Chain-length dependence in surface stresses of alkanethiolate-covered Au(111)*

We have recently shown~\footnote{Phys. Rev. Lett. {\bf 101}, 185504 (2008)} that adsorption-induced stresses in alkanethiolate-covered Au(111) contribute significantly to the stress-response in nano-mechanical cantilever sensors. In particular, we proposed a local stress relief (LSR) mechanism whereby charge removal by the Au-S bond from the Au surface promotes a stress reducing rearrangement of surface Au atoms. Since LSR depends on the nature of the Au-S bond it was unclear how the contribution to the stress- response would depend on the alkanethiolate structure. We present a first-principles study of the chain-length dependence in surface stresses of alkanethiolate-covered Au(111). We find that the surface stress upon adsorption is anisotropic and tensile, increasing in magnitude with the chain-length. We analyze this trend in the context of the LSR mechanism and inter-adsorbate interactions.