Author(s) Urso, C.; Barawi, M.; Gaspari, R.; Sirigu, G.; Kriegel, I.; Zavelani-Rossi, M.; Scotognella, F.; Manca, M.; Prato, M.; De Trizio, L.; Manna, L.
Journal J Am Chem Soc
Date Published 2016 Dec 22

We report the colloidal synthesis of ~5.5nm 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 ~1000nm, as confirmed by our pump-probe absorption measurements. Our fitting of experimental LSPR curves revealed that off-stoichiometric GFO NCs have a concentration of free carriers in the order of 1022cm-3, in line with common/typical extrinsically doped metal oxide materials. Noteworthy, the photo-electrochemical 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 follow: 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 conducive oxides and as plasmonic semiconductors.

DOI 10.1021/jacs.6b11063
ISSN 1520-5126
Citation J Am Chem Soc. 2016.

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