Molecular electronics

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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. The unifying feature is use of molecular building blocks to fabricate electronic components. Due to the prospect of size reduction in electronics offered by molecular-level control of properties, molecular electronics has generated much excitement. It provides a potential means to extend Moore's Law beyond the foreseen limits of small-scale conventional silicon integrated circuits. [1]

Contents

Molecular scale electronics

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 components. Because single molecules constitute the smallest stable structures possible, this miniaturization is the ultimate goal for shrinking electrical circuits.

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. That is, instead of creating structures by removing or applying material after a pattern scaffold, the atoms are put together in a chemistry lab. The molecules used have properties that resemble traditional electronic components such as a wire, transistor, or rectifier. This concept of using a molecule as a traditional electronic component was first presented by Aviram and Ratner in 1974, when they proposed a theoretical molecular rectifier composed of donor and acceptor sites which are insulated from one another. [2]

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 in the quantum realm of 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 electrons 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.

Graphical representation of a rotaxane, useful as a molecular switch Rotaxane cartoon.jpg
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 put into use. These include molecular-sized gaps called break junctions, in which a thin electrode is stretched until it breaks. One of the way to over come the gap size issue is by trapping molecular functionalized nanoparticles (internanoparticle spacing is match able to the size of molecules) and later target molecule by place exchange reaction. [3] 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. [4] 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. [5] 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 nanowires with an embedded segment of the wider bandgap material indium phosphide used as an electronic barrier to be bridged by molecules. [6]

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 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, [7] 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. [8] 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

Chemical structures of some conductive polymers. From top left clockwise: polyacetylene; polyphenylene vinylene; polypyrrole (X = NH) and polythiophene (X = S); and polyaniline (X = NH/N) and polyphenylene sulfide (X = S). ConductivePoly.png
Chemical structures of some conductive polymers. From top left clockwise: polyacetylene; polyphenylene vinylene; polypyrrole (X = NH) and polythiophene (X = S); and polyaniline (X = NH/N) and polyphenylene sulfide (X = S).

The biggest advantage of conductive polymers is their processability, mainly by dispersion. Conductive polymers are not plastics, 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 [9] and of advanced dispersion. [10]

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 cells and transistors. [9]

Conducting polymers have backbones of contiguous sp2 hybridized carbon centers. One valence electron on each center resides in a pz 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 p-orbitals form a one-dimensional 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. [11]

Due to their poor processability, conductive polymers have few large-scale applications. They have some promise in antistatic materials [9] 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 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. [10] 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. [12] [13] For example, two orders of magnitude current intensity enhancement was achieved by inserting cationic molecules into the cavity of pillar[5]arene. [14]

See also

Related Research Articles

<span class="mw-page-title-main">Organic electronics</span> Field of materials science

Organic electronics is a field of materials science concerning the design, synthesis, characterization, and application of organic molecules or polymers that show desirable electronic properties such as conductivity. Unlike conventional inorganic conductors and semiconductors, organic electronic materials are constructed from organic (carbon-based) molecules or polymers using synthetic strategies developed in the context of organic chemistry and polymer chemistry.

Indium tin oxide (ITO) is a ternary composition of indium, tin and oxygen in varying proportions. Depending on the oxygen content, it can be described as either a ceramic or an alloy. Indium tin oxide is typically encountered as an oxygen-saturated composition with a formulation of 74% In, 8% Sn, and 18% O by weight. Oxygen-saturated compositions are so typical that unsaturated compositions are termed oxygen-deficient ITO. It is transparent and colorless in thin layers, while in bulk form it is yellowish to gray. In the infrared region of the spectrum it acts as a metal-like mirror.

<span class="mw-page-title-main">Conductive polymer</span> Organic polymers that conduct electricity

Conductive polymers or, more precisely, intrinsically conducting polymers (ICPs) are organic polymers that conduct electricity. Such compounds may have metallic conductivity or can be semiconductors. The main advantage of conductive polymers is that they are easy to process, mainly by dispersion. Conductive polymers are generally not thermoplastics, i.e., they are not thermoformable. But, like insulating polymers, they are organic materials. They can offer high electrical conductivity but do not show similar mechanical properties to other commercially available polymers. The electrical properties can be fine-tuned using the methods of organic synthesis and by advanced dispersion techniques.

Organic semiconductors are solids whose building blocks are pi-bonded molecules or polymers made up by carbon and hydrogen atoms and – at times – heteroatoms such as nitrogen, sulfur and oxygen. They exist in the form of molecular crystals or amorphous thin films. In general, they are electrical insulators, but become semiconducting when charges are either injected from appropriate electrodes, upon doping or by photoexcitation.

<span class="mw-page-title-main">Supramolecular electronics</span> Field of chemistry

Supramolecular electronics is the experimental field of supramolecular chemistry that bridges the gap between molecular electronics and bulk plastics in the construction of electronic circuitry at the nanoscale. In supramolecular electronics, assemblies of pi-conjugated systems on the 5 to 100 nanometer scale are prepared by molecular self-assembly with the aim to fit these structures between electrodes. With single molecules as researched in molecular electronics at the 5 nanometer scale this would be impractical. Nanofibers can be prepared from polymers such as polyaniline and polyacetylene. Chiral oligo(p-phenylenevinylene)s self-assemble in a controlled fashion into (helical) wires. An example of actively researched compounds in this field are certain coronenes.

<span class="mw-page-title-main">PEDOT:PSS</span> Polymer

poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is a polymer mixture of two ionomers. One component in this mixture is made up of polystyrene sulfonate which is a sulfonated polystyrene. Part of the sulfonyl groups are deprotonated and carry a negative charge. The other component poly(3,4-ethylenedioxythiophene) (PEDOT) is a conjugated polymer and carries positive charges and is based on polythiophene. Together the charged macromolecules form a macromolecular salt.

<span class="mw-page-title-main">Unimolecular rectifier</span>

A unimolecular rectifier is a single organic molecule which functions as a rectifier of electric current. The idea was first proposed in 1974 by Arieh Aviram, then at IBM, and Mark Ratner, then at New York University. Their publication was the first serious and concrete theoretical proposal in the new field of molecular electronics (UE). Based on the mesomeric effect of certain chemical compounds on organic molecules, a molecular rectifier was built by simulating the pn junction with the help of chemical compounds.

Nanoelectronics refers to the use of nanotechnology in electronic components. The term covers a diverse set of devices and materials, with the common characteristic that they are so small that inter-atomic interactions and quantum mechanical properties need to be studied extensively. Some of these candidates include: hybrid molecular/semiconductor electronics, one-dimensional nanotubes/nanowires or advanced molecular electronics.

A polymer-based battery uses organic materials instead of bulk metals to form a battery. Currently accepted metal-based batteries pose many challenges due to limited resources, negative environmental impact, and the approaching limit of progress. Redox active polymers are attractive options for electrodes in batteries due to their synthetic availability, high-capacity, flexibility, light weight, low cost, and low toxicity. Recent studies have explored how to increase efficiency and reduce challenges to push polymeric active materials further towards practicality in batteries. Many types of polymers are being explored, including conductive, non-conductive, and radical polymers. Batteries with a combination of electrodes are easier to test and compare to current metal-based batteries, however batteries with both a polymer cathode and anode are also a current research focus. Polymer-based batteries, including metal/polymer electrode combinations, should be distinguished from metal-polymer batteries, such as a lithium polymer battery, which most often involve a polymeric electrolyte, as opposed to polymeric active materials.

Molecular wires are molecular chains that conduct electric current. They are the proposed building blocks for molecular electronic devices. Their typical diameters are less than three nanometers, while their lengths may be macroscopic, extending to centimeters or more.

<span class="mw-page-title-main">PEDOT-TMA</span> Chemical compound

Poly(3,4-ethylenedioxythiophene)-tetramethacrylate or PEDOT-TMA is a p-type conducting polymer based on 3,4-ethylenedioxylthiophene or the EDOT monomer. It is a modification of the PEDOT structure. Advantages of this polymer relative to PEDOT are that it is dispersible in organic solvents, and it is non-corrosive. PEDOT-TMA was developed under a contract with the National Science Foundation, and it was first announced publicly on April 12, 2004. The trade name for PEDOT-TMA is Oligotron. PEDOT-TMA was featured in an article entitled "Next Stretch for Plastic Electronics" that appeared in Scientific American in 2004. The U.S. Patent office issued a patent protecting PEDOT-TMA on April 22, 2008.

Organic photovoltaic devices (OPVs) are fabricated from thin films of organic semiconductors, such as polymers and small-molecule compounds, and are typically on the order of 100 nm thick. Because polymer based OPVs can be made using a coating process such as spin coating or inkjet printing, they are an attractive option for inexpensively covering large areas as well as flexible plastic surfaces. A promising low cost alternative to conventional solar cells made of crystalline silicon, there is a large amount of research being dedicated throughout industry and academia towards developing OPVs and increasing their power conversion efficiency.

<span class="mw-page-title-main">Organic solar cell</span> Type of photovoltaic

An organic solar cell (OSC) or plastic solar cell is a type of photovoltaic that uses organic electronics, a branch of electronics that deals with conductive organic polymers or small organic molecules, for light absorption and charge transport to produce electricity from sunlight by the photovoltaic effect. Most organic photovoltaic cells are polymer solar cells.

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 components. Because single molecules constitute the smallest stable structures imaginable, this miniaturization is the ultimate goal for shrinking electrical circuits.

<span class="mw-page-title-main">Transparent conducting film</span> Optically transparent and electrically conductive material

Transparent conducting films (TCFs) are thin films of optically transparent and electrically conductive material. They are an important component in a number of electronic devices including liquid-crystal displays, OLEDs, touchscreens and photovoltaics. While indium tin oxide (ITO) is the most widely used, alternatives include wider-spectrum transparent conductive oxides (TCOs), conductive polymers, metal grids and random metallic networks, carbon nanotubes (CNT), graphene, nanowire meshes and ultra thin metal films.

<span class="mw-page-title-main">Mark Ratner</span> American physical chemist

Mark A. Ratner is an American chemist and professor emeritus at Northwestern University whose work focuses on the interplay between molecular structure and molecular properties. He is widely credited as the "father of molecular-scale electronics" thanks to his groundbreaking work with Arieh Aviram in 1974 that first envisioned how electronic circuit elements might be constructed from single molecules and how these circuits might behave.

<span class="mw-page-title-main">Photoconductive atomic force microscopy</span> Type of atomic force microscopy

Photoconductive atomic force microscopy (PC-AFM) is a variant of atomic force microscopy that measures photoconductivity in addition to surface forces.

A chemically modified electrode is an electrical conductor that has its surface modified for different electrochemical functions. Chemically modified electrodes are made using advanced approaches to electrode systems by adding a thin film or layer of certain chemicals to change properties of the conductor according to its targeted function.

<span class="mw-page-title-main">Pseudocapacitance</span> Storage of electricity within an electrochemical cell

Pseudocapacitance is the electrochemical storage of electricity in an electrochemical capacitor known as a pseudocapacitor. This faradaic charge transfer originates by a very fast sequence of reversible faradaic redox, electrosorption or intercalation processes on the surface of suitable electrodes. Pseudocapacitance is accompanied by an electron charge-transfer between electrolyte and electrode coming from a de-solvated and adsorbed ion. One electron per charge unit is involved. The adsorbed ion has no chemical reaction with the atoms of the electrode since only a charge-transfer takes place.

<span class="mw-page-title-main">Chemiresistor</span>

A chemiresistor is a material that changes its electrical resistance in response to changes in the nearby chemical environment. Chemiresistors are a class of chemical sensors that rely on the direct chemical interaction between the sensing material and the analyte. The sensing material and the analyte can interact by covalent bonding, hydrogen bonding, or molecular recognition. Several different materials have chemiresistor properties: metal-oxide semiconductors, some conductive polymers, and nanomaterials like graphene, carbon nanotubes and nanoparticles. Typically these materials are used as partially selective sensors in devices like electronic tongues or electronic noses.

References

  1. ↑ Petty, M.C.; Bryce, M.R. & Bloor, D. (1995). Introduction to Molecular Electronics. New York: Oxford University Press. pp. 1–25. ISBN   0-19-521156-1.
  2. ↑ Aviram, Arieh; Ratner, Mark A. (15 November 1974). "Molecular rectifiers". Chemical Physics Letters. 29 (2): 277–283. Bibcode:1974CPL....29..277A. doi:10.1016/0009-2614(74)85031-1.
  3. ↑ Jafri, S H M; Blom, T; Leifer, K; Strømme, M; Löfås, H; Grigoriev, A; Ahuja, R; Welch, K (29 October 2010). "Assessment of a nanoparticle bridge platform for molecular electronics measurements". Nanotechnology. 21 (43): 435204. Bibcode:2010Nanot..21Q5204J. doi:10.1088/0957-4484/21/43/435204. PMID   20890018. S2CID   29398313.
  4. ↑ Gimzewski, J.K.; Joachim, C. (1999). "Nanoscale science of single molecules using local probes". Science. 283 (5408): 1683–1688. Bibcode:1999Sci...283.1683G. doi:10.1126/science.283.5408.1683. PMID   10073926.
  5. ↑ Sørensen, J.K. Archived 2016-03-29 at the Wayback Machine . (2006). "Synthesis of new components, functionalized with (60)fullerene, for molecular electronics". 4th Annual meeting - CONT 2006, University of Copenhagen.
  6. ↑ Schukfeh, Muhammed Ihab; Storm, Kristian; Mahmoud, Ahmad; Søndergaard, Roar R.; Szwajca, Anna; Hansen, Allan; Hinze, Peter; Weimann, Thomas; Fahlvik Svensson, Sofia; Bora, Achyut; Dick, Kimberly A.; Thelander, Claes; Krebs, Frederik C.; Lugli, Paolo; Samuelson, Lars; Tornow, Marc (2013). "Conductance Enhancement of InAs/InP Heterostructure Nanowires by Surface Functionalization with Oligo(phenylene vinylene)s". ACS Nano. 7 (5): 4111–4118. doi:10.1021/nn400380g. PMID   23631558.
  7. ↑ Von Hippel, Arthur R.; Landshoff, Rolf (October 1959). "Molecular Science and Molecular Engineering". Physics Today. 12 (10): 48. Bibcode:1959PhT....12j..48V. doi:10.1063/1.3060522.
  8. ↑ Aviram, Arieh; Ratner, Mark A. (November 1974). "Molecular rectifiers". Chemical Physics Letters. 29 (2): 277–283. Bibcode:1974CPL....29..277A. doi:10.1016/0009-2614(74)85031-1.
  9. 1 2 3 Naarmann, Herbert (2000). "Polymers, Electrically Conducting". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a21_429. ISBN   978-3-527-30673-2.
  10. 1 2 Wessling, B. (2000). "Conductive polymers as organic nanometals". Handbook of Nanostructured Materials and Nanotechnology. Vol. 5. pp. 501–575. doi:10.1016/B978-012513760-7/50062-9. ISBN   978-0-12-513760-7.
  11. ↑ Skotheim, T., Elsenbaumer, R., Reynolds, J., Eds.; Handbook of Conducting Polymers, 2nd ed.; Marcel Dekker, Inc.: New York, NY, USA, 1998[ page needed ]
  12. ↑ Chen, Hongliang; Fraser Stoddart, J. (September 2021). "From molecular to supramolecular electronics". Nature Reviews Materials. 6 (9): 804–828. Bibcode:2021NatRM...6..804C. doi:10.1038/s41578-021-00302-2. S2CID   232766622.
  13. ↑ Yao, Yifan; Zhang, Lei; Orgiu, Emanuele; Samorì, Paolo (June 2019). "Unconventional Nanofabrication for Supramolecular Electronics" (PDF). Advanced Materials. 31 (23): 1900599. Bibcode:2019AdM....3100599Y. doi:10.1002/adma.201900599. PMID   30941813. S2CID   205290060.
  14. ↑ Li, Xiaobing; Zhou, Siyuan; Zhao, Qi; Chen, Yi; Qi, Pan; Zhang, Yongkang; Wang, Lu; Guo, Cunlan; Chen, Shigui (21 February 2023). "Supramolecular Enhancement of Charge Transport through Pillar[5]arene-Based Self-Assembled Monolayers". Angewandte Chemie International Edition. 62 (19): e202216987. doi:10.1002/anie.202216987. PMID   36728903. S2CID   256502098.

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