Molecular electronics
Encyclopedia
Molecular electronics, sometimes called moletronics, involves the study and application of molecular building blocks for the fabrication of electronic components. This includes both bulk applications of conductive polymers as well as single-molecule electronic components for nanotechnology
.
An interdisciplinary pursuit, molecular electronics spans physics, chemistry, and materials science. The unifying feature is the use of molecular building blocks for the fabrication of electronic components. This includes both passive (e.g. resistive wires) and active components such as transistors and molecular-scale switches. Due to the prospect of size reduction in electronics offered by molecular-level control of properties, molecular electronics has aroused much excitement both in science fiction and among scientists. Molecular electronics provides means to extend Moore's Law
beyond the foreseen limits of small-scale conventional silicon integrated circuits.
Molecular electronics is split into two related but separate subdisciplines: molecular materials for electronics utilizes the properties of the molecules to affect the bulk properties of a material, while molecular scale electronics focuses on single-molecule applications.
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.
Conventional electronics have traditionally been made from bulk materials. With the bulk approach having inherent limitations in addition to becoming increasingly demanding and expensive, the idea was born that the components could instead be built up atom for 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 utilized have properties that resemble traditional electronic components such as a wire
, transistor
or rectifier
.
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 limitations 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 obtaining 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. The miniaturization down to single molecules brings the scale down to a regime where quantum effects
are important. As opposed to the case in conventional electronic components, where electrons can be filled in or drawn out more or less like a continuous flow of 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.
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. 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. Another popular way to anchor molecules to the electrodes is to make use of sulfur
's high 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 has 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.
One of the biggest hindrances for single molecule electronics to be commercially exploited is the lack of techniques to connect a molecular sized circuit to bulk electrodes in a way that gives reproducible results. Also problematic is the fact that some measurements on single molecules are carried out in cryogenic temperatures
(close to absolute zero) which is very energy consuming.
Molecular materials for electronics is a term used to refer to bulk applications of conductive polymers. Conductive polymers or, more precisely, intrinsically conducting polymers (ICPs) are organic polymers that conduct electricity
in their bulk state. Such compounds may have metallic conductivity or can be semiconductor
s. The biggest advantage of conductive polymers is their processability, mainly by dispersion
. Conductive polymers are not plastic
s, i.e., they are not thermoformable, but they are organic polymers, like (insulating) polymers. They can offer high electrical conductivity but do not show mechanical properties as other commercially used polymers do. The electrical properties can be fine-tuned using the methods of organic synthesis
and by advanced dispersion techniques.
The linear-backbone "polymer blacks" (polyacetylene
, polypyrrole
, and polyaniline
) and their copolymers are the main class of conductive polymers. Historically, these are known as melanins. PPV and its soluble derivatives have similarly emerged as the prototypical electroluminescent semiconducting polymers. Today, poly(3-alkylthiophenes) are the archetypical materials for solar cells and transistors.
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 partially emptied. Despite intensive research, the relationship between morphology, chain structure and conductivity is poorly understood yet.
Due to their poor processability, conductive polymers enjoy few large-scale applications . They have some promise in antistatic materials and they have been incorporated into commercial displays and batteries, but there have had limitations due to the manufacturing costs, material inconsistencies, toxicity, poor solubility in solvents, and inability to directly melt process. Nevertheless, conducting polymers are rapidly gaining attraction in new applications with increasingly processable materials with better electrical and physical properties and lower costs. With the availability of stable and reproducible dispersions, PEDOT and polyaniline
have gained some large scale applications. While PEDOT (poly(3,4-ethylenedioxythiophene)
) is mainly used in antistatic applications and as a transparent conductive layer in form of PEDOT:PSS dispersions (PSS=polystyrene sulfonic acid
), polyaniline is widely used for printed circuit board manufacturing – in the final finish, for protecting copper from corrosion and preventing its solderability. The new nanostructured forms of conducting polymers particularly, provide fresh air to this field with their higher surface area and better dispersability.
Nanotechnology
Nanotechnology is the study of manipulating matter on an atomic and molecular scale. Generally, nanotechnology deals with developing materials, devices, or other structures possessing at least one dimension sized from 1 to 100 nanometres...
.
An interdisciplinary pursuit, molecular electronics spans physics, chemistry, and materials science. The unifying feature is the use of molecular building blocks for the fabrication of electronic components. This includes both passive (e.g. resistive wires) and active components such as transistors and molecular-scale switches. Due to the prospect of size reduction in electronics offered by molecular-level control of properties, molecular electronics has aroused much excitement both in science fiction and among scientists. Molecular electronics provides means to extend Moore's Law
Moore's Law
Moore's law describes a long-term trend in the history of computing hardware: the number of transistors that can be placed inexpensively on an integrated circuit doubles approximately every two years....
beyond the foreseen limits of small-scale conventional silicon integrated circuits.
Molecular electronics is split into two related but separate subdisciplines: molecular materials for electronics utilizes the properties of the molecules to affect the bulk properties of a material, while molecular scale electronics focuses on single-molecule applications.
Molecular scale electronics
Molecular scale electronics, also called single molecule electronics, is a branch of nanotechnologyNanotechnology
Nanotechnology is the study of manipulating matter on an atomic and molecular scale. Generally, nanotechnology deals with developing materials, devices, or other structures possessing at least one dimension sized from 1 to 100 nanometres...
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.
Conventional electronics have traditionally been made from bulk materials. With the bulk approach having inherent limitations in addition to becoming increasingly demanding and expensive, the idea was born that the components could instead be built up atom for atom in a chemistry lab (bottom up) as opposed to carving them out of bulk material (top down
Top Down
"Top Down" is a song by American hip hop record producer and recording artist Swizz Beatz. Featured as the eighth track on his debut studio album One Man Band Man...
). 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 utilized have properties that resemble traditional electronic components such as a wire
Wire
A wire is a single, usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various...
, transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
or rectifier
Rectifier
A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
.
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 limitations 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 obtaining reliable and reproducible contacts between the molecular components and the bulk material of the electrodes.
Molecular electronics operates in the quantum realm
Quantum realm
Quantum realm is a term of art in physics referring to scales where quantum mechanical effects become important . Typically, this means distances of 100 nanometers or less. Not coincidentally, this is the same scale as nanotechnology....
of distances less than 100 nanometers. The miniaturization down to single molecules brings the scale down to a regime where quantum effects
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
are important. As opposed to the case in conventional electronic components, where electrons can be filled in or drawn out more or less like a continuous flow of charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
, 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.
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. Another method is to use the tip of a scanning tunneling microscope
Scanning tunneling microscope
A scanning tunneling microscope is an instrument for imaging surfaces at the atomic level. Its development in 1981 earned its inventors, Gerd Binnig and Heinrich Rohrer , the Nobel Prize in Physics in 1986. For an STM, good resolution is considered to be 0.1 nm lateral resolution and...
(STM) to contact molecules adhered at the other end to a metal substrate. Another popular way to anchor molecules to the electrodes is to make use of sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
's high affinity
Chemical affinity
In chemical physics and physical chemistry, chemical affinity is the electronic property by which dissimilar chemical species are capable of forming chemical compounds...
to gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
; though useful, the anchoring is non-specific and thus anchors the molecules randomly to all gold surfaces, and the contact resistance
Contact resistance
The term contact resistance refers to the contribution to the total resistance of a material which comes from the electrical leads and connections as opposed to the intrinsic resistance, which is an inherent property, independent of the measurement method...
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 has 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.
One of the biggest hindrances for single molecule electronics to be commercially exploited is the lack of techniques to connect a molecular sized circuit to bulk electrodes in a way that gives reproducible results. Also problematic is the fact that some measurements on single molecules are carried out in cryogenic temperatures
Cryogenics
In physics, cryogenics is the study of the production of very low temperature and the behavior of materials at those temperatures. A person who studies elements under extremely cold temperature is called a cryogenicist. Rather than the relative temperature scales of Celsius and Fahrenheit,...
(close to absolute zero) which is very energy consuming.
Molecular materials for electronics
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
in their bulk state. Such compounds may have metallic conductivity or can be semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
s. The biggest advantage of conductive polymers is their processability, mainly by dispersion
Dispersion (chemistry)
A dispersion is a system in which particles are dispersed in a continuous phase of a different composition . See also emulsion. A dispersion is classified in a number of different ways, including how large the particles are in relation to the particles of the continuous phase, whether or not...
. Conductive polymers are not plastic
Plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...
s, i.e., they are not thermoformable, but they are organic polymers, like (insulating) polymers. They can offer high electrical conductivity but do not show mechanical properties as other commercially used polymers do. The electrical properties can be fine-tuned using the methods of organic synthesis
Organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
and by advanced dispersion techniques.
The linear-backbone "polymer blacks" (polyacetylene
Polyacetylene
Polyacetylene is an organic polymer with the repeat unit n. The high electrical conductivity discovered for these polymers beginning in the 1960's accelerated interest in the use of organic compounds in microelectronics...
, polypyrrole
Polypyrrole
Polypyrrole is a chemical compound formed from a number of connected pyrrole ring structures. For example a tetrapyrrole is a compound with four pyrrole rings connected. Methine-bridged cyclic tetrapyrroles are called porphyrins. Polypyrroles are conducting polymers of the rigid-rod polymer host...
, and polyaniline
Polyaniline
Polyaniline is a conducting polymer of the semi-flexible rod polymer family. Although the compound itself was discovered over 150 years ago, only since the early 1980s has polyaniline captured the intense attention of the scientific community. This is due to the rediscovery of its high electrical...
) and their copolymers are the main class of conductive polymers. Historically, these are known as melanins. PPV and its soluble derivatives have similarly emerged as the prototypical electroluminescent semiconducting polymers. Today, poly(3-alkylthiophenes) are the archetypical materials for solar cells and transistors.
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
Conjugated system
In chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in compounds with alternating single and multiple bonds, which in general may lower the overall energy of the molecule and increase stability. Lone pairs, radicals or carbenium ions may be part of the...
form a one-dimensional electronic band
Electronic band structure
In solid-state physics, the electronic band structure of a solid describes those ranges of energy an electron is "forbidden" or "allowed" to have. Band structure derives from the diffraction of the quantum mechanical electron waves in a periodic crystal lattice with a specific crystal system and...
, and the electrons within this band become mobile when it is partially emptied. Despite intensive research, the relationship between morphology, chain structure and conductivity is poorly understood yet.
Due to their poor processability, conductive polymers enjoy few large-scale applications . They have some promise in antistatic materials and they have been incorporated into commercial displays and batteries, but there have had limitations due to the manufacturing costs, material inconsistencies, toxicity, poor solubility in solvents, and inability to directly melt process. Nevertheless, conducting polymers are rapidly gaining attraction in new applications with increasingly processable materials with better electrical and physical properties and lower costs. With the availability of stable and reproducible dispersions, PEDOT and polyaniline
Polyaniline
Polyaniline is a conducting polymer of the semi-flexible rod polymer family. Although the compound itself was discovered over 150 years ago, only since the early 1980s has polyaniline captured the intense attention of the scientific community. This is due to the rediscovery of its high electrical...
have gained some large scale applications. While PEDOT (poly(3,4-ethylenedioxythiophene)
Poly(3,4-ethylenedioxythiophene)
Poly or PEDOT is a conducting polymer based on 3,4-ethylenedioxylthiophene or EDOT monomer. Advantages of this polymer are optical transparency in its conducting state, high stability and moderate band gap and low redox potential...
) is mainly used in antistatic applications and as a transparent conductive layer in form of PEDOT:PSS dispersions (PSS=polystyrene sulfonic acid
Sodium polystyrene sulfonate
Sodium polystyrene sulfonate is a type of polymer and ionomer based on polystyrene. It is the sodium salt of polystyrene sulfonic acid.-Chemical properties:...
), polyaniline is widely used for printed circuit board manufacturing – in the final finish, for protecting copper from corrosion and preventing its solderability. The new nanostructured forms of conducting polymers particularly, provide fresh air to this field with their higher surface area and better dispersability.
See also
- Software for molecular modeling
- Molecular conductanceMolecular conductanceMolecular Conductance , or the conductance of a single molecule, is a physical quantity in molecular electronics. Molecular conductance is dependent on the surrounding conditions , as well as the properties of measuring device...
- Molecular wiresMolecular wiresMolecular wires are molecular-scale objects which conduct electrical current. They are the fundamental building blocks for molecularelectronic devices...
- Organic Semiconductors
- Single-molecule magnetSingle-molecule magnetSingle-molecule magnets or SMMs are a class of metalorganic compounds, that show superparamagnetic behavior below a certain blocking temperature at the molecular scale. In this temperature range, SMMs exhibit magnetic hysteresis of purely molecular origin...
- Spin transitionSpin transitionThe spin transition is an example of transition between two electronic states in molecular chemistry. The ability of an electron to transit from a stable to another stable electronic state in a reversible and detectable fashion, makes these molecular systems appealing in the field of molecular...