Direct Covalent Functionalization of H-Terminated 2D Germanane with Thiolated Molecules: Passivation and Tuning of Optoelectronic Properties

Abstract

Covalent molecular functionalization allows the physicochemical properties of 2D materials to be precisely tuned and modulated on-demand. Nonetheless, research on the molecular functionalization of 2D monoelemental graphene-like materials─known as Xenes─remains scarce, being mainly restricted to a specific type of solid-state chemical reaction based on the topotactic transformation of bulkier Zintl phases. Herein, a robust and general chemical approach is reported for the direct functionalization of commercially available H-terminated 2D germanene (2D-GeH) with thiolated molecules (R-SH) via Ge-S bond formation. While the material characterization data provide direct experimental evidence of the Ge-S chemical bonding, density functional theory (DFT) calculations also predict its existence. Remarkably, the anchored thiolated molecules also favor the passivation of the 2D Xene against air oxidation, enlarging its benefits for real implementation. As a proof-of-principle, a redox-responsive molecular moiety such as 6-(ferrocenyl)hexanethiol (Fc6-SH) has been exploited to induce changes in the optoelectronic properties of the resulting 2D-GeFc6 heterostructure by simply modulating the external bias potential, making it possible to optically and electrically read out a molecular switch on 2D Xene via implanting molecular responsiveness. Remarkably, the ON/OFF ratio has been shown to be dependent on the distance between the redox-responsive Fc moiety and the 2D Xene surface through the alkyl chain length. Overall, the reported a-la-carte molecular engineering approach provides the basis toward the rapid development of stable 2D-GeR derivatives exhibiting molecule-programmable properties.