Polyhedral Oligomeric Silsesquioxanes Based Ultralow-k Materials: The Effect of Cage Size

Polyhedral oligomeric silsesquioxanes (POSS) are of considerable interest as building blocks for preparing low-k materials. To date T₈ POSS has been extensively investigated while the potential of larger POSS cages remain an unexplored area. Herein, the first known contribution to map the role of POSS cage size on the dielectric and other comprehensive properties of hybrid materials with identical chemical compositions is described. First, three vinyl POSS (T₈, T₁₀, and T₁₂) species are isolated from a commercial POSS mixture. Then, they are converted to benzocyclobutene functionalized and thermo-crosslinked hybrid materials. It is found that the cage size can strongly affect their k values, more importantly, showing a linear decrease while increasing the cage volumes (k  = 2.24, 2.02, and 1.83 for c-T₈B₈, c-T₁₀B₁₀, and c-T₁₂B₁₂, respectively). This finding highlights a profound influence of POSS cage changes on dielectric properties and could be used to predict ultralow-k (1.5–1.1) materials by extrapolating to larger T₁₄, T₁₆, and T₁₈ POSS cages. Meanwhile, varying the cage size has no obvious effect on the materials’ other properties, and all of them exhibit good comprehensive properties. Moreover, such low-k values can persist at high temperature and high humidity conditions, which affords some promising (ultra)low-k dielectrics for modern integrated circuit development.     

Epitaxial Growth of Ternary Topological Insulator Bi2Te2Se 2D Crystals on Mica

Nanostructures of ternary topological insulator (TI) Bi₂Te₂Se are, in principle, advantageous to the manifestation of topologically nontrivial surface states, due to significantly enhanced surface‐to‐volume ratio compared with its bulk crystals counterparts. Herein, the synthesis of 2D Bi₂Te₂Se crystals on mica via the van der Waals epitaxy method is explored and systematically the growth behaviors during the synthesis process are investigated. Accordingly, 2D Bi₂Te₂Se crystals with domain size up to 50 µm large and thickness down to 2 nm are obtained. A pronounced weak antilocalization effect is clearly observed in the 2D Bi₂Te₂Se crystals at 2 K. The method for epitaxial growth of 2D ternary Bi₂Te₂Se crystals may inspire materials engineering toward enhanced manifestation of the subtle surface states of TIs and thereby facilitate their potential applications in next‐generation spintronics.     

A Leaky‐Integrate‐and‐Fire Neuron Analog Realized with a Mott Insulator

During the last half century, the tremendous development of computers based on von Neumann architecture has led to the revolution of the information technology. However, von Neumann computers are outperformed by the mammal brain in numerous data‐processing applications such as pattern recognition and data mining. Neuromorphic engineering aims to mimic brain‐like behavior through the implementation of artificial neural networks based on the combination of a large number of artificial neurons massively interconnected by an even larger number of artificial synapses. In order to effectively implement artificial neural networks directly in hardware, it is mandatory to develop artificial neurons and synapses. A promising advance has been made in recent years with the introduction of the components called memristors that might implement synaptic functions. In contrast, the advances in artificial neurons have consisted in the implementation of silicon‐based circuits. However, so far, a single‐component artificial neuron that will bring an improvement comparable to what memristors have brought to synapses is still missing. Here, a simple two‐terminal device is introduced, which can implement the basic functions leaky integrate and fire of spiking neurons. Remarkably, it has been found that it is realized by the behavior of strongly correlated narrow‐gap Mott insulators subject to electric pulsing.     

220kV电网开展直升机巡检的探索与研究

针对传统巡检工作的缺陷,提出针对220kV电网开展直升机巡检的工作方式。     

Characterization of non‐conducting materials: a promise of quality and productivity improvements in various industrial fields

An increase in reliability and a decrease in production cost of systems that include insulators require a better knowledge of the origins of friction, wear, adhesion, fracture and electric breakdown. By doing this, for example, a manufacturer could adapt his fabrication to the user's operational conditions whereas, at present, he only can offer a catalogue of standard specified products. The macroscopic behaviour of a material depends on the defects therein; these in turn determine the electric charge distribution and the internal energy of the material. Seen in this light, many pioneering results can be understood and used to clarify the experiments required for validating new theories of breakdown or wear. Several laboratories have worked together to demonstrate the direct relationships that exist between the microscopic parameters of charge and energy localization or relaxation and macroscopic behaviour. One essential point was to find how to carry out experiments whilst preserving the material's intrinsic defect state during sample preparation and how to characterize defects related to stresses applied to the material. Another point was to investigate the phenomena that occur at the early stage of wear, fracture and breakdown before catastrophic failure. Therefore the use of time‐ and spatially‐resolved measurements has been essential. This work reviews some pioneering results, pointing out the importance of polarization and conduction on apparently different behaviour (wear, adhesion and breakdown). These results have been explained from classical solid state physics and electrostatics backgrounds. When the experiments were carried out, however, characterization techniques for interpretating the results were missing, so that they did not receive the attention they deserved. Surprisingly, and as discussed in the second part, because these new characterization techniques have not been sufficiently disseminated in industry, these pioneering results and theories have not been practically applied. As a consequence, standard insulator specifications still do not reflect our actual knowledge. In the third part, some examples illustrate technological progress that can be made by combining some new characterization techniques with the standard ones. © 2001 Society of Chemical Industry     

Atomic‐Scale Control of Electronic Structure and Ferromagnetic Insulating State in Perovskite Oxide Superlattices by Long‐Range Tuning of BO6 Octahedra

Control of BO₆ octahedral rotations at the heterointerfaces of dissimilar ABO₃ perovskites has emerged as a powerful route for engineering novel physical properties. However, its impact length scale is constrained at 2–6 unit cells close to the interface and the octahedral rotations relax quickly into bulk tilt angles away from interface. Here, a long‐range (up to 12 unit cells) suppression of MnO₆ octahedral rotations in La_0.9Ba_0.1MnO₃ through the formation of superlattices with SrTiO₃ can be achieved. The suppressed MnO₆ octahedral rotations strongly modify the magnetic and electronic properties of La_0.9Ba_0.1MnO₃ and hence create a new ferromagnetic insulating state with enhanced Curie temperature of 235 K. The emergent properties in La_0.9Ba_0.1MnO₃ arise from a preferential occupation of the out‐of‐plane Mn d_3z²_?r² orbital and a reduced Mn e_g bandwidth, induced by the suppressed octahedral rotations. The realization of long‐range tuning of BO₆ octahedra via superlattices can be applicable to other strongly correlated perovskites for exploring new emergent quantum phenomena.     

MoTe2 is a good match for GeI by preserving quantum spin Hall phase

Quantum spin Hall (QSH) insulator is a new class of materials that is quickly becoming mainstream in condensed-matter physics.The main obstacle for the development of QSH insulators is that their strong interactions with substrates make them difficult to study experimentally.In this study,using density functional theory,we discovered that MoTe2 is a good match for a GeI monolayer.The thermal stability of a van der Waals GeI/MoTe2 heterosheet was examined via molecular-dynamics simulations.Simulated scanning tunneling microscopy revealed that the GeI monolayer perfectly preserves the bulked honeycomb structure of MoTe2.The GeI on MoTe2 was confirmed to maintain its topological band structure with a sizable indirect bulk bandgap of 0.24 eV by directly calculating the spin Chern number to be-1.As expected,the electron mobility of the GeI is enhanced by MoTe2 substrate restriction.According to deformation-potential theory with the effective-mass approximation,the electron mobility of GeI/MoTe2 was estimated as 372.7 cm2.s-1.V-1 at 300 K,which is 20 times higher than that of freestanding GeI.Our research shows that traditional substrates always destroy the topological states and hinder the electron transport in QSH insulators,and pave way for the further realization and utilization of QSH insulators at room temperature.     

Epitaxial EuO thin films by pulsed laser deposition monitored by in situ x-ray photoelectron spectroscopy

We have grown EuO thin films on silicon [001] and yttrium aluminate [110] from a europium metal target using pulsed laser deposition. In situ x-ray photoelectron spectroscopy has been used to determine the parameter window for stoichiometric EuO deposition. EuO is observed to grow in the relatively high pressure regime of 10⁻⁶-10⁻⁵ mbar, due to the large Eu flux during ablation. EuO is proven to grow epitaxially on yttrium aluminate [110]. Magnetization measurements confirm the stoichiometry of the film.     

Electron‐ and X‐ray‐induced electron emissions from insulators

The electron emission yields of insulators irradiated with ultra‐short pulsed X‐ray and electron beams differ significantly from those of metals. The main experimental results are reviewed and it is shown that the use of simple models for the transport and for the escape of the generated secondary electrons accounts for the role of the temperature and of the crystalline state on these yields (besides the role of the chemical composition of the insulator of interest). When permanent irradiation is used, charging and damaging effects occur and correlated mechanisms are then considered. The present analysis concerns a wide variety of insulators (from oxides and halides to polymers) investigated with a wide variety of techniques (XPS, AES, SEM, etc.) and various practical consequences are also deduced. © 2001 Society of Chemical Industry