Low-cost, fast and easy production of germanium nanostructures and interfacial electron transfer dynamics of BODIPY–germanium nanostructure system

Germanium nanostructures are prepared by electrochemical etching of n-type Sb-doped (100) oriented germanium (Ge) substrates with resistivity of 0.01 Ω cm. Ge substrates are etched in an electrochemical double cell containing hydrofluoric acid and ethanol solution at room temperature. Although the use of illumination source is essential for etching of an n-type semiconductor material, the influence of illumination source type on the germanium surface morphology has not yet been investigated. In this work, the illumination effect is studied by halogen lamp, white LED, 470- and 405-nm pulsed diode laser. It is demonstrated that different Ge surface morphologies such as nanocone, nanorod, nanoplate and nanopyramid are obtained using different illumination source. The current density, anodization time and pulsed laser power density effects on Ge nanopyramid are investigated in order to optimize anodization conditions. The most uniform and continuous Ge nanopyramid array is obtained at the current density of 30 mA/cm² for 45 min under cathode side illumination with 470-nm pulsed diode laser. It is observed that the nanostructured Ge surfaces exhibit a broad photoluminescence band between 400 and 650 nm. Time-resolved fluorescence spectroscopy studies of electron transfer process between BODIPY dye and Ge nanostructures are reported. The obtained fluorescence lifetime data are analyzed in the light of the Marcus electron transfer theory to determine the conduction band energy level of nanostructured germanium substrates.