Nanowires from Pores: How Replicas of Nanostructures Are Made
Nanowires from pores come about in three steps. A single ion crosses a polymer film and leaves a track a few nanometers wide. A pore develops from it, and the pore then serves as a mold for a metal. Through irradiation, etching and electrodeposition, invisible damage in the material ends up as a wire.
Where do the pores in the film come from?
While crossing a solid, an ion keeps giving off energy. This release per unit of path length is called the energy loss. For fast, heavy ions the interaction with the electrons of the material dominates. If the energy loss exceeds a material-dependent threshold, a track forms, and its length depends on the ion's energy and on the material.
Polymer films, some glasses, mica and a few crystals are the best studied. Particle accelerators carry out the irradiation, and irradiation time and beam current set the track density. Because the material along the track is chemically more reactive, an etching solution removes it faster than the surrounding material. That is how the track becomes a pore.
Depending on the etchant, concentration, etching time, temperature and additives, the pore can turn out cylindrical, cone-shaped or double-conical. The same polymer therefore gives different pores when the etching conditions are changed.
The Template Method: From Pores to Wires
In the template method the etched membrane acts as a mold. First, one side of the membrane receives a conductive layer. A metal is then deposited electrochemically in the pores, and starting from the conductive layer it fills the empty channels. Once the polymer is dissolved, only the metal structures remain: wires, tubes or cones. The shape of the resulting object matches the shape of the pore.
That is why people speak of replicas of nanostructures: the metal reproduces the inside of the template. A cylindrical pore gives a wire, and a cone-shaped pore gives a cone. The template method rests on exactly this idea, with the membrane as the original and the metal as its image.
Why are nanowires from pores regular?
The pores in a film share a similar diameter because every track was made by the same physical process. Each ion loses its energy by the same laws and in the same material. On top of that, the same etching conditions apply across the whole membrane. For this reason the nanowires grown in the pores of one film resemble each other.
This regularity follows from the physical sequence and does not arise by chance. It is one reason replicas of nanostructures from one membrane can be treated as a set with similar dimensions.
Why study pores and nanowires?
The pores etched into membranes have several uses. They include filtration and sensing with single nanopores, and they also serve as detectors for particle tracks. Geology makes use of fission tracks in minerals.
Templates for nanostructures form a group of their own. They produce wires, tubes and cones of a given shape, and the main tool for this is the template method. The same kind of template can be used for different replicas if the conditions during irradiation and etching are changed.
In short, nanowires from pores reproduce the tracks that ions left in the material. They arise from regular templates, and that regularity follows directly from the physics of the process.