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[[File:Etched Tracks.gif|right|thumb|300px|Etched ion tracks in polyethylene terephthalate]]
'''Ion tracks''' are damage-trails created by [[swift heavy ion|swift heavy]] [[ion]]s penetrating through solids, which may be sufficiently-contiguous for chemical etching in a variety of crystalline, glassy, and/or polymeric solids.<!-- Commenting out for now... this could potentially be useful for covering historical aspects
<ref>
{{cite book
|author=H. Baumhauer
|year=1894
|title=Die Resultate der Aetzmethode in der krystallographischen Forschung, an einer Reihe von krystallisirten Körpern dargestellt
|publisher=[[Wilhelm Engelmann]]
|url=http://www.archive.org/stream/dieresultateder00baumgoog
}} {{De icon}}</ref>
--><ref>
{{cite journal
|author=D.A. Young
|year=1958
|title=Etching of radiation damage in lithium fluoride
|journal=[[Nature (journal)|Nature]]
|volume=182 |issue=4632 |pages=375–377
|bibcode = 1958Natur.182..375Y
|doi=10.1038/182375a0
|pmid=13577844
|s2cid=4282512
}}</ref><ref name="Fleischer1975" /> They are associated with cylindrical damage-regions several [[nanometer]]s in diameter<ref name="Seitz1956">
{{cite journal
|author1=F. Seitz |author2=J.S. Koehler |year=1956
|editor1=F. Seitz |editor2=D. Turnbull |title=Solid State Physics
|url=https://archive.org/details/solidstatephysic03unkn |url-access=limited |page=[https://archive.org/details/solidstatephysic03unkn/page/n309 307]
|publisher=[[Academic Press]]
|lccn=55012299
}}</ref><ref>
{{cite journal
|author1=M. Toulemonde |author2=C. Dufour |author3=A. Meftah |author4=E. Paumier |year=2000
|title=Transient thermal processes in heavy ion irradiation of crystalline inorganic insulators
|journal=[[Nuclear Instruments and Methods B]]
|volume=166–167 |pages=903–912
|bibcode = 2000NIMPB.166..903T
|doi=10.1016/S0168-583X(99)00799-5
}}</ref> 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]].<ref>
{{Cite journal
|author1=G. Remmert |author2=Y. Eyal |author3=B.E. Fischer |author4=R. Spohr |year = 1995
|title = Gas permeability and cross section of latent ion tracks in polymers
|journal = [[Nuclear Instruments and Methods|Nuclear Instruments and Methods B]]
|volume = 105
|issue =1–4
|pages = 197–199
|doi = 10.1016/0168-583X(95)00576-5
|bibcode = 1995NIMPB.105..197R }}</ref>
==Ion track technology==
Ion track technology deals with the production and application of ion tracks in [[microtechnology]] and [[nanotechnology]].<ref>{{cite book
|author=R. Spohr
|year=1990
|title=Ion tracks and microtechnology.
|url=http://ion-tracks.de/iontracktechnology/index.html
|publisher=[[Vieweg+Teubner Verlag|Vieweg Verlag]]
|isbn=978-3-528-06330-6
|access-date=2011-09-18
|archive-date=2011-09-16
|archive-url=https://web.archive.org/web/20110916012603/http://ion-tracks.de/iontracktechnology/index.html
|url-status=dead
}}</ref> Ion tracks can be selectively etched in many insulating solids, leading to cones or cylinders, down to 8 nanometers in diameter.<ref name="Williams1984">
{{cite journal
|author1=W. D. Williams |author2=N. Giordano |year=1984
|title=Fabrication of 80 Å metal wires
|journal=[[Review of Scientific Instruments]]
|volume=55 |issue=3 |pages=410–412
|bibcode=1984RScI...55..410W
|doi=10.1063/1.1137752
}}</ref> Etched track cylinders can be used as [[filtration|filter]]s,<ref name="Fleischer1963">
{{cite journal
|author1=R.L. Fleischer |author2=P.B. Price |author3=R.M. Walker |year=1963
|title=Method of forming fine holes of near atomic dimensions
|journal=[[Review of Scientific Instruments]]
|volume=34 |issue=5 |pages=510–512
|bibcode=1963RScI...34..510F
|doi=10.1063/1.1718419
}}</ref><ref>
{{cite journal
|author=P. Apel
|year=2003
|title=Swift ion effects in polymers: industrial applications |journal=[[Nuclear Instruments and Methods in Physics Research| Nuclear Instruments and Methods in Physics Research Section B]]
|volume=208|pages=11–20
|bibcode = 2003NIMPB.208...11A
|doi=10.1016/S0168-583X(03)00634-7
}}</ref> [[Coulter counter]] microchannels,<ref name="DeBlois1970">
{{cite journal
|author1=R.W. DeBlois |author2=C.P. Bean |year=1970
|title=Counting and sizing of submicron particles by the resistive pulse technique
|journal=[[Review of Scientific Instruments]]
|volume=41 |issue=7 |pages=909–916
|bibcode = 1970RScI...41..909D
|doi=10.1063/1.1684724
}}</ref> be modified with [[monolayer]]s,<ref name="Petzny">
{{cite journal
|author1=W.J. Petzny |author2=J.A. Quinn |year=1969
|title=Calibrated membranes with coated pore walls
|journal=[[Science (journal)|Science]]
|volume=166 |issue=3906 |pages=751–753
|bibcode = 1969Sci...166..751P
|doi=10.1126/science.166.3906.751
|pmid=5823313 |s2cid=1807195 }}</ref> or be filled by [[electroplating]].<ref name="Possin">
{{cite journal
|author=G.E. Possin
|year=1970
|title=A method for forming very small diameter wires
|journal=[[Review of Scientific Instruments]]
|volume=41 |issue=5 |pages=772–774
|bibcode = 1970RScI...41..772P
|doi=10.1063/1.1684640
}}</ref><ref>
{{cite web
|author=J. Vetter
|title=Free standing metal whiskers
|url=http://ion-tracks.de/
|publisher=[[Gesellschaft für Schwerionenforschung|GSI Darmstadt]]
|access-date=2010-04-27
}}</ref>
Ion track technology has been developed to fill certain niche areas where conventional [[nanolithography]] fails, including:
*Direct shaping of [[radiation resistance|radiation-resistant]] [[mineral]]s, [[glass]]es and [[polymer]]s<ref name=Fleischer1975>
{{cite book
|author1=R.L. Fleischer
|author2=P.B. Price
|author3=R.M. Walker
|year=1975
|title=Nuclear tracks in solids
|journal=Scientific American
|volume=220
|issue=6
|pages=30–9
|url=https://archive.org/details/bub_gb_yfTBvben3GoC
|publisher=[[University of California Press]]
|doi=10.1038/scientificamerican0669-30
|pmid=5769561
|isbn=978-0-520-02665-0
|url-access=registration
}}</ref>
*Generation of elongated structures with a [[image resolution|resolution]] limit down to 8 nanometers<ref name="Williams1984" />
*Direct generation of holes in thin films without any development process<ref>{{Cite journal
|author1=Y. Eyal |author2=K. Gassan |year = 1999
|title = Observation of latent heavy-ion tracks in polyimide by means of transmission electron microscopy
|journal = [[Nuclear Instruments and Methods in Physics Research|Nuclear Instruments and Methods in Physics Research B]]
|volume = 156
|issue = 1–4
|pages = 183–190
|doi = 10.1016/S0168-583X(99)00269-4
|bibcode = 1999NIMPB.156..183E }}
</ref>
*Defining structural depth by [[Stopping and Range of Ions in Matter|ion range]] rather than by target thickness<ref>{{cite book
|author=J.F. Ziegler
|year=1980
|title=Handbook of Stopping Cross-Sections for Energetic Ions in All Elements
|publisher=[[Pergamon Press]]
|isbn=978-0080216072
}}
</ref><ref>{{cite web|url=http://www.srim.org/ |title=Stopping and Range Calculation |publisher=Srim.org |access-date=2013-01-21}}</ref>
*Generating structures with [[photolithography|aspect ratio]] (depth divided by width) up to 10<sup>4</sup>.<ref name="Fleischer1975" />
*Shaping rigid and flexible materials at a defined cutting angle<ref>{{Cite journal
|author1=M. Lindeberg |author2=K. Hjort |year = 2004
|title = A comprehensive study of ion track enabled high aspect ratio microstructures in flexible circuit boards
|journal = Microsystem Technologies
|volume = 10
|issue = 8–9
|pages = 608–621
|doi = 10.1007/s00542-003-0339-2
|s2cid=109327888 }}
</ref>
*Exploring the realm of aligned textures with defined inclination angles<ref name="Spohr2010A">{{Cite journal
|author1=R. Spohr |author2=G. Sharma |author3=P. Forsberg |author4=M. Karlsson |author5=A. Hallén |author6=L. Westerberg |year = 2010
|title = Stroke Asymmetry of Tilted Superhydrophobic Ion Track Textures
|journal = [[Langmuir (journal)|Langmuir]]
|volume = 26
|issue = 9
|pages = 6790–6796
|doi = 10.1021/la904137t
|pmid=20085343 }}
</ref>
*Generation of random patterns consisting of partially overlapping single tracks<ref>{{Cite journal
|author1=C. Riedel |author2=R. Spohr |year = 1980
|title = Transmission Properties of Nuclear Track Filters
|journal = [[Journal of Membrane Science]]
|volume = 7
|issue = 2
|pages = 225–234
|doi = 10.1016/S0376-7388(00)80083-6
}}
</ref>
*Generation of large numbers of individual single track structures<ref name="Spohr2010B">{{Cite journal
|author1=R. Spohr |author2=C. Zet |author3=B.E. Fischer |author4=H. Kiesewetter |author5=P. Apel |author6=I. Gunko |author7=L. Westerberg |year = 2010
|title = Controlled fabrication of ion track nanowires and channels
|journal = [[Nuclear Instruments and Methods in Physics Research|Nuclear Instruments and Methods in Physics Research B]]
|volume = 268
|issue = 6
|pages = 676–686
|doi = 10.1016/j.nimb.2009.12.017
|bibcode = 2010NIMPB.268..676S |hdl=10069/32233 |hdl-access =free}}
</ref>
*Generation of aimed patterns consisting of individual single tracks<ref name="Fischer2003">
{{cite journal
|author1=B.E. Fischer |author2=M. Heiss |author3=M. Cholewa |year=2003
|title=About the art to shoot with single ions
|journal=[[Nuclear Instruments and Methods in Physics Research B]]
|volume=210|pages=285–291
|doi=10.1016/S0168-583X(03)01038-3
|bibcode = 2003NIMPB.210..285F }}
</ref>
==Materials susceptible to ion track recording==
The class of ion track recording materials is characterized by the following properties:<ref name="Fleischer1975"/>
*'''High [[homogeneity and heterogeneity|homogeneity]]''': Local density variations of the pristine material must be small in comparison to the density deficit of the ion track core. Optically [[transparency and translucency|translucent materials]], such as [[polycarbonate]] and [[polyvinylidene fluoride]], have this property. Grained polymers such as [[polytetrafluoroethylene]] do not have this property.
*'''High [[electrical resistance and conductance|electrical resistance]]''': Non-conducting [[dielectric]] minerals, glasses, and polymers have this property, while highly conducting metals and alloys do not have this property. In metals, the [[thermal diffusivity]] is coupled with the [[electrical resistance and conductance|electrical conductivity]], 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 diffusion|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]], depending on the [[atomic diffusion|atomic mobility]]. Ion tracks can be annealed. Erasing is faster in [[glass]]es compared to [[ionic crystals]].
==Irradiation apparatus and methods==
Several types of [[swift heavy ion]] generators and irradiation schemes are currently used:<br />
{|
|- valign="top"
| '''[[Radionuclide|Alpha and fission sources]]'''<ref>{{cite web|url=http://atom.kaeri.re.kr/ |title=Table of Nuclides |publisher=Atom.kaeri.re.kr |access-date=2013-01-21}}</ref><ref>{{cite web |url=http://www.nndc.bnl.gov/chart/reCenter.jsp?z=6&n=6 |title=Interactive Chart of Nuclides |publisher=Nndc.bnl.gov |access-date=2013-01-21 |archive-date=2016-01-09 |archive-url=https://web.archive.org/web/20160109122547/http://www.nndc.bnl.gov/chart/reCenter.jsp?z=6&n=6 |url-status=dead }}</ref> provide low intensity beams with broad angular-, mass-, and energy distribution. The [[ion range|range]] of the emitted fission fragments is limited to about 15 [[micrometre|micrometer]]s in polymers. Weak [[californium]]-252 or [[americium]]-241 sources<ref>10<sup>2</sup> fission events/s</ref> are used for scientific and technological explorations. They are compact, inexpensive, and can be handled safely. ||[[Image:Irradiation using radionuclide.jpg|thumb|Irradiation using radionuclide]]
|- valign="top"
| '''[[Nuclear reactors]]''' provide [[fission fragment]]s with broad angular-, mass-, and energy distributions. Similar to [[radionuclide|alpha and fission sources]], the penetration [[ion range|range]] of the emitted [[fission fragment]]s is limited to about 15 micrometers in polymers. [[Nuclear reactors]] are used for [[filtration|filter]] production.
|| [[Image:Irradiation at nuclear reactor.jpg|thumb|Irradiation at nuclear reactor]]
|- valign="top"
| '''Heavy ion [[particle accelerator]]s''' provide parallel-beam [[irradiation]]s at high [[luminosity]] with ions of defined mass, energy, and tilt angle.<ref>{{Cite web |url=http://tvdg10.phy.bnl.gov/species.html |title=Brookhaven Tandem Van de Graaf |access-date=2011-12-29 |archive-date=2013-02-13 |archive-url=https://web.archive.org/web/20130213222601/http://tvdg10.phy.bnl.gov/species.html |url-status=dead }}</ref><ref>[http://www.gsi.de/forschung/mf/Service_e.htm GSI Irradiation Facilities] {{webarchive |url=https://web.archive.org/web/20080313081908/http://www.gsi.de/forschung/mf/Service_e.htm |date=March 13, 2008 }}</ref><ref>{{cite web|url=http://www.highvolteng.com/Ion_Accelerator_Systems.html |title=High Volage Accelerator Systems |publisher=Highvolteng.com |access-date=2013-01-21}}</ref> Intensities are available in wide ranges, even up to billions of ions per second. Depending on the available energy, track lengths between a few and several hundred micrometers can be produced. [[Particle accelerator|Accelerator]]s are used in [[microtechnology|micro-]] and [[nanotechnology]]. [[Radioactive contamination]] is absent at ion energies below the [[Coulomb barrier]].<ref>{{cite web|url=http://physicsconsult.de/06_Tools/Irradiation_Tools/Coulomb_Barrier/_Coulomb_Barrier.php |title=Estimate Coulomb barrier |publisher=Physicsconsult.de |access-date=2013-01-21}}</ref> ||
[[Image:Irradiation at ion accelerator.jpg|thumb|Irradiation at ion accelerator]]
|- valign="top"
| '''Single ion [[irradiation]]s''' are used to fabricate individual micro- and nanostructures such as cones, channels, pins and wires.<ref name="Spohr2010B"/> The technique requires a weak ion beam which can be switched off after one ion has penetrated the target foil.||
[[Image:Irradiation by single ion.jpg|thumb|Single ion system]]
|- valign="top"
| '''Ion [[microbeam]]s''' offer the highest level of control of the irradiation process. These restrict the output of a heavy ion [[particle accelerator|accelerator]] to a small filament which can be scanned over the sample surface. Scribing with individual swift heavy ions is possible with an aiming precision of about one micrometer.<ref name="Fischer2003" />||
[[Image:Irradiation by ion microbeam.jpg|thumb|Ion microbeam system]]
|}
==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 collision approximation|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]]). Due to the large [[proton-to-electron mass ratio]], the energy of the ''projectile'' decreases gradually and the ''projectile path'' is straight.<ref>For iron, the mass ratio M<sub>Fe</sub>/m<sub>e</sub>~ 10<sup>5</sup></ref> 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'''<ref>
{{cite journal
|author1=R. L. Fleischer |author2=P.B. Price |author3=R.M. Walker |year=1965
|title=Ion Explosion Spike Mechanism for Formation of Charged-Particle Tracks In Solids
|journal=[[Journal of Applied Physics]]
|volume=36 |pages=3645–3652
|bibcode = 1965JAP....36.3645F
|doi=10.1063/1.1703059
|issue=11
}})</ref> the ''primary'' ionization induces an atomic [[collision cascade]],<ref>
{{cite journal
|author=K. Nordlund, M. Ghaly, R. S. Averback, M. Caturla, T. Diaz de la Rubia, J. Tarus
|year=1998
|title=Defect production in collision cascades in elemental semiconductors and FCC metals
|journal=[[Physical Review|Physical Review B]]
|volume=57|page=7556
|bibcode =1998PhRvB..57.7556N
|doi=10.1103/PhysRevB.57.7556
|issue=13
}})</ref> resulting in a disordered zone around the ion trajectory.
*According to the '''electron [[collision cascade]] model''' the ''secondary'' electrons induce a radiation effect in the material, similar to a spatially-confined electron irradiation.<ref>
{{cite journal
|author=R. Katz
|year=1978
|title=Track Structure Theory In Radiobiology and In Radiation Detection
| journal=Nuclear Track Detection
|volume=2
| pages=1–28
|doi=10.1016/0145-224X(78)90002-9
|issue=1}}</ref> The electron [[collision cascade]] 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]] leaves behind an amorphous state with decreased density. Its disorder corresponds to the ion track.<ref name="Seitz1956" /><ref>
{{cite journal
|author1=M. Toulemonde |author2=C. Dufour |author3=A. Meftah |author4=E. Paumier |year=2000
|title=Transient thermal processes In heavy ion irradiation of crystalline inorganic insulators
|journal=[[Nuclear Instruments and Methods B]]
|volume=166-167
| pages=903–912
|bibcode = 2000NIMPB.166..903T
|doi=10.1016/S0168-583X(99)00799-5
}}</ref>
The thermal spike model suggests the [[radiation sensitivity]] of different materials depends on their thermal conductivity and their melting temperature.
<gallery>
File:Thermal_Spike.gif| '''Thermal spike model'''<br />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]] [[mica]]. Depending on the stopping power of the projectile ion the track width is between 4 and 10 nanometer.
Image:10kevau au.gif|[[Molecular dynamics]] simulation of [[collision cascade]] 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]] FeBSiC included for comparison.
</gallery>
==Etching methods==
===Selective ion etching===
''Selective ion track etching''<ref name="Fleischer1975" /> is closely related to the selective etching of [[grain boundaries]] and crystal [[dislocation]]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.<ref>
{{cite journal
|author=P.Y.Apel, I.V. Blonskaya, A.Y. Didyk, S.N. Dmitriev, O.L. Orelovitch, D. Root, L.I. Samoilova, V.A. Vutsadakis
|year=2001
|title=Surfactant-enhanced control of track-etch pore morphology
|journal=[[Nuclear Instruments and Methods in Physics Research|Nuclear Instruments and Methods in Physics Research Section B]]
|volume=179|issue= 1|pages=55–62
|bibcode= 2001NIMPB.179...55A
|doi=10.1016/S0168-583X(00)00691-1
}}</ref> It is based on [[self-organization|self-organized]] [[monolayer]]s.<ref name="Petzny"/> The [[monolayer]]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]].<ref>{{cite journal
|author1=L.C.T. Man |author2=P. Apel |author3=T. Cheung |author4=L. Westerberg |author5=K.N. Yu |author6=C. Zet |author7=R. Spohr |year=2007
|title=Influence of a surfactant on single ion track etching. Preparing and manipulating individual cylindrical micro wires
|journal=[[Nuclear Instruments and Methods in Physics Research|Nuclear Instruments and Methods in Physics Research Section B]]
|volume=265|issue= 2|pages=621–625 |bibcode= 2007NIMPB.265..621M
|doi=10.1016/j.nimb.2007.09.029
}}</ref>
===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.
<gallery>
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).
</gallery>
{| class="wikitable"
|+ Track etching of common polymers<ref>{{cite web |url=http://ion-tracks.de/introduction1/index.html |title=P. Apel, R. Spohr: Introduction to ion track etching in polymers |publisher=Ion-tracks.de |access-date=2013-01-21 |archive-date=2012-04-26 |archive-url=https://web.archive.org/web/20120426081504/http://ion-tracks.de/introduction1/index.html |url-status=dead }}</ref>
! Material!! pH!! Wet etchant!! Sensitizer<sup>1)</sup> !!Desensitizer<sup>2)</sup> !! T/°C<sup>3)</sup>!!Speed<sup>4)</sup>!!Selectivity<sup>5)</sup>
|-
| [[Polycarbonate|PC]]|| basic|| NaOH|| UV|| Alcohols|| 50-80||Fast|| 100-10000
|-
| [[Polyethylene terephthalate|PET]]|| basic|| NaOH|| UV, DMF|| Alcohols|| 50-90||Fast||10-1000
|-
| || basic|| K<sub>2</sub>CO<sub>3</sub>|| || || 80|| Slow|| 1000
|-
| [[Polyimide|PI]]|| basic|| NaOCl || || NaOH|| 50-80||Fast|| 100-1000
|-
| [[CR-39]]|| basic|| NaOH|| UV|| || 50-80|| Fast|| 10-1000
|-
| [[Polyvinylidene fluoride|PVDF]]<sup>6)</sup>|| basic|| KMnO<sub>4</sub> + NaOH|| |||| 80||Medium||10-100
|-
| [[Poly(methyl methacrylate)|PMMA]]<sup>6)</sup>|| acidic|| KMnO<sub>4</sub> + H<sub>2</sub>SO<sub>4</sub>|| ||||50-80|| Medium|| 10
|-
| [[Polypropylene|PP]]<sup>6)</sup>|| acidic|| CrO<sub>3</sub> + H<sub>2</sub>SO<sub>4</sub>|| || ||80||Fast||10-100
|}
<sup>1)</sup> Sensitizers increase the track etch ratio by breaking bonds or by increasing the free volume.<br />
<sup>2)</sup> Desensitizers decrease the track etch ratio. Alternatively ion tracks can be thermally annealed.<br />
<sup>3)</sup> Typical etch bath temperature range. Etch rates increase strongly with concentration and temperature. <br />
<sup>4)</sup> Axial etching depends on track etch speed v<sub>t</sub>, radial etching depends on general etch speed v<sub>g</sub>.<br />
<sup>5)</sup> Selectivity (aspect ratio, track etch ratio) = track etch speed / general etch speed = v<sub>t</sub> / v<sub>g</sub>.<br />
<sup>6)</sup> This method requires to remove remaining metal oxide deposits by aqueous HCl solutions.
==Replication==
Etched ion tracks can be [[replica (disambiguation)|replicated]] by [[polymer]]s<ref>
{{cite journal
|author1=P.B. Price |author2=G.M. Comstock |author3=R.L. Fleischer |author4=W.R. Giard |author5=H.R. Hart |author6=G.E. Nichols |year=1971
|title=Cosmic-Ray Tracks in Plastics: The Apollo Helmet Dosimetry Experiment
|journal=[[Science (journal)|Science]]
|volume=172 |pages=154–157
|bibcode = 1971Sci...172..154C
|doi=10.1126/science.172.3979.154
|pmid=17735223
|issue=3979
|s2cid=13108585 }}</ref> or [[metal]]s.<ref name="Possin"/>
<ref>See: [[plating]] and [[electroplating]]</ref> [[Replica]] and [[die (manufacturing)|template]] can be used as [[composite material|composite]]. A replica can be separated from its [[die (manufacturing)|template]] mechanically or chemically. '''[[Polymer]] [[replica (disambiguation)|replica]]s''' are obtained by filling the etched track with a liquid [[precursor (chemistry)|precursor]] of the [[polymer]] and [[curing (chemistry)|curing]] it. Curing can be activated by a [[catalysis|catalyst]], by ultraviolet [[radiation]], or by [[heat]]. '''[[Metal]] [[replica (disambiguation)|replica]]s''' can be obtained either by [[electroplating|electroless deposition]] or by [[plating|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.<ref>
{{cite journal
|author1=M. Rauber |author2=I. Alber |author3=S. Müller |author4=R. Neumann |author5=O. Picht |author6=C. Roth |author7=A. Schöckel |author8=M.E. Toimil-Molares |author9=W. Ensinger |year=2011
|title=Highly-Ordered Supportless Three-Dimensional Nanowire Networks with Tunable Complexity and Interwire Connectivity for Device Integration
|journal=[[Nano Letters]]
|volume=11|issue=6|pages=2304–2310
|bibcode = 2011NanoL..11.2304R
|doi=10.1021/nl2005516
|pmid=21608990
}}</ref>
'''Segmented nanowires''' are fabricated by alternating the polarity during electro-deposition.<ref>
{{cite journal
|author1=M. Rauber |author2=J. Brötz |author3=J. Duan |author4=J. Liu |author5=S. Müller |author6=R. Neumann |author7=O. Picht |author8=M.E. Toimil-Molares |author9=W. Ensinger |year=2010
|title=Segmented All-Platinum Nanowires with Controlled Morphology through Manipulation of the Local Electrolyte Distribution in Fluidic Nanochannels during Electrodeposition
|journal=[[Journal of Physical Chemistry C]]
|volume=114|pages=22502–22507
|doi=10.1021/jp108889c
|issue=51
}}</ref> The segment length is adjusted by the pulse duration. In this way electrical, thermal, and optical properties can be tuned.
<gallery>
File:Copper_wires_Johann_Vetter.jpg|Free standing metal replica of etched ion tracks in [[polycarbonate|PC]]
File:Markus Rauber Exp_179 - 36.jpg|Interpenetrating wire network
File:Markus_Rauber_Exp_169p3_-_20.jpg|Bundle of segmented platinum nanowires
</gallery>
==Applications==
'''[[Microtechnology]]''': The common mechanical [[tool]]s of the macroworld are being supplemented and complemented, and in some applications replaced by, [[particle beam]]s. Here, beams of [[photon]]s and [[electrons]] modify the [[solubility]] of [[radiation sensitivity|radiation-sensitive]] [[polymer]]s, so-called "[[resist]]s", while masking protects a selected area from exposure to [[irradiation|radiation]], [[etching|chemical attack]], and [[sputtering|erosion by atomic impact]]. Typical products produced in this way are [[integrated circuit]]s and [[microelectromechanical systems|microsystems]]. At present, the field of [[microtechnology]] is expanding toward [[nanotechnology]]. A recent branch of [[microfabrication]] is based on manipulation of individual [[ion]]s.
'''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]]
'''Filters''': Homoporous [[filtration|filters]] were among the first applications<ref name="Fleischer1963" /> of ion track technology, and are now fabricated by several companies.<ref>{{cite web|url=http://physicsconsult.de/07_Links/_Links.php |title=Ion track companies |publisher=Physicsconsult.de |date=2011-07-04 |access-date=2013-01-21}}</ref> Mica membranes with ion track pores were used by Beck and Schultz to determine the mechanism of hindered diffusion in nanopores.<ref>{{Cite journal|last1=Beck|first1=R. E.|last2=Schultz|first2=J. S.|date=1970-12-18|title=Hindered Diffusion in Microporous Membranes with Known Pore Geometry|journal=Science|language=en|volume=170|issue=3964|pages=1302–1305|doi=10.1126/science.170.3964.1302|pmid=17829429|bibcode=1970Sci...170.1302B|s2cid=43124555|issn=0036-8075}}</ref><ref>{{Cite journal|last1=Beck|first1=Robert E.|last2=Schultz|first2=Jerome S.|date=January 1972|title=Hindrance of solute diffusion within membranes as measured with microporous membranes of known pore geometry|journal=Biochimica et Biophysica Acta (BBA) - Biomembranes|language=en|volume=255|issue=1|pages=273–303|doi=10.1016/0005-2736(72)90028-4|pmid=4334681|hdl=2027.42/34175|hdl-access=free}}</ref>
'''Classifying micro- and nanoparticles''': The resistance of a channel filled by an [[electrolyte]] depends on the volume of the particle passing through it.<ref name="DeBlois1970" /> 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]] have a [[surface conductivity]], in addition to the regular [[conductivity (electrolytic)|volume conductivity]], of the electrolyte. Ions attached to a charged surface attract a cloud of mobile [[counterion]]s. Fixed and mobile ions form a [[double layer (interfacial)|double layer]]. For small channels, [[surface conductivity]] is responsible for most of the charge transport. For small channels, surface conductivity exceeds [[conductivity (electrolytic)|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.<ref>
{{cite journal
|author1=A. Wolf |author2=N. Reber |author3=P. Yu. Apel |author4=B.E. Fischer |author5=R. Spohr |year=1995
|title=Electrolyte transport in charged single ion track capillaries
|journal=[[Nuclear Instruments and Methods in Physics Research|Nuclear Instruments and Methods in Physics Research B]]
|volume=105 |issue=1–4 |pages=291–293
|bibcode= 1995NIMPB.105..291W
|doi=10.1016/0168-583X(95)00577-3
}}</ref>
'''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.<ref>
{{cite journal
|author=P.Y. Apel, Y.E. Korchev, Z. Siwy, Z.; R. Spohr, M. Yoshida
|year=2001
|title=Diode-like single-ion track membrane prepared by electro-stopping
|journal=[[Nuclear Instruments and Methods in Physics Research|Nuclear Instruments and Methods in Physics Research B]]
|volume=184|issue=3|pages=337–346
|bibcode= 2001NIMPB.184..337A
|doi=10.1016/S0168-583X(01)00722-4
}}</ref><ref>
{{cite journal
|author1=P. Ramirez |author2=P.Yu. Apel |author3=J. Cervera |author4=S. Mafe |year=2008
|title=Pore structure and function of synthetic nanopores with fixed charges: tip shape and rectification properties
|journal=[[Nanotechnology (journal)|Nanotechnology]]
|volume=19|page=315707
|bibcode= 2008Nanot..19E5707R
|doi=10.1088/0957-4484/19/31/315707
|issue=31
|pmid=21828799 |s2cid=43193256 }}</ref>
'''[[Temperature-responsive polymer|Thermo-responsive]] channel''': Obtained by lining a channel with a [[Poly(N-isopropylacrylamide)|thermo-responsive gel]].<ref>
{{cite journal
|author1=M. Tamada |author2=M. Yoshida |author3=M. Asano |author4=H. Omichi |author5=R. Katakai |author6=R. Spohr |author7=J. Vetter |year=1992
|title=Thermo-response of ion track pores in copolymer films of methacryloyl-L-alaninemethylester and diethyleneglycol-bis-allylcarbonate (CR-39)
|journal=[[Polymer]]
|volume=33|pages=3169–3172
|doi=10.1016/0032-3861(92)90230-T
|issue=15
}}</ref>
'''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.<ref>
{{cite journal
|author1=L.T. Sexton |author2=L.P. Horne |author3=C.R. Martin |year=2007
|title=Developing synthetic conical nanopores for biosensing applications
|journal=[[Molecular BioSystems]]
|volume=3|pages=667–685
|doi=10.1039/b708725j
|issue=10
|pmid=17882330 }}</ref>
'''Anisotropic conduction''': A platform covered with many free standing wires acts as large area field emitter.<ref>
{{cite journal
|author1=F. Maurer |author2=A. Dangwal |author3=D. Lysenkov |author4=G. Müller |author5=M.E. Toimil-Molares |author6=C. Trautmann |author7=J. Brötz |author8=H. Fuess |year=2006
|title=Field emission of copper nanowires grown in polymer ion-track membranes
|journal=[[Nuclear Instruments and Methods in Physics Research|Nuclear Instruments and Methods in Physics Research B]]
|volume=245|issue=1 |pages=337–341
|bibcode= 2006NIMPB.245..337M
|doi=10.1016/j.nimb.2005.11.124
}}</ref>
'''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 (the "GMR effect").<ref>
{{cite journal
|author1=L. Piraux |author2=J.M. George |author3=J.F. Despres |author4=C. Leroy |author5=E. Ferain |author6=R. Legras |author7=K. Ounadjela |author8=A. Fert |year=1994
|title=Giant magnetoresistance in magnetic multilayered nanowires
|journal=[[Applied Physics Letters]]
|volume=65|issue=19|pages=2484–2486
|bibcode= 1994ApPhL..65.2484P
|doi=10.1063/1.112672
}}</ref>
'''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.<ref>
{{cite journal
|author1=B. Doudin |author2=J.P. Ansermet |year=1997
|title=Nanostructuring materials for spin electronics
|journal=[[Europhysics News]]
|volume=28|issue=1|pages=14–17
|bibcode= 1997ENews..28...14D
|doi= 10.1007/s00770-997-0014-8
|s2cid=123078833 |doi-access=free}}</ref>
'''Textures''': Tilted textures with a hydrophobic coating are at the same time superhydrophobic and anisotropic,<ref name="Spohr2010A" /> and show a preferred direction of transport. The effect has been demonstrated to convert vibration into translation.<ref>{{cite web|url=http://physicsconsult.de/02_home/My_Efforts/Images/Converting_Vibration_into_Motion_2.mp4 |title=Converting vibration into translation |access-date=2013-01-21}}</ref>
<gallery>
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'''. <br />''Low magnetic field'': antiparallel orientation and high resistance. <br />''High magnetic field'': parallel orientation and low resistance.
File:Spin Analyzer.gif|'''Spin Analyzer'''<br />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.
</gallery>
==Notes==
{{Reflist|30em}}
==External links==
*{{Commons category-inline}}
[[Category:Microtechnology]]
[[Category:Nanotechnology]]
[[Category:Radiation effects]]