Molecular Simulation of Alkyl monolayers on Si(111)

1. Introduction:

Self-assembled monolayers (SAMs) are one of strategies used for minimizing stiction and reducing adhesion and friction in micro- or nano-electromechanical systems (MEMS/NEMS). SAMs on silicon traditionally rely on siloxane chemistry on oxidized surfaces. The alternative approach is the direct thermal reaction between alkene and H-terminated silicon surface to form stable organic films via Si-C linkage. Other methods such as electrochemistry were used by various groups. This new monolayer coating via Si-C linkage has several key advantages over the previously reported silane-based SAMs. Alkyl molecular chains are covalently bound to Si(111) surface, resulting in stable monolayers. Surface functionalization and modification can be further performed on the monolayers for various applications, such as biosensors and MEMS/NEMS.

Molecular mechanics and molecular dynamics simulations were performed to search for the optimal packing structure of C18 alkyl monolayers on Si(111) surface. The optimal molecular packing was found based on the consideration of three major factors: molecular substitution, substitution pattern, and molecular orientation on Si(111) surface. Ab initio quantum chemical calculations were also preformed to validate the force field used for alkyl/Si(111) system. It was shown that the optimal packing structure was size-independent when it was extended to large systems. Molecular dynamics simulation results based on the optimal packing structure in a temperature range from 50 to 500 K agree with those from experimental measurements for various properties, including system and molecular tilt angles, film thickness, and gauche defects. The optimal packing structure found in this work is a basis for further molecular simulation study of alkyl monolayers on silicon for various engineering applications, such as adhesion and friction reduction in MEMS.


2. Molecular substitutions and energies:


3. Substitution patterns and molecular orientations:


4. Optimal packing structure [C18-alkyl/Si(111)]:


5. MD results from the optimal structure:


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January 21, 2002.