Ion tracks are created by swift heavy ions penetrating through solids.[1][2] They correspond to transformed zones >6 nanometer in diameter[3][4] and can be studied by Rutherford Backscattering Spectrometry (RBS), Transmission Electron Microscopy (TEM), Small-Angle Neutron Scattering (SANS), Small-Angle X-ray Scattering (SAXS) or gas permeation.[5] Ion tracks can be selectively etched in many insulating solids leading to cones or cylinders down to 8 nanometer in diameter.[6] In minerals, ion tracks can remain unaltered for millions of years. Their density tells about the time when the mineral has solidified from its melt and is used as a geological clock in fission track dating. Ion track technology deals with the application of ion tracks in micro- and nanotechnology.[7] Etched track cylinders can be used as filters,[8][9] Coulter counter apertures,[10] be modified with monolayers[11] or filled by electroplating.[12][13]
There is a 2000 year long way from the Greek/Indian concept of an atom to verifying that our world consists of atoms and, ultimately, that atoms can be used as a microtool. In microtechnology, the common mechanical tools of the macroworld have been gradually replaced by particle beams. Here, beams of photons and electrons modify the solubility of radiation sensitive polymers, so-called resists, while masking protects a selected area from exposure to radiation, chemical attack and erosion by atomic impact. Typical products are integrated circuits and microsystems. At present, the field of microtechnology expands toward nanotechnology. A recent branch of microfabrication is based on individual ions.
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Ion track technology fills a niche where conventional lithography fails:
The class of ion track recording materials[2] is characterized by the following properties:
Several types of swift heavy ion generators and irradiation schemes are used:
| Alpha and fission sources[22][23] provide low intensity beams with broad angular-, mass- and energy distribution. The range of the emitted fission fragments is limited to about 15 micrometers in polymers. Weak Californium-252 or Americium-241 sources[24] are used for scientific and technological explorations. They are compact, inexpensive and can be handled safely. | |
| Nuclear reactors provide fission fragments with broad angular-, mass-and energy distribution. Similar to alpha and fission sources, the penetration range of the emitted fission fragments is limited to about 15 micrometers in polymers. Nuclear reactors are used for filter production. | |
| Heavy ion accelerators provide parallel-beam irradiations at high luminosity with ions of defined mass, energy and tilt angle.[25][26][27] Intensities range up to billions of ions per second. Depending on the available energy, track lengths between few and several hundred micrometers are reached. Accelerators are used in micro- and nanotechnology. Radioactive contamination is absent at ion energies below the Coulomb barrier.[28] | |
| Single ion irradiations are used to fabricate individual micro- and nanostructures such as cones, channels, pins and wires.[18] The technique requires a weak ion beam which is switched-off after one ion has penetrated the target foil. | |
| Ion microbeams offer the highest control of the irradiation process. They restrict the output of a heavy ion accelerator to a small filament which is scanned over the sample surface. Scribing with individual swift heavy ions is possible with an aiming precision of about one micrometer.[20] |
When a swift heavy ion penetrates through a solid it leaves behind a trace of modified material confined to a cylinder of few nanometers in diameter. The energy transfer between the heavy projectile ion and the target electrons occurs in binary collisions. The knocked-off primary electrons leave a charged region behind. They induce a secondary electron collision cascade involving an increasing number of electrons of decreasing energy. The electron collision cascade stops when ionization is no longer possible. The remaining energy leads to atomic excitation and vibration (heat). Due to the large proton-to-electron mass ratio the energy of the projectile decreases gradually and the projectile path is straight.[29] A small fraction of the transferred energy remains as ion track in the solid. The diameter of the ion track increases with increasing radiation sensitivity of the material. Several models are used to describe ion track formation.
According to the thermal spike model the radiation sensitivity of different materials depends on their thermal conductivity and their melting temperature.
Selective ion track etching[2] is closely related with the selective etching of grain boundaries and crystal dislocations. The etch process must be sufficiently slow to discriminate between the irradiated and the pristine material. The resulting shape depends on the type of material, the concentration of the etchant and the temperature of the etch bath. In crystals and glasses, selective etching is due to the reduced density of the ion track. In polymers, selective etching is due to polymer fragmentation in the ion track core. The core zone is surrounded by a track halo in which cross-linking can impede track etching. After removal of the cross-linked track halo, the track radius grows linear in time. The result of selective etching is a trough, pore or channel.
Surfactant enhanced etching is used to modify ion track shapes.[35] It is based on self-organized monolayers.[11] The monolayers are semipermeable for the solvated ions of the etch medium and reduce surface attack. Depending on the relative concentration of the surfactant and the etch medium, barrel or cylindrical shaped ion track pores are obtained. The technique can be used to increase the aspect ratio.[36]
Repeated irradiation and processing. A two step irradiation and etching process is used to create perforated wells.
Arbitrary irradiation angles enforce an anisotropy along one specific symmetry axis.
Multiangular channels are interpenetrating networks consisting of two or more channel arrays at different directions.
| Material | pH | Wet etchant | Sensitizer1) | Desensitizer2) | T/C3) | Speed4) | Selectivity5) |
|---|---|---|---|---|---|---|---|
| PC | basic | NaOH | UV | Alcohols | 50-80 | Fast | 100-10000 |
| PET | basic | NaOH | UV, DMF | Alcohols | 50-90 | Fast | 10-1000 |
| basic | K2CO3 | 80 | Slow | 1000 | |||
| PI | basic | NaOCl | NaOH | 50-80 | Fast | 100-1000 | |
| CR39 | basic | NaOH | UV | 50-80 | Fast | 10-1000 | |
| PVDF6) | basic | KMnO4 + NaOH | 80 | Medium | 10-100 | ||
| PMMA6) | acidic | KMnO4 + H2SO4 | 50-80 | Medium | 10 | ||
| PP6) | acidic | CrO3 + H2SO4 | 80 | Fast | 10-100 |
1) Sensitizers increase the track etch ratio by breaking bonds or by increasing the free volume.
2) Desensitizers decrease the track etch ratio. Alternatively ion tracks can be thermally annealed.
3) Typical etch bath temperature range. Etch rates increase strongly with concentration and temperature.
4) Axial etching depends on track etch speed vt, radial etching depends on general etch speed vg.
5) Selectivity (aspect ratio, track etch ratio) = track etch speed / general etch speed = vt / vg.
6) This method requires to remove remaining metal oxide deposits by aqueous HCl solutions.
Etched ion tracks can be replicated by polymers[38] or metals[12] .[39] Replica and template can be used as composite. A replica can be separated from its template mechanically or chemically. Polymer replicas are obtained by filling the etched track with a liquid precursor of the polymer and curing it. Curing can be activated by a catalyst, by uv radiation or by heat. Metal replicas can be obtained either by electroless deposition or by electro-deposition. For replication of through-pores, a cathode film is deposited on one side of the membrane. The membrane is immersed in a metal salt solution. The cathode film is negatively charged with respect to the anode which is placed on the opposite side of the membrane. The positive metal ions are pulled toward the cathode where they catch electrons and precipitate as a compact metal film. During electro-deposition the channels fill gradually with metal. The length of the nano wires is controlled by the deposition time. Rapid deposition leads to polycrystalline wires. Slow deposition leads to single crystalline wires. A free standing replica is obtained by removing the template after deposition of a bearing film on the anode side of the membrane.
Interpenetrating wire networks are fabricated by electro-deposition in multi-angle track-etched membranes. Free-standing three-dimensional networks with tunable complexity and interwire connectivity are obtained.[40]
Segmented nanowires are fabricated by alternating the polarity during electro-deposition.[41] The segment length is adjusted by the pulse duration. In this way electrical, thermal, and optical properties can be tuned.
Filters: Homoporous filters were among the first applications[8] of ion track technology and are fabricated by several companies.[42]
Classifying micro- and nanoparticles: The resistance of a channel filled by an electrolyte depends on the volume of the particle passing through it.[10] The technique is applied to counting and sizing of red blood cells, bacteria and virus particles.
pH Sensor: Charged channels filled with an electrolyte have a surface conductivity in addition to the regular volume conductivity of the electrolyte. Ions attached to a charged surface attract a cloud of mobile counterions. Fixed and mobile ions form a double layer. For small channels surface conductivity is responsible for most of the charge transport. For small channels, surface conductivity exceeds volume conductivity. Negative surface charges can be occupied by firmly bound protons. At low pH (high proton concentration) the wall charge is completely neutralized. Surface conductivity vanishes. Due to the dependence of surface conductivity on pH the channel becomes a pH sensor.[43]
Current rectifying pores: Asymmetric pores are obtained by one-sided etching. The geometric asymmetry translates into a conduction asymmetry. The phenomenon is similar to an electrical valve. The pore has two characteristic conduction states, open and closed. Above a certain voltage the valve opens. Below a certain voltage the valve closes.[44][45]
Thermo-responsive channel: Obtained by lining a channel with a thermo-responsive gel.[46]
Bio-sensor: Chemical modification of the channel wall changes its interaction with passing particles. Different wall claddings bind to specific molecules and delay their passage. In this sense, the wall recognizes the passing particle. As an example, DNA fragments are selectively bound by their complementary fragments. The attached molecules reduce the channel volume. The induced resistance change reflects the molecule's concentration.[47]
Anisotropic conduction: A platform covered with many free standing wires acts as large area field emitter.[48]
Magnetic multilayers: Nano-wires consisting of alternating magnetic/nonmagnetic layers act as magnetic sensors. As an example, cobalt/copper nanowires are obtained from an electrolyte containing both metals. At low voltage, pure copper is deposited while cobalt resists electro-deposition. At high voltage both metals are deposited as an alloy. If the electrolyte contains predominantly cobalt, a magnetic cobalt-copper alloy is deposited with a high fraction of cobalt. The electrical conductivity of the multilayer wire depends on the applied external magnetic field. The magnetic order of the cobalt layers increases with the applied field. Without magnetic field, neighboring magnetic layers prefer the anti-parallel order. With magnetic field, the magnetic layers prefer the orientation parallel with the magnetic field. The parallel orientation corresponds to a reduced electrical resistance. The effect is used in reading heads of magnetic storage media (GMR effect).[49]
Spintronics: Spin valve structure consisting of two magnetic layers of different thickness. The thick layer has a higher magnetic stability and is used as polarizer. The thin layer acts as analyzer. Depending on its magnetization direction with respect to the polarizer (parallel or antiparallel) its conductivity is low or high, respectively.[50]
Textures: Tilted textures with a hydrophobic coating are at the same time superhydrophobic and anisotropic.[18] They show a preferred direction of transport. The effect has been demonstrated to convert vibration into translation.[51]