How far the Chemistry of the Self-Assembly Monolayers on Gold Surfaces Affects their Work Function?

Self-assembled monolayers composed of various long-chain aliphatic molecules and different tail functional groups have been synthesized on the Au(111) surface and characterized by Kelvin Probe Force Microscopy and Ultraviolet Photoelectron Spectroscopy. Carboxy, amino, thio and methyl terminal groups have been considered in the design of self-assembled monolayers with different aliphatic chain lengths (from C6 to C16). The work function measurements by Kelvin Probe Force Microscopy have been carried out under controled and room atmosphere. Remarkably, a reduction of the relative humidity from 40 % to 3 % has induced a work function shift up to 0.3 eV. As expected, the changes of the chain length of the aliphatic moiety and of the tail group have a significant impact on the tuning of the measured work function (3.90 eV for dodecanethiol versus 4.57 eV for mercaptohexadecylamine). Surprisingly, the change of the net dipole moment of the tail group (sign and amplitude) does not dominate the work function variations. In contrast, the change of the chain length and the possibility of the tail group to form a complex hydrogen-bond network between molecules, lead to significant modulations of the work function. In order to interpret these original findings, density functional theory models of equivalent self-assembled monolayers adsorbed on the Au(111) surface have been developed at an unprecedented level of description with large supercells including simultaneously 27 coadsorbed molecules and weak van der Waals interactions between them. Such large systems have allowed the theoretical modeling of complex hydrogen-bond networks between molecules when possible (carboxy tail group). The comparison between computed and measured work functions shows a striking agreement, thus allowing the disentanglement of the previously mentioned competing effects. This consistency between experiment and theory will help designing the electronic properties of self-assembled monolayers in the context of molecular electronics and organic transistors.

Léo Bossard-Giannesini , Luis Cardenas , Hervé Cruguel , Aude Demessence , David Loffreda and Olivier Pluchery, Nanoscale, 2023 doi