Assembled monolayers of Mo3S44+ clusters on well-defined surfaces

Abstract

A class of inorganic monolayers formed by assembling the molybdenum–sulfur cluster, Mo₃S₄⁴⁺, onto a well-defined Au(111) surface is presented. The monolayers have been comprehensively characterized by electrochemistry, X-ray photoelectron spectroscopy (XPS), and in situ scanning tunneling microscopy (in situ STM). The voltammetric data show strong reductive and oxidative desorption signals from Au–S interactions, supported by the presence of both S and Mo signals in XPS. In situ STM shows many small pits in the dense adlayers uniformly spread over the surface, which is a typical feature of self-assembled monolayers (SAMs) of alkanethiols. The density of the pits is ca. 23 (±5)% and is significantly higher than for straight-chain alkanethiol SAMs with a single –SH group. The pit shapes are irregular, suggesting multiple Au–S interactions from Mo₃S₄⁴⁺. High resolution images disclose bright round spots of ca. 8 Å diameter representing individual molecules in the SAM. This is the first example of in situ monolayer formation by a metal–chalcogenide cluster directly anchored onto the gold surface through core ligands and offers a simple way to prepare a new class of functionalized inorganic monolayers.