Resistive Avalanches in La1-xSrxCoO3-δ (x = 0, 0.3) Thin Films and Their Reversible Evolution by Tuning Lattice Oxygen Vacancies (δ).

Strong correlations are often manifested by exotic electronic phases and phase transitions. LaCoO3-δ (LCO) is a system that exhibits such strong electronic correlations with lattice-spin-charge-orbital degrees of freedom. Here, we show that mesoscopic oxygen-deficient LCO films show resistive avalanches of about 2 orders of magnitude due to the metal-insulator transition (MIT) of the film at about 372 K for the 25 W RF power-deposited LCO film on the Si/SiO2 substrate. In bulk, this transition is otherwise gradual and occurs over a very large temperature range. In thin films of LCO, the oxygen deficiency (0 < δ < 0.5) is more easily reversibly tuned, resulting in avalanches. The avalanches disappear after vacuum annealing, and the films behave like normal insulators (δ ∼0.5) with Co2+ in charge ordering alternatively with Co3+. This oxidation state change induces spin state crossovers that result in a spin blockade in the insulating phase, while the conductivity arises from hole hopping among the allowed cobalt Co4+ ion spin states at high temperature. The chemical pressure (strain) of 30% Sr2+ doping at the La3+ site results in reduction in the avalanche magnitude as well as their retention in subsequent heating cycles. The charge nonstoichiometry arising due to Sr2+ doping is found to contribute toward hole doping (i.e., Co3+ oxidation to Co4+) and thereby the retention of the hole percolation pathway. This is also manifested in energies of crossover from the 3D variable range hopping (VRH) type transport observed in the temperature range of 300-425 K, while small polaron hopping (SPH) is observed in the temperature range of 600-725 K for LCO. On the other hand, Sr-doped LCO does not show any crossover and only the VRH type of transport. The strain due to Sr2+ doping refrains the lattice from complete conversion of δ going to 0.5, retaining the avalanches.

Structural and Biochemical Studies on Klebsiella Pneumoniae Enoyl-ACP Reductase (FabI) Suggest Flexible Substrate Binding Site.

Klebsiella pneumoniae, a bacterial pathogen infamous for antibiotic resistance, is included in the priority list of pathogens by various public health organizations due to its extraordinary ability to develop multidrug resistance. Bacterial fatty acid biosynthesis pathway-II (FAS-II) has been considered a therapeutic drug target for antibacterial drug discovery. Inhibition of FAS-II enzyme, enoyl-acyl carrier protein reductase, FabI, not only inhibits bacterial infections but also reverses antibiotic resistance. Here, we characterized Klebsiella pneumoniae FabI (KpFabI) using complementary experimental approaches including, biochemical, x-ray crystallography, and molecular dynamics simulation studies. Biophysical studies shows that KpFabI organizes as a tetramer molecular assembly in solution as well as in the crystal structure. Enzyme kinetics studies reveal a distinct catalytic property towards crotonyl CoA and reducing cofactor NADH. Michaelis-Menten constant (Km) values of substrates show that KpFabI has higher preference towards NADH as compared to crotonyl CoA. The crystal structure of tetrameric apo KpFabI folds into a classic Rossman fold in which ß-strands are sandwiched between α-helices. A highly flexible substrate binding region is located toward the interior of the tetrameric assembly. Thermal stability assay on KpFabI with its substrate shows that the flexibility is primarily stabilized by cofactor NADH. Moreover, the molecular dynamics further supports that KpFabI has highly flexible regions at the substrate binding site. Together, these findings provide evidence for highly dynamic substrate binding sites in KpFabI, therefore, this information will be vital for specific inhibitors discovery targeting Klebsiella pneumoniae.

Computational analysis of the effect of Gly100Ala mutation on the thermostability of SazCA.

Maintaining the protein stability upon mutation is a challenging task in protein engineering. In the present computational study, we induced a single point Gly100Ala mutation in SazCA and examined the factors governing the stability and flexibility of the mutated form, and compared it to that of the wildtype using molecular dynamics simulations. We observed higher structural stability and lesser residual mobility in the mutated SazCA. Improved H-bonding due to Gly100Ala was observed. Ala100 was responsible for the increased helical contents in the mutated SazCA while Gly100 compromised the secondary structure contents in the wildtype. A strong network of salt bridges and high local ordering of the solvent molecules at the protein surface contributed to the enhanced stability of the mutated protein. Our simulations conclusively highlight Gly100Ala mutation as a step towards designing a more robust and thermostable SazCA.Communicated by Ramaswamy H. Sarma.

The New Ag-S Cluster [Ag50S13(StBu)20][CF3COO]4 with a Unique hcp Ag14 Kernel and Ag36 Keplerian-Shell-Based Structural Architecture and Its Photoresponsivity.

In metal nanoclusters (NCs), the kernel geometry and the nature of the surface protecting ligands are very crucial for their structural stability and properties. The synthesis and structural elucidation of Ag NCs is challenging because the zerovalent oxidation state of Ag is very reactive and prone to oxidization. Here, we report the NC [Ag50S13(StBu)20][CF3COO]4 with a hexagonal close-packed (hcp) cagelike Ag14 kernel. A truncated cubic shell and an octahedral shell encapsulate the hcp-layered kernel via an interstitial S2- anionic shell to form an Ag36 Keplerian outer shell of the NC. A theoretical study indicates the stability of this NC in its 4+ charge state and the charge distribution between the kernel and Keplerian shell. The unprecedented electronic structure facilitates its application toward sustainable photoresponse properties. The new insights into this novel Ag NC kernel and Keplerian shell structure may pave the way to understanding the unique structure and developing electronic structure-based applications.

Simple, reversible gradient Seebeck coefficient measurement system for 300-600 K with COMSOL simulations.

Seebeck measurement is a crucial step for characterizing thermoelectric samples, as measuring the accurate value with a simpler system design is challenging. Here, we report a simple design of the Seebeck coefficient measurement system, which can measure the thermo-emf (Seebeck coefficient) of the sample, under a limited temperature range of 300-600 K. Unlike the majority of the reported instrumental designs, the system does not have a hot walled chamber. The sample is sandwiched between two brass block supported heaters, which are controlled separately. Thus, this type of system is suitable for a window of the temperature range near room temperature. In this paper, we report the system that can measure the Seebeck coefficient up to 600 K. The heaters touch the sample through 1 mm thick silver caps, which offer insignificant thermal resistance and a stable temperature, as seen through experiment as well as COMSOL simulations. A typical sample has, at maximum, a diameter of 10 mm and a thickness of 2-3 mm. A reversible temperature gradient is applied in quasi-static direct current mode. By virtue of its design, the sample holder ensures a minimum thermal and electrical contact resistance during a measurement cycle. The combination of metals used for measurement (Ag and Cu) shows negligible junction contribution. The variance up to ±2% and accuracy up to 8% at a high temperature have been obtained using calibration sample reference data of state-of-the-art commercial systems.

Selective Enhancement in Phonon Scattering Leads to a High Thermoelectric Figure-of-Merit in Graphene Oxide-Encapsulated ZnO Nanocomposites.

ZnO is a promising candidate for use as an environmentally friendly thermoelectric (TE) material. However, high thermal conductivity leading to a poor TE figure-of-merit (zT) needs to be addressed to achieve a significant TE efficiency for commercial applications. Here, we demonstrate that selective enhancement in phonon scattering leads to an increase in the zT of ZnO because of Al doping and reduced graphene oxide (RGO) encapsulation. These nanocomposites are synthesized via a facile and scalable method. The incorporation of 1 at% Al with 1.5 wt % RGO into ZnO has been found to show significant improvement in zT (0.52 at 1100 K), which is an order of magnitude larger compared to that of bare undoped ZnO. Photoluminescence and X-ray photoelectron spectroscopy measurements confirm that RGO encapsulation significantly quenches surface oxygen vacancies in ZnO along with nucleation of new interstitial Zn donor states. Tunneling spectroscopy performed on bare as well as composite particles reveals that the band gap of ∼3.4 eV for bare ZnO reduces effectively to ∼0.5 eV upon RGO encapsulation, facilitating charge transport. The electrical conductivity also benefits from high densification (>95%) achieved using the spark plasma sintering method, which also aids in reduction of graphene oxide into RGO. The same Al doping and RGO capping synergistically bring about drastic reduction of thermal conductivity, through enhanced interfacial and point-defect-phonon scatterings. These opposing effects on electrical and thermal conductivities lead to enhancement in the power factors as well as the zT value. Overall, a practically viable route has been demonstrated for the synthesis of oxide-RGO TE materials, which could find their potential applications in high-temperature TE power generation.

A photocatalyst-enzyme coupled artificial photosynthesis system for solar energy in production of formic acid from CO2.

The photocatalyst-enzyme coupled system for artificial photosynthesis process is one of the most promising methods of solar energy conversion for the synthesis of organic chemicals or fuel. Here we report the synthesis of a novel graphene-based visible light active photocatalyst which covalently bonded the chromophore, such as multianthraquinone substituted porphyrin with the chemically converted graphene as a photocatalyst of the artificial photosynthesis system for an efficient photosynthetic production of formic acid from CO(2). The results not only show a benchmark example of the graphene-based material used as a photocatalyst in general artificial photosynthesis but also the benchmark example of the selective production system of solar chemicals/solar fuel directly from CO(2).

Studies on the sensing behaviour of nanocrystalline CuGa(2)O(4) towards hydrogen, liquefied petroleum gas and ammonia.

In the present article, the gas sensing behaviour of nanocrystalline CuGa(2)O(4) towards H(2), liquefied petroleum gas (LPG) and NH(3) has been reported for the first time. Nanocrystalline powders of CuGa(2)O(4) having average particle sizes in the range of 30-60nm have been prepared through thermal decomposition of an aqueous precursor solution comprising copper nitrate, gallium nitrate and triethanol amine (TEA), followed by calcination at 750 degrees C for 2h. The synthesized nanocrystalline CuGa(2)O(4) powders have been characterised through X-ray diffraction (XRD), transmission electron microscopy (TEM), field-emission scanning electron microscopy (FESEM) study, energy dispersive X-ray (EDX) analysis and BET (Brunauer-Emmett-Teller) surface area measurement. The synthesized CuGa(2)O(4) having spinel structure with specific surface area of 40m(2)/g exhibits maximum sensitivity towards H(2), LPG, and NH(3) at 350 degrees C.