Resolving the Structure of a Well-Ordered Hydroxyl Overlayer on In₂O₃(111): Nanomanipulation and Theory

Changes in chemical and physical properties resulting from water adsorption play an important role in the characterization and performance of device-relevant materials. Studies of model oxides with well-characterized surfaces can provide detailed information that is vital for a general understanding of water-oxide interactions. In this work, we study single crystals of indium oxide, the prototypical transparent contact material that is heavily used in a wide range of applications and most prominently in optoelectronic technologies. Water adsorbs dissociatively already at temperatures as low as 100 K, as confirmed by scanning tunneling microscopy (STM), photoelectron spectroscopy, and density functional theory. This dissociation takes place on lattice sites of the defect-free surface. While the In₂O₃(111)-(1 × 1) surface offers four types of surface oxygen atoms (12 atoms per unit cell in total), water dissociation happens exclusively at one of them together with a neighboring pair of 5-fold coordinated In atoms. These O-In groups are symmetrically arranged around the 6-fold coordinated In atoms at the surface. At room temperature, the In₂O₃(111) surface thus saturates at three dissociated water molecules per unit cell, leading to a well-ordered hydroxylated surface with (1 × 1) symmetry, where the three water O_WH groups plus the surface O_SH groups are imaged together as one bright triangle in STM. Manipulations with the STM tip by means of voltage pulses preferentially remove the H atom of one surface O_SH group per triangle. The change in contrast due to strong local band bending provides insights into the internal structure of these bright triangles. The experimental results are further confirmed by quantitative simulations of the STM image corrugation.