*Review* **Halogen Bonds Fabricate 2D Molecular Self-Assembled Nanostructures by Scanning Tunneling Microscopy**

## **Yi Wang, Xinrui Miao \* and Wenli Deng \***

College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China; 201810103602@mail.scut.edu.cn

**\*** Correspondence: msxrmiao@scut.edu.cn (X.M.); wldeng@scut.edu.cn (W.D.)

Received: 30 September 2020; Accepted: 9 November 2020; Published: 20 November 2020

**Abstract:** Halogen bonds are currently new noncovalent interactions due to their moderate strength and high directionality, which are widely investigated in crystal engineering. The study about supramolecular two-dimensional architectures on solid surfaces fabricated by halogen bonding has been performed recently. Scanning tunneling microscopy (STM) has the advantages of realizing in situ, real-time, and atomic-level characterization. Our group has carried out molecular self-assembly induced by halogen bonds at the liquid–solid interface for about ten years. In this review, we mainly describe the concept and history of halogen bonding and the progress in the self-assembly of halogen-based organic molecules at the liquid/graphite interface in our laboratory. Our focus is mainly on (1) the e ffect of position, number, and type of halogen substituent on the formation of nanostructures; (2) the competition and cooperation of the halogen bond and the hydrogen bond; (3) solution concentration and solvent e ffects on the molecular assembly; and (4) a deep understanding of the self-assembled mechanism by density functional theory (DFT) calculations.

**Keywords:** halogen bonding; σ-hole interactions; self-assembly; scanning tunneling microscopy
