Syntheses of Colloidal F:In₂O₃ Cubes: Fluorine-Induced Faceting and Infrared Plasmonic Response

Abstract

Cube-shaped nanocrystals (NCs) of conventional metals like gold and silver generally exhibit localized surface plasmon resonance (LSPR) in the visible region with spectral modes determined by their faceted shapes. However, faceted NCs exhibiting LSPR response in the infrared (IR) region are relatively rare. Here, we describe the colloidal synthesis of nanoscale fluorine-doped indium oxide (F:In2O3) cubes with LSPR response in the IR region, wherein fluorine was found to both direct the cubic morphology and act as an aliovalent dopant. Single crystalline 160 nm F:In2O3 cubes terminated by (100) facets and concave cubes were synthesized using a colloidal heat-up method. The presence of fluorine was found to impart higher stabilization to the (100) facets through density functional theory (DFT) calculations that evaluated the energetics of F-substitution at surface oxygen sites. These calculations suggest that the cubic morphology results from surface binding of F-atoms. In addition, fluorine acts as an anionic aliovalent dopant in the cubic bixbyite lattice of In2O3, introducing a high concentration of free electrons leading to LSPR. We confirmed the presence of lattice fluorine dopants in these cubes using solid-state 19 F and 115 In nuclear magnetic resonance (NMR) spectroscopy. The cubes exhibit narrow, shape-dependent multimodal LSPR extinction peaks due to corner-and edge-centered modes. The spatial origin of these different contributions to the spectral response are directly visualized by electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). Experimental Methods Materials: Indium (III) acetylacetonate (In(acac)3, 99.99%), Indium (III) fluoride (InF3, > 99.9%), Indium (III) chloride (InCl3, 99.999%), Indium (III) bromide (InBr3, 99.999%), Oleic acid (OA, 90%, technical grade), Octylamine (OcAm, 99%), Trioctylamine (TOA, 98%) and tetrachloroethylene (TCE, ≥ 99%) were purchased from Sigma-Aldrich. Toluene (99.5%) was purchased from Fisher Chemical. All chemicals were used as received without any further purification. Fluorine-Doped Indium Oxide (F:In2O3) Cube (3% InF3) Synthesis: All synthesis procedures were carried out using standard Schlenk line techniques aided by a nitrogen-filled glovebox. For the growth of 3% doped F:In2O3 cubes, a mixture of In(acac)3 (399.78 mg, 0.97 mmol), InF3 (5.15 mg, 0.03 mmol), OA (1 ml), OcAm (0.5 ml) and TOA (3.5 ml) was loaded in a three-neck round-bottom flask in the glovebox. This precursor mixture was then stirred with a magnetic bar at 600 rpm and degassed under vacuum at 120 °C for 15 min. The mixture turned transparent during this operation signifying the formation of indium oleate. Thereafter, the flask was filled with nitrogen and further heated at a ramp rate of 15 °C/min to 320 °C. The reaction mixture turned cloudy and opaque which signified cube growth and was designated as the growth reaction start time. The growth reaction was allowed to run for 5 min at 320 °C or the desired set point temperature. Subsequently, growth was...

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