Determination of the structures of small gold clusters on stepped magnesia by density functional calculations

The structural modifications of small supported gold clusters caused by realistic surface defects (steps) in the MgO(001) support are investigated by computational methods. The most stable gold cluster structures on a stepped MgO(001) surface are searched for in the size range up to 24 Au atoms, and locally optimized by density-functional calculations. Several structural motifs are found within energy differences of 1 eV: inclined leaflets, arched leaflets, pyramidal hollow cages and compact structures. We show that the interaction with the step clearly modifies the structures with respect to adsorption on the flat defect-free surface. We find that leaflet structures clearly dominate for smaller sizes. These leaflets are either inclined and quasi-horizontal, or arched, at variance with the case of the flat surface in which vertical leaflets prevail. With increasing cluster size pyramidal hollow cages begin to compete against leaflet structures. Cage structures become more and more favourable as size increases. The only exception is size 20, at which the tetrahedron is found as the most stable isomer. This tetrahedron is however quite distorted. The comparison of two different exchange-correlation functionals (Perdew-Burke-Ernzerhof and local density approximation) show the same qualitative trends.     

Discrimination of Escherichia coli strains using glycan cantilever array sensors

Carbohydrate-based sensors, that specifically detect sugar binding molecules or cells, are increasingly important in medical diagnostic and drug screening. Here we demonstrate that cantilever arrays functionalized with different mannosides allow the real-time detection of several Escherichia coli strains in solution. Cantilever deflection is thereby dependent on the bacterial strain studied and the glycan used as the sensing molecule. The cantilevers exhibit specific and reproducible deflection with a sensitivity range over four orders of magnitude.     

Fast and air stable near-infrared organic detector based on squaraine dyes

In search for an organic material suitable for the detection of near-infrared electromagnetic radiation and at the same time capable of air stable operation of related devices, so to address the many applications envisaged with this technology (remote control, chemical/biological sensing, optical communication, spectroscopic and medical instruments), we explore the performance of a blend of hydrazone end-capped symmetric squaraines and phenyl C₆₁ butyric acid methyl ester. We succeed in developing air stable solution-processed devices with external quantum efficiency in the NIR as high as 3.5% and response times of few hundreds of nanoseconds. Essential to these achievements has been a detailed characterization of the devices performed by correlating the optoelectronic performances to the morphology of the layers (extracted from AFM measurements) and to the charge carrier mobility (extracted from transistor structures), enabling their optimization at the chemical level, by tailoring the squaraine substitution pattern, and at the device level, by tuning the blend composition. We show that a good balance between holes and electrons mobility is essential for high EQE and fast response speed, and that a smooth morphology is mandatory to achieve long term air stability and operability with no need for encapsulation.     

The influence of hydrogen and nitrogen on the formation of Si nanoclusters embedded in sub-stoichiometric silicon oxide layers

This work reports the study concerning the influence of the preparation conditions on the structure of silicon rich oxide (SRO) deposited by PECVD method by which the structural properties of the film are strictly related. In particular we investigated the role of reactant gases N₂O and SiH₄ on the total Si concentration, Si excess concentration, Si clustered concentration and size of nanoclusters formed by high annealing temperature. We payed particular attention on the role of the hydrogen and nitrogen during the Si agglomeration.The presence of hydrogen atoms on the as-deposited specimen, confirmed by the Si–H bonds peak on the FTIR analysis, has been directly correlated to the silicon excess concentration in the layer. The silicon, oxygen and nitrogen atomic density has been calculated from RBS analysis. These information were coupled to the ones obtained using methodology based on electron energy loss spectroscopy combined with energy filtered images, which allowed us to quantify the clustered silicon concentration in annealed sub-stoichiometric silicon oxide layers (SiO_x). We have verified that the nitrogen dissolved in the layer inhibits the Si excess clustering so that the efficiency of silicon agglomeration process decreases as the nitrogen content increases.     

Mathematical model for the hemodynamic response to venous occlusion measured with near-infrared spectroscopy in the human forearm

We propose a mathematical model to describe the hemodynamic changes induced by a venous occlusion in a human limb. These hemodynamic changes, which include an increase in blood volume, a reduction in blood flow, and modifications to the oxygen saturation of hemoglobin, can all be measured noninvasively with near-infrared spectroscopy (NIRS). To test the model, we have performed NIRS measurements on the human forearm, specifically on the brachioradialis muscle, during venous occlusion induced by a pneumatic cuff inflated around the upper arm to pressures within the range 10-60 mmHg. We have found a good agreement between parameters measured by NIRS (total hemoglobin concentration and hemoglobin saturation) and the corresponding model parameters (capacitor voltage and arterial/capillary branch current). In particular, model and experiment indicate that the time constant for blood accumulation during venous occlusion (approximately 73-79 s) is much slower than the time constant for blood drainage following cuff release (approximately 5 s). These results indicate that this mathematical model can be a valuable analytical tool to characterize, optimize, and further develop diagnostic measurement schemes that use venous occlusion approaches.     

Cognitive radio CDMA networking with spectrum sensing

In this paper, the performance of cognitive radio (CR) code division multiple access (CDMA) systems is analyzed. More precisely, CR users belong to a cognitive radio network (CRN), which coexists with a primary radio network (PRN). Both CRN and PRN are CDMA‐based, with colocated base stations. Soft hand off and power control are considered in both the CRN and the PRN. Upon the development of an accurate simulator for a representative three‐cell cellular scenario, we evaluate the performance of the proposed CR system in terms of outage probability, blocking probability and average data rate of secondary users. Three different spectrum sensing techniques are. Two new schemes, based on interference limit, are proposed and compared with an existing adaptive spectrum sensing scheme. Spectrum activity measurements and spectrum sharing decisions have been considered for evaluating the performance of the three schemes. The paper proposes a new CR‐CDMA networking model and a simulation testbed for evaluating performances of secondary users and primary users in terms of outage, blocking, BER and average data rate in the presence of soft hand‐off and power control. For comparison purposes, the analysis in the absence of spectrum sensing is also investigated.Copyright © 2012 John Wiley & Sons, Ltd.     

State‐of‐the‐Art Neutral Tint Multichromophoric Polymers for High‐Contrast See‐Through Electrochromic Devices

Two new multichromophoric electrochromic polymers featuring a conjugated EDOT/ProDOT copolymer backbone (PXDOT) and a reversible Weitz‐type redox active small molecule electrochrome (WTE) tethered to the conjugated chain are reported here. The careful design of the WTEs provides a highly reversible redox behavior with a colorless red switching that complements the colorless blue switching of the conjugated backbone. Subtractive color mixing successfully provides high performing solution processable polymeric layers with colorless neutral tint switchable limiting states for application in see‐through electrochromic devices. Design, synthesis, comprehensive chemical and spectroelectrochemical characterization as well as the preparation of a proof‐of‐concept device are discussed.     

Investigation of High-Electric-Field Degradation Effects in AlGaN/GaN HEMTs

High-electric-field degradation phenomena are investigated in GaN-capped AlGaN/GaN HEMTs by comparing experimental data with numerical device simulations. Under power- and OFF-state conditions, 150-h DC stresses were carried out. Degradation effects characterizing both stress experiments were as follows: a drop in the dc drain current, the amplification of gate-lag effects, and a decrease in the reverse gate leakage current. Numerical simulations indicate that the simultaneous generation of surface (and/or barrier) and buffer traps can account for all of the aforementioned degradation modes. Experiments also showed that the power-state stress induced a drop in the transconductance at high gate-source voltages only, whereas the OFF-state stress led to a uniform transconductance drop over the entire gate-source-voltage range. This behavior can be reproduced by simulations provided that, under the power-state stress, traps are assumed to accumulate over a wide region extending laterally from the gate edge toward the drain contact, whereas, under the OFF-state stress, trap generation is supposed to take place in a narrower portion of the drain-access region close to the gate edge and to be accompanied by a significant degradation of the channel transport parameters.