A. Kiv

Conducting swift heavy ion track networks

In this paper, the electronic behavior of conducting swift heavy ion track networks is studied. On the one hand, the transient conductivity of ion tracks in metal oxides on silicon in status nascendi is exploited for this purpose, and on the other hand, conducting tracks are produced by ion irradiation of insulating membranes (either self-supported or deposited onto silicon substrates), subsequent etching and finally inserting conducting materials of whatever provenience (in this work preferentially electrolytes). Depending on their manufacture, the conducting tracks either act as electronically active or passive elements. When applying a voltage across individual tracks in the first case, one observes current spikes with negative differential resistances. These tracks interact among themselves, leading to phase-locked synchronous coupled oscillations with complex patterns that are quite similar to those emerging from neural networks. The other case corresponds to networks of electronically passive conducting tracks which become overall electronically active only through their collective interactions. Though the aforementioned effects had been experimentally described earlier, they are re-visited here to make clear that the corresponding systems have to be considered as being artificial neural networks. On this occasion, some new findings are added.

Strategies towards advanced ion track-based biosensors

Three approaches towards ion track-based biosensors appear to be feasible. The development of the first one began a decade ago [Siwy, Z.; Trofin, L.; Kohl, P.; Baker, L.A.; Martin, C.R.; Trautmann, C. J. Am. Chem. Soc. 2005, 127, 5000–5001; Siwy, Z.S.; Harrell, C.C.; Heins, E.; Martin, C.R.; Schiedt, B.; Trautmann, C.; Trofin, L.; Polman, A. Presented at the 6th International Conference on Swift Heavy Ions in Matter, Aschaffenburg, Germany, May 28–31, 2005] and makes use of the concept that the presence of certain biomolecules within liquids can block the passage through narrow pores if being captured there, thus switching off the pores electrical conductivity. The second, having been successfully tested half a year ago [Fink, D.; Klinkovich, I.; Bukelman, O.; Marks, R.S.; Fahrner, W.; Kiv, A.; Fuks, D.; Alfonta, L. Biosens. Bioelectron. 2009, 24, 2702–2706], is based on the accumulation of enzymatic reaction products within the confined volume of narrow etched ion tracks which modifies the pores electrical conductivity. The third and most elegant, at present under development, will exploit the charge transfer from enzymes to semiconductors embedded within etched tracks, enabling the enzymes undergoing specific reactions with the biomolecules to be detected. These strategies can be realized either within carrier-free nanoporous polymeric membranes embedded in the corresponding bioliquids, or within contacted nanoporous insulating layers on semiconducting substrates, the so-called TEMPOS structures [Fink, D.; Petrov, A.; Hoppe, H.; Fahrner, W.R.; Papaleo, R.M.; Berdinsky, A.; Chandra, A.; Biswas, A.; Chadderton, L.T. Nucl. Instrum. Methods B 2004, 218, 355–361]. The latter have the advantage of exhibiting a number of peculiar electronic properties, such as the ability for logic and/or combination of input signals, tunable polarity, negative differential resistances, tunability by external parameters such as light, magnetic fields, etc. and self-pulsations, which should enable one to design intelligent autonomous biosensors. It also appears possible to let the enzymatic reactions take place on the surface of carbon nanotubes embedded within such TEMPOS structures. The advantages and disadvantages of all these approaches will be compared with each other, in respect to detection selectivity, sensitivity and accuracy, as well as sensor reproducibility, reusability and stability.

Current spikes in polymeric latent and funnel-type ion tracks

High-fluence swift heavy ion-irradiated polymer foils, either as-obtained or partially etched, were embedded in electrolytes. Upon application of a sinusoidal electric field across them, pronounced current spikes emerge. Their spectra exhibit peculiar features which depend on parameters such as the amplitude and frequency of the applied electric field and the sample size. A theory is established which explains the most important experimental findings.

Coupled chemical reactions in dynamic nanometric confinement: V. The influence of Li+ and F− ions on etching of nuclear tracks in polymers

Etching of continuous nuclear tracks in thin polymer foils from both sides is known to lead to the formation of double-conical nanopores. In this work and related ones we try to find out how this etching kinetics is modified when materials are added which react with each other upon their contact towards some new product that influences the etching. For that purpose we have chosen here Li+ and F− ions as the additions, which react with each other to form LiF precipitations. The coupled etching and precipitation kinetics is recorded by measuring the electrical current that is transmitted through the foils upon application of a low-frequency alternating sinusoidal voltage. Depending on the etchant concentrations, the etching temperature and the time of Li+ and F− addition, different effects are found that range from (a) no alteration of the transmitted current at all, via (b) the emergence of an alternating current with a temperature-dependent amplitude, and (c) the complete vanishing of any transmitted current at all, towards (d) chaotic transmitted current histories with phases with strong current spike emission and (e) rather quiet phases, alternating with each other in a rather unsystematic way. The observed effects are ascribed to (a) the enhanced penetration efficiency of both the Li+ and F− ions through the polymeric bulk and/or latent ion tracks after the removal of the polymers protective surface layer by the etchant, (b) the high mobility of preferentially the F− ions within the polymer, (c) the LiF precipitation within the polymer or on its surface upon encounter of Li+ and F− ions, (d) the nanofluidic properties of narrow etched tracks covered with Li+ ions on the wall surfaces and F− ions beyond, and/or (e) the formation of LiF membranes within the etched tracks.

Swift heavy ion irradiation as a tool for creating novel nanoelectronic structures

An overview is given about the strategies to create a novel ‘Swift-heavy Ion Track Electronics (SITE)’ and an ‘Electrolytic Electronics with Etched Tracks (E3T)’, by combining swift heavy ion tracks in thin insulating membranes with conventional semiconductor-based electronic skills and eventually also with nanoparticles on the one hand, and with electrolytes on the other hand. In this way, novel multiparametric and multilevel ambient-sensing and decision-making electronic elements can be constructed which offer a range of unusual properties. The most interesting feature in both types of ion track electronics is the frequent occurrence of two different working states, eventually leading to negative differential resistances (NDRs). NDRs can show up as well within individual track structures, as via track-to-track interaction; in the latter case, the NDRs are tunable. An interesting consequence of NDRs are self-pulsating tracks. Structures with NDRs are thought to be the primary active elements in both types of electronics. We are now working to improve the reproducibility of these structures, for reliable technological applications.

The stability of structures with icosahedral local order in Al-based alloys with transition metals

Abstract The paper considers the stability of the phases formed in alloys of aluminum with transition metals (TM) in terms of the theory of coordination compounds. The structural stability is related to the degree of distortion of the first-neighbor coordination icosahedra commonly found around TM atoms. Changes in the symmetry of coordination polyhedrons are explained in terms of the Jahn–Teller theorem. The stability of the structures may be correlated with the effective charge of the TM atom and with the electronegativity of its neighbors: the phase transforms to the structure with lower symmetry when the electronegativity or effective charge decrease.

Computer model of the trapping media in micro FLASH® memory cells

A computer model for the dielectric trapping layer in the microFLASH memory transistor is developed. Due to local trapping of injected charges in corresponding devices the problem of lateral charge migration in the plane parallel to the transistor channel becomes of principal importance. Molecular Dynamics method was used to design a cluster of atoms with dielectric properties and to perform computer simulation of the redistribution of the injected charges in the program/erase processes. The charge distributions obtained on the basis of proposed model are strongly influenced by Coulomb repulsion between the trapped charge carriers. This effect leads to non-Gaussian discrete space distribution of trapped charges and significantly influences the endurance of the memory device. We demonstrate that large densities of traps and injected carriers are strongly correlated, limiting the amount of charge that can be accumulated in the programming process. The model allows select optimum parameters of the trapping layer to ensure high retention properties of the memory cells.

Coupled chemical reactions in dynamic nanometric confinement: Ag2O membrane formation during ion track etching

In this study, continuous swift heavy ion tracks in thin polymer foils were etched from both sides to create two conical nanopores opposing each other. Shortly before both cones merged, one of the nanopores was filled with a silver salt solution, whereas etching of the other cone continued. At the moment of track breakthrough, the etchant reacted with the silver salt solution by forming an impermeable and insulating membrane. Continued etching around the thus-created obstacle led to repetitive {etchant – silver salt solution} interactions. The coupling of the two chemical reactions, {etchant – polymer} and {etchant – silver salt solution}, within the confinement of etched tracks, with continuously changing shapes, showed a highly dynamic nature as recorded by measuring both the electrical current and the optical transmission across the foils. At low etching speeds, a central membrane that grew in radius and thickness with time until, at a critical thickness, the membrane became rather impermeable was formed. However, at high etching speeds, the emerging reaction products exhibited a sponge-like consistency, which allowed for their infinite growth. This precipitation was accompanied by a pronounced current spike formation. A simple theoretical model explains, at a minimum, the basic features.

Symphony and cacophony in ion track etching: how to control etching results

In general, etching of two identical ion-irradiated polymer foils in the same vessel with the same etchant for the same times does not lead to identical track shapes in both foils. In contrast, the track shapes, the etching speeds, and consequently also the etchant consumption of the two foils diverge increasingly with increasing etching times, unless this is prevented by forceful external equilibration of the system. This tendency toward divergence of a system of multiple ion tracks originates from its lack of self-synchronization during etching. A theory has been developed for this case that also shows general applicability to other diverging effects in human life.

Radiation effects in SnO2–Si sensor structures

The radiation resistance of SnO2–Si sensor structures irradiated by fast electrons and γ rays was studied. The radiation-induced structural changes were investigated using the Fourier transform infrared (FTIR) spectroscopy method. FTIR spectroscopy was used with grazing angles of light incidence on the surface of SnO2–Si structure. New bands or any other modifications in spectra for irradiated SnO2 films were not observed. It was found that SnO2 films reveal a high resistance to irradiation while structural changes were observed in the silicon substrate.