Colloidal Chromium-Doped In₂O₃ Nanocrystals as Building Blocks for High-T_C Ferromagnetic Transparent Conducting Oxide Structures

Colloidal free-standing Cr³⁺-doped In₂O₃ nanocrystals were synthesized in oleylamine from indium (III) and chromium (III) acetylacetonate precursors. The nanocrystals were treated with trioctylphosphine oxide to remove surface-bound dopant ions and ensure internal doping. The lattice resolved transmission electron microscopy images reveal that nanocrystals are faceted and highly crystalline, with no evidence of a secondary phase formation. The average doping concentration estimated with energy dispersive X-ray spectroscopy at the single nanocrystal level agrees with the average doping concentration from the analogous nanocrystal ensemble measurement. Ligand-field electronic absorption spectroscopy suggests that Cr³⁺ dopants are preferentially substituted for In³⁺ ions in their trigonally distorted octahedral (b) sites in In₂O₃ nanocrystals. Nanocrystalline films, prepared under mild conditions using colloidal Cr³⁺-doped In₂O₃ nanocrystals as building blocks, exhibit robust room temperature ferromagnetism. Structural and compositional analyses combined with the ligand-field spectroscopy indicate intrinsic ferromagnetism in this material. The ability to rationally synthesize and manipulate a new form of transition-metal-doped In₂O₃ nanocrystals opens up new opportunities for spintronics applications and may provide a framework for understanding the origin of ferromagnetism in transparent conducting oxides.