, therefore, the local index, is dependent on the local light-induced dewetting temperature. We demonstrate printing of a free-form pattern engraved into a fused silica glass substrate using a laser raster scan. Large-scale spatially controlled engraving of metasurfaces has implications on other technological fields beyond optics, such as surface fluidics, acoustics, and thermomechanics.

Additively manufactured hierarchical stainless steels with high strength and ductility.

Many traditional approaches for strengthening steels typically come at the expense of useful ductility, a dilemma known as strength-ductility trade-off. New metallurgical processing might offer the possibility of overcoming this. Here we report that austenitic 316L stainless steels additively manufactured via a laser powder-bed-fusion technique exhibit a combination of yield strength and tensile ductility that surpasses that of conventional 316L steels. High strength is attributed to solidification-enabled cellular structures, low-angle grain boundaries, and dislocations formed during manufacturing, while high uniform elongation correlates to a steady and progressive work-hardening mechanism regulated by a hierarchically heterogeneous microstructure, with length scales spanning nearly six orders of magnitude. In addition, solute segregation along cellular walls and low-angle grain boundaries can enhance dislocation pinning and promote twinning. This work demonstrates the potential of additive manufacturing to create alloys with unique microstructures and high performance for structural applications.

Probing the Active Surface Sites for CO Reduction on Oxide-Derived Copper Electrocatalysts.

CO electroreduction activity on oxide-derived Cu (OD-Cu) was found to correlate with metastable surface features that bind CO strongly. OD-Cu electrodes prepared by H2 reduction of Cu2O precursors reduce CO to acetate and ethanol with nearly 50% Faradaic efficiency at moderate overpotential. Temperature-programmed desorption of CO on OD-Cu revealed the presence of surface sites with strong CO binding that are distinct from the terraces and stepped sites found on polycrystalline Cu foil. After annealing at 350 °C, the surface-area corrected current density for CO reduction is 44-fold lower and the Faradaic efficiency is less than 5%. These changes are accompanied by a reduction in the proportion of strong CO binding sites. We propose that the active sites for CO reduction on OD-Cu surfaces are strong CO binding sites that are supported by grain boundaries. Uncovering these sites is a first step toward understanding the surface chemistry necessary for efficient CO electroreduction.

Simultaneous in-situ synthesis and characterization of Co@Cu core-shell nanoparticle arrays.

Core-shell particle ensembles are fabricated by pulsed-laser-induced dewetting of initially continuous, ultrathin alloy films through a combination of morphological and chemical instability. The synthesis of these arrays is monitored in situ with high spatial and temporal resolutions, which, when combined with ex situ composition analysis, provides insight to the morphological and chemical evolution pathways leading to core-shell particle formation.

A (S)TEM gas cell holder with localized laser heating for in situ experiments.

The advent of aberration correction for transmission electron microscopy has transformed atomic resolution imaging into a nearly routine technique for structural analysis. Now an emerging frontier in electron microscopy is the development of in situ capabilities to observe reactions at atomic resolution in real time and within realistic environments. Here we present a new in situ gas cell holder that is designed for compatibility with a wide variety of sample type (i.e., dimpled 3-mm discs, standard mesh grids, various types of focused ion beam lamellae attached to half grids). Its capabilities include localized heating and precise control of the gas pressure and composition while simultaneously allowing atomic resolution imaging at ambient pressure. The results show that 0.25-nm lattice fringes are directly visible for nanoparticles imaged at ambient pressure with gas path lengths up to 20 µm. Additionally, we quantitatively demonstrate that while the attainable contrast and resolution decrease with increasing pressure and gas path length, resolutions better than 0.2 nm should be accessible at ambient pressure with gas path lengths less than the 15 µm utilized for these experiments.

Real-time observation of nanosecond liquid-phase assembly of nickel nanoparticles via pulsed-laser heating.

Using pump-probe electron microscopy techniques, the dewetting of thin nickel films exposed to a pulsed nanosecond laser was monitored at tens of nanometers spatial and nanosecond time scales to provide insight into the liquid-phase assembly dynamics. Thickness-dependent and correlated time and length scales indicate that a spinodal instability drives the assembly process. Measured lifetimes of the liquid metal are consistent with finite-difference simulations of the laser-irradiated film and are consistent with estimated and observed spinodal time scales. These results can be used to design improved synthesis and assembly routes toward achieving advanced functional nanomaterials and devices.

Approaches for ultrafast imaging of transient materials processes in the transmission electron microscope.

The growing field of ultrafast materials science, aimed at exploring short-lived transient processes in materials on the microsecond to femtosecond timescales, has spawned the development of time-resolved, in situ techniques in electron microscopy capable of capturing these events. This article gives a brief overview of two principal approaches that have emerged in the past decade: the stroboscopic ultrafast electron microscope and the nanosecond-time-resolved single-shot instrument. The high time resolution is garnered through the use of advanced pulsed laser systems and a pump-probe experimental platforms using laser-driven photoemission processes to generate time-correlated electron probe pulses synchronized with laser-driven events in the specimen. Each technique has its advantages and limitations and thus is complementary in terms of the materials systems and processes that they can investigate. The stroboscopic approach can achieve atomic resolution and sub-picosecond time resolution for capturing transient events, though it is limited to highly repeatable (>10(6) cycles) materials processes, e.g., optically driven electronic phase transitions that must reset to the material's ground state within the repetition rate of the femtosecond laser. The single-shot approach can explore irreversible events in materials, but the spatial resolution is limited by electron source brightness and electron-electron interactions at nanosecond temporal resolutions and higher. The first part of the article will explain basic operating principles of the stroboscopic approach and briefly review recent applications of this technique. As the authors have pursued the development of the single-shot approach, the latter part of the review discusses its instrumentation design in detail and presents examples of materials science studies and the near-term instrumentation developments of this technique.