Mesoscopic modelling of conducting and semiconducting polymers

We present generalized Monte Carlo calculations to assess the effects of texture and related key factors on the properties of polymer-based light emitting diodes. We, describe one class of mesoscopic model giving specific realizations of the polymer network. The model, with simple physically based rules, shows the effects of polymer structural order on current flow, trapping and radiative and non-radiative charge recombination within the polymer layer. Interactions between charges are included explicitly, as are image interactions with the electrodes. It is important that these Coulomb interactions are not simplified to an averaged space charge, since the local interactions can lead to effective trapping of charge, even in the absence of defective chains or impurity trapping. There proves to be an important role for trapping, in which charges are localized for times long compared with transit times. The competition between current flow, trapping and radiative and non-radiative charge recombination means that some of the trends are not intuitively obvious. For example, if radiative recombination occurs only on short polymer chains, as is the case for certain polymer systems, the internal efficiency appears to saturate for a concentration of these shorter luminescent chains of about 20-30%. As the proportion of shorter chains increases, trapping increases, whereas current efficiency decreases. Our approach provides a natural link between atomistic models of individual polymer molecules and the macroscopic descriptions of device modelling. Such mesoscopic models provide a means to design better film structures, and hence to optimize the effectiveness of new organic materials in a range of applications

Comment on: "Revealing common artifacts due to ferromagnetic inclusions in highly oriented pyrolytic graphite", by M. Sepioni, R.R. Nair, I.-Ling Tsai, A.K. Geim and I.V. Grigorieva, EPL 97 (2012) 47001

This comment addresses several issues in the paper by Sepioni et al., where it is stated that the ferromagnetism in pristine highly oriented pyrolytic graphite (HOPG) reported by several groups in the previous years is most likely due to impurity contamination. In this comment, clear arguments are given why this statement is not justified. Furthermore, it is pointed out, that there are already measurements using element-sensitive microscopic techniques, e.g. X-ray Magnetic Circular Dichroism (XMCD) that directly proved the intrinsic origin of the ferromagnetism in graphite, also in pristine HOPG.Comment: 1, 0 figures, 9 reference

DISCOVERY OF RR LYRAE STARS IN THE NUCLEAR BULGE OF THE MILKY WAY

Indexación: Web of ScienceGalactic nuclei, such as that of the Milky Way, are extreme regions with high stellar densities, and in most cases, the hosts of a supermassive black hole. One of the scenarios proposed for the formation of the Galactic nucleus is merging of primordial globular clusters. An implication of this model is that this region should host stars that are characteristically found in old Milky Way globular clusters. RR Lyrae stars are primary distance indicators, well known representatives of old and metal-poor stellar populations, and therefore are regularly found in globular clusters. Here we report the discovery of a dozen RR Lyrae type ab stars in the vicinity of the Galactic center, i.e., in the so-called nuclear stellar bulge of the Milky Way. This discovery provides the first direct observational evidence that the Galactic nuclear stellar bulge contains ancient stars (>10 Gyr old). Based on this we conclude that merging globular clusters likely contributed to the build-up of the high stellar density in the nuclear stellar bulge of the Milky Way.http://iopscience.iop.org/article/10.3847/2041-8205/830/1/L14/meta;jsessionid=2531FBFFF06C9ECBA4852FB9D1F89851.c1.iopscience.cld.iop.or

Theoretical study of metal-polyimide interfacial properties

The aim of this work is to achieve a better understanding of metal-polyimide interfacial properties, such as bonding strength between two materials and charge transfer, and to assess the consequences for the adhesion. The effect of surface modification on adhesion is also discussed. These interfacial studies of PMDA-ODA polyimide with evaporated chromium (Cr), copper (Cu) and nickel (Ni) atoms have been performed using a self-consistent molecular dynamics method with semi-empirical quantum chemistry at the complete neglect of differential overlap level. All metals were predicted to bond strongly at six-fold rings of polyimide. Despite actual charge transfer between metal atoms and polyimide is small, the bond formation induces charge rearrangement among the polyimide atoms far from the reaction site. The adhesion of polyimide to a Ni(100) surface is predicted to be somewhat weaker than that of nickel atoms to polyimide at six-fold sites

Variable stars in the VVV globular clusters

Indexación: Scopus.The VVV survey observed some of the most crowded and most obscured regions in the inner Milky Way during the last years. A significant sample of the less known globular clusters in our galaxy lie there. Combining the high-resolution, wide-field, near infrared capabilities of the survey camera, the use of 5 different filters, and multi-epoch observations, we are able to overcome many of the previous challenges that prevented a proper study of these objects. Particularly, the identification of the RR Lyrae stars in these globular clusters is proving to be a fundamental tool to establish accurately their distances and reddenings, and to infer information about the Oosterhoff dichotomy that Galactic globular clusters seem to follow. © The Authors, published by EDP Sciences, 2017.https://www.epj-conferences.org/articles/epjconf/pdf/2017/21/epjconf_puls2017_01022.pd

The Impact on Maine's Budget of Allowing Same-Sex Couples to Marry

This analysis estimates the impact of allowing same-sex couples to marry on Maine's state budget. We estimate that allowing same-sex couples to marry will result in a net gain of approximately $7.9 million each year for the State. This net impact will be the result of savings in expenditures on state means-tested public benefits programs and an increase in revenue from state sales and income taxes and marriage license fees. Throughout this report, we estimate the economic impact of weddings conservatively. In other words, we choose assumptions that are cautious from the State's perspective in that they tend to produce lower revenues and higher expenditures given the range of possibilities. Even so, we find that the effect of allowing same-sex couples to marry in Maine is an annual positive fiscal impact of approximately$7.9 million

Charge induced-defects in Poly (P-Phenylene Vinylene) (PPV)

We use a molecular dynamics method with semi-empirical quantum chemistry at the complete neglect of differential overlap (CNDO) level to study the charge-induced structural relaxation of poly(p-phenylene vinylene) (PPV) backbone, as a result of charge injection in light emitting diodes. Our results suggest that structural relaxation of PPV is accompanied by a local change in the electronic structure of the polymer leading to commonly known defects. The mobility of these defects along the polymer chains of different finite length was estimated from computer experiments. The charge rearrangement among the PPV atoms is also discussed

Mesoscopic modelling of charge evolution in conducting polymers

We address here some of the issues relating to conducting polymer based devices. We examine the effects of polymer disorder on charge injection, transport, trapping and recombination in light-emitting diodes (LEDs) using a mesoscopic model which includes specific realizations of the electroluminescent polymer network. A key point of this model is to consider both the intermolecular and the interdomain charge carrier transport which are strongly influenced by structural polymer disorder. Simulations of bipolar charge evolution (electrons and holes) through a polydiacetylene (PDA) film, which are injected in the polymer layer from the appropriated electrodes, have been used to give some insights to the issue concerning the fraction of polymer molecules contributing to the conduction process as well as to light emission. The effect of charge traps, such as cross-links, on space charge and recombination is also discussed