Ion track

Ion tracks are produced by fast heavy ions on their way through a solid. They correspond structurally altered areas with a diameter of 6-8 nanometers and can be analyzed using the following techniques: Rutherford backscattering (RBS ), transmission electron microscopy ( TEM), small angle neutron scattering (SANS ), X-ray small -angle scattering (SAXS ), gas permeation. Ion tracks can be selectively etched in many insulating solids. The result consists of cones or cylinders with diameters down to 8 nanometers. Ion tracks in minerals can survive for millions of years. From their density can be determined to which the mineral is solidified from its melt time. In the fission track dating of ion tracks serve as geologic clocks.

Ion track technology is the application of ion tracks in micro-and nanotechnology. Etched track cylinders can be used as a filter, as Zählöffnungen, they can be modified by monolayers, or filled by electrochemical deposition.

The classical paradigm of the atom as indivisible basic building block of matter was confirmed by experiments in the 19th century. Beginning of the 20th century were detected atoms. From the middle of the 20th century, individual atoms were used as micro tool. Of this, the present article is.

In the micro technology, the mechanical tools of macro world are increasingly being replaced by the irradiation process. Photons and electrons are used to increase the solubility of radiation-sensitive polymers, so-called resist, or out. As a structuring element masks are used. They protect the resist prior to irradiation. The unmasked areas change their wet chemical solubility and their resistance to removal by ion sputtering. Typical products of micro-technology are integrated circuits and microelectromechanical systems (MEMS). Currently, the micro technology is refined to nanotechnology. Here, increasing ions. A new branch of this technology is based on the structuring by high-energy heavy ions. Due to their high energy density can be produced with single ion micro-and nanostructures.

Areas of application

Ion track technology was developed for niche areas where the conventional lithography fails:

• Processing of radiation-resistant minerals, glasses and polymers
• Production of slender structures with a resolution limit down to 8 nanometers
• Direct perforation of thin films without any development process
• Structures, the depth of which is not dictated by the range of the ions through the layer thickness
• Fabrication of structures with high aspect ratio (length: width) to 104
• Design of rigid and flexible materials under defined cutting angle
• Aligned textures with a defined angle of inclination
• Preparation of random patterns in partially overlapping ion traces
• Production of large numbers of individual ions structures
• Targeted production of patterns of individual ion tracks

Materials used

The class of ion track sensitive materials is characterized by the following properties:

• Large Homogeneity: The local density fluctuations of the original material must be small compared to the change in density of the core region of the ion track. Optically transparent (non- opaque ) material such as polycarbonate, polyvinylidene fluoride, have this property. Granular polymers such as Teflon ( polytetrafluoroethylene) are opaque and do not have this property.

• High electrical resistance (low conductance ): Non-conductive dielectric minerals, glasses and polymers have this property, while highly conductive metals and alloys do not have this property. In metals the thermal conductivity is coupled to the electrical resistance. This suppresses the formation of metal in a heated zone in the vicinity of the ion trajectory.

• High radiation sensitivity: polymers have a high radiation sensitivity compared to glasses and ionic crystals. The irradiation effect in polymers is caused by a triggered during ion passage secondary electron avalanche. This causes the network of the polymer both strand breaks and cross-links. Strand breaks prevail in the core of the ion track before (radius 6-8 nm) and cross-connections in the outer region of the ion track (up to about 100 nanometers).

• Low mobility of the atoms: For selective etching of ion tracks in the density contrast between the latent ion track and the original material must be high. By diffusion of the contrast fades. This is due to the thermally excited migration of atoms. Ion tracks can be thermally cured. In glasses this happens faster than in hard ionic crystals.

Radiation techniques

Several types of ion accelerators and radiation techniques are used:

Ion track formation

If a fast Schwerion penetrates a solid, it leaves behind a trail of modified material. The modified portion has a diameter of a few nanometers. The energy transfer between the heavy projectile ion and the easy target electrons occurs in two collisions. The ejected primary electrons leave an electrically charged zone and trigger a secondary electron avalanche that detects an avalanche rising number of electrons whose energy decays rapidly. The electron avalanche comes to a halt as soon as the energy is no longer sufficient for ionization. The residual in the solid state energy is converted into atomic excitation and vibration and largely converted into heat. Due to the large mass ratio between proton and electron, the energy of the projectile decreases continuously. The projectile path is straight. Only a small fraction of the transferred energy remains as a trace ion in the solid. The diameter of the ion track increases with increasing radiation sensitivity of the material. Several models are used to describe the ion track formation.

• After the ion explosion model Primärionisation leads to an atomic collision cascade. This results in an amorphous zone with a reduced density around the ion path.

• After the electron cascade model, the secondary electrons cause radiation effect in the material, comparable to a localized irradiation with electrons. The electron cascade model is particularly suitable for polymers.

• After thermal spike model, electron avalanche is responsible for the energy transfer between the projectile and the target electrons. Once the melting point of the target material is exceeded, forms a melt. This caused by the cold ambient rapid quenching of the melt leaving an amorphous ( glassy ) state decreased density. The remaining disorder corresponds to the ion track.

It follows from the thermal spike model that the radiation sensitivity of solids increases with decreasing thermal conductivity and decreasing melting point.

Deferred ion track in mica. Depending on the braking force of the track Projektilions diameter is between 4 and 10 nanometers.

Molecular dynamics simulation of a collision cascade in gold. Development of a collision cascade initiated in the center of the image with color-coded atomic kinetic energy.

The Spurätzschwelle is the energy input required for the selective etching of ion track. In ionic crystals, the threshold increases with the thermal conductivity.

Etching

Single-stage method

The selective etching of ion tracks is closely related to the selective etching of grain boundaries and dislocation lines. The etching process must be sufficiently slow to distinguish between the irradiated and non-irradiated material. The resulting shape will depend on the nature of the material, the concentration of the etchant and the temperature of the Ätzbads. In crystals and glasses, selective etching is due to the decreased density of the material in the ion track. In polymers, the selective etching depends on the fragmentation of the polymer in the core area of the ion track. The core region is surrounded by a partially cross-linked region of lower etchability ( Halo ). Outside the networked halos the ion track increases linearly with time. The result of selective etching, a recess, a pore or a channel.

The network- Funded etching is used to shoaling of trace forms. The method relies on the self-assembly of monolayers on the inner wall of the etched channel. The monolayers are a barrier to the solute in the aqueous medium ( solvated ) ions. It hinders the etching of the surface. Depending on the concentration of the wetting agent and the etchant caused barrel or cylindrical shapes. The technique is used to increase the aspect ratio.

Multi-stage process

The Stepwise irradiation and processing: A multi-stage process with two exposures and two etchings for the production of perforated wells.

Arbitrary irradiation angle force an anisotropy along a particular axis of symmetry.

Multi-angle channels are mutually interpenetrating networks of two or more channel droves in different directions.

Asymmetric channels with constriction at the upper end.

Perforated micro-container.

Influence of the irradiation angle ( 45 and 90 degrees).

Channel overlap. Three membranes with crisscross ion tracks ( ± 10, ± 20, ± 45 degrees).

1) sensitizers increase the Spurätzverhältnis by breaking chemical bonds and increase the free space. 2) Desensitizer reduce the Spurätzverhältnis. Another possibility is thermal treatment. 3 ) Typical field of etching temperature. The etch rate increases with concentration and temperature of the etching medium. 4) depends on the axial etching Spurätzrate VT, the radial etch rate etch of the general vg. 5 ) Selectivity (aspect ratio, Spurätzverhältnis ) = Spurätzgeschwindigkeit / General etch rate = vt / vg. 6) The method requires the removal of the remaining metal oxide on the sample by hydrochloric acid.

Impression

Etched ion tracks can be molded by polymers or metals. The molded micro parts can be used as a composite material. A replica can be separated mechanically or chemically by its shape. Polymerreplikate are prepared that fills the etched ion track with a liquid precursor of the polymer and this then hardens. Curing can be carried out catalytically, by UV irradiation or heat. Metallreplikate can be made either by electrodeposition or electroless deposition. The impression of continuous pores first, a metal film is deposited on the porous membrane, which serves as cathode during the electrodeposition. Subsequently, the membrane is immersed in a corresponding metal salt solution. The cathode film is negatively charged with respect to the anode. The anode is placed on the opposite side of the membrane. The solvated positively charged metal atoms are attracted to the cathode. There they are neutralized by escaping electrons and deposited as a metal film. During the electro-deposition to fill the channels, starting at the applied cathode film, with metal. The length of the microwires can be preset by the duration of the deposition. A rapid deposition results in polycrystalline, a slow deposition of single crystal wires. Detached replicas are made ​​by depositing a viable cathode layer by removing the template.

Be passing through networks of wires are prepared by electrodeposition in mehrwinkelbestrahlte etched membranes. In this way, free-standing 3-D networks adjustable penetration can be produced.

Segmented nanowires are produced by alternating reversal of polarity during the deposition. The segmentation length is controlled by the pulse duration of the deposition. In this way, the electrical, thermal and optical properties of the composite can be set.

Mutually interpenetrating network of wires.

Set of segmented wires, obtained by reversing the polarity.

Applications

Filters: with a uniform pore size and shape are among the first applications of ion track technology. They are offered by several manufacturers.

Size classification of the micro-and nanoparticles: The resistance of a Kananals, which is filled with a saline solution depends on the volume of the hatching through said channel particle. This technique is used to determine the number and size of cells, bacteria, and virus particles.

PH sensor: filled with a saline solution channels have a " bulk conductivity " and in an electrically charged surface, in addition a " surface conductivity ." The surface conductivity results from the fact that the ions bound to the wall of the channel attract a cloud of mobile ions of opposite charge. In this way, an electric double layer forms. In the fine channel, the surface conductivity outweighs the volume conductivity. Characterized the electrical resistance of the channel of the interaction of the transported ions is influenced by the channel wall. This effect can be used in sensor technology. Negative surface charges can be occupied by relatively tightly bound protons. At low pH ( high concentration of protons ), the wall charge is completely neutralized by the protons. The channel is for the pH sensor.

Electric rectifying pores: Asymmetric pores can be achieved by unilateral etching embedded in the membrane ion tracks. The geometrical asymmetry results in an asymmetry of the electric conductivity. The phenomenon can be compared with an electric valve. The pore has two distinct conduction states: open and closed. Above a certain voltage, the valve opens. Below a certain voltage the valve closes.

Thermoresponsive channel: Manufactured by lining a passageway with thermoresponsivem poly (N-isopropylacrylamide ) gel.

Biosensor: The chemical modification of the channel changes its interaction with durchschlüpfenden particles. Different panels affect the passage of time. In this sense, to "recognize" the channel wall, the passing particles. For example, DNA fragments are selectively bound to their complementary fragments. The attached molecules reduce the channel volume. The increased electrical Widerstend of the channel reflects the concentration of the " recognized " resist molecules.

PH sensor: The selective etching of the polymer in ion traces electrically negatively charged channels can be obtained. If these are filled with a dilute salt solution so formed near the surface of a positive, mobile counter charge cloud. This increases the electrical conductivity of the channels. Since the wall charges are neutralized by the protons, the conductivity of the proton concentration, the pH, the solution depends on.

Anisotropic electrical conductivity: A free-standing wires with studded metal plate for large-area field emitters.

Magnetic multilayers: nanowires which consist of alternating layers of magnetic and non-magnetic layers to act as magnetic field sensors. For example, cobalt, copper layers can be obtained from a salt solution which contains the ions of both metals. At low voltage, the ( noble ) of copper is deposited while the lighter oxidizable ( noble ) cobalt remains in the solution. At higher voltage, the two metals, the copper and cobalt are precipitated. When in the salt solution, the concentration of cobalt in comparison with the copper is very high, at the high voltage especially cobalt is eliminated. The result is a magnetically active layer. The magnetic order of cobalt layers increases with increasing external magnetic field. The magnetic orientation is determined by the external field. The individual cobalt layers are magnetically oriented in parallel. In the absence of external magnetic field, the preferred magnetic order is anti-parallel. Since the state of lower order corresponds to a higher electrical resistance, can be used as the magnetic field sensor. The effect is used in the read / write heads of magnetic storage devices (GMR effect).

Spintronics: a spin valve structure is formed of two magnetic layers of different thickness. The thick layer has higher magnetic stability than the thin layer. It is used as a polarizer. The thin layer is used as an analyzer. The electrical resistance depends on the mutual orientation of the magnetic layers. In parallel order of the resistance is small and large antiparallel order.

Textures: Tilted needle textures are coated with a water-repellent, Teflon-like film. You are superhydrophobic, as water droplets just touch the tips. At the same time the textures have a preferred direction of transport due to the tilt. The possibility of conversion of vibration in translation was demonstrated.

Particle transit channel. The transient current slump is the particle volume proportionally.

PH Sensor: The electrical conductivity depends on the density of mobile charge cloud.

The asymmetric pore allows positive ions preferably from right to left.

The heat-sensitive channel. The hydrogel -lined canal opens above and closes below the critical temperature.

Biospecific sensor. The wall cladding binds complementary biomolecules. The resistance increases with their concentration.

Field-emitter array

Shingled magnetic field sensor. High field parallel order, small electric resistance.

Spin analyzer The energy loss of spin-polarized electrons depends on the magnetic order of the analyzer. Links: polarizer (blue: spin up ). Right: analyzer (blue: spin up, red: spin down ).

Tilted track texture with asymmetric sliding properties.