No abstract

Evaporation of ultrathin (1.3–10 nm nominal thickness) gold films onto quartz or mica leads to the formation of a layer of rather uniform gold islands on the transparent support. The morphology of ultrathin gold island films of various thicknesses was studied by using atomic force microscopy (AFM) and scanning electron microscopy (SEM) imaging. The surface plasmon (SP) absorption characteristic of such films is highly sensitive to the surrounding medium, with the plasmon band changing in intensity and wavelength upon binding of various molecules to the surface. The binding process can be monitored quantitatively by measuring the changes in the gold SP absorption, by using transmission UV/Vis spectroscopy. The method, termed transmission surface plasmon resonance (T‐SPR) spectroscopy, is shown to be applicable to both chemically and physically adsorbed molecules, in liquid or gas phase, with measurements carried out either ex situ or in situ (real‐time measurements) using a variety of molecular probes. Binding to a preformed molecular layer on the Au surface produces a similar response, suggesting the possible use of T‐SPR for selective sensing. The sensitivity of T‐SPR spectroscopy in detecting molecular binding to the gold depends strongly on the film preparation conditions, and may be comparable to that obtained in surface plasmon resonance (SPR) sensing.

No abstract

We have used in-situ scanning tunneling microscopy and cyclic voltammetry to study self-assembled monolayers of ethanethiol on Au(111) electrodes. The adlayer was found to consist of domains of two different ordered structures, one corresponding to a (p × √3), well-established for other short-chain alkanethiols, and the other to an oblique primitive (4 × 3) superstructure, not previously reported for nonfunctionalized alkanethiols. At potentials slightly negative of 0 V vs SCE the adlayer undergoes a structural transformation that eventually leads to the formation of small pits and islands on the surface. Electrochemical studies in 0.1 M H2SO4 have revealed that around −0.31 V vs SCE the ethanethiol adlayer is reductively desorbed. Oxidative desorption of ethanethiol takes place at 1.15 V. The cathodic as well as the anodic desorption of the monolayer was monitored by scanning tunneling microscopy.

Copper electrodeposition from sulfuric acid solutions onto ethanethiol-modified Au(111) electrodes was studied by in-situ scanning tunneling microscopy and cyclic voltammetry. The ethanethiol adlayer undergoes an order−disorder transition before Cu underpotential deposition starts around +0.20 V versus SCE. Five percent of a monolayer is deposited positive of the Nernst potential at a sweep rate of 10 mV s-1. At low overpotentials the Cu deposit exhibits a ramified monatomic high morphology, if the ethanethiol adlayer is dense. In all cases three-dimensional growth nucleating at large substrate defects is found in addition. Cyclic voltammetry revealed two characteristic deposition features:  firstly, a sharp cathodic peak at −0.18 V which is ascribed to the insertion of a Cu monolayer between Au and the organic adlayer and, secondly, a current loop due to Cu bulk deposition. The corresponding stripping peaks are found at 0.08 and 0.35 V, respectively.