Rare‐Earth‐Element‐Ytterbium‐Substituted Lead‐Free Inorganic Perovskite Nanocrystals for Optoelectronic Applications

Lead‐(Pb‐) halide perovskite nanocrystals (NCs) are interesting nanomaterials due to their excellent optical properties, such as narrow‐band emission, high photoluminescence (PL) efficiency, and wide color gamut. However, these NCs have several critical problems, such as the high toxicity of Pb, its tendency to accumulate in the human body, and phase instability. Although Pb‐free metal (Bi, Sn, etc.) halide perovskite NCs have recently been reported as possible alternatives, they exhibit poor optical and electrical properties as well as abundant intrinsic defect sites. For the first time, the synthesis and optical characterization of cesium ytterbium triiodide (CsYbI₃) cubic perovskite NCs with highly uniform size distribution and high crystallinity using a simple hot‐injection method are reported. Strong excitation‐independent emission and high quantum yields for the prepared NCs are verified using photoluminescence measurements. Furthermore, these CsYbI₃ NCs exhibit potential for use in organic–inorganic hybrid photodetectors as a photoactive layer. The as‐prepared samples exhibit clear on–off switching behavior as well as high photoresponsivity (2.4 × 10³ A W^?1) and external quantum efficiency (EQE, 5.8 × 10⁵%) due to effective exciton dissociation and charge transport. These results suggest that CsYbI₃ NCs offer tremendous opportunities in electronic and optoelectronic applications, such as chemical sensors, light emitting diodes (LEDs), and energy conversion and storage devices.     

Two‐Terminal Multibit Optical Memory via van der Waals Heterostructure

2D van der Waals (vdWs) heterostructures exhibit intriguing optoelectronic properties in photodetectors, solar cells, and light‐emitting diodes. In addition, these materials have the potential to be further extended to optical memories with promising broadband applications for image sensing, logic gates, and synaptic devices for neuromorphic computing. In particular, high programming voltage, high off‐power consumption, and circuital complexity in integration are primary concerns in the development of three‐terminal optical memory devices. This study describes a multilevel nonvolatile optical memory device with a two‐terminal floating‐gate field‐effect transistor with a MoS₂/hexagonal boron nitride/graphene heterostructure. The device exhibits an extremely low off‐current of ≈10^?14 A and high optical switching on/off current ratio of over ≈10⁶, allowing 18 distinct current levels corresponding to more than four‐bit information storage. Furthermore, it demonstrates an extended endurance of over ≈10⁴ program–erase cycles and a long retention time exceeding 3.6 × 10⁴ s with a low programming voltage of ?10 V. This device paves the way for miniaturization and high‐density integration of future optical memories with vdWs heterostructures.     

Dual Catalytic and Self-Assembled Growth of Two-Dimensional Transition Metal Dichalcogenides Through Simultaneous Predeposition Process

The recent introduction of alkali metal halide catalysts for chemical vapor deposition (CVD) of transition metal dichalcogenides (TMDs) has enabled remarkable two-dimensional (2D) growth. However, the process development and growth mechanism require further exploration to enhance the effects of salts and understand the principles. Herein, simultaneous predeposition of a metal source (MoO₃) and salt (NaCl) by thermal evaporation is adopted. As a result, remarkable growth behaviors such as promoted 2D growth, easy patterning, and potential diversity of target materials can be achieved. Step-by-step spectroscopy combined with morphological analyses reveals a reaction path for MoS₂ growth in which NaCl reacts separately with S and MoO₃ to form Na₂SO₄ and Na₂Mo₂O₇ intermediates, respectively. These intermediates provide a favorable environment for 2D growth, including an enhanced source supply and liquid medium. Consequently, large grains of monolayer MoS₂ are formed by self-assembly, indicating the merging of small equilateral triangular grains on the liquid intermediates. This study is expected to serve as an ideal reference for understanding the principles of salt catalysis and evolution of CVD in the preparation of 2D TMDs.     

Tailored Heterojunction Active Sites for Oxygen Electrocatalyst Promotion in Zinc-Air Batteries

Rechargeable zinc-air batteries (ZABs) are promising energy storage systems due to their low-cost and safety. However, the working principle of ZABs is based on oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), which display sluggish kinetic and low stability. Herein, this work proposes a novel method to design a heterogeneous CoP/CoO electrocatalyst on mesopore nanobox carbon/carbon nanotube (CoP/CoO@MNC-CNT) that enriched active sites and synergistic effect. Moreover, the well-defined heterointerfaces could lower the energy barrier for intermediate species adsorption and promote OER and ORR electrochemical performances. The CoP/CoO@MNC-CNT electrocatalyst presents a high half-wave potential of 0.838 V for ORR and a small overpotential of 270 mV for OER. The ZABs-based CoP/CoO@MNC-CNT air-cathode shows an open-circuit voltage of 1.409 V, the long-term cycle life of 500 h with a small voltage difference change of 7.7%. Additionally, the flexible ZABs exhibit highly mechanical stability, demonstrating their application potential in wearable electronic devices.     

Development of a Shape‐Memory Tube to Prevent Vascular Stenosis

Inserting a graft into vessels with different diameters frequently causes severe damage to the host vessels. Poor flow patency is an unresolved issue in grafts, particularly those with diameters less than 6 mm, because of vessel occlusion caused by disturbed blood flow following fast clotting. Herein, successful patency in the deployment of an ≈2 mm diameter graft into a porcine vessel is reported. A new library of property‐tunable shape‐memory polymers that prevent vessel damage by expanding the graft diameter circumferentially upon implantation is presented. The polymers undergo seven consecutive cycles of strain energy‐preserved shape programming. Moreover, the new graft tube, which features a diffuser shape, minimizes disturbed flow formation and prevents thrombosis because its surface is coated with nitric‐oxide‐releasing peptides. Improved patency in a porcine vessel for 18 d is demonstrated while occlusive vascular remodeling occurs. These insights will help advance vascular graft design.     

Improvement of mechanical properties of cast pure titanium by repeated heat treatment

ABSTRACT

Pure titanium components fabricated by casting have a coarse grain microstructure. To improve the mechanical strength of pure titanium components by refining the grain size, the cast samples were repeatedly heat-treated. During the heat treatment, the titanium samples were repeatedly heated above the alpha-to-beta (α–β) transition temperature and cooled to room temperature to undergo phase transformation. The heating cycle was performed 1, 3, 5, and 7 times. As the number of heating cycles increased, the grain size decreased. The tensile strength was 267.9?MPa in the as-cast sample and improved to 343.4?MPa after 7 heat-treatment cycles owing to the grain size refinement, while the elongation was maintained during the heat treatment.

This paper is part of a thematic issue on Titanium.     

Chemically Tuned p‐ and n‐Type WSe2 Monolayers with High Carrier Mobility for Advanced Electronics

Monolayers of transition metal dichalcogenides (TMDCs) have attracted a great interest for post‐silicon electronics and photonics due to their high carrier mobility, tunable bandgap, and atom‐thick 2D structure. With the analogy to conventional silicon electronics, establishing a method to convert TMDC to p‐ and n‐type semiconductors is essential for various device applications, such as complementary metal‐oxide‐semiconductor (CMOS) circuits and photovoltaics. Here, a successful control of the electrical polarity of monolayer WSe₂ is demonstrated by chemical doping. Two different molecules, 4‐nitrobenzenediazonium tetrafluoroborate and diethylenetriamine, are utilized to convert ambipolar WSe₂ field‐effect transistors (FETs) to p‐ and n‐type, respectively. Moreover, the chemically doped WSe₂ show increased effective carrier mobilities of 82 and 25 cm² V^?1s^?1 for holes and electrons, respectively, which are much higher than those of the pristine WSe₂. The doping effects are studied by photoluminescence, Raman, X‐ray photoelectron spectroscopy, and density functional theory. Chemically tuned WSe₂ FETs are integrated into CMOS inverters, exhibiting extremely low power consumption ( ≈ 0.17 nW). Furthermore, a p‐n junction within single WSe₂ grain is realized via spatially controlled chemical doping. The chemical doping method for controlling the transport properties of WSe₂ will contribute to the development of TMDC‐based advanced electronics.     

Direct Synthesis of a Self‐Assembled WSe2/MoS2 Heterostructure Array and its Optoelectrical Properties

Functional van der Waals heterojunctions of transition metal dichalcogenides are emerging as a potential candidate for the basis of next‐generation logic devices and optoelectronics. However, the complexity of synthesis processes so far has delayed the successful integration of the heterostructure device array within a large scale, which is necessary for practical applications. Here, a direct synthesis method is introduced to fabricate an array of self‐assembled WSe₂/MoS₂ heterostructures through facile solution‐based directional precipitation. By manipulating the internal convection flow (i.e., Marangoni flow) of the solution, the WSe₂ wires are selectively stacked over the MoS₂ wires at a specific angle, which enables the formation of parallel‐ and cross‐aligned heterostructures. The realized WSe₂/MoS₂‐based p–n heterojunction shows not only high rectification (ideality factor: 1.18) but also promising optoelectrical properties with a high responsivity of 5.39 A W^?1 and response speed of 16 µs. As a feasible application, a WSe₂/MoS₂‐based photodiode array (10 × 10) is demonstrated, which proves that the photosensing system can detect the position and intensity of an external light source. The solution‐based growth of hierarchical structures with various alignments could offer a method for the further development of large‐area electronic and optoelectronic applications.