Self-assembly of molecules at the solid-liquid interface on silver and gold surfaces : the challenge of the complex interplay of interactions

Abstract

Self assembled monolayers (SAMs) provide a convenient way of modifying surface properties, making them increasingly useful as technology is miniaturised. To use SAMs for sophisticated applications such as molecular electronics, greater control over molecular position, orientation and functionality is needed. However, more complex molecular architectures affect the SAM-determining thermodynamic and kinetic factors. This thesis investigates multifunctional molecules with controlled intermolecular interactions, nonideal molecular geometries, and multipodal anchoring. Whether ordered SAMs, which retain the planned functionality, can be formed from these molecules using solution deposition was explored.

The assembly of biphenyl-2,2′,4,6,6′-pentacarboxylic acid (BP-PCA) resulted in the formation of the first 2D, hydrogen-bonded network of upright-standing molecules. Five carboxylic acid groups provide the means of intermolecular separation and surface anchoring. This proof of concept provides a basis for designing SAMs not confined to dense packing, and therefore retaining space for conformational freedom or hosting.

2-(3′,5′-Dithiatricyclo[5.2.1.0²˒⁶]decane-4′-ylidene)-1,3-dithiole-4,5-dicarboxylic acid (DHTTF-DCA) incorporated an electron donor moiety and two carboxylic acid groups for anchoring, but had an irregular shape not well-suited to forming crystalline SAMs. Prepared layers showed net polar orientation but no crystallinity. Despite the presence of dihydrotetrathiafulvalene (DHTTF), the electron donor, no significant rectifying properties were found.

Finally, 1,8,13-tris(mercaptomethyl)-10-ethynyl(C₆₀) triptycene (C₆₀-Trip) provided an interesting case in terms of assembly and functionality because of the tripodal base architecture and large molecule size, in addition to incorporating a fullerene. The parent triptycene tripod and C₆₀ form highly ordered layers with compatible periodicity, but C₆₀-Trip, unexpectedly, does not form an ordered SAM. Evidence of multiple bonding regimes suggested a lack of tripodal bonding. Electrochemical analysis of the layers points towards a polymerisation between sulfur and C₆₀.

Structural characterisation was carried out using scanning tunnelling microscopy (STM) with complementary spectroscopic analysis using X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS). Conductivity measurements and electrochemical experiments provided further analysis.