The Exciton Model for Molecular Materials:Past, Present and Future?

In assemblies of identical molecules or chromophores, electronic excitations can be described as excitons, bound electron-hole pairs that can move from site to site as a pair in a coherent manner. The understanding of excitons is crucial when trying to engineer favorable photophysical properties through structuring organic molecular matter. In recent decades, limitations of the concept of an exciton have become clear. The exciton can hybridize with phonon and photons. To clarify these issues, the exciton is discussed within the broader context of the gauge properties of the electromagnetic force.</p

Analyse quantifiée et membre supérieur : progrès et perspectives

Von Garteninspektor Meinhard

Simulation of electrical conductivity in a pi-conjugated polymeric conductor with infrared light

Irradn. with IR light is found to stimulate the elec. cond. of a film of an org. polymeric conductor [poly(3,4-ethylenedioxythiopene) with polystyrene sulfonate]. The change in cond. is found to be linear in the intensity of the irradn. (4-400 mW/cm2). Both frequency and time domain measurements reveal that the change in resistance induced by irradn., relaxes according to DR(t) ~ (1/t)0.6, with t as the time after excitation. As a possible mechanism for this relaxation, the authors model the diffusion of heat from the polymer film to the supporting glass substrate. By assuming that the change in resistance is linear with the raise in temp. caused by the IR irradn., one predicts a DR(t) ~ (1/t)0.5 dependence. The similarity between the model and exptl. behavior is taken as an indication that the relaxation is limited by heat transport from the polymer film and that the thermalization of the charge carriers occurs on a shorter time scale. Elec. characterization is complemented with optical measurements. These show IR-induced transient absorption of the polymer film with practically the same relaxation behavior as the change in resistance. This suggests that the optical transients are also due to thermal excitations. In the sub-ps time domain, measurements of the change in optical transmission (DT/T) induced by the IR pulse show a very short-lived component with a lifetime close to the instrumental resoln. (.apprx.500 fs). The rapid response is followed by a slow component that decays according to (DT/T)(t) ~ (1/t)0.65. This is interpreted in terms of cooling of the excited charge carriers limited by heat transport, indicating that the thermalization of the carriers occurs on the sub-ps time scal

Unipolar resistive switching in metal oxide/organic semiconductor non-volatile memories as a critical phenomenon

Diodes incorporating a bilayer of an organic semiconductor and a wide bandgap metal oxide can show unipolar, non-volatile memory behavior after electroforming. The prolonged bias voltage stress induces defects in the metal oxide with an areal density exceeding 10(17) m(-2). We explain the electrical bistability by the coexistence of two thermodynamically stable phases at the interface between an organic semiconductor and metal oxide. One phase contains mainly ionized defects and has a low work function, while the other phase has mainly neutral defects and a high work function. In the diodes, domains of the phase with a low work function constitute current filaments. The phase composition and critical temperature are derived from a 2D Ising model as a function of chemical potential. The model predicts filamentary conduction exhibiting a negative differential resistance and nonvolatile memory behavior. The model is expected to be generally applicable to any bilayer system that shows unipolar resistive switching. (C) 2015 Author(s).Dutch Polymer Institute (DPI), BISTABLE [704]; Fundacao para Ciencia e Tecnologia (FCT) through the research Instituto de Telecommunicacoes (IT-Lx); project Memristor based Adaptive Neuronal Networks (MemBrAiNN) [PTDC/CTM-NAN/122868/2010]; European Community Seventh Framework Programme FP7', ONE-P [212311]; Dutch Ministry of Education, Culture and Science (Gravity Program) [024.001.035]info:eu-repo/semantics/publishedVersio

Economics of End-of-Life Materials Recovery: A Study of Small Appliances and Computer Devices in Portugal

The challenges brought on by the increasing complexity of electronic products, and the criticality of the materials these devices contain, present an opportunity for maximizing the economic and societal benefits derived from recovery and recycling. Small appliances and computer devices (SACD), including mobile phones, contain significant amounts of precious metals including gold and platinum, the present value of which should serve as a key economic driver for many recycling decisions. However, a detailed analysis is required to estimate the economic value that is unrealized by incomplete recovery of these and other materials, and to ascertain how such value could be reinvested to improve recovery processes. We present a dynamic product flow analysis for SACD throughout Portugal, a European Union member, including annual data detailing product sales and industrial-scale preprocessing data for recovery of specific materials from devices. We employ preprocessing facility and metals pricing data to identify losses, and develop an economic framework around the value of recycling including uncertainty. We show that significant economic losses occur during preprocessing (over $70 M USD unrecovered in computers and mobile phones, 2006–2014) due to operations that fail to target high value materials, and characterize preprocessing operations according to material recovery and total costs.Portuguese Foundation for International Cooperation in Science, Technology and Higher EducationMIT Portugal Progra

Programming and Dynamic Control of the Circular Polarization of Luminescence from an Achiral Fluorescent Dye in a Liquid Crystal Host by Molecular Motors

Circular polarized light is utilized in communication and display technologies and a major challenge is to develop systems that can be switched between left and right circular polarized luminescence with high degrees of polarization and enable multiple addressable stable states. Luminescent dyes in Liquid Crystal (LC) cholesteric phases are attractive systems to generate, amplify and modulate circularly polarized luminescence (CPL). In the present study, we employ light‐driven molecular motors as photo‐controlled chiral dopants in LCs to switch the handedness of the LC and the circular polarization of luminescence from an achiral dye embedded in the mesogenic material. Tuning of the color of the CPL and the retention time of the photoprogrammed helicity is demonstrated making use of a variety of motors and dyes. The flexibility offered by the design based on inherently chiral unidirectional rotary motors provides full control over CPL non‐invasively by light, opening possibilities for pixilated displays with externally addressable polarization

Resistive Switching in Metal Oxide/Organic Semiconductor Nonvolatile Memories

Diodes incorporating a bilayer of a metal oxide and an organic semiconductor can show unipolar, nonvolatile memory behavior after electroforming. Electroforming involves dielectric breakdown induced by prolonged bias voltage stress. When the power dissipated during breakdown is limited, electroforming is reversible and involves formation of defects at the organic-oxide interface that can heal spontaneously. When the power dissipation during breakdown exceeds a certain threshold, electroforming becomes irreversible. The fully electroformed diodes show electrical bistability, featuring (meta)stable states with low and high conduction that can be programmed by voltage pulses. The high conduction results from current flowing via filamentary paths. The bistability is explained by the coexistence of two thermodynamically stable phases at the interface between semiconductor and oxide. One phase contains mainly ionized defects and has a low work function, while the other phase has mainly neutral defects and a high work function. In the diodes, domains of the phase with low work function give rise to current filaments. In the filaments, Joule heating will raise temperature locally. When the temperature exceeds the critical temperature, the filament will switch off. The switching involves a collective recombination of charge carriers trapped at the defects as evidenced by bursts of electroluminescence

Age-related DNA methylation changes: Potential impact on skeletal muscle aging in humans

Human aging is accompanied by a decline in muscle mass and muscle function, which is commonly referred to as sarcopenia. Sarcopenia is associated with detrimental clinical outcomes, such as a reduced quality of life, frailty, an increased risk of falls, fractures, hospitalization, and mortality. The exact underlying mechanisms of sarcopenia are poorly delineated and the molecular mechanisms driving the development and progression of this disorder remain to be uncovered. Previous studies have described age-related differences in gene expression, with one study identifying an age-specific expression signature of sarcopenia, but little is known about the influence of epigenetics, and specially of DNA methylation, in its pathogenesis. In this review, we will focus on the available knowledge in literature on the characterization of DNA methylation profiles during skeletal muscle aging and the possible impact of physical activity and nutrition. We will consider the possible use of the recently developed DNA methylation-based biomarkers of aging called epigenetic clocks in the assessment of physical performance in older individuals. Finally, we will discuss limitations and future directions of this field

Stretch Evoked Potentials in Healthy Subjects and After Stroke: A Potential Measure for Proprioceptive Sensorimotor Function

Sensory feedback is of vital importance in motor control, yet rarely assessed in diseases with impaired motor function like stroke. Muscle stretch evoked potentials (StrEPs) may serve as a measure of cortical sensorimotor activation in response to proprioceptive input. The aim of this study is: 1) to determine early and late features of the StrEP and 2) to explore whether StrEP waveform and features can be measured after stroke. Consistency of StrEP waveforms and features was evaluated in 22 normal subjects. StrEP features and similarity between hemispheres were evaluated in eight subacute stroke subjects. StrEPs of normal subjects had a consistent shape across conditions and sessions (mean cross correlation waveforms > 0.75). Stroke subjects showed heterogeneous StrEP waveforms. Stroke subjects presented a normal early peak (40 ms after movement onset) but later peaks had abnormal amplitudes and latencies. No significant differences between stroke subjects with good and poor motor function were found (P > 0.14). With the consistent responses of normal subjects the StrEP meets a prerequisite for potential clinical value. Recording of StrEPs is feasible even in subacute stroke survivors with poor motor function. How StrEP features relate to clinical phenotypes and recovery needs further investigatio