Mechanism of the potential-triggered surface transformation of germanium in acidic medium studied by ATR-IR...

In acidic solution, germanium surfaces undergo a transformation to hydrogen-terminated surfaces at sufficiently negative electrode potentials. Herein, we used in situ and operando attenuated total reflection infrared (ATR-IR) spectroscopy coupled to electrochemical experiments to study the details of this surface transformation on Ge(111) and Ge(100) in 0.1 M HClO₄. The ATR-IR data gathered during the surface transformation are consistent with an interpretation according to which an intermediate state exists of a surface with mixed termination. In the mixed termination, both H and OH are bound to the surface, which showed a Ge-H stretching mode at ∼2025-2030 cm^-1. At sufficiently negative potentials, the surfaces became fully hydrogen terminated. ATR-IR spectra can be understood by assigning the peak at ∼1977-1990 cm^-1 to the stretching mode of GeH₁ species on Ge(111), and the peak at ∼2000-2015 cm^-1 to a stretching mode of GeH₂ species on Ge(100). Measurements of the linear dichroism showed the GeH₁ species to be oriented predominantly upright. The transition dipole moment of the GeH₂ species was oriented parallel to the surface, as expected for an antisymmetric stretching mode.