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  • American Institute of Physics (AIP)  (154,867)
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  • 1
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    American Institute of Physics (AIP)
    Publication Date: 2016-04-15
    Description: We report the magnetic and optical properties of CuCr 2 O 4 thin films fabricated by atomic layer deposition (ALD) from Cu(thd) 2 , Cr(acac) 3 , and ozone; we deposit 200 nm thick films and anneal them at 700 °C in oxygen atmosphere to crystallize the spinel phase. A ferrimagnetic transition at 140 K and a direct bandgap of 1.36 eV are determined for the films from magnetic and UV-vis spectrophotometric measurements. Electrical transport measurements confirm the p-type semiconducting behavior of the films. As the ALD technique allows the deposition of conformal pin-hole-free coatings on complex 3D surfaces, our CuCr 2 O 4 films are interesting material candidates for various frontier applications.
    Electronic ISSN: 2166-532X
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  • 2
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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 + .
    Electronic ISSN: 2166-532X
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  • 3
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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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  • 4
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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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  • 5
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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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  • 6
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    American Institute of Physics (AIP)
    Publication Date: 2016-08-17
    Description: Hierarchical polymer fibers with long-range ordering have been straightforwardly fabricated employing a macroscale patterned mesh comprising microscale metallic filaments as a conductive collector, in an otherwise conventional electrospinning apparatus. Using electrostatic simulations, we elucidate that the patterning electric field is extremely confined to the immediate vicinity of the mesh collector surface. This lensing phenomenon is controlling the fiber patterning effect, and its strength decays with height above the patterned surface. Our study sheds new light on the physical mechanism underpinning electrospinning and offers a new approach for engineering fiber architectures where a precise control of in-plane physical properties is sought.
    Electronic ISSN: 2166-532X
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  • 7
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    American Institute of Physics (AIP)
    Publication Date: 2016-08-16
    Description: Cu 12 Sb 4 S 13 -based tetrahedrites are high-performance thermoelectrics that contain earth-abundant and environmentally friendly elements. At present, the mechanistic understanding of their low lattice thermal conductivity (
    Electronic ISSN: 2166-532X
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  • 8
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    American Institute of Physics (AIP)
    Publication Date: 2016-08-16
    Description: Composition-dependent charge transport and temperature-dependent density of state effective mass-dependent Seebeck coefficient were investigated in Bi 2− x Sb x Te 3 ( x = 1.56-1.68) compounds. The compounds were prepared by the spark plasma sintering of high-energy ball-milled powder. High-temperature Hall measurements revealed that the charge transport in the compounds was governed dominantly by phonon scattering and influenced additionally by alloy scattering depending on the amount of Sb. Contrary effects of Sb content on the Seebeck coefficient were discussed in terms of carrier concentration and density of state effective mass, and it was elucidated by temperature-normalized Pisarenko plot for the first time.
    Electronic ISSN: 2166-532X
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  • 9
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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.
    Electronic ISSN: 2166-532X
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  • 10
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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.
    Electronic ISSN: 2166-532X
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  • 11
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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.
    Electronic ISSN: 2166-532X
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  • 12
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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.
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  • 13
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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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  • 14
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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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  • 15
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    American Institute of Physics (AIP)
    Publication Date: 2016-09-02
    Description: In this work, transparent conducting oxides (TCOs) have been employed as a back contact instead of Mo on Cu 2 ZnSnSe 4 (CZTSe) thin-film solar cells in order to examine the feasibility of bifacial Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells based on a vacuum process. It is found that the interfacial reaction between flourine doped tin oxide (FTO) or indium tin oxide (ITO) and the CZTSe precursor is at odds with the conventional CZTSe/Mo reaction. While there is no interfacial reaction on CZTSe/FTO, indium in CZTSe/ITO was significantly diffused into the CZTSe layers; consequently, a SnO 2 layer was formed on the ITO substrate. Under bifacial illumination, we achieved a power efficiency of 6.05% and 4.31% for CZTSe/FTO and CZTSe/ITO, respectively.
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  • 16
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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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  • 17
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    American Institute of Physics (AIP)
    Publication Date: 2016-09-07
    Description: Due to the high conversion efficiency, organic-inorganic hybrid perovskite (CH 3 NH 3 PbI 3 ) solar cells are investigated for new practical energy resources for our future society. However, the perovskite solar cells have been quite unstable devices. In this manuscript, the history of perovskite solar cells is introduced, and then, the efforts to improve and understand the stability issue of perovskite solar cells are summarized.
    Electronic ISSN: 2166-532X
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  • 18
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    American Institute of Physics (AIP)
    Publication Date: 2016-09-21
    Description: We report the significant increase in the electrical conductivity, by five orders of magnitude, of Re-doped β -MoO 3 epitaxial films grown using reactive DC magnetron sputtering. Re atoms were successfully incorporated into the films by using Mo-Re alloy targets, although the film compositions deviated significantly from those of the targets. Pristine β -MoO 3 exhibited a conductivity below 10 −7 S/cm, whereas β -Mo 0.982 Re 0.018 O 3 exhibited a conductivity of 1.2 × 10 −2 S/cm and a high optical transmittance of 67% (average) in the visible region. Furthermore, transparent conducting properties were reproduced in a polycrystalline film grown on a glass substrate, indicating the high potential of these films for use as transparent conductors.
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  • 19
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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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  • 20
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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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  • 21
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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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  • 22
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    American Institute of Physics (AIP)
    Publication Date: 2016-06-29
    Electronic ISSN: 2166-532X
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  • 23
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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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  • 24
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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.
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  • 25
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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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  • 26
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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 .
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  • 27
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    American Institute of Physics (AIP)
    Publication Date: 2016-03-08
    Description: Computational capability has enabled materials design to evolve from trial-and-error towards more informed methodologies that require large amounts of data. Expert-designed tools and their underlying databases facilitate modern-day high throughput computational methods. Standard data formats and communication standards increase the impact of traditional data, and applying these technologies to a high throughput experimental design provides dense, targeted materials data that are valuable for material discovery. Integrated computational materials engineering requires both experimentally and computationally derived data. Harvesting these comprehensively requires different methods of varying degrees of automation to accommodate variety and volume. Issues of data quality persist independent of type.
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  • 28
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    American Institute of Physics (AIP)
    Publication Date: 2016-03-11
    Description: We demonstrate the selective fabrication of Ruddlesden-Popper (RP) type SrIrO 3 , Sr 3 Ir 2 O 7 , and Sr 2 IrO 4 epitaxial thin films from a single SrIrO 3 target using pulsed laser deposition (PLD). We identified that the growth conditions stabilizing each phase directly map onto the phase diagram expected from thermodynamic equilibria. This approach allows precise cation stoichiometry control as evidenced by the stabilization of single phase Sr 3 Ir 2 O 7 for the first time, overcoming the close thermodynamic stability between neighboring RP phases. Despite the non-equilibrium nature of PLD, these results highlight the importance of thermodynamic guiding principles to strategically synthesize the targeted phase in complex oxide thin films.
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  • 29
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    American Institute of Physics (AIP)
    Publication Date: 2016-03-11
    Description: We established a process for growing highly ordered MoS 2 thin films. The process consists of four steps: MoO 3 thermal evaporation, first annealing, sulfurization, and second annealing. The main feature of this process is that thermally deposited MoO 3 thin films are employed as a precursor for the MoS 2 films. The first deposition step enabled us to achieve precise control of the resulting thickness of the MoS 2 films with high uniformity. The crystalline structures, surface morphologies, and chemical states at each step were characterized by X-ray diffraction, atomic force microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. Based on these characterizations and a careful optimization of the growth conditions, we successfully produced a highly oriented MoS 2 thin film with a thickness of five monolayers over an entire one-centimeter-square sapphire substrate.
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  • 30
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    American Institute of Physics (AIP)
    Publication Date: 2016-03-16
    Description: The molecular beam epitaxy (MBE) technique is well known for producing atomically smooth thin films as well as impeccable interfaces in multilayers of many different materials. In particular, molecular beam epitaxy is well suited to the growth of complex oxides, materials that hold promise for many applications. Rapid synthesis and high throughput characterization techniques are needed to tap into that potential most efficiently. We discuss our approach to doing that, leaving behind the traditional one-growth-one-compound scheme and instead implementing combinatorial oxide molecular beam epitaxy in a custom built system.
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  • 31
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    American Institute of Physics (AIP)
    Publication Date: 2016-03-16
    Description: Polymer nanocomposites are a designer class of materials where nanoscale particles, functional chemistry, and polymer resin combine to provide materials with unprecedented combinations of physical properties. In this paper, we introduce NanoMine, a data-driven web-based platform for analysis and design of polymer nanocomposite systems under the material genome concept. This open data resource strives to curate experimental and computational data on nanocomposite processing, structure, and properties, as well as to provide analysis and modeling tools that leverage curated data for material property prediction and design. With a continuously expanding dataset and toolkit, NanoMine encourages community feedback and input to construct a sustainable infrastructure that benefits nanocomposite material research and development.
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  • 32
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    American Institute of Physics (AIP)
    Publication Date: 2016-03-16
    Description: Soft matter embodies a wide range of materials, which all share the common characteristics of weak interaction energies determining their supramolecular structure. This complicates structure-property predictions and hampers the direct application of data-driven approaches to their modeling. We present several aspects in which these methods play a role in designing soft-matter materials: drug design as well as information-driven computer simulations, e.g., histogram reweighting. We also discuss recent examples of rational design of soft-matter materials fostered by physical insight and assisted by data-driven approaches. We foresee the combination of data-driven and physical approaches a promising strategy to move the field forward.
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  • 33
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    American Institute of Physics (AIP)
    Publication Date: 2016-03-16
    Description: We investigate spin relaxation in graphene by systematically comparing the roles of spin absorption, other contact-induced effects (e.g., fringe fields), and bulk spin relaxation for graphene spin valves with MgO barriers, Al 2 O 3 barriers, and transparent contacts. We obtain effective spin lifetimes by fitting the Hanle spin precession data with two models that include or exclude the effect of spin absorption. Results indicate that additional contact-induced spin relaxation other than spin absorption dominates the contact effect. For tunneling contacts, we find reasonable agreement between the two models with median discrepancy of ∼20% for MgO and ∼10% for Al 2 O 3 .
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  • 34
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    American Institute of Physics (AIP)
    Publication Date: 2016-03-16
    Description: Organic-inorganic halide perovskites, especially methylammonium lead halide, have recently led to remarkable advances in photovoltaic devices. However, due to environmental and stability concerns around the use of lead, research into lead-free perovskite structures has been attracting increasing attention. In this study, a layered perovskite-like architecture, (NH 4 ) 3 Bi 2 I 9 , is prepared from solution and the structure solved by single crystal X-ray diffraction. The band gap, which is estimated to be 2.04 eV using UV-visible spectroscopy, is lower than that of CH 3 NH 3 PbBr 3 . The energy-minimized structure obtained from first principles calculations is in excellent agreement with the X-ray results and establishes the locations of the hydrogen atoms. The calculations also point to a significant lone pair effect on the bismuth ion. Single crystal and powder conductivity measurements are performed to examine the potential application of (NH 4 ) 3 Bi 2 I 9 as an alternative to the lead containing perovskites.
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  • 35
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    American Institute of Physics (AIP)
    Publication Date: 2016-04-08
    Description: A key element of materials discovery and design is to learn from available data and prior knowledge to guide the next experiments or calculations in order to focus in on materials with targeted properties. We suggest that the tight coupling and feedback between experiments, theory and informatics demands a codesign approach, very reminiscent of computational codesign involving software and hardware in computer science. This requires dealing with a constrained optimization problem in which uncertainties are used to adaptively explore and exploit the predictions of a surrogate model to search the vast high dimensional space where the desired material may be found.
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  • 36
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    American Institute of Physics (AIP)
    Publication Date: 2016-04-08
    Description: The underlying mechanism behind the blue/red color-switchable luminescence in the C 8 carbon quantum dots (CQDs)/organic hybrid light-emitting devices (LEDs) is investigated. The study shows that the increasing bias alters the energy-level spatial distribution and reduces the carrier potential barrier at the CQDs/organic layer interface, resulting in transition of the carrier transport mechanism from quantum tunneling to direct injection. This causes spatial shift of carrier recombination from the organic layer to the CQDs layer with resultant transition of electroluminescence from blue to red. By contrast, the pure CQDs-based LED exhibits green–red electroluminescence stemming from recombination of injected carriers in the CQDs.
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  • 37
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    American Institute of Physics (AIP)
    Publication Date: 2016-04-08
    Description: Mn 0.05 Ge 0.95 quantum dots (QDs) samples were grown by molecular beam epitaxy on Si substrates and 15-nm-thick fully strained Si 0.8 Ge 0.2 virtual substrates, respectively. The QDs samples grown on the Si 0.8 Ge 0.2 virtual substrates show a significant ferromagnetism with a Curie temperature of 227 K, while the QDs samples grown on the Si substrates are non-ferromagnetic. Microstructures of the QDs samples were characterized by high resolution transmission electron microscopy and synchrotron radiation X-ray diffraction. Interdependence between microstructure and ferromagnetism of Mn-doped Ge QDs was investigated. For the QDs sample grown on the strained Si 0.8 Ge 0.2 virtual substrate, although the ferromagnetic phase Mn 5 Ge 3 clusters were found to be formed in small dome-shaped dots, the significant ferromagnetism observed in that sample is attributed to ferromagnetic phase Mn-doped large dome-shaped Ge QDs, rather than to the ferromagnetic phase Mn 5 Ge 3 clusters. The fully strained Si 0.8 Ge 0.2 virtual substrates would result in a residual strain into the QDs and an increase in Ge composition in the QDs. Both consequences favor the formations of ferromagnetic phase Mn-doped Ge QDs from points of view of quantum confinement effect as well as Mn doping at substitutional sites.
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  • 38
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    American Institute of Physics (AIP)
    Publication Date: 2016-04-14
    Description: In artificial multiferroics hybrids consisting of ferromagnetic La 0.7 Sr 0.3 MnO 3 (LSMO) and ferroelectric BaTiO 3 epitaxial layers, net Ti moments are found from polarized resonant soft x-ray reflectivity and absorption. The Ti dichroic reflectivity follows the Mn signal during the magnetization reversal, indicating exchange coupling between the Ti and Mn ions. However, the Ti dichroic reflectivity shows stronger temperature dependence than the Mn dichroic signal. Besides a reduced ferromagnetic exchange coupling in the interfacial LSMO layer, this may also be attributed to a weak Ti-Mn exchange coupling that is insufficient to overcome the thermal energy at elevated temperatures.
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  • 39
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    American Institute of Physics (AIP)
    Publication Date: 2016-11-05
    Description: Using atomistic simulations, several semi-coherent cube-on-cube bimetal interfaces are comparatively investigated to unravel the combined effect of the character of misfit dislocations, the stacking fault energy difference between bimetal pairs, and their lattice mismatch on the dissociation of interfacial misfit dislocations. Different dissociation paths and features under loadings provide several unique deformation mechanisms that are critical for understanding interface strengthening. In particular, applied strains can cause either the formation of global interface coherency by the migration of misfit dislocations from an interface to an adjoining crystal interior or to an alternate packing of stacking faults connected by stair-rod dislocations.
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  • 40
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    American Institute of Physics (AIP)
    Publication Date: 2016-11-08
    Description: In this work, combining experimental results and first principles calculations, we show that interstitial nitrogen not only serves for inducing tetragonality in α ′-Fe 8 N x but is also essential for achieving a high degree of perpendicular magneto-crystalline anisotropy, K . Our results demonstrate that the orbital magnetic moments of the iron atoms above and below N in the direction of magnetization are much more susceptible to the applied magnetic field than their in-plane counterparts, leading to a giant value of K as compared to a hypothetical distorted material without N.
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  • 41
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    American Institute of Physics (AIP)
    Publication Date: 2016-11-22
    Electronic ISSN: 2166-532X
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  • 42
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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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  • 43
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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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  • 44
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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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  • 45
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    American Institute of Physics (AIP)
    Publication Date: 2016-11-29
    Description: We report the fabrication of a set of new rare-earth-free magnetic compounds, which form the Fe 3 Co 3 Ti 2 -type hexagonal structure with P -6 m 2 symmetry. Neutron powder diffraction shows a significant Fe/Co anti-site mixing in the Fe 3 Co 3 Ti 2 structure, which has a strong effect on the magnetocrystalline anisotropy as revealed by first-principle calculations. Increasing substitution of Fe atoms for Co in the Fe 3 Co 3 Ti 2 lattice leads to the formation of Fe 4 Co 2 Ti 2 , Fe 5 CoTi, and Fe 6 Ti 2 with significantly improved permanent-magnet properties. A high magnetic anisotropy (13.0 Mergs/cm 3 ) and saturation magnetic polarization (11.4 kG) are achieved at 10 K by altering the atomic arrangements and decreasing Fe/Co occupancy disorder.
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  • 46
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    American Institute of Physics (AIP)
    Publication Date: 2016-04-16
    Description: Our ability to collect “big data” has greatly surpassed our capability to analyze it, underscoring the emergence of the fourth paradigm of science, which is data-driven discovery. The need for data informatics is also emphasized by the Materials Genome Initiative (MGI), further boosting the emerging field of materials informatics. In this article, we look at how data-driven techniques are playing a big role in deciphering processing-structure-property-performance relationships in materials, with illustrative examples of both forward models (property prediction) and inverse models (materials discovery). Such analytics can significantly reduce time-to-insight and accelerate cost-effective materials discovery, which is the goal of MGI.
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  • 47
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    American Institute of Physics (AIP)
    Publication Date: 2016-04-29
    Description: The creation of new materials “by design” is a process that starts from desired materials properties and proceeds to identify requirements for the constituent components. Such process is challenging because it inverts the typical modeling approach, which starts from given micro-level components to predict macro-level properties. We describe how to tackle this inverse problem using concepts from evolutionary computation. These concepts have widespread applicability and open up new opportunities for design as well as discovery. Here we apply them to design tasks involving two very different classes of soft materials, shape-optimized granular media and nanopatterned block copolymer thin films.
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  • 48
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    American Institute of Physics (AIP)
    Publication Date: 2016-04-29
    Description: The global trend of miniaturization and concomitant increase of functionality in microelectronics, microoptics, and various other fields in microtechnology leads to an emerging demand for temperature control at small scales. In this realm, elastocaloric cooling is an interesting alternative to thermoelectrics due to the large latent heat and good down-scaling behavior. Here, we investigate the elastocaloric effect due to a stress-induced phase transformation in binary TiNi and quaternary TiNiCuCo films of 20 μ m thickness produced by DC magnetron sputtering. The mesoscale mechanical and thermal performance, as well as the fatigue behavior are studied by uniaxial tensile tests combined with infrared thermography and digital image correlation measurements. Binary films exhibit strong features of fatigue, involving a transition from Lüders-like to homogeneous transformation behavior within three superelastic cycles. Quaternary films, in contrast, show stable Lüders-like transformation without any signs of degradation. The elastocaloric temperature change under adiabatic conditions is −15 K and −12 K for TiNi and TiNiCuCo films, respectively. First-of-its-kind heat pump demonstrators are developed that make use of out-of-plane deflection of film bridges. Owing to their large surface-to-volume ratio, the demonstrators reveal rapid heat transfer. The TiNiCuCo-based devices, for instance, generate a temperature difference of 3.5 K within 13 s. The coefficients of performance of the demonstrators are about 3.
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  • 49
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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.
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  • 50
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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.
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  • 51
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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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  • 52
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    American Institute of Physics (AIP)
    Publication Date: 2016-05-19
    Description: To experimentally (dis)prove ferroelectric effects on the properties of lead-halide perovskites and of solar cells, based on them, we used second-harmonic-generation spectroscopy and the periodic temperature change (Chynoweth) technique to detect the polar nature of methylammonium lead bromide (MAPbBr 3 ). We find that MAPbBr 3 is probably centrosymmetric and definitely non-polar; thus, it cannot be ferroelectric. Whenever pyroelectric-like signals were detected, they could be shown to be due to trapped charges, likely at the interface between the metal electrode and the MAPbBr 3 semiconductor. These results indicate that the ferroelectric effects do not affect steady-state performance of MAPbBr 3 solar cells.
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  • 53
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    American Institute of Physics (AIP)
    Publication Date: 2016-03-23
    Description: The magnetic and transport properties of Fe 3 O 4 films with a series of thicknesses are investigated. For the films with thickness below 15 nm, the saturation magnetization (M s ) increases and the coercivity decreases with the decrease in films’ thickness. The M s of 3 nm Fe 3 O 4 film is dramatically increased to 1017 emu/cm 3 . As for films’ thickness more than 15 nm, M s is tending to be close to the Fe 3 O 4 bulk value. Furthermore, the Verwey transition temperature (T v ) is visible for all the films, but suppressed for 3 nm film. We also find that the ρ of 3 nm film is the highest of all the films. The suppressed T v and high ρ may be related to the islands morphology in 3 nm film. To study the structure, magnetic, and transport properties of the Fe 3 O 4 films, we propose that the giant magnetic moment most likely comes from the spin of Fe ions in the tetrahedron site switching parallel to the Fe ions in the octahedron site at the surface, interface, and grain boundaries. The above results are of great significance and also provide a promising future for either device applications or fundamental research.
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  • 54
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    American Institute of Physics (AIP)
    Publication Date: 2016-03-29
    Description: Two structural phase transitions are investigated in highly strained BiFeO 3 thin films as a function of film thickness and temperature via synchrotron x-ray diffraction. Both transition temperatures (upon heating: monoclinic M C to monoclinic M A to tetragonal) decrease as the film becomes thinner. A film-substrate interface layer, evidenced by half-order peaks, contributes to this behavior, but at larger thicknesses (above a few nanometers), the temperature dependence results from electrostatic considerations akin to size effects in ferroelectric phase transitions, but observed here for structural phase transitions within the ferroelectric phase. For ultra-thin films, the tetragonal structure is stable to low temperatures.
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  • 55
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    American Institute of Physics (AIP)
    Publication Date: 2016-04-01
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  • 56
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    American Institute of Physics (AIP)
    Publication Date: 2016-04-12
    Description: Single crystals of Mott-Hubbard insulator LaVO 3 exhibit spin and orbital ordering along with a structural change below ≈140 K. The occurrence of orbital ordering in epitaxial LaVO 3 films has, however, been little investigated. By temperature-dependent Raman scattering spectroscopy, we probed and evidenced the transition to orbital ordering in epitaxial LaVO 3 film samples fabricated by pulsed-laser deposition. This opens up the possibility to explore the influence of different epitaxial strain (compressive vs . tensile) and of epitaxy-induced distortions of oxygen octahedra on the orbital ordering, in epitaxial perovskite vanadate films.
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  • 57
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    American Institute of Physics (AIP)
    Publication Date: 2016-04-12
    Description: The process of taking a new material from invention to deployment can take 20 years or more. Since the announcement of the Materials Genome Initiative in 2011, new attention has been paid to accelerating this timeframe to address key challenges in industries from energy, to biomedical materials, to catalysis, to polymers, particularly in the development of new materials discovery techniques. Materials informatics, or algorithmically analyzing materials data at scale to gain novel insight, has been lauded as a path forward in this regard. An equal challenge to discovery, however, is the acceleration from discovery to market. In this paper, we address application of an informatics approach to materials selection, manufacturing, and qualification and identify key opportunities and challenges in each of these areas with a focus on reducing time to market for new advanced materials technologies.
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  • 58
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    American Institute of Physics (AIP)
    Publication Date: 2016-04-22
    Description: Near infrared light emitting nanocrystals are known to lose efficiency when embedded in a polymer matrix. One of the factors leading to reduced efficiency is the labile nature of the ligands that may desorb off the nanocrystal surface when the nanocrystals are in the polymer solution. We show that adding trioctylphosphine to the nanocrystal-poly(methylmethacrylate) solution prior to film casting enhances the photoluminescence efficiency. The solid films’ photoluminescence quantum efficiency values are reduced by less than a factor of two in the solid form compared to the solution case. We demonstrate record efficiency values of 25% for lead sulfide nanocrystals solid films emitting at 1100 nm.
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  • 59
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    American Institute of Physics (AIP)
    Publication Date: 2016-04-26
    Description: This review focuses on the application of triblock copolymers as designed templates to synthesize nanoporous materials with various compositions. Asymmetric triblock copolymers have several advantages compared with symmetric triblock copolymers and diblock copolymers, because the presence of three distinct domains can provide more functional features to direct the resultant nanoporous materials. Here we clearly describe significant contributions of asymmetric triblock copolymers, especially polystyrene- block -poly(2-vinylpyridine)- block -poly(ethylene oxide) (abbreviated as PS- b -P2VP- b -PEO).
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  • 60
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    American Institute of Physics (AIP)
    Publication Date: 2016-11-09
    Description: We present a first evaluation of the potential for spin crossover (SCO) compounds to be considered as a new class of giant mechanocaloric effect materials. From literature data on the variation of the spin crossover temperature with pressure, we estimate the maximum available adiabatic temperature change for several compounds and the relatively low pressures that may be required to observe these effects.
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  • 61
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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.
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  • 62
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    American Institute of Physics (AIP)
    Publication Date: 2016-05-19
    Description: A multilayer capacitor comprising 19 layers of 38 μ m-thick 0.9Pb(Mg 1/3 Nb 2/3 )O 3 –0.1PbTiO 3 has elsewhere been shown to display electrocaloric temperature changes of 2.2 K due to field changes of 24 V μ m −1 , near ∼100 °C. Here we demonstrate temperature changes of 1.2 K in an equivalent device with 2.6 times the thermal mass, i.e., 49 layers that could tolerate 10.3 V μ m −1 . Breakdown was compromised by the increased number of layers, and occurred at 10.5 V μ m −1 near the edge of a near-surface inner electrode. Further optimization is required to improve the breakdown strength of large electrocaloric multilayer capacitors for cooling applications.
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  • 63
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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.
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  • 64
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    American Institute of Physics (AIP)
    Publication Date: 2016-05-21
    Description: In this letter, we report a NiAl buffer layer as a template for the integration of epitaxial current-perpendicular-plane-giant magnetoresistive (CPP-GMR) devices on a Si(001) single crystalline substrate. By depositing NiAl on a Si wafer at an elevated temperature of 500 °C, a smooth and epitaxial B 2-type NiAl(001) layer was obtained. The surface roughness was further improved by depositing Ag on the NiAl layer and applying subsequent annealing process. The epitaxial CPP-GMR devices grown on the buffered Si(001) substrate present a large magnetoresistive output comparable with that of the devices grown on an MgO(001) substrate, demonstrating the possibility of epitaxial spintronic devices with a NiAl templated Si wafer for practical applications.
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  • 65
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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.
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  • 66
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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.
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  • 67
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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.
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  • 68
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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.
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  • 69
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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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  • 70
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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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  • 71
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    American Institute of Physics (AIP)
    Publication Date: 2016-06-09
    Description: Pulsed laser deposited thin Y Ba 2 Cu 3 O 7−x (YBCO) films with pinning additions of 5 at. % Ba 2 Y TaO 6 (BYTO) were compared to films with 2.5 at. % Ba 2 Y TaO 6 + 2.5 at. % Ba 2 Y NbO 6 (BYNTO) additions. Excellent magnetic flux-pinning at 77 K was obtained with remarkably high irreversibility fields greater than 10 T (YBCO-BYTO) and 11 T (YBCO-BYNTO), representing the highest ever achieved values in YBCO films.
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  • 72
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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.
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  • 73
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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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  • 74
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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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  • 75
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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.
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  • 76
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    American Institute of Physics (AIP)
    Publication Date: 2016-08-09
    Description: Sputter deposition, while varying the substrate temperature, is employed to isochemically control the structural state and concomitant mechanical response in a Pd-based metallic glass at the time of glass formation. Increasing the deposition temperature from 333 K to 461 K results in a 33.5% increase in hardness to 9.69 GPa for amorphous films. Further increasing the temperature leads to a decrease in hardness, indicating low and high temperature deposition regimes where increased surface mobility allows access to a more relaxed and more rejuvenated structure, respectively. Through this mechanism we access the range of achievable structural states, from ultrastable to highly liquid-like glasses.
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  • 77
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    American Institute of Physics (AIP)
    Publication Date: 2016-08-16
    Description: We demonstrate crack-free ZnO/GaN distributed Bragg reflectors (DBRs) grown by hybrid plasma-assisted molecular beam epitaxy using the same growth chamber for continuous growth of both ZnO and GaN without exposure to air. This is the first time these ZnO/GaN DBRs have been demonstrated. The Bragg reflectors consisted up to 20 periods as shown with cross-sectional transmission electron microscopy. The maximum achieved reflectance was 77% with a 32 nm wide stopband centered at 500 nm. Growth along both (0001) and (000 1 ̄ ) directions was investigated. Low-temperature growth as well as two-step low/high-temperature deposition was carried out where the latter method improved the DBR reflectance. Samples grown along the (0001) direction yielded a better surface morphology as revealed by scanning electron microscopy and atomic force microscopy. Reciprocal space maps showed that ZnO(000 1 ̄ )/GaN reflectors are relaxed whereas the ZnO(0001)/GaN DBRs are strained. The ability to n-type dope ZnO and GaN makes the ZnO(0001)/GaN DBRs interesting for various optoelectronic cavity structures.
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  • 78
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    American Institute of Physics (AIP)
    Publication Date: 2016-08-19
    Description: We have investigated the tunneling transport of mono- and few-layers of MnPS 3 by using conductive atomic force microscopy. Due to the band alignment of indium tin oxide/MnPS 3 /Pt-Ir tip junction, the key features of both Schottky junction and Fowler-Nordheim tunneling (FNT) were observed for all the samples with varying thickness. Using the FNT model and assuming the effective electron mass (0.5 m e ) of MnPS 3 , we estimate the tunneling barrier height to be 1.31 eV and the dielectric breakdown strength as 5.41 MV/cm.
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  • 79
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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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  • 80
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    American Institute of Physics (AIP)
    Publication Date: 2016-08-23
    Description: Pb 0.9 La 0.1 (Zr 0.52 Ti 0.48 )O 3 (PLZT) relaxor-ferroelectric thin films were grown on SrRuO 3 /SrTiO 3 /Si substrates by pulsed laser deposition. A large recoverable storage density ( U reco ) of 13.7 J/cm 3 together with a high energy efficiency ( η ) of 88.2% under an applied electric field of 1000 kV/cm and at 1 kHz frequency was obtained in 300-nm-thick epitaxial PLZT thin films. These high values are due to the slim and asymmetric hysteresis loop when compared to the values in the reference undoped epitaxial lead zirconate titanate Pb(Zr 0.52 Ti 0.48 )O 3 ferroelectric thin films ( U reco = 9.2 J/cm 3 and η = 56.4%) which have a high remanent polarization and a small shift in the hysteresis loop, under the same electric field.
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  • 81
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    American Institute of Physics (AIP)
    Publication Date: 2016-08-24
    Description: Nanostructured Mn 3 Ge ribbons with a composition ranging from 77 to 74 at.% Mn were prepared using induction melting, melt-spinning, and subsequent heat treatment. The hard magnetic properties of the ribbons originate from the highly anisotropic tetragonal D 0 22 structure of Mn 3 Ge. Depending on the composition and the amount of ferrimagnetic Mn 5 Ge 2 as a secondary phase, a coercivity of up to μ 0 H C = 2.62 T was obtained for the Mn 3 Ge ribbons. Microstructure investigations by transmission electron microscopy confirmed the formation of the secondary phase. All samples show isotropic coercive fields, i.e., independent of the direction of the applied magnetic field in contrast to already known epitaxial thin films. The Curie temperature was found to be higher than 800 K, which is the temperature of the phase transition from the tetragonal D 0 22 structure to the hexagonal D 0 19 structure.
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  • 82
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    American Institute of Physics (AIP)
    Publication Date: 2016-08-26
    Description: The power-conversion efficiency of perovskite solar cells has soared up to 22.1% earlier this year. Within merely five years, the perovskite solar cell can now compete on efficiency with inorganic thin-film technologies, making it the most promising of the new, emerging photovoltaic solar cell technologies. The next grand challenge is now the aspect of stability. The hydrophilicity and volatility of the organic methylammonium makes the work-horse material methylammonium lead iodide vulnerable to degradation through humidity and heat. Additionally, ultraviolet radiation and oxygen constitute stressors which can deteriorate the device performance. There are two fundamental strategies to increasing the device stability: developing protective layers around the vulnerable perovskite absorber and developing a more resilient perovskite absorber. The most important reports in literature are summarized and analyzed here, letting us conclude that any long-term stability, on par with that of inorganic thin-film technologies, is only possible with a more resilient perovskite incorporated in a highly protective device design.
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  • 83
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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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  • 84
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    American Institute of Physics (AIP)
    Publication Date: 2016-09-28
    Description: We take advantage of scanning transmission electron microscopy and electron energy loss spectroscopy to investigate the changes in near-surface electronic structure and quantify the degree of local degradation of Ni-based cathode materials with the layered structure (LiNi 0.8 Mn 0.1 Co 0.1 O 2 and LiNi 0.4 Mn 0.3 Co 0.3 O 2 ) after 20 cycles of delithiation and lithiation. Reduction of transition metals occurs in the near-surface region of cathode materials: Mn is the major element to be reduced in the case of relatively Mn-rich composition, while reduction of Ni ions is dominant in Ni-rich materials. The valences of Ni and Mn ions are complementary, i.e., when one is reduced, the other is oxidized in order to maintain charge neutrality. The depth of degradation zone is found to be much deeper in Ni-rich materials. This comparative analysis provides important insights needed for the devising of new cathode materials with high capacity as well as long lifetime.
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  • 85
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    American Institute of Physics (AIP)
    Publication Date: 2016-09-28
    Description: Over the last few years, tremendous progress has been made in the research field of perovskite solar cells. Not only are record power conversion efficiencies now exceeding 20%, but our understanding about the different mechanisms leading to this extraordinary performance has improved phenomenally. The aim of this special issue is to review the current state-of-the-art understanding of perovskite solar cells. Most of the presented articles are research updates giving a succinct overview over different aspects concerning perovskite solar cells.
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  • 86
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    American Institute of Physics (AIP)
    Publication Date: 2016-10-04
    Description: First order magnetocaloric materials reach high entropy changes but at the same time exhibit hysteresis losses which depend on the sample’s microstructure. We use non-destructive 3D X-ray microtomography to understand the role of surface morphology for the magnetovolume transition of LaFe 11.8 Si 1.2 . The technique provides unique information on the spatial distribution of the volume change at the transition and its relationship with the surface morphology. Complementary Hall probe imaging confirms that on a morphologically complex surface minimization of strain energy dominates. Our findings sketch the way for a tailored surface morphology with low hysteresis without changing the underlying phase transition.
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  • 87
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    American Institute of Physics (AIP)
    Publication Date: 2016-10-04
    Description: The power conversion efficiency of lead halide perovskite solar cells recently surpassed 22.1%. In this study, we suggest the perovskite absorber growth mechanism of the two-step process could be explained by an Ostwald ripening growth model for planar-structure perovskite solar cells. We attempt to find out the source of two main problems such as unreacted PbI 2 and non-uniformed morphology by the proposed ripening growth mechanism and experimental results. This growth mechanism opens the way toward understanding a key aspect of the photovoltaic operation of high-efficiency, two-step perovskite solar cells.
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  • 88
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    American Institute of Physics (AIP)
    Publication Date: 2016-10-05