**3. Conclusions**

This review briefly documented the definition, research history, and essential properties of the XBs. The XB, as a "specific supramolecular interaction" has been widely used in the preparation of complex 2D self-assembly motifs. This review includes 28 molecules that were designed and home-synthesized carefully, and which were grouped by the similarity of π-conjugated cores. Those molecules within the same family are designed by changing the position and number of the halogen substituents on the aromatic conjugated cores and are used to explore the formation of XBs under different solvents and concentrations at the liquid/solid interface. STM observation and DFT calculations show that an X–X contact often is accompanied by a concomitant HB, and the two bonds act as the collaborative forces to stabilize the 2D adlayers. Moreover, varying the type, number and position of halogen substitutes on the π-conjugated cores can induce the rearrangemen<sup>t</sup> of the electronic density distribution of the molecules, which can give rise to new molecular arrangements on surfaces. However, to confirm the fabrication of networks driven by XBs, significant efforts need to be involved in the design of 2D crystal engineering. Furthermore, the dominant force of molecule–substrate interactions on the formation of self-assembled nanostructures is difficult to quantify in terms of the real contribution of XBs for stabilizing the supramolecular networks. Fortunately, the high-resolution STM images could support precise determination of interatomic distances (bond length) and angles (bond angle), which thus allows better insight into XBs and X–X interactions.

**Author Contributions:** Writing—original draft preparation, Y.W.; writing—review and editing, X.M.; supervision, W.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Natural Science Foundation of Guangdong Province gran<sup>t</sup> number 2018A030313452, the Science and Technology Program of Guangzhou gran<sup>t</sup> number 202002030083, and the Fundamental Research Funds for the Central Universities (SCUT).

**Acknowledgments:** Financial support from the Natural Science Foundation of Guangdong Province (2018A030313452), the Science and Technology Program of Guangzhou (202002030083), and the Fundamental Research Funds for the Central Universities (SCUT) is gratefully acknowledged.

**Conflicts of Interest:** The authors declare no conflict of interest.
