Editing Nanowire (section)

== 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 | bibcode=2016TAML....6..195L}}</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]]. Nanowires can grow 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 |issue=1 | doi-access = free |bibcode=2018MicNa...417084G }}</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 ===

[[Optical lithography]] is a simple method to produce nanowires .<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.