W掺杂二氧化钒的水热晶化机理及其相变性能

以硫酸氧钒为钒源,采用沉淀-胶溶法制备VO_2溶胶。然后向溶胶中加入偏钒酸铵,利用溶胶水热晶化制备出W掺杂二氧化钒(W-VO_2,M相)粉体。通过XRD,FESEM和DSC对合成产物的物相组成、形貌和相变性能进行研究。结果表明:在280℃条件下水热处理4~48h,VO_2溶胶经过水热晶化生成长约1~2μm、直径约100~200nm棒状W-VO_2(B)晶体,伴随着B相向M相晶型转变,W-VO_2(B)逐渐消溶,而W-VO_2(M)逐渐长大,形貌由棒状转变为片状或雪花状;W-VO_2(M)相变温度随着W掺杂量增加而降低,当名义掺杂量为6.0%(原子分数)时,相变温度降低到28℃。根据水热晶化和形貌演变过程,提出了W-VO_2(M)可能的"形核-生长-转化-熟化"形成机理。     

VO2/TiN Plasmonic Thermochromic Smart Coatings for Room‐Temperature Applications

Vanadium dioxide/titanium nitride (VO₂/TiN) smart coatings are prepared by hybridizing thermochromic VO₂ with plasmonic TiN nanoparticles. The VO₂/TiN coatings can control infrared (IR) radiation dynamically in accordance with the ambient temperature and illumination intensity. It blocks IR light under strong illumination at 28 °C but is IR transparent under weak irradiation conditions or at a low temperature of 20 °C. The VO₂/TiN coatings exhibit a good integral visible transmittance of up to 51% and excellent IR switching efficiency of 48% at 2000 nm. These unique advantages make VO₂/TiN promising as smart energy‐saving windows.     

MBE技术蓝宝石衬底上生长VO2薄膜及其太赫兹和金属-绝缘体相变特性研究

采用分子束外延(MBE)技术在单晶蓝宝石衬底上生长了高质量化学计量比二氧化钒(VO₂)薄膜, 通过该技术实现薄膜厚度15~60 nm精确控制。对于优化条件下VO₂薄膜, 实现了电阻率变化超过4个数量级的优异金属-绝缘体相变, 近似于之前报道高质量单晶VO₂相变特性。特别是通过太赫兹时域光谱分析了不同厚度的VO₂薄膜在太赫兹波段的光学特性。结果表明: VO₂薄膜的厚度对其在太赫兹波段的光学特性有很大影响。因此, 为了获得更优的可靠性和重复性能, VO₂薄膜的厚度必须得到精确控制。本研究结果对于下一步VO₂基太赫兹器件研究具有重要意义。     

VOC2O4·H2O热分解制备纳米VO2粉体及相变特性

提出了一种采用草酸氧钒(VOC2O4·H2O)热分解制备纳米VO2的方法.通过热分析(TG-DSC)、XRD和TEM手段,分析了草酸氧钒前驱体的热分解过程,纳米VO2的结构和形貌,测定了纳米VO2粉体的电阻-温度曲线.实验结果表明VOC2O4·H2O热分解开始温度为343℃,在380℃,真空度为20～50Pa的条件下,热分解获得VO2的平均尺寸为22nm,相变温度为69℃.     

Thickness‐Dependent In‐Plane Polarization and Structural Phase Transition in van der Waals Ferroelectric CuInP2S6

van der Waals (vdW) layered materials have rather weaker interlayer bonding than the intralayer bonding, therefore the exfoliation along the stacking direction enables the achievement of monolayer or few layers vdW materials with emerging novel physical properties and functionalities. The ferroelectricity in vdW materials recently attracts renewed interest for the potential use in high‐density storage devices. With the thickness becoming thinner, the competition between the surface energy, depolarization field, and interfacial chemical bonds may give rise to the modification of ferroelectricity and crystalline structure, which has limited investigations. In this work, combining the piezoresponse force microscope scanning, contact resonance imaging, the existence of the intrinsic in‐plane polarization in vdW ferroelectrics CuInP₂S₆ single crystals is reported, whereas below a critical thickness between 90 and 100 nm, the in‐plane polarization disappears. The Young's modulus also shows an abrupt stiffness at the critical thickness. Based on the density functional theory calculations, these behaviors are ascribed to a structural phase transition from monoclinic to trigonal structure, which is further verified by transmission electron microscope technique. These findings demonstrate the foundational importance of structural phase transition for enhancing the rich functionality and broad utility of vdW ferroelectrics.     

Structural Phase Transition Effect on Resistive Switching Behavior of MoS2‐Polyvinylpyrrolidone Nanocomposites Films for Flexible Memory Devices

The 2H phase and 1T phase coexisting in the same molybdenum disulfide (MoS₂) nanosheets can influence the electronic properties of the materials. The 1T phase of MoS₂ is introduced into the 2H‐MoS₂ nanosheets by two‐step hydrothermal synthetic methods. Two types of nonvolatile memory effects, namely write‐once read‐many times memory and rewritable memory effect, are observed in the flexible memory devices with the configuration of Al/1T@2H‐MoS₂‐polyvinylpyrrolidone (PVP)/indium tin oxide (ITO)/polyethylene terephthalate (PET) and Al/2H‐MoS₂‐PVP/ITO/PET, respectively. It is observed that structural phase transition in MoS₂ nanosheets plays an important role on the resistive switching behaviors of the MoS₂‐based device. It is hoped that our results can offer a general route for the preparation of various promising nanocomposites based on 2D nanosheets of layered transition metal dichalcogenides for fabricating the high performance and flexible nonvolatile memory devices through regulating the phase structure in the 2D nanosheets.     

Decompression‐Driven Superconductivity Enhancement in In2Se3

An unexpected superconductivity enhancement is reported in decompressed In₂Se₃. The onset of superconductivity in In₂Se₃ occurs at 41.3 GPa with a critical temperature (T_c) of 3.7 K, peaking at 47.1 GPa. The striking observation shows that this layered chalcogenide remains superconducting in decompression down to 10.7 GPa. More surprisingly, the highest T_c that occurs at lower decompression pressures is 8.2 K, a twofold increase in the same crystal structure as in compression. It is found that the evolution of T_c is driven by the pressure‐induced R‐3m to I‐43d structural transition and significant softening of phonons and gentle variation of carrier concentration combined in the pressure quench. The novel decompression‐induced superconductivity enhancement implies that it is possible to maintain pressure‐induced superconductivity at lower or even ambient pressures with better superconducting performance.     

Impurity‐Induced First‐Order Phase Transitions in Highly Crystalline V2O3 Nanocrystals

A first‐order phase transition in a bulk material is generally considered to arise at extended defects such as grain boundaries or dislocations, where the energetic barrier between the two phases is reduced. Downsizing a crystal to the nanoscale can exclude the number of defects, leading to enhanced kinetic stabilization of the metastable phase. Here, the disappearance of the first‐order metal–insulator transition in defect‐free V₂O₃ nanocrystals and the revival of the transition by introducing a certain Cr or Ti impurity content are investigated. The hysteresis width of the transition corresponding to the barrier height decreases with the impurity content. It is proposed that homogeneous impurity doping is a universal method that can control the occurrence of a first‐order phase transition in nanoscale materials.     

Nano‐TiO2 Drives Epithelial–Mesenchymal Transition in Intestinal Epithelial Cancer Cells

The majority of cancer mortality is associated with cancer metastasis. Epithelial‐to‐mesenchymal transition (EMT) is a process by which cells attain migratory and invasive properties, eventually leading to cancer metastasis. Here, it is shown that titanium dioxide nanoparticles (nano‐TiO₂), a common food additive, can induce the EMT process in colorectal cancer cells. Nano‐TiO₂ exposure is observed to activate transforming growth factor‐β (TGF‐β)/mitogen‐activated protein kinase (MAPK) and wingless (Wnt) pathways, and drive the EMT process. Similarly, silica nanoparticles (nano‐SiO₂) and hydroxyapatite nanoparticles (nano‐HA), as food‐based additives, can be ingested and accumulated in the stomach, and are found to be able to induce the EMT progression. The implication of this work can be profound for colorectal cancer patients where these food additives may unknowingly and unnecessarily hasten the progression of their cancers.     

Truly Concomitant and Independently Expressed Short‐ and Long‐Term Plasticity in a Bi2O2Se‐Based Three‐Terminal Memristor

Concomitance of diverse synaptic plasticity across different timescales produces complex cognitive processes. To achieve comparable cognitive complexity in memristive neuromorphic systems, devices that are capable of emulating short‐term (STP) and long‐term plasticity (LTP) concomitantly are essential. In existing memristors, however, STP and LTP can only be induced selectively because of the inability to be decoupled using different loci and mechanisms. In this work, the first demonstration of truly concomitant STP and LTP is reported in a three‐terminal memristor that uses independent physical phenomena to represent each form of plasticity. The emerging layered material Bi₂O₂Se is used for memristors for the first time, opening up the prospects for ultrathin, high‐speed, and low‐power neuromorphic devices. The concerted action of STP and LTP allows full‐range modulation of the transient synaptic efficacy, from depression to facilitation, by stimulus frequency or intensity, providing a versatile device platform for neuromorphic function implementation. A heuristic recurrent neural circuitry model is developed to simulate the intricate “sleep–wake cycle autoregulation” process, in which the concomitance of STP and LTP is posited as a key factor in enabling this neural homeostasis. This work sheds new light on the development of generic memristor platforms for highly dynamic neuromorphic computing.