Growth kinetics and thermodynamic stability of octadecyltrichlorosilane self-assembled monolayer on Si(100) substrate

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 (Δ G_ads) change amounts to -4.2 kcal/mol suggesting thermodynamic stability of OTS monolayer on a silicon surface. Partial monolayer formation by a 'uniform' 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 (γ _c) of 20.7 dyn/cm for OTS monolayer on Si (100) surface.