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{{Short description|Wire with a diameter in the nanometres}}
{{Lead too short|date=February 2022}}
[[file:SnSe@SWCNT.jpg|thumb|upright=1.2|Crystalline 2×2-atom [[tin selenide]] nanowire grown inside a single-wall [[carbon nanotube]] (tube diameter ≈1 nm).<ref name="wire">{{cite journal |last1=Carter |first1=Robin |last2=Suyetin |first2=Mikhail |last3=Lister |first3=Samantha |last4=Dyson |first4=M. Adam |last5=Trewhitt |first5=Harrison |last6=Goel |first6=Sanam |last7=Liu |first7=Zheng |last8=Suenaga |first8=Kazu |last9=Giusca |first9=Cristina |last10=Kashtiban |first10=Reza J. |last11=Hutchison |first11=John L. |last12=Dore |first12=John C. |last13=Bell |first13=Gavin R. |last14=Bichoutskaia |first14=Elena |author-link14=Elena Besley |last15=Sloan |first15=Jeremy |year=2014 |title=Band gap expansion, shear inversion phase change behaviour and low-voltage induced crystal oscillation in low-dimensional tin selenide crystals |journal=Dalton Trans |volume=43 |issue=20 |pages=7391–9 |doi=10.1039/C4DT00185K |pmid=24637546 |doi-access=free}}</ref>]]
{{Nanoelec}}

A '''nanowire''' is a [[nanostructure]] in the form of a [[wire]] with the diameter of the order of a nanometre (10<sup>−9</sup>&nbsp;m). More generally, nanowires can be defined as structures that have a thickness or diameter constrained to tens of [[nanometer]]s or less and an unconstrained length. At these scales, quantum mechanical effects are important—which coined the term "[[quantum wire]]s".

Many different types of nanowires exist, including superconducting (e.g. [[Yttrium barium copper oxide|YBCO]]<ref>{{cite journal|doi=10.1126/science.1251594 |pmid=24812400 |title=In Situ TEM Observation of a Microcrucible Mechanism of Nanowire Growth |journal=Science |volume=344 |issue=6184 |pages=623–6 |year=2014 |last1=Boston |first1=R. |last2=Schnepp |first2=Z. |last3=Nemoto |first3=Y. |last4=Sakka |first4=Y. |last5=Hall |first5=S. R. |s2cid=206555658 |bibcode=2014Sci...344..623B |hdl=1983/8f23c618-23f8-46e1-a1d9-960a0b491b1f |url=https://research-information.bris.ac.uk/en/publications/in-situ-tem-observation-of-a-microcrucible-mechanism-of-nanowire-growth(8f23c618-23f8-46e1-a1d9-960a0b491b1f).html |hdl-access=free }}</ref>), metallic (e.g. [[nickel|Ni]], [[platinum|Pt]], [[gold|Au]], [[silver|Ag]]), semiconducting (e.g. [[Silicon nanowire|silicon nanowires (SiNWs)]], [[indium phosphide|InP]], [[gallium nitride|GaN]]) and insulating (e.g. [[Silicon dioxide|SiO<sub>2</sub>]], [[Titanium dioxide|TiO<sub>2</sub>]]).

[[Molecular nanowires]] are composed of repeating molecular units either organic (e.g. [[DNA]]) or inorganic (e.g. Mo<sub>6</sub>S<sub>9−''x''</sub>I<sub>''x''</sub>).

== Characteristics ==
[[file:HgTe@SWCNT.png|thumb|A noise-filtered HRTEM image of a [[HgTe]] extreme nanowire embedded down the central pore of a SWCNT. The image is also accompanied by a simulation of the crystal structure<ref name=ExtremeNanowire>{{cite journal|doi=10.1021/nn5023632 |pmid=25163005 |title=Raman Spectroscopy of Optical Transitions and Vibrational Energies of ≈1 nm HgTe Extreme Nanowires within Single Walled Carbon Nanotubes |journal=ACS Nano |volume=8|issue=9 |pages=9044–52 |year=2014 |last1=Spencer |first1=Joseph |last2=Nesbitt |first2=John |last3=Trewhitt |first3=Harrison |last4=Kashtiban |first4=Reza |last5=Bell |first5=Gavin |last6=Ivanov |first6=Victor |last7=Faulques |first7=Eric |last8=Smith |first8=David|url=https://eprints.soton.ac.uk/401309/1/HgTe%2540SWNT_ACSNano_Final.pdf }}</ref>]]

Typical nanowires exhibit aspect ratios (length-to-width ratio) of 1000 or more. As such they are often referred to as one-dimensional (1-D) materials. Nanowires have many interesting properties that are not seen in bulk or 3-D (three-dimensional) materials. This is because [[electron]]s in nanowires are [[quantum]] confined laterally and thus occupy energy levels that are different from the traditional continuum of energy levels or bands found in bulk materials.

A consequence of this [[quantum confinement]] in nanowires is that they exhibit discrete values of the [[electrical conductance]]. Such discrete values arise from a quantum mechanical constraint on the number electronic transport channels at the nanometer scale, and they are often approximately equal to [[integer]] multiples of the [[quantum of conductance]]:
: <math>\frac{2e^2}{h} \simeq 77.41\; \mu S</math>

This conductance is twice the reciprocal of the resistance unit called the [[von Klitzing constant]], {{physconst|RK|symbol=yes|after=,}} defined as {{nowrap|1=''R''<sub>K</sub> = ''h''/''e''<sup>2</sup>}} and named for [[Klaus von Klitzing]], the discoverer of the [[quantum Hall effect|integer quantum Hall effect]].

Examples of nanowires include inorganic molecular nanowires (Mo<sub>6</sub>S<sub>9−''x''</sub>I<sub>''x''</sub>, Li<sub>2</sub>Mo<sub>6</sub>Se<sub>6</sub>), which can have a diameter of 0.9&nbsp;nm and be hundreds of micrometers long. Other important examples are based on semiconductors such as InP, Si, GaN, etc., dielectrics (e.g. SiO<sub>2</sub>,TiO<sub>2</sub>), or metals (e.g. Ni, Pt).

There are many applications where nanowires may become important in electronic, opto-electronic and nanoelectromechanical devices, as additives in advanced composites, for metallic interconnects in nanoscale quantum devices, as field-emitters and as leads for biomolecular nanosensors.

== Synthesis ==
[[file:Epitaxial Nanowire Heterostructures SEM image.jpg|thumb|upright|An [[Scanning electron microscope|SEM]] image of epitaxial nanowire heterostructures grown from catalytic gold nanoparticles]]

There are two basic approaches to synthesizing nanowires: [[Silicon Nanowire#Top Down Synthesis Methods|top-down]] and [[Silicon Nanowire#Bottom-up Synthesis Methods|bottom-up]]. A top-down approach reduces a large piece of material to small pieces, by various means such as [[lithography]],<ref>{{cite journal|author=Shkondin, E.|title=Large-scale high aspect ratio Al-doped ZnO nanopillars arrays as anisotropic metamaterials. |journal=Optical Materials Express |volume=7 |pages=1606–1627 |date=2017|last2= Takayama, O. |first2= Aryaee Panah, M. E. |last3= Liu, P. |first3= Larsen, P. V. |last4= Mar, M. D. |first4= Jensen, F. |last5= Lavrinenko, A. V. |issue=5 |doi=10.1364/OME.7.001606 |bibcode=2017OMExp...7.1606S |url=https://backend.orbit.dtu.dk/ws/files/131570679/ome_7_5_1606.pdf |doi-access=free }}</ref><ref>{{cite journal|author=Shkondin, E.|title=Fabrication of hollow coaxial Al2O3/ZnAl2O4 high aspect ratio freestanding nanotubes based on the Kirkendall effect. |journal=Journal of Vacuum Science & Technology A |volume=38 |pages=1606–1627 |date=2020|last2= Alimadadi, H. |first2= Takayama, O. |last3= Jensen, F. |first3= Lavrinenko, A. V. |issue=1 |doi=10.1116/1.5130176 |bibcode=2020JVSTA..38a3402S |s2cid=209898658 |url=https://backend.orbit.dtu.dk/ws/files/204375048/1.5130176.pdf }}</ref> [[Ball mill|milling]] or [[thermal oxidation]]. A bottom-up approach synthesizes the nanowire by combining constituent [[adatom]]s. Most synthesis techniques use a bottom-up approach. Initial synthesis via either method may often be followed by a [[Silicon Nanowire#Thermal Oxidation of Silicon Nanowires|nanowire thermal treatment step]], often involving a form of self-limiting oxidation, to fine tune the size and aspect ratio of the structures.<ref name="slo">{{cite journal| last1= Liu| first1=M.| last2= Peng |first2=J.| display-authors= etal |title=  Two-dimensional modeling of the self-limiting oxidation in silicon and tungsten nanowires | journal= Theoretical and Applied Mechanics Letters  | year= 2016 |  volume=6 | issue=5 | pages=195–199 | url= https://www.researchgate.net/publication/306273009 | doi= 10.1016/j.taml.2016.08.002 |arxiv=1911.08908  | doi-access= free }}</ref> After the bottom-up synthesis, nanowires can be integrated using pick-and-place techniques.<ref>{{Cite journal |last1=Ali |first1=Utku Emre |last2=Yang |first2=He |last3=Khayrudinov |first3=Vladislav |last4=Modi |first4=Gaurav |last5=Cheng |first5=Zengguang |last6=Agarwal |first6=Ritesh |last7=Lipsanen |first7=Harri |last8=Bhaskaran |first8=Harish |date=September 2022 |title=A Universal Pick-and-Place Assembly for Nanowires |url=https://onlinelibrary.wiley.com/doi/10.1002/smll.202201968 |journal=Small |language=en |volume=18 |issue=38 |pages=2201968 |doi=10.1002/smll.202201968 |pmid=35938750 |s2cid=251399932 |issn=1613-6810}}</ref>

Nanowire production uses several common laboratory techniques, including suspension, electrochemical deposition, vapor deposition, and [[vapor–liquid–solid method|VLS]] growth. [[Ion track technology (track replication)|Ion track technology]] enables growing homogeneous and segmented nanowires down to 8&nbsp;nm diameter. As nanowire oxidation rate is controlled by diameter, [[thermal oxidation]] steps are often applied to tune their morphology.

=== Suspension ===
A suspended nanowire is a wire produced in a high-vacuum chamber held at the longitudinal extremities. Suspended nanowires can be produced by:
* The chemical etching of a larger wire
* The bombardment of a larger wire, typically with highly energetic ions
* Indenting the tip of a [[scanning tunneling microscope|STM]] in the surface of a metal near its melting point, and then retracting it

=== VLS growth ===
A common technique for creating a nanowire is [[vapor–liquid–solid method]] (VLS), which was first reported by Wagner and Ellis in 1964 for silicon whiskers with diameters ranging from hundreds of nm to hundreds of μm.<ref>{{cite journal|last=Wagner |first=R. S.|author2=Ellis, W. C. |year=1964|title=Vapor-liquid-solid mechanism of single crystal growth|journal=Appl. Phys. Lett.|volume=4|issue=5|pages=89|doi=10.1063/1.1753975|bibcode=1964ApPhL...4...89W}}</ref> This process can produce high-quality crystalline nanowires of many semiconductor materials, for example,  VLS–grown single crystalline [[Silicon Nanowire|silicon nanowires (SiNWs)]] with smooth surfaces could have excellent properties, such as ultra-large elasticity.<ref>{{cite journal| last1=Zhang | first1= H. | display-authors=etal |title= Approaching the ideal elastic strain limit in silicon nanowires | journal=Science Advances | year=2016| volume=2 |issue=8| pages=e1501382 | doi=10.1126/sciadv.1501382| pmid= 27540586 | pmc= 4988777 | bibcode=2016SciA....2E1382Z}}</ref> This method uses a source material from either laser [[ablation|ablated]] particles or a feed gas such as [[silane]].

VLS synthesis requires a catalyst. For nanowires, the best catalysts are liquid metal (such as [[gold]]) [[nanocluster]]s, which can either be self-assembled from a thin film by [[dewetting]], or purchased in colloidal form and deposited on a substrate.

The source enters these nanoclusters and begins to saturate them. On reaching supersaturation, the source solidifies and grows outward from the nanocluster. Simply turning off the source can adjust the final length of the nanowire. Switching sources while still in the growth phase can create compound nanowires with super-lattices of alternating materials. For example, a method termed ENGRAVE (Encoded Nanowire GRowth and Appearance through VLS and Etching)<ref>{{cite journal |last1=Christesen |first1=Joseph D. |last2=Pinion |first2=Christopher W. |last3=Grumstrup |first3=Erik M. |last4=Papanikolas |first4=John M. |last5=Cahoon |first5=James F. |title=Synthetically Encoding 10 nm Morphology in Silicon Nanowires |journal=Nano Letters |date=2013-12-11 |volume=13 |issue=12 |pages=6281–6286 |doi=10.1021/nl403909r |pmid=24274858 |bibcode=2013NanoL..13.6281C |issn=1530-6984|doi-access=free }}</ref> developed by the Cahoon Lab at [[University of North Carolina at Chapel Hill|UNC-Chapel Hill]] allows for nanometer-scale morphological control via rapid ''in situ'' dopant modulation.

A single-step vapour phase reaction at elevated temperature synthesises inorganic nanowires such as Mo<sub>6</sub>S<sub>9−''x''</sub>I<sub>''x''</sub>.  From another point of view, such nanowires are cluster [[polymer]]s.

Similar to VLS synthesis, VSS (vapor-solid-solid) synthesis of nanowires (NWs) proceeds through thermolytic decomposition of a silicon precursor (typically phenylsilane). Unlike VLS, the catalytic seed remains in solid state when subjected to high temperature annealing of the substrate. This such type of synthesis is widely used to synthesise metal silicide/germanide nanowires through VSS alloying between a copper substrate and a silicon/germanium precursor.

=== Solution-phase synthesis ===
Solution-phase synthesis refers to techniques that grow nanowires in solution. They can produce nanowires of many types of materials. Solution-phase synthesis has the advantage that it can produce very large quantities, compared to other methods. In one technique, the [[polyol]] synthesis, ethylene glycol is both solvent and reducing agent. This technique is particularly versatile at producing nanowires of gold,<ref>{{cite journal|last1=Yin|first1=Xi|last2=Wu|first2=Jianbo|last3=Li|first3=Panpan|last4=Shi|first4=Miao|last5=Yang|first5=Hong|title=Self-Heating Approach to the Fast Production of Uniform Metal Nanostructures|journal=ChemNanoMat|date=January 2016|volume=2|issue=1|pages=37–41|doi=10.1002/cnma.201500123}}</ref> lead, platinum, and silver.

The supercritical fluid-liquid-solid growth method<ref>{{cite journal|pmid=10688792|year=2000|last1=Holmes|first1=J. D.|title=Control of thickness and orientation of solution-grown silicon nanowires|journal=Science|volume=287|issue=5457|pages=1471–3|last2=Johnston|first2=K. P.|last3=Doty|first3=R. C.|last4=Korgel|first4=B. A.|bibcode=2000Sci...287.1471H|doi=10.1126/science.287.5457.1471}}</ref><ref>{{cite journal|doi=10.1021/cm2007704|title=Rapid SFLS Synthesis of Si Nanowires Using Trisilane with in situ Alkyl-Amine Passivation|journal=Chemistry of Materials|volume=23|issue=11|pages=2697–2699|year=2011|last1=Heitsch|first1=Andrew T.|last2=Akhavan|first2=Vahid A.|last3=Korgel|first3=Brian A.}}</ref> can be used to synthesize semiconductor nanowires, e.g., Si and Ge. By using metal nanocrystals as seeds,<ref>{{cite journal|doi=10.1002/adma.200390101|title=Supercritical Fluid–Liquid–Solid (SFLS) Synthesis of Si and Ge Nanowires Seeded by Colloidal Metal Nanocrystals|journal=Advanced Materials|volume=15|issue=5|pages=437–440|year=2003|last1=Hanrath|first1=T.|last2=Korgel|first2=B.A.|bibcode=2003AdM....15..437H |s2cid=137573988 }}</ref> Si and Ge organometallic precursors are fed into a reactor filled with a supercritical organic solvent, such as [[toluene]]. [[Thermal decomposition|Thermolysis]] results in degradation of the precursor, allowing release of Si or Ge, and dissolution into the metal nanocrystals.  As more of the semiconductor solute is added from the supercritical phase (due to a concentration gradient), a solid crystallite precipitates, and a nanowire grows uniaxially from the nanocrystal seed.

=== Liquid Bridge Induced Self-assembly ===
Protein nanowires in spider silk have been formed by rolling a droplet of spider silk solution over a superhydrophobic pillar structure.<ref>{{cite journal | author1= Gustafsson, L. | author2= Jansson, R. | author3= Hedhammar, M. | author4= van der Wijngaart, W. | date= 2018 |title= Structuring of Functional Spider Silk Wires, Coatings, and Sheets by Self-Assembly on Superhydrophobic Pillar Surfaces |journal= Advanced Materials|volume= 30 |issue= 3 |doi= 10.1002/adma.201704325 | pmid= 29205540 | bibcode= 2018AdM....3004325G | s2cid= 205283504 }}</ref><ref>{{cite journal | author1= Gustafsson, L. | author2= Kvick, M. | author3= Åstrand, C. | author4= Ponsteen, N. | author5= Dorka, N. | author6= Hegrová, V. | author7= Svanberg, S. | author8= Horák, J. | author9= Jansson, R. | author10= Hedhammar, M. | author11= van der Wijngaart, W. | date= 2023 |title= Scalable Production of Monodisperse Bioactive Spider Silk Nanowires |journal= Macromolecular Bioscience | volume= 23 | issue= 4 | pages= e2200450 | doi= 10.1002/mabi.202200450| pmid= 36662774 | s2cid= 256032679 | doi-access= free }}</ref>

=== Non-catalytic growth ===
[[File:Nanowire growth.png|thumb|In situ observation of CuO nanowire growth]]

The vast majority of nanowire-formation mechanisms are explained through the use of catalytic nanoparticles, which drive the nanowire growth and are either added intentionally or generated during the growth. However, nanowires can be also grown without the help of catalysts, which gives an advantage of pure nanowires and minimizes the number of technological steps. The mechanisms for catalyst-free growth of nanowires (or whiskers) were known from 1950s.<ref>{{cite journal |last1=Sears |first1=G.W. |year=1955 |title=A Growth Mechanism for Mercury Whiskers |journal=Acta Metall |volume=3 |issue=4 |pages=361–366 |doi=10.1016/0001-6160(55)90041-9}}</ref>

The simplest methods to obtain metal oxide nanowires use ordinary heating of the metals, e.g. metal wire heated with battery, by [[Joule heating]] in air<ref>{{cite journal | last1 = Rackauskas | first1 = S. | last2 = Nasibulin | first2 = A. G. | last3 = Jiang | first3 = H. | last4 = Tian | first4 = Y. | last5 = Kleshch | first5 = V. I. | last6 = Sainio | first6 = J. | last7 = Obraztsova | first7 = E. D. | last8 = Bokova | first8 = S. N. | last9 = Obraztsov | first9 = A. N. | last10 = Kauppinen | first10 = E. I. | s2cid = 3529748 | year = 2010 | title = A Novel Method for Metal Oxide Nanowire Synthesis | doi = 10.1088/0957-4484/20/16/165603 | pmid = 19420573 | journal = Nanotechnology | volume = 20 | issue = 16| page = 165603 | bibcode = 2009Nanot..20p5603R }}</ref> can be easily done at home. Spontaneous nanowire formation by non-catalytic methods were explained by the [[dislocations|dislocation]] present in specific directions<ref>{{cite journal | last1 = Frank | first1 = F. C. | s2cid = 53512926 | year = 1949 | title = The influence of dislocations on crystal growth| doi = 10.1039/df9490500048 | journal = Discussions of the Faraday Society| volume = 5 | page = 48 }}</ref><ref>{{cite journal | last1 = Burton | first1 = W. K. | last2 = Cabrera | first2 = N. | last3 = Frank | first3 = F. C. | s2cid = 119643095 | year = 1951 | title = The Growth of Crystals and the Equilibrium Structure of Their Surfaces | doi = 10.1098/rsta.1951.0006 | journal = Philos. Trans. R. Soc. Lond. A | volume = 243 | issue = 866| pages = 299–358 | bibcode = 1951RSPTA.243..299B }}</ref> or the growth anisotropy of various [[crystal|crystal faces]]. More recently, after microscopy advancement, the nanowire growth driven by [[screw dislocation]]s<ref>{{cite journal | last1 = Morin | first1 = S. A. | last2 = Bierman | first2 = M. J. | last3 = Tong | first3 = J. | last4 = Jin | first4 = S. | s2cid = 30955349 | year = 2010 | title = Mechanism and Kinetics of Spontaneous Nanotube Growth Driven by Screw Dislocations | doi = 10.1126/science.1182977 | journal = Science | volume = 328 | issue = 5977| pages = 476–480 | pmid=20413496| bibcode = 2010Sci...328..476M }}</ref><ref>{{cite journal | last1 = Bierman | first1 = M. J. | last2 = Lau | first2 = Y. K. A. | last3 = Kvit | first3 = A. V | last4 = Schmitt | first4 = A. L. | last5 = Jin | first5 = S. | s2cid = 20919593 | year = 2008 | title = Dislocation-Driven Nanowire Growth and Eshelby Twist | doi = 10.1126/science.1157131 | pmid = 18451264 | journal = Science | volume = 320 | issue = 5879| pages = 1060–1063 | bibcode = 2008Sci...320.1060B }}</ref> or [[Twin boundary|twin boundaries]]<ref>{{cite journal | last1 = Rackauskas | first1 = S. | last2 = Jiang | first2 = H. | last3 = Wagner | first3 = J. B. | last4 = Shandakov | first4 = S. D. | last5 = Hansen | first5 = T. W. | last6 = Kauppinen | first6 = E. I. | last7 = Nasibulin | first7 = A. G. | year = 2014 | title = In Situ Study of Noncatalytic Metal Oxide Nanowire Growth | doi = 10.1021/nl502687s | pmid = 25233273 | journal = Nano Lett. | volume = 14 | issue = 10| pages = 5810–5813 | bibcode = 2014NanoL..14.5810R }}</ref> were demonstrated. The picture on the right shows a single atomic layer growth on the tip of CuO nanowire, observed by in situ [[Transmission electron microscopy|TEM microscopy]] during the non-catalytic synthesis of nanowire.

Atomic-scale nanowires can also form completely self-organised without need for defects. For example, [[Rare-earth element|rare-earth]] silicide (RESi<sub>2</sub>) nanowires of few nm width and height and several 100&nbsp;nm length form on silicon([[Miller index|001]]) substrates which are covered with a sub-monolayer of a rare earth metal and subsequently annealed.<ref>{{Cite journal |last1=Preinesberger |first1=C. |last2=Becker |first2=S. K. |last3=Vandré |first3=S. |last4=Kalka |first4=T. |last5=Dähne |first5=M. |date=February 2002 |title=Structure of DySi2 nanowires on Si(001) |url=http://aip.scitation.org/doi/10.1063/1.1430540 |journal=Journal of Applied Physics |language=en |volume=91 |issue=3 |pages=1695–1697 |doi=10.1063/1.1430540 |bibcode=2002JAP....91.1695P |issn=0021-8979}}</ref> The lateral dimensions of the nanowires confine the electrons in such a way that the system resembles a (quasi-)one-dimensional metal.<ref>{{Cite journal |last1=Holtgrewe |first1=Kris |last2=Appelfeller |first2=Stephan |last3=Franz |first3=Martin |last4=Dähne |first4=Mario |last5=Sanna |first5=Simone |date=2019-06-10 |title=Structure and one-dimensional metallicity of rare-earth silicide nanowires on Si(001) |url=https://link.aps.org/doi/10.1103/PhysRevB.99.214104 |journal=Physical Review B |language=en |volume=99 |issue=21 |pages=214104 |doi=10.1103/PhysRevB.99.214104 |bibcode=2019PhRvB..99u4104H |s2cid=197525473 |issn=2469-9950}}</ref>  Metallic RESi<sub>2</sub> nanowires form on silicon(''[[Miller index|hhk]]'') as well. This system permits tuning the dimensionality between two-dimensional and one-dimensional by the coverage and the tilt angle of the substrate.<ref>{{Cite journal |last1=Appelfeller |first1=Stephan |last2=Holtgrewe |first2=Kris |last3=Franz |first3=Martin |last4=Freter |first4=Lars |last5=Hassenstein |first5=Christian |last6=Jirschik |first6=Hans-Ferdinand |last7=Sanna |first7=Simone |last8=Dähne |first8=Mario |date=2020-09-24 |title=Continuous crossover from two-dimensional to one-dimensional electronic properties for metallic silicide nanowires |url=https://link.aps.org/doi/10.1103/PhysRevB.102.115433 |journal=Physical Review B |language=en |volume=102 |issue=11 |pages=115433 |doi=10.1103/PhysRevB.102.115433 |bibcode=2020PhRvB.102k5433A |s2cid=224924918 |issn=2469-9950}}</ref>

=== DNA-templated metallic nanowire synthesis ===

An emerging field is to use DNA strands as scaffolds for metallic nanowire synthesis. This method is investigated both for the synthesis of metallic nanowires in electronic components and for biosensing applications, in which they allow the transduction of a DNA strand into a metallic nanowire that can be electrically detected. Typically, ssDNA strands are stretched, whereafter they are decorated with metallic nanoparticles that have been functionalised with short complementary ssDNA strands.<ref>{{cite journal | author = Guo|display-authors=etal| title = Efficient DNA-assisted synthesis of trans-membrane gold nanowires | doi=10.1038/micronano.2017.84 | journal = Microsystems & Nanoengineering| volume = 4 | pages = 17084 | year = 2018 | doi-access = free }}</ref><ref>{{cite journal | journal = Langmuir | volume = 32 | issue = 40 | pages = 10159–10165 | title = Temperature-Dependent Charge Transport through Individually Contacted DNA Origami-Based Au Nanowires | doi=10.1021/acs.langmuir.6b01961| pmid = 27626925 | year = 2016 | last1 = Teschome | first1 = Bezu | last2 = Facsko | first2 = Stefan | last3 = Schönherr | first3 = Tommy | last4 = Kerbusch | first4 = Jochen | last5 = Keller | first5 = Adrian | last6 = Erbe | first6 = Artur }}</ref><ref>{{cite journal | journal = Physical Review Letters | volume = 86 | issue = 16 | pages = 3670–3 | title = Metallic Conduction through Engineered DNA: DNA Nanoelectronic Building Blocks | doi=10.1103/PhysRevLett.86.3670| pmid = 11328050 | bibcode=2001PhRvL..86.3670R| last1 = Rakitin | first1 = A | last2 = Aich | first2 = P | last3 = Papadopoulos | first3 = C | last4 = Kobzar | first4 = Yu | last5 = Vedeneev | first5 = A. S | last6 = Lee | first6 = J. S | last7 = Xu | first7 = J. M | year = 2001 }}</ref><ref>{{cite journal | journal = Advanced Materials | volume = 16| issue = 20| pages = 1799–1803| title = DNA-Templated Assembly of a Protein-Functionalized Nanogap Electrode| doi=10.1002/adma.200400244| year = 2004| last1 = Ongaro| first1 = A| last2 = Griffin| first2 = F| last3 = Nagle| first3 = L| last4 = Iacopino| first4 = D| last5 = Eritja| first5 = R| last6 = Fitzmaurice| first6 = D| bibcode = 2004AdM....16.1799O| s2cid = 97905129}}</ref>

=== Crack-Defined Shadow Mask Lithography ===

A simple method to produce nanowires with defined geometries has been recently reported using conventional optical lithography.<ref name="Enrico 2019 8217–8226">{{cite journal | author = Enrico|display-authors=etal| title = Scalable Manufacturing of Single Nanowire Devices Using Crack-Defined Shadow Mask Lithography | doi=10.1021/acsami.8b19410 | journal = ACS Appl. Mater. Interfaces| volume = 11 | pages = 8217–8226 | year = 2019 |issue=8|pmid=30698940|pmc=6426283| doi-access = free }}</ref> In this approach, optical lithography is used to generate nanogaps using controlled crack formation.<ref>{{cite journal | author = Dubois|display-authors=etal| title = Crack-Defined Electronic Nanogaps | doi=10.1002/adma.201504569 | journal = Advanced Materials | volume = 28 | pages = 2172178–2182 | year = 2016 |issue=11|pmid=26784270|bibcode=2016AdM....28.2178D |s2cid=205265220 |url=http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-182341}}</ref> These nanogaps are then used as shadow mask for generating individual nanowires with precise lengths and widths. This technique allows to produce individual nanowires below 20&nbsp;nm in width in a scalable way out of several metallic and metal oxide materials.

== Physics ==

=== Conductivity ===
[[Image:Wire15micron.jpg|thumb|An [[Scanning electron microscope|SEM]] image of a 15 micrometer nickel wire]]

Several physical reasons predict that the conductivity of a nanowire will be much less than that of the corresponding bulk material. First, there is scattering from the wire boundaries, whose effect will be very significant whenever the wire width is below the free electron mean free path of the bulk material. In copper, for example, the mean free path is 40&nbsp;nm. Copper nanowires less than 40&nbsp;nm wide will shorten the mean free path to the wire width. Silver nanowires have very different electrical and thermal conductivity from bulk silver.<ref>{{Cite journal|last1=Cheng|first1=Zhe|last2=Liu|first2=Longju|last3=Xu|first3=Shen|last4=Lu|first4=Meng|last5=Wang|first5=Xinwei|date=2015-06-02|title=Temperature Dependence of Electrical and Thermal Conduction in Single Silver Nanowire|url= |journal=Scientific Reports|language=en|volume=5|issue=1|pages=10718|doi=10.1038/srep10718|pmid=26035288|pmc=4451791|arxiv=1411.7659|bibcode=2015NatSR...510718C|issn=2045-2322}}</ref>

Nanowires also show other peculiar electrical properties due to their size. Unlike single wall carbon nanotubes, whose motion of electrons can fall under the regime of [[ballistic transport]] (meaning the electrons can travel freely from one electrode to the other), nanowire conductivity is strongly influenced by edge effects. The edge effects come from atoms that lay at the nanowire surface and are not fully bonded to neighboring atoms like the atoms within the bulk of the nanowire. The unbonded atoms are often a source of defects within the nanowire, and may cause the nanowire to conduct electricity more poorly than the bulk material. As a nanowire shrinks in size, the surface atoms become more numerous compared to the atoms within the nanowire, and edge effects become more important.{{citation needed|date=December 2023}}

The conductance in a nanowire is described as the sum of the transport by separate ''channels'', each having a different electronic wavefunction normal to the wire. The thinner the wire is, the smaller the number of channels available to the transport of electrons. As a result, wires that are only one or a few atoms wide exhibit quantization of the conductance: i.e. the conductance can assume only discrete values that are multiples of the [[conductance quantum]] {{nowrap|1=''G''<sub>0</sub> = 2''e''<sup>2</sup>/''h''}} (where ''e'' is the [[elementary charge]] and ''h'' is the [[Planck constant]]) (see also ''[[Quantum Hall effect]]''). This quantization has been observed by measuring the conductance of a nanowire suspended between two electrodes while pulling it progressively longer: as its diameter reduces, its conductivity decreases in a stepwise fashion and the plateaus correspond approximately to multiples of ''G''<sub>0</sub>.<ref name="Yanson 1998">{{cite journal | last1=Yanson | first1=A. I. | last2=Bollinger | first2=G. Rubio | last3=van den Brom | first3=H. E. | last4=Agraït | first4=N. | last5=van Ruitenbeek | first5=J. M. | title=Formation and manipulation of a metallic wire of single gold atoms | journal=Nature | volume=395 | issue=6704 | date=1998 | issn=0028-0836 | doi=10.1038/27405 | pages=783–785| arxiv=cond-mat/9811093 | bibcode=1998Natur.395..783Y }}</ref><ref name="Rodrigues 2000">{{cite journal | last1=Rodrigues | first1=Varlei | last2=Fuhrer | first2=Tobias | last3=Ugarte | first3=Daniel | title=Signature of Atomic Structure in the Quantum Conductance of Gold Nanowires | journal=Phys. Rev. Lett. | volume=85 | issue=19 | date=2000-11-06 | issn=0031-9007 | doi=10.1103/PhysRevLett.85.4124 | pages=4124–4127| pmid=11056640 | bibcode=2000PhRvL..85.4124R }}</ref>

The quantization of conductivity is more pronounced in semiconductors like Si or GaAs than in metals, because of their lower electron density and lower effective mass. It can be observed in 25&nbsp;nm wide silicon fins, and results in increased [[threshold voltage]].  In practical terms, this means that a [[MOSFET]] with such nanoscale silicon fins, when used in digital applications, will need a higher gate (control) voltage to switch the transistor on.<ref>{{cite journal|doi=10.1103/PhysRevB.68.075311 |journal=Physical Review B|volume=68|issue=7|page=075311|year=2003|title=Quantum interference in a one-dimensional silicon nanowire|last1=Tilke|first1=A. T.|last2=Simmel|first2=F. C.|last3=Lorenz|first3=H.|last4=Blick|first4=R. H.|last5=Kotthaus|first5=J. P.|bibcode=2003PhRvB..68g5311T}}</ref>

=== Welding ===

To incorporate nanowire technology into industrial applications, researchers in 2008 developed a method of welding nanowires together: a [[sacrificial metal]] nanowire is placed adjacent to the ends of the pieces to be joined (using the manipulators of a [[scanning electron microscope]]); then an electric current is applied, which fuses the wire ends. The technique fuses wires as small as 10&nbsp;nm.<ref>{{cite journal|author=Halford, Bethany |title=Wee Welding with Nanosolder|journal=Chemical & Engineering News|volume=86|issue=51 |year=2008|pages=35}}</ref>

For nanowires with diameters less than 10&nbsp;nm, existing welding techniques, which require precise control of the heating mechanism and which may introduce the possibility of damage, will not be practical. Recently scientists discovered that single-crystalline ultrathin gold nanowires with diameters ≈&nbsp;3–10&nbsp;nm can be "cold-welded" together within seconds by mechanical contact alone, and under remarkably low applied pressures (unlike macro- and micro-scale [[cold welding]] process).<ref>{{cite journal|doi=10.1038/nnano.2010.4|pmid=20154688|title=Cold welding of ultrathin gold nanowires|journal=Nature Nanotechnology|volume=5|issue=3|pages=218–24|year=2010|last1=Lu|first1=Yang|last2=Huang|first2=Jian Yu|last3=Wang|first3=Chao|last4=Sun|first4=Shouheng|last5=Lou|first5=Jun|bibcode=2010NatNa...5..218L}}</ref> High-resolution [[transmission electron microscopy]] and [[in situ]] measurements reveal that the welds are nearly perfect, with the same crystal orientation, strength and electrical conductivity as the rest of the nanowire. The high quality of the welds is attributed to the nanoscale sample dimensions, oriented-attachment mechanisms and mechanically assisted fast [[surface diffusion]]. Nanowire welds were also demonstrated between gold and silver, and silver nanowires (with diameters ≈&nbsp;5–15&nbsp;nm) at near room temperature, indicating that this technique may be generally applicable for ultrathin metallic nanowires. Combined with other nano- and microfabrication technologies,<ref>{{cite journal|doi=10.1126/science.1090899 |pmid=14631034 |title=Nanowire Crossbar Arrays as Address Decoders for Integrated Nanosystems |journal=Science |volume=302 |issue=5649 |pages=1377–9 |year=2003 |last1=Zhong |first1=Z. |last2=Wang |first2=D |last3=Cui |first3=Y |last4=Bockrath |first4=M. W. |last5=Lieber |first5=C. M. |s2cid=35084433 |bibcode=2003Sci...302.1377Z |url=https://authors.library.caltech.edu/51862/7/zhong.SOM.pdf }}</ref><ref>{{cite journal|doi=10.1126/science.1162193 |pmid=18703709 |title=Polymer Pen Lithography |journal=Science |volume=321 |issue=5896 |pages=1658–60 |year=2008 |last1=Huo |first1=F. |last2=Zheng |first2=Z. |last3=Zheng |first3=G. |last4=Giam |first4=L. R. |last5=Zhang |first5=H. |last6=Mirkin |first6=C. A. |s2cid=354452 |bibcode=2008Sci...321.1658H |url=https://dr.ntu.edu.sg/bitstream/10220/8553/1/48.%20Polymer%20Pen%20Lithography.pdf |pmc=8247121 |hdl=10356/94822 }}</ref> [[cold welding]] is anticipated to have potential applications in the future [[Top-down and bottom-up design#Nanotechnology|bottom-up]] assembly of metallic one-dimensional nanostructures.

=== Mechanical properties ===
[[File:Si NanoWire Failure.gif|thumb|Simulation of a nanowire [[Fracture|fracturing]]]]
The study of nanowire mechanics has boomed since the advent of the [[Atomic force microscopy|atomic force microscope]] (AFM), and associated technologies which have enabled direct study of the response of the nanowire to an applied load.<ref name=":0">{{Cite journal|last1=Wang|first1=Shiliang|last2=Shan|first2=Zhiwei|last3=Huang|first3=Han|date=2017-01-03|title=The Mechanical Properties of Nanowires|journal=Advanced Science|volume=4|issue=4|pages=1600332|doi=10.1002/advs.201600332 |pmc=5396167|pmid=28435775}}</ref> Specifically, a nanowire can be clamped from one end, and the free end displaced by an AFM tip. In this cantilever geometry, the height of the AFM is precisely known, and the force applied is precisely known. This allows for construction of a force vs. displacement curve, which can be converted to a [[Stress–strain curve|stress vs. strain]] curve if the nanowire dimensions are known. From the stress-strain curve, the elastic constant known as the [[Young's modulus|Young's Modulus]] can be derived, as well as the [[toughness]], and degree of [[strain-hardening]].

==== Young's modulus ====
[[File:Stress Strain Ductile Material.pdf|thumb|upright=1.2|Stress-strain curve provides all the relevant mechanical properties including; tensile modulus, yield strength, ultimate tensile strength, and fracture strength]]

The elastic component of the stress-strain curve described by the Young's Modulus, has been reported for nanowires, however the modulus depends very strongly on the microstructure. Thus a complete description of the modulus dependence on diameter is lacking. Analytically, [[continuum mechanics]] has been applied to estimate the dependence of modulus on diameter: <math>E=E_{0}[1+4(E_{0}/E_{s}-1)(r_{s}/D-r_{s}^{2}/D^{2})]</math> in tension, where <math>E_{0}
</math> is the bulk modulus, <math>r_{s}</math> is the thickness of a shell layer in which the modulus is surface dependent and varies from the bulk, <math>E{s}</math> is the surface modulus, and <math>D</math> is the diameter.<ref name=":0" /> This equation implies that the modulus increases as the diameter decreases. However, various computational methods such as molecular dynamics have predicted that modulus should decrease as diameter decreases.

Experimentally, gold nanowires have been shown to have a Young's modulus which is effectively diameter independent.<ref name=":1">{{Cite journal|last1=Wu|first1=Bin|last2=Heidelberg|first2=Andreas|last3=Boland|first3=John J.|s2cid=34828518|date=2005-06-05|title=Mechanical properties of ultrahigh-strength gold nanowires|journal=Nature Materials|volume=4|issue=7|pages=525–529|doi=10.1038/nmat1403|pmid=15937490|issn=1476-1122|bibcode=2005NatMa...4..525W}}</ref> Similarly, [[Nanoindentation|nano-indentation]] was applied to study the modulus of silver nanowires, and again the modulus was found to be 88 GPa, very similar to the modulus of bulk Silver (85 GPa)<ref>{{Cite journal|last1=Li|first1=Xiaodong|last2=Gao|first2=Hongsheng|last3=Murphy|first3=Catherine J.|last4=Caswell|first4=K. K.|date=Nov 2003|title=Nanoindentation of Silver Nanowires|journal=Nano Letters|volume=3|issue=11|pages=1495–1498|doi=10.1021/nl034525b|issn=1530-6984|bibcode=2003NanoL...3.1495L}}</ref> These works demonstrated that the analytically determined modulus dependence seems to be suppressed in nanowire samples where the crystalline structure highly resembles that of the bulk system.

In contrast, Si solid nanowires have been studied, and shown to have a decreasing modulus with diameter<ref>{{Cite journal|last1=Wang|first1=Zhong Lin|last2=Dai|first2=Zu Rong|last3=Gao|first3=Ruiping|last4=Gole|first4=James L.|s2cid=53588258|date=2002-03-27|title=Measuring the Young's modulus of solid nanowires byin situTEM|journal=Journal of Electron Microscopy|volume=51|issue=suppl 1|pages=S79–S85|doi=10.1093/jmicro/51.Supplement.S79|issn=0022-0744}}</ref> The authors of that work report a Si modulus which is half that of the bulk value, and they suggest that the density of point defects, and or loss of chemical stoichiometry may account for this difference.

==== Yield strength ====

The plastic component of the stress strain curve (or more accurately the onset of plasticity) is described by the [[Yield (engineering)|yield strength]]. The strength of a material is increased by decreasing the number of defects in the solid, which occurs naturally in [[nanomaterials]] where the volume of the solid is reduced. As a nanowire is shrunk to a single line of atoms, the strength should theoretically increase all the way to the molecular tensile strength.<ref name=":0" /> Gold nanowires have been described as 'ultrahigh strength' due to the extreme increase in yield strength, approaching the theoretical value of ''E''/10.<ref name=":1" /> This huge increase in yield is determined to be due to the lack of [[dislocation]]s in the solid. Without dislocation motion, a 'dislocation-starvation' mechanism is in operation. The material can accordingly experience huge stresses before dislocation motion is possible, and then begins to strain-harden. For these reasons, nanowires (historically described as 'whiskers') have been used extensively in composites for increasing the overall strength of a material.<ref name=":0" /> Moreover, nanowires continue to be actively studied, with research aiming to translate enhanced mechanical properties to novel devices in the fields of [[Microelectromechanical systems|MEMS]] or [[Nanoelectromechanical systems|NEMS]].

== Possible applications ==

=== Electronic devices ===
[[file:Threshold formation nowatermark.gif|thumb|Atomistic simulation result for formation of inversion channel (electron density) and attainment of threshold voltage (IV) in a nanowire MOSFET. Note that the threshold voltage for this device lies around 0.45V]]

Nanowires have been proposed for use as [[MOSFET]]s (MOS [[field-effect transistors]]). [[MOS transistor]]s are used widely as fundamental building elements in today's electronic circuits.<ref name="triumph">{{cite web |title=Triumph of the MOS Transistor |url=https://www.youtube.com/watch?v=q6fBEjf9WPw | archive-url=https://ghostarchive.org/varchive/youtube/20211211/q6fBEjf9WPw| archive-date=2021-12-11 | url-status=live|website=[[YouTube]] |publisher=[[Computer History Museum]] |access-date=21 July 2019 |date=6 August 2010}}{{cbignore}}</ref><ref name="Raymer">{{cite book |last1=Raymer |first1=Michael G. |title=The Silicon Web: Physics for the Internet Age |date=2009 |publisher=[[CRC Press]] |isbn=9781439803127 |page=365 |url=https://books.google.com/books?id=PLYChGDqa6EC&pg=PA365}}</ref> As predicted by [[Moore's law]], the dimension of MOS [[transistors]] is shrinking smaller and smaller into nanoscale. One of the key challenges of building future nanoscale MOS transistors is ensuring good gate control over the channel. In general, having a wider gate relative to the total transistor length affords greater gate control. Therefore, the high aspect ratio of nanowires potentially allows for good gate control.

Due to their one-dimensional structure with unusual optical properties, the nanowire are of interest for photovoltaic devices.<ref>{{Cite journal|last1=Yu|first1=Peng|last2=Wu|first2=Jiang|last3=Liu|first3=Shenting|last4=Xiong|first4=Jie|last5=Jagadish|first5=Chennupati|last6=Wang|first6=Zhiming M.|date=2016-12-01|title=Design and fabrication of silicon nanowires towards efficient solar cells|journal=Nano Today|volume=11|issue=6|pages=704–737|doi=10.1016/j.nantod.2016.10.001|url=http://discovery.ucl.ac.uk/1536157/}}</ref> Compared with its bulk counterparts, the nanowire solar cells are less sensitive to impurities due to bulk recombination, and thus silicon wafers with lower purity can be used to achieve acceptable efficiency, leading to the reduction on material consumption.<ref>{{Cite journal|last1=Kayes|first1=Brendan M.|last2=Atwater|first2=Harry A.|last3=Lewis|first3=Nathan S.|date=2005-05-23|title=Comparison of the device physics principles of planar and radial p-n junction nanorod solar cells|journal=Journal of Applied Physics|volume=97|issue=11|pages=114302–114302–11|doi=10.1063/1.1901835|issn=0021-8979|bibcode=2005JAP....97k4302K|url=https://authors.library.caltech.edu/4165/1/KAYjap05.pdf}}</ref>

After p-n junctions were built with nanowires, the next logical step was to build [[logic gates]]. By connecting several p-n junctions together, researchers have been able to create the basis of all logic circuits: the [[AND gate|AND]], [[OR gate|OR]], and [[NOT gate|NOT]] gates have all been built from semiconductor nanowire crossings.

In August 2012, researchers reported constructing the first [[NAND gate]] from undoped silicon nanowires. This avoids the problem of how to achieve precision doping of complementary nanocircuits, which is unsolved. They were able to control the [[Schottky barrier]] to achieve low-resistance contacts by placing a [[silicide]] layer in the metal-silicon interface.<ref>{{cite journal|doi=10.1021/nl300930m|pmid=22594644|arxiv=1208.1465|title=Multifunctional Devices and Logic Gates with Undoped Silicon Nanowires|journal=Nano Letters|volume=12|issue=6|pages=3074–9|year=2012|last1=Mongillo|first1=Massimo|last2=Spathis|first2=Panayotis|last3=Katsaros|first3=Georgios|last4=Gentile|first4=Pascal|last5=De Franceschi|first5=Silvano|s2cid=22112655|bibcode=2012NanoL..12.3074M}}</ref>

It is possible that semiconductor nanowire crossings will be important to the future of digital computing. Though there are other uses for nanowires beyond these, the only ones that actually take advantage of physics in the nanometer regime are electronic.<ref>{{cite journal|title= Toward nanowire electronics|journal= IEEE Transactions on Electron Devices|volume= 55|issue= 11|pages= 2827–2845|url= https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1169&context=nanodocs&sei-redir=1&referer=https%3A%2F%2Fscholar.google.com%2Fscholar%3Fhl%3Dzh-CN%26q%3Dnanowire%2Belectronics%26btnG%3D%26lr%3D#search=%22nanowire%20electronics%22|doi=10.1109/TED.2008.2008011|year= 2008|last1= Appenzeller|first1= Joerg|last2= Knoch|first2= Joachim|last3= Bjork|first3= Mikael T.|last4= Riel|first4= Heike|author4-link=Heike Riel|last5= Schmid|first5= Heinz|last6= Riess|first6= Walter|s2cid= 703393|bibcode= 2008ITED...55.2827A}}</ref>

In addition, nanowires are also being studied for use as photon ballistic waveguides as interconnects in [[quantum dot]]/quantum effect well photon logic arrays. Photons travel inside the tube, electrons travel on the outside shell.

When two nanowires acting as photon waveguides cross each other the juncture acts as a [[quantum dot]].

Conducting nanowires offer the possibility of connecting molecular-scale entities in a molecular computer. Dispersions of conducting nanowires in different polymers are being investigated for use as transparent electrodes for flexible flat-screen displays.

Because of their high [[Young's moduli]], their use in mechanically enhancing composites is being investigated. Because nanowires appear in bundles, they may be used as tribological additives to improve friction characteristics and reliability of electronic transducers and actuators.

Because of their high aspect ratio, nanowires are also suited to [[Dielectrophoresis|dielectrophoretic]] manipulation,<ref>{{cite journal|author = Wissner-Gross, A. D.|title = Dielectrophoretic reconfiguration of nanowire interconnects|journal = Nanotechnology|volume = 17|issue = 19|pages = 4986–4990|year = 2006|url = http://www.alexwg.org/publications/Nanotechnology_17-4986.pdf|doi = 10.1088/0957-4484/17/19/035|bibcode = 2006Nanot..17.4986W|s2cid = 4590982}}</ref><ref>{{cite news|url=http://nanotechweb.org/articles/journal/5/10/2/1|date=October 19, 2006|title=Nanowires get reconfigured|work=nanotechweb.org|access-date=January 18, 2007|archive-url=https://web.archive.org/web/20070522224736/http://nanotechweb.org/articles/journal/5/10/2/1|archive-date=May 22, 2007|url-status=dead}}</ref><ref>{{cite journal |author1=Grange, R. |author2=Choi, J.W. |author3=Hsieh, C.L. |author4=Pu, Y. |author5=Magrez, A. |author6=Smajda, R. |author7=Forro, L. |author8=Psaltis, D. |title=Lithium niobate nanowires: synthesis, optical properties and manipulation |journal=Applied Physics Letters |volume=95 |issue=14 |pages=143105 |year=2009 |url=http://link.aip.org/link/?APPLAB/95/143105/1 |doi=10.1063/1.3236777 |bibcode=2009ApPhL..95n3105G |url-status=dead |archive-url=http://arquivo.pt/wayback/20160514132623/http://link.aip.org/link/?APPLAB/95/143105/1 |archive-date=2016-05-14 }}</ref> which offers a low-cost, bottom-up approach to integrating suspended dielectric metal oxide nanowires in electronic devices such as UV, water vapor, and ethanol sensors.<ref>{{Cite journal | last1 = Vizcaíno | first1 = J. L. P. | last2 = Núñez | first2 = C. G. A. | s2cid = 124474608 | title = Fast, effective manipulation of nanowires for electronic devices | doi = 10.1117/2.1201312.005260 | journal = SPIE Newsroom | year = 2013 }}</ref>

Due to their large surface-to-volume ratio, physico-chemical reactions are facilitated on the surface of nanowires.<ref>{{Cite journal|last1=Coradini|first1=Diego S. R.|last2=Tunes|first2=Matheus A.|last3=Kremmer|first3=Thomas M.|last4=Schön|first4=Claudio G.|last5=Uggowitzer|first5=Peter J.|last6=Pogatscher|first6=Stefan|date=2020-11-05|title=Degradation of Cu nanowires in a low-reactive plasma environment|journal=npj Materials Degradation|language=en|volume=4|issue=1|pages=1–8|doi=10.1038/s41529-020-00137-2|s2cid=226248533|issn=2397-2106|doi-access=free|hdl=20.500.11850/454060|hdl-access=free}}</ref>

=== Single nanowire devices for gas and chemical sensing ===

The high aspect ratio of nanowires makes this nanostructures suitable for electrochemical sensing with the potential for ultimate sensitivity. One of the challenge for the use of nanowires in commercial products is related to the isolation, handling, and integration of nanowires in an electrical circuit when using the conventional and manual pick-and-place approach, leading to a very limited throughput. Recent developments in the nanowire synthesis methods now allow for parallel production of single nanowire devices with useful applications in electrochemistry, photonics, and gas- and biosensing.<ref name="Enrico 2019 8217–8226"/>

=== Nanowire lasers ===
[[File:Nanowire laser.png|thumb|upright|Nanowire lasers for ultrafast transmission of information in light pulses]]

[[Nanowire lasers]] are nano-scaled [[lasers]] with potential as optical interconnects and optical data communication on chip. Nanowire lasers are built from III–V semiconductor heterostructures, the high refractive index allows for low optical loss in the nanowire core. Nanowire lasers are subwavelength lasers of only a few hundred nanometers.<ref>{{cite journal |last1=Koblmüller |first1=Gregor |first2=Benedikt |last2=Mayer |first3=Thomas |last3=Stettner |display-authors=1 |title=GaAs–AlGaAs core–shell nanowire lasers on silicon: invited review |journal=Semiconductor Science and Technology |volume=32 |issue=5 |year=2017 |at=053001 |doi=10.1088/1361-6641/aa5e45 |bibcode=2017SeScT..32e3001K |s2cid=99074531 }}</ref><ref>{{cite journal |last1=Yan |first1=Ruoxue |first2=Daniel |last2=Gargas |first3=Peidong |last3=Yang |title=Nanowire photonics |journal=Nature Photonics |volume=3 |issue=10 |year=2009 |pages=569–576 |doi=10.1038/nphoton.2009.184 |bibcode=2009NaPho...3..569Y |s2cid=2481816 }}</ref> Nanowire lasers are Fabry–Perot resonator cavities defined by the end facets of the wire with high-reflectivity, recent developments have demonstrated repetition rates greater than 200&nbsp;GHz offering possibilities for optical chip level communications.<ref>{{cite journal |last1=Mayer |first1=B. |first2=L. |last2=Janker |first3=B. |last3=Loitsch |display-authors=1 |title=Monolithically integrated high-β nanowire lasers on silicon |journal=Nano Letters |volume=16 |issue=1 |year=2015 |pages=152–156 |doi=10.1021/acs.nanolett.5b03404 |pmid=26618638 }}</ref><ref>{{cite journal |last1=Mayer |first1=B. |first2=A. |last2=Regler |first3=S. |last3=Sterzl |s2cid=1099474 |display-authors=1 |title=Long-term mutual phase locking of picosecond pulse pairs generated by a semiconductor nanowire laser |journal=Nature Communications |volume=8 |year=2017 |at=15521 |doi=10.1038/ncomms15521 |pmid=28534489 |pmc=5457509 |arxiv=1603.02169 |bibcode=2017NatCo...815521M }}</ref>

=== Sensing of proteins and chemicals using semiconductor nanowires ===

In an analogous way to FET devices in which the modulation of conductance (flow of electrons/holes) in the semiconductor, between the input (source) and the output (drain) terminals, is controlled by electrostatic potential variation (gate-electrode) of the charge carriers in the device conduction channel, the methodology of a Bio/Chem-FET is based on the detection of the local change in charge density, or so-called "field effect", that characterizes the recognition event between a target molecule and the surface receptor.

This change in the surface potential influences the Chem-FET device exactly as a 'gate' voltage does, leading to a detectable and measurable change in the device conduction. When these devices are fabricated using semiconductor nanowires as the transistor element the binding of a chemical or biological species to the surface of the sensor can lead to the depletion or accumulation of charge carriers in the "bulk" of the nanometer diameter nanowire i.e. (small cross section available for conduction channels). Moreover, the wire, which serves as a tunable conducting channel, is in close contact with the sensing environment of the target, leading to a short response time, along with orders of magnitude increase in the sensitivity of the device as a result of the huge S/V ratio of the nanowires.

While several inorganic semiconducting materials such as Si, Ge, and metal oxides (e.g. In<sub>2</sub>O<sub>3</sub>, SnO<sub>2</sub>, ZnO, etc.) have been used for the preparation of nanowires, Si is usually the material of choice when fabricating nanowire FET-based chemo/biosensors.<ref>{{cite book |title=Semiconductor Nanowires |series=Smart Materials Series |editor-first=Wei |editor-last=Lu |editor2-first=Jie |editor2-last=Xiang |publisher=Royal Society of Chemistry |location=Cambridge |year=2015 |doi=10.1039/9781782626947 |isbn=978-1-84973-826-2 |url=https://pubs.rsc.org/en/content/ebook/978-1-78262-694-7 }}</ref>

Several examples of the use of [[Silicon Nanowire|silicon nanowire]](SiNW) sensing devices include the ultra sensitive, real-time sensing of biomarker proteins for cancer, detection of single virus particles, and the detection of nitro-aromatic explosive materials such as 2,4,6-tri-nitrotoluene (TNT) in sensitives superior to these of canines.<ref>{{cite journal |last1=Engel |first1=Yoni |last2=Elnathan |first2=Roey |last3=Pevzner |first3=Alexander |last4=Davidi |first4=Guy |last5=Flaxer |first5=Eli |last6=Patolsky |first6=Fernando |title=Supersensitive Detection of Explosives by Silicon Nanowire Arrays |journal=Angewandte Chemie International Edition |date=2010 |volume=49 |issue=38 |pages=6830–6835 |doi=10.1002/anie.201000847 |pmid=20715224|doi-access=free }}</ref>
Silicon nanowires could also be used in their twisted form, as electromechanical devices, to measure intermolecular forces with great precision.<ref>{{cite journal|last1=Garcia|first1=J. C.|last2=Justo|first2=J. F.|s2cid=118792981|title=Twisted ultrathin silicon nanowires: A possible torsion electromechanical nanodevice|journal=Europhys. Lett.|date=2014|volume=108|issue=3|page=36006|doi=10.1209/0295-5075/108/36006|bibcode=2014EL....10836006G|arxiv=1411.0375}}</ref>

==== Limitations of sensing with [[silicon nanowire]] FET devices ====
Generally, the charges on dissolved molecules and macromolecules are screened by dissolved counterions, since in most cases molecules bound to the devices are separated from the sensor surface by approximately 2–12&nbsp;nm (the size of the receptor proteins or DNA linkers bound to the sensor surface). As a result of the screening, the electrostatic potential that arises from charges on the analyte molecule decays exponentially toward zero with distance. Thus, for optimal sensing, the [[Debye length]] must be carefully selected for nanowire FET measurements.
One approach of overcoming this limitation employs fragmentation of the antibody-capturing units and control over surface receptor density, allowing more intimate binding to the nanowire of the target protein. This approach proved useful for dramatically enhancing the sensitivity of [[Cardiac marker|cardiac biomarkers]] (e.g. [[Troponin]]) detection directly from serum for the diagnosis of acute myocardial infarction.<ref>{{cite journal|last=Elnathan|first=Roey |author2=Kwiat, M. |author3=Pevzner, A. |author4=Engel, Y. |author5=Burstein, L. |author6=Khatchtourints, A. |author7=Lichtenstein, A. |author8=Kantaev, R. |author9=Patolsky, F.|title=Biorecognition Layer Engineering: Overcoming Screening Limitations of Nanowire-Based FET Devices|journal=Nano Letters|date=10 September 2012|volume=12|issue=10|pages=5245–5254|doi=10.1021/nl302434w|pmid=22963381|bibcode=2012NanoL..12.5245E }}</ref>

=== Nanowire assisted transfer of sensitive TEM samples ===

For a minimal introduction of stress and bending to [[transmission electron microscopy]] (TEM) samples ([[Lamella (materials)|lamellae]], [[thin films]], and other mechanically and beam sensitive samples), when transferring inside a [[focused ion beam]] (FIB), flexible metallic nanowires can be attached to a typically rigid [[micromanipulator]].

The main advantages of this method include a significant reduction of sample preparation time (quick welding and cutting of nanowire at low beam current), and minimization of stress-induced bending, Pt contamination, and ion beam damage.<ref>{{cite journal |last1=Gorji |first1=Saleh |last2=Kashiwar |first2=Ankush |last3=Mantha |first3=Lakshmi S |last4=Kruk |first4=Robert |last5=Witte |first5=Ralf |last6=Marek |first6=Peter |last7=Hahn |first7=Horst |last8=Kübel |first8=Christian |last9=Scherer |first9=Torsten |title=Nanowire facilitated transfer of sensitive TEM samples in a FIB |date=December 2020 |journal=Ultramicroscopy |volume=219 |page=113075 |doi=10.1016/j.ultramic.2020.113075 |pmid=33035837 |s2cid=222255773 |url=https://www.researchgate.net/publication/342952945}}</ref>
This technique is particularly suitable for [[in situ electron microscopy]] sample preparation.

== Corn-like nanowires ==

Corn-like nanowire is a one-dimensional nanowire with interconnected nanoparticles on the surface, providing a large percentage of reactive facets. TiO<sub>2</sub> corn-like nanowires were first prepared by a surface modification concept using surface tension stress mechanism through a two consecutive hydrothermal operation, and showed an increase of 12% in dye-sensitized solar cell efficiency the light scattering layer.<ref>{{cite journal |first1=A. M. |last1=Bakhshayesh |first2=M. R. |last2=Mohammadi |first3=H. |last3=Dadar |first4=D. J. |last4=Fray |title=Improved efficiency of dye-sensitized solar cells aided by corn-like TiO<sub>2</sub> nanowires as the light scattering layer |journal=Electrochimica Acta |volume=90 |issue=15 |pages=302–308 |year=2013 |doi=10.1016/j.electacta.2012.12.065 }}</ref> CdSe corn-like nanowires grown by chemical bath deposition and corn-like γ-Fe<sub>2</sub>O<sub>3</sub>@SiO<sub>2</sub>@TiO<sub>2</sub> photocatalysts induced by magnetic dipole interactions have been also reported previously.<ref>{{cite journal |first1=H. M. |last1=Gubur |first2=F. |last2=Septekin |first3=S. |last3=Alpdogan |first4=B. |last4=Sahan |first5=B. K. |last5=Zeyrek |s2cid=137884561 |title=Structural properties of CdSe corn-like nanowires grown by chemical bath deposition |journal=Journal of Materials Science: Materials in Electronics |volume=27 |year=2016 |issue=7 |pages=7640–7645 |doi=10.1007/s10854-016-4748-2 }}</ref><ref>{{cite journal |first1=F. |last1=Wang |first2=M. |last2=Li |first3=L. |last3=Yu |first4=F. |last4=Sun |first5=Z. |last5=Wang |first6=L. |last6=Zhang |first7=H. |last7=Zeng |first8=X. |last8=Xu |s2cid=6058050 |title=Corn-like, Recoverable γ-Fe<sub>2</sub>O<sub>3</sub>@SiO<sub>2</sub>@TiO<sub>2</sub> Photocatalyst Induced by Magnetic Dipole Interactions |journal=Sci. Rep. |year=2017 |volume=7 |issue=1 |at=6960 |doi=10.1038/s41598-017-07417-z |pmid=28761085 |pmc=5537353 |bibcode=2017NatSR...7.6960W |doi-access=free }}</ref>

== See also ==
{{Portal|Science|Technology}}

* [[Bacterial nanowires]]
* [[Molecular wire]]
* [[Nanoantenna]]
* [[Nanorod]]
* [[Nanowire battery]]
* [[Non-carbon nanotube]]
* [[Silicon nanowire]]
* [[Solar cell]]

== References ==

{{reflist}}

== External links ==
{{Wiktionary}}

* [http://www.nanohedron.com Nanohedron.com | Nano Image Gallery] several images of nanowires are included in the galleries.
* [http://news-service.stanford.edu/news/2008/january9/nanowire-010908.html Stanford's nanowire battery holds 10 times the charge of existing ones] {{Webarchive|url=https://web.archive.org/web/20100107182920/http://news-service.stanford.edu/news/2008/january9/nanowire-010908.html |date=2010-01-07 }}
* [https://web.archive.org/web/20040918144456/http://www.fkf.mpg.de/klitzing/publications/abstracts/vk00xx/vk0000.html Original article on the Quantum Hall Effect: K. v. Klitzing, G. Dorda, and M. Pepper; Phys. Rev. Lett. 45, 494–497 (1980).]
* [http://www.abc.net.au/science/news/stories/2006/1813845.htm Strongest theoretical nanowire produced at Australia's University of Melbourne.]
* [https://web.archive.org/web/20071012024713/http://www.upenn.edu/pennnews/article.php?id=1217 Penn Engineers Design Electronic Computer Memory in Nanoscale Form That Retrieves Data 1,000 Times Faster.]
* [https://web.archive.org/web/20081013224151/http://pne.tnw.utwente.nl/ One atom thick, hundreds of nanometers long Pt-nanowires are one of the best examples of self-assembly. (University of Twente)]

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