Conductivity Of Molecules & Single-electron Transport

Conductivity of Molecules & Single-Electron Transport

Can

we

manipulate

single

molecules

so

that

their

electronic

capabilities can be tested and then applied? In nanostructures the electrical properties can be markedly different from their macroscopic equivalents thereby revealing many novel effects. Various companies at the present are working on developing a quantum standard of current and capacitance using single electron transport. Also, there are other researches in similar technologies which are likely to play important part in future electronic devices, such as the fabrication of atomic wires, the study of spin-polarised electronics

and

magnetic

nano-structures.

Other

possible

e-

applications could range from quantum computing and so-called secure quantum communication, to devices for single-particle sensor technologies, nano-scale frequency standards and the study of adatom-surface interactions. The quantum effects on which most of these devices are based are very

weak

and

the

measurement

technology

is

of

paramount

importance. A number of UK Companies have already extensive low temperature facilities dedicated to the research on electrical nano-structures and have developed special ultra low noise measurement systems, such as cryogenic current comparators and SQUID sensor technology.

The Problem of 'CONTACT' The results of various researches related to the conductivity of molecules attached to wires are very well known, e.g. DNA has been found to be everything from an insulator to a superconductor, however, up to now, has anyone been able to reliably connect a single molecule?

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Previous work to measure the electrical properties of small numbers of molecules has given a wide range of values for their conductivities. Most previous studies have relied on a 'mechanical' contact between molecules and a metallic wire, where the two are simply pushed together. What we need is a way to connect individual molecules on a molecular circuit board. In a paper in the October 18, 2001 issue of the journal Science, the team reports a method for creating through-bond electrical contacts with

single

molecules

and

the

achievement

of

reproducible

measurements of the molecules' conductivity. The above work started with a uniform atomic layer of gold atoms, and attached long, octanethiol 'insulator' molecules to it through chemical bonds, forming a fur-like coating of aligned molecules. They removed a few of the insulators using a solvent and replaced them with molecules of 1,8-octanedithiol, a molecule that is similar, but is capable of bonding with gold at both ends and acting as a molecular 'wire'. Around ‘two nanometre’ gold particles size were then added to the solvent, where they bonded to the free ends of the 1,8octanedithiol molecules, thus creating a bonded metallic 'contact' at either end of the conducting molecules. A gold-coated conducting atomic force microscope probe, i.e. a conducting probe with an atomsized tip, was then run across the surface and conductivity was measured when it made contact with the gold particles. When electrical measurements were made on over 4000 gold particles, virtually all measurements fell into one of five groups (five distinct conductivity curves). The conductivity curves were distinct whole-number

multiples

of

a

single,

'fundamental'

curve.

The

fundamental curve represents conduction by a single molecule of octanedithiol attached to the two gold contacts. When more than a single molecule was bound, each additional molecule increased the current capacity by the single unit amount of current that could be carried by one molecule. When the probe encountered octanethiol 'insulator' molecules, which could not bond with a gold particle, a much higher electrical resistance was recorded.

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Single supramolecular structures can be used to create switches and storage media. As has been shown already with DNA molecules, the trend towards ‘molecule’ will include biological macromolecules as well. The ability to manipulate and characterise single molecules is an important

first

step

for

the

exploration

of

suitable

molecular

functions. A fully functional chip, however, requires the ability to assemble the molecules with high precision into a functional network.

Najib Altawell

Altawell © 2009

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