"Nature" and "Science" Week (2.13-2.19) Frontiers of Materials Science

Magnetic heterostructure topological insulator
(A magneticheterostructure of topological insulators as a candidate for an axion insulator) An axial insulator that can exhibit a singularly quantized magnetoelectric effect is one of the most interesting quantum phases of a three-dimensional topological insulator (TI). The axial insulator state is expected to occur in the magnetically doped TIs and the magnetization is directed inward and outward from the respective surface. To achieve axial insulators, Mogi et al. developed a TI heterostructure in which magnetic ions (Cr) are only modulated to be doped near the top and bottom surfaces of a TI((Bi,Sb)2Te3) film. The separation layer between the two magnetic layers weakens the interlayer coupling between them, enabling magnetization reversal of the respective layers. In the electrical transmission characteristics, the realization of the shaft insulator was demonstrated by observing the zero Hall platform (ZHP) in which the Hall and longitudinal conductance became zero, and other possible causes of ZHP were excluded. The performance of axial insulators can lead to a new phase of research on new magnetoelectric responses in topological materials. (Nature Materials DOI: 10.1038/NMAT4855)

2. Controllable chiral horizontal carbon nanotube array growth
(Arrays ofhorizontal carbon nanotubes of controlled chirality grown using designedcatalysts) The semiconductor industry is increasingly thinking that Moore's Law (predicting the number of transistors in a single microprocessor chip doubling every two years) is coming to an end. Therefore, the search for alternative semiconductor materials for nanoelectronic devices, including single-walled carbon nanotubes (SWNTs), is continuing. Horizontal nanotube arrays are particularly attractive for technical applications because of optimized current output. However, direct growth of controllable chiral horizontal SWNT arrays that can be applied to a wider range of applications and ensure device uniformity is not yet achievable. Zhang et al. reported that a horizontal array of metal SWNTs can be grown on the surface of a solid carbide catalyst by controlling the symmetry of the surface of the active catalyst. They obtained horizontally aligned metal SWNT arrays with an average density greater than 20 tubes/μm, 90% of which had a chirality index of (12,6) and a semiconductor SWNT array with an average density greater than 10 tubes/μm Among them, 80% of the nanotubes have a chiral index of (8, 4). Nanotubes are grown using uniform size Mo2C and WC solid catalysts. Thermodynamically, SWNTs are selectively nucleated by matching their structural symmetry and diameter to the structural symmetry and diameter of the catalyst. The chiral index of nanotubes grown by Zhang et al. is (2m, m) (where m is a positive integer), which can be increased by increasing the concentration of carbon to maximize the kinetic growth rate in the chemical vapor deposition process. Compared to previously reported methods, such as cloning, seeding, and specific structure-matched growth, Zhang et al.'s strategy for controlling thermodynamics and kinetics provides greater freedom to adjust the chirality of growing SWNTs in the array, and also Can be used to predict growth conditions that achieve the desired chirality. (Nature DOI: 10.1038/nature21051)

3. Contact passivation to achieve efficient and stable solution treatment of planar perovskite solar cells
(Efficient andstable solution-processed planar perovskite solar cells via contact passivation) Planar perovskite solar cells (PSCs) fabricated entirely by solution treatment at low temperatures (<150 °C) offer promise for simple, flexible substrate compatibility and perovskite-based tandem devices. However, these PSCs require a similarly processed electronic selection layer. Tan et al. reported a contact passivation strategy that uses end group chlorinated TiO2 colloidal nanocrystalline films to mitigate interfacial recombination and improve interfacial bonding in low temperature planar solar cells. Solar cells with an effective area of ​​0.049 cm 2 and 1.1 cm 2 and an empirical efficiency of 20.1% and 19.5% were prepared by low temperature solution treatment. The solar cell with an efficiency greater than 20% is operated under a sunlight (1 sunlight is defined as AM1.5, or 1 kW/m2 standard illumination), and it operates continuously for 500 hours at room temperature at the maximum power point. After performance can be maintained at 90% (97% after dark recovery). (Science DOI: 10.1126/science.aai9081)

4. Wigner-Huntington conversion to achieve atomic metal hydrogen
(Observation of the Wigner-Huntington transition to metallic hydrogen) The production of metallic hydrogen is a huge challenge in condensed matter physics. Metal hydrogen can act as a room temperature superconductor and can be metastable after pressure release, and also has an important impact on energy and rocket science. Dias et al. studied solid molecular hydrogen at low temperatures and pressures. At a pressure of 495 gigapascals, hydrogen becomes metallic with a reflectivity of up to 0.91. Dias et al. used the Drude free electron model to fit the reflectance and determined a plasma frequency of 32.5 ± 2.1 eV at a temperature of 5.5 Kelvin. The corresponding electron carrier density was 7.7 ± 1.1 × 1023 particles per cubic centimeter. This result is consistent with the theoretical estimate of atomic density. The nature is the nature of atomic metals. And they have produced Wigner-Huntington conversion to atomic metal hydrogen in the lab. (Science DOI: 10.1126/science.aal1579)

5. Simultaneous polymerization, crystallization and self-assembly to achieve uniform one-dimensional nanoparticles
(Scalable and uniform1D nanoparticles by synchronous polymerization, crystallization and self-assembly) The use of solution processing techniques on a commercial scale to prepare soft material-based nanoparticles is a very important challenge. The self-assembly of block copolymers to selectively solvate one of the segments provides a promising approach to achieving nuclear-corona nanoparticles (micelles) with a wide range of potential uses. Even so, there are significant limitations to this approach. For example, solution processing of block copolymers generally follows a separate synthetic step and is typically carried out at high dilutions. In addition, non-spherical micelles that are expected to be used in many applications are often difficult to achieve because the sample is polydisperse and it is not possible to precisely control the size. Boott et al. demonstrated the formation of solids of sheet-like cylindrical micelles at a concentration of up to 25% via one-pot synthesis (from monomer initiation-binding polymerization induction and crystallization driven self-assembly). In the presence of small seed micelles, the process can scale up low-dispersion samples of cylindrical micelles up to three microns in length. (Nature Chemistry DOI: 10.1038/NCHEM.2721)

6. Synthesis and characterization of triangulene
(Synthesis and characterization of triangulene) Triangulene, the smallest triplet polyphenylene (also known as carat hydrocarbon), has been a mysterious molecule since it was first assumed to exist. Although it contains an even number of carbon atoms (22 in six fused benzene rings), it is impossible to derive a Kekulé resonance structure for the entire molecule: any attempt will result in two unpaired valence electrons. Although the addition of substituents can achieve the stabilization and synthesis of the triangulene core, the triple ground state of triangulene is determined by electron paramagnetic resonance, but due to its extremely high reactivity, the synthesis and characterization of unsubstituted triangulene has not been achieved. Pavliček et al. demonstrated the surface formation of unsubstituted triangulene consisting of six fused benzene rings and the dehydrogenation of precursor molecules using a combination of scanning tunneling microscopy and atomic force microscopy (STM / AFM). STM measurement combined with density functional theory (DFT) calculations confirmed that triangulene retains its free molecular properties on the surface, while AFM measurements resolve the planar triple-symmetric molecular structure. The unique topology of this non-Kekulé hydrocarbon results in open-shell π-conjugated graphene fragments, which produce high spin ground states and may be used in organic spintronic devices. This synthetic method by Pavliček et al. enables multiple experiments to study triangulene and related open-shell fragments at the single molecule level. (Nature Nanotechnology DOI: 10.1038/NNANO.2016.305)

7. The general lower limit of vortex creep of superconductors
(Universallower limit on vortex creep in superconductors) Superconductivity is a good test platform for studying vortex, topological excitation, liquid crystal and Bose-Einstein condensate. The vortex motion can be destructive: it can cause phase changes, pulsar rotation abrupt changes, and losses in superconducting microwave circuits, and it also limits the current carrying capacity of the superconductor. Understanding vortex dynamics is important both fundamentally and technically, and the competition between thermal and energy barriers defined by material disorder is not fully understood. Specifically, early measurements of thermally activated vortex motion (creep) in iron-based superconductors revealed a fast rate (S) comparable to that of YBa2Cu3O7-δ. This is confusing because S is considered to have a relationship with the Ginzburg number (Gi) and Gi is significantly lower than the value in YBa2Cu3O7-δ in most iron-based superconductors. Eley et al. reported a very slow creep in the BaFe2(As0.67P0.33)2 film and proposed the existence of a universal minimum S to Gi1/2 (T/Tc) (Tc is the superconducting transition temperature). Already implemented in their films and a few other materials. This limitation provides new clues to the use of slow creep and the interaction of material parameters with vortex dynamics to design materials. (Nature Materials DOI: 10.1038/NMAT4840)
The hydrogen evolution reaction (HER) is a key step in the electrolysis of water, requiring efficient, stable and inexpensive electrocatalysts. The catalyst must initiate proton reduction with minimal overpotential and has rapid reaction kinetics. The most effective catalyst in acidic solutions is based on Pt because the strength of the Pt-H bond is related to the rapid reaction rate of HER. However, the use of Pt involves problems of cost and stability. Recently, research on non-precious metal-based catalysts has been reported, but such catalysts are very sensitive to acid corrosion and have slightly worse performance than Pt-based catalysts, exhibiting higher overpotentials and lower stability. Compared with Pt, Ru has a bond strength similar to that of Pt-H when it bonds with H (~65 kcal/mol), but little research has been done on it as a HER catalyst. The Baek group in South Korea reported the catalytic performance of Ru-based catalysts for HER in acidic and alkaline environments. This catalyst is a pore-containing two-dimensional carbon structure (Ru@C2N) in which Ru nanoparticles are dispersed in nitriding. This catalyst exhibits a high conversion frequency at 25 mV (0.67 H2/s in 0.5 MH2SO4; 0.75 H2/s in 1.0 M KOH species) and a small overpotential at 10 mA/cm2 current density (at 0.5 M) 13.5 mV in H2SO4; 17 mV in 1.0M KOH) and good stability. This performance is comparable to or better than commercial Pt/C. (Nature Nanotechnology, DOI:10.1038/NNANO.2016.304)

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