Structural Phase Transitions of Molecular Self-Assembly Driven by Nonbonded Metal Adatoms

Accurate Simulations of Scanning Tunneling Microscope: Both Tip and Substrate States Matter

Scanning tunneling microscope (STM) provides an atomic-scale characterization tool. To this end, high-resolution measurements and accurate simulations must closely cooperate. Emerging experimental techniques, e.g., substrate spacers and tip modifications, suppress metallic couplings and improve the resolution. On the other hand, development of STM simulation methods was inactive in the past decade. Conventional simulations focus on the electronic structure of the substrate, often overlooking detailed descriptions of the tip states. Meanwhile, the overwhelming usage of periodic boundary conditions ensures effective simulations of only neutral systems. In this Perspective, we highlight the recent progress that takes the effects of both tip and substrate into account under either Tersoff-Hamann or Bardeen's approximation, which provides an accurate analysis of measured high-resolution STM results, uncovers underlying concepts, and rationally designs experimental protocols for important chemical systems. We hope this Perspective will stimulate broad interest in advanced STM simulations, highlighting the way forward for STM investigations that involve complex geometrical and electronic structures.

On-Surface Synthesis of a Carbon Nanoribbon Composed of 4-5-6-8-Membered Rings

From the structure point of view, there are a number of ways of tiling a carbon sheet with different polygons, resulting in prospects of tailoring electronic structures of low-dimensional carbon nanomaterials. However, up to now, the experimental fabrication of such structures embedded with periodic nonhexagon carbon polygons, especially ones with more than three kinds, is still very challenging, leaving their potential properties unexplored. Here we report the bottom-up synthesis of a nanoribbon composed of 4-5-6-8-membered rings via lateral fusion of polyfluorene chains on Au(111). Scanning probe microscopy unequivocally determines both the geometric structure and the electronic properties of such a nanoribbon, revealing its semiconducting property with a bandgap of ∼1.4 eV on Au(111). We expect that this work could be helpful for designing and synthesizing complicated carbon nanoribbons.

Concentration-modulated global organizational chirality at the liquid/solid interface

Understanding the origin of homochirality in macroscopic assemblies and manipulating organizational chirality still remain a challenge. Herein, homochirality is achieved by combination of the majority-rules principle and concentration-dependent molecular assembly at the liquid/solid interface. A lower molecular concentration in solution facilitates more efficient amplification of chirality, which is formulated by a cooperative equilibrium model based on the Langmuir adsorption isotherm. Our results contribute to gain a new insight into chiral amplification in supramolecular assemblies. Particularly, a homochiral monolayer can be obtained just through modulating the molecular concentration in mixed enantiomer systems.

Real-space visualization of sequential debromination of polybrominated benzenes on Ag(111)

By a combination of scanning tunneling microscopy imaging and density functional theory calculations, dehalogenation processes of symmetric polyhalogenated benzenes were explored on Ag(111), and a series of intermediate states were captured and visualized in real space. These results reveal a sequential dehalogenation scenario of symmetric polybrominated aromatics, which will broaden the understanding of on-surface dehalogenation reactions.

On-Surface Self-Assembled Structural Transformation Induced by Schiff Base Reaction and Hydrogen bonds

By utilizing scanning tunneling microscopy (STM), the self-assembled nanostructures of three characteristic aldehydes have been examined at the solution-solid interface. By introducing the active reactant 5-aminoisophthalic acid (5-AIPA), we succeeded in changing the self-assembled molecular structures through the condensation reaction and obtained the information on structural transformation in real time. The corresponding carboxyl conjugated derivatives were formed in situ and developed into the closely packed and ordered molecular architectures via hydrogen bonds at the solution-solid surface. The relevant simulations have been utilized to interpret the mechanisms of forming the nanostructures. The corresponding theoretical calculation is used to explain the reaction mechanism. Compared with the traditional ways, the on-surface condensation reaction in situ could not only provide a more convenient method for regulating the self-assembled architectures but also offer a promising strategy for building functional nanostructures and devices.