The first direct comparison of self-assembly and Langmuir–Blodgett deposition techniques: Two routes to highly organized monolayers

Abstract

Langmuir–Blodgett (LB) monolayers of two thiol derivatives—hexadecanethiol (HDM) and octadecanethiol (ODM)—were investigated with regards to behaviour previously reported for self-assembled monolayers (SAMs) of the same compounds (, S. H. Gyepi-Garbrah and R. Šilerová, Phys. Chem. Chem. Phys., 2001, 3, 2117), in order to compare the effects of each film-forming strategy. Langmuir films of HDM and ODM were studied at the air/water interface on a 0.01 M ammonium chloride subphase between 8 and 20 °C. The films were more stable at lower temperatures. Monolayers of HDM and ODM were transferred onto gold electrodes at 8, 12 and 16 °C by the LB technique. Transfer at 8 °C was the most successful, and so the electrical properties of LB films deposited at this temperature were studied by electrochemical methods as a function of temperature from 10 to 65 °C in 0.1 M KCl/3 mM [Fe(CN)₆]^4−/3−. Cyclic voltammetry and ac impedance spectroscopy revealed temperature dependent behaviour similar to that reported earlier in analogous SAMs of HDM and ODM (, S. H. Gyepi-Garbrah and R. Šilerová, Phys. Chem. Chem. Phys., 2001, 3, 2117). In particular, a transition temperature characteristic for each film was observed as in the SAMs, above and below which the film's electrical properties differed markedly. These temperatures appear to be independent of the method of film formation (LB deposition or self-assembly). 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 more highly organized than those produced by the self-assembly process, in which chemisorption precedes molecular organization.