"Nature" and "Science" Week (9.18-9.24) Frontiers of Materials Science

1. Layer-by-layer assembly of two-dimensional materials into wafer-level heterostructures with high-performance semiconductor films with vertical structures, and the atomic-scale precision achieved by the design lays the foundation for the discovery of modern integrated circuits and new materials. Layer-by-layer assembly is a method of realizing such a film in which two-dimensional building blocks of atomic thickness are stacked vertically and bonded together by van der Waals interaction. In this way, graphene and transition metal dichalcogenes, which represent two and three atomic thickness two-dimensional structural units, respectively, are used to achieve heterogeneous structures with unique physical properties that were previously unachievable. However, there is currently no large-scale assembly method capable of maintaining the inherent characteristics of these two-dimensional members while generating the original interlayer interface, and thus the layer-by-layer assembly method is limited to the small-scale verification demonstration. Kang et al. reported the production of wafer level semiconductor films with very high levels of spatial uniformity and original interface. The vertical composition and properties of these films are designed on the atomic scale using a stacked assembly of two-dimensional structural units under vacuum. They have manufactured several large-scale, high-quality heterostructure films and devices, including superlattice films with layer-by-layer molding, mass-produced resistors that can tuned four orders of magnitude array of tunnel devices, and band-engineered engineering heterogeneity Structured tunnel diodes, and millimeter-scale ultra-thin films with windows. The stacked film is detachable, hangable, and compatible with water or plastic surfaces, which allows it to be integrated with advanced optical and mechanical systems. (Nature DOI: 10.1038/nature23905)
2. Large-scale anisotropic observation cosmic rays in the direction of arrival of cosmic rays exceeding 8 × 1018 eV are the highest energy nuclei observed in the natural space from the external space. The clues about their origin are mainly derived by studying the distribution of the direction of arrival. Using the Pierre Auger Observatory to record a total exposure of 76,800 km2 sr yr with an energy of more than 8 × 1018 eV of 3 × 104 cosmic rays, Pierre Ogg cosmic rays The cooperation of the station personnel cooperated to determine the existence of anisotropy in the direction of cosmic ray arrival. The detected anisotropy exceeds the significant level of 5.2 σ, which can be described by a dipole with a relative eclipse αd = 100 ± 10 degrees and a declination δd = -24 + 12 -23 degrees with a amplitude of 6.5 + 13 - 0.9 %. . This direction indicates the origin of the extragalactic galaxies of these ultra-high energy particles. (Science DOI: 10.1126/science.aan4338) 3. The ordered partial destruction of ionic liquids in carbon nanopores is composed of equal amounts of positive and negative ions. In the case of bulk materials, the neutrality of these liquids is electrically neutral due to Coulomb order, with alternating charge ion shells formed around the central ions. The understanding of their structure in closed structures is unclear. This hinders the widespread use of ionic liquids in technical applications. Futamura et al. used a scattering experiment to analyze the ionic liquid (EMI-TFSI) whose structure was widely used in nanoporous carbon. The results show that Coulomb order decreases when the pores can only accommodate a single layer of ions. Instead, an isocharged ion pair is formed due to the induced reverse potential in the carbon pore walls. This non-Coulomb ordering is further enhanced in the presence of an external potential. This discovery opened the door to the design of better electrochemical applications. (Nature Materials DOI: 10.1038/NMAT4974) The effectiveness of molecular-based light collection depends on the transport of excitons to charge transfer sites. However, measuring exciton migration has been challenging due to the mismatch between migration length and diffraction limit at the nanoscale. Penwell et al. did not use a bulk matrix quenching method, but instead defined the quenching boundary with sub-diffraction resolution to fully characterize the natural nano and picosecond spatiotemporal exciton migration. The migration length of 16 nm in the poly(2,5-bis(hexyloxy)cyano-p-xylylene) conjugated polymer film was measured by converting the stimulated radiation loss microscope into a time-resolved ultra-fast method. Combined with the Monte Carlo exciton jump simulation, it is shown that the migration in these films is essentially diffuse due to the energy turbulence of the inner chromophore and the uneven chromophore broadening. This approach enables local material structures that were previously uninterrelated to be associated with exciton migration characteristics, not only for photovoltaic or display organic semiconductors, but also for explaining typical exciton migration exhibited in photosynthesis. (Nature Materials DOI: 10.1038/NMAT4975) 5. Reversible insertion of magnesium ion and aluminum ion woven textile photovoltaic devices in cationically deficient anatase TiO2 plays a key role in wearable devices as a continuous power source. Compared to other types of energy harvesters, woven photovoltaic power generation can use the cloth itself as a photovoltaic power generation platform to obtain sufficient power (approximately milliwatts) for use in wearable devices. There are three important features for textile photovoltaic technology, namely environmental stability, adequate energy efficiency and mechanical stability. However, due to the low gas barrier properties of the substrate and the ultra-thin cover, it is still difficult to achieve these three items simultaneously. Jinno et al. reported ultra-flexible organic photovoltaics coated with elastomer on both sides, while achieving stretchability and stability in water while maintaining a high efficiency of 7.9%. The double-coated device was only 5.4% less efficient after immersion in water for 120 minutes. In addition, the efficiency of the device remains at 80% of the initial value even after 100 minutes in water and undergoing a cycle of 52% mechanical compression. (Nature Energy DOI: 10.1038/s41560-017-0001-3) 7. Exponential growth and selection in self-replicating materials Self-replication and evolution under selective pressure are inherent phenomena in life, but few artificial systems exhibit These phenomena. He et al. designed a DNA folding system that replicates the source pattern exponentially, doubling the copy during each of the similar day and night temperatures and UV exposure cycles, producing more than 7 million copies in 24 cycles. It also demonstrates environmental choices in growing populations by introducing pH-sensitive binding in two subpopulations. In the first species, a pH-sensitive triple DNA bond enables the formation of a parent-child template, while in the second species, triple binding inhibits the formation of a double-stranded DNA template. At pH 5.3, the replication rate of species I was 1.3-1.4 times faster than that of species II. At pH 7.8, the replication rate is reversed. When mixed in a vial, the progeny of species I preferentially replicate at pH 7.8; similarly at pH 5.3, the descendants of species II take over the system. This addressable selectivity should be suitable for the selection and evolution of multi-component self-replicating materials ranging from nanoscale to microscopic size. (Nature Materials DOI: 10.1038/NMAT4986) 8. High-Resolution Assessment of Global Hydropower Technology and Economic Potential Hydropower is by far the most important renewable energy source, accounting for more than 72% of global renewable energy. However, only limited information can be used for the global potential supply of hydropower and related costs. Gernaat et al. provide a high-resolution chart that assesses the technical and economic potential of hydropower in a global scale. Using a 15" x 15" discharge and a 3" x 3" digital terrain height map, a virtual hydropower station was built at 3.8 million sites around the world and its potential was calculated by cost optimization. In this way, more than 60,000 suitable sites were identified, which together showed the only remaining global potential of 9.49 PWh yr-1 and cost less than US$ 0.50 kWh-1. The remaining potential areas are Asia Pacific (39%), South America (25%) and Africa (24%), a large portion of which can be produced at low cost (< US$ 0.10 kWh-1). In the context of ecological security, this potential has fallen to 5.67 PWh yr-1. (Nature Energy DOI: 10.1038/s41560-017-0006-y)

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