Tailoring Electron‐Transfer Barriers for Zinc Oxide/C60 Fullerene Interfaces

The interfacial electronic structure between oxide thin films and organic semiconductors remains a key parameter for optimum functionality and performance of next‐generation organic/hybrid electronics. By tailoring defect concentrations in transparent conductive ZnO films, we demonstrate the importance of controlling the electron transfer barrier at the interface with organic acceptor molecules such as C₆₀. A combination of electron spectroscopy, density functional theory computations, and device characterization is used to determine band alignment and electron injection barriers. Extensive experimental and first principles calculations reveal the controllable formation of hybridized interface states and charge transfer between shallow donor defects in the oxide layer and the molecular adsorbate. Importantly, it is shown that removal of shallow donor intragap states causes a larger barrier for electron injection. Thus, hybrid interface states constitute an important gateway for nearly barrier‐free charge carrier injection. These findings open new avenues to understand and tailor interfaces between organic semiconductors and transparent oxides, of critical importance for novel optoelectronic devices and applications in energy‐conversion and sensor technologies.     

Conjugated Polymers: High‐Resolution Scanning Near‐Field Optical Lithography of Conjugated Polymers (Adv. Funct. Mater. 17/2010)

The fabrication of high‐resolution nanostructures in both poly(p‐phenylene vinylene), PPV, and a crosslinkable derivative of poly(9,9′‐dioctylfluorene), F8, using scanning near‐field optical lithography, is reported. The ability to draw complex, reproducible structures with 65000 pixels and lateral resolution below 60 nm (< λ/5) is demonstrated over areas up to 20 μm × 20 μm. Patterning on length‐scales of this order is desirable for realizing applications both in organic nanoelectronics and nanophotonics. The technique is based on the site‐selective insolubilization of a precursor polymer under exposure to the confined optical field present at the tip of an apertured near‐field optical fiber probe. In the case of PPV, a leaving‐group reaction is utilized to achieve insolubilization, whereas the polyfluorene is insolubilized using a photoacid initiator to create a crosslinked network in situ. For PPV, resolubilization of the features is observed at high exposure energies. This is not seen for the crosslinked F8 derivative, r‐F8Ox, allowing us to pattern structures up to 200 nm in height.     

Interfacial Strain Gradients Control Nanoscale Domain Morphology in Epitaxial BiFeO3 Multiferroic Films

Domain switching pathways fundamentally control performance in ferroelectric thin film devices. In epitaxial bismuth ferrite (BiFeO₃) films, the domain morphology is known to influence the multiferroic orders. While both striped and mosaic domains have been observed, the origins of the latter have remained unclear. Here, it is shown that domain morphology is defined by the strain profile across the film–substrate interface. In samples with mosaic domains, X‐ray diffraction analysis reveals strong strain gradients, while geometric phase analysis using scanning transmission electron microscopy finds that within 5 nm of the film–substrate interface, the out‐of‐plane strain shows an anomalous dip while the in‐plane strain is constant. Conversely, if uniform strain is maintained across the interface with zero strain gradient, striped domains are formed. Critically, an ex situ thermal treatment, which eliminates the interfacial strain gradient, converts the domains from mosaic to striped. The antiferromagnetic state of the BiFeO₃ is also influenced by the domain structure, whereby the mosaic domains disrupt the long‐range spin cycloid. This work demonstrates that atomic scale tuning of interfacial strain gradients is a powerful route to manipulate the global multiferroic orders in epitaxial films.     

A Novel Magnetic Hardening Mechanism for Nd-Fe-B Permanent Magnets Based on Solid-State Phase Transformation

Permanent magnets based on neodymium-iron-boron (Nd-Fe-B) alloys provide the highest performance and energy density, finding usage in many high-tech applications. Their magnetic performance relies on the intrinsic properties of the hard-magnetic Nd₂Fe₁₄B phase combined with control over the microstructure during production. In this study, a novel magnetic hardening mechanism is described in such materials based on a solid-state phase transformation. Using modified Nd-Fe-B alloys of the type Nd₁₆Fe_bal-x-y-zCo_xMo_yCu_zB₇ for the first time it is revealed how the microstructural transformation from the metastable Nd₂Fe₁₇B_x phase to the hard-magnetic Nd₂Fe₁₄B phase can be thermally controlled, leading to an astonishing increase in coercivity from ≈200 kAm⁻¹ to almost 700 kAm⁻¹. Furthermore, after thermally treating a quenched sample of Nd₁₆Fe₅₆Co₂₀Mo₂Cu₂B₇, the presence of Mo leads to the formation of fine FeMo₂B₂ precipitates, in the range from micrometers down to a few nanometers. These precipitates are responsible for the refinement of the Nd₂Fe₁₄B grains and so for the high coercivity. This mechanism can be incorporated into existing manufacturing processes and can prove to be applicable to novel fabrication routes for Nd-Fe-B magnets, such as additive manufacturing.     

Performance of adaptive token allocation in the leaky bucket

The leaky bucket is a popular method that can regulate traffic into an ATM broadband network. This paper examines a simple but innovative modification that would also provide priority to access the network. This is done by requiring cells of different classes to obtain different numbers of tokens before receiving their services. As a step further, a dynamic scheme can be used in which the tokens allocated to each class are changed according to the traffic load. Performance evaluations of mean cell delays and cell loss probabilities are obtained to provide insight into the behaviour of the system and to provide guideline for furture design.     

BETaaS: A Platform for Development and Execution of Machine-to-Machine Applications in the Internet of Things

Wireless Personal Communications - The integration of everyday objects into the Internet represents the foundation of the forthcoming Internet of Things (IoT). Smart objects will be the building...     

Doping Molecular Monolayers: Effects on Electrical Transport Through Alkyl Chains on Silicon

n‐Si/C_nH_{2n + 1}/Hg junctions (n = 12, 14, 16 and 18) can be prepared with sufficient quality to assure that the transport characteristics are not anymore dominated by defects in the molecular monolayers. With such organic monolayers we can, using electron, UV and X‐ray irradiation, alter the charge transport through the molecular junctions on n‐ as well as on p‐type Si. Remarkably, the quality of the self‐assembled molecular monolayers following irradiation remains sufficiently high to provide the same very good protection of Si from oxidation in ambient atmosphere as provided by the pristine films. Combining spectroscopic (UV photoemission spectroscopy (UPS), X‐ray photoelectron spectroscopy (XPS), Auger, near edge‐X‐ray absorption fine structure (NEXAFS)) and electrical transport measurements, we show that irradiation induces defects in the alkyl films, most likely C?C bonds and C? C crosslinks, and that the density of defects can be controlled by irradiation dose. These altered intra‐ and intermolecular bonds introduce new electronic states in the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap of the alkyl chains and, in the process, dope the organic film. We demonstrate an enhancement of 1–2 orders of magnitude in current. This change is clearly distinguishable from the previous observed difference between transport through high quality and defective monolayers. A detailed analysis of the electrical transport at different temperatures shows that the dopants modify the transport mechanism from tunnelling to hopping. This study suggests a way to extend significantly the use of monolayers in molecular electronics.     

Physical Layer Development Framework for OsmocomBB

The open source GSM protocol stack of the OsmocomBB project offers a versatile development environment regarding the data link and network layer. There is no solution available for developing physical layer baseband algorithms in combination with the data link and network layer. In this paper, a baseband development framework architecture with a suitable interface to the protocol stack of OsmocomBB is presented. With the proposed framework, a complete GSM protocol stack can be run and baseband algorithms can be evaluated in a closed system. It closes the gap between physical layer signal processing implementations in Matlab and the upper layers of the OsmocomBB GSM protocol stack. An embedded version of the system has been realized with FPGA and PowerPC to enable real-time operation. The functionality of the system has been verified with a testbed comprising an OpenBTS base-station emulator, a receiver board with RF transceiver and our developed physical layer signal processing system.     

Adaptive AQM controllers for IP routers with a heuristic monitor on TCP flows

We propose adaptive proportional (P) and proportional‐integral (PI) controllers for Active Queue Management (AQM) in the Internet. We apply the classical control theory in the controller design and choose a proper phase margin to achieve good performance of AQM. We have identified a simple heuristic parameter that can monitor the changes of network environment. Our adaptive controllers would self‐tune only when the dramatic change in the network parameters drift the monitoring parameter outside its specified interval. When compared to P controller, a PI controller has the advantage of regulating the TCP source window size by adjusting the packet drop probability based on the knowledge of instantaneous queue size, thus steadying the queue size around a target buffer occupancy. We have verified our controllers by OPNET simulation, and shown that with an adaptive PI controller applied, the network is asymptotically stable with good robustness. Copyright © 2005 John Wiley & Sons, Ltd.     

Toward Stretchable Self‐Powered Sensors Based on the Thermoelectric Response of PEDOT:PSS/Polyurethane Blends

The development of new flexible and stretchable sensors addresses the demands of upcoming application fields like internet‐of‐things, soft robotics, and health/structure monitoring. However, finding a reliable and robust power source to operate these devices, particularly in off‐the‐grid, maintenance‐free applications, still poses a great challenge. The exploitation of ubiquitous temperature gradients, as the source of energy, can become a practical solution, since the recent discovery of the outstanding thermoelectric properties of a conductive polymer, poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) (PEDOT:PSS). Unfortunately the use of PEDOT:PSS is currently constrained by its brittleness and limited processability. Herein, PEDOT:PSS is blended with a commercial elastomeric polyurethane (Lycra), to obtain tough and processable self‐standing films. A remarkable strain‐at‐break of ≈700% is achieved for blends with 90 wt% Lycra, after ethylene glycol treatment, without affecting the Seebeck voltage. For the first time the viability of these novel blends as stretchable self‐powered sensors is demonstrated.