Ultrathin films and interfacial phenomena - a comparison of different molecular organization processes
Gyepi-Garbrah, Samuel Hilton
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The objective of this thesis was to investigate what effect the method of preparing a monolayer had on its organization and electrical properties. Gold electrodes were modified with alkanethiols, n - hexadecanethiol (HDM) n-octadecanethiol (ODM), by (i) self-assembly (SA) from a dilute solution of the alkanethiols, and (ii) Langmuir-Blodgett (LB) deposition techniques from Langmuir films at the air-water interface, in order to compare the effects of each film-forming strategy on the resulting Self-Assembled Monolayers (SAMs) and LB films. In addition to the alkanethiol SAMs, self-assembled 11mercaptoundecanoic acid (MUA) was also studied so as to observe the effect of the terminal group. Prior to LB deposition, Langmuir films of HDM and ODM were studied at the air-water interface on a 0.01M ammonium chloride subphase between 8 and 20°C. The films were found to be more stable at lower temperatures. The modified electrodes were studied by Alternating Current Electrochemical Impedance Spectroscopy (AC-EIS) and Cyclic Voltammetry (CV) in 3mM [Fe(CN)6]4-/3- / 0.1M KCI over a temperature range from 15°C to 65°C. A strong temperature dependence of the films' electrical features was observed for each film. Upon heating, all films show markedly different behaviour below and above a transition temperature (Ttr) that is characteristic for each film. From the impedance data, it was possible to model bare gold and the alkanethiol-modified electrodes with the Randles circuit over the whole temperature range examined, but a different equivalent circuit was required to describe the SAM of MUA-modified electrodes before heating above the Ttr. The results are interpreted in terms of a phase transition in these monolayers, and the possible nature of this two-dimensional (2D) phase transition is discussed. CV measurements exhibited the same transition temperatures for a given compound in each type of film (SAM and LB film). However, a comparison of the absolute electrical properties of the LB films and the SAMs suggests that the LB deposition process, in which molecular organization precedes chemisorption, produces films that are less permeable, and therefore more highly organized than those produced by the SA process, in which chemisorption precedes molecular organization. Surprisingly, in the SAMs of HDM, and ODM, there was a second inflexion (transition) at higher temperatures that was not observed in the LB films. In order to explain this temperature dependant phase transition, gold colloids capped with HDM and ODM, three-dimensional (3D) analogues of SAMs, were synthesized and studied by Differential Scanning Calorimetry (DSC) and Nuclear Magnetic Resonance (NMR). In general, DSC revealed the same temperature dependant phase behaviour that was observed in the 2D analogues by CV. However, in the 3D systems, the low temperature transition was observed only in the HDM system and not in the ODM system. It was thus concluded that this transition was less pronounced in the longer chain ODM system due to the curvature in the 3D system, which causes the end groups of ODM to be further apart than those of HDM in the colloid configuration. Thus the first or (lower temperature) transition was attributed to the disordering of the alkyl chains starting from the end closest to the terminal groups.