Harnessing “Click”‐Type Chemistry for the Preparation of Novel Electronic Materials

Sequence‐independent or “click”‐type chemistry is applied for the preparation of novel π‐conjugated oligomers. A variety of bi‐functional monomers for Wittig–Horner olefination are developed and applied in a sequential protection–deprotection process for the preparation of structurally similar π‐conjugated oligomers. Selected oligomers are incorporated as the organic semiconductors in light‐emitting diodes and a field‐effect transistor, demonstrating the potential of the approach.     

Interface Modifications of InAs Quantum‐Dots Solids and their Effects on FET Performance

InAs nanocrystals field‐effect transistors with an ON/OFF ratio of 10⁵ are reported. By tailoring the interface regions in the active layer step‐by‐step, the evolution of the ON/OFF ratio can be followed from approximately 5 all the way to around 10⁵. The formation of a semiconducting solid from colloidal nanocrystals is achieved through targeted design of the nanocrystal–nanocrystal interaction. The manipulation characteristics of the nanocrystal interfaces include the matrix surrounding the inorganic core, the interparticle distance, and the order of nanocrystals in the 3D array. Through careful analysis of device characteristics following each treatment, the effect of each on the physical properties of the films are able to be verified. The enhanced performance is related to interparticle spacing, reduction in sub‐gap states, and better electronic order (lower σ parameter). Films with enhanced charge transport qualities retain their quantum‐confined characteristics throughout the procedure, thus making them useful for optoelectronic applications.     

C beam elements based on the Refined Zigzag Theory for multilayered composite and sandwich laminates

The paper deals with the development and computational assessment of three- and two-node beam finite elements based on the Refined Zigzag Theory (RZT) for the analysis of multilayered composite and sandwich beams. RZT is a recently proposed structural theory that accounts for the stretching, bending, and transverse shear deformations, and which provides substantial improvements over previously developed zigzag and higher-order theories. This new theory is analytically rigorous, variationally consistent, and computationally attractive. The theory is not affected by anomalies of most previous zigzag and higher-order theories, such as the vanishing of transverse shear stress and force at clamped boundaries. In contrast to Timoshenko theory, RZT does not employ shear correction factors to yield accurate results. From the computational mechanics perspective RZT requires C⁰-continuous shape functions and thus enables the development of efficient displacement-type finite elements. The focus of this paper is to explore several low-order beam finite elements that offer the best compromise between computational efficiency and accuracy. The initial attention is on the choice of shape functions that do not admit shear locking effects in slender beams. For this purpose, anisoparametric (aka interdependent) interpolations are adapted to approximate the four independent kinematic variables that are necessary to model the planar beam deformations. To achieve simple two-node elements, several types of constraint conditions are examined and corresponding deflection shape-functions are derived. It is recognized that the constraint condition requiring a constant variation of the transverse shear force gives rise to a remarkably accurate two-node beam element. The proposed elements and their predictive capabilities are assessed using several elastostatic example problems, where simply supported and cantilevered beams are analyzed over a range of lamination sequences, heterogeneous material properties, and slenderness ratios.     

Neuropilin-1 is a placenta growth factor-2 receptor

Placenta growth factor (PlGF) belongs to the family of vascular endothelial growth factors (VEGFs). It binds to the flt-1 VEGF receptor but not to the KDR/flk-1 receptor which is thought to mediate most of the angiogenic and proliferative effects of VEGF. Three PlGF isoforms are produced by alternative splicing. PlGF-1 and PlGF-3 differ from PlGF-2 since they lack the exon 6 encoded peptide which bestows upon PlGF-2 its heparin binding properties. Cross-linking experiments revealed that 125I-PlGF-2 binds to two endothelial cell surface receptors in a heparin dependent fashion. The binding of 125I-PlGF-2 to these receptors was inhibited by an excess of PlGF-2 and by the 165-amino acid form of VEGF (VEGF165), but not at all by VEGF121 and very marginally if at all by PlGF-1. The apparent molecular weight and the binding characteristics of these receptors correspond to those of the recently identified VEGF165 specific receptor neuropilin-1, and we therefore conclude that neuropilin-1 is a receptor for PlGF-2. The binding of 125I-PlGF-2 as well as the binding of 125I-VEGF165 to these receptors was inhibited by a synthetic peptide derived from exon 6 of PlGF. Furthermore, the binding of 125I-PlGF-2, but not that of 125I-VEGF165, was also inhibited by a synthetic peptide derived from exon 7 of PlGF. These observations indicate that the peptides encoded by these exons probably participate in the formation of the domain which mediates the binding of PlGF-2 to these receptors. We have also determined, using chemically modified heparin species, that the presence of sulfate moieties on the glucosamine-O-6 and on the iduronic acid-O-2 groups of heparin was required for the potentiation of 125I-PlGF-2 binding to these receptors. To determine if PlGF-2 is able to induce biological responses that are not induced by PlGF-1, we compared the effects of PlGF-1 and PlGF-2 on the migration and proliferation of endothelial cells. Both PlGF forms induced migration of endothelial cells. However, there was no quantitative difference between the response to PlGF-2 and the response to PlGF-1. Furthermore, neither PlGF-1 nor PlGF-2 had any effect upon the proliferation of the endothelial cells.     

An efficient, conforming axisymmetric shell element including transverse shear and rotary inertia

A conforming, two-node conical element is presented incorporating both transverse shear deformation and rotary inertia effects. The success of the formulation is based on the C⁰ continuity shape functions that, in addition to providing full kinematic compatibility, satisfy the interdependent variable interpolation requirement. This requirement points toward a particular dependence of the kinematic variable interpolations which allow for the satisfaction of the Kirchhoff constraint everywhere in the element domain. There exists an internal degree-of-freedom reduction achieved by means of a continuous shear constraint condition. In the derivation of element matrices, Hamilton's principle is invoked with Gaussian quadratures ensuring exact integration throughout. Excellent convergence characteristics and high accuracy results attainable with the element are demonstrated in static and free vibration problems for thin and moderately thick plates and shells.     

Stress recovery and error estimation for shell structures

The Penalized Discrete Least‐Squares (PDLS) stress recovery (smoothing) technique developed for two‐dimensional linear elliptic problems [1–3] is adapted here to three‐dimensional shell structures. The surfaces are restricted to those which have a 2‐D parametric representation, or which can be built‐up of such surfaces. The proposed strategy involves mapping the finite element results to the 2‐D parametric space whichdescribes the geometry, and smoothing is carried out in the parametric space using the PDLS‐based Smoothing Element Analysis (SEA). Numerical results for two well‐known shell problems are presented to illustrate the performance of SEA/PDLS for these problems. The recovered stresses are used in the Zienkiewicz–Zhu a posteriori error estimator. The estimated errors are used to demonstrate the performance of SEA‐recovered stresses in automated adaptive mesh refinement of shell structures. The numerical results are encouraging. Further testing involving more complex, practical structures is necessary. Copyright © 2000 John Wiley & Sons, Ltd.     

Unraveling the Physics of Vertical Organic Field Effect Transistors through Nanoscale Engineering of a Self-Assembled Transparent Electrode

While organic transistors' performances are continually pushed to achieve lower power consumption, higher working frequencies, and higher current densities, a new type of organic transistors characterized by a vertical architecture offers a radically different design approach to outperform its traditional counterparts. Naturally, the distinct vertical architecture gives way to different governing physical ground rules and structural key features such as the need for an embedded transparent electrode. In this paper, we make use of a zero-frequency electric field-transparent patterned electrode produced through block-copolymer self-assembly based lithography to control the performances of the vertical organic field effect transistor (VOFET) and to study its governing physical mechanisms. Unlike other VOFET structures, this design, involving well-defined electrode architecture, is fully tractable, allowing for detailed modeling, analysis, and optimization. We provide for the first time a complete account of the physics underpinning the VOFET operation, considering two complementary mechanisms: the virtual contact formation (Schottky barrier lowering) and the induced potential barrier (solid-state triode-like shielding). We demonstrate how each mechanism, separately, accounts for the link between controllable nanoscale structural modifications in the patterned electrode and the VOFET performances. For example, the ON/OFF current ratio increases by up to 2 orders of magnitude when the perforations aspect ratio (height/width) decreases from ～0.2 to ～0.1. The patterned electrode is demonstrated to be not only penetrable to zero-frequency electric fields but also transparent in the visible spectrum, featuring uniformity, spike-free structure, material diversity, amenability with flexible surfaces, low sheet resistance (20-2000 Ω sq(-1)) and high transparency (60-90%). The excellent layer transparency of the patterned electrode and the VOFET's exceptional electrical performances make them both promising elements for future transparent and/or efficient organic electronics.     

Conference diary

A variational higher-order theory for bending and stretching of linearly elastic orthotropic beams including the deformations due to transverse shearing and stretching of the transverse normal fibre is presented. The theory assumes a linear distribution for the longitudinal displacement and a parabolic variation of the transverse displacement across the thickness. Additionally, independent expansions are introduced for the through-thickness displacement gradients with the requirement of a least-square compatibility for the transverse strains and the satisfaction of exact stress boundary conditions at the top/bottom beam surfaces. The theory is shown to be well suited for finite element development requiring simple C⁰- and C^?1- continuous displacement interpolation fields. To demonstrate the computational utility of the theory, a simple two-node stretching-bending finite element is formulated. The analytic and finite element results are obtained for a simple bending problem for which an exact elasticity solution is available. It is shown that the inclusion of the transverse normal deformation in the present theory enables improved displacement, strain and stress predictions, particularly, in the analysis of deep beams.     

REDRAW — a diagrammatic reasoning system for qualitative structural analysis

For preliminary analysis of structures, human engineers often employ diagrams as a visual language to study and to gain intuitive understanding about the behavior of structures. This paper reports a preliminary study and the development of a prototype system for diagrammatic reasoning to better emulate the intuitive visual problem solving techniques of human engineers. Diagrammatic reasoning is a type of reasoning in which the primary means of inference is the direct manipulation and inspection of a diagram. Diagrammatic reasoning is prevalent in human problem-solving behavior, especially for problems involving spatial relationships among physical objects. Our research examines the relationship between diagrammatic reasoning and symbolic reasoning in a computational framework. We have built a system called REDRAW, that emulates the human capability for reasoning with pictures for qualitative analysis of simple frame structures. Diagrammatic representations provide an environment where inferences about the physical results of proposed structural configurations can take place in a more intuitive manner than that possible through purely symbolic representations.