Biomimetic Bidirectional Switchable Adhesive Inspired by the Gecko

The gecko adhesive system has attracted significant attention since the discovery that van der Waals interactions, which are always present between surfaces, are predominantly responsible for their adhesion. The unique anisotropic frictional–adhesive capabilities of the gecko adhesive system originate from complex hierarchical structures and just as importantly, the anisotropic articulation of the structures. Here, by cleverly engineering asymmetric polymeric microstructures, a reusable switchable gecko‐like adhesive can be fabricated yielding steady high adhesion ( ≈ 1.25 N/cm²) and friction ( ≈ 2.8 N/cm²) forces when actuated for “gripping”, yet release easily with minimal adhesion ( ≈ 0.34 N/cm²) and friction (≈ 0.38 N/cm²) forces during detachment or “releasing”, over multiple attachment/detachment cycles, with a relatively small normal preload of 0.16 N/cm² to initiate the adhesion. These adhesives can also be used to reversibly suspend weights from vertical (e.g., walls), and horizontal (e.g., ceilings) surfaces by simultaneously and judiciously activating anisotropic friction and adhesion forces. This design opens the way for new gecko‐like adhesive surfaces and articulation mechanisms that do not rely on intensive nanofabrication in order to recover the anisotropic tribological property of gecko adhesive pads, albeit with lower adhesive forces compared to geckos.     

Controllable Anisotropic Dry Adhesion in Vacuum: Gecko Inspired Wedged Surface Fabricated with Ultraprecision Diamond Cutting

Gecko adhesion has inspired the fabrication of various dry adhesive surfaces, most of which are developed to be used under atmospheric conditions. However, applications of gecko‐inspired surfaces can be expanded to vacuum and even space environment due to the characteristics of van der Waals interactions, which are always present between materials regardless of the surrounding environment. In this paper, a controllable, anisotropic dry adhesion in vacuum is demonstrated with gecko‐inspired wedged dry adhesive surfaces fabricated using an ultraprecision diamond cutting mold. The adhesion and friction properties of the wedge‐structured surfaces are systematically characterized in loading–pulling mode and loading–dragging–pulling mode. The surfaces show significant anisotropic adhesion (P_ad ≈ 10.5 kPa vs P_ad ≈ 0.7 kPa) and friction (P_f ≈ 50 kPa vs P_f ≈ 30 kPa) when actuated in gripping and releasing direction, respectively. The wedge‐structured surfaces in vacuum show comparable properties as exposed in atmosphere. A three‐legged gripper is designed to pick up, hold, and release a patterned silicon wafer in vacuum. The study demonstrates a green, high‐yield, and low‐cost method to fabricate a reliable and durable mold for gecko inspired anisotropic dry adhesive surfaces and the potential application of dry adhesive surface in vacuum.     

Insights into the Adhesive Mechanisms of Tree Frogs using Artificial Mimics

Elastic, microstructured surfaces (hydrophobic and hydrophilic) mimicking the surface structure of tree‐frog toe‐pads are fabricated. Their adhesion and friction behaviour in the presence of a liquid layer is evaluated and compared to flat controls. Tree‐frog‐like patterns are beneficial for wet adhesion only if the liquid does not wet the surface. The situation is different in friction, where the surface structure lead to significantly higher friction forces only if the liquid does wet the surface. Taking into account that tree‐frog attachment pads are hydrophilic and that their secretion wets all kind of surfaces, our results indicate that the surface structure in tree‐frog toe‐pads has been developed for climbing, when shear (friction) forces are involved. These results evidence the benefits and limitations of the surface design (microstructure and hydrophilicity) for adhesion and friction under wet conditions.     

Fluorinated Polyimide as a Novel High‐Voltage Binder for High‐Capacity Cathode of Lithium‐Ion Batteries

An increase in the energy density of lithium‐ion batteries has long been a competitive advantage for advanced wireless devices and long‐driving electric vehicles. Li‐rich layered oxide, xLi₂MnO₃?(1?x)LiMn_1?y?zNi_yCo_zO₂, is a promising high‐capacity cathode material for high‐energy batteries, whose capacity increases by increasing charge voltage to above 4.6 V versus Li. Li‐rich layered oxide cathode however suffers from a rapid capacity fade during the high‐voltage cycling because of instable cathode–electrolyte interface, and the occurrence of metal dissolution, particle cracking, and structural degradation, particularly, at elevated temperatures. Herein, this study reports the development of fluorinated polyimide as a novel high‐voltage binder, which mitigates the cathode degradation problems through superior binding ability to conventional polyvinylidenefluoride binder and the formation of robust surface structure at the cathode. A full‐cell consisting of fluorinated polyimide binder‐assisted Li‐rich layered oxide cathode and conventional electrolyte without any electrolyte additive exhibits significantly improved capacity retention to 89% at the 100th cycle and discharge capacity to 223–198 mA h g^?1 even under the harsh condition of 55 °C and high charge voltage of 4.7 V, in contrast to a rapid performance fade of the cathode coated with polyvinylidenefluoride binder.     

Thermoelectric Properties of Tix(HfyZr1?y)1?xNiSn0.998Sb0.002 Half-Heusler Ribbons

Ribbons of Ti _x (Hf _y Zr_1−y )_1−x NiSn_1−z Sb _z (x = 0.1 to 1, y = 0.1 to 0.9, z = 0, 0.002, 0.004) were prepared by spin casting and annealed for 1 h at T _a = 1000 K, 1050 K, 1073 K, and 1100 K. The crystal phase of the ribbons was investigated by x-ray diffraction analysis and transmission electron microscopy. All the ribbons consisted of a phase with a half-Heusler structure. The Seebeck coefficient, electrical conductivity, thermal conductivity, power factor, and figure of merit ZT at room temperature were clarified experimentally as a function of x, y, z, and T _a. Despite the large thermal conductivity, the power factor and figure of merit were remarkably large at x = 0.5, y = 0.5, z = 0.002, and T _a = 1073 K, because the Seebeck coefficient and electrical conductivity were large.     

Mobile‐to‐mobile MIMO transmit–receive diversity systems: analysis and performance in three‐dimensional double‐correlated channels

Mobile‐to‐mobile (M‐to‐M) communications are expected to play a crucial role in future wireless systems and networks. In this paper, we consider M‐to‐M multiple‐input multiple‐output (MIMO) maximal ratio combining system and assess its performance in spatially correlated channels. The analysis assumes double‐correlated Rayleigh‐and‐Lognormal fading channels and is performed in terms of average symbol error probability, outage probability, and ergodic capacity. To obtain the receive and transmit spatial correlation functions needed for the performance analysis, we used a three‐dimensional (3D) M‐to‐M MIMO channel model, which takes into account the effects of fast fading and shadowing. The expressions for the considered metrics are derived as a function of the average signal‐to‐noise ratio per receive antenna in closed‐form and are further approximated using the recursive adaptive Simpson quadrature method. Numerical results are provided to show the effects of system parameters, such as distance between antenna elements, maximum elevation angle of scatterers, orientation angle of antenna array in the x–y plane, angle between the x–y plane and the antenna array orientation, and degree of scattering in the x–y plane, on the system performance. Copyright © 2011 John Wiley & Sons, Ltd.     

Solid solution InxGa1−x AsySbzP1−y−z : A new material for infrared optoelectronics. I. Thermodynamic analysis of the conditions for obtaining solid solutions, isoperiodic to InAs and GaSb substrates, by liquid-phase epitaxy

The diagrams of melt-solid solution (fusibility curves) and solid solution (I)-solid solution (II) (surfaces of spinodal decomposition of solid solutions) phase equilibria in the five-component system In-Ga-As-Sb-P (the solid solutions are isoperiodic to the GaSb and InAs substrates) are calculated. The concentration ranges of the isovalent substitution solid solutions In_xGa_1−x As_ySb_zP_1−y−z , which are accessible for synthesis by liquid-phase epitaxy, are calculated. Fiz. Tekh. Poluprovodn. 31, 410–415 (April 1997)     

Long‐term energy production of III–V triple‐junction solar cells on the Martian surface

Based on theoretical considerations, optimum triple‐junction bandgap combinations are determined in order to achieve highest electrical energy production for a mission on the Martian surface. The solar cell structures analysed in this contribution are based on the Ga_yIn_1–yP, Ga_xIn_1–xAs and Ge material system. A comparison of theoretical and already realised triple‐junction solar cell structures is presented. For the evaluation of long‐term electrical energy production, different geographic and climatic Martian scenarios are considered. Copyright © 2010 John Wiley & Sons, Ltd.     

Nitrogen implantations for rapid thermal oxinitride layers

Oxidation of nitrogen implanted substrates results in so called silicon-oxinitride layers (Si_xO_yN_z layers) which are dependent on implantation dose and energy always thinner than pure silicon-oxides (SiO₂) produced under the same oxidation conditions. Elastic recoil detection profiles indicate that the implanted nitrogen diffuses out of the substrate into the silicon-oxide layer what improves the electrical quality of these insulators. The Si_xO_yN_z layers show lower Fowler–Nordheim tunnelling currents as well as lower interface state densities (D_it) than the corresponding SiO₂ layers or N₂O–silicon-oxinitride insulators. NH₃–Si_xO_yN_z layers show the lowest D_it values because of H₂-annealing effects but contain fixed charges.     

MXene Electrode for the Integration of WSe2 and MoS2 Field Effect Transistors

Recently, MXenes, which are 2D early transition metal carbides and carbonitrides, have attracted wide attention because of their excellent conductivities. Here, the electrode applications of Ti₂C(OH)_xF_y, one member of the MXene family, in WSe₂ and MoS₂ field effect transistors (FETs) are assessed. Kelvin probe force microscopy analysis is performed to determine its work function, which is estimated to be ≈4.98 eV. Devices based on WSe₂/Ti₂C(OH)_xF_y and MoS₂/Ti₂C(OH)_xF_y heterostructures are fabricated with the mechanical transfer method and their electronic performances evaluated. The temperature‐dependent current–voltage transfer characteristics of the devices are determined to extract their Schottky barrier heights. The hole barrier between WSe₂ and Ti₂C(OH)_xF_y is estimated to be ≈0.23 eV and the electron barrier between the MoS₂ band and Ti₂C(OH)_xF_y is ≈0.19 eV, which indicates that the pinning effect occurs at the MoS₂/Ti₂C(OH)_xF_y interface but not at the WSe₂/Ti₂C(OH)_xF_y interface; this difference arises because of the difference between the band structures of WSe₂ and MoS₂. A complementary metal–oxide–semiconductor inverter based on these electrode properties of Ti₂C(OH)_xF_y with MoS₂ (n‐channel) and WSe₂ (p‐channel) is fabricated, which demonstrates that Ti₂C(OH)_xF_y is a promising electrode for future nanoelectronics applications.     

Electronic structure of Ge1 − x − ySixSny ternary alloys for multijunction solar cells

Ternary group‐IV alloys have a wide potential for applications in infrared devices and optoelectronics. In connection with photovoltaic applications, they are among the most promising materials for inclusion in the next generation of high‐efficiency multijunction solar cells, because they can be lattice matched to substrates as GaAs and Ge, offering the possibility of a range of band gaps complementary to III–V semiconductors. Apart from the full decoupling of lattice and band structures in Ge_{1 − x − y}Si_xSn_y alloys, experimentally confirmed, they allow preparation in a controllable and large range of compositions, thus enabling to tune their band gap. Recently, optical experiments on ternary alloy‐based films, photodetectors measured the direct absorption edges and probed the compositional dependence of the direct gap. The nature of the fundamental gap of Ge_{1 − x − y}Si_xSn_y alloys is still unknown, as neither experimental data on the indirect edges nor electronic structure calculations are available, as yet. Here, we report a first calculation of the electronic structure of Ge_{1 − x − y}Si_xSn_y ternary alloys, employing a combined tight‐binding and virtual crystal approximation method, which proved to be useful to describe group‐IV semiconductor binary alloys. Our results confirm predictions and experimental indications that a 1eV band gap is indeed attainable with these ternary alloys, as required for the fourth layer plan to be added to present‐day record‐efficiency triple‐junction solar cells, to further increase their efficiency, for example, for satellite applications. When lattice matched to Ge, we find that Ge_{1 − x − y}Si_xSn_y ternary alloys have an indirect gap with a compositional dependence reflecting the presence of two competing minima in the conduction band. Copyright © 2013 John Wiley & Sons, Ltd.     

Controlling Phase Assemblage in a Complex Multi‐Cation System: Phase‐Pure Room Temperature Multiferroic (1−x)BiTi(1−y)/2FeyMg(1−y)/2O3–xCaTiO3

A room temperature magnetoelectric multiferroic is of interest as, e.g., magnetoelectric random access memory. Bulk samples of the perovskite (1?x)BiTi_(1?y)/2Fe_yMg_(1?y)/2O₃–xCaTiO₃ (BTFM–CTO) are simultaneously ferroelectric, weakly ferromagnetic, and magnetoelectric at room temperature. In BTFM–CTO, the volatility of bismuth oxide, and the complex subsolidus reaction kinetics, cause the formation of a microscopic amount of ferrimagnetic spinel impurity, which complicates the quantitative characterization of their intrinsic magnetic and magnetoelectric properties. Here, a controlled synthesis route to single‐phase bulk samples of BTFM–CTO is devised and their intrinsic properties are determined. For example, the composition x = 0.15, y = 0.75 shows a saturated magnetization of 0.0097μ_B per Fe, a linear magnetoelectric susceptibility of 0.19(1) ps m^?1, and a polarization of 66 μC cm^?2 at room temperature. The onset of weak ferromagnetism and linear magnetoelectric coupling are shown to coincide with the onset of bulk long‐range magnetic order by neutron diffraction. The synthesis strategy developed here will be invaluable as the phase diagram of BTFM–CTO is explored further, and as an example for the synthesis of other compositionally complex BiFeO₃‐related materials.     

Structure and optical properties of heterostructures based on MOCVD (Al x Ga1 − x As1 − y P y )1 − z Si z alloys

Epitaxial heterostructures produced by MOCVD on the basis of Al _x Ga_{1 ? x} As ternary alloys with the composition parameter x ≈ 0.20–0.50 and doped to a high Si and P atomic content are studied. Using the high-resolution X-ray diffraction technique, scanning electron microscopy, X-ray microanalysis, Raman spectroscopy, and photoluminescence spectroscopy, it is shown that the epitaxial films grown by MOCVD are formed of five-component (Al _x Ga_{1 ? x} As_{1 ? y} P _y )_{1 ? z} Si _z alloys.     

Local State‐of‐Charge Mapping of Lithium‐Ion Battery Electrodes

Current lithium‐ion battery technology is gearing towards meeting the robust demand of power and energy requirements for all‐electric transportation without compromising on the safety, performance, and cycle life. The state‐of‐charge (SOC) of a Li‐ion cell can be a macroscopic indicator of the state‐of‐health of the battery. The microscopic origin of the SOC relates to the local lithium content in individual electrode particles and the effective ability of Li‐ions to transport or shuttle between the redox couples through the cell geometric boundaries. Herein, micrometer‐resolved Raman mapping of a transition‐metal‐based oxide positive electrode, Li_1‐x(Ni_yCo_zAl_1‐y‐z)O₂, maintained at different SOCs, is shown. An attempt has been made to link the underlying changes to the composition and structural integrity at the individual particle level. Furthermore, an SOC distribution at macroscopic length scale of the electrodes is presented.     

Specific Heat Capacities of Sn-Zn-Based Solders and Sn-Ag-Cu Solders Measured Using Differential Scanning Calorimetry

The specific heat capacities (C _p) of Sn-Zn-based solders and Sn-Ag-Cu solders have been studied using differential scanning calorimetry. The procedure of measuring the specific heat capacity followed the standard test method designed by the American Society for Testing and Materials (ASTM) E1269-05. The results of this work are lists of specific heat capacities of Sn-9Zn, Sn-9Zn-xAg (x = 0.1, 0.5, 1, 2, and 3), Sn-9Zn-0.5Ag-yAl (y = 0.1, 0.2, and 0.5), Sn-9Zn-0.5Ag-yGa (y = 0.1, 0.2, and 0.5), Sn-8.5Zn-0.5Ag-0.01Al-0.1Ga, and Sn-zAg-0.5Cu (z = 1.0, 2.0, 3.0, and 3.5). The study also found that C _p increased with increasing heating temperature. Furthermore, the lead-free solders investigated have a higher specific heat capacity than the traditional Sn-37Pb solder. Among the studied lead-free solders, Sn-3.5Ag-0.5Cu has the lowest C _p and Sn-9Zn-0.1Ag has the highest C _p. Increased silver content in the Sn-9Zn-xAg and Sn-xAg-0.5Cu solder alloys was also found to effectively lower their C _p.     

Electromigration of Cu/low dielectric constant interconnects

Electromigration in damascene Cu/low dielectric constant interconnects with overlayers of CoWP, Ta/TaN, SiN_x or SiC_xN_yH_z and Cu(Ti) interconnects capped with SiN_x was studied. The results showed that the migration fast path in the bamboo-like lines primarily occurred at the interface. Cu lines fabricated with various forms of TaN/Ta liner including PVD TaN, ALD TaN, and PVD body centered cubic α- or tetragonal β-Ta liners were also investigated. Both thin surface layers of CoWP or Ta/TaN and the addition of Ti in the Cu lines significantly reduced the Cu/cap interface diffusivity and remarkably improved the electromigration lifetime when compared with Cu lines capped with SiN_x or SiC_xN_yH_z. Activation energies for electromigration were found to be 1.9–2.4 eV, 1.4 eV, 0.85–1.1 eV, and 1.3 eV for the bamboo-like Cu lines capped with CoWP, Ta/TaN, and SiN_x or SiC_xN_yH_z, and Cu(Ti) bamboo lines capped with SiN_x, respectively. The structural phase of the Ta was found to have an insignificant effect on the Cu mass flow rate. A large via size, thicker liner and/or stable connected exposed liner can provide a longer lifetime and tighter lifetime distribution, at the expense of chip density or effective Cu line conductivity.     

NixSy Nanowalls/Nitrogen‐Doped Graphene Foam Is an Efficient Trifunctional Catalyst for Unassisted Artificial Photosynthesis

Solar‐to‐hydrogen (STH) conversion through unassisted artificial photosynthesis (APS) devices is one of the promising and environmentally friendly strategies for sustainable development. However, the practical large‐scale application of the unassisted APS devices is impeded by the need for expensive noble metal‐based catalysts in photovoltaics and/or electrolyzers. Herein, well‐aligned 2D Ni_xS_y nanowalls (2D Ni_xS_y NWs) on a 3D nitrogen‐doped graphene foam (3D NGF) are synthesized and further employed it in unassisted APS. Due to the positive synergistic effect between the highly electrocatalytic activity of Ni_xS_y NW and excellent conductivity of NGF, this low cost material of (2D/3D) Ni_xS_y NW/NGF is highly efficient as a multifunctional catalyst in various applications: a counterelectrode for dye‐sensitized solar cell (DSSC) and a “bifunctional” electrocatalyst for oxygen and hydrogen evolution for electrocatalytic overall water splitting. Furthermore, three Ni_xS_y NW/NGF‐based DSSCs as a tandem cell for unassisted solar‐driven overall water splitting is connected, using Ni_xS_y NW/NGF itself on nickel foams as the anode and cathode. Impressively, such integrated photovoltaic‐electrolyzer APS device can achieve an STH efficiency of 3.2% with an excellent stability and low cost. This work opens an avenue to advanced multifunctional materials for the low‐cost and unassisted solar‐driven overall water splitting.     

Preparation and luminescent properties of novel red phosphors for white-light emitting diodes (W-LEDs) application

A series of Eu³⁺–Gd³⁺ co-doped solid solution of Ca_0.54Sr_{0.46–1.5x–1.5z}Eu_zGd_x (MoO₄)_y (WO₄)_1−y (x=0.01–0.20, y=0–1.0, z=0.01–0.30) have been prepared by solid-state reactions. It is found that appropriate amount of Mo⁶⁺ or W⁶⁺, Eu³⁺ and Gd³⁺concentrations can enhance the luminescent intensity and improve crystal structure. These phosphors can be effectively excited by ultraviolet light at 394 nm and blue light at 465 nm (f–f transition) and emits red light (616 nm) with line spectrum. The wavelengths at 394 and 465 nm are nicely fitted in with the widely applied output wavelengths of ultraviolet or blue LED chips.     

Effect of carbon containing SiNx antireflection coating on the screen‐printed contact and low illumination performance of silicon solar cell

Screen‐printed metal contact formation through a carbon containing antireflection coating was investigated for silicon solar cells by fabricating conventional carbon‐free SiN_x and carbon‐rich SiC_xN_y film. An appreciable difference was found in the average shunt resistance (R_sh), which was about an order of magnitude higher for SiC_xN_y‐coated solar cells relative to the counterpart SiN_x‐coated solar cells. Series resistance (R_s) and fill factor (FF) were comparable for both antireflection coatings but the starting efficiency of SiC_xN_y‐coated cell was ~0·2% lower because of slightly inferior surface passivation. However, SiC_xN_y‐coated solar cells showed less degradation under lower illumination (<1000 W/m²) compared with the SiN_x‐coated cells due to reduced FF degradation under low illumination. Theoretical calculations in this paper support that this is a direct result of high R_sh. Detailed photovoltaic system and cost modeling is performed to quantify the enhanced energy production and the reduced levelized cost of electricity due to higher shunt resistance of the SiC_xN_y‐coated cells. It is shown that R_sh value below 30 Ω (7000 Ω cm² for 239 cm² cell) can lead to appreciable loss in energy production in regions of low solar insolation. Copyright © 2011 John Wiley & Sons, Ltd.     

Analysis of the resonant cavities using the FDTD scheme with effective permittivities formulae

An efficient expression of the effective permittivities along x-, y-, z-direction is obtained by analogy with the circuit concept of series–parallel connection of capacitance. Under the condition of numerical stability, the approximate formulae of effective permittivities for various inhomogeneous cell mesh are combined with the Finite Difference Time Domain (FDTD) scheme of arbitrary spatial cell size to calculate resonant frequency of 3-D cavity. The results are in agreement with that of conventional FDTD method, but the proposed method can save computing time and memory.