Surface Dipoles and Work Functions of Alkylthiolates and Fluorinated Alkylthiolates on Au(111)

Abstract

We study the dipole formation at the surface formed by -CH 3 and -CF 3 terminated shortchain alkyl-thiolate monolayers on Au(111). In particular, we monitor the change in work function upon chemisorption using density functional theory calculations. We separate the surface dipole into two contributions, resulting from the gold-adsorbate interaction and the intrinsic dipole of the adsorbate layer, respectively. The two contributions turn out to be approximately additive. Adsorbate dipoles are defined by calculating dipole densities of free-standing molecular monolayers. The gold-adsorbate interaction is to a good degree determined by the Au-S bond only. This bond is nearly apolar and its contribution to the surface dipole is relatively small. The surface dipole of the self-assembled 2 monolayer is then dominated by the intrinsic dipole of the thiolate molecules. Alkyl-thiolates increase the work function of Au(111), whereas fluorinated alkyl-thiolates decrease it.

Of interfaces and spin transport across nanoscale layers

“…The results are reported in Table 3.1. The adsorption energies are in general agreement with those obtained in previous studies [75,71,79,78,80,82,1]. Some of these structures are too close in energy to uniquely identify a single lowest energy structure.…”

“…These binding sites are close to bridge and on-top sites of the substrate, but usually slightly displaced from these ideal sites, see Ref. [71]. to the calculations, the monothiolate motif is less stable than the dithiolate (2ad) or vacancy (4vac) motifs by a sizeable 0.5 eV per molecule.…”

“…Assuming that the Au(111) surface is essentially not reconstructed upon SAM adsorption, DFT studies on √ 3 × √ 3 packing densities invariably give bridge sites as the most favorable adsorption sites, where the sulfur head-group binds to two Au surface atoms, see Fig. 3.1(4bri) [115,116,117,118,119,120,71]. Moreover, whereas adsorption at a hollow site is calculated to be less stable than at a bridge site, adsorption at an on-top site is mostly found to be unstable, i.e.…”

“…The structure labeled (4bri) refers to an unreconstructed surface with the molecules absorbed on bridge sites ‡ , which has ( √ 3 × √ 3)R30 • periodicity and serves as a reference structure, see Ref. 71. Structures (4vac) and (2ad) are the lowest energy structures from Refs.…”

“…This facilitates matching the Fermi level of the metal to the energy levels of an organic semiconductor in order to minimize the barrier for electron or hole injection. In this context several groups have studied the work function of thiolate SAMs on the Au(111) surface by firstprinciples density functional theory (DFT) calculations [70,71,72,5,73,74]. A SAM sets up a dipole layer that results in a potential step, which modifies the work function.…”