(303g) Reactive Molecular Dynamics Study of Alkylsilane Monolayers On Realistic Amorphous Silica Substrates

Alkylsilane monolayers are used in many applications, including as lubricants for micro- and nano-electromechanical systems (MEMS and NEMS), where they are employed to reduce stiction and protect surfaces from oxidation and wear. Molecular simulation has begun to play an important role in understanding the molecular-level properties and behavior of these systems and has aided in the understanding and design of monolayer-based nanolubricants (see e.g. references [1,2]).  While simulation studies typically use idealized surfaces on which the alkysilane monolayers are attached, previous work has shown that monolayers assembled on crystalline versus amorphous silica exhibit different frictional forces at identical monolayer densities as a result of voids present in the monolayers assembled on amorphous silica [3]. In this work, in an effort to get even closer to the systems studied experimentally, we study monolayers assembled on an amorphous hydroxide surface layer.  This closely mimics the post-synthesis processing of amorphous silica with “piranha” solution in order to generate surface hydroxide groups that serve as active sites for the attachment of the alkylsilane chains in the experimental preparation of SAMs.

To examine the impact of silica processing on the properties of monolayers, a synthesis mimetic scheme (SMS) was developed that uses the ReaxFF reactive force field [4]. In this study, amorphous silica substrates were exposed to hydrogen peroxide to create an amorphous hydroxide surface layer to which alkylsilane chains were ultimately bonded. Monolayer properties (including nematic order parameter and tilt angle) were investigated as a function of monolayer density and chain length and compared to the properties of idealized systems (including crystalline substrates and non-processed amorphous substrates). In all cases, the SMS-based systems are considerably less uniform than their idealized counterparts; additional simulations demonstrate that this trend toward disorder results from both a non-regular in-plane arrangement of chains in the monolayer and the processing-induced atomic-scale surface roughness of the silica substrate. This study highlights the importance of considering the synthesis and processing steps associated with the creation of monolayers; including these effects is an important step toward the overall goal of using simulation as a method to screen and predict the next generation of monolayer-based lubricants.

[1] Chandross, M., Grest, G., and Stevens, M. “Friction between Alkylsilane Monolayers: Molecular Simulation of Ordered Monolayers,” Langmuir, vol. 18, January 2002.

[2] Booth, B., Vilt, S., Lewis, J., Rivera, J., Buehler, E., McCabe, C., Jennings, K. “Tribological Durability of Silane Monolayers on Silicon,” Langmuir, vol. 27, April 2011.

[3] Lorenz, C., Chandross, M., Grest, G., Stevens, M., and Webb, E. “Tribological Properties of Alkylsilane Self-Assembled Monolayers,” Languir, vol. 21, September 2005.

[4] van Duin, A., Dasgupta, S., Lorant, F., and Goddard, W. “ReaxFF: A Reactive Force Field for Hydrocarbons,” Journal of Physical Chemistry, vol. 105. 2001.