Single Gold Atom Containing Oligo(phenylene)ethynylene: Assembly into LB Films and Electrical Characterization

Abstract

Monomolecular films of an Oligo(phenylene)ethynylene (OPE) derivative [2isocyano-1,3-dimethylbenzene][4-(4′-amino-phenylethynyl)-phenylethynyl]-gold, 1, containing a gold atom in the molecule backbone have been prepared by the Langmuir-Blodgett (LB) method in order to study how the electrical properties can be modulated in monolayers of OPEs by the incorporation of a gold center in their structures. UV-vis reflection spectra of Langmuir monolayers of 1 at the air-water interface reveal strong aurophilic interactions between neighbouring molecules that increase upon compression. Monolayer Langmuir-Blodgett (LB) films were readily fabricated by the transfer of Langmuir films of 1 onto solid substrates. Quartz crystal microbalance (QCM) experiments conclusively demonstrate the formation of monolayer LB films with a high surface coverage. The morphology of these films was analysed by atomic force microscopy (AFM), revealing the formation of homogeneous layers with an optimum surface pressure of transference of 6 mN•m-1. Film homogeneity and integrity was confirmed by cyclic voltammetry, with efficient blocking of gold electrodes by these well-formed monolayers of 1. The electrical properties of LB films of 1 were investigated by scanning tunnelling microscopy (STM) using a 'tip-to-contact' method. Symmetrical, sigmoidal-shaped I-V curves were observed, with analysis of the pseudolinear (Ohmic) region giving a conductance value G = 3.9 10-5 G 0 , which is relatively high for an OPE derivative and may indicate a beneficial role of metal atom incorporation within the wire-like-system.

Electrical characterization of single molecule and Langmuir–Blodgett monomolecular films of a pyridine-terminated oligo(phenylene-ethynylene) derivative

“…The electrical properties of monomolecular LB films of 1 deposited on Au(111) as described above were studied using a scanning tunneling microscope (STM) and the “STM touch-to-contact” method [23–24 30,44–45]. The “STM touch-to-contact” method requires the STM tip to be positioned immediately above and just touching the LB film, avoiding both penetration of the STM tip into the film or a significant gap between the STM tip and the monolayer.…”

“…The thickness of the monolayer (1.70 ± 0.05 nm) was determined using the attenuation of the Au 4f signal in the XPS spectra as described in the Experimental section. The calibration of the tip–substrate distance was carried out by relating the STM set-point parameters (set point current, I 0 , and tip bias, U t ) to an absolute tip-to-substrate separation as previously described [30,44–45 77,96–97]. Current–distance retraction scans ( I ( s ) curves) were recorded by first setting the STM tunneling parameters ( I 0 = 60 nA and U t = 0.6 V) so that the tip approaches relatively close to the surface and is thereby embedded within the LB film.…”

“…LB films of 1 were prepared in a similar manner to other LB films incorporating oligo(phenylene-ethynylene) derivatives [28,44–45 55,113]. In particular, a Nima Teflon trough with dimensions 720 × 100 mm 2 , which was housed in a constant temperature (20 ± 1 °C) clean room, was employed to prepare the Langmuir films.…”

“…LB films also offer the possibility of exploration of a large number of metal–organic interfaces involving either physi- or chemi-sorbed films [31], and also permits the fabrication of directionally oriented monolayers when the molecule contains two different terminal groups that each have affinity for the substrate [30]. In particular, LB films have been used to analyze different properties and explore potential applications including molecular switching behavior [35–36], rectifying molecular junctions [37–38], exciton migration control [39], top-contact metallization [24,40–41], optical and opto-electronic applications [42–43], modulation of the electrical properties of the junction [24], inclusion of a metal atom in the organic structure of a molecular wire [44], and electrical measurements of both molecular ensembles and single molecules in the constrained environment of the film [24,30,45]. …”

Electrical characterization of single molecule and Langmuir–Blodgett monomolecular films of a pyridine-terminated oligo(phenylene-ethynylene) derivative

“…LB films also offer the possibility of exploration of a large number of metal-organic interfaces involving either physi-or chemisorbed films [31], and also permits the fabrication of directionally oriented monolayers when the molecule contains two different terminal groups that each have affinity for the substrate [30]. In particular, LB films have been used to analyze different properties and explore potential applications including molecular switching behavior [35,36], rectifying molecular junctions [37,38], exciton migration control [39], top-contact metallization [24,40,41], optical and opto-electronic applications [42,43], modulation of the electrical properties of the junction [24], inclusion of a metal atom in the organic structure of a molecular wire [44], and electrical measurements of both molecular ensembles and single molecules in the constrained environment of the film [24,30,45].…”