Design of Anisotropically Shaped Plasmonic Nanocrystals from Ultrasmall Sn-Decorated In₂O₃ Nanoclusters Used as Seed Materials

Abstract

Ultrasmall inorganic nanoclusters (<2.0 nm in diameter) bridge the gap between individual molecules and large nanocrystals (NCs) and provide the critical foundation to design and prepare new solid-state nanomaterials with previously unknown properties and functions. Herein, for the first time, we report the monodispersed colloidal synthesis and successful isolation of metastable, rhombohedral-phase, <2.0 nm indium oxide (In2O3) nanoclusters. Ultrasmall nanocluster formation is controlled by a kinetically driven growth process, as evaluated through the variation of metal-to-passivating ligand concentrations. Although <2.0 nm-diameter In2O3 nanoclusters are synthesized in the presence of tin (Sn) precursors, they do not display typical localized surface plasmon resonance (LSPR) properties, which are commonly observed in Sn-doped In2O3 (Sn:In2O3) NCs. Our Raman and X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy (HRTEM) analyses support the existence of Sn-decorated In2O3 nanoclusters, where Sn complexes reside on the surface of the nanocluster as Z-type ligands, as opposed to the formation of Sn:In2O3 nanoclusters, which behave as wide band gap (∼5.5 eV) nanomaterials. The experimentally determined band gap is in good agreement with the theoretical effective mass calculations. The newly synthesized Sn-decorated, 1.7 nm-diameter In2O3 nanoclusters are further used as reactive monomers for the seeded growth synthesis of bcc-phase, plasmonic Sn:In2O3 NCs via ex situ injection of In precursors without the addition of any Sn precursors. The LSPR peak of Sn:In2O3 NCs, which appear to form nanoflower assemblies, is tunable in the 1800–4000 nm region and possibly even the deep-IR region. In addition to altering the size and assembly of the spherical Sn:In2O3 NCs by introducing different amounts of indium acetylacetonate, injection of indium chloride precursors in the reaction mixture results in the formation of rod-shaped NCs. Surprisingly, Sn-decorated, <1.5 nm-diameter In2O3 nanoclusters do not grow into large plasmonic Sn:In2O3 NCs. Taken together, the results presented here contribute to the fundamental understanding of the surface free energy of ultrasmall metal oxide nanoclusters and further advance the knowledge on the phase transformation and growth of plasmonic NCs.

UV-resonant magnetoplasmonic properties of chemically synthesized indium nanoparticles

“…According to the In 3d XPS spectra, on the other hand, every spectrum showed two typical peaks that could be fitted by two pairs (In(0) and In 3+ ) of doublets, that is, characteristic peaks located at the binding energies of 443.8 (3d 5/2 ) and 451.3 (3d 3/2 ) eV were assigned to metallic In(0), and those at 445.1 and 452.6 eV corresponded to In 3+ of In(OH) 3 . 32,33 With regard to sample NP-2p, particularly, even though the In 3d XPS spectrum was composed of strong In 3+ and weak In(0) peaks, the presence of metallic In 0 was obvious on the basis of the XRD analysis. Hence the obtained XPS and XRD data along with the TEM images suggest that the nanoparticles can have a core-shell structure; that is, metallic In 0 cores can be covered with amorphous In(OH) 3 layers.…”