ALBERT

Description: In semiconductors almost all heat is conducted by phonons (lattice vibrations), which is limited by their quasi-particle lifetimes. Phonon-phonon interactions represent scattering mechanisms that produce thermal resistance. In thermoelectric materials, this resistance due to anharmonicity should be maximised for optimal performance. We use a first-principles lattice-dynamics approach to explore the changes in lattice dynamics across an isostructural series where the average atomic mass is conserved: ZnS to CuGaS 2 to Cu 2 ZnGeS 4 . Our results demonstrate an enhancement of phonon interactions in the multernary materials and confirm that lattice thermal conductivity can be controlled independently of the average mass and local coordination environments.

Description: Normally we think of the glassy state as a single phase and therefore crystallization from chemically identical amorphous precursors should be identical. Here we show that the local structure of an amorphous precursor is distinct depending on the initial deposition conditions, resulting in significant differences in the final state material. Using grazing incidence total x-ray scattering, we have determined the local structure in amorphous thin films of vanadium oxide grown under different conditions using pulsed laser deposition (PLD). Here we show that the subsequent crystallization of films deposited using different initial PLD conditions result in the formation of different polymorphs of VO 2 . This suggests the possibility of controlling the formation of metastable polymorphs by tuning the initial amorphous structure to different formation pathways.

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.

Description: The hybrid perovskite CH 3 NH 3 PbI 3 (MAPI) exhibits long minority-carrier lifetimes and diffusion lengths. We show that slow recombination originates from a spin-split indirect-gap. Large internal electric fields act on spin-orbit-coupled band extrema, shifting band-edges to inequivalent wavevectors, making the fundamental gap indirect. From a description of photoluminescence within the quasiparticle self-consistent GW approximation for MAPI, CdTe, and GaAs, we predict carrier lifetime as a function of light intensity and temperature. At operating conditions we find radiative recombination in MAPI is reduced by a factor of more than 350 compared to direct gap behavior. The indirect gap is retained with dynamic disorder.

Description: III-nitride semiconductors hold tremendous promise for realizing high efficiency photoelectrodes. However, previously reported InGaN photoelectrodes generally exhibit very low photocurrent densities, due to the presence of extensive defects, dislocations, and indium phase separation. Here, we show that In 0.5 Ga 0.5 N nanowires with nearly homogeneous indium distribution can be achieved by plasma-assisted molecular beam epitaxy. Under AM1.5G one sun illumination, the InGaN nanowire photoanode exhibits a photocurrent density of 7.3 mA/cm 2 at 1.2 V ( vs . NHE) in 1M HBr. The incident-photon-to-current efficiency is above 10% at 650 nm, which is significantly higher than previously reported values of metal oxide photoelectrodes.

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.

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.

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.

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.

Description: Strong exchange bias (EB) in perpendicular direction has been demonstrated in vertically aligned nanocomposite (VAN) (La 0.7 Sr 0.3 MnO 3 ) 1−x : (LaFeO 3 ) x (LSMO:LFO, x = 0.33, 0.5, 0.67) thin films deposited by pulsed laser deposition. Under a moderate magnetic field cooling, an EB field as high as ∼800 Oe is achieved in the VAN film with x = 0.33, suggesting a great potential for its applications in high density memory devices. Such enhanced EB effects in perpendicular direction can be attributed to the high quality epitaxial co-growth of vertically aligned ferromagnetic LSMO and antiferromagnetic LFO phases, and the vertical interface coupling associated with a disordered spin-glass state. The VAN design paves a powerful way for integrating perpendicular EB effect within thin films and provides a new dimension for advanced spintronic devices.

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.

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 .

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.

Description: The electronic structure of Heusler alloys having mixed magnetic phases, comprising of vicinal anti-ferromagnetic and ferromagnetic orders, is of great significance. We present the results of an electronic structure study on Ni x Cu 1− x MnSb Heusler alloys, using Mn-2p core-level photoemission spectroscopy. Room temperature data in the paramagnetic phase reveal a non-monotonic variation of both electron correlation strength and conduction-band hybridization such that the former enhances while the latter weakens for compositions showing a mixed phase relative to compositions at the phase boundaries to the ordered phases. The results suggest a possible electronic driving force for settling mixed-magnetic phases.

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.

Description: We review the spin-Seebeck and magnon-electron drag effects in the context of solid-state energy conversion. These phenomena are driven by advective magnon-electron interactions. Heat flow through magnetic materials generates magnetization dynamics, which can strongly affect free electrons within or adjacent to the magnetic material, thereby producing magnetization-dependent (e.g., remnant) electric fields. The relative strength of spin-dependent interactions means that magnon-driven effects can generate significantly larger thermoelectric power factors as compared to classical thermoelectric phenomena. This is a surprising situation in which spin-based effects are larger than purely charge-based effects, potentially enabling new approaches to thermal energy conversion.

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.

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.

Description: In this study, a series of copper sulfides Cu x S with x spanning from 1.8 to 1.96 was prepared and their crystal structures, elemental valence states, and thermoelectric properties were systematically studied. The valence state of Cu in Cu x S is unchanged as the ratio of Cu/S varies, while the thermoelectric properties are very sensitive to the deficiency of Cu. In addition, the type of sulfur arrangement in the crystal structure also plays an important role on the electrical transports. Finally, the optimum Cu/S atomic ratios in the binary Cu x S system were identified for high power factor and thermoelectric figure of merit.

Description: Disorder in the potential-energy landscape presents a major obstacle to the more rapid development of semiconductor quantum device technologies. We report a large-magnitude source of disorder, beyond commonly considered unintentional background doping or fixed charge in oxide layers: nanoscale strain fields induced by residual stresses in nanopatterned metal gates. Quantitative analysis of synchrotron coherent hard x-ray nanobeam diffraction patterns reveals gate-induced curvature and strains up to 0.03% in a buried Si quantum well within a Si/SiGe heterostructure. Electrode stress presents both challenges to the design of devices and opportunities associated with the lateral manipulation of electronic energy levels.

Description: Thermoelectric modules based on half-Heusler compounds offer a cheap and clean way to create eco-friendly electrical energy from waste heat. Here we study the impact of the period composition on the electrical and thermal properties in non-symmetric superlattices, where the ratio of components varies according to (TiNiSn) n :(HfNiSn) 6−n , and 0 ⩽ n ⩽ 6 unit cells. The thermal conductivity ( κ ) showed a strong dependence on the material content achieving a minimum value for n = 3, whereas the highest value of the figure of merit ZT was achieved for n = 4. The measured κ can be well modeled using non-symmetric strain relaxation applied to the model of the series of thermal resistances.

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.

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.

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.

Description: The experimental search for new thermoelectric materials remains largely confined to a limited set of successful chemical and structural families, such as chalcogenides, skutterudites, and Zintl phases. In principle, computational tools such as density functional theory (DFT) offer the possibility of rationally guiding experimental synthesis efforts toward very different chemistries. However, in practice, predicting thermoelectric properties from first principles remains a challenging endeavor [J. Carrete et al. , Phys. Rev. X 4 , 011019 (2014)], and experimental researchers generally do not directly use computation to drive their own synthesis efforts. To bridge this practical gap between experimental needs and computational tools, we report an open machine learning-based recommendation engine ( http://thermoelectrics.citrination.com ) for materials researchers that suggests promising new thermoelectric compositions based on pre-screening about 25 000 known materials and also evaluates the feasibility of user-designed compounds. We show this engine can identify interesting chemistries very different from known thermoelectrics. Specifically, we describe the experimental characterization of one example set of compounds derived from our engine, RE 12 Co 5 Bi ( RE = Gd, Er), which exhibits surprising thermoelectric performance given its unprecedentedly high loading with metallic d and f block elements and warrants further investigation as a new thermoelectric material platform. We show that our engine predicts this family of materials to have low thermal and high electrical conductivities, but modest Seebeck coefficient, all of which are confirmed experimentally. We note that the engine also predicts materials that may simultaneously optimize all three properties entering into zT ; we selected RE 12 Co 5 Bi for this study due to its interesting chemical composition and known facile synthesis.

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.

Description: A Microwave-Induction Heating (MIH) scheme is proposed for the poly(4-vinylphenol) (PVP) gate insulator cross-linking process to replace the traditional oven heating cross-linking process. The cross-linking time is significantly decreased from 1 h to 5 min by heating the metal below the PVP layer using microwave irradiation. The necessary microwave power was substantially reduced to about 50 W by decreasing the chamber pressure. The MIH scheme is a good candidate to replace traditional thermal heating for cross-linking of PVP as the gate insulator for organic thin-film-transistors.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Description: Magnetic interfaces have been identified as promising systems upon which to base next-generation spintronic devices. In these nearly two-dimensional systems, deviations from bulk electronic structure and competition between nearly degenerate magnetic ground states allow the stabilization of widely tunable emergent properties. However, ever smaller length scales pose new challenges which must be overcome in order to understand and control magnetic properties at the atomic level. Using recent examples in oxide heterostructures and topological insulators, we discuss how combining techniques such as neutron scattering, X-ray scattering, X-ray spectroscopy, and transmission electron microscopy enables the probing of magnetism on the Angstrom scale.

Description: Searchable, interactive, databases of material properties, particularly those relating to functional materials (magnetics, thermoelectrics, photovoltaics, etc.) are curiously missing from discussions of machine-learning and other data-driven methods for advancing new materials discovery. Here we discuss the manual aggregation of experimental data from the published literature for the creation of interactive databases that allow the original experimental data as well additional metadata to be visualized in an interactive manner. The databases described involve materials for thermoelectric energy conversion, and for the electrodes of Li-ion batteries. The data can be subject to machine-learning, accelerating the discovery of new materials.

Description: The dynamics of a magnetic domain wall (DW) under a transverse magnetic field H y are investigated in two-dimensional (2D) Co/Ni microstrips, where an interfacial Dzyaloshinskii-Moriya interaction (DMI) exists with DMI vector D lying in + y direction. The DW velocity exhibits asymmetric behavior for ± H y ; that is, the DW velocity becomes faster when H y is applied antiparallel to D . The key experimental results are reproduced in a 2D micromagnetic simulation, which reveals that the interfacial DMI suppresses the periodic change of the average DW angle φ even above the Walker breakdown and that H y changes φ , resulting in a velocity asymmetry. This suggests that the 2D DW motion, despite its microscopic complexity, simply depends on the average angle of the DW and thus can be described using a one-dimensional soliton model. These findings provide insight into the magnetic DW dynamics in 2D systems, which are important for emerging spin-orbitronic applications.

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.

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.

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.

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.

Description: Temperature-dependent strength of Bi-Sb-Te under uniaxial compression is investigated. Bi-Sb-Te samples were produced by three methods: vertical zone-melting, hot extrusion, and spark plasma sintering (SPS). For zone-melted and extruded samples, the brittle-ductile transition occurs over a temperature range of 200-350 °C. In nanostructured samples produced via SPS, the transition is observed in a narrower temperature range of 170-200 °C. At room temperature, the strength of the nanostructured samples is higher than that of zone-melted and extruded samples, but above 300 °C, all samples decrease to roughly the same strength.

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.

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 (

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Description: The paramagnetic fluctuations are investigated in La(Fe 0.88 Si 0.12 ) 13 . The disordered local moment (DLM) is represented by the first principle calculations. With a reduction of the volume, the DLM amplitude decreases gradually. In the temperature dependence of electrical resistivity ρ under hydrostatic pressure, an upturn in the variation of ρ in the paramagnetic state was observed with decreasing temperature, which is originated from the Curie-Weiss-type DLM fluctuations. In the vicinity of the critical pressure for disappearance of the DLM, the variation of ρ as a function of log( T ) was observed.

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.

Description: We report on the transport properties of hybrid devices obtained by depositing graphene on a LaAlO 3 /SrTiO 3 oxide junction hosting a 4 nm-deep 2-dimensional electron system. At low graphene-oxide inter-layer bias, the two electron systems are electrically isolated, despite their small spatial separation. A very efficient reciprocal gating of the two neighboring 2-dimensional systems is shown. A pronounced rectifying behavior is observed for larger bias values and ascribed to the interplay between electrostatic field-effects and tunneling across the LaAlO 3 barrier. The relevance of these results in the context of strongly coupled bilayer systems is discussed.

Description: Many important breakthroughs and significant engineering developments have been achieved during the past two decades in the field of caloric materials. In this review, we address ferroelectrics emerging as ideal materials which permit both giant elastocaloric and/or electrocaloric responses near room temperature. We summarize recent strategies for improving caloric responses using geometrical optimization, maximizing the number of coexisting phases, combining positive and negative caloric responses, introducing extra degree of freedom like mechanical stress/pressure, and multicaloric effect driven by either single stimulus or multiple stimuli. This review highlights the promising perspective of ferroelectrics for developing next-generation solid-state refrigeration.

Description: With their ability to rapidly elucidate composition-structure-property relationships, high-throughput experimental studies have revolutionized how materials are discovered, optimized, and commercialized. It is now possible to synthesize and characterize high-throughput libraries that systematically address thousands of individual cuts of fabrication parameter space. An unresolved issue remains transforming structural characterization data into phase mappings. This difficulty is related to the complex information present in diffraction and spectroscopic data and its variation with composition and processing. We review the field of automated phase diagram attribution and discuss the impact that emerging computational approaches will have in the generation of phase diagrams and beyond.

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.

Description: Here we report the thermoelectric properties of NbCoSn-based n-type half-Heuslers (HHs) that were obtained through arc melting, ball milling, and hot pressing process. With 10% Sb substitution at the Sn site, we obtained enhanced n-type properties with a maximum power factor reaching ∼35 μ W cm −1 K −2 and figure of merit (ZT) value ∼0.6 in NbCoSn 0.9 Sb 0.1 . The ZT is doubled compared to the previous report. In addition, the specific power cost ($ W −1 ) is decreased by ∼68% comparing to HfNiSn-based n-type HH because of the elimination of Hf.

Description: Large crystalline molecular shells buckle spontaneously into icosahedra while multicomponent shells buckle into various polyhedra. Continuum elastic theory explains the buckling of closed shells with one elastic component into icosahedra. A generalized elastic model, on the other hand, describes the spontaneous buckling of inhomogeneous shells into regular and irregular polyhedra. By co-assembling water-insoluble anionic (−1) amphiphiles with cationic (3+) amphiphiles, we realized ionic vesicles. Results revealed that surface crystalline domains and the unusual shell shapes observed arise from the competition of ionic correlations with charge-regulation. We explain here the mechanism by which these ionic membranes generate a mechanically heterogeneous vesicle.

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.

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.

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.

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.

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.

Description: We use calorimetry to identify pressure-driven isothermal entropy changes in ceramic samples of the prototypical ferroelectric BaTiO 3 . Near the structural phase transitions at ∼400 K (cubic-tetragonal) and ∼280 K (tetragonal-orthorhombic), the inverse barocaloric response differs in sign and magnitude from the corresponding conventional electrocaloric response. The differences in sign arise due to the decrease in unit-cell volume on heating through the transitions, whereas the differences in magnitude arise due to the large volumetric thermal expansion on either side of the transitions.

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.

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.

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.

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.

Description: In this letter, we report on the structural and transport measurements of ultrathin MgB 2 films grown by hybrid physical-chemical vapor deposition followed by low incident angle Ar ion milling. The ultrathin films as thin as 1.8 nm, or 6 unit cells, exhibit excellent superconducting properties such as high critical temperature ( T c ) and high critical current density ( J c ). The results show the great potential of these ultrathin films for superconducting devices and present a possibility to explore superconductivity in MgB 2 at the 2D limit.

Description: High-mobility perovskite BaSnO 3 films are of significant interest as new wide bandgap semiconductors for power electronics, transparent conductors, and as high mobility channels for epitaxial integration with functional perovskites. Despite promising results for single crystals, high-mobility BaSnO 3 films have been challenging to grow. Here, we demonstrate a modified oxide molecular beam epitaxy (MBE) approach, which supplies pre-oxidized SnO x . This technique addresses issues in the MBE of ternary stannates related to volatile SnO formation and enables growth of epitaxial, stoichiometric BaSnO 3 . We demonstrate room temperature electron mobilities of 150 cm 2 V −1 s −1 in films grown on PrScO 3 . The results open up a wide range of opportunities for future electronic devices.

Description: We calculate the critical thickness for misfit dislocation (MD) formation in lattice mismatched semipolar and nonpolar III-nitride wurtzite semiconductor layers for the case of MDs originated from prismatic slip (PSMDs). It has been shown that there is a switch of stress relaxation modes from generation of basal slip originated MDs to PSMDs after the angle between c -axis in wurtzite crystal structure and the direction of semipolar growth reaches a particular value, e.g., ∼70° for Al 0.13 Ga 0.87 N/GaN ( h 0 h ̄ 1 ) semipolar heterostructures. This means that for some semipolar growth orientations of III-nitride heterostructures biaxial relaxation of misfit stress can be realized. The results of modeling are compared to experimental data on the onset of plastic relaxation in Al x Ga 1−x N/GaN heterostructures.

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.

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.

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.

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.

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.

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.

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.

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 .

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.

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.

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.

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.

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.

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.

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.

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.

Description: Thin films of homogeneous mixture of amorphous silicon and aluminum were produced with magnetron sputtering using 2-phase Al–Si targets. The films exhibited variable compositions, with and without the presence of hydrogen, aSi 1− x Al x and aSi 1− x Al x H y . The structure and optical properties of the films were investigated using transmission electron microscopy, X-ray photoelectron spectroscopy, UV-Vis NIR spectrometry, ellipsometry, and atomistic modeling. We studied the effect of alloying aSi with Al (within the range 0–25 at. %) on the optical band gap, refractive index, transmission, and absorption. Alloying aSi with Al resulted in a non-transparent film with a low band gap ( 1 eV. Variations of the Al and hydrogen content allowed for tuning of the optoelectronic properties. The films are stable up to a temperature of 300 °C. At this temperature, we observed Al induced crystallization of the amorphous silicon and the presence of large Al particles in a crystalline Si matrix.

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.

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.