Single Molecule Conductance and Surface Electrochemical Characterisation of Molecular Monolayers

Abstract

A key aspect of molecular electronics is understanding and applying the electrical properties of single molecules and their nanometer-scale assemblies. This thesis examines the influence of structural properties and assembly conditions on electron transfer through self-assembled monolayers (SAMs) on gold using a combined approach of single-molecule conductance determined using the STM-BJ and electrochemical techniques (cyclic voltammetry and impedance spectroscopy). A number of heterocyclic molecular systems have been explored in this research including 5 (3-pyridyl)-1,3,4-oxadiazole-2-thiol (3PODT), 5-(4-pyridyl)-1,3,4-oxadiazole-2-thiol (4PODT), and 5-(4-pyridyl)-1H-1,2,4-triazole-3-thiol, with this work being presented in Chapter 3. These molecular targets with conjugated molecular backbones and differing terminal anchoring groups have not previously been studied using single-molecule conductance methodologies. In the next chapter (Chapter 4) a detailed evaluation is undertaken of mixed SAMs of aliphatic 1,8-octanedithiol (ODT) with 1-octanethiol (OT). This is then followed by a complementary study of mixed aromatic SAMs of biphenyl-4,4′-dithiol (BPDT) and biphenyl-4-thiol (BPT). Chapter 5 then presents studies of carbodithioate derivatives such as dimethyl [1,1′-biphenyl]-4,4′-bis(carbodithioate) (DMBPBC), methyl [1,1′-biphenyl]-4-carbodithioate (MBPC), and methyl 4-(phenylethynyl)benzodithioate (MPEB). The influence of the adsorption medium was studied on the molecular adsorption with a focus on molecular organisation and electron transport, with all SAMs being formed in either ethanol or aqueous solutions of Triton X-100 (TX-100 )to facilitate a direct comparison of these contrasting environments. Further summaries of these chapters are given in the following paragraphs. Chapter 3 describes heterocyclic compounds with asymmetrically bound molecules. STM-BJ measurements confirmed the formation of stable junctions by 3PODT and 4PODT from ethanol and TX-100 adsorption media. Para-substituted 4PODT gave higher conductance and more pronounced conductance peaks than meta-substituted 3PODT, indicating both improved electronic coupling and more effective junction formation. PTT did not form measurable junctions, most likely due to poor anchoring on the gold surface. The integrity, compactness and assembly of the SAMs was assessed by their ability to inhibit (or “block”) electron transfer to a water-soluble redox probe as assessed by electrochemical methods. Electrochemical characterisation (CV and EIS) revealed that 4PODT SAMs exhibited optimal blocking of charge transfer to the redox probe in solution and charge transfer resistance, followed by 3PODT, with PTT exhibiting the poorest behaviour with less stable and less blocking monolayers. TX-100 was used for the first time in single-molecule conductance measurements and showed comparable results to those of organic solvents in molecule assembly. Chapter 4 analysed the formation, electrochemical and electrical properties of aliphatic (ODT:OT) and aromatic (BPDT:BPT) thiol mixed SAMs. Mixed monolayers have been barely studied in the molecular electronics literature even though they provide a valuable way of controlling the structural and electrical properties of SAMs for application in molecular electronics and molecular devices. STM-BJ measurements showed that robust molecular junctions were formed predominantly by the dithiols. Increasing the ratio of the monothiol component in both the aliphatic and aromatic mixed SAM systems decreased conductance because monothiols acted as diluents within the SAM, disrupting the ability of dithiols to consistently form stable junctions between electrodes. Aliphatic mixtures showed weaker, broader signals at lower dithiol ratio, and aromatic mixtures produced strong, homogeneous conductance features presumably added by intermolecular π-stacking within the monolayers. Electrochemical measurements show that TX-100 based aqueous surfactant adsorption medium improved the blocking ability of aliphatic SAMs, especially at large dithiol ratios, and aromatic SAMs exhibited improved blocking in ethanol that suggests better monolayer quality. Results show the significance of molecular assembly medium, composition, and structure for tuning conductance and interfacial properties. Chapter 5 discusses studies of a number of carbodithioate derivatives: DMBPBC, MBPC and MPEB. Of the three derivatives, only the symmetric bidentate-anchored DMBPBC formed stable and reproducible molecular junctions in ethanol and TX-100. MBPC and MPEB are monodentate derivatives and with just a single surface anchoring group they give no measurable conductance signals. Electrochemical measurements indicated that DMBPBC had the highest charge transfer resistance and blocking efficiency. The molecular symmetry and bidentate anchoring plays a key role in the assembly of both the SAM and the molecular junction. Overall, this work shows that single-molecule conductance, SAM packing, and electron transfer through the interface strongly depend on the molecular structure, anchoring-group design, and assembly environment. The data shows that the aqueous surfactant adsorption system (TX-100) can be used as an alternative medium to prepare stable and well-ordered monolayers and produce reproducible molecular electronic platforms.