Two‐Dimensional Assembly of Symmetric Colloidal Dimers under Electric Fields

Like atoms and molecules with directional interactions, anisotropic particles could potentially assemble into a much wider range of crystalline arrays and meso‐structures than spherical particles with isotropic interactions. In this paper, the electric‐field directed assembly of geometrically anisotropic particles–colloidal dimers is studied. Rich phase behavior and different assembly regimes are found, primarily arising from the broken radial symmetry in particles. The orientations of individual dimers depend on the frequency of the electric field, the ramping direction of frequency, and the salt concentration. The competition and balance between the hydrodynamic, electric, and Brownian torques determine the orientation of individual particles, while the competition between the electrohydrodynamic force and dipolar interaction determines the aggregation of aligned particles at a given experimental condition. The field distribution near the electrode is critical to understand the orientation and assembly behavior of colloidal dimers on a conducting substrate. This study also demonstrates the effectiveness, the reversibility, and potential opportunity of applying electric field to control the orientation and direct the assembly of non‐spherical particles. In particular, two dimensional close‐packed crystals of perpendicularly aligned dimers are obtained, which shows promise in fabricating 3D photonic crystals based on dimer‐like colloids and field‐directed display.     

Remote Steering of Self‐Propelling Microcircuits by Modulated Electric Field

The principles and design of “active” self‐propelling particles that can convert energy, move directionally on their own, and perform a certain function is an emerging multidisciplinary research field, with high potential for future technologies. A simple and effective technique is presented for on‐demand steering of self‐propelling microdiodes that move electroosmotically on water surface, while supplied with energy by an external alternating (AC) field. It is demonstrated how one can control remotely the direction of diode locomotion by electronically modifying the applied AC signal. The swimming diodes change their direction of motion when a wave asymmetry (equivalent to a DC offset) is introduced into the signal. The data analysis shows that the ability to control and reverse the direction of motion is a result of the electrostatic torque between the asymmetrically polarized diodes and the ionic charges redistributed in the vessel. This novel principle of electrical signal‐coded steering of active functional devices, such as diodes and microcircuits, can find applications in motile sensors, MEMs, and microrobotics.     

TRBR: Flight body posture compensation for transverse ricochetal brachiation robot

Transverse ricochetal brachiation is a sophisticated locomotion style that mimics athletes swinging their bodies with their hands on a ledge in order to propel themselves for a leap to a target ledge. This paper describes the development of a transverse ricochetal brachiation robot (TRBR) and outlines motion control strategies for active flight body posture compensation. The crucial design parameters were obtained by formulating an optimization problem with the goal of maximizing flight distance. Shoulder joints with switchable stiffness were used to enable resonance excitation via the swinging of a robot tail during the swing phase, while enabling tight arm-and-body engagement during the flight phase. Novel electric grippers were designed to provide the required holding forces as well as quick-release functionality to ensure that the kinetic energy accumulated during the swing phase could be transferred smoothly to the flight phase. The reference trajectory of the robot tail was obtained using an optimization procedure based on a dynamic model of the swing phase. We also adopted a dynamic model for the flight phase to elucidate the effects of midair body rotation with the aim of developing body posture compensation methods. Simulation and experimental results demonstrate the efficacy of the proposed body posture compensation method based on a successive loop closure design in improving flight body posture during transverse ricochetal brachiation. The integration of arm swing motion with tail compensation also proved highly effective in enhancing hang time and travel distance.     

Cooperative Switching in Large‐Area Assemblies of Magnetic Janus Particles

Magnetic Janus particles (MJPs) have received considerable attention for their rich assembly behavior and their potential technological role in applications ranging from simple magnetophoretic displays to smart cloaking devices. However, further progress is hampered by the lack of predictive understanding of the cooperative self‐assembly behavior of MJPs and appropriate dynamic control mechanisms. In this paper, a detailed experimental and theoretical investigation into the magnetically directed spatiotemporal self‐assembly and switching of MJPs is presented. For this purpose, a novel type of MJPs with defined hemispherical compartments carrying superparamagnetic iron oxide nanoparticles as well as a novel simulation model to describe their cooperative switching behavior is established. Combination of the theoretical and experimental work culminates in a simple method to direct assemblies of MJPs, even at high particle concentrations. In addition, a magnetophoretic display with switchable MJPs is developed on the basis of the theoretical findings to demonstrate the potential usefulness of controlled large‐area assemblies of magnetic Janus particles.     

Electrohydrodynamically Printed High‐Resolution Full‐Color Hybrid Perovskites

Hybrid perovskites show enormous potential for display due to their tunable emission, high color purity, strong photoluminescence and electroluminescence. For display applications, full‐color and high‐resolution patterning is compulsory, however, current perovskite processing such as spin‐coating fails to meet these requirements. Here, electrohydrodynamic (EHD) printing, with the unique advantages of high‐resolution patterning and large scalability, is introduced to fabricate full‐color perovskite patterns. Perovskite inks via simple precursor mixing are prepared to in situ crystallize tunable‐ and bright‐photoluminescence perovskite arrays without adding antisolvent. Through optimizing the EHD printing process, a high‐resolution dot matrix of 5 µm is achieved. The as‐printed patterns and pictures show full color and high controllability in micrometer dimension, indicating that the EHD printing is a competitive technique for future halide perovskite‐based high‐quality display.     

An All‐Solid‐State Flexible Piezoelectric High‐k Film Functioning as Both a Generator and In Situ Storage Unit

An all‐solid‐state flexible generator–capacitor polymer composite film converts low‐frequency biomechanical energy into stored electric energy. This design, which combines the functionality of a generator with a capacitor, is realized by employing poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐HFP) in the simultaneous dual role of piezoelectric generator and polymer matrices of the flexible capacitor. Proper surface modification of the reduced graphene oxide (rGO) fillers in the polymeric matrices is indispensable in achieving the superior energy storage performance of the composite film. The heightened dielectric performance stems from enhanced compatibility of the rGO fillers and PVDF‐HFP matrices, and a microcapacitor model properly explains the dielectric behaviors. A device that is easily fabricated using our film allows timely decoupled motion energy harvest and output of the motion‐generated electricity. This report opens new design possibilities in the fields of motion sensors, information storage and high‐voltage output by accumulating low‐frequency random biological motions.     

The Multistep Tunneling Analogue of Conductivity Mismatch in Organic Spin Valves

Carbon‐based, molecular semiconductors offer several attractive attributes for spintronics, such as exceptionally weak spin‐orbit coupling and compatibility with bottom‐up nanofabrication. In spite of the promising properties of organic spin valves, however, the physical mechanisms governing spin‐polarized conduction remain poorly understood. An experimental study of C₆₀‐based spin valves is presented and their behavior is modeled with spin‐polarized tunneling via multiple intermediate states with a Gaussian energy distribution. It is shown that, analogous to conductivity mismatch in the diffusive regime, the magnetoresistance decreases with the number of intermediate tunnel steps, regardless of the value of the spin lifetime. This mechanism has been largely overlooked in previous studies of organic spin valves. In addition, using measurements of the temperature and bias dependence of the magnetoresistance, inhomogeneous magnetostatic fields resulting from interfacial roughness are identified as a source for spin relaxation and dephasing. These findings constitute a comprehensive understanding of the processes underlying spin‐polarized transport in these structures and shed new light on previous studies of organic spin valves.     

Hierarchical Microswarms with Leader–Follower‐Like Structures: Electrohydrodynamic Self‐Organization and Multimode Collective Photoresponses

Swarming micro/nanomotors can self‐organize into cohesive groups to execute cooperative tasks. To date, research work has focused on the construction of egalitarian microswarms composed of similar individuals. The construction and collective behaviors of hierarchical leader–follower‐like microswarms are demonstrated. By inducing converging electrohydrodynamic flows under an AC electric field, dielectric microparticles with different sizes and dielectric properties can hierarchically self‐organize into leader–follower‐like microswarms under attractive electrohydrodynamic interactions, and show novel emergent collective behaviors. First, different from immobile single constituents or egalitarian clusters, the hierarchical microswarms autonomously move with tunable speed. Second, they exhibit multimode collective photoresponses emerging from different behaviors of the constituents in response to light signals. With a vertical UV signal, the photoresponsive followers tend to surround the leader and stop the microswarm. In response to sidewise UV signals, the constituents with stronger phototaxis would migrate to the position away from light stimuli, and thus the microswarms reorient parallel/antiparallel to the light direction and perform collective positive/negative phototaxis. Due to differential roles and huge design spaces of constituents, the hierarchical microswarms are envisioned to possess merits of high‐efficiency, multiresponsiveness, and multifunctions, and may serve as intelligent micro/nanorobot systems for biomedicine and microengineering.     

A Versatile Light‐Switchable Nanorod Memory: Wurtzite ZnO on Perovskite SrTiO3

Integrating materials with distinct lattice symmetries and dimensions is an effective design strategy toward realizing novel devices with unprecedented functionalities, but many challenges remain in synthesis and device design. Here, a heterojunction memory made of wurtzite ZnO nanorods grown on perovskite Nb‐doped SrTiO₃ (NSTO) is reported, the electronic properties of which can be drastically reconfigured by applying a voltage and light. Despite of the distinct lattice structures of ZnO and NSTO, a consistent nature of single crystallinity is achieved in the heterojunctions via the low‐temperature solution‐based hydrothermal growth. In addition to a high and persistent photoconductivity, the ZnO/NSTO heterojunction diode can be turned into a versatile light‐switchable resistive switching memory with highly tunable ON and OFF states. The reversible modification of the effective interfacial energy barrier in the concurrent electronic and ionic processes most likely gives rise to the high susceptibility of the ZnO/NSTO heterojunction to external electric and optical stimuli. Furthermore, this facile synthesis route is promising to be generalized to other novel functional nanodevices integrating materials with diverse structures and properties.     

Electric‐Driven Rotation of Silicon Nanowires and Silicon Nanowire Motors

In this work, the first highly controllable assembly and rotation of silicon nanowires and nanomotors in suspension are reported. Si and Si composite nanowires are fabricated with precisely controlled dimensions via colloidal assisted catalytic etching. The nanowires can be rotated with deterministic speed and chirality. The rotation speed and orientation not only depend on the applied AC electric frequency, but also on the electronic type, geometry, surface coating, as well as the electric conductance of suspension mediums. Theoretical analysis is used to understand the rotation of Si nanowires, and also the electric resistivity of Si nanowires is determined from their mechanical rotation. The Si nanowires are precisely assembled into nanomotors that can be rotated with controlled speeds and orientations at prescribed locations. This work provides a new paradigm for designing and actuating various Si‐based nanoelectromechanical system (NEMS) devices, which are relevant to man‐made nanomotors, nanorobots, and nanoengines.     

Spin‐Torque Manipulation for Hydrogen Sensing

External manipulation of spin‐orbit torques (SOTs) promises not only energy‐efficient spin‐orbitronic devices but also versatile applications of spin‐based technologies in diverse fields. However, the external electric‐field control, widely used in semiconductor spintronics, is known to be ineffective in conventional metallic spin‐orbitronic devices due to the very short screening length. Here, an alternative approach to control the SOTs by using gases is shown. It is demonstrated that the spin‐torque generation efficiency of a Pd/Ni₈₁Fe₁₉ bilayer can be reversibly manipulated by the absorption and desorption of H₂ gas, which appears concomitantly with the change of the electrical resistance. It is found that compared with the change of the Pd resistance induced by the H₂ absorption, the change of the spin‐torque generation efficiency is almost an order of magnitude larger. This result provides a new method to externally manipulate the SOTs and paves a way for developing more sensitive hydrogen sensors based on the spin‐orbitronic technology.     

双折射晶体微粒光致旋转受其半径影响分析

利用双折射晶体微粒在具有自旋角动量的光束作用下可产生围绕自身光轴旋转的特性,在光镊实验平台上实现了双折射晶体微粒的光致旋转。为了提高晶体微粒的旋转频率,从理论和实验上对双折射晶体微粒的旋转频率受其半径的影响进行了分析。用MATLAB模拟出CaCO3晶体微粒和SiO2晶体微粒的旋转频率与其半径的三次方成反比的关系曲线,并测得相应的实验关系曲线,其结果与理论分析相吻合。在相同的激光功率下,CaCO3晶体微粒的最高旋转频率可达15.1 Hz,SiO2晶体微粒的最高旋转频率可达11.4 Hz。该结论可用于光致旋转在实际应用中晶体微粒大小的选择和其旋转频率的优化控制。     

利用偏振光实现双折射微粒的光致旋转

利用偏振光与双折射性微粒相互作用时自旋角动量的传递 ,实现了光镊悬浮的CaCO3 晶体微粒的光致旋转 ,并利用四象限探测器 (QD)测量了微粒转动周期及其与偏振状态的关系 ,还分析讨论了微粒在光镊作用下的转动特性 ,给出一些定性结论。     

Stimuli‐Responsive Liquid Marbles: Controlling Structure,Shape, Stability,and Motion

“Liquid marbles” are liquid‐in‐gas dispersed systems stabilized by hydrophobic solid particles adsorbed at the gas‐liquid interface. The structure, stability and movement of these liquid marbles can be controlled by external stimuli such as pH, temperature, light, magnetic and electric fields, ultrasonic, mechanical stress and organic solvents. Stimuli‐responsive modes can be categorized into five classes: (i) liquid marbles whose stability can be controlled by adsorption/desorption of solid particles to/from liquid surfaces, (ii) liquid marbles that can open and close their particle‐coated surface by moving particles to and from the gas‐liquid surface, (iii) liquid marbles that can move, (iv) liquid marbles that can change their shape and (v) liquid marbles that can be split. As a result of these stimuli‐responsive characteristics, liquid marbles offer potential in the areas of controlled encapsulation, delivery and release.     

Simulations of polaron spin inversion in an organic semiconductor: Comparison of two mechanisms

We present a model study of the effects of two mechanisms, the Rashba spin–orbit coupling and the spin-flip term, on the polaron spin inversion in an organic semiconductor. We find that, while both mechanisms can impact the polaron spin by changing the polaron level from a spin eigenstate to a spin superposition state, substantial difference can be observed in the static and dynamical properties of the polaron. Given the values of model parameters relevant to conjugated polymers, the magnitude of the polaron spin inversion caused by the spin–orbit coupling is much smaller than that by the spin-flip term. When the dynamical properties of the polaron are considered, spin oscillations induced by both mechanisms are observed when the polaron moves along the polymer chain driven by external electric field. Interestingly, the length of the polaron motion during one spin oscillation period remains constant in the case of spin–orbit coupling, while it is enhanced with increasing the driven electric field in the case of spin-flip term, in which larger spin diffusion length and longer spin relaxation time can be expected.     

利用具有自旋角动量的光束实现微粒的旋转

从理论上分析了偏振光束与双折射晶体粒子的相互作用过程,讨论了由于光束自旋角动量向晶体粒子的传递所导致的光致旋转效应的原理,通过MATLAB仿真分析,研究了粒子的旋转频率随激光功率的变化关系,得出粒子转动频率与激光功率成正比.利用光镊装置,采用波长为632.8 nm的He-Ne激光器,在不同的激光功率下实现对不同半径双折射粒子的旋转,测量了光致旋转的转动频率,最高转速可达5 r/s,并得出了不同粒子的旋转频率随激光功率的变化关系,实验结果和理论分析基本一致.分析了产生误差的原因,其中载波片底面的摩擦影响最大.     

A contactless electrical energy transmission system

Most mains-operated equipment in use today is connected to the supply via plugs and sockets. These are generally acceptable in benign environments, but can be unsafe or have limited life in the presence of moisture. In explosive atmospheres and in undersea applications, special connectors must be used. This paper describes a technique, the contactless energy transfer system (CETS), by which electrical energy may be transmitted, without electrical connection or physical contact, through nonmagnetic media of low conductivity. The CETS, which has been used to transfer up to 5 kW across a 10 mm gap, employs high-frequency magnetic coupling and enables plug-in power connections to be made in wet or hazardous environmental conditions without the risk of electric shock, short circuiting, or sparking. Energy may be transmitted without the necessity for accurately manufactured “plug-and-socket” mechanisms and may be transmitted from source to load, even when there is relative motion. Load-source voltage matching may be made inherent to the system     

A Switchable Microstrip Patch Antenna for Dual Frequency Operation

A novel design for equilateral‐triangular microstrip antennas with switchable resonant frequency is proposed. For dual‐frequency operation, the proposed design is achieved by loading a pair of slits in the triangular patch, and two PIN diodes are utilized to switch the slits on or off. By increasing the length of the slits, the lower resonant frequency can be varied in the range from 1.22 GHz to 1.72 GHz whereas the upper resonant frequency remains unchanged.     

A new class of multi‐bandgap high‐efficiency photovoltaics enabled by broadband diffractive optics

A semiconductor absorber with a single bandgap is unable to convert broadband sunlight into electricity efficiently. Photons with energy lower than the bandgap are not absorbed, whereas those with energy far higher than the bandgap lose energy via thermalization. In this Article, we demonstrate an approach to mitigate these losses via a thin, efficient broadband diffractive micro‐structured optic that not only spectrally separates incident light but also concentrates it onto multiple laterally separated single‐junction semiconductor absorbers. A fully integrated optoelectronic device model was applied in conjunction with a nonlinear optimization algorithm to design the optic. An experimental demonstration is presented for a dual‐bandgap design using GaInP and GaAs solar cells, where a 20% increase in the total electric power is measured compared with the same cells without the diffractive optic. Finally, we demonstrate that this framework of broadband diffractive optics allows us to independently design for the number of spectral bands and geometric concentration, thereby enabling a new class of multi‐bandgap photovoltaic devices with ultra‐high energy conversion efficiencies. Copyright © 2014 John Wiley & Sons, Ltd.     

Light‐Driven Electrohydrodynamic Instabilities in Liquid Crystals

The induction of electrohydrodynamic instabilities in nematic liquid crystals through light illumination are reported. For this purpose, a photochromic spiropyran is added to the liquid crystal mixture. When an electrical field is applied in the absence of UV light, the homeotropic liquid crystal reorients perpendicular to the electrical field driven by its negative dielectric anisotropy. Upon exposure to UV light, the nonionic spiropyran isomerizes to the zwitterionic merocyanine form inducing electrohydrodynamic instabilities which turns the cell from transparent into highly scattering. The reverse isomerization to closed‐ring spiropyran form occurs thermally or under visible light, which stops the electrohydrodynamic instabilities and the cell becomes transparent again. It is demonstrated that the photoionic electrohydrodynamic instabilities can be used for light regulation. Local exposure, either to drive the electrohydrodynamics or to remove them enables the formation of colored images.