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Single molecule electronics

Single molecule electronics is a branch of molecular electronics that uses single molecules as electronic components. Because single molecules constitute the smallest stable structures imaginable this miniaturization is the ultimate goal for shrinking electrical circuits.

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Concepts

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.

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 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.

Because of the small size of the molecules, quantum mechanics put severe restrains on the states (or orbitals) the electrons can be in on the molecule. These states determine the energy and spatial distribution that an electron can have and hence the electronic properties of the setup. Unfortunately, even though the molecules seem small and simple when drawn schematically, the possible electronic states can only be deduced approximately and this limits the predictability of the molecular electronic properties.

Further, connecting single molecules reliably to a larger scale circuit has proven to be a great challenge and constitute a significant hindrance to commercialization.

History

Conventionally, the electronics are made up of bulk material. Ever since its invention in 1958 the performance and complexity of integrated circuits has been growing exponentially (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 will have to be controlled to a precision of a few atoms in order 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 build 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.

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. It wasn’t until 1997 that the first measurements on the conductance of a single molecule were published .

Examples

Common for molecules utilized in molecular electronics is that the structures contain a lot of alternating double and single bonds (see also Conjugated system). 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

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 to be 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. (Some of the molecules that have been examined will be displayed in a figure below.)

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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). 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.

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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.

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 donor (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. A recent (2004) example of a molecular rectifier made by Geoffrey J. Ashwell and coworkers is shown in the figure below.
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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 is 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 electromigration. 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.

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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 . The connection obtained this way is, however, not stable enough to conduct the advanced measurements possible with the above techniques.

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Anchoring

A popular way to anchor molecules to the electrodes is to make use of sulfurs’ 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 .

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Problems

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 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 measurements on single molecules are carried out in cryogenic temperatures (close to absolute zero) which is very energy consuming. This is done in order to reduce the signal noise to a degree where the faint currents of single molecules can be measured.

Perspectives

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.

See also

References

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