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A common motif in molecular electronics is to sandwich one or more

molecules between two electrodes to form a metal-molecule-metal junction (Figure 1-7). A MMM junction has five components: two electrodes, two contacts which link the molecule and electrodes, and the molecule. Each of these components can be changed to tune the properties of a molecular junction and impart functionality to a device.

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When Ratner and Avirem first theorized a single molecule rectifier there was no way to make and test a single molecule device, but with the invention of the STM in 1981, a useful tool to make and test conductances of single molecule devices presented itself.35 _{Single molecule devices measured by STM are analytic in nature,}

forming briefly and only long enough to be measured.36 _{Another single molecule}

device configuration is a break junction, which forms a device by breaking a gold wire in the presence of organic molecules and measuring the current through the junction that forms when a molecule or molecules bridges the gap.37 _{In single}

molecule devices the conformation of a molecule is difficult to control when in contact with electrodes. Analyzing these devices is necessarily highly statistical. A single molecule with only 15 atoms can have as many as 1060 _{conformations, each}

with a different conductance.35 _{Thus, several thousand measurements of single}

Figure 1-7: Schematic of Metal Molecule Metal Junction

A metal molecule junction has five distinct components. There are two metallic electrodes, a molecular unit, and two contacts or linkers to attach the molecule to the electrodes. Junctions can have or many molecules sandwiched between two electrodes. The chemical species of metal electrodes and spacers may be identical or different.

Electrode 1

Electrode 2

Contact 1

Molecular Unit

Contact 2

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molecules must be made, and histograms of conductances are analyzed to glean a statistical understanding of the conductance and charge transport through a molecule (Figure 1-8).36, 38

Another approach is to use an ensemble of molecules or SAM to measure hundreds to thousands of molecules at a time. One such approach is to use a

conductive AFM tip to contact molecules analogous to the STM type measurement, but instead of measuring the current through a single molecule as with the STM the

Figure 1-8: Single Molecule Electronic Devices

Measuring conductance through single molecules is a highly statistical endeavor.

Molecules can adopt many conformations which affect charge transport and conductance through the molecule.

(A) Schematic of STM junction of a single oligothiophene molecule. The attachment between molecule and STM tip is temporary, and the conformation of the molecule during attachment, and the atom of Au the molecule attaches to significantly influence conductance

(B) Schematic of break junction formed by breaking a thin Au wire in the presence of analyte molecule. The MMM junction forms with one or few molecules and is broken when the distance between Au wire segments is too large. Images copyright their respective owners.

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AFM measures many molecules at once.39,40,41,42,43,44,39 _{Devices formed this way}

measure conductance over several molecules at once, averaging over many

molecular conformations. Though there is less variance in conductance than seen in single molecule measurements, there is still significant variation in the

measurement due to molecular conformations and imperfect contact with the cAFM probe.

Though using a cAFM probe to form a top electrode in a molecular junction is a reliable way to test molecules, it is a temporary junction that is formed and

broken to make a measurement. Making permanent contact with single molecule and single molecule layers is much more difficult due to the destructive nature of traditional metal deposition.45 _{Evaporation and sputtering penetrate thin organic}

layers causing devices to short, so a more reliable must be used to form a top contact. One way to mitigate this is to protect the single molecule layer with an organic conductor either transferred or spuncast on top of the molecules.46,47,48 _This

is a reliable method to form devices, but it adds an extra organic layer between metal electrodes, which can complicate measurements that require coherence between electrodes and molecules.

Another motif that has seen recent research interest is transfer printing a metal electrode on top of a layer of molecules. In this approach, a metallic layer is deposited on a donor material, and a polymeric stamp is used to bring the metal into contact with a molecular layer, so that the molecular layer is never subjected to the harsh conditions of metal evaporation.49,50 _{For example, in nanoTranfer printing}

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(nTP), metal is evaporated directly on a patterned PFPE stamp, which is brought into contact with SAM. The deposited metal has a greater affinity for the SAM than the PFPE, due to a covalent bond that forms between the metal and the SAM and the low surface energy of the polymer, so that when the stamp is removed, the patterned electrode is left behind.

Device schematics of the ensemble devices are summarized in (Figure 1-9).46- 47, 49-51 _{When measuring molecular electronic transport of a polymer system, an}

Figure 1-9: Large-Area Molecular Electronics Device Schematics

(A) Device made with a temporary cAFM electrode formed by bringing a cAFM tip in contact with a SAM

(B) Device made by evaporating metal contact on top of PEDOT:PSS buffer layer. (C) Device made by nTP top contact directly on top of SAM

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ensemble device is the best option due to extremely low currents measured in devices. Many single molecule oligomer devices push the practical limits of

measurement, and in the case of oligothiophenes, thiophene chains up to only six units can be measured.36