Funnel-type etched ion tracks in polymers

Abstract

It is shown that conical track etching is a much more complicated process than generally assumed. The choice of the corresponding parameters (i.e. the ratios of concentrations and diffusion coefficients of both etchant (e.g. NaOH) and stopping solutions (e.g. HCl) and the etching temperature) determines the ratio of polymer dissolution to etchant penetration. The latter value controls the counterplay of diffusion, etching, ionic conductivity, field emission and capacitive effects, which is decisive for both the final track shapes and their electronic properties. The stages of track evolution during etching under different conditions are outlined in detail. Both transparent conical nanopores and “funnel-type” tracks can be obtained, the latter consisting of a shorter cone and a residual latent track. Depending on the internal structure of that latent track segment, such funnel-type tracks either allow smooth transmission of the rectified currents or they emit unipolar current spikes. Not only the study of electronic properties of single ion tracks, but also of a multitude of tracks makes sense. Depending on the applied parameters, the individual track properties may either just add up, or new effects may be found that emerge from the interaction of the tracks among each other. This is preferentially the case for spike-emitting tracks, where effects such as phase-locked spike synchronization can be found as described by neural network theory.

Keywords:

• tracks
• polymers
• etching
• electrical characterization
• neutron depth profiling
• ion transmission spectrometry

Acknowledgements

D.F. thanks the Universidad Autónoma Metropolitana, Iztapalapa, México City, for the Guest Professorship in the frame of the Cathedra Marcos Menzer Mazzari. We are especially obliged to Prof. S.A. Cruz and Mr N. Camarillo from UAM-Iztapalapa and J.C. Terán from UAM- Cuajimalpa, for their continuous help and discussions and for providing us with adequate working facilities. We are further obliged to Dr P. Apel from JNRI Dubna, Russia for providing us with the ion-irradiated foils.

Notes

This is the case if the latent track just behaves like an ohmic resistance. There exists another case which is not treated here but is subject of another paper 6, in which latent tracks emit current spikes.