Iontronic memories based on ionic redox systems: operation protocols.

A recently developed, new ionic device called the ionic voltage effect soft triode (IVEST) was optimized, tuned and embedded into a memory application concept. The device is an electrochemical micro-cell, consisting of a top electrode and two bottom electrodes. The device controls the concentration and diffusion of ions via the voltage applied on the top electrode. The device showed a memory effect lasting up to 6 hours. Despite the remarkably large stability time, the memory contrast was small in the first device versions. Now, we have increased the memory contrast by introducing a new external electrical circuit layout combined with a new operation protocol. This new investigation also reveals peculiarities of the memory and shows that the IVEST can be used in memory applications. These iontronic memories show a secondary information storage connected with the read-out frequency.

A purely ionic voltage effect soft triode.

We report on the construction and characterization of an ionic soft triode intended to be based on interfacial ion adsorption and redox oxidizer depletion. The soft triode was built in a simple manner with no need for sophisticated or expensive materials. It does not utilize the control of a semiconducting channel, but an electrolyte. In different electrical circuit configurations, it can show amplification or memory effects. The device had an electrical current amplification reaching 52 and memory effects in the electrical resistance lasting for up to 6 h. These values were achieved by tuning the electrode interface, the electrolyte and diffusion properties. They are promising for neuromorphic applications.

Analytical model for transmission dips in self-assembled two-dimensional colloidal crystals.

Self-assembled two-dimensional (2D) colloidal crystals (CCs) are utilized in various optical devices, lasers, biosensors, and light harvesting applications. Optical design tuning capabilities, in terms of sphere refractive index and diameter size, can influence the optical characteristics for the close-packed single-layer or multilayer structures. Often transmission dips in 2D CCs are observed, which cannot be explained by Bragg diffraction as it does for 3D photonic crystals. In this work, an analytical attempt to accurately model the transmission dips observed in the 2D CCs optical spectra is presented, aiming to explain the origin of these dips. The formation of a broad dip was studied experimentally as well. A less than 1% mismatching error was found between experiment and theory for the two blaze peak positions as well as for the transmission intensity ratio. Finally, the 2D CCs were integrated in mesostructured solar cells as light trapping structures.

Modeling disorder in two-dimensional colloidal crystals based on electron microscope measurements.

Self-assembled two-dimensional colloidal crystals (CCs) are critical components in many optical and optoelectronic devices. Such structures usually exhibit various types of disorder, which sometimes can be beneficial for the desired applications. However, disorder poses challenges to the modeling of two-dimensional structures. In this work, two-dimensional CCs employed in optoelectronic devices, especially dye-sensitized solar cells, are investigated. scanning electron microscope (SEM) images were used to quantify the disorder in the studied structures. As a basis for simulations, disordered model patterns were generated with properties extracted from the SEM images of prepared samples. Optical modeling was performed with a finite-difference time-domain simulator. The simulated transmission data are consistent with the experimentally measured spectra.

Observation of nano-dewetting in colloidal crystal drying.

The drying of colloidal crystals is connected with a continuous shrinkage process. However, several minutes after starting the drying, the system seems to take a breath before it shrinks monotonously until its final state after about one day. This short period we call "v"-event because of the shape of the curve characterizing the lattice constant: a decrease followed by a counter-intuitive increase which ends after one hour. This event is found in time-dependent optical spectra. It is assigned to the start of a nano-dewetting process occurring at the colloidal particles.

Opal shell structures: direct assembly versus inversion approach.

Opal shell structures can be fabricated in two ways: By direct assembly from hollow spheres (hs-opal) or by infiltration of precursors into opal templates and inversion. The resulting lattice disturbances were characterized by scanning electron microscopy (SEM), optical microscopy, and transmission spectra. The hs-opal system shows much lower disturbances, for example, a lower number of cracks and lattice deformations. The strong suppression of crack formation in one of these inverse opal structures can be considered as promising candidates for the fabrication of more perfect photonic crystals.

Monodisperse titania microspheres via controlled nanoparticle aggregation.

This paper describes the fabrication of highly monodisperse TiO(2) nanoparticle aggregates (NPAs) by controlled aggregation of nanoparticles in a water-in-oil emulsion. Equally sized drops containing a titanium dioxide nanoparticle suspension are produced in a T-channel device. This procedure has a high tuning potential. Increasing the velocity of the titania suspension phase leads to an enlargement of the droplets, while raising the velocity of the oil phase reduces the drop diameter. The technique enables the preparation of monodisperse (<2%) drops between 150 and 400 µm in diameter. Evaporation of the suspension medium leads to nanoparticle aggregates. There is no significant change in dispersity from emulsion drops to NPAs, if the drying is carried out slowly.

Structure and Optical Properties of Opal Films Made by an Out-of-Plane Electric Field-Assisted Capillary Deposition Method.

Self-assembled opals that are considered as a promising candidate for three-dimensional photonic crystals often suffer from the existence of internal defects. Defects influence optical properties and limit the applicability of opal films. Directed assembly using external fields may offer a certain degree of tunability in the opal formation process. We investigate the effect of an out-of-plane electric field on the formation and optical properties of opal films deposited using the capillary deposition method. The application of an electric field of intermediate strength (20-30 V/cm) can improve opal quality. The quality of opal films was found to depend on the polarity of the bottom substrate resulting from the beneficial influence of an asymmetry between the growths and the interplay with gravity. The negatively charged bottom substrate results in slightly better opal quality. This finding shows the potential of electric fields to tune opal formation in order to reduce the defect content.

Electrodeposition of zinc-silica composite coatings: challenges in incorporating functionalized silica particles into a zinc matrix.

Zinc is a well-known sacrificial coating material for iron and co-deposition of suitable particles is of interest for further improving its corrosion protection performance. However, incorporation of particles that are well dispersible in aqueous electrolytes, such as silica particles, is extremely difficult. Here, we report a detailed study of Zn-SiO2 nanocomposite coatings deposited from a zinc sulfate solution at pH 3. The effect of functionalization of the silica particles on the electro-codeposition was investigated. The best incorporation was achieved for particles modified with SiO2-SH, dithiooxamide or cysteamine; these particles have functional groups that can strongly interact with zinc and therefore incorporate well into the metal matrix. Other modifications (SiO2-NH3+, SiO2-Cl and N,N-dimethyldodecylamine) of the silica particles lead to adsorption and entrapment only.

Reversible Photoalignment of Azobenzene in the SURMOF HKUST-1.

Azobenzene guest molecules in the metal-organic framework structure HKUST-1 show reversible photochemical switching and, in addition, alignment phenomena. Since the host system is isotropic, the orientation of the guest molecules is induced via photo processes by polarized light. The optical properties of the thin films, analyzed by interferometry and UV/vis spectroscopy, reveal the potential of this alignment phenomenon for stable information storage.

Opals: status and prospects.

The beauty of opals results from a densely packed, highly ordered arrangement of silica spheres with a diameter of several hundred nanometers. Such ordered nanostructures are typical examples of materials called photonic crystals, which can be formed by known microstructuring methods and by self-assembly. Opals represent a self-assembly approach to these structured media; such an approach can lead to novel materials for photonics, photocatalysis, and other areas. Although self-assembly leads to many types of defects, resulting in the surprising and very individual appearance of natural opals, it causes also difficulties in technological applications of opal systems.

Post-deposition opal evolution.

The formation of artificial opal films consists of wet opal deposition, drying, and possible transformations in the dry state. The processes after deposition, before the crystals lattice reaches its final equilibrium state, are studied herein. We follow the time evolution of the optical transmission spectra for polystyrene opals with different thicknesses. The evolution of the spectra shows pronounced changes in the Bragg peak position, width and height, as well as changes in the background and, in the beginning of the process, a band related to residual water in the sample. Therefore, a wet and a dry phase can be distinguished in the opal transformations. They are all connected to shrinkage and we associate one of them with a possible new sintering mechanism.

Early stages of ZnS growth studied by stopped-flow UV absorption spectroscopy: effects of educt concentrations on the nanoparticle formation.

The growth of ZnS nanoparticles by precipitation from supersaturated aqueous solution is studied by stopped-flow UV absorption spectroscopy. The average size, size distribution, and concentration of the particles are monitored within the sub-second time regime with a resolution of 1.28 ms. Particle growth at these early stages is governed by pronounced ripening. The UV absorption data strongly suggest that growth occurs by preferential adsorption of HS- anions relative to Zn(2+) or ZnOH(+) cations. Correspondingly, the initial sulfide concentration has a much more pronounced influence on the growth kinetics than the initial zinc concentration. These findings are verified by zeta-potential measurements which confirm that the particle surfaces are negatively charged under near-neutral pH conditions.

Improved controllability of opal film growth using capillaries for the deposition process.

A capillary deposition method for the preparation of opal and inverse opal films has been developed. By this method, one can control the film thickness and the crack arrangement in opal as well as inverse opal structures. This method combines tube capillarity with cell capillarity or with gravity depending on the stability of the suspensions. The combination of tube capillarity with cell capillarity is used to prepare opal films from stable suspensions. The tube capillary transports the suspension, while the cell capillary helps to assemble the spheres. The setup defines the drying fronts, thickness, and crack arrangements of the opal films. The combination of capillarity with gravity is useful for making opal films from unstable suspensions. Opal films of spheres with size up to 1 mum can be easily prepared from this combination. Here, the gravity influences the arrangement of the spheres. The two-capillary setup has also been used to infiltrate the opal films with a titania precursor. After calcination, inverse titania opal films with skeleton structure have been obtained.

Is the charge transport in dye-sensitized solar cells really understood?

Electrical transients following laser excitation of dye-sensitized solar cells, thus far described by a complex model in the literature, are not consistent with this model. In particular, there are always delays of the electrical signal after the laser pulse. Although the correct theory has not yet been found, the present article is a step toward an improved understanding.

Existence of a lower critical radius for incorporation of silica particles into zinc during electro-codeposition.

Recently, it was shown that the surface modification of silica particles with -SH functional groups enables their electro-codeposition with zinc. Here, however, we report that no incorporation into Zn can be observed for such modified particles with diameters of <100 nm, while incorporation is possible for particles with diameters of 225 nm and larger. Furthermore, when silica particles are functionalized with mixtures of -SH and -Cl functional groups, which affect the interface energy at the particle/metal interface differently but have similar interfacial energies for the particle/electrolyte interface, it is found that, for successful incorporation of the particles, a minimum amount of -SH functional groups is needed. An explanation for these observations has been derived based on energetic considerations regarding the interfaces involved in the process.

Release from silica SBA-3-like mesoporous fibers: cross-wall transport and external diffusion barrier.

The transport of guest molecules between adjacent pore channels (cross-wall transport) is the limiting factor in the release of guest molecules from SBA-3-like fibers. This specific mode of diffusion is identified by microscopic observation and studied quantitatively in a UV/Vis-monitored release experiment. Analysis of release curves reveals that the external particle surface offers resistance to the guest molecules passing through it (external diffusion barrier). This barrier is native to as-synthesized fibers and can be effectively modified to slow down the release. Extremely effective slowdown is achieved by deposition of a nanometer-thick layer of sodium silicate, that is, the guest molecules are then safely stored in the particles.

Diffusion in coiled pores - learning from microrelease and microsurgery.

The anisotropic diffusion in coiled pore systems of SBA-3-type microparticles has been studied by the release of guest molecules. The diffusion turns out as an example of the influence of hierarchical structuring on physical properties. Two modes of diffusion, associated with transport along and across the mesopores, can be identified and measured using optical microscopy. Redistribution between the two modes has been achieved by mesopore opening using two methods of "microsurgery"-either focused ion beams (FIB) or mechanical tools. The particles trimmed by FIB have revealed risks of misinterpretation of sample preparation with this tool. Instead of pure pore opening, the cutting by FIB resulted in simultaneous sealing of the mesopores.