Photophysics of Two-Dimensional Perovskites—Learning from Metal Halide Substitution

2D perovskites offers a rich playing field to explore exciton physics and they possess a great potential for a variety of opto-electronic applications. Whilst their photophysics shows intricate interactions of excitons with the lattice, most reports have so far relied on single compound studies. With the exception of variations of the organic spacer cations, the effect of constituent substitution on the photophysics and the nature of emitting species, in particular, have remained largely under-explored. Here PEA₂PbBr₄, PEA₂PbI₄, and PEA₂SnI₄ (where PEA stands for phenylethylammonoium) are studied through a variety of optical spectroscopy techniques to reveal a complex set of excitonic transitions at low temperature. Weak high-energy features are attributed to vibronic transitions breaking Kasha's, for which the responsible phonons cannot be accessed through simple Raman spectroscopy. Bright peaks at lower energy are due to two distinct electronic states, of which the upper is a convolution of the free exciton and a localized dark state and the lower is attributed to recombination involving shallow defects. This study offers deeper insights into the photophysics of 2D perovskites through compositional substitution and highlights critical limits to the communities’ current understanding of processes in these compounds.     

Potential options to reduce GHG emissions in Venezuela

This paper presents a summary of the technologies and practices that could be implemented in Venezuela in order to contribute to both climate change mitigation and national development efforts. The mitigation analysis concentrates on options to reduce CO₂ emissions generated from the energy sector and land-use change.

From the mitigation options analyzed for the energy sector it was determined that the most effective are those in the transportation sector (switching to larger capacity vehicles, reduced private vehicle share, and switching fuels for public transportation from gasoline to natural gas), both in terms of contribution to emissions reduction and costs. Regarding the options for industry, boilers conversion from liquids to natural gas shows negative cost, but to a considerably lower extent that for the transportation sector. Efficiency improvements of natural gas boilers, which presents close to zero cost, is more effective in reducing emissions than boiler conversion. Increase in hydro power generation is the alternative with the highest total cost but it is very effective in reducing emissions.

From the mitigation options analyzed for land-use change, it was established that the forest sector has a considerable potential for reducing CO₂ emissions through the adoption of sustainable forest practices, especially by slowing the rate of forest loss and degradation. Maintenance of already existing biomass in natural forests should be the first priority of forest measures to reduce the amount of carbon released to the atmosphere. Forest protection and management of native forest represent the two options with the highest carbon conservation potential and the lowest carbon unit cost. Expansion of the forest cover through the development of intensive forest plantations also presents a high potential to offset carbon emissions in Venezuela.

An analysis of the barriers to mitigation options implementation shows that in the energy sector, low energy prices represent the main barrier to any mitigation program. Another important limitation to mitigation strategies implementation is the lack of institutional capacity and legal instruments for developing the mitigation measures. In the forest sector the primary causes of forest clearing in the country are not related to forest activities, so the definition of feasible mitigation options will depend upon a good understanding of other economic sectors and how they account for land-use change. Land tenure, rural poverty, political interests, and weak implementation of land-use planning instruments and environmental laws are considered to be the key limitations to any effort dealing with forest conservation. Land tenure, economic factors, and lack of incentives represent some of the most important barriers to the development of forest plantations and agroforestry systems in the country.     

Rhodium complexes containing chiral P-donor ligands as catalysts for asymmetric hydrogenation in non conventional media

Abstract  

Rhodium-catalysed asymmetric hydrogenation using P-donor ligands, such as new fluorinated (R)-BINOL and azadioxaphosphabicyclo[3.3.0]octane derivatives was carried out in different reaction media such as organic solvent (CH₂Cl₂), ionic liquid ([BMI][PF₆]), supercritical carbon dioxide (scCO₂) and [BMI][PF₆]/scCO₂ mixture. The best enantioselectivities were obtained in neat [BMI][PF₆], allowing a recycling up to ten times without activity loss. However, the enantioselectivity was lost due to ligand leaching. The ionic liquid phase containing rhodium molecular species was supported on functionalized multi-walled carbon nanotubes in order to improve the recycling, but unfortunately the asymmetric induction was lost upon catalyst immobilization.     

Improving English verb sense disambiguation performance with linguistically motivated features and clear sense distinction boundaries

This paper presents a high-performance broad-coverage supervised word sense disambiguation (WSD) system for English verbs that uses linguistically motivated features and a smoothed maximum entropy machine learning model. We describe three specific enhancements to our system’s treatment of linguistically motivated features which resulted in the best published results on SENSEVAL-2 verbs. We then present the results of training our system on OntoNotes data, both the SemEval-2007 task and additional data. OntoNotes data is designed to provide clear sense distinctions, based on using explicit syntactic and semantic criteria to group WordNet senses, with sufficient examples to constitute high quality, broad coverage training data. Using similar syntactic and semantic features for WSD, we achieve performance comparable to that of human taggers, and competitive with the top results for the SemEval-2007 task. Empirical analysis of our results suggests that clarifying sense boundaries and/or increasing the number of training instances for certain verbs could further improve system performance.

Client perceptions of the therapeutic relationship as a function of client and counselor age.

32 women pseudoclients from each of 2 age groups (25–35 and 60–70 yrs) met with a woman crisis counselor from 1 of the 2 age groups for a 50-min face-to-face interview. Results suggest that age of clients and counselors did not exert a significant influence on clients' perceptions of counselor facilitative skills (e.g., empathy, congruence, regard) or client satisfaction with the therapeutic interview. Age similarity between the client and counselor did not necessarily improve the working alliance. Clients of a specific age group view some counselors of a particular age as more facilitative and empathic than other counselors of that same age. (21 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

A validated matrix solid-phase dispersion method for the extraction of organophosphorus pesticides from bovine samples

A method based on matrix solid-phase dispersion (MSPD) was developed for the quantitative extraction of five organophosphorus (OPPs) pesticides from bovine samples. The determination was carried out by high performance liquid chromatography (HPLC) with diode array spectrophotometric UV detection. The MSPD extraction with octadecylsilyl (C18) sorbent combined with a silica gel clean-up and acetonitrile elution was optimised for chlorpyrifos, chlorfenvinphos, diazinon, fenitrothion, and parathion-methyl. The method was validated, yielding recovery values higher than 94%, except for chlorfenvinphos in liver (55%), and precision values, expressed as relative standard deviations (RSDs), which were less than or equal to 15% in liver and 11.5% in muscle at spiking levels of 0.25, 2.5 and 5 μg g⁻¹. Linearity was studied from 0.5 to 15 μg g⁻¹, and the limits of detection (LODs) were found to be lower than 0.1 μg g⁻¹_. This method was applied to the analysis of real samples with confirmative analyses performed using gas chromatography–mass spectrometry (GC–MS) in selected ion monitoring mode (SIM).