In situ and ex situ AFM investigation of the formation of octadecylsiloxane monolayers

Abstract

The formation of self-assembled monolayers of octadecylsiloxane adsorbed from dilute solutions of octadecyltrichlorosilane in toluene onto freshly cleaved mica surfaces was investigated using atomic force microscopy (AFM) in tapping mode as a well-suited tool to obtain local information on the adsorption process. Three different measurement methods have been used: ex situ measurements and in situ measurements under stopped flow/deposition as well as continuous flow/deposition conditions. Although valuable information on the growth process can be obtained under stable and reproducible conditions with all methods addressed, in situ measurements bear a number of significant advantages for the investigation of such dynamic processes.

Introduction

Self-assembled monolayers (SAMs) have attracted considerable interest during the last decade. Such films are formed spontaneously by immersing proper substrates in dilute solutions of amphiphilic molecules in an appropriate solvent 1, 2. Today a variety of film/substrate systems like organosulfur compounds on metals such as gold, silver, copper and mercury 3, 4, 5, fatty acids on oxide surfaces 6, 7, alkylsiloxanes on hydroxylated surfaces such as native silicon, glass, alumina and mica 8, 9, 10, 11, 12, and phosphonic acids on ionic surfaces of transition metal salts 13, 14, 15are known to form this type of highly ordered monolayer films. Among these systems, self-assembled monolayers of alkylsiloxanes on hydroxylated surfaces have proven to be particularly versatile and useful for practical applications due to their unique chemical and physical stability and their substrate-independent structure. This type of monolayers is formed from precursor molecules such as alkyltrichlorosilanes (RSiCl₃), alkylaminosilanes (RSi(NR′₂)₃) or alkyltrialkoxysilanes (RSi(OR′)₃) having hydrolyzable SiX-groups. It is commonly believed today that the SiX-groups of the precursor molecules are hydrolyzed in a first step to silanols (Si–OH), e.g., by water in the adsorbate solutions. These silanols condensate then with other silanol molecules and hydroxyl groups of the substrate surface to form a two dimensional lateral network of polysiloxane 16, 17, 18.

Various surface sensitive methods have been used so far to characterize self-assembled monolayers (e.g., FTIR-spectroscopy [19], X-ray techniques 20, 21, 22, 23, diffraction methods 24, 25, 26, contact angle measurements 27, 28, and scanning probe techniques 29, 30, 31to name a few), but with the exception of scanning probe techniques, all of these methods probe large surface areas and therefore do not give access to local information. Among scanning probe techniques atomic force microscopy (AFM) is very valuable for the direct observation of surface processes, since measurements can be applied also on nonconducting samples and under liquids [32].

Whereas there is general agreement today that complete monolayers of these compounds on hydroxylated surfaces represent a highly ordered, crystalline-like phase, in which the hydrocarbon chains are—independent of the particular substrate—oriented close to perpendicular to the surface, the mechanism for nucleation and growth of these films remains a matter of controversy. A continuous growth model has been derived from X-ray reflectivity and IR-ATR data, which indicate that a liquid-like, disordered film is formed initially, which is successively converted into the final, highly ordered structure with increasing coverage. In contrast, AFM and X-ray GID studies provide evidence that submonolayers consist of islands of densely packed film molecules with a local structure similar to the complete monolayer, which are separated by uncovered surface regions. With increasing surface coverage, these islands have been shown to grow laterally and combine under formation of larger aggregates until a complete monolayer is formed. A direct comparison between these seemingly conflicting results of previous studies is also hampered by the fact that several parameters such as the water content in the adsorbate solution, the solvent, the precursor concentration, and the type and the age of the solution, which generally vary among these studies, may influence the growth mechanism of these films to a much larger extent than the properties of the resulting, complete monolayers, which are evidently rather insensitive to the specific conditions of preparation. In addition, in situ and ex situ methods have been used sometimes indiscriminately to obtain information about the monolayer growth, although it seems very plausible and has also been pointed out before, that an ex situ technique, where the substrate is removed from the adsorbate solution, rinsed and dried at different stages of the growth process may yield very different results for the same system than an in situ method, where the film formation is directly monitored at the substrate/solution interface, the latter being indisputably the preferred method.

In extension to our previous work on AFM and FTIR investigations of the adsorption of octadecylsiloxane (ODS) films on mica and silicon 33, 34, we report in this paper the first in situ AFM investigations of these systems from dilute solutions of octadecyltrichlorosilane (OTS) in toluene under continuous flow/deposition conditions.