Black GE based on crystalline/amorphous core/shell nanoneedle arrays

Direct growth of black Ge on low-temperature substrates, including plastics and rubber is reported. The material is based on highly dense, crystalline/amorphous core/shell Ge nanoneedle arrays with ultrasharp tips (˜4 nm) enabled by the Ni catalyzed vapor-solid-solid growth process. Ge nanoneedle arrays exhibit remarkable optical properties. Specifically, minimal optical reflectance (<1%) is observed, even for high angles of incidence (˜75°) and for relatively short nanoneedle lengths (˜1 μm). Furthermore, the material exhibits high optical absorption efficiency with an effective band gap of ˜1 eV. The reported black Ge can have important practical implications for efficient photovoltaic and photodetector applications on nonconventional substrates.