First Principles Study of Fatty Acid Monolayers on Au(111)

First principle density functional theory (DFT) calculations are carried out to study fatty (carboxylic) acid molecules adsorbed on Au(111) via their COOH functional group. Focusing on model systems consisting of monolayers of COOHC6H12X molecules with different terminal groups X (X ) COOH, SH, and CH3), we examine various properties including the following: the adsorption structure and the corresponding energetics, the bond dipole and charge transfer at the carboxyl-gold interface and their dependence on the monolayer density, the modification of the Au work function induced by the adsorbed monolayer, the voltage-dependent tunneling current through the monolayer (I-V characteristics) in a molecular junction with a single chemical contact, and the electronic density of states. Our calculations predict that the carboxyl-gold bond dipole is large, ∼1 Debye in the case of dense monolayers, and gives rise to a substantial (∼1.2 eV) electrostatic potential energy drop at the interface. This is at variance with the case of alkanethiol monolayers, for which the S-Au interface bond dipole is very small. This difference between thiol and carboxylic acid monolayers leads to different alignments of the molecular energy levels relative to the Fermi energy of the Au(111) surface, and affects both the computed work function modifications and the I-V characteristics.