Metal–Molecule–Metal Junctions in Langmuir–Blodgett Films Using a New Linker: Trimethylsilane

Abstract

Herein trimethylsilane (TMS) is demonstrated to be an efficient binding group suitable for construction of metal-molecule-metal (M-mol-M') junctions, in which one of the metal contacts is an atomically flat gold surface and the other a scanning tunnelling microscopy (STM) tip. The molecular component of the M-mol-M' devices is an oligomeric phenylene ethynylene (OPE) derivative Me(3)Si C≡C{C(6)H(4)C≡C}(2)C(6)H(4)NH(2), featuring both Me(3)SiC≡C and NH(2) metal contacting groups. This compound can be assembled into Langmuir-Blodgett (LB) films on Au--substrates by surface binding through the amine groups. Alternatively, low coverage (sub-monolayer) films are formed by adsorption from solution. In the case of condensed monolayers top electrical contacts are formed to STM tips through the TMS end group. In low coverage films, single molecular bridges can be formed between the gold surface and a gold STM tip. The similarity in the I-V response of a one-layer LB film and the single molecule conductance experiments reveals several points of critical importance to the design of molecular components for use in the construction of M-mol-M' junctions. Firstly, the presence of neighbouring π systems does not have a significant effect on the conductance of the M-mol-M' junction. Secondly, in the STM configuration, intermolecular electron hopping does not significantly enhance the junction transport characteristics. Thirdly, the symmetric behaviour of the I-V curves obtained, despite the different metal-molecule contacts, indicates that the molecule is simply an amphiphilic electron-donating wire and not a molecular diode with strong rectifying characteristics. Finally, the conductance values obtained from the amine/TMS-contacted OPE described here are of the same order of magnitude as thiol anchored OPEs, making them attractive alternatives to the more conventionally used thiol-contacting chemistry for OPE molecular wires.

Solvent Dependence of the Single Molecule Conductance of Oligoyne-Based Molecular Wires

“…43,44 The resulting narrow and sharp conductance peaks allow us to measure the shift of the conductance values with greater certainty. Also in the case of other more polar surface contacting groups, such as thiols, solvation of the contacting group has been proposed to have a major effect on conductance.…”

Insulated molecular wires: inhibiting orthogonal contacts in metal complex based molecular junctions

“…[91][92][93][94] As a contacting group in molecular junctions, the CuCSiMe 3 moiety is known to favor only a single, albeit low conductance, contact. 14,[95][96][97] This is due to the fact that the CuCSiMe 3 group makes effective electrical contact to defect sites on the gold electrode surfaces in a restricted range of geometries, limited in part due to the steric bulk of the ancillary methyl groups. 67 Thus, in even π-rich oligophenylene ethynylene based molecules, the large footprint and limited range of conducting configurations that are available to CuCSiMe 3 contacts gives rise to conductance histograms featuring only a single conductance peak without complications from alternative conductive molecular orientations or contacts within the junction 14,14,67,96 As will be seen in the discussion that follows, the relatively weak nature of the CuCSiMe 3 -gold interaction (estimated at ca.…”

Single-Molecule Electronics: Chemical and Analytical Perspectives

“…Similarly, junctions can be formed with a terminal acetylene (alkyne, -C≡C-H) group (73,74). It has also been shown to be possible to form junctions with terminal trimethylsilylethynyl (TMSE, -C≡C-SiMe 3 ) and gold electrodes (75)(76)(77). Trimethylsilylethynyl had been previously shown to be a contacting group for forming stable SAMs on Au (110,(78)(79)(80)(81).…”