The thermal conductivity Λ of wide bandgap semiconductor thin films, such as AlN, affects the performance of high-frequency devices, power devices, and optoelectronics. However, accurate measurements of Λ in thin films with sub-micrometer thicknesses and Λ > 100 W m−1 K−1 is challenging. Widely used pump/probe metrologies, such as time–domain thermoreflectance (TDTR) and frequency–domain thermoreflectance, lack the spatiotemporal resolution necessary to accurately quantify thermal properties of sub-micrometer thin films with high Λ. In this work, we use a combination of magneto-optic thermometry and TiN interfacial layers to significantly enhance the spatiotemporal resolution of pump/probe thermal transport measurements. We use our approach to measure Λ of 100, 400, and 1000 nm AlN thin films. We coat AlN thin films with a ferromagnetic thin-film transducer with the geometry of (1 nm-Pt/0.4 nm-Co)x3/(2 nm-TiN). This PtCo/TiN transducer has a fast thermal response time of <50 ps, which allows us to differentiate between the thermal response of the transducer, AlN thin film, and substrate. For the 100, 400, and 1000 nm thick AlN films, we determine Λ to be 200 ± 80, 165 ± 35, and 300 ± 70 W m−1 K−1, respectively. We conclude with an uncertainty analysis that quantifies the errors associated with pump/probe measurements of thermal conductivity, as a function of transducer type, thin-film thermal conductivity, and thin-film thickness. Time resolved magneto-optic Kerr effect experiments can measure films that are three to five times thinner than is possible with standard pump/probe metrologies, such as TDTR. This advance in metrology will enable better characterization of nanoscale heat transfer in high thermal conductivity material systems like wide bandgap semiconductor heterostructures and devices.

Layered oxides have been intensively studied due to their high degree of freedom in designing various electromagnetic properties and functionalities. While Bi-based layered supercell (LSC) compounds [BinOn+δ]-[MO2] (M = Mn, Mn/Al, Mn/Fe, or Mn/Ni; n = 2, 3) are a group of prospective candidates, all of the reported compounds are insulators. Here, we report on the synthesis of two novel metallic LSC compounds [BinOn+δ]-[RhO2] (n = 2, 3) by pulsed laser deposition and subsequent annealing. With tuning the thickness of the sublattice from Bi2O2+δ to Bi3O3+δ, a dimensionality-dependent electrical transport is revealed from a conventional metallic transport in n = 2 to a localized transport in n = 3. Our successful growth will be an important step for further exploring novel layered oxide compounds.

Rutile GeO2 has been predicted to be an ultra-wide bandgap semiconductor suitable for future power electronic devices, while quartz-like GeO2 shows piezoelectric properties. To explore these crystalline phases for application and fundamental materials investigations, molecular beam epitaxy (MBE) is a well-suited thin film growth technique. In this study, we investigate the reaction kinetics of GeO2 during plasma-assisted MBE using elemental Ge and plasma-activated oxygen fluxes. The growth rate as a function of oxygen flux is measured in situ by laser reflectometry at different growth temperatures. A flux of the suboxide GeO desorbing off the growth surface is identified and quantified in situ by the line-of-sight quadrupole mass spectrometry. Our measurements reveal that the suboxide formation and desorption limits the growth rate under Ge-rich or high temperature growth conditions and leads to etching of the grown GeO2 layer under a Ge flux in the absence of oxygen. The quantitative results fit the sub-compound mediated reaction model, indicating the intermediate formation of the suboxide at the growth front. This model is further utilized to delineate the GeO2-growth window in terms of oxygen-flux and substrate temperature. Our study can serve as a guidance for the thin film synthesis of GeO2 and defect-free mesa etching in future GeO2-device processing.

Direct methanol fuel cells (DMFCs) are energy carriers with a significant high energy density, easy implementation, a low operating temperature, and a convenient methanol fuel storage, rendering them a reasonable alternative for portable applications. However, there are several substantial barriers to the widespread use of DMFCs that must be addressed. Noble metal-based catalysts have long been regarded as outstanding electrocatalysts for fuel cells, but their high cost and low durability have kept them from becoming widely used. Nickel-based electrocatalysts are possible replacements for expensive noble metal catalysts owing to their low price, high durability, and remarkable surface oxidation properties. Herein, we develop an incredibly active and remarkably stable electrocatalyst for the methanol oxidation reaction (MOR) via a simple hydrothermal method coupled with nitridation to prepare highly porous Ni3N nanosheets arrays supported by nickel foam (NF) substrate. The in situ growth of highly porous nanosheets on NF (NSAs/NF) exposes more active sites and allows fast charge/mass transfer, creating synergistic effects between Ni3N and NF. As a result, the strong interaction between Ni3N and NF prevents leaching and renders the catalyst highly stable for over 20 h with a 72.58% retention rate, making it among the best retention rates reported recently for comparable Ni-based catalysts. Based on these findings, nickel nitride appears to be an excellent electrocatalyst for fuel cell applications.

Epitaxial heterostructures composed of complex correlated metal oxides, grown along specific crystallographic orientations, offer a route to investigating emergent phenomena such as topological states and spin liquids through geometrical lattice engineering. A2Ru2O7 pyrochlore ruthenates, in particular, exhibit a metal–insulator transition with varying A cation, whose mechanism is not fully understood. We report on the epitaxial growth, and structural and electrical properties of metallic pyrochlore bismuth ruthenate heterostructures, grown along both the [001] and [111] directions. Ordered pyrochlore thin films were obtained with a highly oriented texture along the [001] and [111] crystallographic directions. Density functional theory calculations of the electronic band structure and density of states indicated that Bi2Ru2O7 is semimetallic and that hybridization of the Ru 4d and Bi 6p orbitals via the anion network at the Fermi energy was responsible for the metallicity. Electrical conductivity measurements confirmed that the compound is weakly metallic, in agreement with the reported conductivity for the stoichiometric bulk compound. The carrier concentration and mobility of the electrons compared favorably with previous reports on bulk material and indicate strong electron–electron interactions. The measured and computed optical conductivities were found to share coincident spectral features and confirm the electronic correlation. Comparison of the electrical and optical properties of the two distinct orientations indicates differences that cannot be attributed to differences in crystalline quality or dislocations and may indicate anisotropy in the electronic structure of Bi2Ru2O7. This study will enable access to the kagome lattice arising naturally in the 111 planes of the pyrochlore B cation sublattice, which may be used to uncover emergent topological properties.

Room-temperature stabilization of skyrmions in magnetic multilayered systems results from a fine balance between several magnetic interactions, namely, symmetric and antisymmetric exchange, dipolar interaction and perpendicular magnetic anisotropy as well as, in most cases, Zeeman through an applied external field. Such field-driven stabilization approach is, however, not compatible with most of the anticipated skyrmion based applications, e.g., skyrmion memories and logic or neuromorphic computing, which motivates a reduction or a cancellation of field requirements. Here, we present a method to stabilize at room-temperature and zero-field, a densely packed skyrmion phase in ferromagnetic multilayers with moderate number of repetitions. To this aim, we finely tune the multilayer parameters to stabilize a dense skyrmion phase. Then, relying on the interlayer electronic coupling to an adjacent bias magnetic layer with strong perpendicular magnetic anisotropy and uniform magnetization, we demonstrate the stabilization of sub-60 nm diameter skyrmions at zero-field with adjustable skyrmion density.

Partially frozen-in states are rather the rule than the exception. Coexistence between equilibrium states and frozen-in states is relevant in view of the diversity and complexity of charge carriers, or sublattices, especially in multinary compounds, but also with respect to differently equilibrated spatial regions. This contribution deals with the open circuit potential of samples where only surface-near regions feel the outer partial pressure, or more generally, the component chemical potentials, established by the electrodes. In view of the significance of such measurements for separating ionic and electronic conductivity contributions, and the kinetic difficulties in getting full equilibration near room-temperature, the value of these considerations is obvious. The necessary relations are derived, or their derivations are sketched within the framework of linear force–flux laws. An account is made of recent emf measurements of hybrid halide perovskites, and a refinement of their standard defect diagram is recommended.

Short, partially complementary, single-stranded (ss)DNA strands can form nanostructures with a wide variety of shapes and mechanical properties. It is well known that semiflexible, linear dsDNA can undergo an isotropic to nematic (IN) phase transition and that sufficiently bent structures can form a biaxial nematic phase. Here, we use numerical simulations to explore how the phase behavior of linear DNA constructs changes as we tune the mechanical properties of the constituent DNA by changing the nucleotide sequence. The IN-phase transition can be suppressed in so-called DNA “nunchakus”: structures consisting of two rigid dsDNA arms, separated by a sufficiently flexible spacer. In this paper, we use simulations to explore what phase behavior to expect for different linear DNA constructs. To this end, we first performed numerical simulations exploring the structural properties of a number of different DNA oligonucleotides using the oxDNA package. We then used the structural information generated in the oxDNA simulations to construct more coarse-grained models of the rod-like, bent-core, and nunchaku DNA. These coarse-grained models were used to explore the phase behavior of suspensions of the various DNA constructs. The approach explored in this paper makes it possible to “design” the phase behavior of DNA constructs by a suitable choice of the constituent nucleotide sequence.

Antiferromagnetic (AFM) materials possess unique properties, such as rapid dynamic response, resistance to external magnetic disturbances, and the absence of a stray field. AFM materials are important members in the field of spintronics, and generating the spin current in AFM materials is one of the vital topics for AFM spintronics. The spin photovoltaic effect (SPVE) is the spin counterpart of the bulk photovoltaic effect (BPVE), where the photocurrent is spin-polarized. This effect can generate spin current in a contactless and ultra-fast way. Recently, SPVE has garnered significant interest due to its potential application in AFM spintronics and rich physical content. In this perspective, the mechanism of SPVE, including the relationship between SPVE and BPVE, and symmetry constraints are reviewed. We also provide an overview of recent progress on SPVE in AFM materials. This perspective also offers a viewpoint on this exciting area of research.

CoPS3 stands out in the family of the van der Waals antiferromagnets XPS3 (X = Mn, Ni, Fe, and Co) due to the unquenched orbital momentum of the magnetic Co2+ ions, which is known to facilitate the coupling of spins to both electromagnetic waves and lattice vibrations. Here, using a time-resolved magneto-optical pump–probe technique, we experimentally study the ultrafast laser-induced dynamics of mutually correlated spins and lattice. It is shown that a femtosecond laser pulse acts as an ultrafast heater and, thus, results in the melting of the antiferromagnetic order. At the same time, the resonant pumping of the 4T1g → 4T2g electronic transition in Co2+ ions effectively changes their orbital momentum, giving rise to a mechanical force that moves the ions in the direction parallel to the orientation of their spins, thus generating a coherent Bg phonon mode at the frequency of about 4.7 THz.

Owing to freedom of design, simplicity, and ability to handle complex structures, additive manufacturing (AM) or 3D printing of ceramics represents a promising enabling technology and has already been used to produce geometrically complex ceramic components and ceramic metamaterials. Consequently, novel applications for additively manufactured ceramics, which leverage their structural, high temperature, and chemical-resistant properties, have been proposed in areas ranging from electrical engineering and micro/nanoelectronics to chemical engineering to biology. Polymer derived ceramics (PDCs) represent a relatively new class of materials within additive manufacturing. PDCs enable the development of ceramic parts patterned via low-cost polymer 3D printing methods followed by pyrolysis in a high temperature process in which the polymer itself forms a ceramic often in the absence of any ceramic filler. PDCs have served as a feedstock for various 3D printing techniques for which a wide range of physiochemical factors can be tailored to optimize the ceramic manufacturing processes. In particular, the silicon and carbon-rich polymeric microstructure of PDCs offers a high degree of tunability and potential to achieve a closely defined combination of functional, thermomechanical, and chemical properties. In this review, we cover mechanisms underlying the design and manufacture of ceramics via 3D printing and pyrolysis of preceramic polymers, focusing on chemical formulations, printing technologies, and the mechanical performance of the ceramic network from microscale to scale. We also summarize experimental data from the literature and present qualitative and quantitative comparisons between different AM routes to provide a comprehensive review for 3D printing of PDCs and to highlight potential future research.

CuMnO2 nanoflakes have been prepared utilizing a hydrothermal technique with nitrilotriacetic acid as a precipitant. The structure, composition, and morphology are characterized by several techniques. Interestingly, the as-prepared sample delivers 993 mAh g−1 after 300 cycles, excellent rate capabilities (523.2, 293.3, and 156.1 mAh g−1 at 0.5, 1.0, and 2.0 A g−1, respectively) as the anode of a Li-half battery, and a high specific capacitance of 403.3 F g−1 even at 12 A g−1, as well as stable cycling, excellent kinetics, and rate capabilities for supercapacitors applications, which are superior to the single Cu2O or Mn2O3, suggesting a great potential for advanced lithium-ion batteries.

When planning for any space mission, shielding against ionizing radiation is essential. Polymers, combined with a nano-filler material to reinforce and enhance the polymer properties, can provide a sufficient radiation shielding function with lower weight and less secondary radiation generation than traditional shielding materials such as aluminum and high-density polyethylene. In this study, poly(2, 5)benzimidazole/multi-walled carbon nanotube (ABPBI/MWCNT) nanocomposites were fabricated and evaluated for their proton radiation shielding capabilities in the low-earth orbit region of space. The radiation shielding effectiveness of the ABPBI/MWCNT nanocomposites was experimentally evaluated by comparing their proton transmission properties and their secondary neutron generation to those of pristine ABPBI. The results showed that adding MWCNTs to the ABPBI matrix further reduced the secondary neutrons generated by the pristine ABPBI. In addition, the depth profile showed that proton penetration into the bulk of the composite decreased as the MWCNT weight percentage loading increased. The MWCNT-loaded composites showed improved resistance to proton radiation-induced damage compared to the pristine ABPBI membrane. This was evident from the visible damage observed in the scanning electron microscopy micrographs of the pre- and post-irradiated ABPBI membranes. Furthermore, composites containing MWCNTs displayed improved thermal stability over the pristine ABPBI for both pre- and post-irradiation composites. The overall characteristics presented have shown ABPBI/MWCNT nanocomposites as an effective material for application in the space industry.

Surfaces play a key role during ferroelastic switching and define the interactions of materials with ionic species and biological systems. Here, we perform molecular dynamics simulations and identify ridges and valleys with rounded singularities around the intersections between twin walls and surfaces. Two dominant length scales stem from the elastic bending of the surface layer (>30 lattice units) and local atomic reshuffles (some five lattice units). For static twin walls, which do not shift laterally under external stress, the intrinsic change in Young’s modulus involves softening near valleys and hardening near ridges. The boundary-induced changes in the surface Young’s modulus are of the order of 0.7%.

Artificial multiferroics consist of two types of ferroic materials, typically a ferroelectric and a ferromagnet, often coupled interfacially by magnetostriction induced by the lattice elongations in the ferroelectric. In BaTiO3, the magnitude of strain induced by these elongations is heavily temperature dependent, varying greatly between each of the polar crystal phases and exerting a huge influence over the properties of a coupled magnetic film. Here, we demonstrate that temperature and, thus, strain are effective means of controlling the magnetic anisotropy in BaTiO3(111)/CoFeB heterostructures. We investigate the three polar phases of BaTiO3: tetragonal (T) at room temperature, orthorhombic (O) below 280 K, and rhombohedral (R) below 190 K across a total range of 77–420 K. We find two distinct responses: a step-like change in the anisotropy across the low-temperature phase transitions and a sharp high-temperature reduction around the ferroelectric Curie temperature, measured from hard axis hysteresis loops. Using our measurements of this anisotropy strength, we are then able to show by micromagnetic simulation the behavior of all possible magnetic domain wall states and determine their scaling as a function of temperature. The most significant changes occur in the head-to-head domain wall states, with a maximum change of 210 nm predicted across the entire range, effectively doubling the size of the domain wall as compared to room temperature. Notably, similar changes are seen for both high and low temperatures, which suggests different routes for potential control of magnetic anisotropy and elastically pinned magnetic domain walls.

Metal-halide perovskites (MHPs) exhibit excellent properties for application in optoelectronic devices. The bottleneck for their incorporation is the lack of long-term stability such as degradation due to external conditions (heat, light, oxygen, moisture, and mechanical stress), but the occurrence of phase transitions also affects their performance. Structural phase transitions are often influenced by phonon modes. Hence, an insight into both the structure and lattice dynamics is vital to assess the potential of MHPs. In this study, GIWAXS and Raman spectroscopy are applied, supported by density functional theory calculations, to investigate the apparent manifestation of structural phase transitions in the MHP CsPbBr3. Macroscopically, CsPbBr3 undergoes phase transitions between a cubic (α), tetragonal (β), and orthorhombic (γ) phase with decreasing temperature. However, microscopically, it has been argued that only the γ phase exists, while the other phases exist as averages over length and time scales within distinct temperature ranges. Here, direct proof is provided for this conjecture by analyzing both theoretical diffraction patterns and the evolution of the tilting angle of the PbBr6 octahedra from molecular dynamics simulations. Moreover, sound agreement between experimental and theoretical Raman spectra allowed to identify the Raman active phonon modes and to investigate their frequency as a function of temperature. As such, this work increases the understanding of the structure and lattice dynamics of CsPbBr3 and similar MHPs.

Monolayer (ML) transition-metal dichalcogenides (TMDs) have attracted a lot of research interest in recent years due to their many interesting properties as well as their application promises. Depending on the specific combinations of metals (e.g., Mo and W) with chalcogen elements (e.g., S, Se, and Te), binary TMDs exhibit a wide spectrum of physical characteristics, e.g., from metal to semiconductor and/or superconductor. Extension from binary to ternary compounds and alloys may offer even wider variations of properties and are thus of interest from both fundamental and practical points of view. In this work, we substitute Mo for niobium (Nb) and rhenium (Re) in ML MoSe2 during molecular-beam epitaxy and probe their effects on structural and electrical properties. We find that low-concentration Nb and Re in ML-MoSe2 are both shallow dopants, with Re being an electron donor and Nb acceptor, respectively. By changing Nb(Re)/Mo flux ratios, we can effectively tune the Fermi level by varying electron or hole concentrations in MoSe2. On the other hand, both Nb and Re are found to cause mirror-twin domain boundary defects to proliferate in MoSe2.

Zirconium alloys are critical material components of systems subjected to harsh environments such as high temperatures, irradiation, and corrosion. When exposed to water in high temperature environments, these alloys can thermo-mechanically degrade by forming hydrides that have a crystalline structure that is different from that of zirconium. Cracks can nucleate near these hydrides; hence, these hydrides are a direct link to fracture failure and overall large inelastic strain deformation modes. To fundamentally understand and predict these microstructural failure modes, we interrogated a finite-element database that was deterministically tailored and generated for large strain-dislocation-density crystalline plasticity and fracture modes. A database of 210 simulations was created to randomly sample from a group of microstructural fingerprints that encompass hydride volume fraction, hydride orientation, grain orientation, hydride length, and hydride spacing for a hydride that is physically representative of an aggregate of a hydride population. Machine learning approaches were then used to understand, identify, and characterize the dominant microstructural mechanisms and characteristics. We first used fat-tailed Cauchy distributions to determine the extreme events. A multilayer perceptron was used to learn the mechanistic characteristics of the material response to predefined strain levels and accurately determine the critical fracture stress response and the accumulated shear slips in critical regions. The predictions indicate that hydride volume fraction, a population-level parameter, had a significant effect on localized parameters, such as fracture stress distribution regions, and on the accumulated immobile dislocation densities both within the face centered cubic hydrides and the hexagonal cubic packed h.c.p. matrix.

For applications such as spin accumulation sensors for next-generation hard disk drive read heads, and for fundamental research, it is desirable to increase the spin signal in metallic non-local spin valves, which are central devices in spintronics. To this end, here, we report on the integration of high-spin-polarization Co–Fe binary alloy ferromagnetic injectors and detectors in Al-based non-local spin valves. Room-temperature deposition on amorphous substrates from an alloy target is shown to generate smooth, polycrystalline (110-textured), solid-solution body-centered-cubic Co75Fe25 films, which we characterize by energy dispersive x-ray spectroscopy, x-ray diffraction, x-ray reflectivity, atomic force microscopy, and electronic transport. Simple integration into transparent-interface Al non-local spin valves is then shown to realize up to a factor of ∼5 enhancement of the spin signal relative to Co, with full quantitative analysis yielding strikingly temperature-independent current spin polarizations exceeding 60%. We make a detailed quantitative comparison of these values with prior literature, concluding that Co–Fe alloys present a remarkably facile route to higher spin polarization and spin signals in non-local spin valves, with minimal barrier to adoption.

Although vanadium compounds are well recognized for their ability to change from insulator to metal, they may also be used therapeutically to address significant medical issues. In this study, we used vanadium oxide thin films synthesized by the pulsed laser deposition (PLD) technique to examine human stem cells generated from bone marrow. According to x-ray reflectivity (XRR) measurements, the films’ thickness ranged from 6 to 26 nm. The water contact angle method has been employed to probe the surface energy and wettability of the films, which influence the cell behavior significantly. We also used a variety of techniques, such as differentiation staining, phase contrast microscopy, and real-time reverse transcription-polymerase chain reaction (RT-PCR), to examine the growth, adhesion, proliferation, and differentiation of human bone marrow mesenchymal stem cells (hBMMSCs) on these oxide films over time. Our results indicated that vanadium oxide films alter hBMMSCs adhesion and growth and affect their differentiation. The application of VOx films in biological and medical materials, as well as future research on cells, is all made possible by these findings, which also improve our understanding of the biological actions of vanadium compounds.