Direct Atomic-Scale Observation of Ultrasmall Ag Nanowires that Exhibit fcc, bcc, and hcp Structures under Bending.

Metals usually have three crystal structures: face-centered cubic (fcc), body-centered cubic (bcc), and hexagonal-close packed (hcp) structures. Typically, metals exhibit only one of these structures at room temperature. Mechanical processing can cause phase transition in metals, however, metals that exhibit all the three crystal structures have rarely been approached, even when hydrostatic pressure or shock conditions are applied. Here, through in situ observation of the atomic-scale bending and tensile process of ∼5 nm-sized Ag nanowires (NWs), we show that bending is an effective method to facilitate fcc-structured Ag to access all the above-mentioned structures. The process of transitioning the fcc structure into a bcc structure, then into an hcp structure, and finally into a re-oriented fcc structure under bending has been witnessed in its entirety. This re-oriented fcc structure is twin-related to the matrix, which leads to twin nucleation without the need for partial dislocation activities. The results of this study advance our understanding of the deformation mechanism of small-sized fcc metals.

In Situ Atomic-Scale Quantitative Evidence of Plastic Activities Resulting in Reparable Deformation in Ultrasmall-Sized Ag Nanocrystals.

Permanent structural changes in pure metals that are caused by plastic activity are normally irreparable after unloading. Because of the lack of experimental evidence, it is unclear whether the plastic activity can be repaired as the size of the pure metals decreases to several nanometers; it is also unclear how the metals accommodate the plastic deformation. In this study, the in situ atomic-scale loading and unloading of ∼2 nm Ag nanocrystals was investigated, and three modes of plastic deformation were observed: (i) the phase transition from the face-centered cubic (fcc) phase to the hexagonal close-packed (hcp) phase, (ii) stacking faults, and (iii) deformation twin nucleation. We show that all three modes resulted in structural changes that were reparable, and their generation and restoration during loading and unloading were observed in situ. We discovered that the deformation modes of nanosized metals can be predicted from the ratio of the energy barriers of the fcc-hcp phase transition (ΔγH) and the deformation twin nucleation (ΔγT), which differ from those of the theoretical modes of relatively large-sized metals. The proposed ΔγH/ΔγT criterion provides insights into the deformation mechanism of nanometals.

A Room-Temperature Ferroelectric Resonant Tunneling Diode.

Resonant tunneling is a quantum-mechanical effect in which electron transport is controlled by the discrete energy levels within a quantum-well (QW) structure. A ferroelectric resonant tunneling diode (RTD) exploits the switchable electric polarization state of the QW barrier to tune the device resistance. Here, the discovery of robust room-temperature ferroelectric-modulated resonant tunneling and negative differential resistance (NDR) behaviors in all-perovskite-oxide BaTiO3 /SrRuO3 /BaTiO3 QW structures is reported. The resonant current amplitude and voltage are tunable by the switchable polarization of the BaTiO3 ferroelectric with the NDR ratio modulated by ≈3 orders of magnitude and an OFF/ON resistance ratio exceeding a factor of 2 × 104 . The observed NDR effect is explained an energy bandgap between Ru-t2g and Ru-eg orbitals driven by electron-electron correlations, as follows from density functional theory calculations. This study paves the way for ferroelectric-based quantum-tunneling devices in future oxide electronics.

Tracking the sliding of grain boundaries at the atomic scale.

Grain boundaries (GBs) play an important role in the mechanical behavior of polycrystalline materials. Despite decades of investigation, the atomic-scale dynamic processes of GB deformation remain elusive, particularly for the GBs in polycrystals, which are commonly of the asymmetric and general type. We conducted an in situ atomic-resolution study to reveal how sliding-dominant deformation is accomplished at general tilt GBs in platinum bicrystals. We observed either direct atomic-scale sliding along the GB or sliding with atom transfer across the boundary plane. The latter sliding process was mediated by movements of disconnections that enabled the transport of GB atoms, leading to a previously unrecognized mode of coupled GB sliding and atomic plane transfer. These results enable an atomic-scale understanding of how general GBs slide in polycrystalline materials.

Evolution of the conductive filament system in HfO2-based memristors observed by direct atomic-scale imaging.

The resistive switching effect in memristors typically stems from the formation and rupture of localized conductive filament paths, and HfO2 has been accepted as one of the most promising resistive switching materials. However, the dynamic changes in the resistive switching process, including the composition and structure of conductive filaments, and especially the evolution of conductive filament surroundings, remain controversial in HfO2-based memristors. Here, the conductive filament system in the amorphous HfO2-based memristors with various top electrodes is revealed to be with a quasi-core-shell structure consisting of metallic hexagonal-Hf6O and its crystalline surroundings (monoclinic or tetragonal HfOx). The phase of the HfOx shell varies with the oxygen reservation capability of the top electrode. According to extensive high-resolution transmission electron microscopy observations and ab initio calculations, the phase transition of the conductive filament shell between monoclinic and tetragonal HfO2 is proposed to depend on the comprehensive effects of Joule heat from the conductive filament current and the concentration of oxygen vacancies. The quasi-core-shell conductive filament system with an intrinsic barrier, which prohibits conductive filament oxidation, ensures the extreme scalability of resistive switching memristors. This study renovates the understanding of the conductive filament evolution in HfO2-based memristors and provides potential inspirations to improve oxide memristors for nonvolatile storage-class memory applications.

Co and Pt Dual-Single-Atoms with Oxygen-Coordinated Co-O-Pt Dimer Sites for Ultrahigh Photocatalytic Hydrogen Evolution Efficiency.

The platinum single-atom-catalyst is verified as a very successful route to approach the size limit of Pt catalysts, while how to further improve the catalytic efficiency of Pt is a fundamental scientific question and is challenging because the size issue of Pt is approached at the ultimate ceiling as single atoms. Here, a new route for further improving Pt catalytic efficiency by cobalt (Co) and Pt dual-single-atoms on titanium dioxide (TiO2 ) surfaces, which contains a fraction of nonbonding oxygen-coordinated Co-O-Pt dimers, is reported. These Co-Pt dimer sites originate from loading high-density Pt single-atoms and Co single-atoms, with them anchoring randomly on the TiO2 substrate. This dual-single-atom catalyst yields 13.4% dimer sites and exhibits an ultrahigh and stable photocatalytic activity with a rate of 43.467 mmol g-1 h-1 and external quantum efficiency of ≈83.4% at 365 nm. This activity far exceeds those of equal amounts of Pt single-atom and typical Pt clustered catalysts by 1.92 and 1.64 times, respectively. The enhancement mechanism relies on the oxygen-coordinated Co-O-Pt dimer coupling, which can mutually optimize the electronic states of both Pt and Co sites to weaken H* binding. Namely, the "mute" Co single-atom is activated by Pt single-atom and the activity of the Pt atom is further enhanced through the dimer interaction. This strategy of nonbonding interactive dimer sites and the oxygen-mediated catalytic mechanisms provide emerging rich opportunities for greatly improving the catalytic efficiency and developing novel catalysts with creating new electronic states.

In situ atomic-scale observation of grain size and twin thickness effect limit in twin-structural nanocrystalline platinum.

Twin-thickness-controlled plastic deformation mechanisms are well understood for submicron-sized twin-structural polycrystalline metals. However, for twin-structural nanocrystalline metals where both the grain size and twin thickness reach the nanometre scale, how these metals accommodate plastic deformation remains unclear. Here, we report an integrated grain size and twin thickness effect on the deformation mode of twin-structural nanocrystalline platinum. Above a ∼10 nm grain size, there is a critical value of twin thickness at which the full dislocation intersecting with the twin plane switches to a deformation mode that results in a partial dislocation parallel to the twin planes. This critical twin thickness value varies from ∼6 to 10 nm and is grain size-dependent. For grain sizes between ∼10 to 6 nm, only partial dislocation parallel to twin planes is observed. When the grain size falls below 6 nm, the plasticity switches to grain boundary-mediated plasticity, in contrast with previous studies, suggesting that the plasticity in twin-structural nanocrystalline metals is governed by partial dislocation activities.

Functional Carbon Quantum Dots for Highly Sensitive Graphene Transistors for Cu2+ Ion Detection.

Cu2+ ions play essential roles in various biological events that occur in the human body. It is important to establish an efficient and reliable detection of Cu2+ ions for people's health. The solution-gated graphene transistors (SGGTs) have been extensively investigated as a promising platform for chemical and biological sensing applications. Herein, highly sensitive and highly selective sensor for Cu2+ ion detection is successfully constructed based on SGGTs with gate electrodes modified by functional carbon quantum dots (CQDs). The sensing mechanism of the sensor is that the coordination of CQDs and Cu2+ ions induces the capacitance change of the electrical double layer (EDL) near the gate electrode and then results in the change of channel current. Compared to other metal ions, Cu2+ ions have an excellent binding nature with CQDs that make it an ultrahigh selective sensor. The CQD-modified sensor achieves excellent Cu2+ ion detection with a minimal level of concentration (1 × 10-14 M), which is several orders of magnitude lower than the values obtained from other conventional detection methods. Interestingly, the device also displays a quick response time on the order of seconds. Due to the functionalized nature of CQDs, SGGTs with CQD-modified gate show good prospects to achieve multifunctional sensing platform in biochemical detections.

[Change data capture implementation in HIS].

The paper introduces the Change Data Capture based on change-track-table implementation in hospital information system. It improves efficiency of the change data capture and distribution in the data flat.

[Dimensional modeling analysis for outpatient payments].

This paper introduces a data warehouse model for outpatient payments, which is designed according to the requirements of the hospital financial management while dimensional modeling technique is combined with the analysis on the requirements. This data warehouse model can not only improve the accuracy of financial management requirements, but also greatly increase the efficiency and quality of the hospital management.