Metastable hexagonal close-packed palladium hydride in liquid cell TEM.

Metastable phases-kinetically favoured structures-are ubiquitous in nature1,2. Rather than forming thermodynamically stable ground-state structures, crystals grown from high-energy precursors often initially adopt metastable structures depending on the initial conditions, such as temperature, pressure or crystal size1,3,4. As the crystals grow further, they typically undergo a series of transformations from metastable phases to lower-energy and ultimately energetically stable phases1,3,4. Metastable phases sometimes exhibit superior physicochemical properties and, hence, the discovery and synthesis of new metastable phases are promising avenues for innovations in materials science1,5. However, the search for metastable materials has mainly been heuristic, performed on the basis of experiences, intuition or even speculative predictions, namely 'rules of thumb'. This limitation necessitates the advent of a new paradigm to discover new metastable phases based on rational design. Such a design rule is embodied in the discovery of a metastable hexagonal close-packed (hcp) palladium hydride (PdHx) synthesized in a liquid cell transmission electron microscope. The metastable hcp structure is stabilized through a unique interplay between the precursor concentrations in the solution: a sufficient supply of hydrogen (H) favours the hcp structure on the subnanometre scale, and an insufficient supply of Pd inhibits further growth and subsequent transition towards the thermodynamically stable face-centred cubic structure. These findings provide thermodynamic insights into metastability engineering strategies that can be deployed to discover new metastable phases.

Defect Engineering of Metal Halide Perovskite Nanocrystals via Spontaneous Diffusion of Ag Nanocrystals.

Perovskite nanocrystals (NCs) have emerged as a promising building block for the fabrication of optic-/optoelectronic-/electronic devices owing to their superior characteristics, such as high absorption coefficient, rapid ion mobilities, and tunable energy levels. However, their low structural stability and poor surface passivation have restricted their application to next-generation devices. Herein, a drug delivery system (DDS)-inspired post-treatment strategy is reported for improving their structural stability by doping of Ag into CsPbBr3 (CPB) perovskite NCs; delivery to damaged sites can promote their structural recovery slowly and uniformly, averting the permanent loss of their intrinsic characteristics. Ag NCs are designed through surface-chemistry tuning and structural engineering to enable their circulation in CPB NC dispersions, followed by their delivery to the CPB NC surface, defect-site recovery, and defect prevention. The perovskite-structure healing process through the DDS-type process (with Ag NCs as the drug) is analyzed by a combination of theoretical calculations (with density functional theory) and experimental analyses. The proposed DDS-inspired healing strategy significantly enhances the optical properties and stability of perovskite NCs, enabling the fabrication of white light-emitting diodes.

Self-Assembly of Pulverized Nanoparticles: An Approach to Realize Large-Capacity, Long-Lasting, and Ultra-Fast-Chargeable Na-Ion Batteries.

The fabrication of battery anodes simultaneously exhibiting large capacity, fast charging capability, and high cyclic stability is challenging because these properties are mutually contrasting in nature. Here, we report a rational strategy to design anodes outperforming the current anodes by simultaneous provision of the above characteristics without utilizing nanomaterials and surface modifications. This is achieved by promoting spontaneous structural evolution of coarse Sn particles to 3D-networked nanostructures during battery cycling in an appropriate electrolyte. The anode steadily exhibits large capacity (∼480 mAhg-1) and energy retention capability (99.9%) during >1500 cycles even at an ultrafast charging rate of 12 690 mAg-1 (15C). The structural and chemical origins of the measured properties are explained using multiscale simulations combining molecular dynamics and density functional theory calculations. The developed method is simple, scalable, and expandable to other systems and provides an alternative robust route to obtain nanostructured anode materials in large quantities.

Evaluation of survival rates of airborne microorganisms on the filter layers of commercial face masks.

After the WHO designated COVID-19 a global pandemic, face masks have become a precious commodity worldwide. However, uncertainty remains around several details regarding face masks, including the potential for transmission of bioaerosols depending on the type of mask and secondary spread by face masks. Thus, understanding the interplay between face mask structure and harmful bioaerosols is essential for protecting public health. Here, we evaluated the microbial survival rate at each layer of commercial of filtering facepiece respirators (FFRs) and surgical masks (SMs) using bacterial bioaerosols. The penetration efficiency of bacterial particles for FFRs was lower than that for SMs; however, the microbial survival rate for all tested masks was >13%, regardless of filtration performance. Most bacterial particles survived in the filter layer (44%-77%) (e.g., the core filtering layer); the outer layer also exhibited significant survival rates (18%-29%). Most notably, survival rates were determined for the inner layers (<1% for FFRs, 3%-16% for SMs), which are in contact with the respiratory tract. Our comparisons of the permeability and survival rate of bioaerosols in each layer will contribute to bioaerosol-face mask research, while also providing information to facilitate the establishment of a mask-reuse protocol.

Phase Controlled Growth of Cd3As2 Nanowires and Their Negative Photoconductivity.

The bottom-up synthesis process often allows the growth of metastable phase nanowires instead of the thermodynamically stable phase. Herein, we synthesized Cd3As2 nanowires with a controlled three-dimensional Dirac semimetal phase using a chemical vapor transport method. Three different phases such as the body centered tetragonal (bct), and two metastable primitive tetragonal (P42/nbc and P42/nmc) phases were identified. The conversion between three phases (bct → P42/nbc → P42/nmc) was achieved by increasing the growth temperature. The growth direction is [110] for bct and P42/nbc and [100] for P42/nmc, corresponding to the same crystallographic axis. Field effect transistors and photodetector devices showed the nearly same electrical and photoelectrical properties for three phases. Differential conductance measurement confirms excellent electron mobility (2 × 104 cm2/(V s) at 10 K). Negative photoconductance was first observed, and the photoresponsivity reached 3 × 104 A/W, which is ascribed to the surface defects acting as trap sites for the photogenerated electrons.

Diffusion Along Dislocations Mitigates Self-Limiting Na Diffusion in Crystalline Sn.

The diffusion of carrier ions in alloying anodes often develops compressive stresses in front of the propagating interface, suppressing the carrier-ion diffusion and limiting their full penetration into alloying anodes during battery cycles. This phenomenon, termed "self-limiting diffusion (SLD)", reduces the rate performance of batteries and hinders the full usage of anode materials. However, SLD is mitigated in some systems where tensile residual stresses develop at the interface, causing them to manifest significantly improved rate performance and energy capacity. Here, a comparative study of LiSi and NaSn systems to elucidate how the differing diffusion kinetics displayed by the two systems can influence SLD behaviors and the rate performance of batteries is performed. Experiments show that the Na diffusion into soft Sn crystals induces tensile stresses near the interface, promoting the nucleation of high-density dislocations. Thus-formed dislocations facilitate Na diffusion at ultrafast rates by providing pathways for dislocation pipe diffusion and alleviate SLD, making crystalline Sn suitable for fast-charging anode material. The outcomes of this study, while filling the knowledge gaps on the reasons for SLD, offer some guidelines for the appropriate choice of potential anode materials with superior rate performance and energy capacity suitable for future applications.

Real-time effect of electron beam on MoS2 field-effect transistors.

Irradiation of MoS2 field-effect transistors (FETs) fabricated on Si/SiO2 substrates with electron beams (e-beams) below 30 keV creates electron-hole pairs (EHP) in the SiO2, which increase the interface trap density (Nit ) and change the current path in the channel, resulting in performance changes. In situ measurements of the electrical characteristics of the FET performed using a nano-probe system mounted inside a scanning electron microscope show that e-beam irradiation enables both multilayer and monolayer MoS2 channels act as conductors. The e-beams mostly penetrate the channel owing to their large kinetic energy, while the EHPs formed in the SiO2 layer can contribute to the conductance by flowing into the MoS2 channel or inducing the gate bias effect. The analysis of the device parameters in the initial state and the vent-evacuation state after e-beam irradiation can clarify the effect of the interplay between the e-beam-induced EHPs and ambient adsorbates on the carrier behavior, which depends on the thickness of the MoS2 layer. DC and low frequency noise analysis reveals that the e-beam-induced EHPs increase Nit from 109-1010 to 1011 cm-2 eV-1 in both monolayer and multilayer devices, while the interfacial Coulomb scattering parameter αSC increases by three times in the monolayer and decreases to one-tenth of its original value in the multilayer. In other words, an MoS2 layer with a thickness of ∼30 nm is less sensitive to adsorbates by surface screening. Thus, the carrier mobility in the monolayer device decreases from 45.7 to 40 cm2 V-1 s-1, while in the 30 nm-thick multilayer device, it increases from 4.9 to 5.6 cm2 V-1 s-1. This is further evidenced by simulations of the distribution of interface traps and channel carriers in the MoS2 FET before and after e-beam irradiation, demonstrating that Coulomb scattering decreases as the effective channel moves away from the interface.

Direct-Contact Microelectrical Measurement of the Electrical Resistivity of a Solid Electrolyte Interface.

Because of its effectiveness in blocking electrons, the solid electrolyte interface (SEI) suppresses decomposition reactions of the electrolyte and contributes to the stability and reversibility of batteries. Despite the critical role of SEI in determining the properties of batteries, the electrical properties of SEI layers have never been measured directly. In this paper, we present the first experimental results of the electrical resistivity of a LiF-rich SEI layer measured using a direct-contact microelectrical device mounted in an electron microscope. Measurements show that the SEI layer exhibits high electrical resistivity (2.3 × 105 Ω·m), which is comparable with those of typical insulating materials. Furthermore, a combined technique of advanced analyses and first-principles calculations show that the SEI layer is mainly composed of amorphous LiF and a minute nanocrystalline Li2CO3 compound. The electronic origin responsible for the high resistivity of the SEI layer is elucidated by calculating the band structures of various Li xF compounds and interpreting their effects on the resistivity. This study explains why SEI can prevent the degradation of electrode materials and consumption of Li ions in the electrolyte and thus can be viewed as a stepping stone for developing highly stable and reversible batteries.

Long-term clinical study and multiscale analysis of in vivo biodegradation mechanism of Mg alloy.

There has been a tremendous amount of research in the past decade to optimize the mechanical properties and degradation behavior of the biodegradable Mg alloy for orthopedic implant. Despite the feasibility of degrading implant, the lack of fundamental understanding about biocompatibility and underlying bone formation mechanism is currently limiting the use in clinical applications. Herein, we report the result of long-term clinical study and systematic investigation of bone formation mechanism of the biodegradable Mg-5wt%Ca-1wt%Zn alloy implant through simultaneous observation of changes in element composition and crystallinity within degrading interface at hierarchical levels. Controlled degradation of Mg-5wt%Ca-1wt%Zn alloy results in the formation of biomimicking calcification matrix at the degrading interface to initiate the bone formation process. This process facilitates early bone healing and allows the complete replacement of biodegradable Mg implant by the new bone within 1 y of implantation, as demonstrated in 53 cases of successful long-term clinical study.

Fabrication of Atom Probe Tomography Specimens from Nanoparticles Using a Fusible Bi-In-Sn Alloy as an Embedding Medium.

We propose a new method for preparing atom probe tomography specimens from nanoparticles using a fusible bismuth-indium-tin alloy as an embedding medium. Iron nanoparticles synthesized by the sodium borohydride reduction method were chosen as a model system. The as-synthesized iron nanoparticles were embedded within the fusible alloy using focused ion beam milling and ion-milled to needle-shaped atom probe specimens under cryogenic conditions. An atom probe analysis revealed boron atoms in a detected iron nanoparticle, indicating that boron from the sodium borohydride reductant was incorporated into the nanoparticle during its synthesis.

Formation of Zintl Ions and Their Configurational Change during Sodiation in Na-Sn Battery.

Despite their large theoretical storage capability, Na-Sn batteries exhibit poor round-trip energy efficiencies as compared to Li-Si batteries. Here, we report the results of a comprehensive study to elucidate how and why Na-Sn batteries exhibit such a low energy efficiency. As a convincing evidence for this behavior, we observed that the resistivity of the Sn anode increased by 8 orders of magnitude during in situ sodiation experiments, which is attributed to the formation of electrically resistive Zintl ions in the sodiated Sn. Continual sodiation induced the development of residual stresses at the Sn anode and caused the distortion of Zintl ions from their ideal configuration. This distortion caused a change in the electronic structure, resulting in the increased resistivity of the sodiated Sn. Our findings offer some solutions that can be used to improve the energy efficiency of Na-Sn batteries.

Bent Polytypic ZnSe and CdSe Nanowires Probed by Photoluminescence.

Nanowires (NWs) have witnessed tremendous development over the past two decades owing to their varying potential applications. Semiconductor NWs often contain stacking faults due to the presence of coexisting phases, which frequently hampers their use. Herein, it is investigated how stacking faults affect the optical properties of bent ZnSe and CdSe NWs, which are synthesized using the vapor transport method. Polytypic zinc blende-wurtzite structures are produced for both these NWs by altering the growth conditions. The NWs are bent by the mechanical buckling of poly(dimethylsilioxane), and micro-photoluminescence (PL) spectra were then collected for individual NWs with various bending strains (0-2%). The PL measurements show peak broadening and red shifts of the near-band-edge emission as the bending strain increases, indicating that the bandgap decreases with increasing the bending strain. Remarkably, the bandgap decrease is more significant for the polytypic NWs than for the single phase NWs. This work provides insights into flexible electronic devices of 1D nanostructures by engineering the polytypic structures.

Observation of partial reduction of manganese in the lithium rich layered oxides, 0.4Li2MnO3-0.6LiNi1/3Co1/3Mn1/3O2, during the first charge.

Lithium-rich layered oxides show promise as high-energy harvesting materials due to their large capacities. However, questions remain regarding the large irreversible loss in capacities for the first charge-discharge cycle due to oxygen removal in lattices related to layered Li2MnO3. Herein we present detailed studies on Li-rich Mn-based layered oxides of 0.4Li2MnO3-0.6LiNi1/3Co1/3Mn1/3O2 (Li-rich NCM) electrochemically activated between 2.5 V and 4.3 or 4.7 V vs. Li+/Li. Electron energy loss spectroscopy (EELS) and X-ray absorption spectroscopy (XAS) revealed unusual manganese reduction after the first charge up to a high voltage of 4.7 V. Moreover, the electronic structure did not fully recover to the original pristine of Mn4+ state after discharge. Interestingly, these phenomena were not limited to a single particle, but were observed across the entire electrode. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and electron dispersive spectra (EDS) also showed a dramatic decline in oxygen content with highly porous morphologies, associated with oxygen vacancy formation following oxidation of O2- ions to O2. Our analysis suggests that an unstable manganese valence state with severe defects due to oxygen vacancies may lead to large irreversible capacity loss during the first charge-discharge cycle of Li-rich layered oxides.

Ultrahigh Tensile Strength Nanowires with a Ni/Ni-Au Multilayer Nanocrystalline Structure.

Superior mechanical properties of nanolayered structures have attracted great interest recently. However, previously fabricated multilayer metallic nanostructures have high strength under compressive load but never reached such high strength under tensile loads. Here, we report that our microalloying-based electrodeposition method creates a strong and stable Ni/Ni-Au multilayer nanocrystalline structure by incorporating Au atoms that makes nickel nanowires (NWs) strongest ever under tensile loads even with diameters exceeding 200 nm. When the layer thickness is reduced to 10 nm, the tensile strength reaches the unprecedentedly high 7.4 GPa, approximately 10 times that of metal NWs with similar diameters, and exceeding that of most metal nanostructures previously reported at any scale.

In Situ Temperature-Dependent Transmission Electron Microscopy Studies of Pseudobinary mGeTe·Bi2Te3 (m = 3-8) Nanowires and First-Principles Calculations.

Phase-change nanowires (NWs) have emerged as critical materials for fast-switching nonvolatile memory devices. In this study, we synthesized a series of mGeTe·Bi2Te3 (GBT) pseudobinary alloy NWs-Ge3Bi2Te6 (m = 3), Ge4Bi2Te7 (m = 4), Ge5Bi2Te8 (m = 5), Ge6Bi2Te9 (m = 6), and Ge8Bi2Te11 (m = 8)-and investigated their composition-dependent thermal stabilities and electrical properties. As m decreases, the phase of the NWs evolves from the cubic (C) to the hexagonal (H) phase, which produces unique superlattice structures that consist of periodic 2.2-3.8 nm slabs for m = 3-8. In situ temperature-dependent transmission electron microscopy reveals the higher thermal stability of the compositions with lower m values, and a phase transition from the H phase into the single-crystalline C phase at high temperatures (400 °C). First-principles calculations, performed for the superlattice structures (m = 1-8) of GBT and mGeTe·Sb2Te3 (GST), show an increasing stability of the H phase (versus the C phase) with decreasing m; the difference in stability being more marked for GBT than for GST. The calculations explain remarkably the phase evolution of the GBT and GST NWs as well as the composition-dependent thermal stabilities. Measurement of the current-voltage curves for individual GBT NWs shows that the resistivity is in the range 3-25 mΩ·cm, and the resistivity of the H phase is lower than that of the C phase, which has been supported by the calculations.

Origin of size dependency in coherent-twin-propagation-mediated tensile deformation of noble metal nanowires.

Researchers have recently discovered ultrastrong and ductile behavior of Au nanowires (NWs) through long-ranged coherent-twin-propagation. An elusive but fundamentally important question arises whether the size and surface effects impact the twin propagation behavior with a decreasing diameter. In this work, we demonstrate size-dependent strength behavior of ultrastrong and ductile metallic NWs. For Au, Pd, and AuPd NWs, high ductility of about 50% is observed through coherent twin propagation, which occurs by a concurrent reorientation of the bounding surfaces from {111} to {100}. Importantly, the ductility is not reduced with an increase in strength, while the twin propagation stress dramatically increases with decreasing NW diameter from 250 to 40 nm. Furthermore, we find that the power-law exponent describing the twin propagation stress is fundamentally different from the exponent describing the size-dependence of the yield strength. Specifically, the inverse diameter-dependence of the twin propagation stress is directly attributed to surface reorientation, which can be captured by a surface energy differential model. Our work further highlights the fundamental role that surface reorientations play in enhancing the size-dependent mechanical behavior and properties of metal NWs that imply the feasibility of high efficiency mechanical energy storage devices suggested before.

Polymorphism of GeSbTe superlattice nanowires.

Scaling-down of phase change materials to a nanowire (NW) geometry is critical to a fast switching speed of nonvolatile memory devices. Herein, we report novel composition-phase-tuned GeSbTe NWs, synthesized by a chemical vapor transport method, which guarantees promising applications in the field of nanoscale electric devices. As the Sb content increased, they showed a distinctive rhombohedral-cubic-rhombohedral phase evolution. Remarkable superlattice structures were identified for the Ge(8)Sb(2)Te(11), Ge(3)Sb(2)Te(6), Ge(3)Sb(8)Te(6), and Ge(2)Sb(7)Te(4) NWs. The coexisting cubic-rhombohedral phase Ge(3)Sb(2)Te(6) NWs exhibited an exclusively uniform superlattice structure consisting of 2.2 nm period slabs. The rhombohedral phase Ge(3)Sb(8)Te(6) and Ge(2)Sb(7)Te(4) NWs adopted an innovative structure; 3Sb(2) layers intercalated the Ge(3)Sb(2)Te(6) and Ge(2)Sb(1)Te(4) domains, respectively, producing 3.4 and 2.7 nm period slabs. The current-voltage measurement of the individual NW revealed that the vacancy layers of Ge(8)Sb(2)Te(11) and Ge(3)Sb(2)Te(6) decreased the electrical conductivity.

High performance enzyme fuel cells using a genetically expressed FAD-dependent glucose dehydrogenase α-subunit of Burkholderia cepacia immobilized in a carbon nanotube electrode for low glucose conditions.

FAD-dependent glucose dehydrogenase (FAD-GDH) of Burkholderia cepacia was successfully expressed in Escherichia coli and subsequently purified in order to use it as an anode catalyst for enzyme fuel cells. The purified enzyme has a low Km value (high affinity) towards glucose, which is 463.8 µM, up to 2-fold exponential range lower compared to glucose oxidase. The heterogeneous electron transfer coefficient (Ks) of FAD-GDH-menadione on a glassy carbon electrode was 10.73 s(-1), which is 3-fold higher than that of GOX-menadione, 3.68 s(-1). FAD-GDH was able to maintain its native glucose affinity during immobilization in the carbon nanotube and operation of enzyme fuel cells. FAD-GDH-menadione showed 3-fold higher power density, 799.4 ± 51.44 µW cm(-2), than the GOX-menadione system, 308.03 ± 17.93 µW cm(-2), under low glucose concentration, 5 mM, which is the concentration in normal physiological fluid.

In situ heating transmission electron microscopy observation of nanoeutectic lamellar structure in Sn-Ag-Cu alloy on Au under-bump metallization.

We investigated the microstructural evolution of Sn(96.4)Ag(2.8)Cu(0.8) solder through in situ heating transmission electron microscopy observations. As-soldered bump consisted of seven layers, containing the nanoeutectic lamella structure of AuSn and Au5Sn phases, and the polygonal grains of AuSn2 and AuSn4, on Au-plated Cu bond pads. Here, we found that there are two nanoeutectic lamellar layers with lamella spacing of 40 and 250 nm. By in situ heating above 140°C, the nanoeutectic lamella of AuSn and Au5Sn was decomposed with structural degradation by sphering and coarsening processes of the lamellar interface. At the third layer neighboring to the lamella layer, on the other hand, Au5Sn particles with a zig-zag shape in AuSn matrix became spherical and were finally dissipated in order to minimize the interface energy between two phases. In the other layers except both lamella layers, polycrystal grains of AuSn2 and AuSn4 grew by normal grain growth during in situ heating. The high interface energy of nanoeutectic lamella and polygonal nanograins, which are formed by rapid solidification, acted as a principal driving force on the microstructural change during the in situ heating.