Electrodeposition of Protocrystalline Germanium from Supercritical Difluoromethane

We report results for the electrochemistry of the germanium(II) tri-halide anions, [GeCl3]?, [GeBr3]? and [GeI3]?, in supercritical difluoromethane containing 60?mm [NnBu4][BF4] at 19.1?MPa and 358?K. The voltammetry shows mass-transport-limited currents for reduction to germanium at gold on the first scan. There is no evidence of a germanium stripping peak and, on subsequent scans, the electrode slowly passivates with the deposition of approximately 0.4??m of material. The redox potentials for the reduction of the three tri-halides are in the order [GeCl3]? <[GeBr3]? <[GeI3]?, with the iodide being the most easily reduced complex. Electrodeposition of germanium onto TiN electrodes from supercritical difluoromethane at 19.1?MPa and 358?K, using either 16?mm [EMIM][GeI3] with 60?mm [EMIM][BF4] or 16?mm [NnBu4][GeI3] with 60?mm [NnBu4][BF4], gave deposition rates of 2–3??m?h?1. Raman spectroscopy and transmission electron microscopy showed that the resulting germanium films were protocrystalline, containing nanocrystals of germanium embedded in an amorphous germanium matrix