Growth kinetics and thermodynamic stability of octadecyltrichlorosilane self-assembled monolayer on Si (100) substrate Sneha A. Kulkarni a,b,c , S.A. Mirji a , A.B. Mandale b , R.P. Gupta c , Kunjukrishna P. Vijayamohanan a, * a Physical Chemistry Division, National Chemical Laboratory, Pune 411008, India b Center for Material Characterization, National Chemical Laboratory, Pune 411008, India c Central Electronic Engineering Research Institute, Pilani 333031, India Received 15 February 2005; accepted 8 July 2005 Available online 15 August 2005 Abstract We have studied the growth kinetics and thermodynamic stability of octadecyltrichlorosilane (OTS) self-assembled monolayers on Si (100) substrate in order to understand its role in controlling the adhesion and surface hydrophobicity. Time-dependent contact angle measurements, using water as a function of OTS concentration, show rapid monolayer formation in the initial stage followed by a slow attainment of full coverage and the overall kinetics approximately follows the Langmuir adsorption isotherm. The adsorption rate constant (k a = 150 M  1 s  1 ) is found to be significantly greater than the desorption rate constant (k d = 0.156 s  1 ) while the Gibbs free energy (DG ads ) change amounts to  4.2 kcal/mol suggesting thermodynamic stability of OTS monolayer on a silicon surface. Partial monolayer formation by a Funiform_ growth mechanism, even at low coverage, is revealed by atomic force microscopy (AFM) in conjunction with grazing angle FTIR spectroscopy. Analysis of the interfacial adhesion properties using Zisman plot suggests a critical surface tension (c c ) of 20.7 dyn/cm for OTS monolayer on Si (100) surface. D 2005 Elsevier B.V. All rights reserved. Keywords: Self-assembled monolayers (SAMs); Gibbs’s free energy (DG); Growth kinetics; Zisman plot; Octadecyltrichlorosilane (OTS) 1. Introduction Ultra-thin organic films like self-assembled monolayers (SAMs) and Langmuir–Blodgett (LB) films are extensively studied due to their fundamental importance in surface modification and also for their diverse potential applications in nanotechnology [1–4]. The presence of a simple monomolecular film on a metallic or semiconducting surface can cause dramatic changes in its surface properties and these SAMs are particularly important due to their ability to control wetting, adhesion, lubrication and corro- sion on surfaces and interfaces [1–6]. For example, the presence of SAM on Si (100) surface can tackle the stiction problem of micro-electromechanical systems (MEMS) by providing a suitable low energy surface coating [7–10]. These types of organic monomolecular films acting as a passivation layer cannot only alleviate capillary forces and direct chemical bonding on Si–O surface, but also reduce electrostatic forces if the terminal group is tailored to possess controlled hydrophobicity. The formation of closely packed, covalently bonded monolayer with tunable chain length and hydrophobic terminal group on silicon oxide surface would eliminate the release stiction and reduce the in-use stiction [7–9]. Usually alkyltrichlorosilane based self-assembled monolayers are used for reducing stiction in silicon micromachines and suitable candidates include octadecyltrichlorosilane (OTS), perflorodecyltrichorosilane (FDTS), etc [7–9]. The growth mechanism of OTS-SAM on Si/SiO 2 substrate has been extensively studied in different solvents 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.07.026 * Corresponding author. Fax: +91 20 25893044. E-mail address: vk.pillai@ncl.res.in (K.P. Vijayamohanan). Materials Letters 59 (2005) 3890 – 3895 www.elsevier.com/locate/matlet