Editing Molecular electronics

{{Short description|Branch of chemistry and electronics}}
{{For|[[Quantum mechanics|quantum mechanical]] study of the [[electron]] distribution in a molecule|stereoelectronics}}
{{Missing information|Information visualization methods related to molecular electronics are lacking.|Diagram|date=November 2022}}
{{Use American English|date = April 2019}}
'''Molecular electronics''' is the study and application of molecular building blocks for the fabrication of electronic components. It is an interdisciplinary area that spans [[physics]], [[chemistry]], and [[materials science]]. It provides a potential means to extend [[Moore's Law]] beyond the foreseen limits of small-scale conventional silicon [[integrated circuit]]s.<ref>{{Cite book|last=Petty |first=M.C. |author2=Bryce, M.R. |author3=Bloor, D. |name-list-style=amp |title=Introduction to Molecular Electronics |publisher=Oxford University Press |date=1995 |location=New York |pages=1–25 |isbn=0-19-521156-1}}</ref>

==Molecular scale electronics==
{{main|Molecular scale electronics}}
{{Nanoelectronics}}

Molecular scale [[electronics]], also called single-molecule electronics, is a branch of [[nanotechnology]] that uses single molecules, or nanoscale collections of single molecules, as [[electronic component]]s. Because single molecules constitute the smallest stable structures possible, this miniaturization is the ultimate goal for shrinking [[electrical circuit]]s.

Conventional electronic devices are traditionally made from bulk materials. Bulk methods have inherent limits, and are growing increasingly demanding and costly. Thus, the idea was born that the components could instead be built up atom by atom in a chemistry lab (bottom up) as opposed to carving them out of bulk material (top down). In single-molecule electronics, the bulk material is replaced by single molecules. The molecules used have properties that resemble traditional electronic components such as a [[wire]], [[transistor]], or [[rectifier]].<ref name="Aviram1974">{{Cite journal|last1=Aviram|first1=Arieh|last2=Ratner|first2=Mark A.|date=15 November 1974|title=Molecular rectifiers|journal=Chemical Physics Letters|language=en|volume=29|issue=2|pages=277–283|doi=10.1016/0009-2614(74)85031-1|bibcode=1974CPL....29..277A}}</ref>

Single-molecule electronics is an emerging field, and entire electronic circuits consisting exclusively of molecular sized compounds are still very far from being realized. However, the continuous demand for more computing power, together with the inherent limits of the present day lithographic methods make the transition seem unavoidable. Currently, the focus is on discovering molecules with interesting properties and on finding ways to obtain reliable and reproducible contacts between the molecular components and the bulk material of the electrodes.

Molecular electronics operates at distances less than 100 nanometers. Miniaturization down to single molecules brings the scale down to a regime where [[quantum mechanics]] effects are important. In contrast to the case in conventional electronic components, where [[electron]]s can be filled in or drawn out more or less like a continuous flow of [[electric charge]], the transfer of a single electron alters the system significantly. The significant amount of energy due to charging has to be taken into account when making calculations about the electronic properties of the setup and is highly sensitive to distances to conducting surfaces nearby.

[[Image:Rotaxane cartoon.jpg|thumb|left|Graphical representation of a [[rotaxane]], useful as a molecular switch]]

One of the biggest problems with measuring on single molecules is to establish reproducible electrical contact with only one molecule and doing so without shortcutting the electrodes. Because the current [[photolithographic]] technology is unable to produce electrode gaps small enough to contact both ends of the molecules tested (in the order of nanometers), alternative strategies are used. These include molecular-sized gaps called break junctions, in which a thin electrode is stretched until it breaks. One of the ways to overcome the gap size issue is by trapping molecular functionalized nanoparticles (internanoparticle spacing is matchable to the size of molecules), and later target the molecule by place exchange reaction.<ref>{{cite journal |last1=Jafri |first1=S H M |last2=Blom |first2=T |last3=Leifer |first3=K |last4=Strømme |first4=M |last5=Löfås |first5=H |last6=Grigoriev |first6=A |last7=Ahuja |first7=R |last8=Welch |first8=K |title=Assessment of a nanoparticle bridge platform for molecular electronics measurements |journal=Nanotechnology |date=29 October 2010 |volume=21 |issue=43 |pages=435204 |doi=10.1088/0957-4484/21/43/435204 |pmid=20890018 |bibcode=2010Nanot..21Q5204J |s2cid=29398313 }}</ref>

Another method is to use the tip of a [[scanning tunneling microscope]] (STM) to contact molecules adhered at the other end to a metal substrate.<ref>{{cite journal |last=Gimzewski |first=J.K. |author2=Joachim, C. |title=Nanoscale science of single molecules using local probes |journal=Science |volume=283 |pages=1683–1688 |date=1999 |doi=10.1126/science.283.5408.1683 |pmid=10073926 |issue=5408|bibcode= 1999Sci...283.1683G}}</ref> Another popular way to anchor molecules to the electrodes is to make use of [[sulfur]]'s high [[chemical affinity]] to [[gold]]; though useful, the anchoring is non-specific and thus anchors the molecules randomly to all gold surfaces, and the [[contact resistance]] is highly dependent on the precise atomic geometry around the site of anchoring and thereby inherently compromises the reproducibility of the connection. To circumvent the latter issue, experiments have shown that [[fullerenes]] could be a good candidate for use instead of sulfur because of the large conjugated π-system that can electrically contact many more atoms at once than a single atom of sulfur.<ref>[http://isis.ku.dk/kurser/index.aspx?kursusid=25537&xslt=simple6&param1=140150&param8=false Sørensen, J.K.] {{Webarchive|url=https://web.archive.org/web/20160329183231/https://isis.ku.dk/kurser/index.aspx?kursusid=25537&xslt=simple6&param1=140150&param8=false |date=2016-03-29 }}. (2006). "Synthesis of new components, functionalized with (60)fullerene, for molecular electronics". 4th Annual meeting - CONT 2006, University of Copenhagen.</ref>

The shift from metal electrodes to [[semiconductor]] electrodes allows for more tailored properties and thus for more interesting applications. There are some concepts for contacting organic molecules using semiconductor-only electrodes, for example by using [[indium arsenide]] [[nanowire]]s with an embedded segment of the wider bandgap material [[indium phosphide]] used as an electronic barrier to be bridged by molecules.<ref>{{cite journal |last=Schukfeh |first=Muhammed Ihab |author2=Storm, Kristian |author3=Mahmoud, Ahmad |author4=Søndergaard, Roar R. |author5=Szwajca, Anna| author6=Hansen, Allan |author7=Hinze, Peter |author8=Weimann, Thomas |author9=Fahlvik Svensson, Sofia |author10=Bora, Achyut |author11=Dick, Kimberly A. |author12=Thelander, Claes |author13=Krebs, Frederik C. |author14=Lugli, Paolo |author15=Samuelson, Lars |author16=Tornow, Marc |title=Conductance Enhancement of InAs/InP Heterostructure Nanowires by Surface Functionalization with Oligo(phenylene vinylene)s |journal=ACS Nano |volume=7 |pages=4111–4118 |date=2013 |doi=10.1021/nn400380g |pmid=23631558 |issue=5}}</ref>

One of the biggest hindrances for single-molecule electronics to be commercially exploited is the lack of means to connect a molecular sized circuit to bulk electrodes in a way that gives reproducible results. Also problematic is that some measurements on single molecules are done at [[Cryogenics|cryogenic temperatures]], near absolute zero, which is very energy consuming.

==History==

The first time in history molecular electronics are mentioned was in 1956 by the German physicist Arthur Von Hippel,<ref>{{cite journal |last1=Von Hippel |first1=Arthur R. |last2=Landshoff |first2=Rolf |title=Molecular Science and Molecular Engineering |journal=Physics Today |date=October 1959 |volume=12 |issue=10 |pages=48 |doi=10.1063/1.3060522 |bibcode=1959PhT....12j..48V }}</ref> who suggested a bottom-up procedure of developing electronics from atoms and molecules rather than using prefabricated materials, an idea he named molecular engineering. However the first breakthrough in the field is considered by many the article by Aviram and Ratner in 1974.<ref name="Aviram1974" /> In this article named Molecular Rectifiers, they presented a theoretical calculation of transport through a modified charge-transfer molecule with donor acceptor groups that would allow transport only in one direction, essentially like a semiconductor diode. This was a breakthrough that inspired many years of research in the field of molecular electronics.

==Molecular materials for electronics==
{{further|Conductive polymer|Organic electronics}}

[[File:ConductivePoly.png|300px|thumb|Chemical structures of some conductive polymers. From top left clockwise: [[polyacetylene]]; [[Poly(p-phenylene vinylene)|polyphenylene vinylene]]; polypyrrole (X = NH) and [[polythiophene]] (X = S); and [[polyaniline]] (X = NH/N) and [[Poly(p-phenylene sulfide)|polyphenylene sulfide]] (X = S).]]

The biggest advantage of conductive polymers is their processability, mainly by [[Dispersion (chemistry)|dispersion]]. Conductive polymers are not [[plastic]]s, i.e., they are not thermoformable, yet they are organic polymers, like (insulating) polymers. They can offer high electrical conductivity but have different mechanical properties than other commercially used polymers. The electrical properties can be fine-tuned using the methods of [[organic synthesis]]<ref name=Ullmann>{{cite book |doi=10.1002/14356007.a21_429 |chapter=Polymers, Electrically Conducting |title=Ullmann's Encyclopedia of Industrial Chemistry |year=2000 |last1=Naarmann |first1=Herbert |isbn=978-3-527-30673-2 }}</ref> and of advanced dispersion.<ref name=nalwa/>

The linear-backbone polymers such as [[polyacetylene]], [[polypyrrole]], and [[polyaniline]] are the main classes of conductive polymers. Poly(3-alkylthiophenes) are the archetypical materials for [[Solar cell#Organic/polymer solar cells|solar cells]] and transistors.<ref name=Ullmann/>

Conducting polymers have backbones of contiguous sp<sup>2</sup> hybridized carbon centers. One valence electron on each center resides in a p<sub>z</sub> orbital, which is orthogonal to the other three sigma-bonds. The electrons in these delocalized orbitals have high mobility when the material is ''doped'' by oxidation, which removes some of these delocalized electrons. Thus the [[Conjugated system|conjugated]] p-orbitals form a one-dimensional [[electronic band structure|electronic band]], and the electrons within this band become mobile when it is emptied partly. Despite intensive research, the relationship between morphology, chain structure, and conductivity is poorly understood yet.<ref>Skotheim, T., Elsenbaumer, R., Reynolds, J., Eds.; Handbook of Conducting Polymers, 2nd ed.; Marcel Dekker, Inc.: New York, NY, USA, 1998{{page needed|date=February 2023}}</ref>

Due to their poor processability, conductive polymers have few large-scale applications. They have some promise in antistatic materials<ref name=Ullmann/> and have been built into commercial displays and batteries, but have had limits due to the production costs, material inconsistencies, toxicity, poor solubility in solvents, and inability to directly melt process. Nevertheless, conducting polymers are rapidly gaining attraction in new uses with increasingly processable materials with better electrical and physical properties and lower costs. With the availability of stable and reproducible dispersions, [[poly(3,4-ethylenedioxythiophene)]] (PEDOT) and [[polyaniline]] have gained some large-scale applications. While PEDOT is mainly used in antistatic applications and as a transparent conductive layer in the form of PEDOT and [[Sodium polystyrene sulfonate|polystyrene sulfonic acid]] (PSS, mixed form: PEDOT:PSS) dispersions, polyaniline is widely used to make printed circuit boards, in the final finish, to protect copper from corrosion and preventing its solderability.<ref name=nalwa>{{cite book |doi=10.1016/B978-012513760-7/50062-9 |chapter=Conductive polymers as organic nanometals |title=Handbook of Nanostructured Materials and Nanotechnology |year=2000 |last1=Wessling |first1=B. |volume=5 |pages=501–575 |isbn=978-0-12-513760-7 }}</ref> Newer nanostructured forms of conducting polymers provide fresh impetus to this field, with their higher surface area and better dispersability.

Recently supramolecular chemistry has been introduced to the field, which provide new opportunity for developing next generation of molecular electronics.<ref>{{cite journal |last1=Chen |first1=Hongliang |last2=Fraser Stoddart |first2=J. |title=From molecular to supramolecular electronics |journal=Nature Reviews Materials |date=September 2021 |volume=6 |issue=9 |pages=804–828 |doi=10.1038/s41578-021-00302-2 |bibcode=2021NatRM...6..804C |s2cid=232766622 }}</ref><ref>{{cite journal |last1=Yao |first1=Yifan |last2=Zhang |first2=Lei |last3=Orgiu |first3=Emanuele |last4=Samorì |first4=Paolo |title=Unconventional Nanofabrication for Supramolecular Electronics |journal=Advanced Materials |date=June 2019 |volume=31 |issue=23 |pages=1900599 |doi=10.1002/adma.201900599 |pmid=30941813 |bibcode=2019AdM....3100599Y |s2cid=205290060 |url=https://hal.archives-ouvertes.fr/hal-02130590/file/islandora_78509.pdf }}</ref> For example, two orders of magnitude current intensity enhancement was achieved by inserting cationic molecules into the cavity of pillar[5]arene.<ref>{{cite journal |last1=Li |first1=Xiaobing |last2=Zhou |first2=Siyuan |last3=Zhao |first3=Qi |last4=Chen |first4=Yi |last5=Qi |first5=Pan |last6=Zhang |first6=Yongkang |last7=Wang |first7=Lu |last8=Guo |first8=Cunlan |last9=Chen |first9=Shigui |title=Supramolecular Enhancement of Charge Transport through Pillar[5]arene-Based Self-Assembled Monolayers |journal=Angewandte Chemie International Edition |date=21 February 2023 |volume=62 |issue=19 |pages=e202216987 |doi=10.1002/anie.202216987 |pmid=36728903 |s2cid=256502098 }}</ref>

==See also==
{{colbegin}}
* [[Comparison of software for molecular mechanics modeling]]
* [[Molecular conductance]]
* [[Molecular wires]]
* [[Organic semiconductor]]
* [[Single-molecule magnet]]
* [[Spin transition]]
* [[Unimolecular rectifier]]
* [[Nanoelectronics]]
* [[Molecular scale electronics]]
* [[Mark Ratner]]
* [[Mark Reed (physicist)]]
* [[James Tour]]
* [[Supramolecular chemistry]]
* [[Supramolecular electronics]]
{{Portal|Electronics|Science|Technology
}}
{{colend}}

==References==
{{Reflist}}

==Further reading==
* {{cite journal |last1=Heath |first1=James R. |title=Molecular Electronics |journal=Annual Review of Materials Research |date=1 August 2009 |volume=39 |issue=1 |pages=1–23 |doi=10.1146/annurev-matsci-082908-145401 |bibcode=2009AnRMS..39....1H |url=https://authors.library.caltech.edu/16284/ }}

==External links==
*{{Commons category-inline}}

{{emerging technologies|topics=yes|electronics=yes}}
{{Electronic systems}}
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[[Category:Molecular electronics| ]]
[[Category:Nanoelectronics]]
[[Category:Organic polymers]]
[[Category:Organic semiconductors]]
[[Category:Conductive polymers]]