Real-space visualization of a defect-mediated charge density wave transition.

We study the coupled charge density wave (CDW) and insulator-to-metal transitions in the 2D quantum material 1T-TaS. By applying in situ cryogenic 4D scanning transmission electron microscopy with in situ electrical resistance measurements, we directly visualize the CDW transition and establish that the transition is mediated by basal dislocations (stacking solitons). We find that dislocations can both nucleate and pin the transition and locally alter the transition temperature by nearly ~75 K.

Spontaneous Supercrystal Formation During a Strain-Engineered Metal-Insulator Transition.

Mott metal-insulator transitions possess electronic, magnetic, and structural degrees of freedom promising next-generation energy-efficient electronics. A previously unknown, hierarchically ordered, and anisotropic supercrystal state is reported and its intrinsic formation characterized in-situ during a Mott transition in a CaRuO thin film. Machine learning-assisted X-ray nanodiffraction together with cryogenic electron microscopy reveal multi-scale periodic domain formation at and below the film transition temperature (T ≈ 200-250 K) and a separate anisotropic spatial structure at and above T.

Comparing Ammonium and Tetraaminophosphonium Anion-Exchange Membranes Derived from Vinyl-Addition Polynorbornene Copolymers.

Herein, we systematically examined how composition influenced the properties of vinyl addition polynorbornene anion exchange membranes (AEMs) prepared from 5-hexyl-2-norbornene and 5-(4-bromobutyl)-2-norbornene. Copolymerization kinetics revealed that 5-hexyl-2-norbornene is consumed faster than 5-(4-bromobutyl)-2-norbornene, leading to a portion of the chain being richer in bromoalkyl groups. The alkyl halide pendants can then be converted to either trimethylammonium or tetrakis(dialkylamino)phosphonium cations through straightforward substitution with trimethylamine or a tris(dialkylamino)phosphazene.

Endotaxial stabilization of 2D charge density waves with long-range order.

Charge density waves are emergent quantum states that spontaneously reduce crystal symmetry, drive metal-insulator transitions, and precede superconductivity. In low-dimensions, distinct quantum states arise, however, thermal fluctuations and external disorder destroy long-range order. Here we stabilize ordered two-dimensional (2D) charge density waves through endotaxial synthesis of confined monolayers of 1T-TaS.

Conjugate Multimode Heat Transfer Analysis of Cryogenic EXLO Manipulation.

In this study, a conjugate radiation/conduction multimode heat transfer analysis of cryogenic focused ion beam (FIB) milling steps necessary for producing ex situ lift out specimens under cryogenic conditions (cryo-EXLO) is performed. Using finite volume for transient heat conduction and enclosure theory for radiation heat transfer, the analysis shows that as long as the specimen is attached or touching the FIB side wall trenches, the specimen will remain vitreous indefinitely, while actively cooled at liquid nitrogen (LN2) temperatures. To simulate the time needed to perform a transfer step to move the bulk sample containing the FIB-thinned specimen from the cryo-FIB to the cryo-EXLO cryostat, the LN2 temperature active cooling is turned off after steady-state conditions are reached and the specimen is monitored over time until the critical devitrification temperature is reached.

In operando cryo-STEM of pulse-induced charge density wave switching in TaS.

The charge density wave material 1T-TaS exhibits a pulse-induced insulator-to-metal transition, which shows promise for next-generation electronics such as memristive memory and neuromorphic hardware. However, the rational design of TaS devices is hindered by a poor understanding of the switching mechanism, the pulse-induced phase, and the influence of material defects. Here, we operate a 2-terminal TaS device within a scanning transmission electron microscope at cryogenic temperature, and directly visualize the changing charge density wave structure with nanoscale spatial resolution and down to 300 μs temporal resolution.

Synthesis of graded CdSSe nanoplatelet alloys and heterostructures from pairs of chalcogenoureas with tailored conversion reactivity.

A mixture of ,,'-trisubstituted thiourea and cyclic ,,','-tetrasubstituted selenourea precursors were used to synthesize three monolayer thick CdSSe nanoplatelets in a single synthetic step. The microstructure of the nanoplatelets could be tuned from homogeneous alloys, to graded alloys to core/crown heterostructures depending on the relative conversion reactivity of the sulfur and selenium precursors. UV-visible absorption and photoluminescence spectroscopy and scanning transmission electron microscopy electron energy loss spectroscopy (STEM-EELS) images demonstrate that the elemental distribution is governed by the relative precursor conversion kinetics.

Doping the Undopable: Hybrid Molecular Beam Epitaxy Growth, n-Type Doping, and Field-Effect Transistor Using CaSnO.

The alkaline earth stannates are touted for their wide band gaps and the highest room-temperature electron mobilities among all of the perovskite oxides. CaSnO has the highest measured band gap in this family and is thus a particularly promising ultrawide band gap semiconductor. However, discouraging results from previous theoretical studies and failed doping attempts had described this material as "undopable".

Heterogeneity in Cation Exchange Ag Doping of CdSe Nanocrystals.

Cation exchange is becoming extensively used for nanocrystal (NC) doping in order to produce NCs with unique optical and electronic properties. However, despite its ever-increasing use, the relationships between the cation exchange process, its doped NC products, and the resulting NC photophysics are not well characterized. For example, similar doping procedures on NCs with the same chemical compositions have resulted in quite different photophysics.

Emergent layer stacking arrangements in c-axis confined MoTe.

The layer stacking order in 2D materials strongly affects functional properties and holds promise for next-generation electronic devices. In bulk, octahedral MoTe possesses two stacking arrangements, the ferroelectric Weyl semimetal T phase and the higher-order topological insulator 1T' phase. However, in thin flakes of MoTe, it is unclear if the layer stacking follows the T, 1T', or an alternative stacking sequence.

Linear-in-temperature resistivity for optimally superconducting (Nd,Sr)NiO.

The occurrence of superconductivity in proximity to various strongly correlated phases of matter has drawn extensive focus on their normal state properties, to develop an understanding of the state from which superconductivity emerges. The recent finding of superconductivity in layered nickelates raises similar interests. However, transport measurements of doped infinite-layer nickelate thin films have been hampered by materials limitations of these metastable compounds: in particular, a high density of extended defects.

Influence of Light Atoms on Quantification of Atomic Column Positions in Distorted Perovskites with HAADF-STEM.

Measurement of picometer-scale atomic displacements by aberration-corrected STEM has become invaluable in the study of crystalline materials, where it can elucidate ordering mechanisms and local heterogeneities. HAADF-STEM imaging, often used for such measurements due to its atomic number contrast, is generally considered insensitive to light atoms such as oxygen. Light atoms, however, still affect the propagation of the electron beam in the sample and, therefore, the collected signal.

Resolving the polar interface of infinite-layer nickelate thin films.

Nickel-based superconductors provide a long-awaited experimental platform to explore possible cuprate-like superconductivity. Despite similar crystal structure and d electron filling, however, superconductivity in nickelates has thus far only been stabilized in thin-film geometry, raising questions about the polar interface between substrate and thin film. Here we conduct a detailed experimental and theoretical study of the prototypical interface between NdSrNiO and SrTiO.

Limits to the strain engineering of layered square-planar nickelate thin films.

The layered square-planar nickelates, NdNiO, are an appealing system to tune the electronic properties of square-planar nickelates via dimensionality; indeed, superconductivity was recently observed in NdNiO thin films. Here, we investigate the role of epitaxial strain in the competing requirements for the synthesis of the n = 3 Ruddlesden-Popper compound, NdNiO, and subsequent reduction to the square-planar phase, NdNiO. We synthesize our highest quality NdNiO films under compressive strain on LaAlO (001), while NdNiO on NdGaO (110) exhibits tensile strain-induced rock salt faults but retains bulk-like transport properties.

Geometric frustration of Jahn-Teller order in the infinite-layer lattice.

The Jahn-Teller effect, in which electronic configurations with energetically degenerate orbitals induce lattice distortions to lift this degeneracy, has a key role in many symmetry-lowering crystal deformations. Lattices of Jahn-Teller ions can induce a cooperative distortion, as exemplified by LaMnO (refs. ).

Multidimensional visualization of the dynamic evolution of Li metal via in situ/operando methods.

The growing demands for high-energy density electrical energy storage devices stimulate the coupling of conversion-type cathodes and lithium (Li) metal anodes. While promising, the use of these "Li-free" cathodes brings new challenges to the Li anode interface, as Li needs to be dissolved first during cell operation. In this study, we have achieved a direct visualization and comprehensive analysis of the dynamic evolution of the Li interface.

Atomic-Scale Mapping and Quantification of Local Ruddlesden-Popper Phase Variations.

The Ruddlesden-Popper (ABO) compounds are highly tunable materials whose functional properties can be dramatically impacted by their structural phase . The negligible differences in formation energies for different can produce local structural variations arising from small stoichiometric deviations. Here, we present a Python analysis platform to detect, measure, and quantify the presence of different -phases based on atomic-resolution scanning transmission electron microscopy (STEM) images.

Nanoscale Characterization of Liquid-Solid Interfaces by Coupling Cryo-Focused Ion Beam Milling with Scanning Electron Microscopy and Spectroscopy.

Physical and chemical processes at solid-liquid interfaces play a crucial role in many natural and technological phenomena, including catalysis, solar energy and fuel generation, and electrochemical energy storage. Nanoscale characterization of such interfaces has recently been achieved using cryogenic electron microscopy, thereby providing a new path to advancing our fundamental understanding of interface processes. This contribution provides a practical guide to mapping the structure and chemistry of solid-liquid interfaces in materials and devices using an integrated cryogenic electron microscopy approach.

Multiscale hierarchical structures from a nanocluster mesophase.

Spontaneous hierarchical self-organization of nanometre-scale subunits into higher-level complex structures is ubiquitous in nature. The creation of synthetic nanomaterials that mimic the self-organization of complex superstructures commonly seen in biomolecules has proved challenging due to the lack of biomolecule-like building blocks that feature versatile, programmable interactions to render structural complexity. In this study, highly aligned structures are obtained from an organic-inorganic mesophase composed of monodisperse CdS magic-size cluster building blocks.

Disentangling Coexisting Structural Order Through Phase Lock-In Analysis of Atomic-Resolution STEM Data.

As a real-space technique, atomic-resolution STEM imaging contains both amplitude and geometric phase information about structural order in materials, with the latter encoding important information about local variations and heterogeneities present in crystalline lattices. Such phase information can be extracted using geometric phase analysis (GPA), a method which has generally focused on spatially mapping elastic strain. Here we demonstrate an alternative phase demodulation technique and its application to reveal complex structural phenomena in correlated quantum materials.

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies.

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts.