Jan Philipp Kollender

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 (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.

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 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.

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.

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.

Direct writing of anodic oxides for plastic electronics

Metal oxide thin films for soft and flexible electronics require low cost, room temperature fabrication, and structuring processes. We here introduce an anodic printing process to realize the essential building blocks of electronic circuitry, including resistors, capacitors, field-effect transistors, diodes, rectifiers, and memristors directly on imperceptible plastic substrates. Largely independent on surface properties, we achieve high-quality, few nanometer thin dielectric and semiconducting films even on rough substrates via localized anodisation of valve metals using a scanning droplet cell microscope. We demonstrate printing-like fabrication of 3D multilayer solid-state capacitors with a record-high areal capacity of 4 µF cm−2. Applicable to the whole class of valve metals and their alloys, our method provides a versatile fabrication technique for the circuits that empower the flexible and stretchable electronics of tomorrow.Flexible circuits: anodization makes them all!A simple concept of scanning head-guided anodization is shown to be highly expandable to fabricate various electronic components. A team led by Professor Siegfried Bauer from Johannes Kepler University Linz, Austria develops a universal and patternable printing protocol of anodic oxides for a full range of circuit components for flexible devices. The researchers employ a scanning droplet cell microscope to anodize the pre-deposited thin metal films to form dielectric layers with good control in both lateral dimension and vertical thickness. They demonstrate the versatility of the on-site anodization methods by fabricating oxides-based resistors, diodes, transistors and memristors, and multilayer capacitors with a record-high areal capacity of 4 µF cm−2. The approach is cheap, adaptable, and thus ideal for rapid-prototyping of metal oxides circuits for various applications.

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

ABSTRACT 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. Graphical Abstract

Localized photovoltaic investigations on organic semiconductors and bulk heterojunction solar cells

Abstract 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.