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Design and engineering of effective electrode catalysts represents a critical first step for hydrogen production by electrochemical water splitting. Nanocomposites based on ruthenium atomically dispersed within a carbon scaffold have emerged as viable candidates. In the present study, ruthenium metal centers are atomically embedded within graphitic carbon nitride/reduced graphene oxide nanosheets by thermal refluxing. Subsequent chemical reduction/oxidation leads to ready manipulation of the ruthenium valence state, as evidenced in microscopic and spectroscopic measurements, and hence enhancement/diminishment of the electrocatalytic activity towards hydrogen evolution reaction in both acidic and alkaline media. This is largely ascribed to the increased/reduced contribution of the Ru valence electrons to the density of state near the Fermi level which dictates the binding and reduction of hydrogen. Results from this study highlight the significance of the valence state of metal centers in the manipulation and optimization of the catalytic performance of single atom catalysts.

The conduction of small polarons is impacted by the Fe sub-lattice disorder in atomically doped α-Fe2O3. Larger disorder lowers the polaron mobility.

Carbon nanocomposites with Ru-doped Fe2O3 nanoparticles are derived from a Fe–Ru–Fe trinuclear complex and zeolitic imidazolate framework 8 and exhibit excellent electrocatalytic activity towards oxygen reduction reaction, comparable to that of Pt/C.

Sustainable hydrogen gas production is critical for future fuel infrastructure. Here, a series of phosphorous-doped carbon nitride materials were synthesized by thermal annealing of urea and ammonium hexafluorophosphate, and platinum was atomically dispersed within the structural scaffold by thermal refluxing with Zeise's salt forming Pt–N/P/Cl coordination interactions, as manifested in X-ray photoelectron and absorption spectroscopic measurements. The resulting materials were found to exhibit markedly enhanced electrocatalytic activity towards the hydrogen evolution reaction (HER) in acidic media, as compared to the P-free counterpart. This was accounted for by P doping that led to a significantly improved charge carrier density within C3N4, and the sample with the optimal P content showed an overpotential of only −22 mV to reach the current density of 10 mA cm−2, lower than that of commercial Pt/C (−26 mV), and a mass activity (7.1 mA μg−1Pt at −70 mV vs. reversible hydrogen electrode) nearly triple that of the latter. Results from the present study highlight the significance of P doping in the manipulation of the electronic structures of metal/carbon nitride nanocomposites for high-performance HER electrocatalysis.

Microwave-assisted heating methods have been used to synthesize oleylamine-capped Ge1–xSnx nanocrystals. By varying the reaction temperature while keeping the Ge and Sn precursor concentrations con...

The numerous electronic and optoelectronic applications that rely on semiconductors require tuning their properties through doping. Germanium quantum dots (Ge QDs) were successfully doped with bism...

Iron single atom catalysts have emerged as one of the most active electrocatalysts towards the oxygen reduction reaction (ORR), but the unsatisfactory durability and limited activity for the oxygen evolution reaction (OER) has hampered their commercial applications in rechargeable metal–air batteries. By contrast, cobalt-based catalysts are known to afford excellent ORR stability and OER activity, due to the weak Fenton reaction and low OER Gibbs free energy. Herein, a bimetal hydrogel template is used to prepare carbon aerogels containing Fe–Co bimetal sites (NCAG/Fe–Co) as bifunctional electrocatalysts towards both ORR and OER, with enhanced activity and stability, as compared to the monometal counterparts. High-resolution transmission electron microscopy, elemental mapping and X-ray photoelectron spectroscopy measurements demonstrate homogeneous distributions of the metal centers within defected carbon lattices by coordination to nitrogen dopants. X-ray absorption spectroscopic measurements, in combination with other results, suggest the formation of FeN3 and CoN3 moieties on mutually orthogonal planes with a direct Fe–Co bonding interaction. Electrochemical measurements show that NCAG/Fe–Co delivers a small ORR/OER potential gap of only 0.64 V at the current density of 10 mA cm−2, 60 mV lower than that (0.70 V) with commercial Pt/C and RuO2 catalysts. When applied in a flexible Zn–air battery, the dual-metal NCAG/Fe–Co catalyst also shows a remarkable performance, with a high open-circuit voltage of 1.47 V, a maximum power density of 117 mW cm−2, as well as good rechargeability and flexibility. Results from this study may offer an ingenious protocol in the design and engineering of highly efficient and durable bifunctional electrocatalysts based on dual metal-doped carbons.

Copper compounds have been extensively investigated for diverse applications. However, studies of cuprous hydroxide (CuOH) have been scarce, due to structural metastability. Herein, a facile, wet-chemistry procedure is reported for the preparation of stable CuOH nanostructures via deliberate functionalization with select organic ligands, such as acetylene and mercapto derivatives. The resulting nanostructures are found to exhibit a nanoribbon morphology consisting of small nanocrystals embedded within a largely amorphous nanosheet-like scaffold. The acetylene derivatives are found to anchor onto the CuOH forming Cu-Cࣽ linkages, whereas Cu-S- interfacial bonds are formed with the mercapto ligands. Effective electronic coupling occurs at the ligand-core interface in the former, in contrast to mostly non-conjugated interfacial bonds in the latter, as manifested in spectroscopic measurements and confirmed in theoretical studies based on first principles calculations. Notably, the acetylene-capped CuOH nanostructures exhibit markedly enhanced photodynamic activity in the inhibition of bacteria growth, as compared to the mercapto-capped counterparts, due to a reduced material bandgap and effective photocatalytic generation of reactive oxygen species. Results from this study demonstrate that deliberate structural engineering with select organic ligands is an effective strategy in the stabilization and functionalization of CuOH nanostructures, a critical first step in exploring their diverse applications. This article is protected by copyright. All rights reserved.

X-ray absorption measurements at the U ${L}_{\mathrm{III}}$, Ru $K$, and Fe $K$ edges are reported for the hidden order (HO) material ${\mathrm{URu}}_{2\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{Si}}_{2}$ ($x=0$, 0.05, 0.08, 0.10, 0.12, 0.15, and 0.20) as a function of $x$ and temperature $T$. When Fe is substituted for Ru, the local structure about Fe shrinks slightly and the first neighbor Fe-Si bond length decreases by $\ensuremath{\approx}0.05\phantom{\rule{0.16em}{0ex}}\AA{}$. More importantly excess disorder is observed below 80--100 K (the coherence temperature ${T}^{*}$) in plots of the Debye-Waller factor ${\ensuremath{\sigma}}^{2}$ ($\ensuremath{\sigma}$ is the width of the pair distribution function); at low $T$ the data deviate from the usual Einstein or correlated-Debye model plots. This excess disorder is most prominent for the Ru-Si bond, and ${\ensuremath{\sigma}}^{2}$ actually increases below 80 K. These results suggest a local orthorhombic distortion with ${B}_{1g}$-like symmetry that develops below 80--100 K. A model that describes these local distortions is presented, and discussed in terms of other measurements that indicate a breaking of fourfold symmetry at low $T$. In addition, the square root of the difference between ${\ensuremath{\sigma}}^{2}(T)$ for the Ru-Si pair and a Debye fit to these data serves as an order parameter for this orthorhombic distortion, in the temperature range below 100 K. This quantity is a length related to $a\text{\ensuremath{-}}b$, the difference between the $a$ and $b$ lattice constants in the orthorhombic phase, and provides a connection between this distortion and ${T}^{*}$. X-ray absorption near edge structure (XANES) measurements also show that there are no changes in the edge positions down to 0.1 eV for any edge as a function of $x$, for $T$ in the HO regime.

In this study we investigate the light and heat-induced degradation of methylammonium lead iodide (MAPbI 3 ) perovskite films in an inert atmosphere to exclude the effect of oxygen and humidity. Films aged under solar intensities started to degrade above 75 °C, while films in the dark degraded at 95 °C. To investigate the temperature-induced degradation mechanism, spectroscopic techniques such as Ultraviolet-Visible (UV–Vis) absorption spectroscopy, X-Ray Diffraction (XRD), Extended X-ray Absorption Fine Structure (EXAFS), and Fourier Transform Infrared (FT-IR) were used. Results show that the films aged under light at 75 °C degraded to a mixture of PbI 2 and metallic Pb. In contrast, films aged thermally in the dark, or with light and oxygen, degraded to PbI 2 only. MAPbI 3 solar cells were aged to show the effect of the metallic lead on the charge transfer mechanism.

Lanthanides are multivalent chemical systems whose luminescence activates numerous optical devices. Their multivalent nature sets a complex scenario that challenges widespread single-valent-ion models. Here we study the complexity inherent in the 2+/3+ mixed-valence nature of Yb in CaF2. We report X-ray absorption spectroscopy of CaF2:Yb that shows a maximal, huge Yb3+ X-ray-induced valence reduction at 200 K for low Yb concentration (0.01%) samples, which reverts to the initial state upon warming to 300 K. Although reduction doubles the number of Yb2+ ions, the anomalous emission that is normally observed at low temperatures completely disappears and never recovers. Surprisingly, however, after annealing at 900 K, some anomalous emission is again observed below 150 K. Under X-ray exposure electrons are ejected from the F– ions, including charge compensating interstitials, Fi–. Removing an electron from Fi– leaves a neutral atom which appears metastable. Ab initio calculations of the Fi–-to-Yb3+ electron ...

Yb-substituted CaF2 exhibits an anomalous red-shifted luminescence after UV excitation, attributed to the relaxation of impurity trapped excitons (ITE). CaF2:Yb is the archetype system for this model, in which the Yb2+ ions can be excited into a long-lived (ms) exciton state. Upon de-excitation, the emission intensity should be proportional to the Yb2+ concentration, but that could not be checked when this model was first proposed. Using the X-ray absorption near edge structure (XANES) technique, we determine the fractions of Yb2+ and Yb3+ for low Yb concentrations, 0.01% to 0.1%, and thus determine the net concentration of Yb2+. A comparison with luminescence data shows that the intensity is not proportional to the Yb2+ concentration, and only a fraction of Yb2+ ions contributes to the anomalous luminescence. This is inconsistent with the ITE model and illustrates the importance of checking the dependence of the emission intensity on the Yb2+ concentration.

Here we investigate the local structure present in single-step precursor solutions of methylammonium lead iodide (MAPbI3) perovskite as a function of organic and inorganic precursor ratio, as well as with hydriodic acid (HI), using X-ray absorption spectroscopy. An excess of organic precursor as well as the use of HI as a processing additive has been shown to lead to the formation of smooth, continuous, pinhole free MAPbI3 films, whereas films produced from precursor solutions containing molar equivalents of methylammonium iodide (MAI) and PbI2 lead to the formation of a discontinuous, needlelike morphology. We now show that as the amount of excess MAI in the precursor solution is increased, the iodide coordination of iodoplumbate complexes present in solution increases. The use of HI results in a similar increase in iodide coordination. We therefore offer insight into how solution chemistry can be used to control MAPbI3 thin film morphology by revealing a strong correlation between the lead coordination ...

Electrochemical hydrogen generation is a rising prospect for future renewable energy storage and conversion. Platinum remains a leading choice of catalyst, but because of its high cost and low natural abundance, it is critical to optimize its use. In the present study, platinum oxide nanoparticles of approximately 2 nm in diameter are deposited on carbon nitride (C3N4) nanosheets by thermal refluxing of C3N4 and PtCl2 or PtCl4 in water. These nanoparticles exhibit apparent electrocatalytic activity toward the hydrogen evolution reaction (HER) in acid. Interestingly, the HER activity increases with increasing Pt4+ concentration in the nanoparticles, and the optimized catalyst even outperforms commercial Pt/C, exhibiting an overpotential of only -7.7 mV to reach the current density of 10 mA cm-2 and a Tafel slope of -26.3 mV dec-1 . The results from this study suggest that the future design of platinum oxide catalysts should strive to maximize the Pt4+ sites and minimize the formation of the less active Pt2+ species.

We characterize the properties of ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Yb}}_{x}{\mathrm{RhIn}}_{5}$ single crystals with $0\ensuremath{\le}x\ensuremath{\le}1$ using measurements of powder x-ray diffraction, energy dispersive x-ray spectroscopy, electrical resistivity, magnetic susceptibility, specific heat, x-ray absorption near edge structure (XANES), and neutron diffraction. The Yb valence ${v}_{\text{Yb}}$, calculated from the magnetic susceptibility and measured using XANES, decreases from $3+$ at $x=0$ to $\ensuremath{\sim}2.1+$ at ${x}_{\mathrm{act}}=0.2$, where ${x}_{\mathrm{act}}$ is the measured Yb concentration. A transition from incommensurate to commensurate antiferromagnetism is observed in neutron diffraction measurements along $Q=(0.5,0.5,l)$ between $0.2\ensuremath{\le}{x}_{\mathrm{act}}\ensuremath{\le}0.27$; this narrative is supported by specific-heat measurements in which a second robust feature appears at a temperature ${T}_{I}\phantom{\rule{4pt}{0ex}}({T}_{I}l{T}_{N})$ for the same concentration range. Magnetic susceptibility measurements also reveal features which provide additional evidence of magnetic ordering. The results of this study suggest that the evolution of the Yb valence plays a critical role in tuning the magnetic ground state of ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Yb}}_{x}{\mathrm{RhIn}}_{5}$.

Skutterudites are considered to be good thermoelectrics with high figures of merit, ZT. After synthesis, electrodes are created by grinding and hot pressing the resulting powder. Materials such as NdFe4Sb12 exhibit a significantly greater figure of merit, ZT, (about 43%) when it is ball milled to produce fine powders (inital powder 160 nm; after hot pressing ∼330 nm). This enhancement is typically attributed to the reduced particle size, which in turn decreases the mean free path of phonons, and consequently decreases the thermal conductivity. This work aims to investigate whether there is any damage to the crystal structure in the particles formed by ball milling, which could also affect its thermal conductivity. Using a temperature dependent, Extended X-ray Absorption Fine Structure (EXAFS) analysis of a hand ground and ball milled sample of the skutterudite NdyFe4Sb12, we have determined that ball milling causes no significant damage to the local structure around any site. Consequently further improvements in ZT may be possible with smaller particles.

Lattice distortions corresponding to Ba displacements with respect to the FeAs sublattice are revealed to break the room-temperature tetragonal symmetry in Ba(Fe1-x Cox)2 As2. The displacements yield twin domains of the size of ≈10 nm. The domain size correlates with the magnitude of the local Fe magnetic moment and its non-monotonic dependence on Co concentration.

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.

To optimize a device, you must know what it's actually made of. Cu${}_{2}$ZnSnS${}_{4}$ (CZTS) is appealing as a photovoltaic absorber, but small structural variations in a real sample can have a large impact on device performance and reliability. To understand the structure and stoichiometry of CZTS nanoparticles, the authors measure the extended x-ray absorption fine structure of the constituent metals. They find that as-synthesized nanoparticles are not always of the intended composition, but that copper-poor CZTS does retain the kesterite (or stannite) structure, with the possibility to fine-tune functional properties by controlling antisite defects.

ZnS:Cu contains needle-shaped, CuS conducting nano-precipitates along the [111] planes which amplify the applied electric fields near the tips when the field is reversed, enabling AC electroluminescence for ∼ 50 μ m thick devices at AC voltages of ∼ 100 V . Unfortunately, the degradation process is poorly understood; previous studies using photodiode measurements were only able to determine the overall device degradation. Here, we analyze many individual emission centers using time-lapse microscope images of several degrading devices. We find that most of the individual emission centers have large, sudden, discrete drops in luminosity and many have a luminosity that actually increases significantly, minutes to hours after a drop. This suggests that there are two primary degradation mechanisms: gradual multi-step diffusion of single-atom trap states away from the high-field region (causing the gradual decrease in luminosity) and the sudden break-up and reformation of complex trap states within the high field region (causing the sudden decreases and increases, respectively).

Digital technology holds the potential to transform journalism and the media in several beneficial ways, including new forms of storytelling that might better engage citizens and provide more context, nuance, and texture to reported events and issues. However, the extent to which these benefits have been realized is mixed and subject to debate. In this monograph, we examine how digital technology might transform the content of journalism through augmented reality (AR). The significance of digital storytelling through AR is manifest in its potential to engage a citizenry increasingly disengaged from traditional news and to provide more contextualized information.

We present X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) data of La1−χSrχCoO3, for χ= 0.10-0.30, at the O K edge and then compare the results to local density approximation (LDA) and generalized gradient approximation (GGA) calculations using a 2x2x2 supercell. Increases in the XAS and XMCD are nonlinear with χ, instead appearing correlated with the overall system magnetism, which changes from glass-like to metallic ferromagnetism around χ=0.18. The magnetic states observed in XMCD only occur at the beginning of the pre-edge; higher energy states show no orbital moment, suggesting two distinct types of holes exist. This is consistent with the formation of ferromagnetic clusters growing with χ and eventually overlapping for χ>0.18. Subsequent LDA and GGA calculations find a significant spin magnetic moment on O atoms (which are spinpolarized) and indicate that the doping-induced holes are nearly equally distributed over the Co and O atoms. Calculations of the magnetization for individual O sites indicate that they are not all equivalent, but actually have different moments, depending on the proximity to Sr. This variation in the small fraction of oxygen state with angular momentum may explain in part the unusual magnetic behavior of these materials. Further, the orbitally decomposed density of states reveals that the bands from the traditional picture (localized eg and t2g states) overlap considerably and extend from -6 to 4 eV around E F, suggesting that the local picture is not the best for La1−χSrχCoO3 (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present an extended X-ray absorption fine structure (EXAFS) study of the NdOs4Sb12 system at the Nd and Os LIII edges. Nd is expected to be a “rattler” atom in this skutterudite crystal and we obtain a low Einstein temperature (61 K) as expected. However, although ultrasonic measurements have suggested a large off-center displacement of Nd, the EXAFS data are inconsistent with such a model. An analysis of the first two Os-Sb peaks in the Os LIII data also shows no evidence for unusual disorder. In contrast the Nd-Os peak in the Nd LIII data and the Os-Os peak in the Os LIII data both show significant disorder at 4 K, with the largest disorder found for the Nd-Os pair. This suggests that a distortion of the structure that involves mainly the Os atoms is present; it is likely that this distortion - which may also be dynamic - also causes the anomalies in the ultrasonic data (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

When the well know thermoelectric PbTe is doped with a few percent Tl, the figure of merit, ZT = S2 σ /κ, is dramatically improved (here S is the Seebeck coefficient, σ the electrical conductivity and κ the thermal conductivity). Tl is a valence skipper with valences of +1 and +3 and replaces Pb(+2) in PbTe. Further Tl(+1) has a lone electron pair; consequently, the atom is asymmetric and large local distortions are expected if Tl(+1) is present. Here we we present Extended X-ray Absorption Fine Structure (EXAFS) data for 1% Tl doped PbTe at the Tl and Pb L3 edges and at the Te K edge, to probe the local structure about the dopant, and host atoms. At 10 K the local structure about Pb and Te is well ordered, although disorder of the Pb-Te (Te-Pb) pair distribution function (PDF) increases rapidly with increasing T. In contrast the environment around Tl is more disordered even at 10 K within the well ordered host material. This indicates a large variation of the Tl-Te bond lengths presumably as a result of the presence of some Tl(+1). This increased local disorder will result in additional electron and phonon scattering. We discuss possible models for the disorder about Tl and for PbTe at higher temperature for which neutron PDF measurements suggest Pb moves off-center (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We compare extended x-ray absorption fine structure (EXAFS) data at the Zn K edge for a low concentration of Zn (0.7 mol%) in a stoichiometric crystal with that for higher Zn concentrations (nominally 5 and 9 mol%) in congruent ${\mathrm{LiNbO}}_{3}$ (LNO). Note that stoichiometric and congruent LNO have significantly different optical properties. We find no significant difference in the local structure about Zn out to 4 \AA{} for the two types of crystals and different dopant levels. Although some earlier theoretical models suggest a self-compensation model with 75% of Zn on a Li site and 25% Zn on Nb, we find no clear evidence for a significant fraction of Zn on the Nb site, and estimate at most 2%--3% of Zn might be ${\mathrm{Zn}}_{Nb}$.

Local structures in cubic perovskite-type $(\mathrm{B}{\mathrm{a}}_{0.6}\mathrm{B}{\mathrm{i}}_{0.4})(\mathrm{T}{\mathrm{i}}_{0.6}\mathrm{S}{\mathrm{c}}_{0.4}){\mathrm{O}}_{3}$ solid solutions that exhibit reentrant dipole glass behavior have been studied with variable-temperature x-ray/neutron total scattering, extended x-ray absorption fine structure, and electron diffraction methods. Simultaneous fitting of these data using a reverse Monte Carlo algorithm provided instantaneous atomic configurations, which have been used to extract local displacements of the constituent species. The smaller Bi and Ti atoms exhibit probability density distributions that consist of 14 and 8 split sites, respectively. In contrast, Ba and Sc feature single-site distributions. The multisite distributions arise from large and strongly anisotropic off-center displacements of Bi and Ti. The cation displacements are correlated over a short range, with a correlation length limited by chemical disorder. The magnitudes of these displacements and their anisotropy, which are largely determined by local chemistry, change relatively insignificantly on cooling from room temperature. The structure features a nonrandom distribution of local polarization with low-dimensional polar clusters that are several unit cells in size. In situ measurements of atomic pair-distribution function under applied electric field were used to study field-induced changes in the local structure; however, no significant effects besides lattice expansion in the direction of the field could be observed up to electric-field values of $4\phantom{\rule{0.16em}{0ex}}\mathrm{kV}\phantom{\rule{0.16em}{0ex}}\mathrm{m}{\mathrm{m}}^{\ensuremath{-}1}$.

Going to Ground Frustrated systems, in which the geometry of the crystal lattice stands in the way of achieving an energetic minimum on all lattice sites simultaneously, have the potential to remain disordered down to the lowest temperatures. Numerous experimental efforts to find a material with a truly fluctuating ground state have failed because ordering often sets in at a finite temperature owing to symmetry breaking. Nakatsuji et al. (p. 559 ; see the Perspective by Balents ) identify the compound Ba 3 CuSb 2 O 9 as a promising candidate for this state; the Cu-Sb dipoles reside on a hexagonal structure, forming fluctuating spin singlets. Multiple lines of evidence suggest that the material does not order down to the millikelvin temperature range, remaining magnetically isotropic.

Stable and luminescent Cu-doped ZnSe nanocrystals (NCs) were synthesized in organic solvents with octadecylamine (ODA) as the capping ligand and characterized using a combination of optical and str...

Fluorescent, organically soluble Cu and Br codoped ZnS nanocrystals (NCs) were synthesized and characterized for the potential application of AC electroluminescent (EL) lighting devices. The doped NCs were optically characterized using UV–vis, photoluminescence (PL), Fourier transform infrared (FTIR), and time-resolved PL spectroscopy, and structurally characterized using transmission electron microscopy (TEM) and extended X-ray absorption fine structure (EXAFS). The PL emission of the undoped ZnS NCs upon λex = 280 nm was broad with a peak in the range of 450–470 nm, depending on the preparation of the NCs. Upon doping ZnS NCs with Cu, the PL emission was increased slightly with little change in spectral features. When Br dopant was added in addition to Cu, the codoped ZnS NCs showed much stronger PL emission (∼5 × ) compared to the undoped or Cu-doped sample. In conjunction with time-resolved PL results, this enhanced emission is attributed to a donor–acceptor (D–A) type of transition, between the Br el...

Stable water-suspendable Cu+-doped ZnS nanocrystals (NCs) have been synthesized with mercaptopropionic acid (MPA) as a capping molecule. The nanocrystals have been characterized using a combination of experimental techniques including UV-vis and photoluminescence (PL) spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), inductively coupled plasma (ICP), and extended X-ray absorption fine structure (EXAFS). The UV-vis electronic absorption spectrum shows an excitonic peak at 310 nm, characteristic of quantum-confined ZnS NCs. This excitonic peak does not change noticeably with Cu+ doping. XRD confirms the formation of ZnS nanocrystals, and the average size of the NCs has been determined to be around 6 nm by TEM. The incorporation of Cu+ into the ZnS is manifested as a substantial red-shift of the emission band in the PL spectra upon addition of Cu2+ that was reduced into Cu+ during the synthesis reaction. EXAFS data were obtained to confirm copper doping as well as determine the local structure about Cu+ and Zn2+ in the NCs. Fitting to the EXAFS data for Cu+ suggests that most Cu+ ions are located near the surface within the ZnS NCs and that a significant fraction may be in the form of CuS as found in bulk material. These combined optical and structural studies have provided important new insight into the relevant electronic energy levels and their correlation to the optical and structural properties of ZnS:Cu,Cl NCs. This has important implications in potential applications of this phosphor material for solid state lighting, imaging, and other photonic devices.

The local structure of two skutterudite families---$\mathrm{Ce}{M}_{4}{\mathrm{As}}_{12}$ ($M=\mathrm{Fe}$, Ru, Os) and $Ln{\mathrm{Cu}}_{3}{\mathrm{Ru}}_{4}{\mathrm{O}}_{12}$ ($Ln=\mathrm{La}$, Pr, and Nd)---have been studied using the extended x-ray absorption fine structure (EXAFS) technique with a focus on the lattice vibrations about the rare-earth ``rattler atoms'' and the extent to which these vibrations can be considered local modes, with the rattler vibrating inside a nearly rigid cage. X-ray absorption data at all the metal edges were collected over a temperature range of 4 to 300 K and analyzed using standard procedures. The pair distances from EXAFS results agree quite well with the average structure obtained from diffraction. The cage structure is formed by the $M$ and As atoms in $\mathrm{Ce}{M}_{4}{\mathrm{As}}_{12}$ and by Cu, O, and Ru atoms in $Ln{\mathrm{Cu}}_{3}{\mathrm{Ru}}_{4}{\mathrm{O}}_{12}$. Although some of the bonds within the cage are quite stiff (correlated Debye temperatures, ${\ensuremath{\theta}}_{\mathrm{cD}}$, are $\ensuremath{\sim}500$ K for $\mathrm{Ce}{M}_{4}{\mathrm{As}}_{12}$ and above 800 K for $Ln{\mathrm{Cu}}_{3}{\mathrm{Ru}}_{4}{\mathrm{O}}_{12}$), we show that the structure is not completely rigid. For the rattler atom the nearest-neighbor pairs have a relatively low Einstein temperature, ${\ensuremath{\theta}}_{\mathrm{E}}:\ensuremath{\sim}100--120$ K for Ce-As and $\ensuremath{\sim}130$ K for $Ln\text{\ensuremath{-}}\mathrm{O}$. Surprisingly, the behaviors of the second-neighbor pairs are quite different: for $\mathrm{Ce}{M}_{4}{\mathrm{As}}_{12}$ the second-neighbor pairs $(\mathrm{Ce}\text{\ensuremath{-}}M)$ have a weaker bond while for $Ln{\mathrm{Cu}}_{3}{\mathrm{Ru}}_{4}{\mathrm{O}}_{12}$ the $Ln\text{\ensuremath{-}}\mathrm{Ru}$ second-neighbor pair has a stiffer effective spring constant than the first-neighbor pair. In addition, we show that the ${\mathrm{As}}_{4}$ or ${\mathrm{CuO}}_{4}$ rings are relatively rigid units and that their vibrations are anisotropic within these cubic structures, with stiff restoring forces perpendicular to the rings and much weaker restoring forces in directions parallel to the rings. Consequently vibrations of the rings may also act as ``rattlers'' and help suppress thermal conductivity. In general neither the rigid-cage approximation nor the simple reduced-mass approximation are sufficient for describing rattler behavior.

Ca dopants introduce holes in the Mn e g band which allows electronic transport; such samples exhibit a colossal magnetoresistance (CMR) when the sample becomes ferromagnetic at low T, for x in the range 0.2 < x < 0.5. Previous EXAFS studies on the perovskite manganites La 1 - x Ca x MnO 3 indicated a correlation between changes in the local structure associated with polaron formation, and the sample magnetization for these samples. We have extended the EXAFS measurements to a wider range of Ca concentrations and to very high magnetic fields. Applying a magnetic field reduces the local distortion of each sample for temperatures near T c . These measurements provide clear evidence for a universal relationship between the local structure and the sample magnetization. For at least one sample (x = 0.3), there is still a significant change of σ 2 with T below T c when the sample is fully magnetized, i.e. distortions are still present and continuing to be removed as T is lowered well below T c .

The physical properties of single crystals of filled skutterudite compounds EuT4Sb12 (T = Fe, Ru, Os) have been investigated by means of x-ray diffraction, electrical resistivity, specific heat, magnetization, and x-ray absorption spectroscopy measurements. The Eu-based materials crystallize in the LaFe4P12-type structure (space group ). A small Eu deficiency is encountered for Eu0.95Fe4Sb12, while the homologous compounds, Eu1.0Ru4Sb12 and Eu1.0Os4Sb12, reveal full occupancy of the Eu site. Eu0.95Fe4Sb12 appears to exhibit either canted ferromagnetic or ferrimagnetic order at TC = 88 K whereas EuRu4Sb12 and EuOs4Sb12 are ferromagnets with Curie temperatures of TC = 4 and 9 K, respectively. X-ray absorption near edge spectroscopy measurements reveal a nearly divalent Eu electronic configuration for EuT4Sb12 (T = Fe, Ru, Os) with no significant change with T atom or temperature.

Mn2+-doped ZnSe nanoparticles were synthesized from molecular cluster precursors. Four ZnSe nanoparticle samples, one with low Mn2+ concentration (A), one with an intermediate Mn2+ concentration (B), one with a high Mn2+ concentration (C), and one with no Mn2+, were prepared and characterized using UV−vis, luminescence, electron spin resonance (ESR), and X-ray absorption fine structure (XAFS) techniques. The sample with no Mn2+ had a sharp ZnSe band edge emission peak and a quantum yield of ∼2%. The samples with Mn2+ had a significant decrease in band edge emission. Sample A had no Mn2+ 4T1 → 6A1 emission but showed some ZnSe band edge emission and trap state emission. Sample B had Mn2+ 4T1 → 6A1 emission and a further reduction in ZnSe band edge emission and trap state emission. Sample C showed an increase in the Mn2+ 4T1 → 6A1 emission, a dramatic increase in trap state emission, and essentially no ZnSe band edge emission. The overall emission from all four samples was quenched with time. To better unde...

The low thermal conductivity of the Thermoelectric Clathrates Eu8Ga16Ge30 and Sr8Ga16Ge30 has been attributed to rattling atoms (Eu or Sr) in the Eu2/Sr2 sites. The low Einstein temperature obtained using XAFS for the nearest neighbor Eu-Ga/Ge bonds at the Eu2 sites is consistent with the model. However, the comparable Einstein temperature for the Eu1 site indicates that Eu1 is also a rattler. The fit also confirms that the Eu2 is displaced along either the y or z axis ∼ 0.45A in agreement with neutron diffraction.

The reduction of HAuCl4 by Na2S has been reported to produce gold nanoparticles with an optical absorption in the near-infrared along with its characteristic absorption in the visible. The optical resonances in the visible are due to the gold surface plasma, which are a function of the geometry of the particles. The near-infrared absorption had been attributed to the formation of Au2S/Au core/shell structures. In this report we present new electronic absorption, electron microscopy, and X-ray absorption data in several systems to show that the near-infrared absorption does not involve core/shell structures. We further suggest that the near-infrared adsorption is most likely the result of the formation of aggregates of gold nanoparticles. The identification of the origin of the near-infrared resonance is critical in understanding the optical properties of metal nanoparticle systems.

Lithium niobate, LiNbO3, is an important technological material with good electro-optic, acousto-optic, elasto-optic, piezoelectric and nonlinear properties. EXAFS on Zn-doped LiNbO3 found strong evidences that Zn substitutes primarily at the Li site on highly doped samples. In this work the EXAFS results were revisited using a different approach where the models for simulating the EXAFS results were obtained from the output of defect calculations. The strategy uses the relaxed positions of the ions surrounding the dopants to generate a cluster from where the EXAFS oscillations can be calculated. The defect involves not only the Zn possible substitution at either Li or Nb sites but also the charge compensating defects, when needed. From previous defect modelling, a subset of defects was selected based on the energetics of the defect production in the LiNbO3 lattice. From them, all possible clusters were generated and the simulated EXAFS were computed. The simulated EXAFS were them compared to available EXAFS results in the literature. Based on this comparison different models could be proposed to explain the behaviour of Zn in the LiNbO3 matrix.

Measurements of physical properties show that Yb enters the single crystals systematically and in registry with the nominal Yb concentration x of the starting material dissolved in the molten indium flux., 21 pages, 8 figures

The complex structure and magnetism of Pr$_{2-x}$Bi$_x$Ru$_2$O$_7$ was investigated by neutron scattering and EXAFS. Pr has an approximate doublet ground-state and the first excited state is a singlet. This overall crystal field level scheme is similar to metallic Pr$_2$Ir$_2$O$_7$, which is also reported here. While the B-site (Ru) is well ordered throughout, this is not the case for the A-site (Pr/Bi). A distribution of the Pr-O2 bond length indicates the Pr environment is not uniform even for $x=0$. The Bi environment is highly disordered ostensibly due to the 6s lone pairs on Bi$^{3+}$. Correspondingly we find the non-Kramers doublet ground state degeneracy otherwise anticipated for Pr in the pyrochlore structure is lifted so as to produce a quadrupolar singlet ground state with a spatially varying energy gap. For $x=0$, below T$_N$, the Ru sublattice orders antiferromagnetically, with propagation vector \textbf{k}= (0,0,0), as for Y$_2$Ru$_2$O$_7$. No ordering associated with the Pr sublattice is observed down to 100 mK. The low energy magnetic response of Pr$_{2-x}$Bi$_x$Ru$_2$O$_7$ features a broad spectrum of magnetic excitations associated with inhomogeneous splitting of the Pr quasi-doublet ground state. For $x=0$ ($x=0.97$) the spectrum is temperature dependent (independent). It appears disorder associated with Bi alloying enhances the inhomogeneous Pr crystal field level splitting so that inter-site interactions become irrelevant for $x=0.97$. The structural complexity for the A-site may be reflected in the hysteretic uniform magnetization of B-site ruthenium in the N\'{e}el phase., Comment: 20 pages, 21 figures, submitted to PRB