Electrolyte-Dependent Modification of Resistive Switching in Anodic Hafnia.

Anodic HfO2 memristors grown in phosphate, borate, or citrate electrolytes and formed on sputtered Hf with Pt top electrodes are characterized at fundamental and device levels. The incorporation of electrolyte species deep into anodic memristors concomitant with HfO2 crystalline structure conservation is demonstrated by elemental analysis and atomic scale imaging. Upon electroforming, retention and endurance tests are performed on memristors. The use of borate results in the weakest memristive performance while the citrate demonstrates clear superior memristive properties with multilevel switching capabilities and high read/write cycling in the range of 106. Low temperature heating applied to memristors shows a direct influence on their behavior mainly due to surface release of water. Citrate-based memristors show remarkable properties independent on device operation temperatures up to 100 °C. The switching dynamic of anodic HfO2 memristors is discussed by analyzing high resolution transmission electron microscope images. Full and partial conductive filaments are visualized, and apart from their modeling, a concurrency of filaments is additionally observed. This is responsible for the multilevel switching mechanism in HfO2 and is related to device failure mechanisms.

Gallium-Enhanced Aluminum and Copper Electromigration Performance for Flexible Electronics.

Wide range binary and ternary thin film combinatorial libraries mixing Al, Cu, and Ga were screened for identifying alloys with enhanced ability to withstand electromigration. Bidimensional test wires were obtained by lithographically patterning the substrates before simultaneous vacuum co-deposition from independent sources. Current-voltage measurement automation allowed for high throughput experimentation, revealing the maximum current density and voltage at the electrical failure threshold for each alloy. The grain boundary dynamic during electromigration is attributed to the resultant between the force corresponding to the electron flux density and the one corresponding to the atomic concentration gradient perpendicular to the current flow direction. The screening identifies Al-8 at. % Ga and Cu-5 at. % Ga for replacing pure Al or Cu connecting lines in high current/power electronics. Both alloys were deposited on polyethylene naphthalate (PEN) flexible substrates. The film adhesion to PEN is enhanced by alloying Al or Cu with Ga. Electrical testing demonstrated that Al-8 at. % Ga is more suitable for conducting lines in flexible electronics, showing an almost 50% increase in electromigration suppression when compared to pure Al. Moreover, Cu-5 at. % Ga showed superior properties as compared to pure Cu on both SiO2 and PEN substrates, where more than 100% increase in maximum current density was identified.

Insight into Antimicrobial Properties via Self-Acidification of Compounds from the Molybdenum-Tungsten-Oxygen System.

This communication is focused on the synthesis, characterization and experimental proof of the mechanism of antimicrobial activity of powders from the molybdenum-tungsten-oxygen (Mo-W-O) system. Materials with a discrete ratio of Mo/W ranging from 100% MoO3 to 100% WO3 with a stepwise increase of 5-10 wt % W were synthesized by the spray drying method following calcination. Spherical hollow particles with a broad size distribution were formed and the composition influenced the crystalline phases in such a way that either pure and/or mixed oxides (Mo0.6W0.4O3) were obtained. A good correlation between composition variation and phases present on the antimicrobial activity is obtained and provides a detailed screening of the activity efficiency versus compositional transition. Antimicrobial tests were performed against a model Gram-negative bacterium (Escherichia coli). Furthermore, the mechanism of antimicrobial activity is proven by correlating the medium acidification via pH measurements to the bacteria lifespan at low pH values. The mechanism is additionally supported by the bacterial growth when a buffered nutrient medium was used, together with the evidence that the powder particles have no disruptive effect on the cell wall. Consequently, an extended mechanism is proposed for the mixed oxide, relating both the structure and solubility results. Solubility measurements displayed a steep decrease in metal ions concentration with the addition of W. A narrow compositional range was identified (80 to 60 wt % Mo) where the antimicrobial activity was present, which is concurrent with a very strong decrease in solubility. Materials within this range show adequate features for being implemented into hybrid systems consisting of inorganic materials-polymers/varnishes that can be used for touch surfaces in healthcare settings.

Basic properties mapping of anodic oxides in the hafnium-niobium-tantalum ternary system.

A thin film combinatorial library deposited by co-sputtering of Hf, Nb and Ta was employed to characterise fundamental properties of the Hf-Nb-Ta system. Compositional mappings of microstructure and crystallography revealed similarities in alloy evolution. Distinct lattice distortion was observed upon addition of hexagonal Hf, leading to amorphisation of alloys containing more than 32 at.% Hf and less than 27 and 41 at.% Nb and Ta, respectively. Volta potential and open circuit potential mappings indicated minimal values for the highest Hf concentration. Localised anodisation of the library by scanning droplet cell microscopy revealed valve metal behaviour. Oxide formation factors above 2 nm V-1 were identified in compositional zones with high amounts of Nb and Ta. Fitting of electrochemical impedance spectroscopy data allowed electrical permittivity and resistivity of mixed oxides to be mapped. Their compositional behaviours were attributed to characteristics of the parent metal alloys and particularities of the pure oxides. Mott-Schottky analysis suggested n-type semiconductor properties for all Hf-Nb-Ta oxides studied. Donor density and flat-band potential were mapped compositionally, and their variations were found to be related mainly to the Nb amount. Synergetic effects were identified in mappings of Hf-Nb-Ta parent metals and their anodic oxides.

Silver-, calcium-, and copper molybdate compounds: Preparation, antibacterial activity, and mechanisms.

Developing novel compounds with antimicrobial properties can be an effective approach to decreasing the number of healthcare-associated infections, particularly in the context of medical devices and touch surfaces. A variety of molybdate powders (Ag2MoO4, CaMoO4, CuMoO4 and Cu3Mo2O9) were synthesized and characterized, and Escherichia coli was used as a model gram-negative bacterium to demonstrate their antimicrobial properties. Optical density measurements, bacterial colony growth, and stained gel images for protein expression clearly showed that silver- and copper molybdates inhibit bacterial growth, whereas CaMoO4 exhibited no bactericidal effect. All tests were performed in both daylight and darkness to assess the possible contribution of a photocatalytic effect on the activity observed. The main mechanism responsible for the antibacterial effect observed for Ag2MoO4 is related to Ag+ release in combination with medium acidification, whereas for compounds containing copper, leaching of Cu2+ ions is proposed. All these effects are known to cause damage at the cellular level. A photocatalytic contribution to the antibacterial activity was not clearly observable. Based on the pH and solubility measurements performed for powders in contact with various media (ultrapure water and bacterial growth medium), silver molybdate (Ag2MoO4) was identified as the best antibacterial candidate. This compound has great potential for further use in hybrid powder-polymer/varnish systems for touch surfaces in healthcare settings.

Compositionally Dependent Nonlinear Optical Bandgap Behavior of Mixed Anodic Oxides in Niobium-Titanium System.

Optical bandgap mapping of Nb-Ti mixed oxides anodically grown on a thin film parent metallic combinatorial library was performed via variable angle spectroscopic ellipsometry (VASE). A wide Nb-Ti compositional spread ranging from Nb-90 at.% Ti to Nb-15 at.% Ti deposited by cosputtering was used for this purpose. The Nb-Ti library was stepwise anodized at potentials up to 10 V SHE, and the anodic oxides optical properties were mapped along the Nb-Ti library with 2 at.% resolution. The surface dissimilarities along the Nb-Ti compositional gradient were minimized by tuning the deposition parameters, thus allowing a description of the mixed Nb-Ti oxides based on a single Tauc-Lorentz oscillator for data fitting. Mapping of the Nb-Ti oxides optical bandgap along the entire compositional spread showed a clear deviation from the linear model based on mixing individual Nb and Ti electronegativities proportional to their atomic fractions. This is attributed to the strong amorphization and an in-depth compositional gradient of the mixed oxides. A systematic optical bandgap decrease toward values as low as 2.0 eV was identified at approximately 50 at.% Nb. Mixing of Nb2O5 and TiO2 with both amorphous and crystalline phases is concluded, whereas the possibility of complex NbaTibOy oxide formation during anodization is unlikely.

Photoelectrochemical and Electrochemical Characterization of Sub-Micro-Gram Amounts of Organic Semiconductors Using Scanning Droplet Cell Microscopy.

A model organic semiconductor (MDMO-PPV) was used for testing a modified version of a photoelectrochemical scanning droplet cell microscope (PE-SDCM) adapted for use with nonaqueous electrolytes and containing an optical fiber for localized illumination. The most attractive features of the PE-SDCM are represented by the possibility of addressing small areas on the investigated substrate and the need of small amounts of electrolyte. A very small amount (ng) of the material under study is sufficient for a complete electrochemical and photoelectrochemical characterization due to the scanning capability of the cell. The electrochemical behavior of the polymer was studied in detail using potentiostatic and potentiodynamic investigations as well as electrochemical impedance spectroscopy. Additionally, the photoelectrochemical properties were investigated under illumination conditions, and the photocurrents found were at least 3 orders of magnitude higher than the dark (background) current, revealing the usefulness of this compact microcell for photovoltaic characterizations.

Rhodium-coordinated poly(arylene-ethynylene)-alt-poly(arylene-vinylene) copolymer acting as photocatalyst for visible-light-powered NAD⁺/NADH reduction.

A 2,2'-bipyridyl-containing poly(arylene-ethynylene)-alt-poly(arylene-vinylene) polymer, acting as a light-harvesting ligand system, was synthesized and coupled to an organometallic rhodium complex designed for photocatalytic NAD(+)/NADH reduction. The material, which absorbs over a wide spectral range, was characterized by using various analytical techniques, confirming its chemical structure and properties. The dielectric function of the material was determined from spectroscopic ellipsometry measurements. Photocatalytic reduction of nucleotide redox cofactors under visible light irradiation (390-650 nm) was performed and is discussed in detail. The new metal-containing polymer can be used to cover large surface areas (e.g. glass beads) and, due to this immobilization step, can be easily separated from the reaction solution after photolysis. Because of its high stability, the polymer-based catalyst system can be repeatedly used under different reaction conditions for (photo)chemical reduction of NAD(+). With this concept, enzymatic, photo-biocatalytic systems for solar energy conversion can be facilitated, and the precious metal catalyst can be recycled.

Photoelectrochemical scanning droplet cell microscopy for localized photovoltaic investigations on organic semiconductors.

Photoelectrochemical characterization of the regioregular poly(3-hexylthiophene) (P3HT) was performed using an adapted version of a photoelectrochemical scanning droplet cell microscope (PE-SDCM). The real and imaginary parts of the dielectric function were determined using spectroscopic ellipsometry in order to identify the absorption region of the polymer. Detailed photoelectrochemical experiments were performed for the thin polymer layer contacted with 0.1 M tetrabutylammonium hexafluorophosphate dissolved in propylene carbonate as well as with an electrolyte containing a 5.4 mM ferrocene/ferrocenium redox couple. The effect of the illumination on the P3HT covered WE in contact with both the pure electrolyte and an electrolyte containing a ferrocene/ferrocenium redox couple was studied using dark/illumination sequences. The stability of the photovoltaic effect was characterized using long term current transients. Finally, the photoelectrochemical impedance spectroscopy was applied to determine the electrical properties of the P3HT in the dark and under illumination.

Localized photovoltaic investigations on organic semiconductors and bulk heterojunction solar cells.

Newly synthesized organic electronics materials are often available in submicrogram amounts only. Photoelectrochemical scanning droplet cell microscopy is a powerful method that allows a comprehensive characterisation of such small amounts including oxidation, reduction potentials, doping, determination of charge carriers, band gap, charge capacity, over-oxidation sensitivity and many more. Localized photoelectrochemical characterization of the poly[4,8-bis-substituted-benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl-alt-4-substituted-thieno [3,4-b] thiophene-2,6-diyl] (PBDTTT-c) and PBDTTT-c:PCBM bulk heterojunction was performed using photoelectrochemical scanning droplet cell microscopy (PE-SDCM). The optical properties and the real and imaginary part of the dielectric function, of the polymer were determined using spectroscopic ellipsometry. The photoelectrochemical characterizations were performed in a three and two electrode configuration of PE-SDCM under laser and white light illumination. The effect of illumination was characterized using dark/illumination sequences. The stability of the photocurrent was studied using longer term (600 s) illumination. Finally the effect of cell configuration and illumination conditions on the photovoltage was studied.

Localized photoelectrochemistry on a tungsten oxide-iron oxide thin film material library.

A WO3-Fe2O3 thin film combinatorial library was fabricated using a vapor phase co-deposition method followed by a combined thermal annealing and oxidation process. The scanning electron microscopy (SEM) analysis of the library microstructure combined with X-ray diffraction (XRD) investigations suggested that α-Fe2O3 grains preferentially grow from boundaries of domains, containing finer grains of WO3 and Fe2WO6, forming filiform networks on the surface. The surface density of the hematite networks depends on the amount of Fe present in the library. Photocurrents measured at different applied biases using Photo Electrochemical Scanning Droplet Cell Microscopy (PE-SDCM) were analyzed and mapped along the entire compositional spread. A distinctive photocurrent peak was detected at 21.9 atom % Fe, and its appearance was correlated to the higher amount of hematite present in the library at this specific composition together with a specific WO3 crystallographic orientation ((222) orthorhombic or (400) monoclinic). This finding is confirmed by qualitative and quantitative XPS surface analysis at the photocurrent peak position in the material library. Thus the enhancement of the photocurrent cannot be exclusively attributed to certain surface modifications since only hematite was found on the library surface at the peak composition.

Photoelectrochemical scanning droplet cell microscopy (PE-SDCM).

Principles of localised photoelectrochemistry are summarised and an experimental approach is described that allows the performance of the most important photoelectrochemical experiments within a diameter of 100 µm. Various light sources, such as a continuum emitter with a monochromator, LEDs, and lasers are coupled into a multi-mode fibre to illuminate a small spot that is wetted by the electrolyte from a capillary. Reference electrode, counter electrode, and optical fibre are installed in the capillary system. The performance of this system is demonstrated by photocurrent measurements on n-doped Si and p-doped Si as model substrates. A thickness-graded aluminium thin film for partial shadowing on Si proves the applicability for material library investigations in combinatorial materials science. Further experiments demonstrate the possibility of electrical light chopping as well as impedance spectroscopy with subsequent Mott-Schottky analysis for the determination of charge-carrier concentration and type, flat-band potential, and inversion layer formation. Photoelectrochemical scanning droplet cell microscopy (PE-SDCM) is an extremely versatile tool for the screening of water splitting photoelectrodes, the characterisation of photocatalysts, and high throughput characterisation of microgram amounts of new solar cell materials.