Li iontronics in single-crystalline T-Nb2O5 thin films with vertical ionic transport channels.

The niobium oxide polymorph T-Nb2O5 has been extensively investigated in its bulk form especially for applications in fast-charging batteries and electrochemical (pseudo)capacitors. Its crystal structure, which has two-dimensional (2D) layers with very low steric hindrance, allows for fast Li-ion migration. However, since its discovery in 1941, the growth of single-crystalline thin films and its electronic applications have not yet been realized, probably due to its large orthorhombic unit cell along with the existence of many polymorphs. Here we demonstrate the epitaxial growth of single-crystalline T-Nb2O5 thin films, critically with the ionic transport channels oriented perpendicular to the film's surface. These vertical 2D channels enable fast Li-ion migration, which we show gives rise to a colossal insulator-metal transition, where the resistivity drops by 11 orders of magnitude due to the population of the initially empty Nb 4d0 states by electrons. Moreover, we reveal multiple unexplored phase transitions with distinct crystal and electronic structures over a wide range of Li-ion concentrations by comprehensive in situ experiments and theoretical calculations, which allow for the reversible and repeatable manipulation of these phases and their distinct electronic properties. This work paves the way for the exploration of novel thin films with ionic channels and their potential applications.

Heterointerface Effect on Two-Step Nucleation Mechanism of Bi Particles.

The nucleation and crystallization of Bi particles on two matrices, crystalline bismuth sulfide (c-Bi2S3) and amorphized bismuth titanium oxide (a-Bi12TiO20), were studied by using in situ transmission electron microscopy (TEM) analysis. The atomic structures of the Bi particles were monitored by acquiring high-resolution TEM images in real time. The Bi particles were grown on c-Bi2S3 and a-Bi12TiO20 via a two-step nucleation mechanism; dense liquid clusters were clearly observed at the initial stage of nucleation, and the coalescence of clusters was frequently observed during the growth. However, the nucleation and crystallization behaviors of Bi particles were governed by the matrix; in particular, the evolution of their morphology and atomic structure was confined on c-Bi2S3 but free from matrix effects on a-Bi12TiO20. The matrix effect on the two-step nucleation mechanism was demonstrated from a thermodynamic point of view.

Single- or double-membrane-bound vesicles and P, Ca, and Fe-containing granules in Xanthomonas citri cultured on a solid medium.

The organelle-like structures of Xanthomonas citri, a bacterial pathogen that causes citrus canker, were investigated using an analytical transmission electron microscope. After high-pressure freezing, the bacteria were then freeze-substituted for imaging and element analysis. Miniscule electron-dense structures of varying shapes without a membrane enclosure were frequently observed near the cell poles in a 3-day culture. The bacteria formed cytoplasmic electron-dense spherical structures measuring approximately 50 nm in diameter. Furthermore, X. citri produced electron-dense or translucent ellipsoidal intracellular or extracellular granules. Single- or double-membrane-bound vesicles, including outer-inner membrane vesicles, were observed both inside and outside the cells. Most cells had been lysed in the 3-week X. citri culture, but they harbored one or two electron-dense spherical structures. Contrast-inverted scanning transmission electron microscopy images revealed distinct white spherical structures within the cytoplasm of X. citri. Likewise, energy-dispersive X-ray spectrometry showed the spatial heterogeneity and co-localization of phosphorus, oxygen, calcium, and iron only in the cytoplasmic electron-dense spherical structures, thus corroborating the nature of polyphosphate granules.

Adatom Doping of Transition Metals in ReSe2 Nanosheets for Enhanced Electrocatalytic Hydrogen Evolution Reaction.

Two-dimensional Re dichalcogenide nanostructures are promising electrocatalysts for the hydrogen evolution reaction (HER). Herein, we report the adatom doping of various transition metals (TM = Mn, Fe, Co, Ni, and Cu) in ReSe2 nanosheets synthesized using a solvothermal reaction. As the atomic number of TM increases from Mn to Cu, the adatoms on Re sites become more favored over the substitution. In the case of Ni, the fraction of adatoms reaches 90%. Ni doping resulted in the most effective enhancement in the HER catalytic performance, which was characterized by overpotentials of 82 and 109 mV at 10 mA cm-2 in 0.5 M H2SO4 and 1 M KOH, respectively, and the Tafel slopes of 54 and 81 mV dec-1. First-principles calculations predicted that the adatom doping structures (TMs on Re sites) have higher catalytic activity compared with the substitution ones. The adsorbed H atoms formed a midgap hybridized state via direct bonding with the orbitals of TM adatom. The present work provides a deeper understanding into how TM doping can provide the catalytically active sites in these ReSe2 nanosheets.

An unexpected surfactant role of immiscible nitrogen in the structural development of silver nanoparticles: an experimental and numerical investigation.

Artificially designing the crystal orientation and facets of noble metal nanoparticles is important to realize unique chemical and physical features that are very different from those of noble metals in bulk geometries. However, relative to their counterparts synthesized in wet-chemical processes, vapor-depositing noble metal nanoparticles with the desired crystallographic features while avoiding any notable impurities is quite challenging because this task requires breaking away from the thermodynamically favorable geometry of nanoparticles. We used plasma-generated N atoms as a surface-active agent, a so-called surfactant, to control the structural development of Ag nanoparticles supported on a chemically heterogeneous ZnO substrate. The N-surfactant-facilitated sputter deposition provided strong selectivity for crystalline orientation and facets, leading to a highly flattened nanoparticle shape that clearly deviated from the energetically favorable spherical polyhedra, due to the drastic decreases in the surface free energies of Ag nanoparticles in the presence of the N surfactant. The Ag nanoparticles successively developed a nearly unidirectional (111) orientation aligned by stimulating the crystalline coupling of Ag along the orientation of the ZnO substrate. The experimental and simulation results not only offer new insights into the advantages of N as a surfactant for the orientation and shape-controlled synthesis of Ag nanoparticles via sputter deposition but also provide the first solid evidence validating that immiscible, nonresidual gaseous surfactants can be used in the vapor deposition processes of noble metal nanoparticles to manipulate their surface free energies.

Author Correction: Lattice strain-enhanced exsolution of nanoparticles in thin films.

The original version of this Article contained an error in the Data Availability section, which incorrectly read 'The data that support the findings of this study are available from the corresponding authors upon request.' The correct version replaces this sentence with 'The research data underpinning this publication can be accessed at https://doi.org/10.17630/21d12144-58ef-4f82-acd0-ba3c9a44ed72'. This has been corrected in both the PDF and HTML versions of the Article.

Lattice strain-enhanced exsolution of nanoparticles in thin films.

Nanoparticles formed on oxide surfaces are of key importance in many fields such as catalysis and renewable energy. Here, we control B-site exsolution via lattice strain to achieve a high degree of exsolution of nanoparticles in perovskite thin films: more than 1100 particles µm-2 with a particle size as small as ~5 nm can be achieved via strain control. Compressive-strained films show a larger number of exsolved particles as compared with tensile-strained films. Moreover, the strain-enhanced in situ growth of nanoparticles offers high thermal stability and coking resistance, a low reduction temperature (550 °C), rapid release of particles, and wide tunability. The mechanism of lattice strain-enhanced exsolution is illuminated by thermodynamic and kinetic aspects, emphasizing the unique role of the misfit-strain relaxation energy. This study provides critical insights not only into the design of new forms of nanostructures but also to applications ranging from catalysis, energy conversion/storage, nano-composites, nano-magnetism, to nano-optics.

Origins of High Performance and Degradation in the Mixed Perovskite Solar Cells.

The origins of the high device performance and degradation in the air are the greatest issues for commercialization of perovskite solar cells. Here this study investigates the possible origins of the mixed perovskite cells by monitoring defect states and compositional changes of the perovskite layer over the time. The results of deep-level transient spectroscopy analysis reveal that a newly identified defect formed by Br atoms exists at deep levels of the mixed perovskite film, and its defect state shifts when the film is aged in the air. The change of the defect state is originated from loss of the methylammonium molecules of the perovskite layer, which results in decreased JSC , deterioration of the power conversion efficiency and long-term stability of perovskite solar cells. The results provide a powerful strategy to diagnose and manage the efficiency and stability of perovskite solar cells.

Introduction to the standard reference data of electron energy loss spectra and their database: eel.geri.re.kr.

Electron energy loss spectroscopy (EELS) is an analytical technique that can provide the structural, physical and chemical information of materials. The EELS spectra can be obtained by combining with TEM at sub-nanometer spatial resolution. However, EELS spectral information can't be obtained easily because in order to interpret EELS spectra, we need to refer to and/or compare many reference data with each other. And in addition to that, we should consider the different experimental variables used to produce each data. Therefore, reliable and easily interpretable EELS standard reference data are needed.Our Electron Energy Loss Data Center (EELDC) has been designated as National Standard Electron Energy Loss Data Center No. 34 to develop EELS standard reference (SR) data and to play a role in dissemination and diffusion of the SR data to users. EELDC has developed and collected EEL SR data for the materials required by major industries and has a total of 82 EEL SR data. Also, we have created an online platform that provides a one-stop-place to help users interpret quickly EELS spectra and get various spectral information. In this paper, we introduce EEL SR data, the homepage of EELDC and how to use them.

Switchable ferroelectric photovoltaic effects in epitaxial h-RFeO3 thin films.

Ferroelectric photovoltaics (FPVs) have drawn much attention owing to their high stability, environmental safety, and anomalously high photovoltages, coupled with reversibly switchable photovoltaic responses. However, FPVs suffer from extremely low photocurrents, which is primarily due to their wide band gaps. Here, we present a new class of FPVs by demonstrating switchable ferroelectric photovoltaic effects and narrow band-gap properties using hexagonal ferrite (h-RFeO3) thin films, where R denotes rare-earth ions. FPVs with narrow band gaps suggest their potential applicability as photovoltaic and optoelectronic devices. The h-RFeO3 films further exhibit reasonably large ferroelectric polarizations (4.7-8.5 µC cm-2), which possibly reduces a rapid recombination rate of the photo-generated electron-hole pairs. The power conversion efficiency (PCE) of h-RFeO3 thin-film devices is sensitive to the magnitude of polarization. In the case of the h-TmFeO3 (h-TFO) thin film, the measured PCE is twice as large as that of the BiFeO3 thin film, a prototypic FPV. The effect of electrical fatigue on FPV responses has been further investigated. This work thus demonstrates a new class of FPVs towards high-efficiency solar cell and optoelectronic applications.

Single-Crystalline Nanobelts Composed of Transition Metal Ditellurides.

Following the celebrated discovery of graphene, considerable attention has been directed toward the rich spectrum of properties offered by van der Waals crystals. However, studies have been largely limited to their 2D properties due to lack of 1D structures. Here, the growth of high-yield, single-crystalline 1D nanobelts composed of transition metal ditellurides at low temperatures (T ≤ 500 °C) and in short reaction times (t ≤ 10 min) via the use of tellurium-rich eutectic metal alloys is reported. The synthesized semimetallic 1D products are highly pure, stoichiometric, structurally uniform, and free of defects, resulting in high electrical performances. Furthermore, complete compositional tuning of the ternary ditelluride nanobelts is achieved with suppressed phase separation, applicable to the creation of unprecedented low-dimensional materials/devices. This approach may inspire new growth/fabrication strategies of 1D layered nanostructures, which may offer unique properties that are not available in other materials.

Tunable polaron-induced coloration of tungsten oxide via a multi-step control of the physicochemical property for the detection of gaseous F.

The tunable polaron effect of amorphous tungsten oxide on FTO substrates has been used to detect fluorine in the gas phase via photochemical and gasochromic reactions. By combining photochemical (UV exposure under an H2 atomsphere) and gasochromic (XeF2 exposure) reactions, the detection of gaseous fluorine using amorphous tungsten oxide is described. The effective hydrogenation of WO3 was achieved using UV/H2 exposure to prepare hydrogenated tungsten oxide (H-WO3-x) upon activating the strong polaron-coupling to infrared (IR) light to decrease IR transmission from 70 to 20% at 1000 nm wavelength. This is explained by creation of W 5d unpaired electrons excited by band-edge defect states or W5+ states. The H-WO3-x lattice structure was maintained as an amorphous structure and found to have hydrogen-associated shallow- and oxygen vacancy-associated deep-trap levels with a moderate enhancement of the n-type characteristic. The gasochromic reaction takes place within tens of seconds at room temperature upon exposure to XeF2 gas leading to atomic F insertion. Fluorine, which is one of the most electronegative materials, is combined with the W5+ and W6+ in H-WO3-x to remove H to form volatile HF vapor and the formation of W-F bonds. The global incorporation of fluorine effectively turns H-WO3-x into F-WO3-x structures and deactivates the polaron-IR coupling (IR transmission change from 20 to 70%) since all the band-edge defect states are passivated upon F insertion with a strong n-doping effect. Therefore, this approach, entirely processed at room temperature, is highly applicable to fluorine detecting sensors and devices utilizing the polaron-IR coupling effect.

Enhanced Switchable Ferroelectric Photovoltaic Effects in Hexagonal Ferrite Thin Films via Strain Engineering.

Ferroelectric photovoltaics (FPVs) are being extensively investigated by virtue of switchable photovoltaic responses and anomalously high photovoltages of ∼104 V. However, FPVs suffer from extremely low photocurrents due to their wide band gaps (Eg). Here, we present a promising FPV based on hexagonal YbFeO3 (h-YbFO) thin-film heterostructure by exploiting its narrow Eg. More importantly, we demonstrate enhanced FPV effects by suitably exploiting the substrate-induced film strain in these h-YbFO-based photovoltaics. A compressive-strained h-YbFO/Pt/MgO heterojunction device shows ∼3 times enhanced photovoltaic efficiency than that of a tensile-strained h-YbFO/Pt/Al2O3 device. We have shown that the enhanced photovoltaic efficiency mainly stems from the enhanced photon absorption over a wide range of the photon energy, coupled with the enhanced polarization under a compressive strain. Density functional theory studies indicate that the compressive strain reduces Eg substantially and enhances the strength of d-d transitions. This study will set a new standard for determining substrates toward thin-film photovoltaics and optoelectronic devices.

Wafer-Scale Integration of Highly Uniform and Scalable MoS2 Transistors.

Molybdenum disulfide with atomic-scale flatness has application potential in high-speed and low-power logic devices owing to its scalability and intrinsic high mobility. However, to realize viable technologies based on two-dimensional materials, techniques that enable their large-area growth with high quality and uniformity on wafer cale is a prerequisite. Here, we provide a route toward highly uniform growth of a wafer-scale, four-layered MoS2 film on a 2 in. substrate via a sequential process consisting of the deposition of a molybdenum trioxide precursor film by sputtering followed by postsulfurization using a chemical vapor deposition process. Spatial spectroscopic analyses by Raman and PL mapping validated that the as-synthesized MoS2 thin films exhibit high uniformity on a 2 in. sapphire substrate. The highly uniform MoS2 layers allow a successful integration of devices based on ∼1200 MoS2 transistor arrays with a yield of 95% because of their extreme homogeneity on Si wafers. Moreover, a pulse electrical measurement technique enabled investigation of the inherent physical properties of the atomically thin MoS2 layers by minimizing the charge-trapping effect. Such a facile synthesis method can be possibly applied to other 2D transition metal dichalcogenides to ultimately realize the chip integration of device architectures with all 2D-layered building blocks.

Forming-less and Non-Volatile Resistive Switching in WOX by Oxygen Vacancy Control at Interfaces.

Resistive switching devices are recognized as candidates for next-generation memory devices in that they can replace conventional memory devices. In these devices, a WOX film deposited by RF magnetron sputtering with a significant number of oxygen vacancies exhibits a resistive switching property and does not involve the use of a forming process. The resistive switching mechanism involves the hopping of electrons through the sub-band states of the oxygen vacancies in E-field-driven electromigration. X-ray photoemission spectroscopy, ultra-violet photoemission spectroscopy, and transmission electron microscopy-electron energy loss spectroscopy were performed to analyze local variations in the O-vacancies and in the electronic band structure of a WOX thin film. The band structure is responsible for the correlation between the motion of the electrons under the interface effect at the electrodes with the change in the resistance and the bias-polarity dependence of the I-V property of the device. The optimized metal-insulator-metal structure (Pt/WOX/Au), which has an asymmetric electrode and many oxygen vacancies, gives rise to excellent resistive-switching properties with a high on/off ratio on the order of 105 times, a low set voltage of <0.34 V, and a uniform DC cyclic performance in the order of 1500 cycles at room temperature. These specifications can be further adopted for application to non-volatile memory-device applications.

Direct Observation of Inherent Atomic-Scale Defect Disorders responsible for High-Performance Ti1-x Hfx NiSn1-y Sby Half-Heusler Thermoelectric Alloys.

Structural defects often dominate the electronic- and thermal-transport properties of thermoelectric (TE) materials and are thus a central ingredient for improving their performance. However, understanding the relationship between TE performance and the disordered atomic defects that are generally inherent in nanostructured alloys remains a challenge. Herein, the use of scanning transmission electron microscopy to visualize atomic defects directly is described and disordered atomic-scale defects are demonstrated to be responsible for the enhancement of TE performance in nanostructured Ti1-x Hfx NiSn1-y Sby half-Heusler alloys. The disordered defects at all atomic sites induce a local composition fluctuation, effectively scattering phonons and improving the power factor. It is observed that the Ni interstitial and Ti,Hf/Sn antisite defects are collectively formed, leading to significant atomic disorder that causes the additional reduction of lattice thermal conductivity. The Ti1-x Hfx NiSn1-y Sby alloys containing inherent atomic-scale defect disorders are produced in one hour by a newly developed process of temperature-regulated rapid solidification followed by sintering. The collective atomic-scale defect disorder improves the zT to 1.09 ± 0.12 at 800 K for the Ti0.5 Hf0.5 NiSn0.98 Sb0.02 alloy. These results provide a promising avenue for improving the TE performance of state-of-the-art materials.

Creation of a Short-Range Ordered Two-Dimensional Electron Gas Channel in Al2O3/In2O3 Interfaces.

The tuning of electrical properties in oxides via surface and interfacial two-dimensional electron gas (2DEG) channels is of great interest, as they reveal the extraordinary transition from insulating or semiconducting characteristics to metallic conduction or superconductivity enabled by the ballistic transport of spatially confined electrons. However, realizing the practical aspects of this exotic phenomenon toward short-range ordered and air-stable 2DEG channels remains a great challenge. At the heterointerface formed after deposition of an Al2O3 layer on a nanocrystalline In2O3 layer, a dramatic improvement in carrier conduction equivalent to metallic conduction is obtained. A conductivity increase by a factor of 1013 times that in raw In2O3, a sheet resistance of 850 Ω/cm2, and a room temperature Hall mobility of 20.5 cm2 V-1 s-1 are obtained, which are impossible to achieve by tuning each layer individually. The physicochemical origin of metallic conduction is mainly ascribed to the 2D interfacially confined O-vacancies and semimetallic nanocrystalline InOx (x < 2) phases by the clustered self-doping effect caused by O-extraction from In2O3 to the Al2O3 phase during ALD. Unlike other submetallic oxides, this 2D channel is air-stable by complete Al2O3 passivation and thereby promises applicability for implementation in devices.

Effect of Nb Doping on Chemical Sensing Performance of Two-Dimensional Layered MoSe2.

Here, we report that Nb doping of two-dimensional (2D) MoSe2 layered nanomaterials is a promising approach to improve their gas sensing performance. In this study, Nb atoms were incorporated into a 2D MoSe2 host matrix, and the Nb doping concentration could be precisely controlled by varying the number of Nb2O5 deposition cycles in the plasma enhanced atomic layer deposition process. At relatively low Nb dopant concentrations, MoSe2 showed enhanced device durability as well as NO2 gas response, attributed to its small grains and stabilized grain boundaries. Meanwhile, an increase in the Nb doping concentration deteriorated the NO2 gas response. This might be attributed to a considerable increase in the number of metallic NbSe2 regions, which do not respond to gas molecules. This novel method of doping 2D transition metal dichalcogenide-based nanomaterials with metal atoms is a promising approach to improve the performance such as stability and gas response of 2D gas sensors.

Evaluation of bone healing using rhBMP-2 soaked hydroxyapatite in ridge augmentation: a prospective observational study.

BACKGROUND: The goal of this study is to evaluate complication and effectiveness of alveolar ridge augmentations using a hydroxyapatite-based alloplastic bony substitute with rhBMP-2. METHODS: A total of 10 patients (4 males, 6 females; 58.5 ± 8.6 years) participated in this clinical research. Alveolar ridge augmentations were performed in edentulous (4 maxillary posterior, 5 mandibular posterior, and 1 mandibular anterior) regions. Anorganic bovine bone (ABB; Bio-Oss®, Geistlich Pharma AG, Wolhusen, Switzerland) was used as the bone graft material in the control group (n = 5)) while hydroxyapatite-based alloplastic bony substitute with rhBMP-2(HA+rhBMP-2; NOVOSIS®-Dent, CGBio Inc., Seongnam, Korea) was used in the experimental group (n = 5). In order to evaluate relative changes in bone volume and resorption rate of the bone graft material, CBCT radiographs were taken immediately and at 4 months after the bone graft in all subjects. Among the 10 patients, 8 received dental implants in Seoul National University Bundang Hospital, while the others received in local clinics. Bone specimens for further histomorphometric examinations were gained from these 8 patients using trephine burs during the implant placements. Clinical, radiographic, and histomorphometric evaluations were focused because of the small sample size. RESULTS: When CBCT radiographs were compared between immediately and at 4.07 ± 0.13 months after the bone graft, both alveolar bone widths (ABB 2.52 ± 0.18 mm, HA+rhBMP-2 1.75 ± 0.85 mm) and heights (ABB 1.68 ± 0.17 mm, HA+rhBMP-2 1.57 ± 0.28 mm) increased in the two groups. Resorption rates of transplanted bone graft material in the alveolar bone widths and heights were (ABB 29.7 ± 8.8%, HA+rhBMP-2 31.5 ± 7.4%) and (ABB 39.2 ± 21.8%, HA+rhBMP-2 52.6 ± 6.5%), respectively. Histomorphometrically, ABB group showed bone formation via osteoconduction and HA+rhBMP-2 group via osteoinduction. HA+rhBMP-2 group showed more bone formation around the bone graft materials than the ABB group. Postoperative complications were not found in all subjects. CONCLUSIONS: Our study had following conclusions: (1) Ridge augmentations using HA+rhBMP-2 could be clinically useful to supplement implant placements in edentulous regions. (2) Serious postoperative complications related to the graft material did not occur.

In situ TEM observation on the interface-type resistive switching by electrochemical redox reactions at a TiN/PCMO interface.

The interface-type resistive switching devices exhibiting bipolar and multi-level resistive switching have been considered as the key component for neuromorphic device applications. To directly observe the microscopic details of underlying electrochemical redox reactions occuring at a metal/oxide interface, we implemented in situ resistive switching of TiN/Pr0.7Ca0.3MnO3 (PCMO)/Pt junction devices in a transmission electron microscope (TEM). The in situ TEM observations directly show that an intermediate reaction layer (TiOxNy), growing and shrinking in the thickness range of a few nanometers at the TiN/PCMO interface in response to the applied voltage, mainly determines the device resistance by limiting the transport of charge carriers via the Poole-Frenkel conduction mechanism. A detailed analysis of in situ TEM observations demonstrates that electrochemical redox reactions at the TiN/PCMO interface are facilitated by the electric field driven drift of oxygen as well as Ti ions with a much stronger influence of the oxygen ions. As such, the reaction kinetics are governed by the electric field acting across the TiOxNy reaction layer. This layer defines the critical field for the onset of switching, which is measured to be of the order of 106 V cm-1, a typical value at which the ionic drift velocity starts increasing exponentially with the field according to the nonlinear ionic drift model. The present results indicate that understanding the nature of the electric field driven drift of ions in a nanoscale solid electrolyte is a key to the precise control of the resistive switching of metal/insulator/metal junction devices via voltage stimulations.