Taiwan is situated less than 200 kilometers from the first COVID‐19 outbreak state, China, and it has millions of international visitors yearly. Taiwan's collective efforts to block and eliminate the ...invisible enemy (COVID‐19) from the island have resulted in relatively low infection and death numbers and have been hailed as a successful anomaly amid the global pandemic. This review provides some background on the systems and organizations that helped Taiwan streamline a task force (command center) in a timely manner to launch related initiatives, mobilize the public, and engage private resources to implement strategies and policies that were further enhanced by collaborative behaviors and volunteers. Even subject to threatening conditions such as cruise ship stopover and numerous foreign immigrant workers, there were no outbreaks of community infection in Taiwan similar to those in Singapore, Japan, and other countries. Taiwan's successful measures offer a good example for future comparative studies.
Radical borylation using N‐heterocyclic carbene (NHC)‐BH3 complexes as boryl radical precursors has emerged as an important synthetic tool for organoboron assembly. However, the majority of reported ...methods are limited to reaction modes involving carbo‐ and/or hydroboration of specific alkenes and alkynes. Moreover, the generation of NHC‐boryl radicals relies principally on hydrogen atom ion with the aid of radical initiators. A distinct radical generation method is reported, as well as the reaction pathways of NHC‐boryl radicals enabled by photoredox catalysis. NHC‐boryl radicals are generated via a single‐electron oxidation and subsequently undergo cross‐coupling with the in‐situ‐generated radical anions to yield gem‐difluoroallylboronates. A photoredox‐catalyzed radical arylboration reaction of alkenes was achieved using cyanoarenes as arylating components from which elaborated organoborons were accessed. Mechanistic studies verified the oxidative formation of NHC‐boryl radicals through a single‐electron‐transfer pathway.
NHC‐boryl radicals are generated by single‐electron oxidation of NHC‐BH3 complexes using a photoredox catalytic system. New reaction modes of NHC‐boryl radicals were determined for the facile synthesis of highly elaborated organoboron molecules. Key: N‐heterocyclic carbene (NHC), leaving group (LG).
We report first-principles and strongly correlated calculations of the newly discovered heavy fermion superconductor UTe2. Our analyses reveal three key aspects of its magnetic, electronic, and ...superconducting properties that include (i) a two-leg ladder-type structure with strong magnetic frustrations, which might explain the absence of long-range orders and the observed magnetic and transport anisotropy, (ii) quasi-two-dimensional Fermi surfaces composed of two separate electron and hole cylinders with similar nesting properties as in UGe2, which may potentially promote magnetic fluctuations and help to enhance the spin-triplet pairing, and (iii) a unitary spin-triplet pairing state of strong spin-orbit coupling at zero field, with point nodes presumably on the heavier hole Fermi surface along the kx direction, in contrast to the previous belief of nonunitary pairing. Our proposed scenario is in excellent agreement with latest thermal conductivity measurement and provides a basis for understanding the peculiar magnetic and superconducting properties of UTe2.
In this work, a record high thermoelectric figure‐of‐merit ZT of 1.6 ± 0.2 at 873 K in p‐type polycrystalline Bi0.94Pb0.06CuSe1.01O0.99 by a synergy of rational band manipulation and novel ...nanostructural design is reported. First‐principles density functional theory calculation results indicate that the density of state at the Fermi level that crosses the valence band can be significantly reduced and the measured optical bandgap can be enlarged from 0.70 to 0.74 eV by simply replacing 1% O with 1% Se, both indicating a potentially reduced carrier concentration and in turn, an improved carrier mobility and a boosted power factor up to 9.0 µW cm−1 K−2. Meanwhile, comprehensive characterizations reveal that under Se‐rich condition, Cu2Se secondary microphases and significant lattice distortions triggered by Pb‐doping and Se‐substitution can be simultaneously achieved, contributing to a reduced lattice thermal conductivity of 0.4 W m−1 K−1. Furthermore, a unique shear exfoliation technique enables an effective grain refinement with higher anisotropy of the polycrystalline pellet, leading to a further improved power factor up to 10.9 µW cm−1 K−2 and a further reduced lattice thermal conductivity of 0.30 W m−1 K−1, which gives rise to record high ZT.
A record high thermoelectric figure‐of‐merit ZT of 1.6 ± 0.2 at 873 K in p‐type polycrystalline Bi0.94Pb0.06CuSe1.01O0.99 by a synergy of rational band manipulation and novel nanostructural design is reported.
GaN-Based RF Power Devices and Amplifiers Mishra, Umesh K.; Shen, Likun; Kazior, Thomas E. ...
Proceedings of the IEEE,
02/2008, Volume:
96, Issue:
2
Journal Article
Peer reviewed
The rapid development of the RF power electronics requires the introduction of wide bandgap material due to its potential in high output power density, high operation voltage and high input ...impedance. GaN-based RF power devices have made substantial progresses in the last decade. This paper attempts to review the latest developments of the GaN HEMT technologies, including material growth, processing technologies, device epitaxial structures and MMIC designs, to achieve the state-of-the-art microwave and millimeter-wave performance. The reliability and manufacturing challenges are also discussed.
Organoboron compounds are highly valuable in synthetic chemistry. In particular, α-borylcarbonyl compounds have shown versatile synthetic applications, owing to fruitful chemistries of both the boryl ...and carbonyl moieties. However, the synthesis of these molecules still remains tedious and time-consuming. Here we report a straightforward and practical route to synthesize α-borylcarbonyl molecules based on a regioselective radical α-borylation of α,β-unsaturated carbonyl compounds. The reaction features unusual α-regioselectivity and high functional-group compatibility. Further synthetic applications of new α-borylated products were also demonstrated. DFT and kinetic studies implicated that the α-regioselectivity of β-aryl-α,β-unsaturated carbonyl compounds was determined by the thermodynamically more favorable radical α-addition step, whereas the formation of α-addition products from β-alkyl-α,β-unsaturated carbonyl compounds was driven by an energetically favored hydrogen atom transfer step. Given that α,β-unsaturated carbonyl compounds can be easily obtained in abundance and variety, this method enjoys great advantages in diverse and economical synthesis of valuable α-borylcarbonyl molecules.
This study assessed the diversity and distribution of endophytic fungal communities associated with the leaves and stems of four vascular plant species in the High Arctic using 454 pyrosequencing ...with fungal-specific primers targeting the ITS region. Endophytic fungal communities showed high diversity. The 76,691 sequences obtained belonged to 250 operational taxonomic units (OTUs). Of these OTUs, 190 belonged to Ascomycota, 50 to Basidiomycota, 1 to Chytridiomycota, and 9 to unknown fungi. The dominant orders were Helotiales, Pleosporales, Capnodiales, and Tremellales, whereas the common known fungal genera were Cryptococcus, Rhizosphaera, Mycopappus, Melampsora, Tetracladium, Phaeosphaeria, Mrakia, Venturia, and Leptosphaeria. Both the climate and host-related factors might shape the fungal communities associated with the four Arctic plant species in this region. These results suggested the presence of an interesting endophytic fungal community and could improve our understanding of fungal evolution and ecology in the Arctic terrestrial ecosystems.
A new and practical α‐monomethylation strategy using an amine‐borane/N,N‐dimethylformamide (R3N‐BH3/DMF) system as the methyl source was developed. This protocol has been found to be effective in the ...α‐monomethylation of arylacetonitriles and arylacetamides. Mechanistic studies revealed that the formyl group of DMF delivered the carbon and one hydrogen atoms of the methyl group, and R3N‐BH3 donated the remaining two hydrogen atoms. Such a unique reaction pathway enabled controllable assemblies of CDH2‐, CD2H‐, and CD3‐ units using Me2NH‐BH3/d7‐DMF, Me3N‐BD3/DMF and Me3N‐BD3/d7‐DMF systems, respectively. Further application of this method to the facile synthesis of anti‐inflammatory flurbiprofen and its varied deuterium‐labeled derivatives was demonstrated.
Selective monomethylation: An unprecedented and practical methylation protocol using a R3N‐BH3/DMF system as the methyl source has been developed for the selective α‐monomethylation of arylacetonitriles and arylacetamides. The formyl group of DMF delivers the carbon and one hydrogen atoms of the methyl group, and R3N‐BH3 donates the remaining two hydrogen atoms.
A dual temperature‐ and light‐responsive C2H2/C2H4 separation switch in a diarylethene metal–organic framework (MOF) is presented. At 195 K and 100 kPa this MOF shows ultrahigh C2H2/C2H4 selectivity ...of 47.1, which is almost 21.4 times larger than the corresponding value of 2.2 at 293 K and 100 kPa, or 15.7 times larger than the value of 3.0 for the material under UV at 195 K and 100 kPa. The origin of this unique control in C2H2/C2H4 selectivity, as unveiled by density functional calculations, is due to a guest discriminatory gate‐opening effect from the diarylethene unit.
Photochromic diarylethene units were used to identify C2H2, leading to ultrahigh C2H2/C2H4 selectivity and promising application in C2H2/C2H4 separation at low temperature. The photochromic material can be further used as a temperature‐ and light‐responsive switch for C2H2/C2H4 separation.
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