Electronics and Self-Assembly with Single Molecules

Self-assembled Nanogaps with a Single Molecule Incorporated

One of the important challenges in molecular electronics is the preparation of a 1-2 nm nanogap with a single molecule situated in the nanogap. In a recent report, gold nanorods were used to bridge the gap between the molecular length scale (1-2 nm) and the micrometer length scale, which is readily accessible by simple lithographic techniques.¹¹ Gold nanorods have recently attracted large interest due to their ease of synthesis, unique shape dependant optical properties, and possible biochemical imaging²⁰ and medical applications.²¹ The most widely used preparation method is the seed-mediated and surfactant assisted synthesis, first reported by Murphy and co-workers.²² In brief, citrate-stabilized gold nanoparticle seeds are prepared by the reduction of HAuCl₄ (484385) with NaBH₄ (480886) in an aqueous citrate solution. The seeds are then added to a growth solution containing the surfactant CTAB (H9151), HAuCl₄ and ascorbic acid (A5960). Depending on the specific growth conditions and purification schemes, rods with aspect ratios from 4 to 25 can be prepared.^23,24 Important factors for the successful synthesis are the use of the correct type of CTAB surfactant²⁵ ( H6269), precise temperature control and clean glassware. Gold nanorod dimers can be prepared by addition of a water soluble thiol end-functionalized polyethylene glycol (HS-PEG-SH) to the gold nanoparticle seeds before they are exposed to the growth conditions. By carefully tuning the concentration of HS-PEG-SH the reaction conditions are optimized to yield a high degree of gold nanorod dimers. By diluting the HS-PEG-SH concentration further, one can, to a high degree, expect that a single molecule is acting as the linker between two nanorods (Figure 2).¹¹

In another experiment (Figure 3), the electronic properties of the gold rods were tested. First, gold nanorods were self-assembled to gold nanoparticle-coated tin oxide nanowires. The assembly was facilitated using thiol end-capped oligo(phenylenevinylene)s (OPVs) which are interesting test molecules exceeding low tunneling barriers²⁶ and widely used in molecular electronic experiments.^4,10 Second, the electronic properties of the OPV molecule were probed by conducting atomic force microscopy (C-AFM). The conductive AFM tip was placed directly on the gold nanorod as one electrode, while the much larger tin oxide nanowire acted as the second electrode, which was easily connected to close the electronic circuit around the molecule (Figure 3). I-V characteristics revealed signatures similar to those previously obtained for OPV molecules, thus also confirming that gold nanorods may be promising candidates for bridging the gap between the molecular and micrometer length scale.

Synthesis of a Conducting Molecule with Well Defined Contact Geometry

Control of the contact between a molecule and the electrode surface has proved challenging since minute changes in atomic position of the binding thiol or the atomic structure of the surface may lead to large difference in the electronic properties.^27-29 In an attempt to circumvent the challenges discussed above, C₆₀ was introduced as an alternative anchoring group. The C₆₀ molecule can create a strong contact to the surface through multiple connection sites, thus rendering the details of the contact site less important since the limiting tunneling barrier is now moved from the interface between molecule and electrode to well defined chemical bonds inside the molecule. As a consequence, the detailed contact geometry can then be controlled by chemical synthesis of C₆₀ derivatives (Figure 4). The first electronic measurements taking use of this concept was described by Martin et al.¹⁵ who performed single molecule measurements using the break junction technique, and found that one of the dominant features of the molecule was extremely high stability of the single molecule junctions - even at room temperature.

The C₆₀-anchored molecule (1,4-bis(fullero[c]pyrrolidin-1-yl)benzene) was synthesized via [2+3]cycloaddition reaction of an in situ generated azomethine ylide to a fullerene carbon-carbon double bond. This reaction is known as the Prato-reaction.³⁰ In brief, N,N’-(1,4-phenylene) bisglycine and paraformaldehyde (16005) was sonicated in 1,2-dicholorbenzene (240664) and added to a solution of C₆₀ in 1,2-dichlorobenzene. The reaction mixture was refluxed for 6 hours yielding the black target molecule in 28% yield after purification (Figure 4).¹⁵

Conclusion

In order to be able to study the intrinsic charge transport mechanisms in individual molecules, one of the main challenges is to circumvent problems associated with the specific position of the bonds between electrodes and molecules. A strategy for predefining the metalmolecule interface, and thereby achieving atomic scale precision, by the introduction of new C₆₀-based anchoring groups have been presented. Another challenge is the fact that only one molecular junction is realized per device, hence the realization of molecular circuitry with numerous individual addressable components is highly desirable. Alternative routes that may help overcome these challenges are the use of chemical bottom-up fabrication techniques such as the Langmuir-Blodgett technique or other self-assembly based methods that may aid in the development towards complex circuitry. One possibility is the bottom-up synthesis of electrodes (gold nanorods) with molecules incorporated directly in the nanogap and prepared by the use of simple aqueous solution-based chemistries.