Single molecule electronics
Encyclopedia
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.
The field is often referred to as simply "molecular electronics
", but this term is also used to refer to the distantly related field of conductive polymers and organic electronics
, which uses the properties of molecules to affect the bulk properties of a material. A nomenclature distinction has been suggested so that molecular materials for electronics refers to this latter field of bulk applications, while molecular scale electronics refers to the nanoscale single-molecule applications discussed here.
(a trend also known as Moore’s law) and has forced the feature sizes of the embedded components to shrink accordingly. As the structures become smaller, the sensitivity for deviations increases and in a few generations, when the minimum feature sizes reaches 13 nm, the composition of the devices must be controlled to a precision of a few atoms
for the devices to work. 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
). This idea is the reasoning behind molecular electronics with the ultimate miniaturization being components contained in single molecules.
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. This way billions of billions of copies are made simultaneously (typically more than 10 20 molecules are made at once) while the composition of molecules are controlled down to the last atom. 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.
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. This means that when an electron has been transferred from the source electrode to the molecule, the molecule gets charged up and makes it much harder for the next one to transfer (see also Coulomb blockade
). The significant amount of energy due to charging must be taken into account when making calculations about the electronic properties of the setup and is highly sensitive to distances to conducting surfaces nearby.
The theory of single molecule devices is particularly interesting since the system under consideration is an open quantum system in nonequilibrium (driven by voltage). In the low bias voltage regime, the nonequilibrium nature of the molecular junction can be ignored, and the current-voltage characteristics of the device can be calculated using the equilibrium electronic structure of the system. However, in stronger bias regimes a more sophisticated treatment is required, as there is no longer a variational principle
. In the elastic tunneling case (where the passing electron does not exchange energy with the system), the formalism of Rolf Landauer
can be used to calculate the transmission through the system as a function of bias voltage, and hence the current. In inelastic tunneling, an elegant formalism based on the non-equilibrium Green's functions of Leo Kadanoff
and Gordon Baym
, and independently by Leonid Keldysh was put forth by Ned Wingreen and Yigal Meir. This Meir-Wingreen formulation has been used to great success in the molecular electronics community to examine the more difficult and interesting cases where the transient electron exchanges energy with the molecular system (for example through electron-phonon coupling or electronic excitations).
Further, connecting single molecules reliably to a larger scale circuit has proven a great challenge, and constitutes a significant hindrance to commercialization.
). The reason for this is that such a pattern delocalizes the molecular orbitals making it possible for electrons to move freely over the conjugated area.
The sole purpose of molecular wires
is to electrically connect different parts of a molecular electrical circuit. As the assembly of these and their connection to a macroscopic circuit is still not mastered, the focus of research in single molecule electronics is primarily on the functionalized molecules: molecular wires are characterized by containing no functional groups and are hence composed of plain repetitions of a conjugated building block. Among these are the carbon nanotubes that are quite large compared to the other suggestions but have shown very promising electrical properties.
The main problem with the molecular wires is to obtain good electrical contact with the electrodes so that the electrons can move freely in and out of the wire.
It is the quantization of charge into electrons that is responsible for the markedly different behavior compared to bulk electronics. Because of the size of a single molecule, the charging due to a single electron is significant and provides a mean to turn the transistor ON or OFF (see Coulomb blockade
). For this to work, the electronic orbitals on the transistor molecule cannot be too well integrated with the orbitals on the electrodes. If they are, an electron cannot be said to be located on the molecule or the electrodes and the molecule will function as a wire.
A popular group of molecules, that can work as the semiconducting channel material in a molecular transistor, is the oligopolyphenylenevinylenes (OPVs) that works by the Coulomb blockade mechanism when placed between the source and drain electrode in an appropriate way. Fullerenes work by the same mechanism and have also been commonly utilized.
Semiconducting carbon nanotubes have also been demonstrated to work as channel material but although molecular, these molecules are sufficiently large to behave almost as bulk semiconductors.
The size of the molecules and the low temperature the measurements are being conducted at makes the quantum mechanical states well defined. It is therefore being researched if the quantum mechanical properties can be used for more advanced purposes than simple transistors (e.g. spintronics
).
Physicists at the University of Arizona, in collaboration with chemists from the University of Madrid, have designed a single molecule transistor using a ring-shaped molecule similar to benzene
. Physicists at Canada's National Institute for Nanotechnology have designed a single-molecule transistor using styrene.
Both groups expect (their designs have yet to be experimentally verified) their respective devices to function at room temperature, and to be controlled by a single electron.
(D) in one end and an electron acceptor
(A) in the other. This way, the unstable state D+ – A- will be more readily made than D- – A+. The result is that an electric current
can be drawn through the molecule if the electrons are added through the acceptor end, but not so easily if the reverse is attempted. An example of a molecular rectifier was made by Geoffrey J. Ashwell's Ph.D. students.
. Here a current is lead through a thin wire until it melts and the atoms migrate to produce the gap. Further, the reach of conventional photolithography can be enhanced by chemically etching or depositing metal on the electrodes.
Probably the easiest way to conduct measurements on several molecules is to use the tip of a scanning tunneling microscope
(STM) to contact molecules adhered at the other end to a metal substrate.
high affinity
to gold
. In these setups, the molecules are synthesized
so that sulfur atoms are placed strategically to function as crocodile clips connecting the molecules to the gold electrodes. Though useful, the anchoring is non-specific and thus anchors the molecules randomly to all gold surfaces. Further, 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.
is a semi-metal, a category in between metals and semi-conductors. It has a layered structure, each sheet being one atom thick. Between each sheet, the interactions are weak enough to allow an easy manual cleavage.
Tailoring the graphite
sheet to obtain well defined nanometer-sized objects remains a challenge. However, by the close of the twentieth century, chemists were exploring methods to fabricate extremely small graphitic objects that could be considered single molecules. After studying the interstellar conditions under which carbon is known to form clusters, Richard Smalley's
group (Rice University, Texas) set up an experiment in which graphite was vaporized using laser irradiation. Mass spectrometry revealed that clusters containing specific "magic numbers" of atoms were stable, in particular those clusters of 60 atoms. Harry Kroto, an English chemist who assisted in the experiment, suggested a possible geometry for these clusters – atoms covalently bound with the exact symmetry of a soccer ball. Coined buckminsterfullerenes, buckyballs or C60
, the clusters retained some properties of graphite, such as conductivity. These objects were rapidly envisioned as possible building blocks for molecular electronics.
Applying a voltage drop in the order of volts across a nanometer sized junction results in a very strong electrical field. The field can cause metal atoms to migrate and eventually close the gap by a thin filament, which can be broken again when carrying a current. The two levels of conductance imitate molecular switching between a conductive and an isolating state of a molecule.
Another encountered artifact is when the electrodes undergo chemical reactions due to the high field strength in the gap. When the bias is reversed the reaction will cause hysteresis
in the measurements that can be interpreted as being of molecular origin.
A metallic grain between the electrodes can act as a single electron transistor by the mechanism described above thus resembling the characteristics of a molecular transistor. This artifact is especially common with nanogaps produced by the electromigration technique.
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. This is done to reduce signal noise enough to measure the faint currents of single molecules.
advanced the concept of charge transfer in molecules. They subsequently further refined the study of both charge transfer and energy transfer in molecules. Likewise, a 1974 paper from Mark Ratner
and Ari Aviram 1 illustrated a theoretical molecular rectifier
. In 1988, Aviram described in detail a theoretical single-molecule field-effect transistor
. Further concepts were p roposed by Forrest Carter of the Naval Research Laboratory, including single-molecule logic gate
s. A wide range of ideas were presented, under his aegis, at a conference entitled Molecular Electronic Devices in 1988. These were all theoretical constructs and not concrete devices. The direct measurement of the electronic characteristics of individual molecules awaited the development of methods for making molecular-scale electrical contacts. This was no easy task. Thus, the first experiment directly-measuring the conductance of a single molecule was only reported in 1997 by Mark Reed and co-workers. Since then, this branch of the field has progressed rapidly. Likewise, as it has become possible to measure such properties directly, the theoretical predictions of the early workers have been substantially confirmed.
Recent progress in nanotechnology
and nanoscience has facilitated both experimental and theoretical study of molecular electronics. In particular, the development of the scanning tunneling microscope
(STM) and later the atomic force microscope
(AFM) have facilitated manipulation of single-molecule electronics. In addition, theoretical advances in molecular electronics have facilitated further understanding of non-adibatic charge transfer events at electrode-electrolyte interfaces.
The concept of molecular electronics was first published in 1974 when Aviram and Ratner suggested an organic molecule that could work as a rectifier. Having both huge commercial and fundamental interest much effort was put into proving its feasibility and 16 years later in 1990 the first demonstration of an intrinsic molecular rectifier was realized by Ashwell and coworkers for a thin film of molecules.
The first measurement of the conductance of a single molecule was realised in 1994 by C. Joachim and J. K. Gimzewski and published in 1995 (see the corresponding Phys. Rev. Lett. paper). This was the conclusion of 10 years of research started at IBM TJ Watson, using the scanning tunnelling microscope tip apex to switch a single molecule as already explored by A. Aviram, C. Joachim and M. Pomerantz at the end of the 80's (see their seminal Chem. Phys. Lett. paper during this period). The trick was to use an UHV Scanning Tunneling microscope to allow the tip apex to gently touch the top of a single molecule adsorbed on a Au(110) surface. A resistance of 55 MOhms was recorded together with a low voltage linear I-V. The contact was certified by recording the I-z current distance characteristic, which allows the measurement of the deformation of the cage under contact. This first experiment was followed by the reported result using a mechanical break junction approach to connect two gold electrodes to a sulfur-terminated molecular wire by Mark Reed and James Tour
in 1997.
A single-molecule amplifier was implemented by C. Joachim and J.K. Gimzewski in IBM Zurich. This
experiment involving a single molecule demonstrated that a single molecule can provide gain in a circuit just by playing with through intramolecular quantum interference effects.
A collaboration of researchers at HP
and UCLA, led by James Heath, Fraser Stoddart, R. Stanley Williams, and Philip Kuekes, has developed molecular electronics based on rotaxane
s and catenane
s.
Work is also being done on the use of single-wall carbon nanotubes as field-effect transistors. Most of this work is being done by IBM.
Some specific reports of a field-effect transistor
based on molecular self-assembled monolayer
s were shown to be fraudulent in 2002 as part of the Schön scandal.
Until recently entirely theoretical, the Aviram-Ratner model for a unimolecular rectifier has been unambiguously-confirmed in experiments by a group led by Geoffrey J. Ashwell at Bangor University
, UK. Many rectifying molecules have so far been identified, and the number and efficiency of these systems is expanding rapidly.
Supramolecular electronics
is a new field that tackles electronics at a supramolecular
level.
An important issue in molecular electronics is the determination of the resistance of a single molecule (both theoretical and experimental). For example, Bumm, et al. used STM to analyze a single molecular switch in a self-assembled monolayer
to determine how conductive such a molecule can be. Another problem faced by this field is the difficulty of performing direct characterization since imaging at the molecular scale is often difficult in many experimental devices.
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...
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.
The field is often referred to as simply "molecular electronics
Molecular electronics
Molecular electronics, sometimes called moletronics, involves the study and application of molecular building blocks for the fabrication of electronic components...
", but this term is also used to refer to the distantly related field of conductive polymers and organic electronics
Organic electronics
Organic electronics, plastic electronics or polymer electronics, is a branch of electronics dealing with conductive polymers, plastics, or small molecules. It is called 'organic' electronics because the polymers and small molecules are carbon-based...
, which uses the properties of molecules to affect the bulk properties of a material. A nomenclature distinction has been suggested so that molecular materials for electronics refers to this latter field of bulk applications, while molecular scale electronics refers to the nanoscale single-molecule applications discussed here.
Fundamental concepts
Conventional electronics have traditionally been made from bulk materials. Ever since its invention in 1958 the performance and complexity of integrated circuits has been growing exponentiallyExponential growth
Exponential growth occurs when the growth rate of a mathematical function is proportional to the function's current value...
(a trend also known as Moore’s law) and has forced the feature sizes of the embedded components to shrink accordingly. As the structures become smaller, the sensitivity for deviations increases and in a few generations, when the minimum feature sizes reaches 13 nm, the composition of the devices must be controlled to a precision of a few atoms
for the devices to work. 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...
). This idea is the reasoning behind molecular electronics with the ultimate miniaturization being components contained in single molecules.
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. This way billions of billions of copies are made simultaneously (typically more than 10 20 molecules are made at once) while the composition of molecules are controlled down to the last atom. 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.
Theoretical basis
Molecular electronics operates in the quantum realmQuantum 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. This means that when an electron has been transferred from the source electrode to the molecule, the molecule gets charged up and makes it much harder for the next one to transfer (see also Coulomb blockade
Coulomb blockade
In physics, a Coulomb blockade , named after Charles-Augustin de Coulomb's electrical force, is the increased resistance at small bias voltages of an electronic device comprising at least one low-capacitance tunnel junction. Because of the CB, the resistances of devices are not constant at low bias...
). The significant amount of energy due to charging must be taken into account when making calculations about the electronic properties of the setup and is highly sensitive to distances to conducting surfaces nearby.
The theory of single molecule devices is particularly interesting since the system under consideration is an open quantum system in nonequilibrium (driven by voltage). In the low bias voltage regime, the nonequilibrium nature of the molecular junction can be ignored, and the current-voltage characteristics of the device can be calculated using the equilibrium electronic structure of the system. However, in stronger bias regimes a more sophisticated treatment is required, as there is no longer a variational principle
Variational principle
A variational principle is a scientific principle used within the calculus of variations, which develops general methods for finding functions which minimize or maximize the value of quantities that depend upon those functions...
. In the elastic tunneling case (where the passing electron does not exchange energy with the system), the formalism of Rolf Landauer
Rolf Landauer
Rolf William Landauer was an IBM physicist who in 1961 argued that when information is lost in an irreversible circuit, the information becomes entropy and an associated amount of energy is dissipated as heat...
can be used to calculate the transmission through the system as a function of bias voltage, and hence the current. In inelastic tunneling, an elegant formalism based on the non-equilibrium Green's functions of Leo Kadanoff
Leo Kadanoff
Leo Philip Kadanoff is an American physicist. He is a professor of physics at the University of Chicago and a former President of the American Physical Society . He has contributed to the fields of statistical physics, chaos theory, and theoretical condensed matter physics.-Biography:Kadanoff...
and Gordon Baym
Gordon Baym
Gordon Alan Baym is an American theoretical physicist.Born in New York City, he graduated from the Brooklyn Technical High School, and received his undergraduate degree from Cornell University in 1956. He earned his Ph.D...
, and independently by Leonid Keldysh was put forth by Ned Wingreen and Yigal Meir. This Meir-Wingreen formulation has been used to great success in the molecular electronics community to examine the more difficult and interesting cases where the transient electron exchanges energy with the molecular system (for example through electron-phonon coupling or electronic excitations).
Further, connecting single molecules reliably to a larger scale circuit has proven a great challenge, and constitutes a significant hindrance to commercialization.
Examples
Common for molecules utilized in molecular electronics is that the structures contain a lot of alternating double and single bonds (see also Conjugated systemConjugated 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...
). The reason for this is that such a pattern delocalizes the molecular orbitals making it possible for electrons to move freely over the conjugated area.
Wires
Molecular wires
Molecular wires are molecular-scale objects which conduct electrical current. They are the fundamental building blocks for molecularelectronic devices...
is to electrically connect different parts of a molecular electrical circuit. As the assembly of these and their connection to a macroscopic circuit is still not mastered, the focus of research in single molecule electronics is primarily on the functionalized molecules: molecular wires are characterized by containing no functional groups and are hence composed of plain repetitions of a conjugated building block. Among these are the carbon nanotubes that are quite large compared to the other suggestions but have shown very promising electrical properties.
The main problem with the molecular wires is to obtain good electrical contact with the electrodes so that the electrons can move freely in and out of the wire.
Transistors
Single molecule transistors are fundamentally different than the ones known from bulk electronics. The gate in a conventional (field-emission) transistor determines the conductance between the source and drain electrode by controlling the density of charge carriers between them, whereas the gate in a single molecule transistor controls the feasibility of a single electron to jump on and off the molecule by modifying the energy of the molecular orbitals. One of the effects of this difference is that the single molecule transistor is almost binary: it is either ON or OFF. This opposes its bulk counterparts, which have quadratic responses to gate voltage.It is the quantization of charge into electrons that is responsible for the markedly different behavior compared to bulk electronics. Because of the size of a single molecule, the charging due to a single electron is significant and provides a mean to turn the transistor ON or OFF (see Coulomb blockade
Coulomb blockade
In physics, a Coulomb blockade , named after Charles-Augustin de Coulomb's electrical force, is the increased resistance at small bias voltages of an electronic device comprising at least one low-capacitance tunnel junction. Because of the CB, the resistances of devices are not constant at low bias...
). For this to work, the electronic orbitals on the transistor molecule cannot be too well integrated with the orbitals on the electrodes. If they are, an electron cannot be said to be located on the molecule or the electrodes and the molecule will function as a wire.
A popular group of molecules, that can work as the semiconducting channel material in a molecular transistor, is the oligopolyphenylenevinylenes (OPVs) that works by the Coulomb blockade mechanism when placed between the source and drain electrode in an appropriate way. Fullerenes work by the same mechanism and have also been commonly utilized.
Semiconducting carbon nanotubes have also been demonstrated to work as channel material but although molecular, these molecules are sufficiently large to behave almost as bulk semiconductors.
The size of the molecules and the low temperature the measurements are being conducted at makes the quantum mechanical states well defined. It is therefore being researched if the quantum mechanical properties can be used for more advanced purposes than simple transistors (e.g. spintronics
Spintronics
Spintronics , also known as magnetoelectronics, is an emerging technology that exploits both the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-state devices.An additional effect occurs when a spin-polarized current is...
).
Physicists at the University of Arizona, in collaboration with chemists from the University of Madrid, have designed a single molecule transistor using a ring-shaped molecule similar to benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
. Physicists at Canada's National Institute for Nanotechnology have designed a single-molecule transistor using styrene.
Both groups expect (their designs have yet to be experimentally verified) their respective devices to function at room temperature, and to be controlled by a single electron.
Rectifiers (diodes)
Molecular rectifiers are mimics of their bulk counterparts and have an asymmetric construction so that the molecule can accept electrons in one end but not the other. The molecules have an electron donorElectron donor
An electron donor is a chemical entity that donates electrons to another compound. It is a reducing agent that, by virtue of its donating electrons, is itself oxidized in the process....
(D) in one end and an electron acceptor
Electron acceptor
An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process....
(A) in the other. This way, the unstable state D+ – A- will be more readily made than D- – A+. The result is that an electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
can be drawn through the molecule if the electrons are added through the acceptor end, but not so easily if the reverse is attempted. An example of a molecular rectifier was made by Geoffrey J. Ashwell's Ph.D. students.
Techniques
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.Molecular gaps
One way to produce electrodes with a molecular sized gap between them is break junctions, in which a thin electrode is stretched until it breaks. Another is electromigrationElectromigration
Electromigration is the transport of material caused by the gradual movement of the ions in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms. The effect is important in applications where high direct current densities are used, such as in...
. Here a current is lead through a thin wire until it melts and the atoms migrate to produce the gap. Further, the reach of conventional photolithography can be enhanced by chemically etching or depositing metal on the electrodes.
Probably the easiest way to conduct measurements on several molecules 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.
Anchoring
A popular way to anchor molecules to the electrodes is to make use of sulfurs’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...
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...
. In these setups, the molecules are synthesized
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...
so that sulfur atoms are placed strategically to function as crocodile clips connecting the molecules to the gold electrodes. Though useful, the anchoring is non-specific and thus anchors the molecules randomly to all gold surfaces. Further, 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.
Fullerene nanoelectronics
In polymers, classical organic molecules are composed of both carbon and hydrogen (and sometimes additional compounds such as nitrogen, chlorine or sulphur). They are obtained from petrol and can often be synthesized in large amounts. Most of these molecules are insulating when their length exceeds a few nanometers. However, naturally occurring carbon is conducting. In particular, graphite (recovered from coal or encountered naturally) is conducting. From a theoretical point of view, graphiteGraphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
is a semi-metal, a category in between metals and semi-conductors. It has a layered structure, each sheet being one atom thick. Between each sheet, the interactions are weak enough to allow an easy manual cleavage.
Tailoring the graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
sheet to obtain well defined nanometer-sized objects remains a challenge. However, by the close of the twentieth century, chemists were exploring methods to fabricate extremely small graphitic objects that could be considered single molecules. After studying the interstellar conditions under which carbon is known to form clusters, Richard Smalley's
Richard Smalley
Richard Errett Smalley was the Gene and Norman Hackerman Professor of Chemistry and a Professor of Physics and Astronomy at Rice University, in Houston, Texas...
group (Rice University, Texas) set up an experiment in which graphite was vaporized using laser irradiation. Mass spectrometry revealed that clusters containing specific "magic numbers" of atoms were stable, in particular those clusters of 60 atoms. Harry Kroto, an English chemist who assisted in the experiment, suggested a possible geometry for these clusters – atoms covalently bound with the exact symmetry of a soccer ball. Coined buckminsterfullerenes, buckyballs or C60
Fullerene
A fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and they resemble the balls used in association football. Cylindrical ones are called carbon nanotubes or buckytubes...
, the clusters retained some properties of graphite, such as conductivity. These objects were rapidly envisioned as possible building blocks for molecular electronics.
Artifacts
When trying to measure electronic characteristics of molecules, artificial phenomena can occur that can be hard to distinguish from truly molecular behavior. Before they were discovered these artifacts have mistakenly been published as being features pertaining to the molecules in question.Applying a voltage drop in the order of volts across a nanometer sized junction results in a very strong electrical field. The field can cause metal atoms to migrate and eventually close the gap by a thin filament, which can be broken again when carrying a current. The two levels of conductance imitate molecular switching between a conductive and an isolating state of a molecule.
Another encountered artifact is when the electrodes undergo chemical reactions due to the high field strength in the gap. When the bias is reversed the reaction will cause hysteresis
Hysteresis
Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...
in the measurements that can be interpreted as being of molecular origin.
A metallic grain between the electrodes can act as a single electron transistor by the mechanism described above thus resembling the characteristics of a molecular transistor. This artifact is especially common with nanogaps produced by the electromigration technique.
Commercialization
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. At the current state, the difficulty of connecting single molecules vastly outweighs any possible performance increase that could be gained from such shrinkage. The picture becomes even worse if the molecules are to have a certain spatial orientation and/or have multiple poles to connect.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. This is done to reduce signal noise enough to measure the faint currents of single molecules.
History and recent progress
In their 1940's discussion of so-called "donor-acceptor" complexes, Robert Mulliken and Albert Szent-GyorgiAlbert Szent-Györgyi
Albert Szent-Györgyi de Nagyrápolt was a Hungarian physiologist who won the Nobel Prize in Physiology or Medicine in 1937. He is credited with discovering vitamin C and the components and reactions of the citric acid cycle...
advanced the concept of charge transfer in molecules. They subsequently further refined the study of both charge transfer and energy transfer in molecules. Likewise, a 1974 paper from Mark Ratner
Mark Ratner
Mark A. Ratner is Morrison Professor of Chemistry and Professor of Materials Science and Engineering at Northwestern University...
and Ari Aviram 1 illustrated a theoretical molecular 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...
. In 1988, Aviram described in detail a theoretical single-molecule field-effect transistor
Field-effect transistor
The field-effect transistor is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with...
. Further concepts were p roposed by Forrest Carter of the Naval Research Laboratory, including single-molecule logic gate
Logic gate
A logic gate is an idealized or physical device implementing a Boolean function, that is, it performs a logical operation on one or more logic inputs and produces a single logic output. Depending on the context, the term may refer to an ideal logic gate, one that has for instance zero rise time and...
s. A wide range of ideas were presented, under his aegis, at a conference entitled Molecular Electronic Devices in 1988. These were all theoretical constructs and not concrete devices. The direct measurement of the electronic characteristics of individual molecules awaited the development of methods for making molecular-scale electrical contacts. This was no easy task. Thus, the first experiment directly-measuring the conductance of a single molecule was only reported in 1997 by Mark Reed and co-workers. Since then, this branch of the field has progressed rapidly. Likewise, as it has become possible to measure such properties directly, the theoretical predictions of the early workers have been substantially confirmed.
Recent progress in nanotechnology
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...
and nanoscience has facilitated both experimental and theoretical study of molecular electronics. In particular, the development of the 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) and later the atomic force microscope
Atomic force microscope
Atomic force microscopy or scanning force microscopy is a very high-resolution type of scanning probe microscopy, with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit...
(AFM) have facilitated manipulation of single-molecule electronics. In addition, theoretical advances in molecular electronics have facilitated further understanding of non-adibatic charge transfer events at electrode-electrolyte interfaces.
The concept of molecular electronics was first published in 1974 when Aviram and Ratner suggested an organic molecule that could work as a rectifier. Having both huge commercial and fundamental interest much effort was put into proving its feasibility and 16 years later in 1990 the first demonstration of an intrinsic molecular rectifier was realized by Ashwell and coworkers for a thin film of molecules.
The first measurement of the conductance of a single molecule was realised in 1994 by C. Joachim and J. K. Gimzewski and published in 1995 (see the corresponding Phys. Rev. Lett. paper). This was the conclusion of 10 years of research started at IBM TJ Watson, using the scanning tunnelling microscope tip apex to switch a single molecule as already explored by A. Aviram, C. Joachim and M. Pomerantz at the end of the 80's (see their seminal Chem. Phys. Lett. paper during this period). The trick was to use an UHV Scanning Tunneling microscope to allow the tip apex to gently touch the top of a single molecule adsorbed on a Au(110) surface. A resistance of 55 MOhms was recorded together with a low voltage linear I-V. The contact was certified by recording the I-z current distance characteristic, which allows the measurement of the deformation of the cage under contact. This first experiment was followed by the reported result using a mechanical break junction approach to connect two gold electrodes to a sulfur-terminated molecular wire by Mark Reed and James Tour
James Tour
James M. Tour is a synthetic organic chemist, specializing in nanotechnology.-Career:He is well-known for his work in molecular electronics and molecular switching molecules. He has also been involved in other work, such as the creation of a nanocar and NanoKids, an interactive learning DVD to...
in 1997.
A single-molecule amplifier was implemented by C. Joachim and J.K. Gimzewski in IBM Zurich. This
experiment involving a single molecule demonstrated that a single molecule can provide gain in a circuit just by playing with through intramolecular quantum interference effects.
A collaboration of researchers at HP
Hewlett-Packard
Hewlett-Packard Company or HP is an American multinational information technology corporation headquartered in Palo Alto, California, USA that provides products, technologies, softwares, solutions and services to consumers, small- and medium-sized businesses and large enterprises, including...
and UCLA, led by James Heath, Fraser Stoddart, R. Stanley Williams, and Philip Kuekes, has developed molecular electronics based on rotaxane
Rotaxane
A rotaxane is a mechanically-interlocked molecular architecture consisting of a "dumbbell shaped molecule" which is threaded through a "macrocycle" . The name is derived from the Latin for wheel and axle...
s and catenane
Catenane
A catenane is a mechanically-interlocked molecular architecture consisting of two or more interlocked macrocycles. The interlocked rings cannot be separated without breaking the covalent bonds of the macrocycles. Catenane is derived from the Latin catena meaning "chain"...
s.
Work is also being done on the use of single-wall carbon nanotubes as field-effect transistors. Most of this work is being done by IBM.
Some specific reports of a field-effect transistor
Field-effect transistor
The field-effect transistor is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with...
based on molecular self-assembled monolayer
Self-assembled monolayer
A self assembled monolayer is an organized layer of amphiphilic molecules in which one end of the molecule, the “head group” shows a specific, reversible affinity for a substrate...
s were shown to be fraudulent in 2002 as part of the Schön scandal.
Until recently entirely theoretical, the Aviram-Ratner model for a unimolecular rectifier has been unambiguously-confirmed in experiments by a group led by Geoffrey J. Ashwell at Bangor University
Bangor University
Bangor University is a university based in the city of Bangor in the county of Gwynedd in North Wales-United Kingdom.It was officially known for most of its history as the University College of North Wales...
, UK. Many rectifying molecules have so far been identified, and the number and efficiency of these systems is expanding rapidly.
Supramolecular electronics
Supramolecular electronics
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 1...
is a new field that tackles electronics at a supramolecular
Supramolecular chemistry
Supramolecular chemistry refers to the area of chemistry beyond the molecules and focuses on the chemical systems made up of a discrete number of assembled molecular subunits or components...
level.
An important issue in molecular electronics is the determination of the resistance of a single molecule (both theoretical and experimental). For example, Bumm, et al. used STM to analyze a single molecular switch in a self-assembled monolayer
Self-assembled monolayer
A self assembled monolayer is an organized layer of amphiphilic molecules in which one end of the molecule, the “head group” shows a specific, reversible affinity for a substrate...
to determine how conductive such a molecule can be. Another problem faced by this field is the difficulty of performing direct characterization since imaging at the molecular scale is often difficult in many experimental devices.
See also
- Molecular electronicsMolecular electronicsMolecular electronics, sometimes called moletronics, involves the study and application of molecular building blocks for the fabrication of electronic components...
- 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...
- StereoelectronicsStereoelectronicsStereoelectronics is the study of the interplay between the electronic structure and geometry of a molecule. For example, in the case of reactant molecules with chiral isomers the electron distribution can determine the stereochemistry of the reactions of the different diastereomers....
- Organic Semiconductors
- Conductive polymers
- 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...
- Software for molecular modeling