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

Abstract

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.

Prediction of structure and cation ordering in an ordered normal-inverse double spinel

“…A direct comparison of the measured XRD patterns with those simulated for the ordered double spinel structure and a random distribution of the cations presents strong evidence for the existence of the identified double spinel ordering. Rather than being associated with the specific binary spinels studied here, if the discovered novel double spinel ground state configuration turns out to be a general phenomenon for the entire set of known spinels, the known spinel chemical space can be extended many-fold, thereby opening new avenues to rationally design and tune material functionality for a plethora of applications [55][56][57][58][59][60] .…”

Size Dependence of Doping by a Vacancy Formation Reaction in Copper Sulfide Nanocrystals

“…[3][4][5][6] An important route to achieve additional control of optoelectronic properties is through doping by the introduction of donor or acceptor impurities, [7][8][9] which is the primary method used to control bulk semiconductor properties.A liovalent impurity doping in NCs is challenging because of the difficulty associated with controlled introduction of foreign atoms into the SC NC lattice. [11][12][13][14][15] Va cancy formation in these systems leads to the appearance of alocalized surface plasmon resonance (LSPR) related to the free carriers,and is perhaps the most notable signature of vacancies. This approach is of particular importance when the insertion of impurities is difficult and especially instrumental in NCs made of materials with at endencyt of orm lattice vacancies,s uch as copper chalcogenides and metal oxides.…”

“…This approach is of particular importance when the insertion of impurities is difficult and especially instrumental in NCs made of materials with at endencyt of orm lattice vacancies,s uch as copper chalcogenides and metal oxides. [11][12][13][14][15] Va cancy formation in these systems leads to the appearance of alocalized surface plasmon resonance (LSPR) related to the free carriers,and is perhaps the most notable signature of vacancies. [16][17][18] Thea ppearance of plasmonic behavior within such SC NCs opens apath for investigation of coupled plasmonic-excitonic behavior in quantum confined systems.…”

Transparent Conductive Oxides and Their Applications in Near Infrared Plasmonics

“…Besides, off‐stoichiometry defects (the ratio of A:B:O isn't 2:1:4) can also be used to tuning the doping level of spinel‐type oxides . A GFO (Ga 2 FeO 4 ) nanocrystal system was reported to exhibit LSPR around 1000 nm range . The nanocrystal shows a bipolar characteristic, that holes are majority carriers in Fe‐rich samples and electrons in Ga‐rich ones, and both of those can support SP.…”