Study of Elastic and Structural Properties of BaFe2As2 Ultrathin Film Using Picosecond Ultrasonics.

We obtain the through-thickness elastic stiffness coefficient (C33) in nominal 9 nm and 60 nm BaFe2As2 (Ba-122) thin films by using picosecond ultrasonics. Particularly, we reveal the increase in elastic stiffness as film thickness decreases from bulk value down to 9 nm, which we attribute to the increase in intrinsic strain near the film-substrate interface. Our density functional theory (DFT) calculations reproduce the observed acoustic oscillation frequencies well. In addition, temperature dependence of longitudinal acoustic (LA) phonon mode frequency for 9 nm Ba-122 thin film is reported. The frequency change is attributed to the change in Ba-122 orthorhombicity (a-b)/(a+b). This conclusion can be corroborated by our previous ultrafast ellipticity measurements in 9 nm Ba-122 thin film, which exhibit strong temperature dependence and indicate the structural phase transition temperature Ts.

Improvement of water harvesting performance through collector modification in industrial cooling tower.

Shortages of freshwater have become increasingly common around the world, and various studies have been conducted to solve this problem by collecting and reusing the water in nature or from factories and power plants that produce large fog plumes. Although the shape of a collection screen is strongly related to its harvesting performance, only flat meshes have been considered in previous studies, and research on the effects of collector structure shapes is severely lacking. In this study, we proposed modified collector structures improving harvesting performances in industrial cooling towers. The screen shape was modified in three steps. First, a concave shape was adopted for the mesh screen to increase the aerodynamic characteristics of the collection structure. Next, a sidewall was installed to collect additional fog from defected flows generated by the concave structure. Finally, to reduce loss during the draining of collected water droplets, the discharge direction of the fog flow was changed to follow the same direction as fog-laden flows in nature. Our results are expected to be useful for collector design in terms of increasing harvesting efficiency in various industrial fields in the future.

Resist-Free Lithography for Monolayer Transition Metal Dichalcogenides.

Photolithography and electron-beam lithography are the most common methods for making nanoscale devices from semiconductors. While these methods are robust for bulk materials, they disturb the electrical properties of two-dimensional (2D) materials, which are highly sensitive to chemicals used during lithography processes. Here, we report a resist-free lithography method, based on direct laser patterning and resist-free electrode transfer, which avoids unintentional modification to the 2D materials throughout the process. We successfully fabricate large arrays of field-effect transistors using MoS2 and WSe2 monolayers, the performance of which reflects the properties of the pristine materials. Furthermore, using these pristine devices as a reference, we reveal that among the various stages of a conventional lithography process, exposure to a solvent like acetone changes the electrical conductivity of MoS2 the most. This new approach will enable a rational design of reproducible processes for making large-scale integrated circuits based on 2D materials and other surface-sensitive materials.

Robotic four-dimensional pixel assembly of van der Waals solids.

Van der Waals (vdW) solids can be engineered with atomically precise vertical composition through the assembly of layered two-dimensional materials1,2. However, the artisanal assembly of structures from micromechanically exfoliated flakes3,4 is not compatible with scalable and rapid manufacturing. Further engineering of vdW solids requires precisely designed and controlled composition over all three spatial dimensions and interlayer rotation. Here, we report a robotic four-dimensional pixel assembly method for manufacturing vdW solids with unprecedented speed, deliberate design, large area and angle control. We used the robotic assembly of prepatterned 'pixels' made from atomically thin two-dimensional components. Wafer-scale two-dimensional material films were grown, patterned through a clean, contact-free process and assembled using engineered adhesive stamps actuated by a high-vacuum robot. We fabricated vdW solids with up to 80 individual layers, consisting of 100 × 100 µm2 areas with predesigned patterned shapes, laterally/vertically programmed composition and controlled interlayer angle. This enabled efficient optical spectroscopic assays of the vdW solids, revealing new excitonic and absorbance layer dependencies in MoS2. Furthermore, we fabricated twisted N-layer assemblies, where we observed atomic reconstruction of twisted four-layer WS2 at high interlayer twist angles of ≥4°. Our method enables the rapid manufacturing of atomically resolved quantum materials, which could help realize the full potential of vdW heterostructures as a platform for novel physics2,5,6 and advanced electronic technologies7,8.

Atomically Thin, Optically Isotropic Films with 3D Nanotopography.

Flat optics aims for the on-chip miniaturization of optical systems for high-speed and low-power operation, with integration of thin and lightweight components. Here, we present atomically thin yet optically isotropic films realized by using three-dimensional (3D) topographic reconstruction of anisotropic two-dimensional (2D) films to balance the out-of-plane and in-plane optical responses on the subwavelength scale. We achieve this by conformal growth of monolayer transition metal dichalcogenide (TMD) films on nanodome-structured substrates. The resulting films show an order-of-magnitude increase in the out-of-plane susceptibility for enhanced angular performance, displaying polarization isotropy in the off-axis absorption, as well as improved photoluminescence emission profiles, compared to their flat-film counterparts. We further show that such 3D geometric programming of optical properties is applicable to different TMD materials, offering spectral generalization over for the entire visible range. Our approach presents a powerful platform for advancing the development of atomically thin flat optics with custom-designed light-matter interactions.

Superconductivity in undoped BaFe2As2 by tetrahedral geometry design.

Fe-based superconductors exhibit a diverse interplay between charge, orbital, and magnetic ordering. Variations in atomic geometry affect electron hopping between Fe atoms and the Fermi surface topology, influencing magnetic frustration and the pairing strength through changes of orbital overlap and occupancies. Here, we experimentally demonstrate a systematic approach to realize superconductivity without chemical doping in BaFe2As2, employing geometric design within an epitaxial heterostructure. We control both tetragonality and orthorhombicity in BaFe2As2 through superlattice engineering, which we experimentally find to induce superconductivity when the As-Fe-As bond angle approaches that in a regular tetrahedron. This approach to superlattice design could lead to insights into low-dimensional superconductivity in Fe-based superconductors.

Control of Epitaxial BaFe2As2 Atomic Configurations with Substrate Surface Terminations.

Atomic layer controlled growth of epitaxial thin films of unconventional superconductors opens the opportunity to discover novel high temperature superconductors. For instance, the interfacial atomic configurations may play an important role in superconducting behavior of monolayer FeSe on SrTiO3 and other Fe-based superconducting thin films. Here, we demonstrate a selective control of the atomic configurations in Co-doped BaFe2As2 epitaxial thin films and its strong influence on superconducting transition temperatures by manipulating surface termination of (001) SrTiO3 substrates. In a combination of first-principles calculations and high-resolution scanning transmission electron microscopy imaging, we show that Co-doped BaFe2As2 on TiO2-terminated SrTiO3 is a tetragonal structure with an atomically sharp interface and with an initial Ba layer. In contrast, Co-doped BaFe2As2 on SrO-terminated SrTiO3 has a monoclinic distortion and a BaFeO3- x initial layer. Furthermore, the superconducting transition temperature of Co-doped BaFe2As2 ultrathin films on TiO2-terminated SrTiO3 is significantly higher than that on SrO-terminated SrTiO3, which we attribute to shaper interfaces with no lattice distortions. This study allows the design of the interfacial atomic configurations and the effects of the interface on superconductivity in Fe-based superconductors.

Origin of the emergence of higher T c than bulk in iron chalcogenide thin films.

Fabrication of epitaxial FeSexTe1-x thin films using pulsed laser deposition (PLD) enables improving their superconducting transition temperature (T c) by more than ~40% than their bulk T c. Intriguingly, T c enhancement in FeSexTe1-x thin films has been observed on various substrates and with different Se content, x. To date, various mechanisms for T c enhancement have been reported, but they remain controversial in universally explaining the T c improvement in the FeSexTe1-x films. In this report, we demonstrate that the controversies over the mechanism of T c enhancement are due to the abnormal changes in the chalcogen ratio (Se:Te) during the film growth and that the previously reported T c enhancement in FeSe0.5Te0.5 thin films is caused by a remarkable increase of Se content. Although our FeSexTe1-x thin films were fabricated via PLD using a Fe0.94Se0.45Te0.55 target, the precisely measured composition indicates a Se-rich FeSexTe1-x (0.6 < x < 0.8) as ascertained through accurate compositional analysis by both wavelength dispersive spectroscopy (WDS) and Rutherford backscattering spectrometry (RBS). We suggest that the origin of the abnormal composition change is the difference in the thermodynamic properties of ternary FeSexTe1-x, based on first principle calculations.

Suppression of neovascularization and experimental arthritis by D-form of anti-flt-1 peptide conjugated with mini-PEG(™).

Extensive angiogenesis in the synoviums is a characteristic pathology of rheumatoid arthritis (RA). We have demonstrated that anti-flt-1 hexapeptide, GNQWFI, specifically inhibits the interaction of VEGF or PlGF with its receptor flt-1 (Yoo et al. [13]). In this study, we investigate the feasibility of the synthetic D-form of anti-flt-1 hexapeptide conjugated with 8-amino-3,6-dioxaoctanoic acid (mini-PEG™) for treatment of RA. We first modified the structure of anti-flt-1 peptide from the L-form (GNQWFI) to all D-form (gnqwfi; allD) and then conjugated allD with mini-PEG™ to enhance its stability. The result showed that the allD anti-flt-1 peptide showed an increased stability in the sera without major loss of inhibitory activity. The allD and its mini-PEGylated derivative similarly suppressed wounding migration, chemotaxis, and tube formation of endothelial cells in vitro. However, in the Matrigels assay, the in vivo anti-angiogenic activity of mini-PEGylated allD was stronger than that of native allD or L-form. Moreover, oral and subcutaneous administration of mini-PEGylated allD, but not oral feeding of original L-form, successfully suppressed severity of collagen-induced arthritis. After a single subcutaneous injection, the Cy5-labeled mini-PEGylated allD was found to be distributed systemically and accumulated in arthritic joints of mice, particularly in joints with a severe clinical score. In conclusion, our data suggests that mini-PEGylated allD is more beneficial in the treatment of RA than unmodified anti-flt-1 peptides, since it has increased stability and the possibility of oral delivery, and could be applied to treat angiogenesis-dependent human diseases, including RA.

Tens of centimeter-scale flexible superhydrophobic nanofiber structures through curing process.

Superhydrophobic surfaces have been fabricated for many biological and engineering applications. However, due to the delamination of the nanostructured layer from a handling layer during the fabrication process, we could not have obtained uniform nanostructured surfaces in large areas. Here, we first report tens of centimeter-scale superhydrophobic nanostructures with flexibility, cost-effective, no aging degradation and drag reduction by adopting curing processes, which have uniform superhydrophobic nanostructured surfaces on the size of 30x14 cm2. Such a nanostructure could be a potential platform for many applications such as microfluidic devices for biological studies, and industrial self-cleaning products for automobiles, ships, and houses.