Kinked row-induced chirality driven by molecule-substrate interactions

Nonuniform STM Contrast of Self-Assembled Tri-n-octyl-triazatriangulenium Tetrafluoroborate on HOPG

We have assembled 4,8,12-tri-n-octyl-4,8,12-triazatrianguleniumtetrafluoroborate (TATA-BF4) on highly oriented pyrolytic graphite (HOPG) and have studied the structure and tunneling properties of this self-assembled monolayer (SAM) using scanning tunneling microscopy (STM) under ambient conditions. We show that the triazatriangulenium cations TATA+ form hexagonally packed structures driven by the interaction between the aromatic core and the HOPG lattice, as evidenced by density functional theory (DFT) modeling. According to the DFT results, the three alkyl chains of the platform tend to follow the main crystallographic directions of HOPG, leading to a different STM appearance. The STM contrast of the SAM shows that the monolayer is formed by two types of species, namely, TATA+ with BF4- counterions on top and without them. The cationic TATA+ platform gives rise to a seemingly higher appearance than neutral TATA-BF4, in contrast to observations made on metallic substrates. The variation of the STM tunneling parameters does not change the relative difference of contrast, revealing the stability of both species on HOPG. DFT calculations show that TATA-BF4 on HOPG has sufficient binding energy to resist dissociation into TATA+ and BF4-, which might occur under the action of the electric field in the tunneling gap during STM scanning.

Leveraging Molecular Properties to Tailor Mixed-Dimensional Heterostructures beyond Energy Level Alignment

The surface sensitivity and lack of dielectric screening in two-dimensional (2D) materials provide numerous intriguing opportunities to tailor their properties using adsorbed π-electron organic molecules. These organic-2D mixed-dimensional heterojunctions are often considered solely in terms of their energy level alignment, i.e., the relative energies of the frontier molecular orbitals versus the 2D material conduction and valence band edges. While this simple model is frequently adequate to describe doping and photoinduced charge transfer, the tools of molecular chemistry enable additional manipulation of properties in organic-2D heterojunctions that are not accessible in other solid-state systems. Fully exploiting these possibilities requires consideration of the details of the organic adlayer beyond its energy level alignment, including hybridization and electrostatics, molecular orientation and thin-film morphology, nonfrontier orbitals and defects, excitonic states, spin, and chirality. This Perspective explores how these relatively overlooked molecular properties offer unique opportunities for tuning optical and electronic characteristics, thereby guiding the rational design of organic-2D mixed-dimensional heterojunctions with emergent properties.

Where do the counterions go? Tip-induced dissociation of self-assembled triazatriangulenium-based molecules on Au(111)

Chemical coupling of functional molecules on top of the so-called platform molecules allows the formation of functional self-assembled monolayers (SAMs). An often-used example of such a platform is triazatriangulenium (TATA), which features an extended aromatic core providing good electronic contact to the underlying metal surface. Here, we present a study of the SAM formation of a TATA platform on Au(111) employing scanning tunneling microscopy (STM) under ambient atmospheric conditions. In solution, the TATA platform is stabilized by BF4 counterions, while after deposition on a gold substrate, the localization of the BF4 counterions remains unknown. We used 1,2,4-trichlorobenzene as a solvent of TATA-BF4 to induce SAM formation on a heated (∼50 °C) Au substrate. We show by STM how to detect and distinguish TATA-BF4 from TATA platforms, which lost their BF4 counterions. Finally, we observe a change of the counterion position on the SAM during the STM scanning, which we explain by an electric-field-induced decrease of the electrostatic interaction in TATA-BF4 on the surface. We applied DFT calculations to reveal the influence of the gold lattice and the electric field of the STM tip on the stability of TATA-BF4 physisorbed on the surface.