Publications

International audience

An entry from the Inorganic Crystal Structure Database, the world’s repository for inorganic crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the joint CCDC and FIZ Karlsruhe Access Structures service and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

GaN thin film integrated to polycrystalline diamond substrates is a novel microwave transistor material with significantly improved heat dissipation capability. Due to the thermal and mechanical properties mismatch between GaN and diamond, a natural concern arises in terms of its interfacial stability as currently there is no established method to evaluate the interfacial toughness in GaN-on-diamond material. Using three generations of "GaN-on-Diamond" materials with varying process parameters, a comprehensive study has been carried out to identify the most appropriate fracture mechanics-based methods for reliable evaluation of the interfacial toughness in this novel material system. Several techniques were assessed, and the results are cross-compared; these include an ex situ nanoindentation induced buckling method and two-step indentation approach together with several analytical models. Additionally, a microcantilever bending method was adopted to measure an upper bound for the interfacial fracture toughness. For the three generations of materials, the interfacial toughness, GIc, was determined to be 0.7, 0.9, and 0.6 J·m–2, respectively. Postmortem analysis of the micro- and nanostructure of fractured interfaces indicated that the systems with better heat spreading capability displayed smoother fracture surfaces, i.e., were more brittle due to the lack of active toughening mechanisms. Potential modifications to the interface for improved mechanical stability were proposed based on the experimental results.

The nearby Type Ia supernovae (SNe Ia) 2011fe and 2011by had nearly identical\nphotospheric phase optical spectra, light-curve widths, and photometric\ncolours, but at peak brightness SN 2011by reached a fainter absolute magnitude\nin all optical bands and exhibited lower flux in the near-ultraviolet (NUV).\nBased on those data, Foley & Kirshner (2013) argue that the progenitors of SNe\n2011by and 2011fe were supersolar and subsolar, respectively, and that SN\n2011fe generated 1.7 times the amount of 56Ni as SN 2011by. With this work, we\nextend the comparison of these SNe Ia to 10 days before and 300 days after\nmaximum brightness with new spectra and photometry. We show that the nebular\nphase spectra of SNe 2011fe and 2011by are almost identical, and do not support\na factor of 1.7 difference in 56Ni mass. Instead, we find it plausible that the\nTully-Fisher distance for SN 2011by is an underestimate, in which case these\nSNe Ia may have reached similar peak luminosity, formed similar amounts of\n56Ni, and had lower metallicity progenitors than previously estimated.\nRegardless of the true distance to SN 2011by, we find that the relative\nprogenitor metallicity difference remains well supported by their disparity in\nNUV flux, which we show to be even stronger at pre-maximum epochs --- although\ncontributions from differences in total ejecta mass, viewing angle, or\nprogenitor density cannot be ruled out. We also demonstrate that, independent\nof distance modulus, SN 2011by exhibits a late-time luminosity excess that\ncannot be explained by a light echo, but is more likely to be the result of\ngreater energy trapping by the nucleosynthetic products of SN 2011by.\n

Mercury (Hg) is widely distributed in the environment, and its organic form, methylmercury (MeHg), can extensively bioaccumulate and biomagnify in aquatic and terrestrial food webs. Concentrations of MeHg in organisms are highly variable, and the sources in natural food webs are often not well understood. This study examined stable isotope ratios of MeHg (mass-dependent fractionation, as δ(202)HgMeHg; and mass-independent fractionation, as Δ(199)HgMeHg) in benthic invertebrates, juvenile steelhead trout (Oncorhynchus mykiss), and water striders (Gerris remigis) along a stream productivity gradient, as well as carnivorous terrestrial invertebrates, in a forested watershed at the headwater of South Fork Eel River in northern California. Throughout the sampling sites, δ(202)HgMeHg (after correction due to the effect of MeHg photodegradation) was significantly different between benthic (median = -1.40‰; range, -2.34 to -0.78‰; total number of samples = 29) and terrestrial invertebrates (median = +0.51‰; range, -0.37 to +1.40‰; total number of samples = 9), but no major difference between these two groups was found for Δ(199)HgMeHg. Steelhead trout (52 individual fishes) have MeHg of predominantly aquatic origins, with a few exceptions at the upstream locations (e.g., 1 fish collected in a tributary had a purely terrestrial MeHg source and 4 fishes had mixed aquatic and terrestrial MeHg sources). Water striders (seven pooled samples) derive MeHg largely from terrestrial sources throughout headwater sections. These data suggest that direct terrestrial subsidy (e.g., terrestrial invertebrates falling into water) can be important for some stream predators in headwater streams and could represent an important means of transfer of terrestrially derived MeHg (e.g., in situ methylation within forests, atmospheric sources) to aquatic ecosystems. Moreover, these findings show that terrestrial subsidies can enhance MeHg bioaccumulation of consumers in headwater streams where aqueous MeHg levels are very low.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Copper is an essential nutrient for life, but at the same time, hyperaccumulation of this redox-active metal in biological fluids and tissues is a hallmark of pathologies such as Wilson's and Menkes diseases, various neurodegenerative diseases, and toxic environmental exposure. Diseases characterized by copper hyperaccumulation are currently challenging to identify due to costly diagnostic tools that involve extensive technical workup. Motivated to create simple yet highly selective and sensitive diagnostic tools, we have initiated a program to develop new materials that can enable monitoring of copper levels in biological fluid samples without complex and expensive instrumentation. Herein, we report the design, synthesis, and properties of PAF-1-SMe, a robust three-dimensional porous aromatic framework (PAF) densely functionalized with thioether groups for selective capture and concentration of copper from biofluids as well as aqueous samples. PAF-1-SMe exhibits a high selectivity for copper over other biologically relevant metals, with a saturation capacity reaching over 600 mg/g. Moreover, the combination of PAF-1-SMe as a material for capture and concentration of copper from biological samples with 8-hydroxyquinoline as a colorimetric indicator affords a method for identifying aberrant elevations of copper in urine samples from mice with Wilson's disease and also tracing exogenously added copper in serum. This divide-and-conquer sensing strategy, where functional and robust porous materials serve as molecular recognition elements that can be used to capture and concentrate analytes in conjunction with molecular indicators for signal readouts, establishes a valuable starting point for the use of porous polymeric materials in noninvasive diagnostic applications.