Short range versus long range structure in Cu In,Ga Se2, Cu In,Ga 3Se5, and Cu In,Ga 5Se8

The Cu poor phases Cu In,Ga 3Se5 and Cu In,Ga 5Se8 play an important role both for understanding the Cu In,Ga Se material system and for growing high efficiency Cu In,Ga Se2 thin film solar cells. Using extended X ray absorption fine structure spectroscopy, we have studied the element specific short range structure of Cu In,Ga Se2, Cu In,Ga 3Se5, and Cu In,Ga 5Se8 alloys spanning the entire compositional range. The materials feature different local atomic arrangements and the element specific average bond lengths remain nearly constant despite significant changes of the lattice constants with increasing In to Ga ratio and decreasing Cu content. In particular, the average bond lengths of Cu Se and Ga Se are almost identical while the average In Se bond length is significantly longer in all three phases. The distance between lattice sites with mixed site occupation therefore corresponds to the weighted average of different element specific bond lengths rather than to the individual bond lengths themselves. Furthermore, the increasing number of vacancies with decreasing Cu content lead to both a significant unit cell contraction and a slight bond length expansion. The crystallographic long range structure and the element specific short range structure thus describe different structural aspects that are certainly interrelated but obviously not identica

Reactivity of Cr species grafted on SiO2/Si(100) surface: a Reflection EXAFS study down to the sub-monolayer regime

In situ X-ray absorption near-edge spectroscopy/extended X-ray absorption fine structure (XANES/EXAFS) experiments are conducted for the first time on a highly diluted Cr/SiO2/Si(100) system (2 Cr/nm2, representing a model of the Phillips catalyst for the ethylene polymerization) by exploiting the reflection EXAFS (ReflEXAFS) geometry. This experiment, aimed to give a contribution in bridging the gap between surface science and catalysis, demonstrates that it is possible to follow the reversible red-ox reactivity of surface species grafted on a single well-defined surface, at a concentration limit that is far below the monolayer coverage level and for a highly sensitive sample. A further improvement on the impurity level of the ReflEXAFS chamber is however required in order to be able to follow in situ the polymerization reaction. Our results demonstrate that the red-ox ability of the isolated surface Cr species is not enough to make a polymerization active species

Do dietary patterns determine levels of vitamin B6, folate, and vitamin B12 intake and corresponding biomarkers in European adolescents? The Healthy Lifestyle in Europe by Nutrition in Adolescence (HELENA) study

Objectives: To determine dietary patterns (DPs) and explain the highest variance of vitamin B6, folate, and B12 intake and related concentrations among European adolescents. Methods: A total of 2173 adolescents who participated in the Healthy Lifestyle in Europe by Nutrition in Adolescence study met the eligibility criteria for the vitamin B intake analysis (46% boys) and 586 adolescents for the biomarkers analysis (47% boys). Two non-consecutive, 24-h, dietary recalls were used to assess the mean intakes. Concentrations were measured by chromatography and immunoassay testing. A reduced rank regression was applied to elucidate the combined effect of food intake of vitamin B and related concentrations. Results: The identified DPs (one per vitamin B intake and biomarker and by sex) explained a variability between 34.2% and 23.7% of the vitamin B intake and between 17.2% and 7% of the biomarkers. In the reduced rank regression models, fish, eggs, cheese, whole milk and buttermilk intakes were loaded positively for vitamin B intake in both sexes; however, soft drinks and chocolate were loaded negatively. For the biomarkers, a higher variability was observed in the patterns in terms of food loads such as alcoholic drinks, sugars, and soft drinks. Some food items were loaded differently between intakes and biomarkers such as fish products, which was loaded positively for intakes but negatively for plasma folate in girls. Conclusions: The identified DPs explained up to 34.2% and 17.2% of the variability of the vitamin B intake and plasma concentrations, respectively, in European adolescents. Further studies are needed to elucidate the factors that determine such patterns