The Formation of Two Dimensional Supramolecular Structures and Their Use in Studying Charge Transport at the Single Molecule Level at the Liquid-Solid Interface

Abstract

Understanding charge transport through molecular junctions and factors affecting the conductivity at the single molecule level is the first step in designing functional electronic devices using individual molecules. A variety of methods have been developed to fabricate metal-molecule-metal junctions in order to evaluate Single Molecule Conductance (SMC). Single molecule junctions usually are formed by wiring a molecule between two metal electrodes via anchoring groups that provide efficient electronic coupling and bind the organic molecular backbone to the metal electrodes. We demonstrated a novel strategy to fabricate single molecule junctions by employing the stabilization provided by the long range ordered structure of the molecules on the surface. The templates formed by the ordered molecular adlayer immobilize the molecule on the electrode surface and facilitate conductance measurements of single molecule junctions with controlled molecular orientation. This strategy enables the construction of orientation-controlled single molecule junctions, with molecules lacking proper anchoring groups that cannot be formed via conventional SMC methods. Utilizing Scanning Tunneling Microscopy (STM) imaging and STM break junction (STM-BJ) techniques combined, we employed the molecular assembly of mesitylene to create highly conductive molecular junctions with controlled orientation of benzene ring perpendicular to the STM tip as the electrode. The long range ordered structure of mesitylene molecules imaged using STM, supports the hypothesis that mesitylene is initially adsorbed on the Au(111) with the benzene ring lying flat on the surface and perpendicular to the Au tip. Thus, long range ordered structure of mesitylene facilitates formation of Au-π-Au junctions. Mesitylene molecules do not have standard anchoring groups providing enough contact to the gold electrode and the only assumable geometry for the molecules in the junction is via direct contact between Au and the π system of the benzene ring in mesitylene. SMC measurements for Au/mesitylene/Au junctions results in a molecular conductance value around 0.125Go, two orders of magnitude higher than the measured conductance of a benzene ring connected via anchoring groups. We attributed this conductance peak to charge transport perpendicular to the benzene ring due to direct coupling between the π system and the gold electrode that happens in planar orientation. The conductance we measured for planar orientation of benzene ring is two order of magnitude larger than conductance of junctions formed with benzene derivatives with conventional linkers. Thus, altering the orientation of a single benzene-containing molecule between the two electrodes from planar orientation to the upright attached via the linkers, results in altering the conductivity in a large order. Based on these findings, by utilizing STM imaging and STM-BJ in an electrochemical environment including potential induced self-assembly formation of terephthalic acid, we designed an electrochemical single molecule switch. Terephthalic acid forms large domains of ordered structure on negatively charged Au(111) surface under negative electrochemical surface potentials with the benzene ring lying flat on the surface due to hydrogen bonding between carboxylic acid groups of neighboring molecules. Formation of long range ordered structure facilitates direct contact between the π system of the benzene ring and the gold electrodes resulting in the conductance peak. On positively charged Au(111), deprotonation of carboxylic acid groups leads to absence of long range ordered structure of molecules with planar orientation and absence of the conductance peak. In this case alternating the surface (electrode) potential from negative to positive charge densities induces a transition in the adlayer structure on the surface and switches conductance value. Hence, electrochemical surface potential can, in principle, be employed as an external stimulus to switch single molecule arrangement on the surface and the conductance in the junction. The observation of conductance switching due to molecule’s arrangement in the junction lead to the hypothesis that for any benzene derivative, an orientation-dependent conductance in the junction due to the contact geometry (i.e. electrode-anchoring groups versus direct electrode-π contact) should be expected. Conventional techniques in fabricating single molecule junctions enable accessing charge transport along only one direction, i.e., between two anchoring groups. However, molecules such as benzene derivatives are anisotropic objects and we are able to measure an orientation-dependent conductance. In order to systematically study anisotropic conductivity at single molecule level, we need to measure the conductance in different and well-controlled orientations of single molecules in the junction. We employed the same EC-STM-BJ set up for SMC measurements and utilize electrochemical potential of the substrate (electrode) as the tuning source to variate the orientation of the single molecule in the junction. We investigated single molecule conductance of the benzene rings with carboxylic acid functional groups in two orientations: one with the benzene ring bridging between two electrodes using carboxylic acids as anchoring groups (upright); and one with the molecule lying flat on the substrate perpendicular to the STM tip (planar). Physisorption of these species on the Au (111) single crystal electrode surface at negative electrochemical potentials results in an ordered structure with the benzene ring in a planar orientation. Positive electrochemical potentials cause formation of the ordered structure with molecules standing upright due to coordination of a deprotonated carboxyl groups to the electrode surface. Thus, formation of the single molecule junction and consequently conductivity measurements is facilitated in two directions for the same molecule and anisotropic conductivity can be studied. In engineering well-ordered two-dimensional (2-D) molecular structures with controlled assembly of molecular species, pH can be employed as another tuning source for the molecular structures and adsorption in experiments conducted in aqueous solutions. Based on simple chemical principles, amine (NH2) groups are hydrogen bond acceptors and donors. Amines are soluble in water and protonation results in protonated (NH3+) and unprotonated (NH2) amine groups in acidic and moderately acidic/neutral solutions, respectively. Thus, amines are suitable molecular building blocks for fabricating 2-D supramolecular structures where pH is employed as a knob to manipulate intermolecular hydrogen bonding leading to phase transitions. We investigated pH induced structural changes in the 1,3,5–triaminobenzene (TAB) monolayer and the formation/disruption of hydrogen bonds between neighboring molecules. Our STM images indicate that in the concentrated acidic solution, the protonated amine groups of TAB are not able to form H-bonds and long range ordered structure of TAB does not form on the Au(111) surface. However, in moderately acidic solution (pH ~ 5.5) at room temperature, protonation on the ring carbon atom generates species capable of forming H-bonds leading to the formation of the long range ordered structures of TAB molecules. Utilizing EC-STM set up, we investigated the controllable fabrication of a TAB 2-D supramolecular structure based on amine-amine hydrogen bonding and effect of pH in formation of ordered/disordered TAB network.