Electrostatic forces are long-range interactions that play a key role in most chemical systems in nature. Reactions involving charge-separated processes are considered to be electric field responsive. 1,2 Thus, electric field effects make it possible to manipulate the kinetics and/or thermodynamics of chemical reaction processes. We reasoned that electric field effects could affect boronic acid-based dynamic covalent chemistry (DCC). For this end, the use of scanning tunneling microscopy (STM) was considered a good choice, as it combines localized control of a switchable electric field, and high-resolution imaging. So far, most of the studies about electric-field-induced phase transitions are based on the dynamics of non-covalent interactions. 3,4 To the best of our knowledge, electric-field-induced switchable surfaces based on reversible covalent bonds are not explored yet. Herein, we have designed a surface model system to demonstrate the bidirectional guidance of a DCC system by an external electric field, illustrated in Figure 1. By reversing the direction of the electric field that exists between the STM tip and a conductive solid substrate, one can locally control the on-surface polymerization/depolymerization at a liquid/solid interface. Consequently, the reversible transformation between self-assembled monolayers (SAMs) and covalent organic frameworks (COFs) can be monitored at molecular level.

In this article, we discuss the recent progress on controlled structural modification of 2D materials by means of molecular functionalization, with a focus on scanning probe microscopy techniques for their characterization. For many practical applications of these novel materials, it is necessary to tune their electronic and optical properties, and molecule-based functionalization is a powerful approach to reach this. We discuss recent covalent and non-covalent approaches, for functionalization of graphene, transition metal dichalcogenides, black phosphorus, and hexagonal boron nitride. Nanostructuring approaches and their impact on 2D materials' properties are highlighted.

Based on a low-temperature scanning tunneling microscopy study, we present a direct visualization of a cycloaddition reaction performed for some specific fluorinated maleimide molecules deposited on graphene. Up to now, it was widely admitted that such a cycloaddition reaction can not happen without pre-existing defects. However, our study shows that the cycloaddition reaction can be carried out on a defect-free basal graphene plane at room temperature. In the course of covalently grafting the molecules to graphene, the sp conjugation of carbon atoms was broken, and local sp bonds were created. The grafted molecules perturbed the graphene lattice, generating a standing-wave pattern with an anisotropy which was attributed to a (1,2) cycloaddition, as revealed by T-matrix approximation calculations. DFT calculations showed that while both (1,4) and (1,2) cycloadditions were possible on free-standing graphene, only the (1,2) cycloaddition could be obtained for graphene on SiC(0001). Globally averaging spectroscopic techniques, XPS and ARPES, were used to determine the modification in the elemental composition of the samples induced by the reaction, indicating an opening of an electronic gap in graphene.