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Colloidal Synthesis of Bipolar Off-Stoichiometric Gallium Iron Oxide Spinel-Type Nanocrystals with Near-IR Plasmon Resonance

Urso, Carmine, Barawi, Mariam, Gaspari, Roberto, Sirigu, Gianluca, Kriegel, Ilka, Zavelani-Rossi, Margherita, Scotognella, Francesco, Manca, Michele, Prato, Mirko, De Trizio, Luca, Manna, Liberato
Journal of the American Chemical Society 2017 v.139 no.3 pp. 1198-1206
absorption, cations, electrons, gallium, iron, iron oxides, nanocrystals, oxides, semiconductors, stoichiometry, surface plasmon resonance
We report the colloidal synthesis of ∼5.5 nm inverse spinel-type oxide Ga₂FeO₄ (GFO) nanocrystals (NCs) with control over the gallium and iron content. As recently theoretically predicted, some classes of spinel-type oxide materials can be intrinsically doped by means of structural disorder and/or change in stoichiometry. Here we show that, indeed, while stoichiometric Ga₂FeO₄ NCs are intrinsic small bandgap semiconductors, off-stoichiometric GFO NCs, produced under either Fe-rich or Ga-rich conditions, behave as degenerately doped semiconductors. As a consequence of the generation of free carriers, both Fe-rich and Ga-rich GFO NCs exhibit a localized surface plasmon resonance in the near-infrared at ∼1000 nm, as confirmed by our pump–probe absorption measurements. Noteworthy, the photoelectrochemical characterization of our GFO NCs reveal that the majority carriers are holes in Fe-rich samples, and electrons in Ga-rich ones, highlighting the bipolar nature of this material. The behavior of such off-stoichiometric NCs was explained by our density functional theory calculations as follows: the substitution of Ga³⁺ by Fe²⁺ ions, occurring in Fe-rich conditions, can generate free holes (p-type doping), while the replacement of Fe²⁺ by Ga³⁺ cations, taking place in Ga-rich samples, produces free electrons (n-type doping). These findings underscore the potential relevance of spinel-type oxides as p-type transparent conductive oxides and as plasmonic semiconductors.