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  • 1
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    American Institute of Physics (AIP)
    Publication Date: 2016-10-28
    Electronic ISSN: 2166-532X
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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  • 2
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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)].
    Electronic ISSN: 2166-532X
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  • 3
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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.
    Electronic ISSN: 2166-532X
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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  • 4
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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.
    Electronic ISSN: 2166-532X
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  • 5
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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.
    Electronic ISSN: 2166-532X
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  • 6
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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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  • 7
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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.
    Electronic ISSN: 2166-532X
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  • 8
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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.
    Electronic ISSN: 2166-532X
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  • 9
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    American Institute of Physics (AIP)
    Publication Date: 2016-12-07
    Description: The structure and luminescence mechanisms of the CuInS 2 quantum dots (QDs) after epitaxial growth of ZnS shell are in debate. The light absorption/emission spectroscopy reveals that after ZnS shell growth the cation diffusion at the CuInS 2 /ZnS interface results in formation of the alloyed Cu x Zn 1− x InS 2 /ZnS:Cu QDs. These core/shell QDs exhibit dual-color photoluminescence with abnormal blue shift with decreasing excitation photon energy. The results show that the green and orange emissions originate separately from defects in the core and the shell. The absorption tail of the ZnS QDs turns from Urbach to Halperin-Lax type after Cu doping.
    Electronic ISSN: 2166-532X
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  • 10
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    American Institute of Physics (AIP)
    Publication Date: 2016-12-08
    Description: We record the two-dimensional laser-induced fluorescence (LIF) on multiple plasma constituents in a YBiO 3 plasma. This allows us to directly link the influence of oxygen present in the background gas during pulsed laser deposition to the oxidation of plasma species as well as the formation of epitaxial YBiO 3 films. With spatiotemporal LIF mapping of the plasma species (Y, YO, Bi, and BiO) in different background gas compositions, we find that little direct chemical interaction takes place between the plasma plume constituents and the background gas. However, a strong influence of the background gas composition can be seen on the YBO film growth, as well as a strong correlation between the oxygen fraction in the background gas and the amount of YO in the plasma plume. We assign this correlation to a direct interaction between the background gas and the target in between ablation pulses. In an O 2 background, an oxygen-rich surface layer forms in between ablation pulses, which provides additional oxygen for the plasma plume during target ablation. This differs from our previous observations in STO and LAO plasmas, where species oxidation primarily takes place during propagation of the plasma plume towards the substrate.
    Electronic ISSN: 2166-532X
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  • 11
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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.
    Electronic ISSN: 2166-532X
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  • 12
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    American Institute of Physics (AIP)
    Publication Date: 2016-11-22
    Electronic ISSN: 2166-532X
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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  • 13
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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.
    Electronic ISSN: 2166-532X
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  • 14
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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.
    Electronic ISSN: 2166-532X
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  • 15
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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.
    Electronic ISSN: 2166-532X
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  • 16
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    American Institute of Physics (AIP)
    Publication Date: 2016-01-05
    Description: Transport characteristics of TiN/Ta/TaO x /TiN resistive-switching crossbar devices with amorphous TaO x functional layer have been investigated at cryogenic temperatures. Quasi-DC I - V characteristics at 10 K show a negative differential resistance region followed by a rapid transition to the non-volatile formed state. Accounting for Joule heating, the device temperature at the point of switching was estimated at 150 K. Measurements of transient resistance at low stage temperatures revealed an abrupt drop of resistance delayed by a characteristic incubation time after the leading edge of the voltage pulse. The incubation time was a strong function of applied voltage but did not depend on temperature between 10 K and 100 K. This implies a very low activation energy of the threshold switching process at low temperatures. Both of these observations argue against the involvement of oxygen vacancy motion at the onset of the forming process.
    Electronic ISSN: 2166-532X
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  • 17
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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.
    Electronic ISSN: 2166-532X
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  • 18
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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 .
    Electronic ISSN: 2166-532X
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  • 19
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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.
    Electronic ISSN: 2166-532X
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  • 20
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    American Institute of Physics (AIP)
    Publication Date: 2016-03-25
    Description: Materials innovations enable new technological capabilities and drive major societal advancements but have historically required long and costly development cycles. The Materials Genome Initiative (MGI) aims to greatly reduce this time and cost. In this paper, we focus on data reuse in the MGI and, in particular, discuss the impact of three different computational databases based on density functional theory methods to the research community. We also discuss and provide recommendations on technical aspects of data reuse, outline remaining fundamental challenges, and present an outlook on the future of MGI’s vision of data sharing.
    Electronic ISSN: 2166-532X
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  • 21
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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.
    Electronic ISSN: 2166-532X
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  • 22
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    American Institute of Physics (AIP)
    Publication Date: 2016-04-08
    Description: Metal organic chemical vapor deposition of GaAs on standard nominal 300 mm Si(001) wafers was studied. Antiphase boundary (APB) free epitaxial GaAs films as thin as 150 nm were obtained. The APB-free films exhibit an improvement of the room temperature photoluminescence signal with an increase of the intensity of almost a factor 2.5. Hall effect measurements show an electron mobility enhancement from 200 to 2000 cm 2 /V s. The GaAs layers directly grown on industrial platform with no APBs are perfect candidates for being integrated as active layers for nanoelectronic as well as optoelectronic devices in a CMOS environment.
    Electronic ISSN: 2166-532X
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  • 23
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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.
    Electronic ISSN: 2166-532X
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  • 24
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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.
    Electronic ISSN: 2166-532X
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  • 25
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    American Institute of Physics (AIP)
    Publication Date: 2016-02-02
    Description: The magnetocaloric effect in ferromagnetic single crystal EuTi 0.85 Nb 0.15 O 3 has been investigated using magnetization and heat capacity measurements. EuTi 0.85 Nb 0.15 O 3 undergoes a continuous ferromagnetic phase transition at T C = 9.5 K due to the long range ordering of magnetic moments of Eu 2+ (4 f 7 ). With the application of magnetic field, the spin entropy is strongly suppressed and a giant magnetic entropy change is observed near T C . The values of entropy change Δ S m and adiabatic temperature change Δ T ad are as high as 51.3 J kg −1 K −1 and 22 K, respectively, for a field change of 0–9 T. The corresponding magnetic heating/cooling capacity is 700 J kg −1 . This compound also shows large magnetocaloric effect even at low magnetic fields. In particular, the values of Δ S m reach 14.7 and 23.8 J kg −1 K −1 for field changes of 0–1 T and 0–2 T, respectively. The low-field giant magnetocaloric effect, together with the absence of thermal and field hysteresis makes EuTi 0.85 Nb 0.15 O 3 a very promising candidate for low temperature magnetic refrigeration.
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  • 26
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    American Institute of Physics (AIP)
    Publication Date: 2016-02-13
    Description: We discuss and present search strategies for finding new thermoelectric compositions based on first principles electronic structure and transport calculations. We illustrate them by application to a search for potential n -type oxide thermoelectric materials. This includes a screen based on visualization of electronic energy isosurfaces. We report compounds that show potential as thermoelectric materials along with detailed properties, including SrTiO 3 , which is a known thermoelectric, and appropriately doped KNbO 3 and rutile TiO 2 .
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  • 27
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    American Institute of Physics (AIP)
    Publication Date: 2016-02-25
    Description: Electric field control of magnetization and anisotropy in layered structures with perpendicular magnetic anisotropy is expected to increase the versatility of spintronic devices. As a model system for reversible voltage induced changes of magnetism by magnetoionic effects, we present several oxide/metal heterostructures polarized in an electrolyte. Room temperature magnetization of Fe-O/Fe layers can be changed by 64% when applying only a few volts in 1M KOH. In a next step, the bottom interface of the in-plane magnetized Fe layer is functionalized by an L1 0 FePt(001) underlayer exhibiting perpendicular magnetic anisotropy. During subsequent electrocrystallization and electrooxidation, well defined epitaxial Fe 3 O 4 /Fe/FePt heterostructures evolve. The application of different voltages leads to a thickness change of the Fe layer sandwiched between Fe-O and FePt. At the point of transition between rigid magnet and exchange spring magnet regime for the Fe/FePt bilayer, this induces a large variation of magnetic anisotropy.
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  • 28
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    American Institute of Physics (AIP)
    Publication Date: 2016-03-02
    Description: We report the experimental observation of longitudinal spin Seebeck effect in a multiferroic helimagnet Ba 0.5 Sr 1.5 Zn 2 Fe 12 O 22 . Temperature gradient applied normal to Ba 0.5 Sr 1.5 Zn 2 Fe 12 O 22 /Pt interface generates inverse spin Hall voltage of spin current origin in Pt, whose magnitude was found to be proportional to bulk magnetization of Ba 0.5 Sr 1.5 Zn 2 Fe 12 O 22 even through the successive magnetic transitions among various helimagnetic and ferrimagnetic phases. This finding demonstrates that the helimagnetic spin wave can be an effective carrier of spin current. By controlling the population ratio of spin-helicity domains characterized by clockwise/counter-clockwise manner of spin rotation with use of poling electric field in the ferroelectric helimagnetic phase, we found that spin-helicity domain distribution does not affect the magnitude of spin current injected into Pt. The results suggest that the spin-wave spin current is rather robust against the spin-helicity domain wall, unlike the case with the conventional ferromagnetic domain wall.
    Electronic ISSN: 2166-532X
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  • 29
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    American Institute of Physics (AIP)
    Publication Date: 2016-01-05
    Description: We report an in situ microbeam grazing incidence X-ray scattering study of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C 8 -BTBT) organic semiconductor thin film deposition by hollow pen writing. Multiple transient phases are observed during the crystallization for substrate temperatures up to ≈93 °C. The layered smectic liquid-crystalline phase of C 8 -BTBT initially forms and preceedes inter-layer ordering, followed by a transient crystalline phase for temperature 〉60 °C, and ultimately the stable phase. Based on these results, we demonstrate a method to produce extremely large grain size and high carrier mobility during high-speed processing. For high writing speed (25 mm/s), mobility up to 3.0 cm 2 /V-s has been observed.
    Electronic ISSN: 2166-532X
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  • 30
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    American Institute of Physics (AIP)
    Publication Date: 2016-01-05
    Description: For use in high-magnetic-field coil-based applications, the critical current density ( J c ) of REBa 2 Cu 3 O y (REBCO, where RE = rare earth) coated conductors must be isotropically improved, with respect to the direction of the magnetic field; these improvements must be realized at the operating conditions of these applications. In this study, improvement of the J c for various applied directions of magnetic field was achieved by controlling the morphology of the BaHfO 3 (BHO) nano-rods in a SmBCO film. We fabricated the 3.0 vol. % BHO-doped SmBCO film at a low growth temperature of 720 °C, by using a seed layer technique ( T s = 720 °C film). The low-temperature growth resulted in a morphological change in the BHO nano-rods. In fact, a high number density of (3.1 ± 0.1) × 10 3 μ m −2 of small (diameter: 4 ± 1 nm), discontinuous nano-rods that grew in various directions, was obtained. In J c measurements, the J c of the T s = 720 °C film in all directions of the applied magnetic field was higher than that of the non-doped SmBCO film. The J c min (6.4 MA/cm 2 ) of the former was more than 6 times higher than that (1.0 MA/cm 2 ) of the latter at 40 K, under 3 T. The aforementioned results indicated that the discontinuous BHO nano-rods, which occurred with a high number density, exerted a 3D-like flux pinning at the measurement conditions considered. Moreover, at 4.2 K and under 17 T, a flux pinning force density of 1.6 TN/m 3 was realized; this value was comparable to the highest value recorded, to date.
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  • 31
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    American Institute of Physics (AIP)
    Publication Date: 2016-02-25
    Description: We discuss growth and magnetic properties of high-quality two dimensional (2D) Sn 1−x Mn x Se 2 films. Thin films of this 2D ternary alloy with a wide range of Mn concentrations were successfully grown by molecular beam epitaxy. Mn concentrations up to x ≈ 0.60 were achieved without destroying the crystal structure of the parent SnSe 2 2D system. Most important, the specimens show clear weak ferromagnetic behavior above room temperature, which should be of interest for 2D spintronic applications.
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  • 32
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    American Institute of Physics (AIP)
    Publication Date: 2016-03-02
    Description: The magnetoelectric effects are investigated in a cubic compound SrCuTe 2 O 6 , in which uniform Cu 2+ ( S = 1/2) spin chains with considerable spin frustration exhibit a concomitant antiferromagnetic transition and dielectric constant peak at T N ≈ 5.5 K. Pyroelectric J p ( T ) and magnetoelectric current J ME ( H ) measurements in the presence of a bias electric field are used to reveal that SrCuTe 2 O 6 shows clear variations of J p ( T ) across T N at constant magnetic fields. Furthermore, isothermal measurements of J ME ( H ) also develop clear peaks at finite magnetic fields, of which traces are consistent with the spin-flop transitions observed in the magnetization studies. As a result, the anomalies observed in J p ( T ) and J ME ( H ) curves match well with the field-temperature phase diagram constructed from magnetization and dielectric constant measurements, demonstrating that SrCuTe 2 O 6 is a new magnetoelectric compound with S = 1/2 spin chains.
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  • 33
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    American Institute of Physics (AIP)
    Publication Date: 2016-03-04
    Description: We observe current induced spin transfer torque resonance in permalloy (Py) grown on monolayer MoS 2 . By passing rf current through the Py/MoS 2 bilayer, field-like and damping-like torques are induced which excite the ferromagnetic resonance of Py. The signals are detected via a homodyne voltage from anisotropic magnetoresistance of Py. In comparison to other bilayer systems with strong spin-orbit torques, the monolayer MoS 2 cannot provide bulk spin Hall effects and thus indicates the purely interfacial nature of the spin transfer torques. Therefore our results indicate the potential of two-dimensional transition-metal dichalcogenide for the use of interfacial spin-orbitronics applications.
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  • 34
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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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  • 35
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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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  • 36
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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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  • 37
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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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  • 38
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    American Institute of Physics (AIP)
    Publication Date: 2016-02-12
    Description: GeTe has been predicted to be the father compound of a new class of multifunctional materials, ferroelectric Rashba semiconductors, displaying a coupling between spin-dependent k-splitting and ferroelectricity. In this paper, we report on epitaxial Fe/GeTe(111) heterostructures grown by molecular beam epitaxy. Spin-pumping experiments have been performed in a radio-frequency cavity by pumping a spin current from the Fe layer into GeTe at the Fe ferromagnetic resonance and detecting the transverse charge current originated in the slab due to spin-to-charge conversion. Preliminary experiments indicate that a clear spin to charge conversion exists, thus unveiling the potential of GeTe for spin-orbitronics.
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  • 39
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    American Institute of Physics (AIP)
    Publication Date: 2016-02-09
    Description: Structural and transport properties in the normal and superconducting states are investigated in a Ca 0.8 La 0.2 FeAs 2 single crystal with T c = 27 K, belonging to the newly discovered 112 family of iron based superconductors. The transport critical current density J c for both field directions measured in a focused ion beam patterned microbridge reveals a weakly field dependent and low anisotropic behaviour with a low temperature value as high as J c (B = 0) ∼ 10 5 A/cm 2 . This demonstrates not only bulk superconductivity but also the potential of 112 superconductors towards applications. Interestingly, this superconducting compound undergoes a structural transition below 100 K which is evidenced by temperature-dependent X-ray diffraction measurements. Data analysis of Hall resistance and magnetoresistivity indicate that magnetotransport properties are largely dominated by an electron band, with a change of regime observed in correspondence of the onset of a structural transition. In the low temperature regime, the contribution of a hole band to transport is suggested, possibly playing a role in determining the superconducting state.
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  • 40
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    American Institute of Physics (AIP)
    Publication Date: 2016-02-24
    Description: We discuss our philosophy for implementation of the Materials Genome Initiative through an integrated materials design strategy, exemplified here in the context of electrocatalytic capture and separation of CO 2 gas. We identify for a group of 1:1 X–N graphene analogue materials that electro-responsive switchable CO 2 binding behavior correlates with a change in the preferred binding site from N to the adjacent X atom as negative charge is introduced into the system. A reconsideration of conductive N-doped graphene yields the discovery that the N-dopant is able to induce electrocatalytic binding of multiple CO 2 molecules at the adjacent carbon sites.
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  • 41
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    American Institute of Physics (AIP)
    Publication Date: 2016-02-25
    Description: Solution processable semiconductor oxides have opened a new paradigm for the enhancement of the lifetime of thin film solar cells. Their fabrication by low-cost and environmentally friendly solution-processable methods makes them ideal barrier (hole and electron) transport layers. In this work, we fabricate flexible ITO-free organic solar cells (OPV) by printing methods applying an aqueous solution-processed V 2 O 5 as the hole transport layer (HTL) and compared them to devices applying PEDOT:PSS. The transparent conducting electrode was PET/Ag/PEDOT/ZnO, and the OPV configuration was PET/Ag/PEDOT/ZnO/P3HT:PC 60 BM/HTL/Ag. Outdoor stability analyses carried out for more than 900 h revealed higher stability for devices fabricated with the aqueous solution-processed V 2 O 5 .
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  • 42
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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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  • 43
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    American Institute of Physics (AIP)
    Publication Date: 2016-03-10
    Description: Magnetocaloric materials are promising as solid state refrigerants for more efficient and environmentally friendly cooling devices. The highest effects have been observed in materials that exhibit a first-order phase transition. These transformations proceed by nucleation and growth which lead to a hysteresis. Such irreversible processes are undesired since they heat up the material and reduce the efficiency of any cooling application. In this article, we demonstrate an approach to decrease the hysteresis by locally changing the nucleation barrier. We created artificial nucleation sites and analyzed the nucleation and growth processes in their proximity. We use Ni-Mn-Ga, a shape memory alloy that exhibits a martensitic transformation. Epitaxial films serve as a model system, but their high surface-to-volume ratio also allows for a fast heat transfer which is beneficial for a magnetocaloric regenerator geometry. Nanoindentation is used to create a well-defined defect. We quantify the austenite phase fraction in its proximity as a function of temperature which allows us to determine the influence of the defect on the transformation.
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  • 44
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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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  • 45
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    American Institute of Physics (AIP)
    Publication Date: 2016-03-17
    Description: Electrical manipulation of magnetism has been a long sought-after goal to realize energy-efficient spintronics. During the past decade, multiferroic materials combining (anti)ferromagnetic and ferroelectric properties are now drawing much attention and many reports have focused on magnetoelectric coupling effect through strain, charge, or exchange bias. This paper gives an overview of recent progress on electrical manipulation of magnetism through strain-mediated magnetoelectric coupling in multiferroic heterostructures.
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  • 46
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    American Institute of Physics (AIP)
    Publication Date: 2016-02-02
    Description: We report the epitaxial growth and the mechanism of a higher temperature insulator-to-metal-transition (IMT) of vanadium dioxide (VO 2 ) thin films synthesized on aluminum nitride (AlN)/Si (111) substrates by a pulsed-laser-deposition method; the IMT temperature is T IMT ≈ 350 K. X-ray diffractometer and high resolution transmission electron microscope data show that the epitaxial relationship of VO 2 and AlN is VO 2 (010) ‖ AlN (0001) with VO 2 [101] ‖   AlN   [ 2 1 ̄ 1 ̄ 0 ] zone axes, which results in a substrate-induced tensile strain along the in-plane a and c axes of the insulating monoclinic VO 2 . This strain stabilizes the insulating phase of VO 2 and raises T IMT for 10 K higher than T IMT single crystal ≈ 340 K in a bulk VO 2 single crystal. Near T IMT , a resistance change of about four orders is observed in a thick film of ∼130 nm. The VO 2 /AlN/Si heterostructures are promising for the development of integrated IMT-Si technology, including thermal switchers, transistors, and other applications.
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  • 47
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    American Institute of Physics (AIP)
    Publication Date: 2016-02-04
    Description: We report a direct conversion of hexagonal boron nitride (h-BN) into pure cubic boron nitride (c-BN) by nanosecond laser melting at ambient temperatures and atmospheric pressure in air. According to the phase diagram, the transformation from h-BN into c-BN can occur only at high temperatures and pressures, as the hBN-cBN-Liquid triple point is at 3500 K/9.5 GPa. Using nanosecond laser melting, we have created super undercooled state and shifted this triple point to as low as 2800 K and atmospheric pressure. The rapid quenching from super undercooled state leads to formation of super undercooled BN (Q-BN). The c-BN phase is nucleated from Q-BN depending upon the time allowed for nucleation and growth.
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  • 48
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    American Institute of Physics (AIP)
    Publication Date: 2016-02-13
    Description: Through empirical observations, sodium (Na) has been identified as a benign contaminant in some thin-film solar cells. Here, we intentionally contaminate thermally evaporated tin sulfide (SnS) thin-films with sodium and measure the SnS absorber properties and solar cell characteristics. The carrier concentration increases from 2 × 10 16 cm −3 to 4.3 × 10 17 cm −3 in Na-doped SnS thin-films, when using a 13 nm NaCl seed layer, which is detrimental for SnS photovoltaic applications but could make Na-doped SnS an attractive candidate in thermoelectrics. The observed trend in carrier concentration is in good agreement with density functional theory calculations, which predict an acceptor-type Na Sn defect with low formation energy.
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  • 49
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    American Institute of Physics (AIP)
    Publication Date: 2016-02-19
    Description: The past decade has been especially creative for spintronics since the (re)discovery of various two dimensional (2D) materials. Due to the unusual physical characteristics, 2D materials have provided new platforms to probe the spin interaction with other degrees of freedom for electrons, as well as to be used for novel spintronics applications. This review briefly presents the most important recent and ongoing research for spintronics in 2D materials.
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  • 50
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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.
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  • 51
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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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  • 52
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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.
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  • 53
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    American Institute of Physics (AIP)
    Publication Date: 2016-11-02
    Description: We report the first example of room-temperature rubber-like deformation in thin-film metallic glasses (TFMGs), 260-nm-thick Zr 60 Cu 24 Al 11 Ni 5 layers, under ultra-high shear strain. The TFMGs were deposited, with no external heating, on Zr-based bulk metallic glass (BMG) and Si(001) substrates by rf magnetron sputtering in a 3 mTorr Ar plasma. Cross-sectional transmission electron microscopy (XTEM) analyses and nanoindentation results reveal that the TFMGs undergo an incredibly large shear strain, estimated to be ∼4000%, during fatigue tests, and thickness reductions of up to 61.5%, with no shear-banding or cracking, during extreme nanoindentation experiments extending through the film and into the substrate. TFMG/BMG samples also exhibit film/substrate diffusion bonding during deformation as shown by high-resolution XTEM.
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  • 54
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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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  • 55
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    American Institute of Physics (AIP)
    Publication Date: 2016-11-11
    Description: We study the effect of self-assembled monolayer (SAM) treatment of the SiO 2 dielectric on the electrical characteristics of PbS transistors. Using SAMs, we observe threshold voltage shifts in the electron transport, allowing us to tune the electrical properties of the devices depending on the SAM molecule used. Moreover, the use of a specific SAM improves the charge carrier mobility in the devices by a factor of three, which is attributed to the reduced interface traps due to passivated silanol on the SiO 2 surface. These reduced traps confirm that the voltage shifts are not caused by the trap states induced by the SAMs.
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  • 56
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    American Institute of Physics (AIP)
    Publication Date: 2016-11-15
    Description: We investigate the electromechanical interactions in individual polyvinylidene fluoride-trifluoroethylene nanowires in response to localized electrical poling via a conducting atomic force microscope tip. Spatially resolved measurements of piezoelectric coefficients and elastic moduli before and after poling reveal a striking dependence on the polarity of the poling field, notably absent in thin films of the same composition. These observations are attributed to the unclamped nature of the nanowires and the inherent asymmetry in their chemical and electrical interactions with the tip and underlying substrate. Our findings provide insights into the mechanism of poling/switching in polymer nanowires critical to ferroelectric device performance.
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  • 57
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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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  • 58
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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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  • 59
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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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  • 60
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    American Institute of Physics (AIP)
    Publication Date: 2016-09-02
    Description: In this paper, an electrocaloric (EC) cooler prototype made of 150 ceramic-based Multi-Layer Capacitors (MLCs) has been detailed. This cooler involves a column of dielectric fluid where heat exchange with the MLCs takes place. The maximum variation of temperature in the fluid column due to the EC effect reaches 0.13 K whereas the heat exchanged during one stroke is 0.28 J. Although this prototype requires improvements with respect to heat exchange, the basic principle of creating a temperature gradient in a column of fluid has been validated.
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  • 61
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    American Institute of Physics (AIP)
    Publication Date: 2016-09-08
    Description: Efficiency and dynamics of radiative recombination of carriers are crucial figures of merit for optoelectronic materials. Following the recent success of lead halide perovskites in efficient photovoltaic and light emitting technologies, here we review some of the noted literature on the luminescence of this emerging class of materials. After outlining the theoretical formalism that is currently used to explain the carrier recombination dynamics, we review a few significant works which use photoluminescence as a tool to understand and optimize the operation of perovskite based optoelectronic devices.
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  • 62
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    American Institute of Physics (AIP)
    Publication Date: 2016-09-21
    Description: With the rapid progress in deposition techniques for hybrid organic-inorganic perovskite (HOIP) thin films, this new class of photovoltaic (PV) technology has achieved material quality and power conversion efficiency comparable to those established technologies. Among the various techniques for HOIP thin films preparation, vapor based deposition technique is considered as a promising alternative process to substitute solution spin-coating method for large-area or scale-up preparation. This technique provides some unique benefits for high-quality perovskite crystallization, which are discussed in this research update.
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  • 63
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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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  • 64
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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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  • 65
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    American Institute of Physics (AIP)
    Publication Date: 2016-10-05
    Description: The conventional mechanoluminescence (ML) mechanism of phosphors such as SrAl 2 O 4 :Eu and ZnS:Mn is known to utilize carrier trapping at shallow traps followed by stress (or strain)-induced detrapping, which leads to activator recombination in association with local piezoelectric fields. However, such a conventional ML mechanism was found to be invalid for the ZnS:Cu-embedded polydimethylsiloxane (PDMS) composite, due to the absence of luminescence with a rigid matrix and a negligibly small value of the piezoelectric coefficient (d 33 ) of the composite. An alternative mechanism, namely, the triboelectricity-induced luminescence has been proposed for the mechanically driven luminescence of a ZnS:Cu-PDMS composite.
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  • 66
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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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  • 67
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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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  • 68
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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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  • 69
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    American Institute of Physics (AIP)
    Publication Date: 2016-08-06
    Description: We have grown Mg-doped GaN films with low residual hydrogen concentration using a low-temperature pulsed sputtering deposition (PSD) process. The growth system is inherently hydrogen-free, allowing us to obtain high-purity Mg-doped GaN films with residual hydrogen concentrations below 5 × 10 16 cm −3 , which is the detection limit of secondary ion mass spectroscopy. In the Mg profile, no memory effect or serious dopant diffusion was detected. The as-deposited Mg-doped GaN films showed clear p-type conductivity at room temperature (RT) without thermal activation. The GaN film doped with a low concentration of Mg (7.9 × 10 17 cm −3 ) deposited by PSD showed hole mobilities of 34 and 62 cm 2 V −1 s −1 at RT and 175 K, respectively, which are as high as those of films grown by a state-of-the-art metal-organic chemical vapor deposition apparatus. These results indicate that PSD is a powerful tool for the fabrication of GaN-based vertical power devices.
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  • 70
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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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  • 71
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    American Institute of Physics (AIP)
    Publication Date: 2016-08-11
    Description: We present a detailed study of compressively strained Pb x Sr 1− x TiO 3 thin films grown by off-axis radio frequency magnetron sputtering on (001)-oriented Nb-doped SrTiO 3 substrates. Film tetragonality and the ferroelectric critical temperatures are measured for samples of different composition and thickness and compared with a phenomenological Landau-Devonshire model. 180 ∘ ferroelectric domains are observed using both X-ray diffraction and piezoresponse force microscopy and domain sizes obtained by the two techniques are compared and discussed.
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  • 72
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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 (
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  • 73
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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.
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  • 74
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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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  • 75
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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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  • 76
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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.
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  • 77
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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.
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  • 78
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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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  • 79
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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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  • 80
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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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  • 81
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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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  • 82
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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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  • 83
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    American Institute of Physics (AIP)
    Publication Date: 2016-10-18
    Description: Direct conversion between thermal and electrical energy can be achieved by thermoelectric materials, which provide a viable route for power generation and solid state refrigeration. Here, we use a combination of energetic, electronic, and vibrational first-principles based results to predict the figure of merit performance in hole doped single crystals of SnS and (Pb,Sn)S. We find high ZT values for both materials, specifically for (Pb,Sn)S along the b -axis. Both SnS and (Pb,Sn)S have excellent power factors when doped, due to a combination of increased electrical conductivity (due to doping) and a significantly enhanced Seebeck coefficient obtained by a doping-induced multiband effect. Anharmonic phonon calculations combined with a Debye-Calloway model show that the lattice thermal conductivity of both compounds is low, due to intrinsic anharmonicity, and is lowered further by the random, solid solution nature of the cation sublattice of (Pb,Sn)S. (Pb,Sn)S exhibits a high ZT plateau ranging from 1.3 at 300 K to 1.9 at 800 K. The overall ZT of the hole doped (Pb,Sn)S crystals is predicted to outperform most of the current state-of-the-art thermoelectric sulfide materials.
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  • 84
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    American Institute of Physics (AIP)
    Publication Date: 2016-11-04
    Description: We present a novel absorber material—NaSbS 2 —for solar cells. NaSbS 2 is formed as an unexpected byproduct in the chemical synthesis of Sb 2 S 3 . However, NaSbS 2 has many attractive features for a solar material. Here single phase NaSbS 2 nanoparticles were synthesized through solution processing. NaSbS 2 semiconductor-sensitized solar cells were demonstrated for the first time. The best cell yielded J sc = 10.76 mA/cm 2 , V oc = 0.44 V, FF = 48.6%, and efficiency η = 2.30% under 1 sun. At the reduced 0.1 sun, the η increased to 3.18%—a respectable η for a new solar material.
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  • 85
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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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  • 86
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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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  • 87
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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.
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  • 88
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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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  • 89
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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
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Published by <