Theoretical study of the structure of self-assembled monolayers of short alkylthiolates on Au(111) and Ag(111): the role of induced substrate reconstruction and chain-chain interactions.

We compare the stability of various structures of high coverage self-assembled monolayers (SAMs) of short alkylthiolates, S(CH(2))(n-1)CH(3) (= C(n)), on Ag(111) and Au(111). We employ: (i) the ab initio thermodynamics approach based on density functional theory (DFT) calculations, to compare the stability of SAMs of C(1) (with coverages Θ = 3/7 and 1/3) on both substrates, and (ii) a set of pairwise interatomic potentials derived from second-order Møller-Plesset (MP2) perturbation theory calculations, to estimate the role of chain-chain (Ch-Ch) interactions in the structure and stability of SAMs of longer chain alkylthiolates. For C(1)/Ag(111) (C(1)/Au(111)) the SAM with Θ = 3/7 is more (less) stable than for Θ = 1/3 in a wide range of temperatures and pressures in line with experiments. In addition, for the molecular densities of SAMs corresponding to Θ = 3/7 and 1/3, the MP2-based Ch-Ch interaction potential also predicts the different chain orientations observed experimentally in SAMs of alkylthiolates on Ag(111) and Au(111). Thus, for short length alkylthiolates, a simple model based on first principles calculations that separately accounts for molecule-surface (M-S) and Ch-Ch interactions succeeds in predicting the main structural differences between the full coverage SAMs usually observed experimentally on Ag(111) and Au(111).

Study of the interaction between short alkanethiols from ab initio calculations.

We have developed a force field to describe the interaction of alkanethiols HS(CH(2))(n-1) CH(3) (C(n) for short) by fitting a set of approximately 220 interaction energies for dimers of C(n) (with n = 1,2,...6) and CH(4) molecules obtained from second-order Møller-Plesset perturbation theory calculations. The derived force field, based on a sum of so-called exp-6 pairwise potentials and effective Coulomb interaction potential between the HS- heads, predicts very well the interaction energies for dimers and trimers of alkanethiols not included in the input database for the fit. Also the calculated minimum energy tilt angle of the alkyl chains for a hexagonal arrangement of alkanethiolates with a nearest neighbor distance of 5 A is in good agreement with the available experimental data for a sqrt [3] x sqrt [3] self-assembled monolayer (SAM) on Au(111). Thus, the derived force field might be suitable for large-scale molecular dynamics and/or Monte Carlo simulations to predict the structure, stability and/or kinetics of SAMs on other surfaces.