Direct Covalent Functionalization\nof H‑Terminated\n2D Germanane with Thiolated Molecules: Passivation and Tuning of Optoelectronic\nProperties

Covalent molecular functionalization allows the physicochemical\nproperties of 2D materials to be precisely tuned and modulated on-demand.\nNonetheless, research on the molecular functionalization of 2D monoelemental\ngraphene-like materialsknown as Xenesremains scarce,\nbeing mainly restricted to a specific type of solid-state chemical\nreaction based on the topotactic transformation of bulkier Zintl phases.\nHerein, a robust and general chemical approach is reported for the\ndirect functionalization of commercially available H-terminated 2D\ngermanene (<b>2D-GeH</b>) with thiolated molecules (<b>R-SH</b>) via Ge–S bond formation. While the material characterization\ndata provide direct experimental evidence of the Ge–S chemical\nbonding, density functional theory (DFT) calculations also predict\nits existence. Remarkably, the anchored thiolated molecules also favor\nthe passivation of the 2D Xene against air oxidation, enlarging its\nbenefits for real implementation. As a proof-of-principle, a redox-responsive\nmolecular moiety such as 6-(ferrocenyl)hexanethiol (<b>Fc</b><sub><b>6</b></sub><b>-SH</b>) has been exploited to\ninduce changes in the optoelectronic properties of the resulting <b>2D-GeFc</b><sub><b>6</b></sub> heterostructure by simply\nmodulating the external bias potential, making it possible to optically\nand electrically read out a molecular switch on 2D Xene via implanting\nmolecular responsiveness. Remarkably, the ON/OFF ratio has been shown\nto be dependent on the distance between the redox-responsive Fc moiety\nand the 2D Xene surface through the alkyl chain length. Overall, the\nreported a-la-carte molecular engineering approach provides the basis\ntoward the rapid development of stable <b>2D-GeR</b> derivatives\nexhibiting molecule-programmable properties.