Polar surfaces offer intriguing physical and chemical properties applicable in electronics or catalysis. Cleaving the KTaO$_3$ perovskite along its polar (001) plane provides a well-defined, bulk-terminated surface with KO and TaO$_2$ terminations. As-cleaved surfaces exhibit a high concentration of in-gap states; these electrons predominantly reside at the TaO$_2$-terminated parts of the surface. These electrons can affect surface chemistry, as is demonstrated for CO molecules. CO has two adsorption configurations on the TaO$_2$ termination, and the CO differs in how it couples to the excess electrons. DFT calculations indicate that CO preferentially couples to electron bipolarons.
The work was supported by FWF project P32148-N36, by GACR 20-21727X and GAUK Primus/20/SCI/009.