High-pressure torsion (HPT) is a type of severe plastic deformation (SPD) that is highly suited to produce bulk ultrafine-grained and nanocrystalline materials, as it introduces many grain boundaries ...as well as dislocations and point defects. In this paper, HPT-mediated nanocrystallization was used to reduce the thermal conductivity and enhance the Seebeck coefficient of skutterudites. Both p- and n-type skutterudites have been processed by HPT with 4 and 5GPa at temperatures up to 773K, resulting in a strongly strengthened nanocrystalline structure, revealing oriented, lamellar-shaped crystallites with a size of ∼50nm and an enhanced dislocation density. In comparison with ball-milled plus hot-pressed skutterudites, the HPT-processed samples show a reduction of the thermal conductivity up to 40%. This and the slightly higher Seebeck coefficient are the reasons why HPT proved to enhance the figure of merit (ZT) values up to a factor of 2, in spite of a markedly enhanced electrical resistivity.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
We show how finite-size scaling of a bulk photovoltaic effect-generated electric field in epitaxial ferroelectric insulating BaTiO_{3}(001) films and a photo-Hall response involving the bulk ...photovoltaic current reveal a large room-temperature mean free path of photogenerated nonthermalized electrons. Experimental determination of mesoscopic ballistic optically generated carrier transport opens a new paradigm for hot electron-based solar energy conversion, and for facile control of ballistic transport distinct from existing low-dimensional semiconductor interfaces, surfaces, layers, or other structures.
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CMK, CTK, FMFMET, IJS, NUK, PNG, UM
A significant experimental challenge in testing proposed relationships between structure and properties is the synthesis of targeted structures with atomistic control over both the structure and the ...composition. SnSe2(MoSe2)1.32 was synthesized to test the hypothesis that the low-temperature synthesis of two interleaved structures would result in complete turbostratic disorder and that the disorder would result in ultralow thermal conductivity. SnSe2(MoSe2)1.32 was prepared by depositing elements to form a precursor containing Sn|Se and Mo|Se bilayers, each containing the number of atoms required to form single dichalcogenide planes. The nanoarchitecture of alternating Sn and Mo layers is preserved as the dichalcogenide planes self-assemble at low temperatures. The resulting compound contains well-formed dichalcogenide planes that closely resemble that found in the binary compounds and extensive turbostratic disorder. As expected from proposed structure–property relationships, the thermal conductivity of SnSe2(MoSe2)1.32 is ultralow, ∼0.05 W m–1 K–1.
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IJS, KILJ, NUK, PNG, UL, UM
Selecting specific 2D building blocks and specific layering sequences of van der Waals heterostructures should allow the formation of new materials with designed properties for specific applications. ...Unfortunately, the synthetic ability to prepare such structures at will, especially in a manner that can be manufactured, does not exist. Herein, we report the targeted synthesis of new metal–semiconductor heterostructures using the modulated elemental‐reactant technique to nucleate specific 2D building blocks, control their thickness, and avoid epitaxial structures with long‐range order. The building blocks, VSe2 and GeSe2, have different crystal structures, which inhibits cation intermixing. The precise control of this approach enabled us to synthesize heterostructures containing GeSe2 monolayers alternating with VSe2 structural units with specific sequences. The transport properties systematically change with nanoarchitecture and a charge‐density wave‐like transition is observed.
Layered cake: The modulated elemental‐reactant technique provides a unique route to the targeted synthesis of new van der Waals heterostructures. VSe2 and GeSe2 were selected as 2D building blocks to inhibit cation intermixing. The kinetic control offered by this approach enabled heterostructures with varying stacking sequences to be prepared.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
A basic summary of thermoelectric principles is presented in a historical context, following the evolution of the field from initial discovery to modern day high-zT materials. A specific focus is ...placed on nanocomposite materials as a means to solve the challenges presented by the contradictory material requirements necessary for efficient thermal energy harvest. Misfit layer compounds are highlighted as an example of a highly ordered anisotropic nanocomposite system. Their layered structure provides the opportunity to use multiple constituents for improved thermoelectric performance, through both enhanced phonon scattering at interfaces and through electronic interactions between the constituents. Recently, a class of metastable, turbostratically-disordered misfit layer compounds has been synthesized using a kinetically controlled approach with low reaction temperatures. The kinetically stabilized structures can be prepared with a variety of constituent ratios and layering schemes, providing an avenue to systematically understand structure-function relationships not possible in the thermodynamic compounds. We summarize the work that has been done to date on these materials. The observed turbostratic disorder has been shown to result in extremely low cross plane thermal conductivity and in plane thermal conductivities that are also very small, suggesting the structural motif could be attractive as thermoelectric materials if the power factor could be improved. The first 10 compounds in the (PbSe)1+δm(TiSe2)n family (m, n ≤ 3) are reported as a case study. As n increases, the magnitude of the Seebeck coefficient is significantly increased without a simultaneous decrease in the in-plane electrical conductivity, resulting in an improved thermoelectric power factor.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
A series of semiconducting (SnSe)1.05nMoSe2n compounds where n = 1-4 were prepared from thin film precursors with designed local compositions and nanoarchitectures to promote formation of the desired ...products during low temperature annealing. Specular diffraction patterns of annealed precursors contain only 00l reflections that yield c-lattice parameters indicating the formation of layered intergrowths. The in-plane diffraction patterns contain the independent reflections from the two constituents and the in-plane lattice parameters are independent of n. Electron microscopy images suggest significant turbostratic disorder between the constituent layers and between Se-Mo-Se trilayers within the dichalcogenide constituent. The room temperature cross-plane thermal conductivity was found to be low, between 0.08 and 0.22 W m−1 K−1 for the series of isocompositional compounds investigated, and is independent of the density of interfaces.
The field of ultraviolet (UV)-laser applications is currently experiencing rapid growth in the semiconductor processing, laser micromachining and biomedical markets. Key enablers for these ...technologies are optical coatings used to manipulate and guide laser beams in a targeted manner. As laser power, laser fluence and pulse frequencies increase, the demands on the physical properties of the coating materials become more stringent. Ion beam sputtering is a technique that allows producing optical coatings with the low losses required in these applications. In this study, we investigate the influence of ion beam sputtering (IBS) parameters on the optical properties of HfO
and SiO
single layers as well as the impact of annealing duration at 475 °C for anti-reflective (AR) and highly reflective (HR) optical coatings at 355 nm. For HfO
sputtered from a metal target the O
flow during the coating process is a key parameter to reduce absorption. SiO
single layers exhibit improved transmission in the UV-range as the ion beam energy for the sputtering process is reduced. Furthermore, a complex behavior for film stress, absorption, surface roughness and coating structure was unraveled as a function of annealing duration for AR- and HR-coatings at 355 nm. The reflectance of the HR-mirror after optimized annealing exceeded 99.94% at 355 nm and a high laser induced damage threshold (LIDT) of 6.9 J/cm
was measured after 2 hours of annealing. For the AR-coating a LIDT-value of 15.7 J/cm
was observed after 12 hours of annealing.
The formation of the ferecrystalline compound (SnSe)1.15VSe2 is studied utilizing complementary X‐ray diffraction techniques, differential scanning calorimetry, compositional analysis, scanning ...transmission electron microscopy, and X‐ray absorption spectroscopy. A careful analysis unravels the formation mechanism, where a simultaneous crystallization of the individual constituents goes hand in hand with the formation of the superlattice structure. SnSe2 monolayers form along with SnSe and VSe2 units in the superlattice during the formation of (SnSe)1.15VSe2, with the SnSe2 monolayers coexisting up to 300°C. An annealing temperature of 400°C is required to fully self‐assemble the ferecrystalline compound (SnSe)1.15VSe2, composed of alternating rocksalt‐like SnSe bilayers and VSe2 trilayers. These results demonstrate a complex pathway along a multi‐valley energy landscape for these metastable compounds, which in turn offers a rich platform to synthesize targeted layering sequences by precisely controlling the composition of the precursors as well as the annealing conditions.
The authors trace the reaction pathway of a chalcogenide based heterostructure from a designed precursor to the self–assembly of a metastable superlattice. The complexity of the formation pathway along a multi–valley energy landscape bears the potential of accessing a plethora of different heterostructures by controlling composition and annealing conditions.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Electrically tunable dielectric thin films in active circuits and systems are challenged by capacitance-induced delays and impedance matching requiring a lower dielectric constant. Here an approach ...to increasing the intrinsic tunability of compounds containing TiO6 octahedra by considering the influence of different connectivity among these octahedra is presented. Such connectivity variants in nanocrystalline monoclinic BaTi2O5 thin films enable a two orders of magnitude enhancement in Ti anharmonic interaction, thereby permitting a ≈65% decrease in dielectric constant to 70 at room temperature without sacrificing tunability. Edge-sharing TiO6 octahedra possess a much shorter Ti-Ti distance of only 2.91 Å as compared to the perovskite structure (~4 Å), permitting large field-induced structural re-arrangement and intrinsic tunability.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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