Scientific publications

Crystalline indium doped zinc oxide (IZO) nanopowders were synthesized using hydrothermal treatment processing. Increasing the doping ratio of indium in the zinc oxide significantly influences the phase structure and shape of the nanopowders resulting in nanorod to nanoparticulate morphologies. As the doping profile increases, the crystallite size decreases, the band gap energy blue shifts and the specific surface area increases (measured by BET method). Additionally Raman spectroscopy exhibited shifts of several peaks, as well as revealed new peaks, confirming the substitution of indium ions within the zinc oxide lattice sites. An IZO suspension made of IZO nanoparticles (In/Zn = 3 atm.%) with a zeta potential greater than 30 mV at pH = 6 was successfully spin-coated on glass substrates for to make transparent conductive coatings evincing sheet resistances as low as 35 kΩ□ (ρ = 4.9 × 10−3 Ω m,) with transmission in the visible range as high as 90 %.

We report a novel fabrication method for ordered arrays of metal nanoparticles that exploits the uniform arrangement of polymer beads deposited as close-packed monolayers. In contrast to colloidal lithography that applies particles as masks, we used thermal decomposition of the metal-covered particles to precisely define metal structures. Large arrays of noble metal (Au, Ag, Pt) nanoparticles were produced in a three-step process on silicon, fused silica and sapphire substrates, demonstrating the generality of this approach. Polystyrene spheres with diameters ranging between 110 nm and 1 µm were convectively assembled into crystalline monolayers, coated with metal and annealed in a resistive furnace or using an ethanol flame. The thermal decomposition of the polymer microspheres converted the metal layer into particles arranged in hexagonal arrays that preserved the order of the original monolayer. Both the particle size and the interparticle distance were adjusted via the thickness of the metal coating and the sphere diameter, respectively.

The growth of nanoscale gold dendrites was studied in situ in a thin liquid film with transmission electron microscopy (TEM) using a liquid cell with silicon nitride (SiN) windows. Gold nanoparticle seeds were covered by a thin liquid layer containing precursor solution. Dendrite nucleation was induced by the electron beam leading to an initial burst of growth. The growth then settled at tip velocities between 0.1 and 2.0 nm/s for different dendrites. Tip velocities fluctuated as different dendrite geometries grew from the tips. Those dendrites showing granularities in their structure experienced the largest growth speed. Comparison of the observed velocities with diffusion-limited growth rates suggests that dendrite growth in thin films at this scale is limited by diffusion. The described method may find application in research on the mechanisms behind dendrite growth and also to study other types of anisotropic growth of nanomaterials driven by crystal and twin geometries.

The aim of the present paper is to give a clear and straightforward derivation of the displacement field, stress function tensor and interaction energy of dislocations in the theory of incompatible anisotropic elasticity. The displacement field caused by a dislocation loop in an elastically anisotropic medium can be written as the sum of a line integral plus a purely geometric part. This is the anisotropic generalization of the Burgers formula. Formulae for the first-order stress function tensor and the interaction energy between two dislocation loops in anisotropic media are derived.

In the protist world, the ciliate Coleps hirtus (phylum Ciliophora, class Prostomatea) synthesizes a peculiar biomineralized test made of alveolar plates, structures located within alveolar vesicles at the cell cortex. Alveolar plates are arranged by overlapping like an armor and they are thought to protect and/or stiffen the cell. Although their morphology is species-specific and of complex architecture, so far almost nothing is known about their genesis, their structure and their elemental and mineral composition. We investigated the genesis of new alveolar plates after cell division and examined cells and isolated alveolar plates by electron microscopy, energy-dispersive X-ray spectroscopy, FTIR and X-ray diffraction. Our investigations revealed an organic mesh-like structure that guides the formation of new alveolar plates like a template and the role of vesicles transporting inorganic material. We further demonstrated that the inorganic part of the alveolar plates is composed out of amorphous calcium carbonate. For stabilization of the amorphous phase, the alveolar vesicles, the organic fraction and the element phosphorus may play a role.

Molluscs with various shell architectures evolved around 542-525 million years ago, as part of a larger phenomenon related to the diversification of metazoan phyla. Molluscs deposit minerals in a chitin matrix. The mollusc chitin is synthesized by transmembrane enzymes that contain several unique extracellular domains. Here we investigate the assembly mechanism of the chitin synthase Ar-CS1 via its extracellular domain ArCS1_E22. The corresponding transmembrane protein ArCS1_E22TM accumulates in membrane fractions of the expression host Dictyostelium discoideum. Soluble recombinant ArCS1_E22 proteins can be purified as monomers only at basic pH. According to confocal fluorescence microscopy experiments, immunolabeled ArCS1_E22 proteins adsorb preferably to aragonitic nacre platelets at pH 7.75. At pH 8.2 or pH 9.0 the fluorescence signal is less intense, indicating that protein-mineral interaction is reduced with increasing pH. Furthermore, ArCS1_E22 forms regular nanostructures on cationic substrates as revealed by atomic force microscopy (AFM) experiments on modified mica cleavage planes. These experiments suggest that the extracellular domain ArCS1_E22 is involved in regulating the multiple enzyme activities of Ar-CS1 such as chitin synthesis and myosin movements by interaction with mineral surfaces and eventually by protein assembly. The protein complexes could locally probe the status of mineralization according to pH unless ions and pCO2 are balanced with suitable buffer substances. Taking into account that the intact enzyme could act as a force sensor, the results presented here provide further evidence that shell formation is coordinated physiologically with precise adjustment of cellular activities to the structure, topography and stiffness at the mineralizing interface.

Onion-like carbon, also known as carbon onions, is a highly conductive material enabling supercapacitor electrodes with a very high power density. However, the moderate specific capacitance (circa 30 F/g) is insufficient for many energy storage applications. In our study, we show how decoration of carbon onions with quinones provides a facile method to increase the energy density up to one order of magnitude, namely, from 0.5 Wh/kg to 4.5 Wh/kg, while retaining a high power density and long lifetime. We present data for carbon onions modified with three different kinds of quinones: 1,4-naphthoquinone, 9,10-phenanthrenequinone, and 4,5-pyrenedione. Quinone-decorated carbon onion electrodes are investigated considering the actual quinone loading and the resulting electrochemical performance is probed in 1 M H2SO4 as the electrolyte using cyclic voltammetry and galvanostatic charge/discharge. The maximum capacitance, 264 F/g, is found for carbon onions modified with 4,5-pyrenedione, which also shows the smallest fade in specific capacitance, namely 3%, over 10,000 charge and discharge cycles at a high current density of 1.3 A/g.

Electrochemical double layer capacitors (EDLC) are rapidly emerging as a promising energy storage technology offering extremely large power densities. Despite significant experimental progress, nanoscale operation mechanisms of the EDLCs remain poorly understood and it is difficult to separate processes at multiple time and length scales involved in operation including that of double layer charging and ionic mass transport. Here we explore the functionality of EDLC microporous carbon electrodes using a combination of classical electrochemical measurements and scanning probe microscopy based dilatometry, thus separating individual stages in charge/discharge processes based on strain generation. These methods allowed us to observe two distinct modes of EDLC charging, one fast charging of the double layer unassociated with strain, and another much slower mass transport related charging exhibiting significant sample volume changes. These studies open the pathway for the exploration of electrochemical systems with multiple processes involved in the charge and discharge, and investigation of the kinetics of those processes.

A recently introduced, novel electrical energy storage concept, the electrochemical flow capacitor (EFC), holds much promise for grid-scale energy storage applications. The EFC combines the principles behind the operation of flow batteries and supercapacitors, and enables rapid charging/discharging and decoupled energy/power ratings. Electrical charge is stored in a flowable carbon slurry composed of low-cost and abundantly available carbon particles in pH-neutral, aqueous electrolyte. Charge storage and transfer is analogous to solid carbon electrodes in conventional supercapacitors. Here, the effects of carbon particle solid fraction, shape, and size on the electrochemical and rheological properties of slurry electrodes are investigated. A static cell configuration is utilized for studying the electrochemical properties of the flowable electrodes. The electrochemical properties of the slurry electrodes tested in a static cell are found to be similar to that of solid electrodes in conventional supercapacitors for both, large spherical and anisometric activated carbons. Flow properties of the slurry electrodes are obtained for shear rates corresponding to pumping shear rates by rheometry. Results indicate that electrochemical and rheological properties of slurries depend on their concentration, shape and size of the carbon particles used in the slurries. For a range of concentrations, slurries based on spherical carbon particles show lower viscosities compared to anisometric activated carbon based slurries while performing similar electrochemically.