Microscopic Characterization of the Interface between Aromatic Isocyanides and Au(111):  A First-Principles Investigation

Abstract

Molecular assemblies comprising aromatic rings have received widespread attention as possible components of molecular electronic devices. An essential prerequisite to understand their stability and transport properties is the microscopic characterization of the interface formed with metallic leads. We present a comprehensive study, based on density functional theory (DFT) of the interface between the Au(111) surface and 1,4-phenylenediisocyanide, the simplest representative of aromatic isocyanides. We find that low coverage is favored at low temperature; this prediction is accessible to experimental validation as the metal work function shift upon adsorption has opposite signs, depending on the coverage. The computed binding energy between the isocyanide groups and the surface (∼0.5 eV) is smaller than that obtained for benzenethiols, but the presence of surface defects may considerably increase binding, by about 0.8 eV. The computed N⋮C stretching frequencies on atop sites show a blue shift, compared with the gas phase, consistent with experiments. In addition, our calculations show that energy differences between geometries with straight and those with tilted molecules on the surface are small compared with room temperature, which may explain the disordered patterns recently inferred from spectroscopy measurements. This suggests that longer isocyanide chains may be better suited to produce ordered self-assembled monolayers (SAMs) on Au(111). Finally, we discuss the electronic properties at the organic/metal interface by including self-energy corrections through many-body perturbation theory and surface polarization effects. Our results indicate that electronic structure calculations beyond DFT are required to make a correct, qualitative assessment of energy level alignment between SAMs and the metallic leads.

Creation of Low-Coordination Gold Sites on Au(111) Surface by 1,4-phenylene Diisocyanide Adsorption

“…At the highest PDI exposures (180 s dosing time), the appearance of a shoulder at *2123 cm -1 , corresponding to a free isocyanide, is assigned to PDI that is adsorbed on the low-coordination gold sites in the one-dimensional chain in a vertical orientation, with one of the isocyanide groups bonded to the exposed gold atom. These species in an upright orientation have been reported several times in the literature for this systems when self-assembled monolayers were formed from solution [22][23][24][25][26].…”

“…In addition, the presence of a single CN vibrational mode is consistent with the proposed structure, since the isocyanide groups are symmetrically equivalent and will therefore only give rise to a single, infrared-allowed vibrational mode. The isocyanide stretching frequency is shifted from the gas-phase value of 2123 cm -1 , consistent with the formation of an isocyanide bond with the surface [22][23][24][25][26][39][40][41][42][43][44][45][46][47][48][49]. Both the C-H bending and CN stretching modes shift somewhat with coverage, and this effect is likely to be due to vibrational coupling between modes in adjacent one-dimensional chains as the coverage increases, and they come closer together.…”

“…This shows peaks at 2137 and 813 cm -1 at a saturation PDI coverage. These modes can be assigned quite straightforwardly to the CN stretching mode (at *2137 cm -1 ) [22][23][24][25][26][39][40][41][42][43][44][45][46][47][48][49], and an out-of-plane C-H bending mode for the benzene ring (813 cm -1 ), which is the only detectable mode in the CH fingerprint region. The out-of-plane bending mode, according to the surface infrared selection rules [21], should be allowed for a benzene ring lying parallel to the surface.…”

“…RAIRS is also used to explore the adsorption of carbon monoxide on a PDI-saturated Au(111) surface. Previous experiments of adsorption of PDI on Au(111) from solution indicated that PDI molecules adsorb in a up-right orientation, with only one of the isocyanide groups bonded to the surface [22][23][24][25][26]. It was recently shown that these PDI molecules are actually a second layer adsorbed on top of the gold low coordination sites linking the flat lying species [1].…”

Energy level alignment at molecule-metal interfaces from an optimally tuned range-separated hybrid functional

“…First, due to its high computational cost, even with today's computing power it is still far more challenging to perform GW calculations of several hundred atoms with periodic boundary conditions, as is the case for molecule-metal interfaces. [31][32][33][34] Moreover, several benchmarking studies, including work on smaller molecule-metal systems, also showed that results from perturbative GW calculations can be challenging and expensive to converge numerically. 23,35,36 Furthermore, it is by now well known that single-shot GW calculations often depend on the underlying starting point, 22,37,38 which adds additional complications for efficient GW calculations of large-scale molecule-metal interfaces and their functional properties.…”