Material science and Engineering

We report on a micromagnetic numerical simulation study of dynamic coupling between neighboring skyrmions periodically arranged in narrow-width nanostrips. We explored the coupled gyration modes and their characteristic dispersions in terms of the interdistance between the neighboring skyrmions. The application of perpendicular magnetic fields allows for the control and modification of the dispersion of the coupled gyration modes. The coupled gyration modes of individual skyrmions might provide a new type of information carrier in narrow-width straight and curved nanostrips, as driven by magnetic interactions in such continuous thin films. Results Intrinsic dynamic modes of single skyrmion in nano-square dot. In the simulation, first, to determine the intrinsic dynamic modes of an isolated skyrmion, we employed, as shown in Fig. 1(a), a single skyrmion of downward core orientation in a 40 nm-wide square dot of thickness of h = 1 nm. Sinc-function fields, denoted as H = H 0 sin[ω H (t − t 0)]/[ω H (t − t 0)] with H 0 = 10 Oe, ω H = 2π × 50 GHz, and t 0 = 1 ns were applied along the y or

We studied the stability of skyrmion formation and its abnormal dynamic motions in magnetic nanotubes with perpendicular magnetic anisotropy with respect to the tube surface. Unlike planar geometry, skyrmions, which are formed in an elliptical shape, are stabilized, to a greater or lesser degree, by the curved geometry, depending on the sign of the Dzyaloshinskii-Moriya Interaction (DMI) constant. The positive and negative signs of the DMI constant play a crucial role in the formation of skyrmions on the curved surface. These effects become stronger as the diameter of the tube decreases, quantitatively evidencing that the curvature-induced DM-like interaction also contributes to the stability of skyrmion formation. The dynamic modes of the skyrmion in the tubes are found to be as follows: two in-plane gyration modes in the counterclockwise (CCW) and clockwise (CW) rotation senses, and one out-of-plane breathing mode. On the other hand, in contrast to the planar geometry, the initial CCW gyration motion turns out to be a linear translational motion in the circumferential direction of the tube for cases where the strength of axial AC magnetic fields is greater than a threshold field strength. The direction of the translational motions is determined by the skyrmion helicity governed by the sign of the interfacial DMI constant. This work provides further physical insight into the static and dynamic properties of skyrmions formed in curved-geometry systems and also suggests its potential application to information processing devices.

Magnetic skyrmions have been intensively studied because they are promising as a potential candidate for information-storage and -processing devices owing to their robust features including nano-scale size, topological stability, and ultra-low threshold current density necessary for their motions [1]. Also, the gyration and breathing dynamic modes of single skyrmions are found[2-4]. Therefore, collective excitations of those modes in oneor two-dimensional arrays of skyrmions are of increasing interest from both fundamental and technological aspects. Here, we report on a delicate study, using micromagnetic numerical simulations, of dynamic coupling between neighboring skyrmions in narrow-width nanostrips where two or more skyrmions are periodically arranged. We found that there exist strongly coupled modes of both the gyration and breathing excitations that exhibit their characteristic dispersions in nanostrips. Moreover, the application of perpendicular magnetic fields allows for th...

We explored strong coupling of dynamic breathing modes in one-dimensional (1D) skyrmion lattices periodically arranged in thin-film nanostrips. The coupled breathing modes exhibit characteristic concave-down dispersions that represent the in-phase high-energy mode at zero wavenumber (k=0) and the anti-phase low-energy mode at the Brillouin zone boundary (k=kBZ). The band width of the allowed modes increases with decreasing inter-distance between nearest-neighboring skyrmions. Furthermore, the collective breathing modes propagate very well through the thin-film nanostrips, as fast as 200 ~ 700 m/s, which propagation is controllable by the strength of magnetic fields applied perpendicularly to the film plane. The breathing modes in 1D skyrmion lattices potentially formed in such nanostrips possibly can be used as information carriers in information processing devices.

We report on a micromagnetic numerical simulation study of dynamic couplings between neighboring skyrmions in narrow-width nanostrips. We explored the coupled gyration modes and their characteristic dispersions in terms of the interdistance between the neighboring skyrmions. The application of perpendicular magnetic fields, importantly, allows for control/modification of the dispersion of the coupled modes. Coupled skyrmion gyration modes provide a new type of efficient, reliable, fast, low-power-consumption informationsignal propagation in narrow-width straight and curved nanostrips, as driven predominantly by the exchange interaction between individual nano-scale skyrmions.

Direct control of microRNA (miRNA) expression by oncogenic and tumor suppressor networks results in frequent dysregulation of miRNAs in cancer cells and contributes to tumorigenesis. We previously demonstrated that activation of the c-Myc oncogenic transcription factor (Myc) broadly influences miRNA expression and in particular leads to widespread miRNA down-regulation. miRNA transcripts repressed by Myc include several with potent tumor suppressor activity such as miR-15a/16 -1, miR-34a, and let-7 family members. In this study, we have investigated mechanisms downstream of Myc that contribute to miRNA repression. Consistent with transcriptional down-regulation, Myc activity results in the decreased abundance of multiple miRNA primary transcripts. Surprisingly, however, primary transcripts encoding several let-7 miRNAs are not reduced in the high Myc state, suggesting a posttranscriptional mechanism of repression. The Lin-28 and Lin-28B RNA binding proteins were recently demonstrated to negatively regulate let-7 biogenesis. We now show that Myc induces Lin-28B expression in multiple human and mouse tumor models. Chromatin immunoprecipitation and reporter assays reveal direct association of Myc with the Lin-28B promoter resulting in transcriptional transactivation. Moreover, we document that activation of Lin-28B is necessary and sufficient for Myc-mediated let-7 repression. Accordingly, Lin-28B loss-of-function significantly impairs Myc-dependent cellular proliferation. These findings highlight an important role for Lin-28B in Myc-driven cellular phenotypes and uncover an orchestration of transcriptional and posttranscriptional mechanisms in Myc-mediated reprogramming of miRNA expression.

A ferrohydrodynamic energy harvesting system based on magnetorheology was investigated. A physical model was introduced to understand the ferrohydrodynamic system with four interactive forces including buoyancy, interfacial tension, drag, and electromotive yield forces. The magnetic flux density increases with increase in the magnetic field strength, while the rate of change of the magnetic flux density decreases with increase in the yield stress due to retardation of the air droplet motion. A numerical simulation was performed to understand the fundamental mechanism, especially one single up-and-down peak. The results show that the air droplet motion in the magnetic field determines the characteristics of the harvested energy by disturbing the magnetic flux density. The output voltage and current are generated according to Faraday's law of induction.

Hybrid organic-inorganic perovskite solar cells recently have received much interest for next generation photovoltaic devices due to simple manufacturing process, low-cost, and high energy conversion efficiency. Relatively large bandgap of perovskite materials enables the fabrication of tandem solar cells with small bandgap Si or thin film solar cells. However, Si cell usually has random pyramid texture which makes it hard to form compact and crystalline perovskite layer on top of Si layer through conventional solution spin-coating technique. Electrodeposition method is scalable, low-cost process, and widely used way to form the various film regardless of substrate morphology.

quinolin-2-ylmethyl)ethane-1,2-diamine (L5), tris(benzimidazol-2-ylmethyl)amine (L6) and tris(pyrid-2ylmethyl)amine (L7), have been isolated and characterized using CHN analysis, UV-Visible spectroscopy and mass spectrometry. The single-crystal X-ray structures of the complexes [Ni(L1)(CH 3 CN)(H 2 O)](ClO 4 ) 2 1a, [Ni(L2)(CH 3 CN) 2 ](BPh 4 ) 2 2, [Ni(L3)(CH 3 CN) 2 ](BPh 4 ) 2 3 and [Ni(L4)(CH 3 CN) 2 ](BPh 4 ) 2 4 have been determined. All these complexes possess a distorted octahedral coordination geometry in which Ni(II) is coordinated to four nitrogen atoms of the tetradentate ligands and two CH 3 CN (2, 3, 4) or one H 2 O and one CH 3 CN (1a) are located in cis positions. The Ni-N py bond distances (2.054(2)-2.078(3) Å ) in 1a, 2 and 3 are shorter than the Ni-N amine bonds (2.127(2)-2.196(3) Å ) because of sp 2 and sp 3 hybridizations of the pyridyl and tertiary amine nitrogens respectively. In 3 the Ni-N im bond (2.040(5) Å ) is shorter than the Ni-N py bond (2.074(4) Å ) due to the stronger coordination of imidazole compared with the pyridine donor. In dichloromethane/acetonitrile solvent mixture, all the Ni(II) complexes possess an octahedral coordination geometry, as revealed by the characteristic ligand field bands in the visible region. They efficiently catalyze the hydroxylation of alkanes when m-CPBA is used as oxidant with turnover number (TON) in the range of 340-620 and good alcohol selectivity for cyclohexane (A/K, 5-9). By replacing one of the pyridyl donors in TPA by a weakly coordinating -NMe 2 or -NEt 2 donor nitrogen atom the catalytic activity decreases slightly with no change in the selectivity. In contrast, upon replacing the pyridyl nitrogen donor by the strongly s-bonding imidazolyl or sterically demanding quinolyl/benzimidazolyl nitrogen donor, both the catalytic activity and selectivity decrease, possibly due to destabilization of the intermediate [(4N)(CH 3 CN)Ni-O ∑ ] + radical species. Adamantane is selectively (3 • /2 • , 12-17) oxidized to 1-adamantanol, 2-adamantanol and 2-adamantanone while cumene is selectively oxidized to 2-phenyl-2-propanol. In contrast to cyclohexane oxidation, the incorporation of sterically hindering quinolyl/benzimidazolyl donors around Ni(II) leads to a high 3 • /2 • bond selectivity for adamantane oxidation. A linear correlation between the metal-ligand covalency parameter (b) and the turnover number has been observed.

Electrochemical water oxidation is considered as one of the most important reactions for a sustainable future. However, in heterogeneous water oxidation, atomistic understanding of this fourelectron reaction is still elusive. In particular, the mechanism of O− O bond formation, the coupled transfer of proton and electron, and the hopping of high-valent metal-oxo intermediates are challenging issues. To date, binding energy related descriptors have been used successfully to predict the catalytic activities by matching the experimental data and quantitatively comparing the performances. In this Perspective, we attempt to emphasize the significance of entropic contribution, like enthalpy, by highlighting recent literature and calculating the entropy from the available data. We envision that this Perspective can suggest a new research direction toward the catalyst design to control the entropy during water oxidation and be technologically useful for characterizing the temperature effects.

Experimental methods Materials Ni(CH3COO)3-4H2O, 5-hydroxymethylfurfural (HMF, 98%) were purchased from Acros and used as received without additional purification. Mn(CH3COO)3-2H2O (99%), Co(CH3COO)3-4H2O (≥98%), myristic acid (CH3(CH2)12COOH, ≥99%), 1-octadecene (CH3(CH2)15CH=CH2, 90%), 1-decanol (CH3(CH2)8CH2OH, ≥98%), 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), 5-formyl-2-furoic acid (FFCA) were purchased from Sigma Aldrich. 2,5diformylfuran (DFF, 98%) was purchased from Tokyo Chemical Industry. Potassium bicarbonate was purchased form Daejung Chemical & Metals. All chemicals were used as received without additional purification. Toray carbon paper (TGP-H-60) and Ni foil (0.1 mm thick, 99.5%) were purchased from Alfa Aesar, and Fluorine-doped tin oxide glass (FTO-2207A) with sheet resistance of 8 ohms/sq were manufactured by Wooyang GMS (Korea). Synthesis of NiO, Mn3O4, Co3O4 nanoparticles (NPs) Three kinds of transition metal oxide NPs (NiO, Mn3O4 and Co3O4 NPs) were synthesized by a hot injection method by modifying synthetic methods in previous reports. 1-3 First, two different mixtures were made: one (Solution 1) was 1 mmol of nickel acetate (manganese or cobalt acetate) and 2 mmol of myristic acid in 20 ml of 1-octadecene, and the other (Solution 2) was 3 mmol of 1-decanol in 1 ml of 1-octadecene. These two mixtures were degassed at 110 ℃ for 2 hours with vigorous stirring. After degassing, the Solution 1 was heated up to 295 ℃ under argon atmosphere. When the temperature of 295 ℃, the Solution 2 was rapidly injected into the Solution 1 to cause the burst nucleation along with super-saturation. The mixture was maintained at 295 ℃ for 1 hour under argon atmosphere and then, cooled down to room temperature. S2 Electrode preparation After the synthesis of the solution with the NiO, Mn3O4, Co3O4 NPs, the mixture of the Co3O4 NPs solution, acetone, and toluene in the volume ratio of 1:2:1 were centrifuged 3 times to purify and obtain nanoparticles. The precipitate of nanoparticles was dispersed in hexane. Then, 40 ul of the dispersion was drop-coated on the carbon paper or spin-coated on FTO (for UV

Manganese plays multiple role in many biological redox reactions in which it exists in different oxidation states from Mn(II) to Mn(IV). Among them the high-valent manganese-oxo intermediate plays important role in the activity of certain enzymes and lessons from the natural system provide inspiration for new developments of artificial systems for a sustainable energy supply and various organic conversions. This review describes recent advances and key lessons learned from the nature on high-valent Mn-oxo intermediates. Also we focus on the elemental science developed from the natural system, how the novel strategies are realised in nano particles and molecular sites at heterogeneous and homogeneous reaction conditions respectively. Finally, perspectives on the utilisation of the high-valent manganese-oxo species towards other organic reactions are proposed.

The development of active water oxidation catalysts is critical to achieve high efficiency in overall water splitting. Recently, sub-10-nm-sized monodispersed partially-oxidized manganese oxide nanoparticles were shown to exhibit not only superior catalytic performance for oxygen evolution, but also unique electrokinetics, compared to their bulk counterparts. In the present work, the water-oxidizing mechanism of partially-oxidized MnO nanoparticles was investigated using integrated in-situ spectroscopic and electrokinetic analyses. We successfully demonstrated that, in contrast to previously reported manganese (Mn)-based catalysts, Mn(III) species are stably generated on the surface of MnO nanoparticles via a proton-coupled electron transfer pathway. Furthermore, we confirmed as to MnO nanoparticles that the one-electron oxidation step from Mn(II) to Mn(III) is no longer the rate determining step for water oxidation and that Mn(IV)=O species are generated as reaction intermediates d...