Direct identification of monolayer rhenium diselenide by an individual diffraction pattern

In the current extensive studies of layered two-dimensional (2D) materials,compared to hexagonal structures such as graphene,hBN,and MoS2,lowsymmetry 2D materials have shown great potential for applications in anisotropic devices.Rhenium diselenide (ReSe2) possesses the bulk space group P(1) and belongs to the triclinic crystal system with a deformed cadmium-iodide-type structure.Here,we propose an electron diffraction-based method to distinguish the monolayer ReSe2 membrane from multilayer ReSe2 and its two different vertical orientations.Our method is also applicable to other low-symmetry crystal systems,including both triclinic and monoclinic lattices,as long as their third unit-cell basis vectors are not perpendicular to the basal plane.Our experimental results are well explained by kinematical electron diffraction theory and the corresponding simulations.Generalization of our method to other 2D materials,such as graphene,is also discussed.     

A PWM plus phase-shift control bidirectional DC-DC converter

A pulse-width modulation (PWM) plus phase-shift control bidirectional dc-dc converter is proposed. In this converter, PWM control and phase-shift control are combined to reduce current stress and conduction losses, and to expand ZVS range. The operation principle and analysis of the converter are explained, and ZVS condition is derived. A prototype of PWM plus phase-shift bidirectional dc-dc converter is built to verify the analysis.     

从科学普及局到中国科学技术协会

中国近现代科普所经历的3次科学大普及(或3次科普高潮),都跟中国科技团体和科普事业机构的兴衰密切相关.本文介绍诞生于科普第二次大发展时期中国最重要的科技团体--中国科协成立之前的一些情况,以及相关科普活动.     

末道次大压下率轧制工艺对提高微碳深冲板成品性能的研究

对微碳深冲板进行末道次大压下率轧制的新工艺试验研究以期提高其成品性能。试验结果表明：采用新工艺可显著细化热轧显微组织，并大大促进冷轧有利织构〈111〉的形成，从而大幅度提高冷轧成品的杯突值、应变硬化指数及塑性应变比。同时，由于采用新工艺而使变形速度提高，变形量加大，使得金属质点均匀流动而获得满意的板形。     

基于MEMS工艺的电学TEM芯片研发与应用

透射电子显微镜利用电子束与样品相互作用成像,可提供原子分辨率的结构和成分信息、原子尺度的空间结构和化学成分等信息,是研究材料微观结构与性能关系的有效手段.随着材料尺寸的不断缩小,其电学性质受结构、形貌的影响显著,在纳米乃至原子尺度下研究纳米材料的电学性能十分必要.通过半导体微制造工艺,设计、制备了一款原位电镜用四电极电学芯片,并结合原位样品杆和高精度数字源表,测量了不同条件下制备的单层二硫化钼材料的电流-电压特性曲线.     

Morphology Engineering in Monolayer MoS2‐WS2 Lateral Heterostructures

In recent years, heterostructures formed in transition metal dichalcogenides (TMDs) have attracted significant attention due to their unique physical properties beyond the individual components. Atomically thin TMD heterostructures, such as MoS₂‐WS₂, MoS₂‐MoSe₂, MoS₂‐WSe₂, and WSe₂‐WS₂, are synthesized so far via chemical vapor deposition (CVD) method. Engineering the morphology of domains including size and shape, however, still remains challenging. Here, a one‐step CVD strategy on the morphology engineering of MoS₂ and WS₂ domains within the monolayer MoS₂‐WS₂ lateral heterostructures through controlling the weight ratio of precursors, MoO₃ and WO₃, as well as tuning the reaction temperature is reported. Not only can the size ratio in terms of area between WS₂ and MoS₂ domains be easily controlled from less than 1 to more than 20, but also the overall heterostructure size can be tuned from several to hundreds of micrometers. Intriguingly, the quantum well structure, a WS₂ stripe embedded in the MoS₂ matrix, is also observed in the as‐synthesized heterostructures, offering opportunities to study quantum confinement effects and quantum well applications. This approach paves the way for the large‐scale fabrication of MoS₂‐WS₂ lateral heterostructures with controllable domain morphology, and shall be readily extended to morphology engineering of other TMD heterostructures.     

In situ electrical measurements of polytypic silver nanowires

Novel 4H structure silver nanowires (4H-AgNWs) have been reported to coexist with the usual face-centered cubic (FCC) ones. Here we report the electrical properties of these polytypic AgNWs for the first time. AgNWs with either 4H or FCC structures in the diameter range of 20-80?nm were measured in situ inside a transmission electron microscope (TEM). Both kinds of AgNW in the diameter range show metallic conductance. The average resistivity of the 4H-AgNWs is 19.9?μΩ?cm, comparable to the 11.9?μΩ?cm of the FCC-AgNWs. The failure current density can be up to ～10(8)?A?cm(-2) for both 4H-and FCC-AgNWs. The maximum stable current density (MSCD) is introduced to estimate the AgNWs' current-carrying ability, which shows diameter-dependence with a peak around 34?nm in diameter. It is attributed to fast annihilation of the current-induced vacancies and the enhanced surface scattering. Our investigations also suggest that the magnetic field of the electromagnetic lens may also introduce some influence on the measurements inside the TEM.     

Vacancy migrations in carbon nanotubes

Activities of vacancy defects in carbon nanotubes have been directly monitored by in situ high-resolution transmission electron microscopy at elevated temperatures. Adatom-vacancy pair defects are first prolific due to the knock-on damage, and then the induced vacancies indeed grow up to 1-2 nm in the size by the following Joule heating. Surprisingly, these large vacancies, or "holes", tend to migrate and coalesce with each other to form even larger ones. It suggests that the activation barrier has been substantially lowered due to the contributions of an electromigration and/or irradiation effect.     

Controlled Growth and Reliable Thickness‐Dependent Properties of Organic–Inorganic Perovskite Platelet Crystal

Organolead halide perovskites (e.g., CH₃NH₃PbI₃) have caught tremendous attention for their excellent optoelectronic properties and applications, especially as the active material for solar cells. Perovskite crystal quality and dimension is crucial for the fabrication of high‐performance optoelectronic and photovoltaic devices. Herein the controlled synthesis of organolead halide perovskite CH₃NH₃PbI₃ nanoplatelets on SiO₂/Si substrates is investigated via a convenient two‐step vapor transport deposition technique. The thickness and size of the perovskite can be well‐controlled from few‐layers to hundred nanometers by altering the synthesis time and temperature. Raman characterizations reveal that the evolutions of Raman peaks are sensitive to the thickness. Furthermore, from the time‐resolved photoluminescence measurements, the best optoelectronic performance of the perovskite platelet is attributed with thickness of ≈30 nm to its dominant longest lifetime (≈4.5 ns) of perovskite excitons, which means lower surface traps or defects. This work supplies an alternative to the synthesis of high‐quality organic perovskite and their possible optoelectronic applications with the most suitable materials.     

滚动直线导轨结构类组合机床的静刚度及寿命研究

随着精密滚动直线导轨的开发和推广,特别是在数控机床驱动工作台及加工中心得以广泛运用,有关该类组合体的各种性能的理论计算成为极其必要.本文提出了在任意力、力矩作用下工作台上任意点的位移计算公式,并对构成工作台的滚动直线导轨寿命进行了计算.