Emergent interface vibrational structure of oxide superlattices.

As the length scales of materials decrease, the heterogeneities associated with interfaces become almost as important as the surrounding materials. This has led to extensive studies of emergent electronic and magnetic interface properties in superlattices1-9. However, the interfacial vibrations that affect the phonon-mediated properties, such as thermal conductivity10,11, are measured using macroscopic techniques that lack spatial resolution. Although it is accepted that intrinsic phonons change near boundaries12,13, the physical mechanisms and length scales through which interfacial effects influence materials remain unclear. Here we demonstrate the localized vibrational response of interfaces in strontium titanate-calcium titanate superlattices by combining advanced scanning transmission electron microscopy imaging and spectroscopy, density functional theory calculations and ultrafast optical spectroscopy. Structurally diffuse interfaces that bridge the bounding materials are observed and this local structure creates phonon modes that determine the global response of the superlattice once the spacing of the interfaces approaches the phonon spatial extent. Our results provide direct visualization of the progression of the local atomic structure and interface vibrations as they come to determine the vibrational response of an entire superlattice. Direct observation of such local atomic and vibrational phenomena demonstrates that their spatial extent needs to be quantified to understand macroscopic behaviour. Tailoring interfaces, and knowing their local vibrational response, provides a means of pursuing designer solids with emergent infrared and thermal responses.

Creation of Multi-Principal Element Alloy NiCoCr Nanostructures via Nanosecond Laser-Induced Dewetting.

The multi-principal element alloy nanoparticles (MPEA NPs), a new class of nanomaterials, present a highly rewarding opportunity to explore new or vastly different functional properties than the traditional mono/bi/multimetallic nanostructures due to their unique characteristics of atomic-level homogeneous mixing of constituent elements in the nanoconfinements. Here, the successful creation of NiCoCr nanoparticles, a well-known MPEA system is reported, using ultrafast nanosecond laser-induced dewetting of alloy thin films. Nanoparticle formation occurs by spontaneously breaking the energetically unstable thin films in a melt state under laser-induced hydrodynamic instability and subsequently accumulating in a droplet shape via surface energy minimization. While NiCoCr alloy shows a stark contrast in physical properties compared to individual metallic constituents, i.e., Ni, Co, and Cr, yet the transient nature of the laser-driven process facilitates a homogeneous distribution of the constituents (Ni, Co, and Cr) in the nanoparticles. Using high-resolution chemical analysis and scanning nanodiffraction, the environmental stability and grain arrangement in the nanoparticles are further investigated. Thermal transport simulations reveal that the ultrashort (≈100 ns) melt-state lifetime of NiCoCr during the dewetting event helps retain the constituent elements in a single-phase solid solution with homogenous distribution and opens the pathway to create the unique MPEA nanoparticles with laser-induced dewetting process.

Tailoring the Angular Mismatch in MoS2 Homobilayers through Deformation Fields.

Ultrathin MoS2 has shown remarkable characteristics at the atomic scale with an immutable disorder to weak external stimuli. Ion beam modification unlocks the potential to selectively tune the size, concentration, and morphology of defects produced at the site of impact in 2D materials. Combining experiments, first-principles calculations, atomistic simulations, and transfer learning, it is shown that irradiation-induced defects can induce a rotation-dependent moiré pattern in vertically stacked homobilayers of MoS2 by deforming the atomically thin material and exciting surface acoustic waves (SAWs). Additionally, the direct correlation between stress and lattice disorder by probing the intrinsic defects and atomic environments are demonstrated. The method introduced in this paper sheds light on how engineering defects in the lattice can be used to tailor the angular mismatch in van der Waals (vdW) solids.

Large-Area Periodic Arrays of Atomically Flat Single-Crystal Gold Nanotriangles Formed Directly on Substrate Surfaces.

The advancement of nanoenabled wafer-based devices requires the establishment of core competencies related to the deterministic positioning of nanometric building blocks over large areas. Within this realm, plasmonic single-crystal gold nanotriangles represent one of the most attractive nanoscale components but where the formation of addressable arrays at scale has heretofore proven impracticable. Herein, a benchtop process is presented for the formation of large-area periodic arrays of gold nanotriangles. The devised growth pathway sees the formation of an array of defect-laden seeds using lithographic and vapor-phase assembly processes followed by their placement in a growth solution promoting planar growth and threefold symmetric side-faceting. The nanotriangles formed in this high-yield synthesis distinguish themselves in that they are epitaxially aligned with the underlying substrate, grown to thicknesses that are not readily obtainable in colloidal syntheses, and present atomically flat pristine surfaces exhibiting gold atoms with a close-packed structure. As such, they express crisp and unambiguous plasmonic modes and form photoactive surfaces with highly tunable and readily modeled plasmon resonances. The devised methods, hence, advance the integration of single-crystal gold nanotriangles into device platforms and provide an overall fabrication strategy that is adaptable to other nanomaterials.

Predictability of Localized Plasmonic Responses in Nanoparticle Assemblies.

Design of nanoscale structures with desired optical properties is a key task for nanophotonics. Here, the correlative relationship between local nanoparticle geometries and their plasmonic responses is established using encoder-decoder neural networks. In the im2spec network, the relationship between local particle geometries and local spectra is established via encoding the observed geometries to a small number of latent variables and subsequently decoding into plasmonic spectra; in the spec2im network, the relationship is reversed. Surprisingly, these reduced descriptions allow high-veracity predictions of local responses based on geometries for fixed compositions and surface chemical states. Analysis of the latent space distributions and the corresponding decoded and closest (in latent space) encoded images yields insight into the generative mechanisms of plasmonic interactions in the nanoparticle arrays. Ultimately, this approach creates a path toward determining configurations that yield the spectrum closest to the desired one, paving the way for stochastic design of nanoplasmonic structures.

Revealing Nanoscale Confinement Effects on Hyperbolic Phonon Polaritons with an Electron Beam.

Hyperbolic phonon polaritons (HPhPs) in hexagonal boron nitride (hBN) enable the direct manipulation of mid-infrared light at nanometer scales, many orders of magnitude below the free-space light wavelength. High-resolution monochromated electron energy-loss spectroscopy (EELS) facilitates measurement of excitations with energies extending into the mid-infrared while maintaining nanoscale spatial resolution, making it ideal for detecting HPhPs. The electron beam is a precise source and probe of HPhPs, which allows the observation of nanoscale confinement in HPhP structures and directly extract hBN polariton dispersions for both modes in the bulk of the flake and modes along the edge. The measurements reveal technologically important nontrivial phenomena, such as localized polaritons induced by environmental heterogeneity, enhanced and suppressed excitation due to 2D interference, and strong modification of high-momenta excitations such as edge-confined polaritons by nanoscale heterogeneity on edge boundaries. The work opens exciting prospects for the design of real-world optical mid-infrared devices based on hyperbolic polaritons.

Enhancing hyperspectral EELS analysis of complex plasmonic nanostructures with pan-sharpening.

Nanoscale hyperspectral techniques-such as electron energy loss spectroscopy (EELS)-are critical to understand the optical response in plasmonic nanostructures, but as systems become increasingly complex, the required sampling density and acquisition times become prohibitive for instrumental and specimen stability. As a result, there has been a recent push for new experimental methodologies that can provide comprehensive information about a complex system, while significantly reducing the duration of the experiment. Here, we present a pan-sharpening approach to hyperspectral EELS analysis, where we acquire two datasets from the same region (one with high spatial resolution and one with high spectral fidelity) and combine them to achieve a single dataset with the beneficial properties of both. This work outlines a straightforward, reproducible pathway to reduced experiment times and higher signal-to-noise ratios, while retaining the relevant physical parameters of the plasmonic response, and is generally applicable to a wide range of spectroscopy modalities.

Spectrally tunable infrared plasmonic F,Sn:In2O3 nanocrystal cubes.

A synthetic challenge in faceted metal oxide nanocrystals (NCs) is realizing tunable localized surface plasmon resonance (LSPR) near-field response in the infrared (IR). Cube-shaped nanoparticles of noble metals exhibit LSPR spectral tunability limited to visible spectral range. Here, we describe the colloidal synthesis of fluorine, tin codoped indium oxide (F,Sn:In2O3) NC cubes with tunable IR range LSPR for around 10 nm particle sizes. Free carrier concentration is tuned through controlled Sn dopant incorporation, where Sn is an aliovalent n-type dopant in the In2O3 lattice. F shapes the NC morphology into cubes by functioning as a surfactant on the {100} crystallographic facets. Cube shaped F,Sn:In2O3 NCs exhibit narrow, shape-dependent multimodal LSPR due to corner, edge, and face centered modes. Monolayer NC arrays are fabricated through a liquid-air interface assembly, further demonstrating tunable LSPR response as NC film nanocavities that can heighten near-field enhancement (NFE). The tunable F,Sn:In2O3 NC near-field is coupled with PbS quantum dots, via the Purcell effect. The detuning frequency between the nanocavity and exciton is varied, resulting in IR near-field dependent enhanced exciton lifetime decay. LSPR near-field tunability is directly visualized through IR range scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS). STEM-EELS mapping of the spatially confined near-field in the F,Sn:In2O3 NC array interparticle gap demonstrates elevated NFE tunability in the arrays.

Two-Dimensional Lateral Epitaxy of 2H (MoSe2)-1T' (ReSe2) Phases.

Two-dimensional (2D) transition metal dichalcogenide (TMDC) heterostructures have been proposed as potential candidates for a variety of applications like quantum computing, neuromorphic computing, solar cells, and flexible field effective transistors. The 2D TMDC heterostructures at the present stage face difficulties being implemented in these applications because of lack of large and sharp heterostructure interfaces. Herein, we address this problem via a CVD technique to grow thermodynamically stable heterostructure of 2H/1T' MoSe2-ReSe2 using conventional transition metal phase diagrams as a reference. We demonstrate how the thermodynamics of mixing in the MoReSe2 system during CVD growth dictates the formation of atomically sharp interfaces between MoSe2 and ReSe2, which can be confirmed by high-resolution scanning transmission electron microscopy imaging, revealing zigzag selenium-terminated interface between the epitaxial 2H and 1T' lattices. Our work provides useful insights for understanding the stability of 2D heterostructures and interfaces between chemically, structurally, and electronically different phases.

Emerging Electron Microscopy Techniques for Probing Functional Interfaces in Energy Materials.

Interfaces play a fundamental role in many areas of chemistry. However, their localized nature requires characterization techniques with high spatial resolution in order to fully understand their structure and properties. State-of-the-art atomic resolution or in situ scanning transmission electron microscopy and electron energy-loss spectroscopy are indispensable tools for characterizing the local structure and chemistry of materials with single-atom resolution, but they are not able to measure many properties that dictate function, such as vibrational modes or charge transfer, and are limited to room-temperature samples containing no liquids. Here, we outline emerging electron microscopy techniques that are allowing these limitations to be overcome and highlight several recent studies that were enabled by these techniques. We then provide a vision for how these techniques can be paired with each other and with in situ methods to deliver new insights into the static and dynamic behavior of functional interfaces.

Polarization- and wavelength-resolved near-field imaging of complex plasmonic modes in Archimedean nanospirals.

Asymmetric nanophotonic structures enable a wide range of opportunities in optical nanotechnology because they support efficient optical nonlinearities mediated by multiple plasmon resonances over a broad spectral range. The Archimedean nanospiral is a canonical example of a chiral plasmonic structure because it supports even-order nonlinearities that are not generally accessible in locally symmetric geometries. However, the complex spiral response makes nanoscale experimental characterization of the plasmonic near-field structure highly desirable. Here we employ high-efficiency, high-spatial-resolution cathodoluminescence imaging in a scanning transmission electron microscope to describe the spatial, spectral, and polarization response of plasmon modes in the nanospiral geometry.

Nanoscale core-shell structure and recrystallization of swift heavy ion tracks in SrTiO3.

It is widely accepted that the interaction of swift heavy ions with many complex oxides is predominantly governed by the electronic energy loss that gives rise to nanoscale amorphous ion tracks along the penetration direction. The question of how electronic excitation and electron-phonon coupling affect the atomic system through defect production, recrystallization, and strain effects has not yet been fully clarified. To advance the knowledge of the atomic structure of ion tracks, we irradiated single crystalline SrTiO3 with 629 MeV Xe ions and performed comprehensive electron microscopy investigations complemented by molecular dynamics simulations. This study shows discontinuous ion-track formation along the ion penetration path, comprising an amorphous core and a surrounding few monolayer thick shell of strained/defective crystalline SrTiO3. Using machine-learning-aided analysis of atomic-scale images, we demonstrate the presence of 4-8% strain in the disordered region interfacing with the amorphous core in the initially formed ion tracks. Under constant exposure of the electron beam during imaging, the amorphous part of the ion tracks readily recrystallizes radially inwards from the crystalline-amorphous interface under the constant electron-beam irradiation during the imaging. Cation strain in the amorphous region is observed to be significantly recovered, while the oxygen sublattice remains strained even under the electron irradiation due to the present oxygen vacancies. The molecular dynamics simulations support this observation and suggest that local transient heating and annealing facilitate recrystallization process of the amorphous phase and drive Sr and Ti sublattices to rearrange. In contrast, the annealing of O atoms is difficult, thus leaving a remnant of oxygen vacancies and strain even after recrystallization. This work provides insights for creating and transforming novel interfaces and nanostructures for future functional applications.

Singular and Nonsingular Transitions in the Infrared Plasmons of Nearly Touching Nanocube Dimers.

Narrow gaps between plasmon-supporting materials can confine infrared electromagnetic energy at the nanoscale, thus enabling applications in areas such as optical sensing. However, in nanoparticle dimers, the nature of the transition between touching (zero gap) and nearly nontouching (nonzero gap ≲15 nm) regimes is still a subject of debate. Here, we observe both singular and nonsingular transitions in infrared plasmons confined to dimers of fluorine-doped indium oxide nanocubes when moving from touching to nontouching configurations depending on the dimensionality of the contact region. Through spatially resolved electron energy-loss spectroscopy, we find a continuous spectral evolution of the lowest-order plasmon mode across the transition for finite touching areas, in excellent agreement with the simulations. This behavior challenges the widely accepted idea that a singular transition always emerges in the near-touching regime of plasmonic particle dimers. The apparent contradiction is resolved by theoretically examining different types of gap morphologies, revealing that the presence of a finite touching area renders the transition nonsingular, while one-dimensional and point-like contacts produce a singular behavior in which the lowest-order dipolar mode in the touching configuration, characterized by a net induced charge in each of the particles, becomes unphysical as soon as they are separated. Our results provide valuable insights into the nature of dimer plasmons in highly doped semiconductors.

Amorphous 1-D nanowires of calcium phosphate/pyrophosphate: A demonstration of oriented self-growth of amorphous minerals.

Amorphous inorganic solids are traditionally isotropic, thus, it is believed that they only grow in a non-preferential way without the assistance of regulators, leading to the morphologies of nanospheres or irregular aggregates of nanoparticles. However, in the presence of (ortho)phosphate (Pi) and pyrophosphate ions (PPi) which have synergistic roles in biomineralization, the highly elongated amorphous nanowires (denoted ACPPNs) form in a regulator-free aqueous solution (without templates, additives, organics, etc). Based on thorough characterization and tracking of the formation process (e.g., Cryo-TEM, spherical aberration correction high resolution TEM, solid state NMR, high energy resolution monochromated STEM-EELS), the microstructure and its preferential growth behavior are elucidated. In ACPPNs, amorphous calcium orthophosphate and amorphous calcium pyrophosphate are distributed at separated but close sites. The ACPPNs grow via either the preferential attachment of ∼2 nm nanoclusters in a 1-dimension way, or the transformation of bigger nanoparticles, indicating an inherent driving force-governed process. We propose that the anisotropy of ACPPNs microstructure, which is corroborated experimentally, causes their oriented growth. This study proves that, unlike the conventional view, amorphous minerals can form via oriented growth without external regulation, demonstrating a novel insight into the structures and growth behaviors of amorphous minerals.

Anomalous isotope effect on the optical bandgap in a monolayer transition metal dichalcogenide semiconductor.

Isotope effects have received increasing attention in materials science and engineering because altering isotopes directly affects phonons, which can affect both thermal properties and optoelectronic properties of conventional semiconductors. However, how isotopic mass affects the optoelectronic properties in 2D semiconductors remains unclear because of measurement uncertainties resulting from sample heterogeneities. Here, we report an anomalous optical bandgap energy red shift of 13 (±7) milli-electron volts as mass of Mo isotopes is increased in laterally structured 100MoS2-92MoS2 monolayers grown by a two-step chemical vapor deposition that mitigates the effects of heterogeneities. This trend, which is opposite to that observed in conventional semiconductors, is explained by many-body perturbation and time-dependent density functional theories that reveal unusually large exciton binding energy renormalizations exceeding the ground-state renormalization energy due to strong coupling between confined excitons and phonons. The isotope effect on the optical bandgap reported here provides perspective on the important role of exciton-phonon coupling in the physical properties of two-dimensional materials.