Nanoscale electrodeposition of germanium on Au(111) from an ionic liquid: an in situ STM study of phase formation

Abstract

The electrodeposition of germanium from the GeBr₄ saturated ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate on Au(111) was investigated with in situ scanning tunneling microscopy and in part also, qualitatively, with in situI/U tunneling spectroscopy. A thin rough layer forms on the gold before the electrode potential for Ge bulk deposition is reached. On this layer thin islands start growing. At overvoltages of 250 mV nanoclusters of approximately 50 nm in diameter and initial heights of some nanometres grow. At an overvoltage of only 50 mV the deposits can transform within several hours into a layered structure with a step height of 330 ± 30 pm, indicative of Ge(111) bilayers. If the deposition is performed at the onset of bulk deposition, triangularly pyramid shaped nanoclusters with side lengths of 10–20 nm and heights between 1 and 3 nm grow very slowly on the time scale of several hours. With further growth the clusters become rather hemispherical in shape and an approximately 15 nm thick layer consists of coherent nanoclusters with diameters of 10–20 nm and heights of up to 5 nm. At overvoltages of 500 mV micrometre thick Ge films can be rapidly obtained that consist in a wider sense of nanosized coherent wires. In situI/U tunneling spectroscopy of bulk deposits with a thickness of 200 to 500 nm shows a bias range of approximately 800 mV with almost zero tunneling currents. The symmetric band gap of 0.7 ± 0.1 eV is in good agreement with the value of 0.67 eV for microcrystalline intrinsic germanium at 300 K.