An extended chiral surface coordination network based on Ag7-clusters

Robust Dipolar Layers between Organic Semiconductors and Silver for Energy-Level Alignment

The interface between a metal electrode and an organic semiconductor (OS) layer has a defining role in the properties of the resulting device. To obtain the desired performance, interlayers are introduced to modify the adhesion and growth of OS and enhance the efficiency of charge transport through the interface. However, the employed interlayers face common challenges, including a lack of electric dipoles to tune the mutual position of energy levels, being too thick for efficient electronic transport, or being prone to intermixing with subsequently deposited OS layers. Here, we show that monolayers of 1,3,5-tris(4-carboxyphenyl)benzene (BTB) with fully deprotonated carboxyl groups on silver substrates form a compact layer resistant to intermixing while capable of mediating energy-level alignment and showing a large insensitivity to substrate termination. Employing a combination of surface-sensitive techniques, i.e., low-energy electron microscopy and diffraction, X-ray photoelectron spectroscopy, and scanning tunneling microscopy, we have comprehensively characterized the compact layer and proven its robustness against mixing with the subsequently deposited organic semiconductor layer. Density functional theory calculations show that the robustness arises from a strong interaction of carboxylate groups with the Ag surface, and thus, the BTB in the first layer is energetically favored. Synchrotron radiation photoelectron spectroscopy shows that this layer displays considerable electrical dipoles that can be utilized for work function engineering and electronic alignment of molecular frontier orbitals with respect to the substrate Fermi level. Our work thus provides a widely applicable molecular interlayer and general insights necessary for engineering of charge injection layers for efficient organic electronics.

Successive Deprotonation Steering the Structural Evolution of Supramolecular Assemblies on Ag(111)

In this study, we demonstrate the structural evolution of a two-dimensional (2D) supramolecular assembly system, which is steered by the thermally activated deprotonation of the primary organic building blocks on a Ag(111) surface. Scanning tunneling microscopy revealed that a variety of structures, featuring distinct structural, chiral, and intermolecular bonding characters, emerged with the gradual thermal treatments. According to our structural analysis, in combination with density function theory calculations, the structural evolution can be attributed to the successive deprotonation of the organic building blocks due to the inductive effect. Our finding offers a facile strategy towards controlling the supramolecular assembly pathways and provides a comprehensive understanding of the 2D crystal engineering on surfaces.

Long-Range Chirality Recognition of a Polar Molecule on Au(111)

Chiral molecular self-assemblies were usually achieved using short-range intermolecular interactions, such as hydrogen-, metal-organic, and covalent bonding. However, unavoidable surface defects, such as step edges, surface reconstructions, or site dislocations may limit the applicability of short-range chirality recognition. Long-range chirality recognition on surfaces would be an appealing but challenging strategy for chiral reservation across surface defects at long distances. Now, long-range chirality recognition is presented between neighboring 3-bromo-naphthalen-2-ol (BNOL) stripes on an inert Au(111) surface across the herringbone reconstruction as investigated by STM and DFT calculations. The key to achieving such recognition is the herringbone reconstruction-induced local dipole accumulation at the edges of the BNOL stripes. The neighboring stripes are then forced to adopt the same chirality to create the opposite edged dipoles and neutralize the neighbored dipole moments.