Photooxidation of self-assembled monolayers by exposure to light of wavelength 254 nm: a static SIMS study.

Self-assembled monolayers (SAMs) of alkanethiols have been photooxidized by exposure to light from a lamp emitting light with a wavelength of 254 nm. The data confirm that SAM oxidation on exposure to UV light sources occurs in the absence of ozone, but also suggest that the mechanism is different from that observed in previous studies using broad-spectrum arc lamps. In particular, for monolayers on both gold and silver, carboxylic acid-terminated SAMs oxidize significantly faster than methyl-terminated SAMs, in contrast to earlier observations for monolayers exposed to light from a mercury arc lamp. The difference in rates of photooxidation for the two classes of monolayer is significantly greater on silver than on gold. These data support our recent suggestion that while methyl-terminated SAMs are able to pack much more closely on silver than on gold, carboxylic acid-terminated thiols are not able to adopt the same close-packed structures, and their rates of photooxidation on silver are similar to, or slightly greater than, those measured for the same adsorbates on gold. Surface potential measurements were made for carboxylic acid- and methyl-terminated SAMs using a Kelvin probe apparatus. It was found that the work functions of carboxylic acid-terminated SAMs are significantly greater than those of methyl-terminated monolayers. It is concluded that these data are consistent with the oxidation reaction being initiated by "hot" electrons generated following the interaction of photons with the metallic substrate.

Friction force microscopy: towards quantitative analysis of molecular organisation with nanometre spatial resolution.

Friction force microscopy (FFM) is a technique based upon scanning force microscopy that provides information on the properties of molecular materials. Continuum mechanics provides models that may be used to conduct quantitative analyses of data. While there are some important unresolved issues associated with the contact mechanics of the tip-sample interaction, there is a growing body of data that demonstrates the sensitivity of FFM to changes in molecular organisation and surface composition. Importantly, FFM provides these data with nm spatial resolution, making it in many respects a unique tool for exploring the structures of organic materials on small length scales. Some of the capabilities of FFM are illustrated by drawing on both the literature and work performed in the authors' laboratory on self-assembled monolayers. For example, the compositions of mixed monolayer systems may be determined, with control of tip chemistry providing an additional element of chemical specificity; the alkyl chain organisation may be investigated; and the rates of surface chemical reactions may be measured. FFM is a powerful tool for the quantitative investigation of nm scale chemistry.

Chemical force microscopy of mixed self-assembled monolayers of alkanethiols on gold: evidence for phase separation.

Mixed self-assembled monolayers formed by the coadsorption of hydroxyl- and methyl-terminated alkanethiols with similar chain lengths have been characterized by friction force microscopy. Friction coefficients have been determined by assuming a fit to Amonton's law. The friction coefficients vary linearly with the fraction of polar-terminated adsorbates in the self-assembled monolayer (SAM). With carboxylic acid-terminated tips, the coefficient of friction increases with the fraction of hydroxyl-terminated thiols, while with methyl-terminated tips it decreases. Similar trends are observed for pull-off forces, which increase and decrease as a function of the fraction of polar-terminated adsorbates for carboxylic acid- and methyl-terminated adsorbates, respectively. Analysis of histograms of adhesion forces has yielded insights into the phase structure of mixed SAMs. Single-component monolayers yield histograms that may be fitted to symmetric Gaussian distributions, irrespective of the nature of the terminal group on either the tip or the SAM. However, mixed monolayers yield broad, asymmetric distributions that could not be fitted with a Gaussian distribution. The best explanation for these data is that mixed SAMs of hydroxyl- and methyl-terminated alkanethiols of similar chain length form phase-separated structures.