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  • Blackwell Publishing Ltd  (406,493)
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  • American Institute of Physics (AIP)  (154,867)
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  • 101
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    American Institute of Physics (AIP)
    Publication Date: 2016-06-29
    Electronic ISSN: 2166-532X
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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  • 102
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    American Institute of Physics (AIP)
    Publication Date: 2016-07-06
    Description: Metastable phases, such as bcc Co or Ni and hcp Fe or Ni, reportedly possess extraordinary magnetic properties for epitaxial ultra-thin films. To understand phase stability of these epitaxy-oriented phases upon substrate lattices, we calculated novel phase diagrams of Co, Fe, and Ni ultrathin films by considering the chemical free energy, elastic strain energy, and surface energy. Verified by experimental data in the literatures, the stable epitaxy-oriented phases are readily identified from the phase diagrams. The stabilization of these metastable phases is determined by the interplay between orientation-dependent elastic strain energy and surface energy.
    Electronic ISSN: 2166-532X
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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  • 103
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    American Institute of Physics (AIP)
    Publication Date: 2016-07-06
    Description: A series of anti-perovskites including Sr 3 PbO are recently predicted to be a three-dimensional Dirac material with a small mass gap, which may be a topological crystalline insulator. Here, we report the epitaxial growth of Sr 3 PbO thin films on LaAlO 3 using molecular beam epitaxy. X-ray diffraction indicates (001) growth of Sr 3 PbO, where [110] of Sr 3 PbO matches [100] of LaAlO 3 . Measurements of the Sr 3 PbO films with parylene/Al capping layers reveal a metallic conduction with p -type carrier density of ∼10 20 cm −3 . The successful growth of high quality Sr 3 PbO film is an important step for the exploration of its unique topological properties.
    Electronic ISSN: 2166-532X
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  • 104
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    American Institute of Physics (AIP)
    Publication Date: 2016-07-08
    Description: Electronic structures and thermoelectric transport properties of α-NaFeO 2 -type d 0 -electron layered complex nitrides AMN 2 (A = Sr or Na; M = Zr, Hf, Nb, Ta) were evaluated using density-functional theory and Boltzmann theory calculations. Despite the layered crystal structure, all materials had three-dimensional electronic structures. Sr(Zr, Hf)N 2 exhibited isotropic electronic transport properties because of the contribution of the Sr 4 d orbitals to the conduction band minimums (CBMs) in addition to that of the Zr 4 d (Hf 5 d ) orbitals. Na(Nb,Ta)N 2 showed weak anisotropic electronic transport properties due to the main contribution of the Nb 4 d (Ta 5 d ) and N 2 p orbitals to the CBMs and no contribution of the Na orbitals.
    Electronic ISSN: 2166-532X
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  • 105
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    American Institute of Physics (AIP)
    Publication Date: 2016-07-08
    Description: We review the spin-Seebeck and magnon-electron drag effects in the context of solid-state energy conversion. These phenomena are driven by advective magnon-electron interactions. Heat flow through magnetic materials generates magnetization dynamics, which can strongly affect free electrons within or adjacent to the magnetic material, thereby producing magnetization-dependent (e.g., remnant) electric fields. The relative strength of spin-dependent interactions means that magnon-driven effects can generate significantly larger thermoelectric power factors as compared to classical thermoelectric phenomena. This is a surprising situation in which spin-based effects are larger than purely charge-based effects, potentially enabling new approaches to thermal energy conversion.
    Electronic ISSN: 2166-532X
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  • 106
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    American Institute of Physics (AIP)
    Publication Date: 2016-07-13
    Description: In semiconductors almost all heat is conducted by phonons (lattice vibrations), which is limited by their quasi-particle lifetimes. Phonon-phonon interactions represent scattering mechanisms that produce thermal resistance. In thermoelectric materials, this resistance due to anharmonicity should be maximised for optimal performance. We use a first-principles lattice-dynamics approach to explore the changes in lattice dynamics across an isostructural series where the average atomic mass is conserved: ZnS to CuGaS 2 to Cu 2 ZnGeS 4 . Our results demonstrate an enhancement of phonon interactions in the multernary materials and confirm that lattice thermal conductivity can be controlled independently of the average mass and local coordination environments.
    Electronic ISSN: 2166-532X
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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  • 107
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    American Institute of Physics (AIP)
    Publication Date: 2016-07-13
    Description: Synthetic minerals and related systems based on Cu–S are attractive thermoelectric (TE) materials because of their environmentally benign characters and high figures of merit at around 700 K. This overview features the current examples including kesterite, binary copper sulfides, tetrahedrite, colusite, and chalcopyrite, with emphasis on their crystal structures and TE properties. This survey highlights the superior electronic properties in the p -type materials as well as the close relationship between crystal structures and thermophysical properties. We discuss the mechanisms of high power factor and low lattice thermal conductivity, approaching higher TE performances for the Cu–S based materials.
    Electronic ISSN: 2166-532X
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  • 108
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    American Institute of Physics (AIP)
    Publication Date: 2016-07-14
    Description: Normally we think of the glassy state as a single phase and therefore crystallization from chemically identical amorphous precursors should be identical. Here we show that the local structure of an amorphous precursor is distinct depending on the initial deposition conditions, resulting in significant differences in the final state material. Using grazing incidence total x-ray scattering, we have determined the local structure in amorphous thin films of vanadium oxide grown under different conditions using pulsed laser deposition (PLD). Here we show that the subsequent crystallization of films deposited using different initial PLD conditions result in the formation of different polymorphs of VO 2 . This suggests the possibility of controlling the formation of metastable polymorphs by tuning the initial amorphous structure to different formation pathways.
    Electronic ISSN: 2166-532X
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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  • 109
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    American Institute of Physics (AIP)
    Publication Date: 2016-07-19
    Description: Strong exchange bias (EB) in perpendicular direction has been demonstrated in vertically aligned nanocomposite (VAN) (La 0.7 Sr 0.3 MnO 3 ) 1−x : (LaFeO 3 ) x (LSMO:LFO, x = 0.33, 0.5, 0.67) thin films deposited by pulsed laser deposition. Under a moderate magnetic field cooling, an EB field as high as ∼800 Oe is achieved in the VAN film with x = 0.33, suggesting a great potential for its applications in high density memory devices. Such enhanced EB effects in perpendicular direction can be attributed to the high quality epitaxial co-growth of vertically aligned ferromagnetic LSMO and antiferromagnetic LFO phases, and the vertical interface coupling associated with a disordered spin-glass state. The VAN design paves a powerful way for integrating perpendicular EB effect within thin films and provides a new dimension for advanced spintronic devices.
    Electronic ISSN: 2166-532X
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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  • 110
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    American Institute of Physics (AIP)
    Publication Date: 2016-07-19
    Description: An attempt was made to tailor the magnetostructural transitions over a wide temperature range under the principle of isostructural alloying. A series of wide Curie-temperature windows (CTWs) with a maximal width of 377 K between 69 and 446 K were established in the Mn 1− y Co y NiGe 1− x Si x system. Throughout the CTWs, the magnetic-field-induced metamagnetic behavior and giant magnetocaloric effects are obtained. The (Mn,Co)Ni(Ge,Si) system shows great potential as multifunctional phase-transition materials that work in a wide range covering liquid-nitrogen and above water-boiling temperatures. Moreover, general understanding of isostructural alloying and CTWs constructed in (Mn,Co)Ni(Ge,Si) as well as (Mn,Fe)Ni(Ge,Si) is provided.
    Electronic ISSN: 2166-532X
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  • 111
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    American Institute of Physics (AIP)
    Publication Date: 2016-07-28
    Description: We report on a new polar interface state between two band insulators: LaInO 3 and BaSnO 3 , where the sheet conductance enhancement in the interface reaches more than the factor of 10 4 depending on the La doping concentration in BaSnO 3 layer, by monitoring the conductance change before and after the polar interface formation as a function of La doping in BaSnO 3 . By eliminating the possibilities of oxygen vacancy involvement and cation diffusion, we show that the conductance enhancement is due to electronic reconstruction in the interface. Furthermore, we have found that the interfaces between BaSnO 3 and the larger bandgap non-polar perovskites BaHfO 3 and SrZrO 3 did not show such a conductance enhancement. We discuss a model for the interface state where the Fermi level plays a critical role and the conductance enhancement is due to the existence of polarization in the polar perovskite, LaInO 3 .
    Electronic ISSN: 2166-532X
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  • 112
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    American Institute of Physics (AIP)
    Publication Date: 2016-05-14
    Description: Materials informatics owes much to bioinformatics and the Materials Genome Initiative has been inspired by the Human Genome Project. But there is more to bioinformatics than genomes, and the same is true for materials informatics. Here we describe the rapidly expanding role of searching for structures of materials using first-principles electronic-structure methods. Structure searching has played an important part in unraveling structures of dense hydrogen and in identifying the record-high-temperature superconducting component in hydrogen sulfide at high pressures. We suggest that first-principles structure searching has already demonstrated its ability to determine structures of a wide range of materials and that it will play a central and increasing part in materials discovery and design.
    Electronic ISSN: 2166-532X
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  • 113
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    American Institute of Physics (AIP)
    Publication Date: 2016-05-17
    Description: In this contribution, we demonstrate the optimization of the microstructures of the Pb 0.85 La 0.1 (Zr 0.65 Ti 0.35 )O 3 (PLZT) relaxor ferroelectric ceramics and subsequent enhancements in their polarization and electrical resistivity by using a hot-pressing process. The resulting superior breakdown strength of hot-pressed PLZT enables the application of high electric field to induce a giant electrocaloric effect, in which the adiabatic change of temperature (Δ T ) and the isothermal change of entropy (Δ S ) are around 2 times greater than those of the samples prepared by the conventional sintering approach using muffle furnace. Moreover, the addition of extra PbO to make up the loss of Pb in the high-temperature sintering leads to the further improvements in the phase composition and electrical properties of PLZT, due to inhibition of the pyrochlore phase formation. The relationship among the sintering conditions, the content of excess PbO, and the microstructure as well as the electrical characteristics of PLZT have been investigated in a systematic manner. This work provides a facile approach to enhanced electrocaloric effect in bulk ceramics.
    Electronic ISSN: 2166-532X
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  • 114
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    American Institute of Physics (AIP)
    Publication Date: 2016-05-18
    Description: The electrocaloric effect (ECE) of two compositions (x = 0.06 and 0.07) of (1 − x)(Na 0.5 Bi 0.5 )TiO 3 -xKNbO 3 in the vicinity of the morphotropic phase boundary is studied by direct measurements. ΔT max = 1.5 K is measured at 125 °C under 70 kV/cm for NBT-6KN while ΔT max = 0.8 K is measured at 75 °C under 55 kV/cm for NBT-7KN. We show that the “shoulder,” T S , in the dielectric permittivity, marks the upper limit of the ECE peak under high applied electric fields. These results imply that the range of temperature with high ECE can be quickly identified for a given composition, which will significantly speed up the process of materials selection for ECE cooling.
    Electronic ISSN: 2166-532X
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  • 115
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    American Institute of Physics (AIP)
    Publication Date: 2016-05-18
    Description: The influence of growth temperature on the surface morphology and luminescence properties of Eu-doped GaN layers grown by organometallic vapor phase epitaxy was investigated. By using a Eu source that does not contain oxygen in its molecular structure, and varying the growth temperature, the local defect environment around the Eu 3+ ions was manipulated, yielding a higher emission intensity from the Eu 3+ ions and a smoother sample surface. The optimal growth temperature was determined to be 960 °C and was used to fabricate a GaN-based red light-emitting diode with a significantly higher output power.
    Electronic ISSN: 2166-532X
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  • 116
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    American Institute of Physics (AIP)
    Publication Date: 2016-05-20
    Description: Ferroelectrics are attractive candidate materials for environmentally friendly solid state refrigeration free of greenhouse gases. Their thermal response upon variations of external electric fields is largest in the vicinity of their phase transitions, which may occur near room temperature. The magnitude of the effect, however, is too small for useful cooling applications even when they are driven close to dielectric breakdown. Insight from microscopic theory is therefore needed to characterize materials and provide guiding principles to search for new ones with enhanced electrocaloric performance. Here, we derive from well-known microscopic models of ferroelectricity meaningful figures of merit for a wide class of ferroelectric materials. Such figures of merit provide insight into the relation between the strength of the effect and the characteristic interactions of ferroelectrics such as dipolar forces. We find that the long range nature of these interactions results in a small effect. A strategy is proposed to make it larger by shortening the correlation lengths of fluctuations of polarization. In addition, we bring into question other widely used but empirical figures of merit and facilitate understanding of the recently observed secondary broad peak in the electrocalorics of relaxor ferroelectrics.
    Electronic ISSN: 2166-532X
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  • 117
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    American Institute of Physics (AIP)
    Publication Date: 2016-05-21
    Description: PbSe is an inexpensive alternative for PbTe as a mid-temperature thermoelectric material, but few investigations have been reported about its intrinsic properties despite recent efforts on doping techniques. In this work, pristine PbSe bulk materials were synthesized by a process combining mechanical alloying and spark plasma sintering, which is increasingly used for processing thermoelectric materials, and their electrical and thermal transport properties as well as thermoelectric performance were investigated in a wide temperature range. A maximum ZT ∼0.83 was obtained at 673 K in nominal composition PbSe + 3 or 4 at. % Pb, leading to nearly 50% enhancement from reported n -type pristine PbSe, mainly benefitting from the improved electrical performance. Furthermore, the potential thermoelectric efficiency was also improved due to the enhanced low-temperature performance, showing a high average ZT of 0.6 that is even comparable to that of commercial n -type Bi 2 Te 3 materials.
    Electronic ISSN: 2166-532X
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  • 118
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    American Institute of Physics (AIP)
    Publication Date: 2016-05-24
    Description: We report a systematic study on the thermoelectric performance of spin Seebeck devices based on Fe 3 O 4 /Pt junction systems. We explore two types of device geometries: a spin Hall thermopile and spin Seebeck multilayer structures. The spin Hall thermopile increases the sensitivity of the spin Seebeck effect, while the increase in the sample internal resistance has a detrimental effect on the output power. We found that the spin Seebeck multilayers can overcome this limitation since the multilayers exhibit the enhancement of the thermoelectric voltage and the reduction of the internal resistance simultaneously, therefore resulting in significant power enhancement. This result demonstrates that the multilayer structures are useful for improving the thermoelectric performance of the spin Seebeck effect.
    Electronic ISSN: 2166-532X
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  • 119
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    American Institute of Physics (AIP)
    Publication Date: 2016-05-26
    Description: We report p-doping of the BaSnO 3 (BSO) by replacing Ba with K. The activation energy of K-dopants is estimated to be about 0.5 eV. We have fabricated pn junctions by using K-doped BSO as a p-type and La-doped BSO as an n-type semiconductor. I-V characteristics of these devices exhibit an ideal rectifying behavior of pn junctions with the ideality factor between 1 and 2, implying high integrity of the BSO materials. Moreover, the junction properties are found to be very stable after repeated high-bias and high-temperature thermal cycling, demonstrating a large potential for optoelectronic functions.
    Electronic ISSN: 2166-532X
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  • 120
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    American Institute of Physics (AIP)
    Publication Date: 2016-05-26
    Description: We present an investigation of the thermoelectric properties of cubic perovskite SrTiO 3 . The results are derived from a combination of calculated transport functions obtained from Boltzmann transport theory in the constant scattering time approximation based on the electronic structure and existing experimental data for La-doped SrTiO 3 . The figure of merit ZT is modeled with respect to carrier concentration and temperature. The model predicts a relatively high ZT at optimized doping and suggests that the ZT value can reach 0.7 at T = 1400 K. Thus ZT can be improved from the current experimental values by carrier concentration optimization.
    Electronic ISSN: 2166-532X
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  • 121
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    American Institute of Physics (AIP)
    Publication Date: 2016-05-28
    Description: The experimental search for new thermoelectric materials remains largely confined to a limited set of successful chemical and structural families, such as chalcogenides, skutterudites, and Zintl phases. In principle, computational tools such as density functional theory (DFT) offer the possibility of rationally guiding experimental synthesis efforts toward very different chemistries. However, in practice, predicting thermoelectric properties from first principles remains a challenging endeavor [J. Carrete et al. , Phys. Rev. X 4 , 011019 (2014)], and experimental researchers generally do not directly use computation to drive their own synthesis efforts. To bridge this practical gap between experimental needs and computational tools, we report an open machine learning-based recommendation engine ( http://thermoelectrics.citrination.com ) for materials researchers that suggests promising new thermoelectric compositions based on pre-screening about 25 000 known materials and also evaluates the feasibility of user-designed compounds. We show this engine can identify interesting chemistries very different from known thermoelectrics. Specifically, we describe the experimental characterization of one example set of compounds derived from our engine, RE 12 Co 5 Bi ( RE = Gd, Er), which exhibits surprising thermoelectric performance given its unprecedentedly high loading with metallic d and f block elements and warrants further investigation as a new thermoelectric material platform. We show that our engine predicts this family of materials to have low thermal and high electrical conductivities, but modest Seebeck coefficient, all of which are confirmed experimentally. We note that the engine also predicts materials that may simultaneously optimize all three properties entering into zT ; we selected RE 12 Co 5 Bi for this study due to its interesting chemical composition and known facile synthesis.
    Electronic ISSN: 2166-532X
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  • 122
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    American Institute of Physics (AIP)
    Publication Date: 2016-06-02
    Description: Here we report the thermoelectric properties of NbCoSn-based n-type half-Heuslers (HHs) that were obtained through arc melting, ball milling, and hot pressing process. With 10% Sb substitution at the Sn site, we obtained enhanced n-type properties with a maximum power factor reaching ∼35 μ W cm −1 K −2 and figure of merit (ZT) value ∼0.6 in NbCoSn 0.9 Sb 0.1 . The ZT is doubled compared to the previous report. In addition, the specific power cost ($ W −1 ) is decreased by ∼68% comparing to HfNiSn-based n-type HH because of the elimination of Hf.
    Electronic ISSN: 2166-532X
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  • 123
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    American Institute of Physics (AIP)
    Publication Date: 2016-06-11
    Description: Large crystalline molecular shells buckle spontaneously into icosahedra while multicomponent shells buckle into various polyhedra. Continuum elastic theory explains the buckling of closed shells with one elastic component into icosahedra. A generalized elastic model, on the other hand, describes the spontaneous buckling of inhomogeneous shells into regular and irregular polyhedra. By co-assembling water-insoluble anionic (−1) amphiphiles with cationic (3+) amphiphiles, we realized ionic vesicles. Results revealed that surface crystalline domains and the unusual shell shapes observed arise from the competition of ionic correlations with charge-regulation. We explain here the mechanism by which these ionic membranes generate a mechanically heterogeneous vesicle.
    Electronic ISSN: 2166-532X
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  • 124
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    American Institute of Physics (AIP)
    Publication Date: 2016-01-13
    Description: A large remanent polarization close to theoretical value 80 μ C/cm 2 of bulk PbTiO 3 is achieved in epitaxial heterostructures of (120–600)-nm-thick PbTiO 3 films grown by pulsed laser deposition on (001) SrTiO 3 substrate using a 100-nm-thick SrRuO 3 bottom electrode layer. The heterostructures employing a 50-nm-thick electrode exhibit a significantly smaller polarization of ≤60 μ C/cm 2 . A detailed x-ray diffraction analysis of the crystal structure allows for relating this large polarization to electrode-controlled relaxation of epitaxial strain in PbTiO 3 . Based on the observed results, we anticipate that the electrode-promoted strain relaxation can be used to enhance polarization in other epitaxial ferroelectric films.
    Electronic ISSN: 2166-532X
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  • 125
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    American Institute of Physics (AIP)
    Publication Date: 2016-04-30
    Description: Single nanowires of two manganese oxide polymorphs (α-MnO 2 and todorokite manganese oxide), which display a controlled size variation in terms of their square structural tunnels, were isolated onto nanofabricated platforms using dielectrophoresis. This platform allowed for the measurement of the electronic conductivity of these manganese oxides, which was found to be higher in α-MnO 2 as compared to that of the todorokite phase by a factor of ∼46. Despite this observation of substantially higher electronic conductivity in α-MnO 2 , the todorokite manganese oxide exhibited better electrochemical rate performance as a Li-ion battery cathode. The relationship between this electrochemical performance, the electronic conductivities of the manganese oxides, and their reported ionic conductivities is discussed for the first time, clearly revealing that the rate performance of these materials is limited by their Li + diffusivity, and not by their electronic conductivity. This result reveals important new insights relevant for improving the power density of manganese oxides, which have shown promise as a low-cost, abundant, and safe alternative for next-generation cathode materials. Furthermore, the presented experimental approach is suitable for assessing a broader family of one-dimensional electrode active materials (in terms of their electronic and ionic conductivities) for both Li-ion batteries and for electrochemical systems utilizing charge-carrying ions beyond Li + .
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  • 126
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    American Institute of Physics (AIP)
    Publication Date: 2016-05-14
    Description: The tailoring of molecular weight distribution and the functional group density of vinyl-terminated polydimethylsiloxane (PDMS) by molecular beam deposition is demonstrated herein. Thermally evaporated PDMS and its residue are characterized using gel permeation chromatography and nuclear magnetic resonance. Thermal fragmentation of vinyl groups occurs for evaporation temperatures above 487 K (214 °C). At a background pressure of 10 −6 mbar, the maximum molecular weight distribution is adjusted from (700 ± 100) g/mol to (6100 ± 100) g/mol with a polydispersity index of 1.06 ± 0.02. The content of vinyl-termination per repeating unit of PDMS is tailored from (2.8 ± 0.2)% to (5.6 ± 0.1)%. Molecular weights of vinyl-terminated PDMS evaporated at temperatures above 388 K (115 °C) correspond to those attributed to trimethyl-terminated PDMS. Side groups of linear PDMS dominate intermolecular interactions and vapor pressure.
    Electronic ISSN: 2166-532X
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  • 127
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    American Institute of Physics (AIP)
    Publication Date: 2016-05-14
    Description: Structural and optical properties of InGaAs quantum well fins (QWFs) selectively grown on Si using the aspect ratio trapping (ART) method in 200 nm deep SiO 2 trenches are studied. A new method combining cathodoluminescence, transmission electron microscopy, and precession electron diffraction techniques is developed to spatially correlate the presence of defects and/or strain with the light emission properties of a single InGaAs QWF. Luminescence losses and energy shifts observed at the nanoscale along InGaAs QWF are correlated with structural defects. We show that strain distortions measured around threading dislocations delimit both high and low luminescent areas. We also show that trapped dislocations on SiO 2 sidewalls can also result in additional distortions. Both behaviors affect optical properties of QWF at the nanoscale. Our study highlights the need to improve the ART growth method to allow integration of new efficient III-V optoelectronic components on Si.
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  • 128
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    American Institute of Physics (AIP)
    Publication Date: 2016-05-27
    Description: Synthesis of inorganic extended solids is a critical starting point from which real-world functional materials and their consequent technologies originate. However, unlike the rich mechanistic foundation of organic synthesis, with its underlying rules of assembly (e.g., functional groups and their reactivities), the synthesis of inorganic materials lacks an underpinning of such robust organizing principles. In the latter case, any such rules must account for the diversity of chemical species and bonding motifs inherent to inorganic materials and the potential impact of mass transport on kinetics, among other considerations. Without such assembly rules, there is less understanding, less predictive power, and ultimately less control of properties. Despite such hurdles, developing a mechanistic understanding for synthesis of inorganic extended solids would dramatically impact the range of new material discoveries and resulting new functionalities, warranting a broad call to explore what is possible. Here we discuss our recent approaches toward a mechanistic framework for the synthesis of bulk inorganic extended solids, in which either embryonic atomic correlations or fully developed phases in solutions or melts can be identified and tracked during product selection and crystallization. The approach hinges on the application of high-energy x-rays, with their penetrating power and large Q-range, to explore reaction pathways in situ . We illustrate this process using two examples: directed assembly of Zr clusters in aqueous solution and total phase awareness during crystallization from K–Cu–S melts. These examples provide a glimpse of what we see as a larger vision, in which large scale simulations, data-driven science, and in situ studies of atomic correlations combine to accelerate materials discovery and synthesis, based on the assembly of well-defined, prenucleated atomic correlations.
    Electronic ISSN: 2166-532X
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  • 129
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    American Institute of Physics (AIP)
    Publication Date: 2016-05-27
    Description: With their ability to rapidly elucidate composition-structure-property relationships, high-throughput experimental studies have revolutionized how materials are discovered, optimized, and commercialized. It is now possible to synthesize and characterize high-throughput libraries that systematically address thousands of individual cuts of fabrication parameter space. An unresolved issue remains transforming structural characterization data into phase mappings. This difficulty is related to the complex information present in diffraction and spectroscopic data and its variation with composition and processing. We review the field of automated phase diagram attribution and discuss the impact that emerging computational approaches will have in the generation of phase diagrams and beyond.
    Electronic ISSN: 2166-532X
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  • 130
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    American Institute of Physics (AIP)
    Publication Date: 2016-10-05
    Description: All-oxide photovoltaics could open rapidly scalable manufacturing routes, if only oxide materials with suitable electronic and optical properties were developed. SnO has exceptional doping and transport properties among oxides, but suffers from a strongly indirect band gap. Here, we address this shortcoming by band-structure engineering through isovalent but heterostructural alloying with divalent cations (Mg, Ca, Sr, and Zn). Using first-principles calculations, we show that suitable band gaps and optical properties close to that of direct semiconductors are achievable, while the comparatively small effective masses are preserved in the alloys. Initial thin film synthesis and characterization support the feasibility of the approach.
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  • 131
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    American Institute of Physics (AIP)
    Publication Date: 2016-10-28
    Electronic ISSN: 2166-532X
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  • 132
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    American Institute of Physics (AIP)
    Publication Date: 2016-10-29
    Description: Thermoelectric properties of nanostructured FeSi 2 , Mg 2 Si, and SiGe are compared with their nanocomposites of SiGe–Mg 2 Si and SiGe–FeSi 2 . It was found that the addition of silicide nanoinclusions to SiGe alloy maintained or increased the power factor while further reduced the thermal conductivity compared to the nanostructured single-phase SiGe alloy. This resulted in ZT enhancement of Si 0.88 Ge 0.12 –FeSi 2 by ∼30% over the broad temperature range of 500-950 °C compared to the conventional Si 0.80 Ge 0.20 alloy. The Si 0.88 Ge 0.12 –Mg 2 Si nanocomposite showed constantly increasing ZT versus temperature up to 950 °C (highest measured temperature) reaching ZT ∼ 1.3. These results confirm the concept of silicide nanoparticle-in-SiGe-alloy proposed earlier by Mingo et al. [Nano Lett. 9 , 711–715 (2009)].
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  • 133
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    American Institute of Physics (AIP)
    Publication Date: 2016-11-01
    Description: The temperature dependence of the thermal conductivity of 27 different single crystal oxides is reported from ≈20 K to 350 K. These crystals have been selected among the most common substrates for growing epitaxial thin-film oxides, spanning over a range of lattice parameters from ≈3.7 Å to ≈12.5 Å. Different contributions to the phonon relaxation time are discussed on the basis of the Debye model. This work provides a database for the selection of appropriate substrates for thin-film growth according to their desired thermal properties, for applications in which heat management is important.
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  • 134
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    American Institute of Physics (AIP)
    Publication Date: 2016-11-02
    Description: CdTe defect chemistry is adjusted by annealing samples with excess Cd or Te vapor with and without extrinsic dopants. We observe that Group I (Cu and Na) elements can increase hole density above 10 16 cm −3 , but compromise lifetime and stability. By post-deposition incorporation of a Group V dopant (P) in a Cd-rich ambient, lifetimes of 30 ns with 10 16 cm −3 hole density are achieved in single-crystal and polycrystalline CdTe without CdCl 2 or Cu. Furthermore, phosphorus doping appears to be thermally stable. This combination of long lifetime, high carrier concentration, and improved stability can help overcome historic barriers for CdTe solar cell development.
    Electronic ISSN: 2166-532X
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  • 135
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    American Institute of Physics (AIP)
    Publication Date: 2016-08-20
    Description: Lead halide perovskite solar cells have shown a tremendous rise in power conversion efficiency with reported record efficiencies of over 20% making this material very promising as a low cost alternative to conventional inorganic solar cells. However, due to a differently severe “hysteretic” behaviour during current density-voltage measurements, which strongly depends on scan rate, device and measurement history, preparation method, device architecture, etc., commonly used solar cell measurements do not give reliable or even reproducible results. For the aspect of commercialization and the possibility to compare results of different devices among different laboratories, it is necessary to establish a measurement protocol which gives reproducible results. Therefore, we compare device characteristics derived from standard current density-voltage measurements with stabilized values obtained from an adaptive tracking of the maximum power point and the open circuit voltage as well as characteristics extracted from time resolved current density-voltage measurements. Our results provide insight into the challenges of a correct determination of device performance and propose a measurement protocol for a reliable characterisation which is easy to implement and has been tested on varying perovskite solar cells fabricated in different laboratories.
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  • 136
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    American Institute of Physics (AIP)
    Publication Date: 2016-08-20
    Description: We report on synthesis and characterization of a new magnetic nanolaminate (V,Mn) 3 GaC 2 , which is the first magnetic MAX phase of a 312 stoichiometry. Atomically resolved energy dispersive X-ray mapping of epitaxial thin films reveals a tendency of alternate chemical ordering between V and Mn, with atomic layers composed of primarily one element only. Magnetometry measurements reveal a ferromagnetic response between 50 K and 300 K, with indication of a magnetic ordering temperature well above room temperature.
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  • 137
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    American Institute of Physics (AIP)
    Publication Date: 2016-08-23
    Description: We have developed a method to detect the presence of small amounts of chemical substances in water, using a Al 2 O 3 nanoparticle thin film covered with phosphonic acid (HDF-PA) self-assembled monolayer. The HDF-PA self-assembled Al 2 O 3 nanoparticle thin film acts as a liquid-vapour separation filter, allowing the passage of chemical vapour while blocking liquids. Prevention of the liquid from contacting the SnO 2 nanowire and source-drain electrodes is required in order to avoid abnormal operation. Using this characteristic, the concentration of chemical substances in water could be evaluated by measuring the current changes in the SnO 2 nanowire transistor covered with the HDF-PA self-assembled Al 2 O 3 nanoparticle thin film.
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  • 138
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    American Institute of Physics (AIP)
    Publication Date: 2016-08-24
    Description: Sputter deposition is a widely used growth technique for a large range of important material systems. Epitaxial films of carbides, nitrides, metals, oxides and more can all be formed during the sputter process which offers the ability to deposit smooth and uniform films from the research level up to an industrial scale. This tunable kinematic deposition process excels in easily adapting for a large range of environments and growth procedures. Despite the vast advantages, there is a significant lack of in situ analysis options during sputtering. In particular, the area of real time atomic layer control is severely deficient. Atomic layer controlled growth of epitaxial thin films and artificially layered superlattices is critical for both understanding their emergent phenomena and engineering novel material systems and devices. Reflection high-energy electron diffraction (RHEED) is one of the most common in situ analysis techniques during thin film deposition that is rarely used during sputtering due to the effect of the strong permanent magnets in magnetron sputter sources on the RHEED electron beam. In this work we have solved this problem and designed a novel way to deter the effect of the magnets for a wide range of growth geometries and demonstrate the ability for the first time to have layer-by-layer control during sputter deposition by in situ RHEED.
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  • 139
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    American Institute of Physics (AIP)
    Publication Date: 2016-08-24
    Description: We present a detailed study of the reaction kinetics and thermodynamics of the plasma-assisted oxide molecular beam epitaxy of the ternary compound (In x Ga 1− x ) 2 O 3 for 0 ≤ x ≤ 1. We measured the growth rate of the alloy in situ by laser reflectrometry as a function of growth temperature T G for different metal-to-oxygen flux ratios r Me , and nominal In concentrations x nom in the metal flux. We determined ex situ the In and Ga concentrations in the grown film by energy dispersive X-ray spectroscopy. The measured In concentration x shows a strong dependence on the growth parameters T G , r Me , and x nom whereas growth on different co-loaded substrates shows that in the macroscopic regime of ∼ μ m 3 x does neither depend on the detailed layer crystallinity nor on crystal orientation. The data unveil that, in presence of In, Ga incorporation is kinetically limited by Ga 2 O desorption the same way as during Ga 2 O 3 growth. In contrast, In incorporation during ternary growth is thermodynamically suppressed by the presence of Ga due to stronger Ga–O bonds. Our experiments revealed that Ga adatoms decompose/etch the In–O bonds whereas In adatoms do not decompose/etch the Ga–O bonds. This result is supported by our thermochemical calculations. In addition we found that a low T G and/or excessively low r Me kinetically enables In incorporation into (In x Ga 1− x ) 2 O 3 . This study may help growing high-quality ternary compounds (In x Ga 1− x ) 2 O 3 allowing band gap engineering over the range of 2.7–4.7 eV.
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  • 140
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    American Institute of Physics (AIP)
    Publication Date: 2016-08-30
    Description: Self-organized AlGaN nanowires by molecular beam epitaxy have attracted significant attention for deep ultraviolet optoelectronics. However, due to the strong compositional modulations under conventional nitrogen rich growth conditions, emission wavelengths less than 250 nm have remained inaccessible. Here we show that Al-rich AlGaN nanowires with much improved compositional uniformity can be achieved in a new growth paradigm, wherein a precise control on the optical bandgap of ternary AlGaN nanowires can be achieved by varying the substrate temperature. AlGaN nanowire LEDs, with emission wavelengths spanning from 236 to 280 nm, are also demonstrated.
    Electronic ISSN: 2166-532X
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  • 141
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    American Institute of Physics (AIP)
    Publication Date: 2016-08-30
    Description: In the quest for more efficient thermoelectric material able to convert thermal to electrical energy and vice versa, composites that combine a semiconductor host having a large Seebeck coefficient with metal nanodomains that provide phonon scattering and free charge carriers are particularly appealing. Here, we present our experimental results on the thermal and electrical transport properties of PbS-metal composites produced by a versatile particle blending procedure, and where the metal work function allows injecting electrons to the intrinsic PbS host. We compare the thermoelectric performance of composites with microcrystalline or nanocrystalline structures. The electrical conductivity of the microcrystalline host can be increased several orders of magnitude with the metal inclusion, while relatively high Seebeck coefficient can be simultaneously conserved. On the other hand, in nanostructured materials, the host crystallites are not able to sustain a band bending at its interface with the metal, becoming flooded with electrons. This translates into even higher electrical conductivities than the microcrystalline material, but at the expense of lower Seebeck coefficient values.
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  • 142
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    American Institute of Physics (AIP)
    Publication Date: 2016-09-02
    Description: We present unusual high hardness (up to 7.7 GPa) achieved in Cu/Al multilayers relative to monolithic Cu and Al films (∼2 GPa and ∼1 GPa, respectively). Nanotwins and stacking faults (SFs) were proposed to be the main contributors of hardness enhancement, especially when h 〈 5 nm. Using molecular dynamics simulations of deposition, we demonstrated that intermixing near Cu/Al interface was paramount in stabilizing the SFs in both Cu and Al layers. Our experimental results indicated that the high strength caused by layer intermixing was in sharp contrast to the general belief that only sharp interface structures could strengthen the multilayers.
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  • 143
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    American Institute of Physics (AIP)
    Publication Date: 2016-09-02
    Description: The Co-Pt nanochessboard is a quasi-periodic, nanocomposite tiling of L1 0 and L1 2 magnetic phases that offers a novel structure for the investigation of exchange coupling, relevant to permanent magnet applications. Periodicity of the tiling is controlled by the rate of cooling through the eutectoid isotherm, resulting in control over the L1 0 -L1 2 exchange coupling. First order reversal curve analysis reveals a transition from partial coupling to nearly complete exchange-coupling in a Co 40.2 Pt 59.8 nanochessboard structured alloy as the periodicity is reduced below the critical correlation length. Micromagnetic simulations give insights into how exchange coupling manifests in the tiling, and its impact on microscopic magnetization reversal mechanisms.
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  • 144
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    American Institute of Physics (AIP)
    Publication Date: 2016-09-07
    Description: Perovskite solar cells (PSCs) marked tremendous progress in a short period of time and offer bright hopes for cheap solar electricity. Despite high power conversion efficiency 〉20%, its poor operational stability as well as involvement of toxic, volatile, and less-abundant materials hinders its practical deployment. The fact that degradation and toxicity are typically observed in the most successful perovskite involving organic cation and toxic lead, i.e., CH 3 NH 3 PbX 3 , requires a deep understanding of their role in photovoltaic performance in order to envisage if a non-toxic, stable yet highly efficient device is feasible. Towards this, we first provide an overview of the basic chemistry and physics of halide perovskites and its correlation with its extraordinary properties such as crystal structure, bandgap, ferroelectricity, and electronic transport. We then discuss device related aspects such as the various device designs in PSCs and role of interfaces in origin of PV parameters particularly open circuit voltage, various film processing methods and their effect on morphology and characteristics of perovskite films, and the origin and elimination of hysteresis and operational stability in these devices. We then identify future perspectives for stable and efficient PSCs for practical deployment.
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  • 145
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    American Institute of Physics (AIP)
    Publication Date: 2016-09-20
    Description: We use calorimetry to identify pressure-driven isothermal entropy changes in ceramic samples of the prototypical ferroelectric BaTiO 3 . Near the structural phase transitions at ∼400 K (cubic-tetragonal) and ∼280 K (tetragonal-orthorhombic), the inverse barocaloric response differs in sign and magnitude from the corresponding conventional electrocaloric response. The differences in sign arise due to the decrease in unit-cell volume on heating through the transitions, whereas the differences in magnitude arise due to the large volumetric thermal expansion on either side of the transitions.
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  • 146
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    American Institute of Physics (AIP)
    Publication Date: 2016-09-20
    Description: Nanocrystalline silicon thermoelectrics can be a solution to improve the cost-effectiveness of thermoelectric technology from both material and integration viewpoints. While their figure-of-merit is still developing, recent advances in theoretical/numerical calculations, property measurements, and structural synthesis/fabrication have opened up possibilities to develop the materials based on fundamental physics of phonon transport. Here, this is demonstrated by reviewing a series of works on nanocrystalline silicon materials using calculations of multiscale phonon transport, measurements of interfacial heat conduction, and synthesis from nanoparticles. Integration of these approaches allows us to engineer phonon transport to improve the thermoelectric performance by introducing local silicon-oxide structures.
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  • 147
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    American Institute of Physics (AIP)
    Publication Date: 2016-09-20
    Description: To bring perovskite solar cells to the industrial world, performance must be maintained at the photovoltaic module scale. Here we present large-area manufacturing and processing options applicable to large-area cells and modules. Printing and coating techniques, such as blade coating, slot-die coating, spray coating, screen printing, inkjet printing, and gravure printing (as alternatives to spin coating), as well as vacuum or vapor based deposition and laser patterning techniques are being developed for an effective scale-up of the technology. The latter also enables the manufacture of solar modules on flexible substrates, an option beneficial for many applications and for roll-to-roll production.
    Electronic ISSN: 2166-532X
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  • 148
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    American Institute of Physics (AIP)
    Publication Date: 2016-10-08
    Description: Metal nanoparticle-semiconductor interfaces are sites of complex light-matter interactions, in particular, the exciton-plasmon coupling which plays a key role in the optical response of such heterostructures. There exists a pathway of photoinduced charge transfer from the semiconductor to the metal, which can be used to controllably vary the driving forces at the interface that leads to tunable optoelectronic properties. In this letter, we report the observation of a dramatic suppression of plasmonic as well as excitonic absorption in a-Ge 24 Se 76 /gold nanoparticle heterostructures by trapped charges. Suppression of the excitonic absorption is strongly correlated with the plasmon wavelength.
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  • 149
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    American Institute of Physics (AIP)
    Publication Date: 2016-10-08
    Description: The elastocaloric effect in a columnar-grained Cu 71.5 Al 17.5 Mn 11 shape memory alloy fabricated by directional solidification was investigated. A large entropy change of 25.0 J/kg K generated by the reversible martensitic transformation was demonstrated. The adiabatic temperature change of 12-13 K was directly measured, covering a wide temperature range of more than 100 K. The low applied stress with a specific elastocaloric ability of 100.8 K/GPa was identified and the potentially attainable operational temperature window as wide as more than 215 K was also discussed. The outstanding elastocaloric refrigeration capability, together with the low applying stress and uniform phase transformation, makes the columnar-grained Cu–Al–Mn shape memory alloy a promising material for solid-state refrigeration.
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  • 150
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    American Institute of Physics (AIP)
    Publication Date: 2016-10-08
    Description: We report the growth of wafer-scale arrays of individually position-controlled and vertically aligned ZnO nanotube arrays on graphene deposited by chemical vapor deposition (CVD-graphene). Introducing two-dimensional layered materials such as graphene as a growth buffer has recently been suggested for growing nanomaterials on traditionally incompatible substrates. However, their growth has been restricted to small areas or had limited controllability. Here, we study the distinct growth behavior of ZnO on CVD-graphene that makes the selective area growth of individual nanostructures on its surface difficult, and propose a set of methods to overcome this. The resulting nanotube arrays, as examined by scanning electron microscopy and transmission electron microscopy, exhibited uniform morphologies and high structural quality over a large area and could be prepared on a broad variety of substrates, including amorphous, metallic, or flexible substrates.
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  • 151
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    American Institute of Physics (AIP)
    Publication Date: 2016-10-19
    Description: Double perovskite Bi 2 FeMnO 6 is a potential candidate for the single-phase multiferroic system. In this work, we study the magnetic, electronic, and optical properties in BFMO by performing the density functional theory calculations and experimental measurements of magnetic moment. We also demonstrate the strain dependence of magnetization. More importantly, our calculations of electronic and optical properties reveal that the onsite local correlation on Mn and Fe sites is critical to the gap opening in BFMO, which is a prerequisite condition for the ferroelectric ordering. Finally, we calculate the x-ray magnetic circular dichroism spectra of Fe and Mn ions (L 2 and L 3 edges) in BFMO.
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  • 152
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    American Institute of Physics (AIP)
    Publication Date: 2016-07-16
    Description: We demonstrated epitaxial growth of GaN (0001) films on nearly lattice-matched Hf (0001) substrates by using a low-temperature (LT) epitaxial growth technique. High-temperature growth of GaN films results in the formation of polycrystalline films due to significant reaction at GaN/Hf heterointerfaces, while LT-growth allowed us to suppress the interfacial reactions and to obtain epitaxial GaN films on Hf substrates with a GaN 11 2 ̄ 0 / / Hf 11 2 ̄ 0 in-plane orientation. LT-grown GaN films can act as buffer layers for GaN growth at high temperatures. The interfacial layer thickness at the LT-GaN/Hf heterointerface was as small as 1 nm, and the sharpness of the contact remained unchanged even after annealing up to approximately 700 °C, which likely accounts for the dramatic improvement in GaN crystalline quality on Hf substrates.
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  • 153
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    American Institute of Physics (AIP)
    Publication Date: 2016-07-22
    Description: The hybrid perovskite CH 3 NH 3 PbI 3 (MAPI) exhibits long minority-carrier lifetimes and diffusion lengths. We show that slow recombination originates from a spin-split indirect-gap. Large internal electric fields act on spin-orbit-coupled band extrema, shifting band-edges to inequivalent wavevectors, making the fundamental gap indirect. From a description of photoluminescence within the quasiparticle self-consistent GW approximation for MAPI, CdTe, and GaAs, we predict carrier lifetime as a function of light intensity and temperature. At operating conditions we find radiative recombination in MAPI is reduced by a factor of more than 350 compared to direct gap behavior. The indirect gap is retained with dynamic disorder.
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  • 154
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    American Institute of Physics (AIP)
    Publication Date: 2016-07-22
    Description: We study the two-dimensional electron gas at the interface of NdTiO 3 and SrTiO 3 to reveal its nanoscale transport properties. At electron densities approaching 10 15 cm −2 , our terahertz spectroscopy data show conductivity levels that are up to six times larger than those extracted from DC electrical measurements. Moreover, the largest conductivity enhancements are observed in samples intentionally grown with larger defect densities. This is a signature of electron transport over the characteristic length-scales typically probed by electrical measurements being significantly affected by scattering by structural defects introduced during growth, and, a trait of a much larger electron mobility at the nanoscale.
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  • 155
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    American Institute of Physics (AIP)
    Publication Date: 2016-07-22
    Description: III-nitride semiconductors hold tremendous promise for realizing high efficiency photoelectrodes. However, previously reported InGaN photoelectrodes generally exhibit very low photocurrent densities, due to the presence of extensive defects, dislocations, and indium phase separation. Here, we show that In 0.5 Ga 0.5 N nanowires with nearly homogeneous indium distribution can be achieved by plasma-assisted molecular beam epitaxy. Under AM1.5G one sun illumination, the InGaN nanowire photoanode exhibits a photocurrent density of 7.3 mA/cm 2 at 1.2 V ( vs . NHE) in 1M HBr. The incident-photon-to-current efficiency is above 10% at 650 nm, which is significantly higher than previously reported values of metal oxide photoelectrodes.
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  • 156
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    American Institute of Physics (AIP)
    Publication Date: 2016-07-23
    Description: Alloying in the system Cu 2 ZnSnSe 4 –CuInSe 2 –ZnSe (CZTISe) is investigated experimentally and theoretically. The goal is to distinguish single-phase and multi-phase regions within the Cu 2 ZnSnSe 4 -2CuInSe 2 -4ZnSe pseudo-ternary phase diagram. CZTISe thin films are prepared by co-evaporation of the chemical elements and are investigated in real-time during growth using in situ angle dispersive X-ray diffraction. The focus is mainly on thin films along the Cu 2 ZnSnSe 4 –2CuInSe 2 isopleth with small ZnSe addition as well as on films along the Cu 2 ZnSnSe 4 -4ZnSe isopleth with small CuInSe 2 addition. For both cases, ab initio calculations with density-functional theory are performed to estimate the stability of the alloy with respect to the formation of secondary phases. Both in experiment and calculation, we find a surprisingly large single-phase region in the Cu 2 ZnSnSe 4 corner of the pseudo-ternary phase diagram slightly off the Cu 2 ZnSnSe 4 -4ZnSe isopleth. This may help avoiding secondary phase formation under Zn-rich conditions and open up new possibilities for the application of CZTISe thin films in solar cells.
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  • 157
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    American Institute of Physics (AIP)
    Publication Date: 2016-07-26
    Description: We report the in-plane thermoelectric properties of suspended (Bi 1− x Sb x ) 2 Te 3 nanoplates with x ranging from 0.07 to 0.95 and thicknesses ranging from 9 to 42 nm. The results presented here reveal a trend of increasing p -type behavior with increasing antimony concentration, and a maximum Seebeck coefficient and thermoelectric figure of merit at x ∼ 0.5. We additionally tuned extrinsic doping of the surface using a tetrafluoro-tetracyanoquinodimethane (F 4 -TCNQ) coating. The lattice thermal conductivity is found to be below that for undoped ultrathin Bi 2 Te 3 nanoplates of comparable thickness and in the range of 0.2–0.7 W m −1 K −1 at room temperature.
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  • 158
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    American Institute of Physics (AIP)
    Publication Date: 2016-08-02
    Description: In recent years, the interest in hybrid organic–inorganic perovskites has increased at a rapid pace due to their tremendous success in the field of thin film solar cells. This area closely ties together fundamental solid state research and device application, as it is necessary to understand the basic material properties to optimize the performances and open up new areas of application. In this regard, the energy levels and their respective alignment with adjacent charge transport layers play a crucial role. Currently, we are lacking a detailed understanding about the electronic structure and are struggling to understand what influences the alignment, how it varies, or how it can be intentionally modified. This research update aims at giving an overview over recent results regarding measurements of the electronic structure of hybrid perovskites using photoelectron spectroscopy to summarize the present status.
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  • 159
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    American Institute of Physics (AIP)
    Publication Date: 2016-08-02
    Description: With molecular beam epitaxy we have grown Cr y (Bi x Sb 1-x ) 2-y Te 3 thin films with homogeneous distribution of Cr dopants and Curie temperature up to 77 K. The films with Cr concentration y ≥ 0.39 are found to be topologically trivial, highly insulating ferromagnets, whose conductivity can be tuned over two orders of magnitude by gate voltage. The ferromagnetic insulators with electrically tunable conductivity can be used to realize the quantum anomalous Hall effect at higher temperature in topological insulator heterostructures and to develop field effect devices for spintronic applications.
    Electronic ISSN: 2166-532X
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  • 160
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    American Institute of Physics (AIP)
    Publication Date: 2016-11-09
    Description: Time domain electric pulse measurements were conducted on a capacitor consisting of a Pt film as the top electrode, atomic-layer-deposited 6.5-nm-thick amorphous Al 2 O 3 as the dielectric layer, and two-dimensional electron gas (2DEG) at the interface between Al 2 O 3 and SrTiO 3 as the bottom electrode. The sample showed highly useful current-voltage characteristics as the selector in cross-bar array resistance switching random access memory. The long-term (order of second) variation in the leakage current when the Pt electrode was positively biased was attributed to the field-induced migration of oxygen vacancies between the interior of the Al 2 O 3 and the 2DEG region. Relaxation of the vacancy concentration occurred even at room temperature.
    Electronic ISSN: 2166-532X
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  • 161
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    American Institute of Physics (AIP)
    Publication Date: 2016-11-18
    Description: We use micro-Raman spectroscopy to study strain in free-standing graphene monolayers anchored to SiN holes of non-circular geometry. We show that a uniform differential pressure load yields measurable deviations from hydrostatic strain, conventionally observed in radially symmetric microbubbles. A pressure load of 1 bar yields a top hydrostatic strain of ≈ 0.7% and a G ± splitting of 10 cm −1 in graphene clamped to elliptical boundaries with axes 40 and 20 μ m, in good agreement with the calculated anisotropy Δ ε ≈ 0.6% and consistently with recent reports on Grüneisen parameters. The implementation of arbitrary strain configurations by designing suitable boundary clamping conditions is discussed.
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  • 162
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    American Institute of Physics (AIP)
    Publication Date: 2016-11-22
    Electronic ISSN: 2166-532X
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  • 163
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    American Institute of Physics (AIP)
    Publication Date: 2016-11-22
    Description: We report a method in making transmission electron microscopy sample for both CVD-grown and exfoliated 2D materials without etching process, thus gentle to those 2D materials that are sensitive to water and reactive etchants. Large-scale WS 2 monolayer grown on glass, NbS 2 atomic layers grown on exfoliated h-BN flakes, and water-sensitive exfoliated TiS 2 flakes are given as representative examples. We show that the as-transferred samples not only retain excellent structural integrity down to atomic scale but also have little oxidations, presumably due to the minimum contact with water/etchants. This method paves the way for atomic scale structural and chemical investigations in sensitive 2D materials.
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  • 164
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    American Institute of Physics (AIP)
    Publication Date: 2016-11-29
    Description: We have investigated the temperature-dependent oxygen behavior in the lithium battery cathode Li x Ni 0.5 Mn 1.5 O 4 (LNMO) materials in the temperature range 30-1000 °C. As the temperature increases, oxygen release occurs and the change of crystal structures from the face centered cubic spinel at 30 °C to other phases follows. The amount of released oxygen and the changed crystalline phases are dependent on Li content and temperature. These phenomena are reversible against temperature in air, but not in vacuum and argon gas environments. This study illustrates the important role of temperature and atmospheric environments in synthesizing the LNMO battery materials.
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  • 165
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    American Institute of Physics (AIP)
    Publication Date: 2016-11-29
    Description: We devised a single-step mechanochemical synthesis/densification procedure for Cu 2 X (X = S, Se) thermoelectric materials via applying a pressure of 3 GPa to a stoichiometric admixture of elemental Cu and X for 3 min at room temperature. The obtained bulk materials were single-phase, nearly stoichiometric structures with a relative packing density of 97% or higher. The structures contained high concentration of atomic scale defects and pores of 20-200 nm diameter. The above attributes gave rise to a high thermoelectric performance: at 873 K, the ZT value of Cu 2 S reached 1.07, about 2.1 times the value typical of samples grown from the melt. The ZT value of Cu 2 Se samples reached in excess of 1.2, close to the state-of-the-art value.
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  • 166
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    American Institute of Physics (AIP)
    Publication Date: 2016-12-01
    Description: Carrier multiplication using singlet exciton fission (SF) to generate a pair of spin-triplet excitons from a single optical excitation has been highlighted as a promising approach to boost the photocurrent in photovoltaics (PVs) thereby allowing PV operation beyond the Shockley-Queisser limit. The applicability of many efficient fission materials, however, is limited due to their poor solubility. For instance, while acene-based organics such as pentacene (Pc) show high SF yields (up to200%), the plain acene backbone renders the organic molecule insoluble in common organic solvents. Previous approaches adding solubilizing side groups such as bis(tri- iso -propylsilylethynyl) to the Pc core resulted in low vertical carrier mobilities due to reduction of the transfer integrals via steric hindrance, which prevented high efficiencies in PVs. Here we show how to achieve good solubility while retaining the advantages of molecular Pc by using a soluble precursor route. The precursor fully converts into molecular Pc through thermal removal of the solubilizing side groups upon annealing above 150 °C in the solid state. The annealed precursor shows small differences in the crystallinity compared to evaporated thin films of Pc, indicating that the Pc adopts the bulk rather than surface polytype. Furthermore, we identify identical SF properties such as sub-100 fs fission time and equally long triplet lifetimes in both samples.
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  • 167
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    American Institute of Physics (AIP)
    Publication Date: 2016-05-28
    Electronic ISSN: 2166-532X
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  • 168
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    American Institute of Physics (AIP)
    Publication Date: 2016-06-02
    Description: Bi 2 Te 3 -based compounds are a well-known class of outstanding thermoelectric materials. β-As 2 Te 3 , another member of this family, exhibits promising thermoelectric properties around 400 K when appropriately doped. Herein, we investigate the high-temperature thermoelectric properties of the β-As 2− x Bi x Te 3 solid solution. Powder X-ray diffraction and scanning electron microscopy experiments showed that a solid solution only exists up to x = 0.035. We found that substituting Bi for As has a beneficial influence on the thermopower, which, combined with extremely low thermal conductivity values, results in a maximum ZT value of 0.7 at 423 K for x = 0.017 perpendicular to the pressing direction.
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  • 169
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    American Institute of Physics (AIP)
    Publication Date: 2016-06-09
    Description: This paper discusses the influence of material strain and strain rate on efficiency and temperature span of elastocaloric cooling processes. The elastocaloric material, a newly developed quaternary Ni-Ti-Cu-V alloy, is characterized at different maximum strains and strain rates. The experiments are performed with a specially designed test setup, which enables the measurement of mechanical and thermal process parameters. The material efficiency is compared to the efficiency of the Carnot process at equivalent thermal operation conditions. This method allows for a direct comparison of the investigated material with other caloric materials.
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  • 170
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    American Institute of Physics (AIP)
    Publication Date: 2016-06-10
    Description: We report on the transport properties of hybrid devices obtained by depositing graphene on a LaAlO 3 /SrTiO 3 oxide junction hosting a 4 nm-deep 2-dimensional electron system. At low graphene-oxide inter-layer bias, the two electron systems are electrically isolated, despite their small spatial separation. A very efficient reciprocal gating of the two neighboring 2-dimensional systems is shown. A pronounced rectifying behavior is observed for larger bias values and ascribed to the interplay between electrostatic field-effects and tunneling across the LaAlO 3 barrier. The relevance of these results in the context of strongly coupled bilayer systems is discussed.
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  • 171
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    American Institute of Physics (AIP)
    Publication Date: 2016-06-22
    Description: Materials’ design for high-performance thermoelectric oxides is discussed. Since chemical stability at high temperature in air is a considerable advantage in oxides, we evaluate thermoelectric power factor in the high temperature limit. We show that highly disordered materials can be good thermoelectric materials at high temperatures, and the effects of strong correlation can further enhance the figure of merit by adding thermopower arising from the spin and orbital degrees of freedom. We also discuss the Kelvin formula as a promising expression for strongly correlated materials and show that the calculation based on the Kelvin formula can be directly compared with the cross-layer thermopower of layered materials.
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  • 172
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    American Institute of Physics (AIP)
    Publication Date: 2016-06-22
    Description: Thermoelectric modules based on half-Heusler compounds offer a cheap and clean way to create eco-friendly electrical energy from waste heat. Here we study the impact of the period composition on the electrical and thermal properties in non-symmetric superlattices, where the ratio of components varies according to (TiNiSn) n :(HfNiSn) 6−n , and 0 ⩽ n ⩽ 6 unit cells. The thermal conductivity ( κ ) showed a strong dependence on the material content achieving a minimum value for n = 3, whereas the highest value of the figure of merit ZT was achieved for n = 4. The measured κ can be well modeled using non-symmetric strain relaxation applied to the model of the series of thermal resistances.
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  • 173
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    American Institute of Physics (AIP)
    Publication Date: 2016-06-23
    Description: In this study, a series of copper sulfides Cu x S with x spanning from 1.8 to 1.96 was prepared and their crystal structures, elemental valence states, and thermoelectric properties were systematically studied. The valence state of Cu in Cu x S is unchanged as the ratio of Cu/S varies, while the thermoelectric properties are very sensitive to the deficiency of Cu. In addition, the type of sulfur arrangement in the crystal structure also plays an important role on the electrical transports. Finally, the optimum Cu/S atomic ratios in the binary Cu x S system were identified for high power factor and thermoelectric figure of merit.
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  • 174
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    American Institute of Physics (AIP)
    Publication Date: 2016-06-24
    Description: Here we demonstrate a method to tune a ferroelectric imprint, which is stable in time, based on the coupling between the non-switchable polarization of ZnO and switchable polarization of PbZr x Ti (1−x) O 3 . SrRuO 3 /PbZr x Ti (1−x) O 3 /ZnO/SrRuO 3 heterostructures were grown with different ZnO thicknesses. It is shown that the coercive voltages and ferroelectric imprint vary linearly with the thickness of ZnO. It is also demonstrated that the ferroelectric imprint remains stable with electric field cycling and electric field stress assisted aging.
    Electronic ISSN: 2166-532X
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  • 175
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    American Institute of Physics (AIP)
    Publication Date: 2016-06-24
    Description: The main ideas in the theory of thermoelectrics are discussed. We discuss power generation, thermoelectric cooling, transport theory, the Seebeck coefficient, and phonon drag.
    Electronic ISSN: 2166-532X
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  • 176
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    American Institute of Physics (AIP)
    Publication Date: 2016-06-24
    Description: In this review, we focus on the celebrated interface between two band insulators, LaAlO 3 and SrTiO 3 , that was found to be conducting, superconducting, and to display a strong spin-orbit coupling. We discuss the formation of the 2-dimensional electron liquid at this interface, the particular electronic structure linked to the carrier confinement, the transport properties, and the signatures of magnetism. We then highlight distinctive characteristics of the superconducting regime, such as the electric field effect control of the carrier density, the unique tunability observed in this system, and the role of the electronic subband structure. Finally we compare the behavior of T c versus 2D doping with the dome-like behavior of the 3D bulk superconductivity observed in doped SrTiO 3 . This comparison reveals surprising differences when the T c behavior is analyzed in terms of the 3D carrier density for the interface and the bulk.
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  • 177
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    American Institute of Physics (AIP)
    Publication Date: 2016-06-29
    Description: Elastocaloric cooling has emerged as a promising alternative to vapor compression in recent years. Although the technology has the potential to be more efficient than current technologies, there are many technical challenges that must be overcome to realize devices with high performance and acceptable durability. We study the effects of surface finish and training techniques on dog bone shaped polycrystalline samples of NiTi. The fatigue life of several samples with four different surface finishes was measured and it was shown that a smooth surface, especially at the edges, greatly improved fatigue life. The effects of training both on the structure of the materials and the thermal response to an applied strain was studied. The load profile for the first few cycles was shown to change the thermal response to strain, the structure of the material at failure while the final structure of the material was weakly influenced by the surface finish.
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  • 178
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    American Institute of Physics (AIP)
    Publication Date: 2016-07-01
    Description: Temperature-dependent strength of Bi-Sb-Te under uniaxial compression is investigated. Bi-Sb-Te samples were produced by three methods: vertical zone-melting, hot extrusion, and spark plasma sintering (SPS). For zone-melted and extruded samples, the brittle-ductile transition occurs over a temperature range of 200-350 °C. In nanostructured samples produced via SPS, the transition is observed in a narrower temperature range of 170-200 °C. At room temperature, the strength of the nanostructured samples is higher than that of zone-melted and extruded samples, but above 300 °C, all samples decrease to roughly the same strength.
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  • 179
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    American Institute of Physics (AIP)
    Publication Date: 2015-06-24
    Description: A series of magnesium vanadates (MgV 2 O 6 , Mg 2 V 2 O 7 , and Mg 3 V 2 O 8 ) were synthesized to investigate the effect of cation concentration on photocatalytic performance. The samples were characterized by X-ray diffraction, field emission-scanning electron microscopy, UV-visible diffuse reflectance spectroscopy, and fluorescence spectroscopy. The photocatalytic O 2 evolution experiments under visible light irradiation showed Mg 2 V 2 O 7 exhibits the best performance, while Mg 3 V 2 O 8 has the lowest activity. The density functional theory calculations indicated that the lowest unoccupied states of Mg 3 V 2 O 8 are the mostly localized by the cation layers. The fluorescence spectra and fluorescence decay curves gave evident performances of excited states of magnesium vanadates and pointed out MgV 2 O 6 has a very short excited electron lift-time. Mg 2 V 2 O 7 performs high photocatalytic activity because of its high electron mobility and long electron life-time.
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  • 180
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    American Institute of Physics (AIP)
    Publication Date: 2015-06-25
    Description: High quality single crystals of BaFe 12 O 19 were grown using the floating zone technique in 100 atm of flowing oxygen. Single crystal neutron diffraction was used to determine the nuclear and magnetic structures of BaFe 12 O 19 at 4 K and 295 K. At both temperatures, there exist local electric dipoles formed by the off-mirror-plane displacements of magnetic Fe 3+ ions at the bipyramidal sites. The displacement at 4 K is about half of that at room temperature. The temperature dependence of the specific heat shows no anomaly associated with long range polar ordering in the temperature range from 1.90 to 300 K. The inverse dielectric permittivity, 1/ε, along the c -axis shows a T 2 temperature dependence between 10 K and 20 K, with a significantly reduced temperature dependence displayed below 10 K. Moreover, as the sample is cooled below 1.4 K there is an anomalous sharp upturn in 1/ε. These features resemble those of classic quantum paraelectrics such as SrTiO 3 . The presence of the upturn in 1/ε indicates that BaFe 12 O 19 is a critical quantum paraelectric system with Fe 3+ ions involved in both magnetic and electric dipole formation.
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  • 181
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    American Institute of Physics (AIP)
    Publication Date: 2015-07-10
    Description: We report the synthesis and crystal structure of a new high-temperature form of Ca 3 P 2 . The crystal structure was determined through Rietveld refinements of synchrotron powder x-ray diffraction data. This form of Ca 3 P 2 has a crystal structure of the hexagonal Mn 5 Si 3 type, with a Ca ion deficiency compared to the ideal 5:3 stoichiometry. This yields a stable, charge-balanced compound of Ca 2+ and P 3− . We also report the observation of a secondary hydride phase, Ca 5 P 3 H, which again is a charge-balanced compound. The calculated band structure of Ca 3 P 2 indicates that it is a three-dimensional Dirac semimetal with a highly unusual ring of Dirac nodes at the Fermi level. The Dirac states are protected against gap opening by a mirror plane in a manner analogous to what is seen for graphene.
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  • 182
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    American Institute of Physics (AIP)
    Publication Date: 2015-07-10
    Description: We present synthesis and characterization of a new magnetic atomic laminate: (Mo 0.5 Mn 0.5 ) 2 GaC. High quality crystalline films were synthesized on MgO(111) substrates at a temperature of ∼530 °C. The films display a magnetic response, evaluated by vibrating sample magnetometry, in a temperature range 3-300 K and in a field up to 5 T. The response ranges from ferromagnetic to paramagnetic with change in temperature, with an acquired 5T-moment and remanent moment at 3 K of 0.66 and 0.35 μ B per metal atom (Mo and Mn), respectively. The remanent moment and the coercive field (0.06 T) exceed all values reported to date for the family of magnetic laminates based on so called MAX phases.
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  • 183
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    American Institute of Physics (AIP)
    Publication Date: 2015-07-10
    Description: While rare-earth based Laves phases are known to exhibit large magnetostriction, the magnetic properties of some binary Laves phases containing transition metals alone are not well known. This is because many of these compounds contain refractory elements that complicate melt processing due to high melting temperatures and extensive phase separation. Here, phase-pure WFe 2 nanoclusters, with the hexagonal C14 Laves structure, were deposited via inert gas condensation, allowing for the first known measurement of ferromagnetism in this phase, with M S of 26.4 emu/g (346 emu/cm 3 ) and a K U of 286 kerg/cm 3 , at 10 K, and a T C of 550 K.
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  • 184
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    American Institute of Physics (AIP)
    Publication Date: 2015-07-17
    Description: We examined the interfacial charge transfer effect on photocatalysts using a patterned CuO thin film deposited on a rutile TiO 2 (110) substrate. Photocatalytic activity was visualized based on the formation of metal Ag particles resulting from the photoreduction of Ag + ions under visible-light illumination. Ag particles were selectively deposited near the edge of CuO film for several nanometer thick CuO film, indicating that interfacial excitation from the valence band maximum of TiO 2 to the conduction band minimum of CuO plays a key role in efficient photocatalytic activity of CuO nanocluster-grafted TiO 2 systems with visible-light sensitivity.
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  • 185
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    American Institute of Physics (AIP)
    Publication Date: 2015-07-21
    Description: A novel heterojunction structured composite photocatalyst CdS/Au/g-C 3 N 4 has been developed by depositing CdS/Au with a core (Au)-shell (CdS) structure on the surface of g-C 3 N 4 . The photocatalytic hydrogen production activity of the developed photocatalyst was evaluated under visible-light irradiation (λ 〉 420 nm) using methanol as a sacrificial reagent. As a result, its activity is about 125.8 times higher than that of g-C 3 N 4 and is even much higher than that of Pt/g-C 3 N 4 . The enhancement in photocatalytic activity is attributed to efficient separation of the photoexcited charges due to the anisotropic junction in the CdS/Au/g-C 3 N 4 system.
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  • 186
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    American Institute of Physics (AIP)
    Publication Date: 2015-07-24
    Description: The oxidation of species in the plasma plume during pulsed laser deposition controls both the stoichiometry as well as the growth kinetics of the deposited SrTiO 3 thin films, instead of the commonly assumed mass distribution in the plasma plume and the kinetic energy of the arriving species. It was observed by X-ray diffraction that SrTiO 3 stoichiometry depends on the composition of the background gas during deposition, where in a relative small pressure range between 10 −2 mbars and 10 −1 mbars oxygen partial pressure, the resulting film becomes fully stoichiometric. Furthermore, upon increasing the oxygen (partial) pressure, the growth mode changes from 3D island growth to a 2D layer-by-layer growth mode as observed by reflection high energy electron diffraction.
    Electronic ISSN: 2