Ex Situ Covalent Functionalization of Germanene via 1,3-Dipolar Cycloaddition: A Promising Approach for the Bandgap Engineering of Group-14 Xenes.

Group-14 Xenes beyond graphene such as silicene, germanene, and stanene have recently gained a lot of attention for their peculiar electronic properties, which can be tuned by covalent functionalization. Up until now, reported methods for the top-down synthesis of covalently functionalized silicene and germanene typically yield multilayered flakes with a minimum thickness of 100 nm. Herein, the ex situ covalent functionalization of germanene (fGe) is reported via 1,3-dipolar cycloaddition of the azomethine ylide generated by the decarboxylative condensation of 3,4-dihydroxybenzaldehyde and sarcosine. Amorphous few-layered sheets (average thickness of ≈6 nm) of dipolarophile germanene (GeX) are produced by thermal dehydrogenation of its fully saturated parent precursor, germanane (GeH). Spectroscopic evidence confirmed the emergence of the dipolarophilic sp domains due to the dehydrogenation of germanane, and their sp hybridization due to the covalent functionalization of germanene. Elemental mapping of the functionalized germanene revealed flakes with a very high abundance of carbon uniformly covering the germanium backbone. The 500 meV increase of the optical bandgap of germanene observed upon functionalization paves the way toward bandgap engineering of other group-14 Xenes, which could potentially be a major turning point in the fields of electronics, electrocatalysis, and photocatalysis.