Color Point Tuning for (Sr,Ca,Ba) Si2O2N2:Eu2+ for White Light LEDs

Color point tuning is an important challenge for improving white light LEDs. In this paper, the possibilities of color tuning with the efficient LED phosphor Sr1−x−y−zCaxBaySi2O2N2:Euz2+ (0 ≤ x, y ≤ 1; 0.005 ≤ z ≤ 0.16) are investigated. The emission color can be tuned in two ways: by changing Eu2+ concentration and by substitution of the host lattice cation Sr2+ by either Ca2+ or Ba2+. The variation in the Eu2+ concentration shows a red shift of the emission upon increasing the Eu concentration above 2%. The red shift is explained by energy migration and energy transfer to Eu2+ ions emitting at longer wavelengths. Along with this (desired) red shift there is an (undesired) lowering of the quantum efficiency and the thermal quenching temperature due to concentration quenching. Partial substitution of Sr2+ by either Ca2+ or Ba2+ also results in a red-shifted Eu2+ emission. For Ca2+ this is expected and the red shift is explained by an increased crystal field splitting for Eu2+ on the (smaller) Ca2+ cation site. For Ba2+, the red shift is surprising. Often, a blue shift of the fd emission is observed in case of substitution of Sr2+ by the larger Ba2+ cation. The Eu2+ emission in the pure BaSi2O2N2 host lattice is indeed blue-shifted. Temperature dependent luminescence measurements show that the quenching temperature drops upon substitution of Sr by Ca, whereas for Ba substitution, the quenching temperature remains high. Color tuning by partial substitution of Sr2+ by Ba2+ is therefore the most promising way to shift the color point of LEDs while retaining the high quantum yield and high luminescence quenching temperature

[Review of] Stephen Castles. Here for Good: Western Europe\u27s New Ethnic Minorities

A disturbing, extremely important book. The wealth of information provided by the authors illuminates the present circumstances of ethnic minorities in Western Europe, principally West Germany, and points to potential dangers and repercussions in the future. The thesis of Here for Good posits the change in status of the guest worker from that of migrant or temporary resident to that of immigrant or permanent settler in one of seven major host countries in Western Europe. This shift should mark a parallel shift in the political and economic policies which address the needs of the immigrants and their impact upon the host countries. The adaptation of political and economic strategies, however, is slow in coming, breeding complex problems at every level. Castles examines the most relevant aspects of the crisis, and it is a crisis, letting the statistical data spell out authoritatively the seriousness of the situation. The book is organized according to seven major concerns, each discussed in a separate chapter and thoroughly documented with statistical tables

Hydrogen bond based noncovalent association in the semi-fluorous solvent perfluorobutyl-methyl ether: Host-host and host-guest association of the host 1-(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-Heptadecafluoro-decyl)-3-pyridin-2-yl-urea

A fluorous pyridyl-urea, 1-(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluoro-decyl)-3-pyridin-2-yl-urea, was prepared to act as a host and analyzed by 1H NMR inCD2Cl2 and perfluorobutyl-methyl ether (HFE7100). Crystals were analyzed by X-ray diffraction. The host molecules were found to form pillar-like structures in the crystal. There is an intramolecular bond between the pyridyl nitrogen and one urea hydrogen. 1H NMR spectra demonstrated that the urea hydrogens’ positions shift as the concentration of the host changes. The dependence of the shifts on concentration are consistent with the formation of a trimer of hosts with a logKeq for formation of trimer from monomer approximately 6. Association of the host with guests octanoic acid, ethyl acetate, N-ethylacetamide, N,N-dimethylacetamide, and acetone, was analyzed by titration of the host with individual guests in HFE7100 solvent. Downfield or upfield shifts of the urea hydrogens were used to indicate hydrogen bond formation with the guest. Acetone and ethyl acetate were unable to overcome the self-association of the host and form host-guest complexes. Octanoic acid binding caused shifts in the 1H NMR spectra of one hydrogen of the urea group. N-ethylacetamide and N,N-dimethylacetamide induced shifts in both urea hydrogens. The results indicate that the host monomer’s favored conformation contains an intramolecular hydrogen bond. This bond is not broken upon association with octanoic acid, but it is broken upon association with the two acetamides

Foreign direct investment and environmental taxes

This paper studies the effect of foreign direct investment (FDI) on environmental policy stringency in a two-country model with trade costs, where FDI could be unilateral and bilateral and both governments address local pollution through environmental taxes. We show that FDI does not give rise to ecological dumping because the host country has an incentive to shift rents away from the source country towards the host country. Environmental policy strategies and welfare effects are studied under the assumption that parameter values support FDI to be profitable. JEL Classification: F12, F18, F23environmental taxes, Foreign Direct Investment, multinational enterprises, plant location

Comparative Metabolomics of Early Development of the Parasitic Plants Phelipanche aegyptiaca and Triphysaria versicolor.

Parasitic weeds of the family Orobanchaceae attach to the roots of host plants via haustoria capable of drawing nutrients from host vascular tissue. The connection of the haustorium to the host marks a shift in parasite metabolism from autotrophy to at least partial heterotrophy, depending on the level of parasite dependence. Species within the family Orobanchaceae span the spectrum of host nutrient dependency, yet the diversity of parasitic plant metabolism remains poorly understood, particularly during the key metabolic shift surrounding haustorial attachment. Comparative profiling of major metabolites in the obligate holoparasite Phelipanche aegyptiaca and the facultative hemiparasite Triphysaria versicolor before and after attachment to the hosts revealed several metabolic shifts implicating remodeling of energy and amino acid metabolism. After attachment, both parasites showed metabolite profiles that were different from their respective hosts. In P. aegyptiaca, prominent changes in metabolite profiles were also associated with transitioning between different tissue types before and after attachment, with aspartate levels increasing significantly after the attachment. Based on the results from 15N labeling experiments, asparagine and/or aspartate-rich proteins were enriched in host-derived nitrogen in T. versicolor. These results point to the importance of aspartate and/or asparagine in the early stages of attachment in these plant parasites and provide a rationale for targeting aspartate-family amino acid biosynthesis for disrupting the growth of parasitic weeds

Setting Incentives for Collaboration Among Agricultural Scientists: Application of Principal-Agent Theory to Team Work

�The USDA is attempting to shift more research funds into competitive grants involving collaboration across disciplines on large projects. This type of research structure raises a host of information and incentive issues. The objective of this paper is to shed new light on principal-agent problems that are likely to arise in this new funding structure.incentives; Principal-agent model; team research; competitive grants; multi-disciplinary research

Exchange bias with Fe substitution in LaMnO_3

The exchange bias (EB) in LaMn_{0.7}Fe_{0.3}O_3 is observed by the negative shift and training effect of the hysteresis loops, while the sample was cooled in external magnetic field. The analysis of cooling field dependence of EB gives the size of the ferromagnetic (FM) cluster ~ 25 Angstrom, where the magnetic anisotropy of FM cluster is found two order of magnitude higher than the FM bulk manganites. We propose that the nanoscale FM clusters are embedded in the glassy magnetic host with EB at the FM/glassy magnetic interface.Comment: 6 figure

Quantum Impurity in a Nearly Critical Two Dimensional Antiferromagnet

We describe the spin dynamics of an arbitrary localized impurity in an insulating two dimensional antiferromagnet, across the host transition from a paramagnet with a spin gap to a Neel state. The impurity spin susceptibility has a Curie-like divergence at the quantum-critical coupling, but with a universal, effective spin which is neither an integer nor a half-odd-integer. In the Neel state, the transverse impurity susceptibility is a universal number divided by the host spin stiffness (which determines the energy cost to slow twists in the orientation of the Neel order). These, and numerous other results for the thermodynamics, Knight shift, and magnon damping have significant applications to experiments on layered transition metal oxides.Comment: 12 pages, 3 figures; Additional details and results are in cond-mat/991202