How Ion Tracks Form: From Energy Loss to the Finished Pore
The formation of ion tracks starts with a simple process. While crossing a solid, an ion continuously loses energy. The energy given up per unit of path length is called the energy loss. This quantity decides whether a lasting track stays behind in the material. The road from damage a few nanometers wide to a membrane with regular pores can be followed one step at a time.
Energy Loss and the Threshold for Track Formation
For fast, heavy ions the interaction with the electrons of the material dominates. The ion hands most of its energy to the electrons rather than to the atomic nuclei. As a result, a narrow zone of strongly disturbed structure appears along its path. Energy loss is therefore the basic quantity for describing the phenomenon.
A track forms as soon as the energy loss exceeds a threshold that depends on the material. Below that value the ion passes through the film and leaves its structure practically unchanged. Above it, the ion leaves a lasting track a few nanometers across. The track's length depends on the ion's energy and on the material. Polymer films, some glasses, mica and a few crystals have been studied best.
The Particle Accelerator as an Ion Source
Particle accelerators do the irradiating. They give the ions the energy needed to pass the threshold in the material at hand. An accelerator therefore lets you control which ions hit the film and with what energy. That produces single tracks and, with suitable irradiation, whole fields of tracks.
Track density is set through the irradiation time and the beam current. A longer exposure or a stronger current puts more tracks on the same area. This matters because the density later fixes the number of pores in the membrane.
Ion Range and Film Thickness
An ion does not travel through a material indefinitely, since its energy drops with every stretch it covers. Its range must therefore be matched to the thickness of the film. If the range exceeds the thickness, the track runs through the entire film. Otherwise the ion gets stuck in the material and the track ends sooner, with its length set by the range. Ion energy and film thickness are thus closely linked.
From Track to Pore: Etching
The material along the track is chemically more reactive, so an etching solution removes it faster than its surroundings. The thin channel becomes a pore. Its shape depends on the type and concentration of the etchant, the temperature, the etching time and any additives. It can be cylindrical, cone-shaped or double-conical.
The pores in a film have similar diameters because each track arose from the same physical process. This evenness is what sets membranes with track pores apart from many other porous materials.
Uses of the Finished Pores
Membranes with track pores are used in filtration and in sensing with single nanopores. Ion tracks also serve in detectors for particle tracks, and in geology they allow fission tracks in minerals to be examined. Templates for nanostructures are another field of use.
In the template method, one side of the membrane is coated with a conductive layer and a metal is deposited electrochemically inside the pores. Dissolving the polymer leaves wires, tubes or cones that match the shape of the pores. The whole chain, from the ion's energy loss to the finished nanostructure, therefore traces back to a single event: the formation of ion tracks in the material.