"Nature" and "Science" Week (8.7-8.13) Frontiers of Materials Science

1. Metal-catalyzed electrochemical diazidation of alkenes O-diamines are structural motifs commonly found in bioactive natural products, therapeutic agents, and molecular catalysts. This stimulates the sustainability of efficient, alternative preparation techniques. Fu et al. reported a simple and environmentally friendly solution for converting olefins and sodium azide (two readily available materials) to 1,2-diazonium. This conversion is driven by electricity and is catalyzed by earth-rich manganese and under mild conditions, exhibiting excellent substrate connectivity and functional group compatibility. Using standard protocols, the resulting 1,2-diazide can be smoothly reduced to an ortho-diamine in a single step with high chemical selectivity. The results of the mechanism study are consistent with the experimental results of metal-involved azide radical transfer as the main pathway, making the formation of double carbon-nitrogen bonds possible. (Science DOI: 10.1126/science.aan6206)

2. Chaotic dynamics in nanoscale NbO2 Mott memristors for analogue computing Currently, machine learning systems use a simplified neuron model that lacks the rich nonlinear phenomena observed in biological systems that exhibit temporal and spatial synergistic dynamics. There is evidence that neurons work in a situation called chaotic edge, which may be at the heart of complexity, learning efficiency, adaptability, and computational (non-boolean) computation in the brain. Neural networks exhibit enhanced computational complexity when running at chaotic edges, and chaotic elemental networks have now been used to solve problems of combinatorial or global optimization. Therefore, the source of controllable chaotic characteristics that can be incorporated into neural heuristic circuits may be an important part of future computing systems. These chaotic components can already be simulated using sophisticated transistor circuits that simulate known chaotic equations, but it has been experimentally difficult to implement chaotic dynamics through a single scalable electronic device. Kumar et al. describe a single NbO2 Mott memristor that is less than 100 nanometers and exhibits a current-controlled negative differential resistance that is both a nonlinear transfer drive and a Mott-transition driven temperature-controlled negative differential resistance. Mott materials have temperature-dependent metal-insulator transitions that can be used as electronic switches. Kumar et al. combined these memristors into a relaxation oscillator to observe the adjustable range of periodic and chaotic self-vibration, demonstrating the coupling of nonlinear current propagation and nanoscale thermal fluctuations to generate chaotic oscillations. Such memristors may be useful in certain types of neuro-heuristic calculations, and by introducing pseudo-random signals that prevent global synchronization, may also help find global minima during constraint search. Incorporating such a memristor into the hardware of the Hopfield computing network can greatly improve the efficiency and accuracy of solutions that conflate difficult-to-calculate problems. (Nature DOI: 10.1038/nature23307)

3. Cooperative carbon-atom abstraction from alkenes in the core of a pentanuclear nickel cluster at the core of the five-nuclear nickel cluster Although the idea of ​​shearing CC bonds through soluble transition metal complexes in unactivated hydrocarbons is still a challenge to date, this reaction has the potential to achieve previously unimaginable skeleton transformation. For example, it is conceivable to disassemble and recombine the CC and CH bonds normally observed in surface catalysis, but will eventually belong to homogeneous catalysis as control increases. Shoshani et al. reported a penta-nuclear nickel cluster that does not react with functional groups such as alcohols, amines, and even water, but is capable of selectively cracking simple olefins such as ethylene, styrene, and isobutylene at temperatures as low as -30 °C. The C=C bond has an almost quantitative yield. In the reaction with styrene and isobutylene, the separation of the intermediates indicated that the five nickel centers synergistically activated the three CH bonds of the olefin substrate prior to the CC bond in the core of the split cluster, resulting in a five-nuclear nickel carbide. Net organic product conversion is the extraction of carbon atoms from olefins. (Nature Chemistry DOI: 10.1038/NCHEM.2840)

4. Magnetic quantum phase transition in Cr-doped Bi2(SexTe1−x)3 driven by the Stark effect in Cr-doped Bi2(SexTe1−x)3 Recent experimental observations of the quantum anomalous Hall effect have led researchers to pay great attention to magnetic topological insulators. In the magnetic counterparts of these conventional topological insulators such as Bi2Te3, long-range ferromagnetic states can be established by chemical doping of transition metal elements. However, more complex electronic phase diagrams may occur, and in the specific example of Cr-doped Bi2(SexTe1−x)3, magnetic quantum phase transitions regulated by actual chemical compositions have been reported. From an application-oriented perspective, the relevance of these results depends on the possibility of manipulating the magnetic and electronic band topology through external disturbances. External disturbances can be achieved by an electric field such as that produced by a gate electrode (similar to what has been achieved in conventional dilute magnetic semiconductors). Zhang et al. studied the magnetic transport properties of Cr-doped Bi2(SexTe1−x)3 with different compositions under gate voltage. Of all the components studied, except for samples close to the topological quantum critical point where the gate voltage reversibly drives the ferromagnetic-paramagnetic phase transition, the effect of the electric field on the magnetic sequence is negligible. Theoretical calculations show that due to the Stark effect, the vertical electric field causes the shift of the electron energy level, which promotes the topological quantum phase transition, which in turn produces a magnetic phase transition. (Nature Nanotechnology DOI: 10.1038/NNANO.2017.149)

5. Measurement of the spin temperature of optically cooled nuclei and GaAs hyperfine constants in GaAs/AlGaAs quantum dots The deep cooling of electrons and nuclear spins is equivalent to achieving nearly 100% polarization, which is a key requirement for solid state quantum information technology. Although the polarization of a single nuclear spin in diamond and SiC is above 99%, the polarization of the core in a quantum dot is limited to 50-65%. The theoretical model attributed this limitation to the coherent "dark" nuclear spin states formed, but the experimental verification is insufficient, especially the accuracy of the polarization measurement is relatively poor. Chekhovich et al. used a new method of manipulating nuclear spin states using radio frequency pulses to measure nuclear polarization in GaAs/AlGaAs quantum dots with high precision, and observed polarizations of up to 80%. This is the highest result of optical cooling of quantum dots reported so far. This value is not limited by the nuclear coherence effect. Chekhovich et al. found that optically cooled cores are well described in traditional spin temperature frameworks. The results of this study laid the foundation for the further development of quantum dot electron spin qubits, which utilize the deep cooling of the mesoscopic nuclear spin system to achieve long qubit coherence. In addition, they also conducted experimental measurements on GaAs super-precision material constants for the first time. (Nature Materials DOI: 10.1038/NMAT4959)

6. Nanostructured organic semiconductor films for molecular detection with surface-enhanced Raman spectroscopy Π-conjugated organic semiconductors have been explored in a variety of optoelectronic devices, but their ability to be used as surface enhanced Raman spectroscopy (SERS) active platforms in molecular detection is unknown. Yilmaz et al. demonstrated that the molecular semiconductor α,ω-diperfluorohexylquaterthiophene (α,ω-diperfluorohexyltetrathiophene) (DFH-4T) can be easily fabricated by vapor phase deposition to have SERS activity, superhydrophobicity and Nanostructured membrane of the spring. This DFH-4T film without any additional plasmonic layer can exhibit up to 3.4 x 103 Raman signal enhancement to the probe molecule methylene blue. Quantum mechanical calculations, comparison experiments with α,ω-dihexylquaterthiophene (α,ω-dihexyltetrathiophene) (DH-4T) without fluorine, and thin film microstructure analysis, the combination of the three proved π- The conjugated core fluorocarbon substitution and the unique DFH-4T film morphology play a leading role in SERS enhancement. In addition, by coating a thin layer of gold on the surface of the DFH-4T film, Raman signal enhancement up to about 1010 and detection of sub-Putomolyl (<10-21 moles) of the analyte can be achieved. The results of this paper provide important guidance for the molecular design of SERS reactive organic semiconductors and the ease of fabrication of SERS platforms for ultrasensitive trace analysis. (Nature Materials DOI: 10.1038/NMAT4957)

7. Photon-triggered nanowire transistors Electronic circuits that implement photon triggering have long been a long-term goal of photonics. Recent demonstrations are either all-optical transistors that photons control other photons, or phototransistors that enhance photon tuning or enhance gate response. However, only a few studies have reported devices that optically convert and amplify current without an electronic gate. Kim et al. show photon-triggered nanowire (NW) transistors, photon-triggered NW logic gates, and a single NW photodetection system. NW is synthesized by a long crystalline silicon (CSi) segment joined by short porous silicon (PSi) segments. In the fabricated device, the electrical contacts at both ends of the NW are connected to a single PSi segment in the middle. Exposing the PSi segment triggers the current in NW and has a high on/off ratio greater than 8×106. Two separate optical input signals can be used to trigger a device that contains two PSi segments along the NW. Using local pump lasers, Kim et al. demonstrated photon-triggered logic gates including AND, OR and NAND gates. A 25 nm diameter photon-triggered NW transistor with a single 100 nm PSi segment requires less than 300 pW of power. In addition, they use high photosensitivity to create sub-micron resolution photodetection systems. Photon trigger transistors offer new options for multi-function device applications such as programmable logic components and ultra-sensitive photodetectors. (Nature Nanotechnology DOI: 10.1038/NNANO.2017.153)

8. Quantifying ligand effects in high-oxidation-state metal catalysis Catalysis using high-valent metals such as titanium (IV) affects our daily lives through reactions similar to olefin polymerization. Optimization in any catalytic reaction involves not only careful selection of the metal, but also careful selection of the ancillary ligand. Because these choices can significantly affect the electronic structure of the system, which in turn has an impact on catalyst performance, there is an urgent need for new tools for developing catalysts. Understanding the ancillary ligand effect can be said to be one of the most critical aspects of catalyst optimization. Although the phosphine parameters of low-cost systems have been used for decades, there is no comparable system for high-priced metals. New ligand electronic parameters experimentally derived from high-valent chromium species are currently available. Billow et al. demonstrated new parameters that can quantitatively determine the effect of ancillary ligands on the rate of catalysis and, in some cases, even provide mechanistic information. Analysis of the reaction in this manner allows for a better catalyst structure and paves the way for the use of these parameters in many high-priced processes. (Nature Chemistry DOI: 10.1038/NCHEM.2843)

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