Photoluminescence dispersion as a probe of structural inhomogeneity in silica

We report time-resolved photoluminescence spectra of point defects in amorphous silicon dioxide (silica), in particular the decay kinetics of the emission signals of extrinsic Oxygen Deficient Centres of the second type from singlet and directly-excited triplet states are measured and used as a probe of structural inhomogeneity. Luminescence activity in sapphire ($\alpha$-Al$_2$O$_3$) is studied as well and used as a model system to compare the optical properties of defects in silica with those of defects embedded in a crystalline matrix. Only for defects in silica, we observe a variation of the decay lifetimes with emission energy and a time dependence of the first moment of the emission bands. These features are analyzed within a theoretical model with explicit hypothesis about the effect introduced by the disorder of vitreous systems. Separate estimations of the homogenous and inhomogeneous contributions to the measured emission linewidth are obtained: it is found that inhomogeneous effects strongly condition both the triplet and singlet luminescence activities of oxygen deficient centres in silica, although the degree of inhomogeneity of the triplet emission turns out to be lower than that of the singlet emission. Inhomogeneous effects appear to be negligible in sapphire

Phase diagram of self-assembled rigid rods on two-dimensional lattices: Theory and Monte Carlo simulations

Monte Carlo simulations and finite-size scaling analysis have been carried out to study the critical behavior in a two-dimensional system of particles with two bonding sites that, by decreasing temperature or increasing density, polymerize reversibly into chains with discrete orientational degrees of freedom and, at the same time, undergo a continuous isotropic-nematic (IN) transition. A complete phase diagram was obtained as a function of temperature and density. The numerical results were compared with mean field (MF) and real space renormalization group (RSRG) analytical predictions about the IN transformation. While the RSRG approach supports the continuous nature of the transition, the MF solution predicts a first-order transition line and a tricritical point, at variance with the simulation results.Comment: 12 pages, 10 figures, supplementary informatio

Alternating current conduction properties of thermally evaporated alpha-nickel phthalocyanine thin films: Effects of oxygen doping and thermal annealing

Published versio

Low-voltage operation of metal-ferroelectric-insulator-semiconductor diodes incorporating a ferroelectric polyvinylidene fluoride copolymer Langmuir-Blodgett film

We report the electrical characteristics of metal-ferroelectric-insulator-semiconductor structures, where the ferroelectric layer is a Langmuir-Blodgett film of a copolymer of 70% vinylidene fluoride and 30% trifluoroethylene. The 36-nm thick copolymer films were deposited on thermally oxidized (10 nm SiO2) p-type silicon and covered with a gold gate electrode. Polarization-field hysteresis loops indicate polarization switching in the polymer film. The device capacitance shows hysteresis when cycling the applied voltage between ±3 V, exhibiting a zero-bias on/off capacitance ratio of over 3:1 and a symmetric memory window 1 V wide, with little evidence of bias that can arise from traps in the oxide. Model calculations are in good agreement with the data and show that film polarization was not saturated. The capacitance hysteresis vanishes above the ferroelectric- paraelectric transition temperature, showing that it is due to polarization hysteresis. The retention time of both the on and off states was approximately 15 min at room temperature, possibly limited by leakage or by polarization instability in the unsaturated film. These devices provide a basis for nonvolatile data storage devices with fast nondestructive readout

Simulations of charge transfer in Electron Multiplying Charge Coupled Devices

Electron Multiplying Charge Coupled Devices (EMCCDs) are a variant of traditional CCD technology well suited to applications that demand high speed operation in low light conditions. On-chip signal amplification allows the sensor to effectively suppress the noise introduced by readout electronics, permitting sub-electron read noise at MHz pixel rates. The devices have been the subject of many detailed studies concerning their operation, however there has not been a study into the transfer and multiplication process within the EMCCD gain register. Such an investigation has the potential to explain certain observed performance characteristics, as well as inform further optimisations to their operation. In this study, the results from simulation of charge transfer within an EMCCD gain register element are discussed with a specific focus on the implications for serial charge transfer efficiency (CTE). The effects of operating voltage and readout speed are explored in context with typical operating conditions. It is shown that during transfer, a small portion of signal charge may become trapped at the semiconductor-insulator interface that could act to degrade the serial CTE in certain operating conditions

X-ray irradiation of soda-lime glasses studied in situ with surface plasmon resonance spectroscopy

We present here a study of hard X-ray irradiation of soda-lime glasses performed in situ and in real time. For this purpose, we have used a Au thin film grown on glass and studied the excitation of its surface plasmon resonance (SPR) while irradiating the sample with X-rays, using a recently developed experimental setup at a synchrotron beamline [Serrano et al., Rev. Sci. Instrum. 83, 083101 (2012)]. The extreme sensitivity of the SPR to the features of the glass substrate allows probing the modifications caused by the X-rays. Irradiation induces color centers in the soda-lime glass, modifying its refractive index. Comparison of the experimental results with simulated data shows that both, the real and the imaginary parts of the refractive index of soda-lime glasses, change upon irradiation in time intervals of a few minutes. After X- ray irradiation, the effects are partially reversible. The defects responsible for these modifications are identified as non-bridging oxygen hole centers, which fade by recombination with electrons after irradiation. The kinetics of the defect formation and fading process are also studied in real time