A comparative study of the growth of octadecyltrichlorosilane and 3-mercaptopropyltrimethoxysilane self-assembled monolayers on hydrophilic silicon surfaces

Abstract

Self-assembled monolayers of two different organosilane precursors, methyl-terminated nonpolar n-octadecyltrichlorosilane (OTS, Cl₃Si(CH₂)₁₇CH₃) and thiol-terminated polar 3-mercaptopropyltrimethoxysilane (MPTMS, (CH₃O)₃SiCH₂CH₂CH₂SH), were prepared separately on hydrophilic silicon surfaces by immersion in millimolar solutions of the respective precursors in toluene at room temperature. Ex situ atomic force microscopy (AFM), lateral force microscopy (LFM) and X-ray photoelectron spectroscopy (XPS) were used to study the growth and the properties of OTS and MPTMS SAMs. For OTS SAMs, generally speaking, small islands surrounded large dendrite-shaped islands. But for MPTMS SAMs, sporadic small round islands appeared, but no dendrites. The impact of the solution age was more significant on the growth of OTS SAMs than MPTMS SAMs. At the same precursor concentration and solution age, the growth of OTS SAMs was much faster than MPTMS SAMs due to the greater hydrolysis ability of Si–Cl bonds in OTS as compared with that of Si–OCH₃ bonds in MPTMS. The difference in hydrolysis ability was confirmed by the absence of a Cl signal in the XP spectrum of OTS SAMs and the existence of a C_1s peak corresponding to unhydrolyzed Si–OCH₃ bonds in the XP spectrum of MPTMS SAMs. This trend together with the difference in alkyl chain length had a strong influence on the surface morphology and coverage of these two SAMs. According to the individual adsorption behavior of the components, the predictable kinetic difficulty of preparing OTS/MPTMS mixed SAMs by co-adsorption is pointed out. Furthermore, a potential reaction condition for stepwise adsorption is suggested.

Introduction

Self-assembled monolayers (SAMs) have gained great interest over the past two decades in many fields such as surface modification, protective coatings, lubricant additives, and biosensors [1], [2]. Among numerous systems that exhibit self-assembly properties, the growth of self-assembled alkylsiloxane monolayers on hydroxylated silicon surfaces (in the following we use the term SiO₂) has received increasing attention because of their complex growth mechanism. Several characteristics of alkylsiloxane SAMs, like chemical composition, film thickness, contact-angle and surface coverage have been investigated by X-ray photoelectron spectroscopy (XPS) [3], X-ray diffraction (XRD) [4], contact-angle measurements [5], ellipsometry [3], atomic force microscopy (AFM) [6], lateral force microscopy (LFM) [7] and so on.

Since Sagiv [8] discovered octadecyltrichlorosilane (OTS) on SiO₂ surface in 1980, numerous studies have pointed out that the preparation of organosilane SAMs depends significantly on various parameters such as solvent [2], [9], reaction temperature [10], [11], precursor concentration [12], solution age (i.e., the time interval between solution preparation and immersion of the substrate) [13], [14], and water content of the solvent [6], [12], [13]. For silicon substrate, toluene is a widely used solvent [14], [15], [16], [17].

Parikh et al. [10] found a critical temperature T_c for alkylsilane deposition. Below this temperature, densely packed islands or highly ordered films have been obtained. But above T_c, only inhomogeneous disordered films exist. Because T_c of OTS (28 ± 4 °C) is close to room temperature, it is not surprising that different research groups have often obtained different, even contradictory results.

Allara et al. [18] found that proper substrate hydration is a prerequisite for the deposition. There exists usually a thin water film of 1.0–1.5-nm thickness on SiO₂. It can serve as a reservoir for the hydrolysis of organosilane molecules. Furthermore, for a fully activated (1 0 0) oriented silicon wafer, the number of surface OH-groups has been estimated to be 5 × 10¹⁴ cm⁻² [3], [19], [20]. These groups serve as binding sites during the adsorption. OTS molecules need trace water to hydrolyze. Hydrolysis in solvents favors self-aggregation, whereas hydrolysis on SiO₂ substrate brings about the formation of SAMs. At low water content, it is more difficult for OTS molecule to hydrolyze and the growth of the adsorbed layer could be incomplete [21], [22]. At high water content, OTS aggregates will preform in the solution before adsorption.

Besides CH₃-terminated nonpolar OTS, SH-terminated polar 3-mercaptopropyltrimethoxysilane (MPTMS) SAMs on glass slides [23], SiO₂ [24], [25] or gold [26] have attracted significant attention because of their strong affinity for noble metals such as gold nanoparticles. They can act as a coupling agent between transition metals and SiO₂ [23], [24], [27], [28] and have potential application in catalysis, protein adsorption and biosensors. To our knowledge, unlike OTS SAMs on SiO₂, which have been studied in detail, reports on MPTMS SAMs on SiO₂, especially those concerned with the initial stage growth or AFM surface morphology, are still relatively rare [24], [25]. Hu et al. [25] indicate that MPTMS molecules show good ability of self-assembly on SiO₂ at the concentration of 5 mM. Above this concentration, the domain is composed of many disordered polymers that are higher than 5 nm. However, the only variable in their study was the concentration and the only reaction time given was 30 min. The solvent they used was benzene which is detrimental to health and the influence of other parameters, for example, the solution age, have not been mentioned.

Although the adsorption of OTS SAMs on SiO₂ has been extensively studied, the reproducibility has always been a problem because of the interplay between different parameters. For example, the coexistence of large dendrites and small round islands was not shown in every report [29], [30]. In this study, it was necessary for us to first investigate such adsorption in our experimental environment to establish the basis for comparison. Then we applied the same reaction variables to the study of MPTMS SAMs on the same substrate. We treated this comparative study as a preparative work for the preparation of OTS/MPTMS mixed SAMs. In view of health considerations, unlike Hu et al. [25], we selected toluene instead of benzene as the solvent. We compared the two adsorbed structures on the basis of their AFM, LFM images and their XP spectra. For the preparation of OTS/MPTMS mixed SAMs, Choi et al. [29] have suggested two methods: a one-step (also called co-adsorption) and a stepwise preparation. Our study suggests possible reaction conditions for stepwise adsorption and points out the kinetic infeasibility of co-adsorption.