Dry Processing and Recycling of Thick Nacre–Mimetic Nanocomposites

Bioinspired nanocomposites with high levels of reinforcement hold great promise for future, green lightweight, and functional engineering materials, but they suffer from slow, tedious, and nonscalable preparation routes, that typically only lead to very thin films. A rapid and facile dry powder processing technique is introduced to generate bioinspired nanocomposite materials at high fractions of reinforcements (50 wt%) and with millimeter scale thickness. The process uses powder drying of vitrimer-coated nanoplatelets (nanoclay and MXene) from aqueous solution and subsequent hot-pressing. As a method of choice in industrial lightweight composite materials engineering, hot-pressing underscores a high potential to translate this approach to actual products. The use of the vitrimer chemistry with temperature-activated bond shuffling is important to facilitate smooth integration into the nanocomposite design, leading to layered nacre-inspired nanocomposites with nanoscale hard/soft order traced by X-ray diffraction and excellent mechanical properties investigated using flexural tests. Recycling by grinding and hot-pressing is possible without property loss. The compatibility with existing composite processing techniques, scalable thickness and dimensions, and recyclability open considerable opportunities for translating bioinspired nanocomposites to real-life applications.     

Ultranarrow Bandwidth Organic Photodiodes by Exchange Narrowing in Merocyanine H‐ and J‐Aggregate Excitonic Systems

Ultranarrowband organic photodiodes (OPDs) are demonstrated for thin film solid state materials composed of tightly packed dipolar merocyanine dyes. For these dyes the packing arrangement can be controlled by the bulkiness of the donor substituent, leading to either strong H‐ or strong J‐type exciton coupling in the interesting blue (H‐aggregate) and NIR (J‐aggregate) spectral ranges. Both bands are shown to arise from one single exciton band according to fluorescence measurements and are not just a mere consequence of different polymorphs within the same thin film. By fabrication of organic thin‐film transistors, these dyes are demonstrated to exhibit hole transport behavior in spin‐coated thin films. Moreover, when used as organic photodiodes in planar heterojunctions with C₆₀ fullerene, they show wavelength‐selective photocurrents in the solid state with maximum external quantum efficiencies of up to 11% and ultranarrow bandwidths down to 30 nm. Thereby, narrowing the linewidths of optoelectronic functional materials by exciton coupling provides a powerful approach to produce ultranarrowband organic photodiodes.     

Superoleophilic Magnetic Iron Oxide Nanoparticles for Effective Hydrocarbon Removal from Water

Various hydrocarbons are efficiently extracted from water by using a new sorbent material based on covalently functionalized magnetic nanoparticles. The functionalization of the magnetite nanoparticles with a self‐assembled monolayer of hexadecylphosphonic acid renders the nanoparticles oleophilic and the magnetic nature of magnetite allows for simple extraction of the hydrocarbon‐soaked sorbent. The sorbent material is capable of extracting single contaminants such as alkanes and aromatics and complex hydrocarbon mixtures such as crude oils in high extraction rates of up to 14 times the sorbent volume. Experimental results are explained by molecular dynamics simulations on the adsorption of single components from a hydrocarbon‐water mixture to the alkylphosphonic acid layer on the nanoparticles. The core–shell sorbent material is highly stable and therefore, reusable over several successive extraction cycles without degradation. The extraction performance is determined at different water temperatures, different water sources, and different magnetic core materials and evaluated compared to heptadecanoic acid functionalized magnetite. The new sorbent material provides the opportunity for an efficient, reliable, inexpensive, and environmental friendly removal of hydrocarbons from water.     

Shallow and Undoped Germanium Quantum Wells: A Playground for Spin and Hybrid Quantum Technology

Buried‐channel semiconductor heterostructures are an archetype material platform for the fabrication of gated semiconductor quantum devices. Sharp confinement potential is obtained by positioning the channel near the surface; however, nearby surface states degrade the electrical properties of the starting material. Here, a 2D hole gas of high mobility (5 × 10⁵ cm² V^?1 s^?1) is demonstrated in a very shallow strained germanium (Ge) channel, which is located only 22 nm below the surface. The top‐gate of a dopant‐less field effect transistor controls the channel carrier density confined in an undoped Ge/SiGe heterostructure with reduced background contamination, sharp interfaces, and high uniformity. The high mobility leads to mean free paths ≈ 6 µm, setting new benchmarks for holes in shallow field effect transistors. The high mobility, along with a percolation density of 1.2 × 10¹¹cm^?2, light effective mass (0.09m_e), and high effective g‐factor (up to 9.2) highlight the potential of undoped Ge/SiGe as a low‐disorder material platform for hybrid quantum technologies.     

A Novel Buffering Technique for Aqueous Processing of Zinc Oxide Nanostructures and Interfaces,and Corresponding Improvement of Electrodeposited ZnO‐Cu2O Photovoltaics

A novel buffering method is presented to improve the stability of zinc oxide processed in aqueous solutions. By buffering the aqueous solution with a suitable quantity of sacrificial zinc species, the dissolution of functional zinc oxide structures and the formation of unwanted impurities can be prevented. The method is demonstrated for ZnO films and nanowires processed in aqueous solutions used for the selective etching of mesoporous anodic alumina templates and the electrochemical deposition of Cu₂O. In both cases, improved ZnO stability is observed with the buffering method. ZnO‐Cu₂O heterojunction solar cells (bilayer and nanowire cells) synthesized using both traditional and buffered deposition methods are characterized by impedance spectroscopy and solar simulation measurements. Buffering the Cu₂O deposition solution is found to reduce unwanted recombination at the heterojunction and improve the photovoltaic performance.     

A study of forward error correction for mobile multicast

3rd Generation Partnership Project (3GPP) has standardized the use of forward error correction (FEC) for the provision of reliable data transmission in the mobile multicast framework. This error control method inevitably adds a constant overhead in the transmitted data. However, it is so simple as to meet a prime objective for mobile multicast services; that is scalability to applications with thousands of receivers. In this paper, we present a study on the impact of application layer FEC on mobile multicast transmissions. We examine whether it is beneficial or not, how the optimal code dimension varies based on network conditions, which parameters affect the optimal code selection, and how this can be done. Additionally, we focus on one of the most critical aspects in mobile multicast transmission, which is power control. The evaluation is performed with the aid of a novel scheme that incorporates the properties of an evolved mobile network, as they are specified by the 3GPP. Copyright © 2010 John Wiley & Sons, Ltd.     

Thermoelectric Coolers with Sintered Silver Interconnects

The fabrication and performance of a sintered Peltier cooler (SPC) based on bismuth telluride with sintered silver interconnects are described. Miniature SPC modules with a footprint of 20 mm² were assembled using pick-and-place pressure-assisted silver sintering at low pressure (5.5 N/mm²) and moderate temperature (250°C to 270°C). A modified flip-chip bonder combined with screen/stencil printing for paste transfer was used for the pick-and-place process, enabling high positioning accuracy, easy handling of the tiny bismuth telluride pellets, and immediate visual process control. A specific contact resistance of (1.4 ± 0.1) × 10^?5 Ω cm² was found, which is in the range of values reported for high-temperature solder interconnects of bismuth telluride pellets. The realized SPCs were evaluated from room temperature to 300°C, considerably outperforming the operating temperature range of standard commercial Peltier coolers. Temperature cycling capability was investigated from 100°C to 235°C over more than 200 h, i.e., 850 cycles, during which no degradation of module resistance or cooling performance occurred.     

Conducting Domain Walls in Lithium Niobate Single Crystals

Ferroic materials play an increasingly important role in novel (nano)electronic devices. Recently, research on domain walls (DWs) receives a big boost by the discovery of DW conductivity (DWC) in BiFeO₃ and Pb(Zr_xTi_1‐x)O₃ ferroic thin films. Here, it is demonstrated that DWC is not restricted to thin films, but equally applies to millimeter‐thick wide‐bandgap, ferroic single crystals, such as LiNbO₃. In this material transport along DWs can be switched by super‐bandgap illumination and tuned by engineering the tilting angle of DWs with respect to the polar axis. The results are consistently obtained using conductive atomic force microscopy to locally map the DWC and macroscopic contacts, thereby in addition investigating the temperature dependence, DW transport activation energies, and relaxation behavior.     

Enhancing the Selectivity and Transparency of the Electron Contact in Silicon Heterojunction Solar Cells by Phosphorus Catalytic Doping

An intrinsic hydrogenated amorphous silicon (a-Si:H(i)) film and a doped silicon film are usually combined in the heterojunction contacts of silicon heterojunction (SHJ) solar cells. In this work, a post-doping process called catalytic doping (Cat-doping) on a-Si:H(i) is performed on the electron selective side of SHJ solar cells, which enables a device architecture that eliminates the additional deposition of the doped silicon layer. Thus, a single phosphorus Cat-doping layer combines the functions of two other layers by enabling excellent interface passivation and high carrier selectivity. The overall thinner layer on the window side results in higher spectral response at short wavelengths, leading to an improved short-circuit current density of 40.31 mA cm⁻² and an efficiency of 23.65% (certified). The cell efficiency is currently limited by sputter damage from the subsequent transparent conductive oxide fabrication and low carrier activation in the a-Si:H(i) with Cat-doping. Numerical device simulations show that the a-Si:H(i) with Cat-doping can provide sufficient field effect passivation even at lower active carrier concentrations compared to the as-deposited doped layer, due to the lower defect density.     

On the Performance of TETRA DMO Short Data Service in Railway VANETs

In this paper we present the results of an empirical study investigating the performance of TETRA (TErrestrial Trunked RAdio) in a Vehicular Ad-hoc Network (VANET) for safety related railway traffic applications. The Short Data Service (SDS) of TETRA in the Direct Mode Operation (DMO) allows an infrastructure-less exchange of traffic relevant information between vehicles in range of the communication. The propagation channel in case of such a direct (base station free) communication between railway vehicles underlies specific effects, which are not equivalent to other well known terrestrial mobile scenarios. We will present measurements covering urban, suburban and rural environments along a regional railway network in the south of Bavaria. Beside different operational conditions such as front, rear, and flank approaches of trains, we investigated several topological scenarios on both, single and double track sections along the line. We will also discuss the observed characteristic changes in narrow band signal attenuation and Doppler spectra for passages through forests, hilly areas, stations and a tunnel. We determined statistics for the transmission delay of messages and the message erasure rates for single and multi user access on a single common carrier for different transmission intervals. Finally, the quality of service for the envisaged safety applications is assessed.