Tuning the Surface Electron Accumulation Layer of In₂ O₃ by Adsorption of Molecular Electron Donors and Acceptors

In₂ O₃ , an n-type semiconducting transparent transition metal oxide, possesses a surface electron accumulation layer (SEAL) resulting from downward surface band bending due to the presence of ubiquitous oxygen vacancies. Upon annealing In₂ O₃ in ultrahigh vacuum or in the presence of oxygen, the SEAL can be enhanced or depleted, as governed by the resulting density of oxygen vacancies at the surface. In this work, an alternative route to tune the SEAL by adsorption of strong molecular electron donors (specifically here ruthenium pentamethylcyclopentadienyl mesitylene dimer, [RuCp*mes]₂ ) and acceptors (here 2,2'-(1,3,4,5,7,8-hexafluoro-2,6-naphthalene-diylidene)bis-propanedinitrile, F₆ TCNNQ) is demonstrated. Starting from an electron-depleted In₂ O₃ surface after annealing in oxygen, the deposition of [RuCp*mes]₂ restores the accumulation layer as a result of electron transfer from the donor molecules to In₂ O₃ , as evidenced by the observation of (partially) filled conduction sub-bands near the Fermi level via angle-resolved photoemission spectroscopy, indicating the formation of a 2D electron gas due to the SEAL. In contrast, when F₆ TCNNQ is deposited on a surface annealed without oxygen, the electron accumulation layer vanishes and an upward band bending is generated at the In₂ O₃ surface due to electron depletion by the acceptor molecules. Hence, further opportunities to expand the application of In₂ O₃ in electronic devices are revealed.