Electrochemical and STM Study of α,ω-alkanedithiols Self-assembled Monolayers

V. C. Ferreira, F. Silva and L. M. Abrantes

Abstract
Self-assembled monolayers (SAMs), prepared by the immersion method, from ethanolic solutions containing α,ω-alkanedithiol, n-alkanethiol or mixed thiol/dithiol solutions, with 6, 9 and 10 carbon atoms in the alkyl chain, have been investigated. The amount of adsorbate and the SAM stability in alkaline medium is evaluated by reductive desorption of the prepared monolayers by cyclic voltammetry. An upright orientation of the dithiol self-assembled molecules and disulfide bonding at the SAM/solution interface are suggested by the higher reductive desorption charge of the dithiol monolayers (relative to thiol SAMs) for n = 6 and 9. The results show that an improvement on the stability of these dithiol SAMs is obtained by the presence of monothiols, resulting in mixed monolayers. Mixed SAMs prepared from longer alkane chain thiols, n = 10, allow to overcome the increased possibility of loop formation and therefore lower surface coverage is obtained for the 1,10-decanedithiol monolayers. Morphological characterisation of the modified electrodes is performed by scanning tunnelling microscopy (STM) ex situ, in air. Typical one atom deep thiol induced depressions are observed in the STM images of the dithiol and mixed SAMs.

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Keywords
Self-assembled monolayers, reductive desorption, scanning tunnelling microscopy, α,ω-alkanedithiols