Highly crystalline Fe₂GeS₄ nanocrystals: Green synthesis and their structural and optical characterization

The olivine Fe₂GeS₄ compound has attracted much attention as a thermodynamically stable derivative of pyrite FeS₂, which has been studied extensively as an earth-abundant light-absorbing candidate material. Nevertheless, reports on nanocrystalline Fe₂GeS₄ and its optoelectronic properties are limited. Herein, Fe₂GeS₄ nanocrystals are synthesized via a solvent-free mechanochemical process. This process not only reduces the synthesis time, but also avoids the use of hazardous solvents, thereby mitigating environmental concerns. The crystallinity of the synthesized nanocrystals is significantly enhanced by a post-heat treatment in a sulfur-containing atmosphere, showing no phase decomposition. Lattice-resolved micrographs reveal that the post-annealed nanocrystals have a hexagonal-faceted platelet structure with (002) base planes. The oxide layer near the surface region is removed by the post-annealing process, most likely due to the replacement of oxygen with sulfur in the controlled atmosphere. The post-annealed Fe₂GeS₄ nanocrystals clearly exhibit an optical band gap of 1.43 eV and near-band-edge photoluminescent emission at 1.41 eV. This is the first experimental demonstration of the Fe₂GeS₄ nanocrystals having optoelectronic properties that are suitable for solar applications.