Solving a Century-Old Mystery through the Synthesis and Crystallization of Semiconducting Germanium (II) Oxide, GeO.

New structural motifs for lead-free semiconductors and photonics are highly desirable but challenging to discover often due to thermodynamic instability. After more than a century of ambiguous literature and misidentification, the metastable germanium (II) oxide, GeO, is crystallized for the first time through a comproportionation-based vapor transport method. GeO and GeO·H2O nanoparticles can also be produced on the gram-scale using a facile benchtop method < 100 0C. The crystal structure is described through X-ray diffraction, and the optical properties, thermal stability, and electronic structure of the new semiconductor are detailed. GeO·H2O can be exfoliated into crystalline monolayers of GeO at using a soft wet-chemical approach as a proof-of-concept. GeO has a visible-light absorbing bandgap (Eg = 2.2 eV), a large birefringence (Δn = 0.18), and is demonstrated to be highly photo-active for the degradation of methylene blue under simulated solar light (AM1.5G), decomposing >95 % within three minutes. These results demonstrate that crystalline Ge(II)-containing oxides are more synthesizable than once thought and have exciting properties for semiconductor and photonic applications.