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Ion tracks are damage-trails created by swift heavy ions penetrating through solids, which may be sufficiently-contiguous for chemical etching in a variety of crystalline, glassy, and/or polymeric solids. They are associated with cylindrical damage-regions several
nanometer 330px, Different lengths as in respect to the molecular scale. The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American and British English spelling differences#-re, ...
s in diameter and can be studied by
Rutherford backscattering spectrometry Rutherford backscattering spectrometry (RBS) is an analytical technique used in materials science. Sometimes referred to as high-energy ion scattering (HEIS) spectrometry, RBS is used to determine the structure and composition of materials by mea ...
(RBS),
transmission electron microscopy Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a ...
(TEM),
small-angle neutron scattering Small-angle neutron scattering (SANS) is an experimental technique that uses elastic neutron scattering at small scattering angles to investigate the structure of various substances at a mesoscopic scale of about 1–100 nm. Small angle n ...
(SANS),
small-angle X-ray scattering Small-angle X-ray scattering (SAXS) is a small-angle scattering technique by which nanoscale density differences in a sample can be quantified. This means that it can determine nanoparticle size distributions, resolve the size and shape of (monodi ...
( SAXS) or gas
permeation In physics and engineering, permeation (also called imbuing) is the penetration of a permeate (a fluid such as a liquid, gas, or vapor) through a solid. It is directly related to the concentration gradient of the permeate, a material's intrins ...
.


Ion track technology

Ion track technology deals with the production and application of ion tracks in microtechnology and
nanotechnology Nanotechnology, also shortened to nanotech, is the use of matter on an atomic, molecular, and supramolecular scale for industrial purposes. The earliest, widespread description of nanotechnology referred to the particular technological goal ...
. Ion tracks can be selectively etched in many insulating solids, leading to cones or cylinders, down to 8 nanometers in diameter. Etched track cylinders can be used as
filter Filter, filtering or filters may refer to: Science and technology Computing * Filter (higher-order function), in functional programming * Filter (software), a computer program to process a data stream * Filter (video), a software component tha ...
s, Coulter counter microchannels, be modified with
monolayer A monolayer is a single, closely packed layer of atoms, molecules, or cells. In some cases it is referred to as a self-assembled monolayer. Monolayers of layered crystals like graphene and molybdenum disulfide are generally called 2D materials. C ...
s, or be filled by
electroplating Electroplating, also known as electrochemical deposition or electrodeposition, is a process for producing a metal coating on a solid substrate through the reduction of cations of that metal by means of a direct electric current. The part to be ...
. Ion track technology has been developed to fill certain niche areas where conventional
nanolithography Nanolithography (NL) is a growing field of techniques within nanotechnology dealing with the engineering (patterning e.g. etching, depositing, writing, printing etc) of nanometer-scale structures on various materials. The modern term reflects on ...
fails, including: *Direct shaping of
radiation-resistant Radiation hardening is the process of making electronic components and circuits resistant to damage or malfunction caused by high levels of ionizing radiation ( particle radiation and high-energy electromagnetic radiation), especially for envir ...
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
s,
glass Glass is a non- crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenchin ...
es and
polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
s *Generation of elongated structures with a resolution limit down to 8 nanometers *Direct generation of holes in thin films without any development process *Defining structural depth by ion range rather than by target thickness *Generating structures with aspect ratio (depth divided by width) up to 104. *Shaping rigid and flexible materials at a defined cutting angle *Exploring the realm of aligned textures with defined inclination angles *Generation of random patterns consisting of partially overlapping single tracks *Generation of large numbers of individual single track structures *Generation of aimed patterns consisting of individual single tracks


Materials susceptible to ion track recording

The class of ion track recording materials is characterized by the following properties: *High
homogeneity Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, size, ...
: Local density variations of the pristine material must be small in comparison to the density deficit of the ion track core. Optically translucent materials, such as
polycarbonate Polycarbonates (PC) are a group of thermoplastic polymers containing carbonate groups in their chemical structures. Polycarbonates used in engineering are strong, tough materials, and some grades are optically transparent. They are easily work ...
and
polyvinylidene fluoride Polyvinylidene fluoride or polyvinylidene difluoride (PVDF) is a highly non-reactive thermoplastic fluoropolymer produced by the polymerization of vinylidene difluoride. PVDF is a specialty plastic used in applications requiring the highest pur ...
, have this property. Grained polymers such as
polytetrafluoroethylene Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. It is one of the best-known and widely applied PFAS. The commonly known brand name of PTFE-based composition is Teflon by Chemo ...
do not have this property. *High
electrical resistance The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual parallel ...
: Non-conducting
dielectric In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the ma ...
minerals, glasses, and polymers have this property, while highly conducting metals and alloys do not have this property. In metals, the
thermal diffusivity In heat transfer analysis, thermal diffusivity is the thermal conductivity divided by density and specific heat capacity at constant pressure. It measures the rate of transfer of heat of a material from the hot end to the cold end. It has the SI ...
is coupled with the
electrical conductivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...
, suppressing the formation of a thermal spike. *High radiation sensitivity: Polymers have high radiation sensitivity as compared to glasses and ionic crystals. The radiation effect in polymers is caused by the secondary electron cascade, inducing both chain scission (dominating in the track core) and cross-linking (dominating in the track halo). *Low atomic mobility: For selective ion track etching, the density contrast between the latent ion track and the pristine material must be high. The contrast fades due to
diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical ...
, depending on the atomic mobility. Ion tracks can be annealed. Erasing is faster in
glass Glass is a non- crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenchin ...
es compared to ionic crystals.


Irradiation apparatus and methods

Several types of swift heavy ion generators and irradiation schemes are currently used:


Formation of ion tracks

When a swift heavy ion penetrates through a solid, it leaves behind a trace of irregular and modified material confined to a cylinder of few nanometers in diameter. The energy transfer between the heavy ''projectile'' ion and the light target electrons occurs in binary collisions. The knocked-off ''primary'' electrons leave a charged region behind, inducing a ''secondary'' electron collision cascade involving an increasing number of electrons of decreasing energy. This electron collision cascade stops when ionization is no longer possible. The remaining energy leads to atomic excitation and vibration, producing (
heat In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
). Due to the large
proton-to-electron mass ratio In physics, the proton-to-electron mass ratio, ''μ'' or ''β'', is the rest mass of the proton (a baryon found in atoms) divided by that of the electron (a lepton found in atoms), a dimensionless quantity, namely: :''μ'' = The number in parenthe ...
, the energy of the ''projectile'' decreases gradually and the ''projectile path'' is straight. A small fraction of the transferred energy remains as an 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 ion explosion spike model the ''primary'' ionization induces an atomic
collision cascade In condensed-matter physics, a collision cascade (also known as a displacement cascade or a displacement spike) is a set of nearby adjacent energetic (much higher than ordinary thermal energies) collisions of atoms induced by an energetic par ...
, resulting in a disordered zone around the ion trajectory. *According to the electron
collision cascade In condensed-matter physics, a collision cascade (also known as a displacement cascade or a displacement spike) is a set of nearby adjacent energetic (much higher than ordinary thermal energies) collisions of atoms induced by an energetic par ...
model the ''secondary'' electrons induce a radiation effect in the material, similar to a spatially-confined electron irradiation. The electron
collision cascade In condensed-matter physics, a collision cascade (also known as a displacement cascade or a displacement spike) is a set of nearby adjacent energetic (much higher than ordinary thermal energies) collisions of atoms induced by an energetic par ...
model is particularly suited for polymers. *According to the thermal spike model, the electron collision cascade is responsible for the energy transfer between the projectile ion and the target nuclei. If the temperature exceeds the melting temperature of the target substance, a liquid is formed. The rapid
quenching In materials science, quenching is the rapid cooling of a workpiece in water, oil, polymer, air, or other fluids to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, such as ...
leaves behind an amorphous state with decreased density. Its disorder corresponds to the ion track. The thermal spike model suggests the radiation sensitivity of different materials depends on their thermal conductivity and their melting temperature. File:Thermal_Spike.gif, Thermal spike model
Ion track corresponds to frozen disorder after rapid quenching of melt zone around ion trajectory. Temperature represented by color. Ion path vertical to image plane. File:Latent Track Vetter Scholz.jpg, Latent ion track in
muscovite Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula K Al2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavag ...
mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...
. Depending on the stopping power of the projectile ion the track width is between 4 and 10 nanometer. Image:10kevau au.gif,
Molecular dynamics Molecular dynamics (MD) is a computer simulation method for analyzing the physical movements of atoms and molecules. The atoms and molecules are allowed to interact for a fixed period of time, giving a view of the dynamic "evolution" of th ...
simulation of
collision cascade In condensed-matter physics, a collision cascade (also known as a displacement cascade or a displacement spike) is a set of nearby adjacent energetic (much higher than ordinary thermal energies) collisions of atoms induced by an energetic par ...
in gold. File:Track Etch Threshold and Thermal Conductivity.jpg, Track etch threshold: energy input required for selective etching. For ionic crystals, the threshold increases with the heat conductivity.
Amorphous metal An amorphous metal (also known as metallic glass, glassy metal, or shiny metal) is a solid metallic material, usually an alloy, with disordered atomic-scale structure. Most metals are crystalline in their solid state, which means they have a high ...
FeBSiC included for comparison.


Etching methods


Selective ion etching

''Selective ion track etching'' is closely related to the selective etching of
grain boundaries In materials science, a grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are two-dimensional defects in the crystal structure, and tend to decrease the electrical and therma ...
and crystal
dislocation In materials science, a dislocation or Taylor's dislocation is a linear crystallographic defect or irregularity within a crystal structure that contains an abrupt change in the arrangement of atoms. The movement of dislocations allow atoms to s ...
s. 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

''Surfactant enhanced etching'' is used to modify ion track shapes. It is based on
self-organized Self-organization, also called spontaneous order in the social sciences, is a process where some form of overall order arises from local interactions between parts of an initially disordered system. The process can be spontaneous when suffic ...
monolayer A monolayer is a single, closely packed layer of atoms, molecules, or cells. In some cases it is referred to as a self-assembled monolayer. Monolayers of layered crystals like graphene and molybdenum disulfide are generally called 2D materials. C ...
s. The
monolayer A monolayer is a single, closely packed layer of atoms, molecules, or cells. In some cases it is referred to as a self-assembled monolayer. Monolayers of layered crystals like graphene and molybdenum disulfide are generally called 2D materials. C ...
s are semi-permeable 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.


Other related terminology

''Repeated irradiation and processing'': A two-step irradiation and etching process 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 in different directions. File:Double sided etching of ion track.gif, alt=Double sided etching of track membrane, Double sided etching of ion track at track etch ratio 5:1. File:Asymmetric ion track channels.jpg, Asymmetric ion track channels with strongly reduced top diameter. File:Perforated microwells.jpg, Microwells with perforated bottom. File:Perforated membranes with different channel angles.gif, Two membranes with different channel inclination (vertical and 45 degrees). File:Membranes perforated at two strutting angles.gif, Three membranes perforated at two strutting angles (±10, ±20, ±45 degrees). 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.


Replication

Etched ion tracks can be replicated by
polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
s or
metal A metal (from ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, e ...
s.
Replica A 1:1 replica is an exact copy of an object, made out of the same raw materials, whether a molecule, a work of art, or a commercial product. The term is also used for copies that closely resemble the original, without claiming to be identical. Al ...
and
template Template may refer to: Tools * Die (manufacturing), used to cut or shape material * Mold, in a molding process * Stencil, a pattern or overlay used in graphic arts (drawing, painting, etc.) and sewing to replicate letters, shapes or designs ...
can be used as
composite Composite or compositing may refer to: Materials * Composite material, a material that is made from several different substances ** Metal matrix composite, composed of metal and other parts ** Cermet, a composite of ceramic and metallic materials ...
. A replica can be separated from its
template Template may refer to: Tools * Die (manufacturing), used to cut or shape material * Mold, in a molding process * Stencil, a pattern or overlay used in graphic arts (drawing, painting, etc.) and sewing to replicate letters, shapes or designs ...
mechanically or chemically.
Polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
replica A 1:1 replica is an exact copy of an object, made out of the same raw materials, whether a molecule, a work of art, or a commercial product. The term is also used for copies that closely resemble the original, without claiming to be identical. Al ...
s are obtained by filling the etched track with a liquid
precursor Precursor or Precursors may refer to: * Precursor (religion), a forerunner, predecessor ** The Precursor, John the Baptist Science and technology * Precursor (bird), a hypothesized genus of fossil birds that was composed of fossilized parts of u ...
of the
polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
and curing it. Curing can be activated by a
catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
, by ultraviolet
radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
, or by
heat In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
.
Metal A metal (from ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, e ...
replica A 1:1 replica is an exact copy of an object, made out of the same raw materials, whether a molecule, a work of art, or a commercial product. The term is also used for copies that closely resemble the original, without claiming to be identical. Al ...
s 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, and 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, and the lengths of the nano-wires are controlled by the deposition time. Rapid deposition leads to polycrystalline wires, while 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. Segmented nanowires are fabricated by alternating the polarity during electro-deposition. The segment length is adjusted by the pulse duration. In this way electrical, thermal, and optical properties can be tuned. File:Copper_wires_Johann_Vetter.jpg, Free standing metal replica of etched ion tracks in PC File:Markus Rauber Exp_179 - 36.jpg, Interpenetrating wire network File:Markus_Rauber_Exp_169p3_-_20.jpg, Bundle of segmented platinum nanowires


Applications

Microtechnology: The common mechanical
tool A tool is an object that can extend an individual's ability to modify features of the surrounding environment or help them accomplish a particular task. Although many animals use simple tools, only human beings, whose use of stone tools dates b ...
s of the macroworld are being supplemented and complemented, and in some applications replaced by,
particle beam A particle beam is a stream of charged or neutral particles. In particle accelerators, these particles can move with a velocity close to the speed of light. There is a difference between the creation and control of charged particle beams and ne ...
s. Here, beams of
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alwa ...
s and
electrons The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...
modify the
solubility In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubi ...
of radiation-sensitive
polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
s, so-called "
resist A resist, used in many areas of manufacturing and art, is something that is added to parts of an object to create a pattern by protecting these parts from being affected by a subsequent stage in the process. Often the resist is then removed. For ...
s", while masking protects a selected area from exposure to
radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
,
chemical attack Chemical warfare (CW) involves using the toxic properties of chemical substances as weapons. This type of warfare is distinct from nuclear warfare, biological warfare and radiological warfare, which together make up CBRN, the military acronym ...
, and erosion by atomic impact. Typical products produced in this way are
integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...
s and microsystems. At present, the field of microtechnology is expanding toward
nanotechnology Nanotechnology, also shortened to nanotech, is the use of matter on an atomic, molecular, and supramolecular scale for industrial purposes. The earliest, widespread description of nanotechnology referred to the particular technological goal ...
. A recent branch of
microfabrication Microfabrication is the process of fabricating miniature structures of micrometre scales and smaller. Historically, the earliest microfabrication processes were used for integrated circuit fabrication, also known as " semiconductor manufacturing ...
is based on manipulation of individual ions. Geology: Ion tracks are useful as they can remain unaltered for millions of years In minerals. Their density yields information about the time when the mineral solidified from its melt, and are used as geological clocks in
fission track dating Fission track dating is a radiometric dating technique based on analyses of the damage trails, or tracks, left by fission fragments in certain uranium-bearing minerals and glasses. Fission-track dating is a relatively simple method of radiomet ...
Filters: Homoporous
filters Filter, filtering or filters may refer to: Science and technology Computing * Filter (higher-order function), in functional programming * Filter (software), a computer program to process a data stream * Filter (video), a software component that ...
were among the first applications of ion track technology, and are now fabricated by several companies. Mica membranes with ion track pores were used by Beck and Schultz to determine the mechanism of hindered diffusion in nanopores. Classifying micro- and nanoparticles: The resistance of a channel filled by an
electrolyte An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon ...
depends on the volume of the particle passing through it. This technique is applied to the counting and sizing of individual red blood cells, bacteria, and virus particles. pH Sensor: Charged channels filled with an
electrolyte An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon ...
have a
surface conductivity Surface conductivity is an additional conductivity of an electrolyte in the vicinity of the charged interfaces. Surface and volume conductivity of liquids correspond to the electrically driven motion of ions in an electric field. A layer of coun ...
, in addition to the regular volume conductivity, of the electrolyte. Ions attached to a charged surface attract a cloud of mobile
counterion 160px, Polystyrene sulfonate, a cation-exchange resin, is typically supplied with as the counterion.">cation-exchange_resin.html" ;"title="Polystyrene sulfonate, a cation-exchange resin">Polystyrene sulfonate, a cation-exchange resin, is typical ...
s. Fixed and mobile ions form a double layer. For small channels,
surface conductivity Surface conductivity is an additional conductivity of an electrolyte in the vicinity of the charged interfaces. Surface and volume conductivity of liquids correspond to the electrically driven motion of ions in an electric field. A layer of coun ...
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. 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. Thermo-responsive channel: Obtained by lining a channel with a thermo-responsive gel. 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. Anisotropic conduction: A platform covered with many free standing wires acts as large area field emitter. 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 An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon ...
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 An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon ...
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 (the "GMR effect"). Spintronics: Spin valve structure consists of two magnetic layers of different thicknesses. 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. Textures: Tilted textures with a hydrophobic coating are at the same time superhydrophobic and anisotropic, and show a preferred direction of transport. The effect has been demonstrated to convert vibration into translation. File:Etched Tracks.gif, Etched ion tracks File:Particle Transit Channel.gif, Particle Transit Channel. The transient current drop is proportional to the particle volume. File:PH Sensor Channel.gif, pH Sensor: The moving circle represents the cross section of a negatively charged channel. Left: At low pH all surface charges are occupied by protons (low conductivity). Right: At high pH all surface charges are available (high conductivity). File:Asymmetric Pore.jpg, Asymmetric pore transmits positive ions preferentially from right to left. File:Thermo Responsive Channel.gif, Thermo-responsive channel. The hydrogel-lined channel opens above and closes below the critical temperature of the hydrogel. File:Biosensor Channel.gif, Biospecific Sensor. The electrical resistance of a channel clad with an immuno reactant depends on the concentration of a specific molecule. File:Field Emitter Array.gif, Field emitter array File:Multilayer Magnetic Sensor.gif, Multilayer Magnetosensor.
''Low magnetic field'': antiparallel orientation and high resistance.
''High magnetic field'': parallel orientation and low resistance. File:Spin Analyzer.gif, Spin Analyzer
The energy loss of spin-polarized electrons depends on the magnetic orientation of the analyzer. Left: polarizer (blue: spin-up). Right: analyzer (blue: spin-up; red: spin-down). File:Tilted Track Texture.gif, Tilted Track Texture with asymmetric transport properties.


Notes


External links

*{{Commons category-inline Microtechnology Nanotechnology Radiation effects