O.L. Orelovitch

Effect of nanopore geometry on ion current rectification

We present the results of systematic studies of ion current rectification performed on artificial asymmetric nanopores with different geometries and dimensions. The nanopores are fabricated by the ion track etching method using surfactant-doped alkaline solutions. By varying the alkali concentration in the etchant and the etching time, control over the pore profile and dimensions is achieved. The pore geometry is characterized in detail using field-emission scanning electron microscopy. The dependence of the ion current rectification ratio on the pore length, tip diameter, and the degree of pore taper is analysed. The experimental data are compared to the calculations based on the Poisson-Nernst-Planck equations. A strong effect of the tip geometry on the diode-like behaviour is confirmed.

Fabrication of nanopores in polymer foils with surfactant-controlled longitudinal profiles

We present a surfactant-controlled etching method which allows the production of asymmetric track-etched nanopore membranes with diode-like ionic conductivity. The asymmetry of the pores is provided by self-assembly of amphiphilic molecules at the pore entrances on one side of the membrane during chemical etching while this process is excluded on the other side. By varying the alkali concentration in the etchant, control over the pore profile is achieved. The pore geometry is characterized in detail using field-emission scanning electron microscopy. The method is equally applicable to membranes with many and with single pores and may constitute an alternative to existing methods of production of resistive-pulse sensors.

Surfactant-enhanced control of track-etch pore morphology

Abstract The influence of surfactants on the process of chemical development of ion tracks in polymers is studied. Based on the experimental data, a mechanism of the surfactant effect on the track-etch pore morphology is proposed. In the beginning of etching the surfactant is adsorbed on the surface and creates a layer that is quasi-solid and partially protects the surface from the etching agent. However, some etchant molecules diffuse through the barrier and react with the polymer surface. This results in the formation of a small hole at the entrance to the ion track. After the hole has attained a few nanometers in diameter, the surfactant molecules penetrate into the track and cover its walls. Further diffusion of the surfactant into the growing pore is hindered. The adsorbed surfactant layer is not permeable for large molecules. In contrast, small alkali molecules and water molecules diffuse into the track and provide the etching process enlarging the pore. At this stage the transport of the surfactant into the pore channel can proceed only due to the lateral diffusion in the adsorbed layer. The volume inside the pore is free of surfactant molecules and grows at a higher rate than the pore entrance. After a more prolonged etching the bottle-like (or “cigar-like”) pore channels are formed. The bottle-like shape of the pore channels depends on the etching conditions such as alkali and surfactant concentration, temperature, and type of the surfactant. The use of surfactants enables one to produce track-etch membranes with improved flow rate characteristics compared with those having cylindrical pores with the same nominal pore diameters.

Effect of nanosized surfactant molecules on the etching of ion tracks: New degrees of freedom in design of pore shape

Abstract A new method to control the geometry of track-etch nano- and micropores is described. Surfactant molecules added to a solution used for etching out ion tracks, create a steric-hindrance effect which is responsible for the formation of “bottleneck” or “cigar-like” pores. New applications are made possible with these new pore geometries.

Asymmetric ion track nanopores for sensor technology. Reconstruction of pore profile from conductometric measurements.

We reconstruct the profile of asymmetric ion track nanopores from an algorithm developed for conductometric measurements of symmetric nanopores. The validity of the reconstruction is supported by FESEM observations. Our analysis reveals that asymmetric pores fabricated by one-sided etching are funnel-like and not conical. The analysis provides the constriction diameter and the pore profile as a function of etching time. The reconstruction of the pore profile defines the starting conditions of asymmetric nanopores at breakthrough. The deviation from the conical shape is most pronounced at the pore tip. This critical zone dominates transport properties relevant to ion conductance, selectivity, current rectification, resistive pulse sensing and biosensors. The classical cone approximation used until now underestimates the tip diameter by a factor of two. As transport processes in nanopores depend in a highly nonlinear way on the constriction diameter the presented reconstruction must be taken into account when studying ionic and molecular transport processes in asymmetric pores.

Ultrasound propagation in the micropores of track membranes

Air-coupled and high-frequency ultrasonic spectroscopy is used to study ultrasound transmission through track membranes (TMs). Observed behavior, anomalous compared to that observed for other membranes, suggests independent ultrasound propagation through the pores. This is proved experimentally by studying frequency dependence of some acoustical parameters and by closing the pore aperture at the TM surface. This changes boundary conditions so that such wave is inhibited. Ultrasound propagation in such small pores opens up a new way to investigate gas behavior under rarefied conditions and provides a new technique to characterize TMs.

Preparation of porous polymer samples for SEM: combination of photo oxidation degradation with a freeze fracture technique

A new method for the preparation of porous polymer samples and investigations of their structure by scanning electron microscopy (SEM) is described. The technique for cleavage preparation is complemented by a preliminary treatment of a polymer with photo oxidation degradation to render it brittle. The advantages of this technique have been demonstrated with polyethylene tere-phthalate (PET) track membranes. The true size and shape of pores in the bulk sample can be seen in the cleavage plane. The degradation procedure does not change the membrane morphology. At the same time it allows one to reveal the characteristic features of the porous structure without its modification. Clear pictures of cross-sections of track membranes with cylindrical and bow-tie-like pore channels are presented.

New methods of track membrane treatment in the preparation of samples for further observation with scanning electron microscopy

Track membranes are porous systems consisting of a polymer foil with thin channels (i.e. pores) piercing it from surface to surface. The creation of non‐cylindrical pores in a track membrane is important for the optimization of membrane characteristics, i.e. the highest productivity at the required selectivity. A new method of cleavage preparation (the irradiation of track membrane samples with accelerated electrons) for the observation of channel shapes directly in the membrane cross‐sections is presented. Diagrams showing the tensile and burst strengths as a function of the irradiation dose, and images of surfaces and cleavages of track membrane samples are presented in this work. The changes in the pore sizes and shapes along the channel were clearly seen. These results can be used for the optimization of track membrane production.

Shedding light on the mechanism of asymmetric track etching: an interplay between latent track structure, etchant diffusion and osmotic flow

The method of producing single track-etched conical nanopores has received considerable attention and found many applications in diverse fields such as biosensing, nanofluidics, information processing and others. The performance of an asymmetric nanopore is largely determined by its geometry, especially by the size and shape of its tip. In this paper we reconstruct the profiles of so-called conical pores fabricated by asymmetric chemical etching of ion tracks in polymer foil. Conductometric measurements during etching and field emission scanning electron microscopy examinations of the resulting pores were employed in order to determine the pore geometry. We demonstrate that the pore constriction geometry evolves through a variety of configurations with advancing time after breakthrough. While immediately after breakthrough the pore tips are trumpet-shaped, further etching is strongly affected by osmotic effects which eventually lead to bullet-shaped pore tips. We evidence that the osmotic flow appearing during asymmetric track etching has a determinative effect on pore formation. A convection-diffusion model is presented that semi-quantitatively explains the effect of osmotic processes under asymmetric track etching conditions. In addition, a phenomenon of reagent contaminant precipitation in nanopores is reported and discussed.

Diffraction filters based on polyimide and poly(ethylene naphthalate) track membranes

The problem of optical filters for soft x rays and extreme ultraviolet that provide a high degree of blocking ultraviolet and visible background radiations is considered. The subject of discussion is the filter based on a track membrane, a polymer film with micrometer and submicrometer pores, rather than the standard thin-film system. It is proposed that the membranes be made of poly(ethylene naphthalate) or polyimide, the UV absorption edge of which lies near the boundary of the visible range. The properties of poly(ethylene naphthalate) and polyimide membranes are contrasted with those of conventional porous poly(ethylene terephthalate) films, which are obtained by ion track etching. The spectral characteristics of poly(ethylene naphthalate) and polyimide films, as well as the formation of “track” pores when the specimens are successively treated by fast ions and chemicals, are studied. The basic parameters of the resulting porous structures are examined, and treatment conditions under which desired optical properties of the membranes are achieved are found. Filters based on poly(ethylene naphthalate) and polyimide track membranes may be applied in x-ray astronomy as constituents of detectors incorporated into solar telescopes and in experiments with the laboratory plasma.