Scott Medling

Homoleptic cerium(III) and cerium(IV) nitroxide complexes: significant stabilization of the 4+ oxidation state.

Electrochemical experiments performed on the complex Ce(IV)[2-((t)BuNO)py]4, where [2-((t)BuNO)py](-) = N-tert-butyl-N-2-pyridylnitroxide, indicate a 2.51 V stabilization of the 4+ oxidation state of Ce compared to [(n)Bu4N]2[Ce(NO3)6] in acetonitrile and a 2.95 V stabilization compared to the standard potential for the ion under aqueous conditions. Density functional theory calculations suggest that this preference for the higher oxidation state is a result of the tetrakis(nitroxide) ligand framework at the Ce cation, which allows for effective electron donation into, and partial covalent overlap with, vacant 4f orbitals with δ symmetry. The results speak to the behavior of CeO2 and related solid solutions in oxygen uptake and transport applications, in particular an inherent local character of bonding that stabilizes the 4+ oxidation state. The results indicate a cerium(IV) complex that has been stabilized to an unprecedented degree through tuning of its ligand-field environment.

Enhanced Cu emission in ZnS:Cu,Cl/ZnS core–shell nanocrystals

ZnS : Cu,Cl/ZnS core-shell nanocrystals (NCs) have been synthesized via a facile aqueous synthesis method. The shell growth of the NCs was observed via a red-shift in the UV-Vis absorption spectra with increasing NC size. The Cu photoluminescence (PL) emission was enhanced by capping with a thin ZnS shell. The ZnS : Cu (0.2%) and ZnS : Cu (0.5%) show a more pronounced red-shift in the apparent PL peak position as well as a 37% and 67% increase in emission intensity, respectively, in comparison to the undoped NCs. The observed red-shift is mainly due to an increase in intensity of the Cu PL emission. The 1% Cu-doped NCs exhibit very little red-shift because the observed emission is dominated by the Cu-dopant and thus nearly independent of the size of the NCs. The increase in Cu emission is evidence that Cu atoms occupying non-emissive surface sites in doped ZnS NCs were encapsulated by the ZnS shell. Extended X-Ray Absorption Fine Structure (EXAFS) data also suggests that the Cu had slightly more neighbors upon growth of a ZnS shell, indicating its encapsulation into the core of the NCs. The EXAFS Zn edge data also indicate greater disorder in the ZnS structure when the shell is grown, which may be attributed to the ZnS shell being more amorphous than the core NCs. This study demonstrates that core-shell structures can be used as a simple and yet powerful strategy to enhance PL properties of doped semiconductor NCs.

Evolution of magnetic oxygen states in Sr-doped LaCoO3.

Magnetism in La(1-x)Sr(x)CoO(3) as a function of doping is investigated with x-ray absorption spectroscopy and x-ray magnetic circular dicrhoism at the O K edge, and corresponding first principles electronic structure calculations. For small x, the spectra are consistent with the formation of ferromagnetic clusters occurring within a nonmagnetic insulating matrix. Sr-induced, magnetic O-hole states form just above E(F) and grow with increasing Sr doping. Density functional calculations for x=0 yield a nonmagnetic ground state with the observed rhombohedral distortion and indicates that doping introduces holes at the Fermi level in magnetic states with significant O 2p and Co t(2g) character for the undistorted pseudocubic structure. Supercell calculations show stronger magnetism on oxygen atoms having more Sr neighbors.

Probing the local structure of dilute Cu dopants in fluorescent ZnS nanocrystals using EXAFS

A local structure study of ZnS nanocrystals, doped with very low concentrations of Cu, was carried out using the EXAFS technique to better understand how Cu substitutes into the host lattice and forms Cu luminescence centers. We show that a large fraction of the Cu have three nearest neighbor S atoms and the Cu-S bond is significantly shortened compared to Zn-S, by ∼0.08 Å. In addition, the second neighbor Cu-Cu peak is extremely small. We propose that Cu occupies an interior site next to a S(2-) vacancy, with the Cu displaced towards the remaining S(2-) and away from the vacancy; such a displacement immediately explains the lack of a significant Cu-Cu peak in the data. There is no evidence for interstitial Cu sites (Cu(i)), indicating that no more than 2% of the Cu are Cu(i.) This study provides new insights into the local structure of the Cu dopant in ZnS without the presence of CuS nanoprecipitates that are present at higher Cu doping levels.

A ligand field series for the 4f-block from experimental and DFT computed Ce(IV/III) electrochemical potentials.

Understanding of the sensitivity of the reduction potential of cerium(IV) cations to ligand field strength has yet to benefit from systematic variation of the ligand environment. Detailed analyses for a series of seven cerium(IV) tetrakis(pyridyl-nitroxide) compounds and their cerium(III) analogues in varying ligand field strengths are presented. Electrochemical, spectroscopic, and computational results reveal a close correlation of electronic properties with ligand substituents. Together with electrochemical data for reported eight-coordinate compounds, DFT calculations reveal a broad range of the cerium(IV/III) redox potentials correlated to ligand field strengths, establishing a semiempirical, predictive model for the modulation of cerium redox thermodynamics and ligand field strengths. Applications over a variety of scientific disciplines make use of the fundamental redox thermodynamics of cerium. Such applications will benefit from a combined experimental and theoretical approach for assessing redox cycling of cerium compounds.

Understanding and improving electroluminescence in mill-ground ZnS : Cu,Cl phosphors

We demonstrate that lightly milled ZnS : Cu,Cl phosphors produce AC electroluminescence (EL) emission in devices half as thick and produce up to 5 times the light output as the thinnest devices made with unground phosphors. We also establish minimum and maximum bounds on micro-milling conditions for producing powders that are still viable for AC EL. To understand the cause of the minimum size, we report extended x-ray absorption fine structure (EXAFS) measurements on phosphors sorted by particle size. The EXAFS data show that grinding preferentially damages the initially embedded CuS nano-precipitate. This suggests that grinding cleaves the ZnS : Cu through the embedded CuS nano-precipitates, leaving the CuS nano-precipitates on the surface to be further broken apart by continued grinding, eventually reducing the effectiveness of the CuS-induced electric field enhancement. (Some figures in this article are in colour only in the electronic version)

Self-irradiation damage to the local structure of plutonium and plutonium intermetallics

The effect of self-irradiation damage on the local structure of δ-Pu, PuAl2, PuGa3, and other Pu intermetallics has been determined for samples stored at room temperature using the extended x-ray absorption fine-structure (EXAFS) technique. These measurements indicate that the intermetallic samples damage at a similar rate as indicated in previous studies of PuCoGa5. In contrast, δ-Pu data indicate a much slower damage accumulation rate. To explore the effect of storage temperature and possible room temperature annealing effects, we also collected EXAFS data on a δ-Pu sample that was held at less than 32 K for a two month period. This sample damaged much more quickly. In addition, the measurable damage was annealed out at above only 135 K. Data from samples of δ-Pu with different Ga concentrations and results on all samples collected from different absorption edges are also reported. These results are discussed in terms of the vibrational properties of the materials and the role of Ga in δ-Pu as a network...

Comparison of local distortions in Ba8Ga16X30 (X = Si, Ge, Sn): an EXAFS study

We report an extended X-ray fine structure (EXAFS) analysis of the type-I clathrates Ba8Ga16X30 (X = Si, Sn) and compare the results with the results of previous studies on X = Ge. The distribution of Ga on the three crystallographic cage sites is not random, with the Ga preferentially having X as the nearest neighbor. Our results show that for X = Si, Sn the average pair distances within the cages (Ga–Sn, Ga–Ga, Ga–Si, Sn–Sn) are significantly different from the distances found in X-ray diffraction, with some much shorter bonds and some much longer bonds. These results suggest a substantial buckling of the cages, particularly for Ba8Ga16Sn30. The environment about Ba, extracted from Ba K edge EXAFS, becomes increasingly disordered from Ge to Si to Sn, and for Ba8Ga16Sn30 the nearest Ba neighbor distance is very short, consistent with severe buckling. This buckling contributes to the increased local disorder for Ba8Ga16Si30 and Ba8Ga16Sn30, and provides an explanation for a higher resistivity and a lower ZT than for Ba8Ga16Ge30.

Background subtraction for fluorescence EXAFS data of a very dilute dopant Z in Z + 1 host.

When conducting EXAFS at the Cu K-edge for ZnS:Cu with very low Cu concentration (<0.04% Cu), a large background was present that increased with energy. This background arises from a Zn X-ray Raman peak, which moves through the Cu fluorescence window, plus the tail of the Zn fluorescence peak. This large background distorts the EXAFS and must be removed separately before reducing the data. A simple means to remove this background is described.

Degradation and local distortions in electroluminescent ZnS:Cu,Cl phosphors

We present time-lapsed microscopy and EXAFS/XANES data on size selected ground ZnS:Cu,Cl and ZnS:Cu,Mn,Cl phosphors to probe long standing problems in using these phosphors in high brightness applications. The time-lapsed study shows that many of the individual emission centers on each particle degrade via large step decreases while < 50% have a monotonic decrease. These large steps suggest a break-up of Cu aggregate centers and this degradation mechanism is likely irreversible. The Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption near Edge Structure (XANES) studies show that during grinding, the CuS precipitates within the ZnS host become highly disordered yet the host material shows no disorder, suggesting that the ZnS:Cu,Cl materials cleave through the CuS precipitates during grinding.