Sb2Te3 and Bi2Te3 Thin Films Grown by Room-Temperature MBE

in Journal of Electronic Materials (2012), 41(6), 1493-1497

Sb2Te3 and Bi2Te3 thin films were grown on SiO2 and BaF2 substrates at room temperature using molecular beam epitaxy. Metallic layers with thicknesses of 0.2 nm were alternately deposited at room ... [more ▼]

Sb2Te3 and Bi2Te3 thin films were grown on SiO2 and BaF2 substrates at room temperature using molecular beam epitaxy. Metallic layers with thicknesses of 0.2 nm were alternately deposited at room temperature and the films were subsequently annealed at 250A degrees C for 2 h. x-Ray diffraction and energy-filtered transmission electron microscopy (TEM) combined with high-accuracy energy-dispersive x-ray spectrometry revealed stoichiometric films, grain sizes of less than 500 nm, and a texture. High-quality in-plane thermoelectric properties were obtained for Sb2Te3 films at room temperature, i.e., low charge carrier density (2.6 x 10(19) cm(-3)), large thermopower (130 V K-1), large charge carrier mobility (402 cm(2) V-1 s(-1)), and resulting large power factor (29 W cm(-1) K-2). Bi2Te3 films also showed low charge carrier density (2.7 x 10(19) cm(-3)), moderate thermopower (-153 V K-1), but very low charge carrier mobility (80 cm(2) V-1 s(-1)), yielding low power factor (8 W cm(-1) K-2). The low mobilities were attributed to Bi-rich grain boundary phases identified by analytical energy-filtered TEM. [less ▲]

Quantitative spatial magnetization distribution in iron oxide nanocubes and nanospheres by polarized small-angle neutron scattering

in New Journal of Physics (2012), 14

By means of polarized small-angle neutron scattering, we have resolved the long-standing challenge of determining the magnetization distribution in magnetic nanoparticles in absolute units. The reduced ... [more ▼]

By means of polarized small-angle neutron scattering, we have resolved the long-standing challenge of determining the magnetization distribution in magnetic nanoparticles in absolute units. The reduced magnetization, localized in non-interacting nanoparticles, indicates strongly particle shape-dependent surface spin canting with a 0.3(1) and 0.5(1) nm thick surface shell of reduced magnetization found for similar to 9 nm nanospheres and similar to 8.5 nm nanocubes, respectively. Further, the reduced macroscopic magnetization in nanoparticles results not only from surface spin canting, but also from drastically reduced magnetization inside the uniformly magnetized core as compared to the bulk material. Our microscopic results explain the low macroscopic magnetization commonly found in nanoparticles. [less ▲]

Microstructure analyses and thermoelectric properties of Ag1-xPb18Sb1+yTe20

in Journal of Solid State Chemistry (2012), 193(SI), 58-63

This study reports microstructural investigations of long-term annealed 18, x=y=0, hereinafter referred to as AgPb18SbTe20) (Lead-Antimony-Silver-Tellurium, LAST-18) as well as of Ag1-xPb18Sb1+yTe20, i.e ... [more ▼]

This study reports microstructural investigations of long-term annealed 18, x=y=0, hereinafter referred to as AgPb18SbTe20) (Lead-Antimony-Silver-Tellurium, LAST-18) as well as of Ag1-xPb18Sb1+yTe20, i.e. Ag-deficient and Sb-excess LAST-18 (x not equal 0, y not equal 0), respectively. Two different length scales are explored. The micrometer scale was evaluated by SEM to analyze the volume fraction and the number of secondary phases as well as the impact of processing parameters on the homogeneity of bulk samples. For AgPb18SbTe20, site-specific FIB liftout of TEM lamellae from thermoelectrically characterized samples was accomplished to investigate the structure on the nanometer scale. High-resolution TEM and energy-filtered TEM were performed to reveal shape and size distribution of nanoprecipitates, respectively. A hypothesis concerning the structure-property relationship is set out within the frame of a gradient annealing experiment. This study is completed by results dealing with inhomogeneities on the micrometer scale of Ag1-xPb18Sb1+yTe20 and its electronic properties. (C) 2012 Elsevier Inc. All rights reserved. [less ▲]

Local ordering and magnetism in Ga0.9Fe3.1N

in Journal of Solid State Chemistry (2011), 184(9), 2315-2321

Prior investigations of the ternary nitride series Ga1-xFe3+xN (0 <= x 1) have indicated a transition from ferromagnetic gamma'-Fe4N to antiferromagnetic ``GaFe3N''. The ternary nitride ``GaFe3N'' has ... [more ▼]

Prior investigations of the ternary nitride series Ga1-xFe3+xN (0 <= x 1) have indicated a transition from ferromagnetic gamma'-Fe4N to antiferromagnetic ``GaFe3N''. The ternary nitride ``GaFe3N'' has been magnetically and spectroscopically reinvestigated in order to explore the weakening of the ferromagnetic interactions through the gradual incorporation of gallium into gamma'-Fe4N. A hysteretic loop at RI reveals the presence of a minority phase of only 0.1-0.2 at\%, in accord with the sound two-step synthesis. The composition of the gallium-richest phase ``GaFe3N'' was clarified by Prompt Gamma-ray Activation Analysis and leads to the berthollide formula Ga0.91(1)Fe3.09(10)N1.05(7). Magnetic measurements indicate a transition around 8 K, further supported by Mossbauer spectral data. The weakening of the ferromagnetic coupling through an increasing gallium concentration is explained by a simple Stoner argument. In Ga0.9Fe3.1N the presence of iron on the gallium site affects the magnetism by the formation of 13-atom iron clusters. (C) 2011 Elsevier Inc. All rights reserved. [less ▲]

Shape Induced Symmetry in Self-Assembled Mesocrystals of Iron Oxide Nanocubes

in Nano Letters (2011), 11(4), 1651-1656

Grazing incidence small-angle scattering,and electron microscopy have been used to show for the first time that nonspherical nanoparticles can assemble into highly ordered body-centered tetragonal ... [more ▼]

Grazing incidence small-angle scattering,and electron microscopy have been used to show for the first time that nonspherical nanoparticles can assemble into highly ordered body-centered tetragonal mesocrystals. Energy models accounting for the directionality and magnitude of the van der Waals and dipolar interactions as a function of the degree of truncation of the nanocubes illustrated the importance of the directional dipolar forces for the formation of the initial nanocube clusters and the dominance of the van der Waals multibody interactions in the dense packed arrays. [less ▲]

Properties of spark plasma sintered nanostructured Zn1+xSb

in Physica Status Solidi A. Applications and Materials Science (2011), 208(8), 1913-1919

Engineering materials with specific physical properties has recently focused on the effect of nanoscopic inhomogeneities at the 10 nm scale. Such features are expected to scatter medium and long ... [more ▼]

Engineering materials with specific physical properties has recently focused on the effect of nanoscopic inhomogeneities at the 10 nm scale. Such features are expected to scatter medium and long-wavelength phonons lowering thereby the thermal conductivity of the system without simultaneously decreasing the charge transport (phonon-glass electron-crystal concept). A new Zn1+xSb nanophase obtained by a wet chemical approach was densified by spark plasma sintering (SPS). Investigations on compounds subsumed as ``Zn4Sb3'' always suffer from its low thermal stability and the contamination of the nanoparticles with solvents and additives used in the synthesis. In order to gain insight into this compound's electronic properties we investigated a material free from remnants of the synthesis but contaminated with a small amount of well-characterized decomposition product, i.e., ZnSb. To investigate the influence of the sintering process on the densified samples, different SPS conditions were applied. Four different conditions were used with heating rates between 160 degrees and 230 degrees C/min, sintering temperatures between 130 and 190 degrees C and sintering times between 3 and 6 min. Powders from the surface of the pellets were subject to powder X-ray diffraction (XRD) yielding information about the surface composition. Small pieces of the pellets were also characterized using high-energy synchrotron radiation scattering in order to reveal the phase compositions inside the pellets. Small changes in the sintering conditions of the samples were found to have a large influence on the resulting sample compositions. In addition, the phase compositions on the surface differ significantly from the ones inside the pellets which show a much higher grade of decomposition. The density and morphology of the obtained pellets have been investigated by means of laser microscopy and scanning electron microscopy (SEM). The low density and porosity of the different pellets is a result of the graphite pressing tool which has to be used to ensure the temperature control during the SPS process. (C) 2011 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim [less ▲]

Wet Chemical Synthesis and a Combined X-ray and Mossbauer Study of the Formation of FeSb2 Nanoparticles

in Inorganic Chemistry (2011), 50(22), 11807-11812

Understanding how solids form is a challenging task, and few strategies allow for elucidation of reaction pathways that are useful for designing the synthesis of solids. Here, we report a powerful ... [more ▼]

Understanding how solids form is a challenging task, and few strategies allow for elucidation of reaction pathways that are useful for designing the synthesis of solids. Here, we report a powerful solution-mediated approach for formation of nanocrystals of the thermoelectrically promising FeSb2 that uses activated metal nanoparticles as precursors. The small particle size of the reactants ensures minimum diffusion paths, low activation barriers, and low reaction temperatures, thereby eliminating solid-solid diffusion as the rate-limiting step in conventional bulk-scale solid-state synthesis. A time- and temperature-dependent study of formation of nanoparticular FeSb2 by X-ray powder diffraction and iron-57 Mossbauer spectroscopy showed the incipient format on of the binary phase in the temperature range of 200-250 degrees C. [less ▲]

Lattice dynamics in the thermoelectric Zintl compound Yb14MnSb11

in Physical Review (2011), 84(18),

The density of phonon states in the thermoelectric material Yb14MnSb11 has been studied first by inelastic neutron scattering and second in an element-specific way by nuclear inelastic x-ray scattering ... [more ▼]

The density of phonon states in the thermoelectric material Yb14MnSb11 has been studied first by inelastic neutron scattering and second in an element-specific way by nuclear inelastic x-ray scattering. The low sound velocity of 1880(50) m/s as obtained from the density of phonon states can be identified as an important reason for the low heat transport in this system. The high melting temperature of Yb14MnSb11 contrasts with the low energy of all phonons (<25 meV) and relates to an unusual lack of softening of phonon modes with temperature, when comparing the phonon density of states observed at ambient temperatures and at 1200 K. We have also measured the density of phonon states of the related Eu14MnSb11 compound and of the thermoelectric Zintl phase Zn4Sb3 in order to compare with related thermodynamic properties of Yb14MnSb11 and to elucidate the different mechanisms of the heat conductivity reduction in Zintl phases. [less ▲]

Spin correlations in the extended kagome system YBaCo3FeO7

in Physical Review. B : Condensed Matter (2011), 84(22),

The transition metal-based oxide YBaCo3FeO7 is structurally related to the mineral Swedenborgite SbNaBe4O7, a polar noncentrosymmetric crystal system. The magnetic Co3Fe sublattice consists of a ... [more ▼]

The transition metal-based oxide YBaCo3FeO7 is structurally related to the mineral Swedenborgite SbNaBe4O7, a polar noncentrosymmetric crystal system. The magnetic Co3Fe sublattice consists of a tetrahedral network containing kagome-like layers with trigonal interlayer sites. This geometry causes frustration effects for magnetic ordering, which were investigated by magnetization measurements, Mossbauer spectroscopy polarized neutron diffraction, and neutron spectroscopy. Magnetization measurement and neutron diffraction do not show long range ordering even at low temperature (1 K), although a strong antiferromagnetic coupling (similar to 2000 K) is deduced from the magnetic susceptibility. Below 590 K we observe two features, a spontaneous weak anisotropic magnetization hysteresis along the polar crystallographic axis and a hyperfine field on the Fe kagome sites, whereas the Fe spins on the interlayer sites remain idle. Below similar to 50 K the onset of a hyperfine field shows the development of moments static on the Mossbauer time scale also for the Fe interlayer sites. Simultaneously, an increase of spin correlations is found by polarized neutron diffraction. The relaxation part of the dynamic response has been further investigated by high-resolution neutron spectroscopy, which reveals that the spin correlations start to freeze below similar to 50 K. Monte Carlo simulations show that the neutron scattering results at lower temperatures are compatible with a recent proposal that the particular geometric frustration in the Swedenborgite structure promotes quasi-one-dimensional partial order. [less ▲]

Fading of modern Prussian blue pigments in linseed oil medium

in Journal of Analytical Atomic Spectrometry [=JAAS] (2011), 26(5), 930

The fading of modern laboratory-synthesized and commercial Prussian blue, iron(III) hexacyanoferrate(II), based pigments in a linseed oil medium during exposure to light has been investigated. The ... [more ▼]

The fading of modern laboratory-synthesized and commercial Prussian blue, iron(III) hexacyanoferrate(II), based pigments in a linseed oil medium during exposure to light has been investigated. The Prussian blue pigments were painted from linseed oil, as a pure pigment and mixed with white lead, (PbCO3)2Pb(OH)2, zinc white, ZnO, or titanium white, TiO2, pigment. The samples were subjected to accelerated ageing for 800 hours and the light fastness of the Prussian blue pigment was evaluated by reference to blue wool standards. Pure Prussian blue is extremely light fast whilst it strongly fades when mixed with a white pigment, especially with lead white or zinc oxide. The painted samples were studied by UV-visible, iron K-edge X-ray absorption, iron-57 transmission Mössbauer, and attenuated total reflectance infrared spectroscopy. X-ray absorption results reveal a decrease in the iron coordination number in aged samples in the presence of white pigment. The Mössbauer spectra of the pure Prussian blue and the unaged and aged mixtures of Prussian blue and lead white or zinc oxide at 1:100 and 1:10 dilution ratios, respectively, indicate the presence of iron(II) and iron(III) in a ratio close to one as expected for the bulk stoichiometric KFeIII[FeII(CN)6]; no change in the spectral parameters was observed upon ageing. Combined with the X-ray near edge absorption and infrared studies, these results suggest reduction of the surface iron ions in the Prussian blue with ageing upon exposure to light. [less ▲]