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.

Metal oxide-based gas sensor array for VOCs determination in complex mixtures using machine learning.

Detection of volatile organic compounds (VOCs) from the breath is becoming a viable route for the early detection of diseases non-invasively. This paper presents a sensor array of 3 component metal oxides that give maximal cross-sensitivity and can successfully use machine learning methods to identify four distinct VOCs in a mixture. The metal oxide sensor array comprises NiO-Au (ohmic), CuO-Au (Schottky), and ZnO-Au (Schottky) sensors made by the DC reactive sputtering method and having a film thickness of 80-100 nm. The NiO and CuO films have ultrafine particle sizes of < 50 nm and rough surface texture, while ZnO films consist of nanoscale platelets. This array was subjected to various VOC concentrations, including ethanol, acetone, toluene, and chloroform, one by one and in a pair/mix of gases. Thus, the response values show severe interference and departure from commonly observed power law behavior. The dataset obtained from individual gases and their mixtures were analyzed using multiple machine learning algorithms, such as Random Forest (RF), K-Nearest Neighbor (KNN), Decision Tree, Linear Regression, Logistic Regression, Naive Bayes, Linear Discriminant Analysis, Artificial Neural Network, and Support Vector Machine. KNN and RF have shown more than 99% accuracy in classifying different varying chemicals in the gas mixtures. In regression analysis, KNN has delivered the best results with an R2 value of more than 0.99 and LOD of 0.012 ppm, 0.015 ppm, 0.014 ppm, and 0.025 ppm for predicting the concentrations of acetone, toluene, ethanol, and chloroform, respectively, in complex mixtures. Therefore, it is demonstrated that the array utilizing the provided algorithms can classify and predict the concentrations of the four gases simultaneously for disease diagnosis and treatment monitoring.

Frequency dependent impedance response analysis of nanocrystalline ZnO chemiresistors.

ZnO is a widely studied gas sensor material and is used in many commercial sensor devices. However, selectivity towards any particular gas remains an issue due to lack of complete knowledge of the gas sensing mechanism of oxide surfaces. In this paper, we have studied the frequency dependent gas sensor response of ZnO nanoparticles of a diameter of nearly 30 nm. A small rise of synthesis temperature from 85 °C to 95 °C in the solvothermal process, shows coarsening by joining and thereby distinct loss of grain boundaries as seen from transmission electron micrographs. This leads to a substantial reduction in impedance, Z (GΩ to MΩ), and rises in resonance frequencyfres(from 1 to 10 Hz) at room temperature. From temperature dependent studies it is observed that the grain boundaries show a Correlated Barrier Hopping mechanism of transport and the hopping range in the grain boundary region is typically 1 nm with a hopping energy of 153 meV. On the other hand, within the grain, it shows a change of transport type from low temperature tunneling to beyond 300 °C as polaron hopping. The presence of disorder (defects) as the hopping sites. The temperature dependence offresagrees with different predicted oxygen chemisorbed species between 200 °C to 400 °C. As opposed to the traditional DC response, the AC response in the imaginary part of (Z″) shows gas specific resonance frequencies for each gas, such as NO2, ethanol, and H2. Among the two reducing gases, ethanol and hydrogen; the former shows good dependence on concentration in Z″ whereas the latter shows a good response infresas well as capacitance. Thus, the results of frequency dependent response allow us to investigate greater details of the gas sensing mechanism in ZnO, which may be exploited for selective gas sensing.

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.

Highly Sensitive and Selective Triethylamine Sensing through High-Entropy Alloy (Ti-Zr-Cr-V-Ni) Nanoparticle-Induced Fermi Energy Control of MoS2 Nanosheets.

A giant enhancement of nearly 100 times is seen in triethylamine response through Ti-Zr-Cr-V-Ni high-entropy alloy nanoparticle (HEA NP)-induced fermi energy control of two-dimensional molybdenum disulfide (MoS2) nanosheets. These Laves-phase HEA NP-decorated MoS2 samples are synthesized using cryomilling followed by 30 h of sonication. The prolonged sonication results in well-exfoliated MoS2 with fairly small (∼10-20 nm) HEA NPs anchored due to cryomilling confirmed by extensive microscopic and spectroscopic examinations. The presence of HEA NPs leads to reduction in edge oxidation of MoS2 as seen from X-ray photoelectron spectroscopy. Moreover, this edge state reduction causes strong Fermi level pinning, which is commonly observed in layered MoS2 with bulk metal electrodes. This leads to target gas-specific carrier-type response and selective oxidation of TEA vapors due to highly catalytically active metals. The resulting composite (MoS2 + NPs) exhibits high response (380% for 2000 ppm TEA vapors) along with selectivity toward TEA at 50 °C. The cross-sensitivity of the composite to other volatile organic compounds and NH3, CO, and H2 has been very minimal. Thus, the highly selective catalytic activity of metal alloy NPs and their Fermi energy control has been proposed as the prime factors for observed large sensitivity and selective response of MoS2 + NP nanocomposites.

Charge transport variation from Bloch-Grüneisen to Mott variable range hopping and transport change due to hydrogenation in Palladium thin films.

We report a systematic investigation of the differences in charge transport mechanism in ultra-thin nano-island like films of palladium with thickness varying between 5 nm and 3 nm. The thicker films were found to be metallic in a large temperature range with a dominant Bloch-Grüneisen mechanism of charge transport arising due to electron-acoustic phonon scattering. These films were also found to exhibit an additional electron-magnon scattering. At temperatures below 20 K, the two films displayed a metal-insulator transition which was explained using Al'tshuler's model of increased scattering in disordered conductors. The thinner films were insulating and were found to exhibit Mott's variable range hopping mechanism of charge transport. The thinnest film showed a linear decrease of resistance with an increase in temperature in the entire temperature range. The island-like thin films were found to display very different response to hydrogenation at room temperature where the metallic films were found to display a decrease of resistance while the insulating films were found to have an increase of resistance. The decrease of resistance was ascribed to a hydrogen induced lattice expansion in the thin films that were at the percolation threshold while the resistance increase to an increase in work function of the films due to an increased adsorption of the hydrogen atoms at the surface sites of palladium.

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.

Thickness induced metal to insulator charge transport and unusual hydrogen response in granular palladium nanofilms.

This work reports a systematic study of the evolution of charge transport properties in granular ultra-thin films of palladium of thicknesses varying between 6 nm and 2 nm. While the films with thickness >4 nm exhibit metallic behaviour, that at 3 nm thickness undergoes a metal-insulator transition at 19.5 K. In contrast, the 2 nm thick film remained insulating at all temperatures, with transport following Mott's variable range hopping. At room temperature, while the thicker films exhibit resistance decrease upon H2 exposure, the insulating film showed an anomalous initial resistance increase before switching to a subsequent decrease. The nanostructure dependent transport and the ensuing H2 response is modeled on a percolation model, which also explores the relevance of film thickness as a macroscopic control parameter to engineer the desired system response in granular metal films.

First example of engineered ß-cyclodextrinylated MEMS devices for volatile pheromone sensing of olive fruit pests.

Olive oil is more preferred than other vegetable oils because of the increasing health concern among people throughout the world. The major hindrance in large-scale production of olive oil is olive fruit pests which cause serious economic damage to the olive orchards. This requires careful monitoring and timely application of suitable remedies before pest infestation. Herein we demonstrate efficacious utilization of covalently functionalized ß-cyclodextrinylated MEMS devices for selective and sensitive detection of female sex pheromone of olive fruit pest, Bactocera oleae. Two of the MEMS devices, silicon dioxide surface-micromachined cantilever arrays and zinc oxide surface-microfabricated interdigitated circuits, have been used to selectively capture the major pheromone component, 1,7-dioxaspiro[5,5]undecane. The non-covalent capture of olive pheromones inside the ß-cyclodextrin cavity leads to the reduction of resonant frequency of the cantilevers, whereas an increase in resistance has been found in case of zinc oxide derived MEMS devices. Sensitivity of the MEMS devices towards the olive pheromone was found to be directly correlated with the increasing availability of ß-cyclodextrin moieties over the surface of the devices and thus the detection limit of the devices has been achieved to a value as low as 0.297 ppq of the olive pheromone when the devices were functionalized with one of the standardized protocols. Overall, the reversible usability and potential capability of the suitably functionalized MEMS devices to selectively detect the presence of female sex pheromone of olive fruit fly before the onset of pest infestation in an orchard makes the technology quite attractive for viable commercial application.

Orthorhombic crystal structure and oxygen deficient cluster distribution model for YBa2Cu3-xAlxO6+δ superconductor.

Single crystal x-ray diffraction measurements on both as-grown as well as oxygenated single crystals of an aluminium doped high temperature superconductor YBa2Cu3-xAlxO6+δ revealed the crystal structure to be orthorhombic with space group Pmmm, in contrast to, tetragonal crystal structures corresponding to space group P4/mmm, previously reported for as-grown YBa2Cu3-xAlxO6+δ, and conflicting structures on oxygenated YBa2Cu3-xAlxO6+δ. The orthorhombic crystal structure was confirmed by powder x-ray diffraction that showed the presence of two peaks corresponding to (020) and (200) reflections associated with orthorhombic structures of space group Pmmm, instead of a single (200) reflection corresponding to tetragonal crystal structures with space group P4/mmm. All the as-grown crystals were found to be superconducting. An oxygen-vacancy cluster distribution model is proposed to explain the differences in the obtained magnetisation hysteresis loop and the broad superconducting transition temperature. The model proposes the existence of two oxygen deficient clusters of (Al-..-Cu-O-Cu)n and (Cu-O-Cu-..)n juxtaposed with each other whose number and size vary as the as-grown single crystals of YBa2Cu3-xAlxO6+δ are subjected to oxygenation. X-ray photoelectron spectroscopy measurements showed the existence of two distinct peaks in each of the spectrum of O, Cu, Y and Ba in YBa2Cu3-xAlxO6+δ crystals corresponding to the two different types of clusters. The relative intensities of each XPS peak was found to decrease in the oxygenated crystals as compared to the as-grown ones confirming the change in the number and size of clusters in the as-grown crystals after oxygenation.