Elsevier

Superlattices and Microstructures

Volume 36, Issues 1–3, July–September 2004, Pages 113-121
Superlattices and Microstructures

Functionalization of oxidized silicon surfaces with methyl groups and their characterization

https://doi.org/10.1016/j.spmi.2004.08.026Get rights and content

Abstract

Oxidized silicon surfaces were functionalized with chemically bonded methyl end groups and characterized by means of Fourier transform infrared (FTIR) spectroscopy with the attenuated total reflection (ATR) method, contact angle measurements, scanning force microscopy (SFM), and thermal desorption spectroscopy (TDS). Detailed results are presented for trimethylsilyl (TMS) and pentamethyldisilyl (PMDS) terminated surfaces, which were prepared by silanization with suitable chloro compounds. The IR spectra of the TMS-terminated surface exhibit two CH stretching peaks at 2904 and 2963 cm−1. In the thermal desorption experiments desorption of trimethylsilanol and methane was observed at 550 C. The IR spectra of the PMDS-terminated surface show two CH stretching peaks at 2898 and 2955 cm−1. The thermal desorption spectra indicate cleavage of Si–Si bonds and desorption of trimethylsilane at 530 C. The wetting behavior, adhesion, and mechanical properties were studied by contact angle measurements and SFM. The results are compared with the well-defined Si(111)-(1×1):H surface and a self-assembled monolayer (SAM) on a silicon surface with long hydrocarbon chains, prepared with octadecyltrichlorosilane (OTS, H3C(CH2)17SiCl3). The water contact angle was 82 for TMS and 85 for PMDS end groups. The friction forces measured for the TMS- and PMDS-terminated surfaces were comparable and about 3 times higher than that of the H-terminated silicon and the OTS-SAM surface. The corresponding friction coefficients were 0.17, 0.18, 0.34, and 0.45 for Si(111)-(1×1):H, OTS SAM, TMS, and PMDS surfaces, respectively.

Introduction

SAM techniques have attracted significant attention in the field of materials and surface science because they can be used to tailor surface properties such as wetting, lubrication, micropatterning, corrosion, and biocompatibility [1], [2], [3]. Alkylsiloxane and alkylthiol SAMs with long hydrocarbon chains have been employed to realize a variety of chemical surface modifications. However, a long hydrocarbon chain may reduce the thermal stability and change the mechanical properties, such as elasticity and hardness, which are critical e.g. in nanotribology. Keeping these facts in mind, small alkylsilane molecules containing only methyl groups were chosen to replace the long hydrocarbon chains. Here, we present functionalization of freshly oxidized silicon surfaces with small methylsilyl groups and the characterization of their spectroscopic, mechanical, and thermal properties. They can be useful in nanotribology, silicon-based material devices, and continuum contact mechanics.

The physical and chemical properties of the methyl-terminated surfaces were investigated by FTIR-ATR spectroscopy, advancing contact angle measurements, and lateral force microscopy (LFM). The results are compared with those of the well-studied OTS SAM with seventeen methylene groups in the alkyl chain.

The FTIR-ATR method, originally developed by Harrick [4], has been used previously for the characterization of monolayer and submonolayer films [5], [6], [7], [8]. Most of the substrates onto which alkylsiloxane monolayers have been anchored consisted of silicon, germanium, or silicon oxide. The ATR crystals made of these materials are transparent to IR radiation.

The invention of the LFM [9] has opened up the possibility to measure forces with a spatial resolution approaching the nanometer scale [10]. Thereby, it has become feasible to study the friction behavior and the underlying mechanisms of friction and lubrication at the nanometer scale [11], [12]. In this article, we are reporting the functionalization of freshly oxidized silicon surfaces with small methylsilyl groups and compare the corresponding properties with those of OTS SAM and the ideal Si(111)-(1×1):H surface. It should be noticed that the radius of a TMS group is approximately ∼0.3 nm. Therefore the change of the topography due to this termination was too small to be resolved by SFM measurements. In the case of TMS and PMDS terminations the formation of a dense monolayer is not possible. This is due to steric hindrance and the fact that crosslinking between these silane molecules is not possible.

Section snippets

Sample preparation

The following solvents and chemicals were purchased from Sigma-Aldrich and used for the preparation of the terminations: toluene (99.9%), ethanol (99.9%), isopropanol (99.9%), heptane (99.8%), trimethylchlorosilane, TMCS (C3H9ClSi, 98%), pentamethylchlorodisilane, PMCDS (C5H15ClSi2, 97%), and octadecyltrichlorosilane, OTS (C18H37Cl3Si, 95%).

To prepare the samples, silicon wafers were cleaned in isopropanol in an ultrasonic bath for 5 min. This process was repeated 3 times in order to remove any

FTIR-ATR spectra

Fig. 1 shows the FTIR-ATR spectra of the TMS and PMDS terminations in comparison with the OTS SAM. FTIR studies have been reported for monolayers of OTS on different substrates [14], [15], [16], [17], [18]. The peak frequencies reported of the methylene (CH2) groups were in the range of 2915–2924 cm−1 and 2846–2850 cm−1 for the asymmetric (νas) and symmetric (νs) CH stretching vibration, respectively. Fig. 1(c) shows the spectrum of the OTS monolayer. An intense peak is obtained at ∼2923 cm−1

Conclusions

We have identified the termination of oxidized silicon surfaces with trimethylsilyl (TMS) and pentamethyldisilyl (PMDS) groups by means of FTIR spectroscopy and TDS. The desorption of trimethylsilanol indicates that on TMS-terminated surfaces there were still silanol groups present. However, water contact angles close to 90 show that they were shielded effectively by methyl groups. The mechanical and wetting properties were similar for the two methyl-terminated surfaces and even their thermal

Acknowledgment

Financial support by Heidelberger Druckmaschinen AG is gratefully acknowledged.

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