Controllable Phase Transition Properties in VO2 Films via Metal-Ion Intercalation.

VO2 has shown great promise for sensors, smart windows, and energy storage devices, because of its drastic semiconductor-to-metal transition (SMT) near 340 K coupled with a structural transition. To push its application toward room-temperature, effective transition temperature (Tc) tuning in VO2 is desired. In this study, tailorable SMT characteristics in VO2 films have been achieved by the electrochemical intercalation of foreign ions (e.g., Li ions). By controlling the relative potential with respect to Li/Li+ during the intercalation process, Tc of VO2 can be effectively and systematically tuned in the window from 326.7 to 340.8 K. The effective Tc tuning could be attributed to the observed strain and lattice distortion and the change of the charge carrier density in VO2 introduced by the intercalation process. This demonstration opens up a new approach in tuning the VO2 phase transition toward room-temperature device applications and enables future real-time phase change property tuning.

Self-Assembled Au Nanoelectrodes: Enabling Low-Threshold-Voltage HfO2-Based Artificial Neurons.

Filamentary-type resistive switching devices, such as conductive bridge random-access memory and valence change memory, have diverse applications in memory and neuromorphic computing. However, the randomness in filament formation poses challenges to device reliability and uniformity. To overcome this issue, various defect engineering methods have been explored, including doping, metal nanoparticle embedding, and extended defect utilization. In this study, we present a simple and effective approach using self-assembled uniform Au nanoelectrodes to controll filament formation in HfO2 resistive switching devices. By concentrating the electric field near the Au nanoelectrodes within the BaTiO3 matrix, we significantly enhanced the device stability and reduced the threshold voltage by up to 45% in HfO2-based artificial neurons compared to the control devices. The threshold voltage reduction is attributed to the uniformly distributed Au nanoelectrodes in the insulating matrix, as confirmed by COMSOL simulation. Our findings highlight the potential of nanostructure design for precise control of filamentary-type resistive switching devices.

High Strength and Low Coercivity of Cobalt with Three-Dimensional Nanoscale Stacking Faults.

Lower coercivity (HC) and magnetic anisotropy (K1) coupled with high mechanical strength are essential properties for Co-based soft magnetic thin films; however, the strength-coercivity trade-off limits their development. Co with face centered cubic structure (fcc) exhibits lower HC and K1 than its grand hexagonal close packed structure (hcp); however, metastable fcc-phase Co is hard to stabilize. Here, by using Cu (100) seed layer, we synthesized micron-thick fcc Co films with self-formed three-dimensional nanoscale stacking faults (3D-nSFs) that could achieve high strengths without sacrificing soft magnetic properties. The 3D-nSFs, induced by the Co/Cu interface, could not only stabilize the metastable fcc Co to yield lower HC but also impede dislocation motion to strengthen Co films. More importantly, we successfully tailored the density of 3D-nSFs and confirmed a large variation in magnetic coercivity (by 100%) and indentation hardness (by 25%). This work provides a new strategy for integrated performance optimization by interface design and strain engineering.

ZnO-AuxCu1-x Alloy and ZnO-AuxAl1-x Alloy Vertically Aligned Nanocomposites for Low-Loss Plasmonic Metamaterials.

Hyperbolic metamaterials are a class of materials exhibiting anisotropic dielectric function owing to the morphology of the nanostructures. In these structures, one direction behaves as a metal, and the orthogonal direction behaves as a dielectric material. Applications include subdiffraction imaging and hyperlenses. However, key limiting factors include energy losses of noble metals and challenging fabrication methods. In this work, self-assembled plasmonic metamaterials consisting of anisotropic nanoalloy pillars embedded into the ZnO matrix are developed using a seed-layer approach. Alloys of AuxAl1-x or AuxCu1-x are explored due to their lower losses and higher stability. Optical and microstructural properties were explored. The ZnO-AuxCu1-x system demonstrated excellent epitaxial quality and optical properties compared with the ZnO-AuxAl1-x system. Both nanocomposite systems demonstrate plasmonic resonance, hyperbolic dispersion, low losses, and epsilon-near-zero permittivity, making them promising candidates towards direct photonic integration.

Nitride-Oxide-Metal Heterostructure with Self-Assembled Core-Shell Nanopillar Arrays: Effect of Ordering on Magneto-Optical Properties.

Magneto-optical (MO) coupling incorporates photon-induced change of magnetic polarization that can be adopted in ultrafast switching, optical isolators, mode convertors, and optical data storage components for advanced optical integrated circuits. However, integrating plasmonic, magnetic, and dielectric properties in one single material system poses challenges since one natural material can hardly possess all these functionalities. Here, co-deposition of a three-phase heterostructure composed of a durable conductive nitride matrix with embedded core-shell vertically aligned nanopillars, is demonstrated. The unique coupling between ferromagnetic NiO core and atomically sharp plasmonic Au shell enables strong MO activity out-of-plane at room temperature. Further, a template growth process is applied, which significantly enhances the ordering of the nanopillar array. The ordered nanostructure offers two schemes of spin polarization which result in stronger antisymmetry of Kerr rotation. The presented complex hybrid metamaterial platform with strong magnetic and optical anisotropies is promising for tunable and modulated all-optical-based nanodevices.

Field-assisted growth of one-dimensional ZnO nanostructures with high defect density.

One-dimensional ZnO nanostructures have shown great potential in electronics, optoelectronics and electromechanical devices owing to their unique physical and chemical properties. Most of these nanostructures were grown by equilibrium processes where the defects density is controlled by thermodynamic equilibrium. In this work, flash sintering, a non-equilibrium field-assisted processing method, has been used to synthesize ZnO nanostructures. By applying a high electric field and limiting a low current flow, ZnO nanorods grew uniformly by a vapor-liquid-solid mechanism due to the extreme temperatures achieved near the hot spot. High density basal stacking faults in the nanorods along with ultraviolet excitonic emission and a red emission under room temperature demonstrate the potential of defect engineering in nanostructures via the field-assisted growth method.

Vertically Aligned AgxAu1-x Alloyed Nanopillars Embedded in ZnO as Nanoengineered Low-Loss Hybrid Plasmonic Metamaterials.

Hybrid plasmonic metamaterials offer a pathway to exotic properties and technologically important applications including subdiffraction imaging and plasmonic energy harvesting. Challenges remain for practical applications including high absorption losses of noble metals and tedious growth/fabrication processes. In this work, a self-assembled hybrid plasmonic metamaterial consisting of anisotropic AgxAu1-x alloy nanopillars embedded in a ZnO matrix has been successfully grown. The chemical composition of the nanoalloy was determined to be Ag61Au39. The microstructure and optical properties arising from ZnO-Ag61Au39 alloyed hybrid systems were investigated and compared with that of the ZnO-Ag particle-in-matrix nanocomposite and the ZnO-Au vertically aligned nanocomposite. The ZnO-Ag61Au39 hybrid system demonstrates anisotropic morphology, excellent epitaxial quality, and enhanced optical properties, including surface plasmon resonance, hyperbolic dispersion, low absorption losses, and numerous epsilon-near-zero permittivity points, making it a promising candidate for practical applications of hybrid plasmonic metamaterials.

Metal-Free Oxide-Nitride Heterostructure as a Tunable Hyperbolic Metamaterial Platform.

Metal-free plasmonic metamaterials with wide-range tunable optical properties are highly desired for various components in future integrated optical devices. Designing a ceramic-ceramic hybrid metamaterial has been theoretically proposed as a solution to this critical optical material demand. However, the processing of such all-ceramic metamaterials is challenging due to difficulties in integrating two very dissimilar ceramic phases as one hybrid system. In this work, an oxide-nitride hybrid metamaterial combining two highly dissimilar ceramic phases, i.e., semiconducting weak ferromagnetic NiO nanorods and conductive plasmonic TiN matrix, has been successfully integrated as a unique vertically aligned nanocomposite form. Highly anisotropic optical properties such as hyperbolic dispersions and strong magneto-optical coupling have been demonstrated under room temperature. The novel functionalities presented show the strong potentials of this new ceramic-ceramic hybrid thin film platform and its future applications in next-generation nanophotonics and magneto-optical integrated devices without the lossy metallic components.

Large-Scale Plasmonic Hybrid Framework with Built-In Nanohole Array as Multifunctional Optical Sensing Platforms.

Light coupling with patterned subwavelength hole arrays induces enhanced transmission supported by the strong surface plasmon mode. In this work, a nanostructured plasmonic framework with vertically built-in nanohole arrays at deep-subwavelength scale (6 nm) is demonstrated using a two-step fabrication method. The nanohole arrays are formed first by the growth of a high-quality two-phase (i.e., Au-TiN) vertically aligned nanocomposite template, followed by selective wet-etching of the metal (Au). Such a plasmonic nanohole film owns high epitaxial quality with large surface coverage and the structure can be tailored as either fully etched or half-way etched nanoholes via careful control of the etching process. The chemically inert and plasmonic TiN plays a role in maintaining sharp hole boundary and preventing lattice distortion. Optical properties such as enhanced transmittance and anisotropic dielectric function in the visible regime are demonstrated. Numerical simulation suggests an extended surface plasmon mode and strong field enhancement at the hole edges. Two demonstrations, including the enhanced and modulated photoluminescence by surface coupling with 2D perovskite nanoplates and the refractive index sensing by infiltrating immersion liquids, suggest the great potential of such plasmonic nanohole array for reusable surface plasmon-enhanced sensing applications.

Novel Layered Supercell Structure from Bi2AlMnO6 for Multifunctionalities.

Layered materials, e.g., graphene and transition metal (di)chalcogenides, holding great promises in nanoscale device applications have been extensively studied in fundamental chemistry, solid state physics and materials research areas. In parallel, layered oxides (e.g., Aurivillius and Ruddlesden-Popper phases) present an attractive class of materials both because of their rich physics behind and potential device applications. In this work, we report a novel layered oxide material with self-assembled layered supercell structure consisting of two mismatch-layered sublattices of [Bi3O3+δ] and [MO2]1.84 (M = Al/Mn, simply named BAMO), i.e., alternative layered stacking of two mutually incommensurate sublattices made of a three-layer-thick Bi-O slab and a one-layer-thick Al/Mn-O octahedra slab in the out-of-plane direction. Strong room-temperature ferromagnetic and piezoelectric responses as well as anisotropic optical property have been demonstrated with great potentials in various device applications. The realization of the novel BAMO layered supercell structure in this work has paved an avenue toward exploring and designing new materials with multifunctionalities.

Self-Assembled Epitaxial Au-Oxide Vertically Aligned Nanocomposites for Nanoscale Metamaterials.

Metamaterials made of nanoscale inclusions or artificial unit cells exhibit exotic optical properties that do not exist in natural materials. Promising applications, such as super-resolution imaging, cloaking, hyperbolic propagation, and ultrafast phase velocities have been demonstrated based on mostly micrometer-scale metamaterials and few nanoscale metamaterials. To date, most metamaterials are created using costly and tedious fabrication techniques with limited paths toward reliable large-scale fabrication. In this work, we demonstrate the one-step direct growth of self-assembled epitaxial metal-oxide nanocomposites as a drastically different approach to fabricating large-area nanostructured metamaterials. Using pulsed laser deposition, we fabricated nanocomposite films with vertically aligned gold (Au) nanopillars (∼20 nm in diameter) embedded in various oxide matrices with high epitaxial quality. Strong, broad absorption features in the measured absorbance spectrum are clear signatures of plasmon resonances of Au nanopillars. By tuning their densities on selected substrates, anisotropic optical properties are demonstrated via angular dependent and polarization resolved reflectivity measurements and reproduced by full-wave simulations and effective medium theory. Our model predicts exotic properties, such as zero permittivity responses and topological transitions. Our studies suggest that these self-assembled metal-oxide nanostructures provide an exciting new material platform to control and enhance optical response at nanometer scales.

Nanovoid formation mechanism in nanotwinned Cu.

Nanotwinned metals have been intensely investigated due to their unique microstructures and superior properties. This work aims to investigate the nanovoid formation mechanism in sputter-deposited nanotwinned Cu. Three different types of epitaxial or polycrystalline Cu films are fabricated by magnetron sputtering deposition technique. In the epitaxial Cu (111) films deposited on Si (110) substrates, high fractions of nanovoids and nanotwins are formed. The void size and density can be tailored by varying deposition parameters, including argon pressure, deposition rate, and film thickness. Interestingly, nanovoids become absent in the polycrystalline Cu film deposited on Si (111) substrate, but they can be regained in the epitaxial nanotwinned Cu (111) when deposited on Si (111) substrate with an Ag seed layer. The nanovoid formation seems to be closely associated with twin nucleation and film texture. Based on the comparative studies between void-free polycrystalline Cu films and epitaxial nanotwinned Cu films with nanovoids, the underlying mechanisms for the formation of nanovoids are discussed within the framework of island coalescence model.

TiN-Au/HfO2 -Au Multilayer Thin Films with Tunable Hyperbolic Optical Response.

Hyperbolic metamaterials (HMM) possess significant anisotropic physical properties and tunability and thus find many applications in integrated photonic devices. HMMs consisting of metal and dielectric phases in either multilayer or vertically aligned nanocomposites (VAN) form are demonstrated with different hyperbolic properties. Herein, self-assembled HfO2 -Au/TiN-Au multilayer thin films, combining both the multilayer and VAN designs, are demonstrated. Specifically, Au nanopillars embedded in HfO2 and TiN layers forming the alternative layers of HfO2 -Au VAN and TiN-Au VAN. The HfO2 and TiN layer thickness is carefully controlled by varying laser pulses during pulsed laser deposition (PLD). Interestingly, tunable anisotropic physical properties can be achieved by adjusting the bi-layer thickness and the number of the bi-layers. Type II optical hyperbolic dispersion can be obtained from high layer thickness structure (e.g., 20 nm), while it can be transformed into Type I optical hyperbolic dispersion by reducing the thickness to a proper value (e.g., 4 nm). This new nanoscale hybrid metamaterial structure with the three-phase VAN design shows great potential for tailorable optical components in future integrated devices.

Achieving room temperature plasticity in brittle ceramics through elevated temperature preloading.

Ceramic materials with high strength and chemical inertness are widely used as engineering materials. However, the brittle nature limits their applications as fracture occurs before the onset of plastic yielding. There has been limited success despite extensive efforts to enhance the deformability of ceramics. Here we report a method for enhancing the room temperature plastic deformability of ceramics by artificially introducing abundant defects into the materials via preloading at elevated temperatures. After the preloading treatment, single crystal (SC) TiO2 exhibited a substantial increase in deformability, achieving 10% strain at room temperature. SC α-Al2O3 also showed plastic deformability, 6 to 7.5% strain, by using the preloading strategy. These preinjected defects enabled the plastic deformation process of the ceramics at room temperature. These findings suggest a great potential for defect engineering in achieving plasticity in ceramics at room temperature.

Additive manufacturing of an ultrastrong, deformable Al alloy with nanoscale intermetallics.

Light-weight, high-strength, aluminum (Al) alloys have widespread industrial applications. However, most commercially available high-strength Al alloys, like AA 7075, are not suitable for additive manufacturing due to their high susceptibility to solidification cracking. In this work, a custom Al alloy Al92Ti2Fe2Co2Ni2 is fabricated by selective laser melting. Heterogeneous nanoscale medium-entropy intermetallic lamella form in the as-printed Al alloy. Macroscale compression tests reveal a combination of high strength, over 700 MPa, and prominent plastic deformability. Micropillar compression tests display significant back stress in all regions, and certain regions have flow stresses exceeding 900 MPa. Post-deformation analyses reveal that, in addition to abundant dislocation activities in Al matrix, complex dislocation structures and stacking faults form in monoclinic Al9Co2 type brittle intermetallics. This study shows that proper introduction of heterogeneous microstructures and nanoscale medium entropy intermetallics offer an alternative solution to the design of ultrastrong, deformable Al alloys via additive manufacturing.

A quantitative evaluation of microstructure by electron back-scattered diffraction pattern quality variations.

Band contrast (BC) is a qualitative measure of electron back-scattered diffraction (EBSD), which is derived from the intensity of the Kikuchi bands. The BC is dependent upon several factors including scanning electron microscope measurement parameters, EBSD camera setup, and the specimen itself (lattice defect and grain orientation). In this study, the effective factors for BC variations and the feasibility of using BC variations for the quantification of microstructure evolutions have been investigated. In addition, the effects of the lattice defect and the grain orientation on the BC variations are studied. Next, a shear-deformed microstructure of 316L stainless steel, which contains nanosized grains and a large portion of twin boundaries, is revealed by BC map and histogram. Recovery and recrystallization of shear-deformed 316L stainless steel are displayed by BC variations during isothermal annealing at 700 and 800°C, respectively. It is observed that the BC turns bright as the shear-deformed crystal structure is recovered or recrystallized.

Self-Assembled Complex Three-Phase Core-Shell Nanostructure of Au-CoFe2-TiN with a Magneto-Optical Coupling Effect.

Nanostructured plasmonic-magnetic metamaterials have gained great research interest due to their enhanced magneto-optical coupling effects. Here, we report a complex three-phase nanocomposite design combining ferromagnetic CoFe2 with plasmonic TiN and Au as a multifunctional hybrid metamaterial using either a cogrowth or a templated method. Via the first method of cogrowing three phases, three different morphologies of Au-CoFe2 core-shell nanopillars were formed in the TiN matrix. Via the second method of sequential deposition of a TiN-Au seed layer and a TiN-CoFe2 layer, highly ordered and uniform single-type core-shell nanopillars (i.e., the CoFe2 shell with a Au core) form in the TiN matrix. Both cogrowth and templated growth TiN-CoFe2-Au hybrid systems exhibit excellent epitaxial quality, hyperbolic dispersion, magnetic anisotropy, and a magneto-optical coupling effect. This study provides an effective approach for achieving highly uniform multiphase vertically aligned nanocomposite structures with well-integrated optical, magnetic, and coupling properties.

Gradient nanostructured steel with superior tensile plasticity.

Nanostructured metallic materials with abundant high-angle grain boundaries exhibit high strength and good radiation resistance. While the nanoscale grains induce high strength, they also degrade tensile ductility. We show that a gradient nanostructured ferritic steel exhibits simultaneous improvement in yield strength by 36% and uniform elongation by 50% compared to the homogenously structured counterpart. In situ tension studies coupled with electron backscattered diffraction analyses reveal intricate coordinated deformation mechanisms in the gradient structures. The outermost nanolaminate grains sustain a substantial plastic strain via a profound deformation mechanism involving prominent grain reorientation. This synergistic plastic co-deformation process alters the rupture mode in the post-necking regime, thus delaying the onset of fracture. The present discovery highlights the intrinsic plasticity of nanolaminate grains and their significance in simultaneous improvement of strength and tensile ductility of structural metallic materials.

In Situ Reactive Formation of Mixed Oxides in Additively Manufactured Cobalt Alloy.

Oxide-dispersion-strengthened (ODS) alloys have long been considered for high temperature turbine, spacecraft, and nuclear reactor components due to their high temperature strength and radiation resistance. Conventional synthesis approaches of ODS alloys involve ball milling of powders and consolidation. In this work, a process-synergistic approach is used to introduce oxide particles during laser powder bed fusion (LPBF). Chromium (III) oxide (Cr2O3) powders are blended with a cobalt-based alloy, Mar-M 509, and exposed to laser irradiation, resulting in reduction-oxidation reactions involving metal (Ta, Ti, Zr) ions from the metal matrix to form mixed oxides of increased thermodynamic stability. A microstructure analysis indicates the formation of nanoscale spherical mixed oxide particles as well as large agglomerates with internal cracks. Chemical analyses confirm the presence of Ta, Ti, and Zr in agglomerated oxides, but primarily Zr in the nanoscale oxides. Mechanical testing reveals that agglomerate particle cracking is detrimental to tensile ductility compared to the base alloy, suggesting the need for improved processing methods to break up oxide particle clusters and promote their uniform dispersion during laser exposure.

Abnormal in-plane epitaxy and formation mechanism of vertically aligned Au nanopillars in self-assembled CeO2-Au metamaterial systems.

Metamaterials present great potential in the applications of solar cells and nanophotonics, such as super lenses and other meta devices, owing to their superior optical properties. In particular, hyperbolic metamaterials (HMMs) with exceptional optical anisotropy offer improved manipulation of light-matter interactions as well as a divergence in the density of states and thus show enhanced performances in related fields. Recently, the emerging field of oxide-metal vertically aligned nanocomposites (VANs) suggests a new approach to realize HMMs with flexible microstructural modulations. In this work, a new oxide-metal metamaterial system, CeO2-Au, has been demonstrated with variable Au phase morphologies from nanoparticle-in-matrix (PIM), nanoantenna-in-matrix, to VAN. The effective morphology tuning through deposition background pressure, and the corresponding highly tunable optical performance of three distinctive morphologies, were systematically explored and analyzed. A hyperbolic dispersion at high wavelength has been confirmed in the nano-antenna CeO2-Au thin film, proving this system as a promising candidate for HMM applications. More interestingly, a new and abnormal in-plane epitaxy of Au nanopillars following the large mismatched CeO2 matrix instead of the well-matched SrTiO3 substrate, was discovered. Additionally, the tilting angle of Au nanopillars, α, has been found to be a quantitative measure of the balance between kinetics and thermodynamics during the depositions of VANs. All these findings provide valuable information in the understanding of the VAN formation mechanisms and related morphology tuning.