Atomic View on the (111) Surface of a Cu2O Single Crystal: Reconstruction, Electronic Properties, and Band-Bending Effects

The nature of the (√3 × √3)R30° reconstruction of the Cu2O(111) surface has been heavily debated for almost 30 years. This work demonstrates that the nanopyramidal reconstruction model that was recently developed for Cu2O(111) thin films is valid also for bulk samples. For this purpose, the surface of a natural Cu2O(111) crystal was prepared by common surface science methods, yielding a robust and reproducible (√3 × √3)R30° superstructure pattern in electron diffraction. Atomically resolved STM images display a regular array of trifold symmetric protrusions, in perfect agreement with the nanopyramidal model in which Cu4O units attach to every third Cu–O six-membered ring of a Cu-depleted (111) surface. STM conductance spectra reveal the p-type character of the oxide with the valence-band top pinned to the Fermi level. From band-bending effects in the tip electric field, an induced carrier concentration of 6 × 1017 cm–3 is determined for the (√3 × √3)R30° phase. This value increases to 4 × 1018 cm–3 on the few (1 × 1) patches covered with a Cu-poor minority phase, indicating a close interplay between atomic structure and local screening response for a given surface termination. Our work clarifies the nature of the (√3 × √3)R30° reconstruction of bulk Cu2O(111) by connecting it to the recently proposed nanopyramidal reconstruction model.