Colloidal Plasmonic Nanostar Antennas with Wide Range Resonance Tunability

Gold nanostars display exceptional field enhancement properties and tunable resonant modes that can be leveraged to create effective imaging tags, phototherapeutic agents, and hot electron-based photocatalytic platforms. Despite having emerged as the cornerstone among plasmonic nanoparticles with respect to resonant strength and tunability, some well-known limitations have hampered their technological implementation. Herein we tackle these recognized intrinsic weaknesses, which stem from the complex, and thus computationally untreatable morphology and the limited sample monodispersity, by proposing a novel 6-spike nanostar, which we have computationally studied and synthetically realized, as the epitome of 3D plasmonic nanoantenna with wide range plasmonic tunability. Our concerted computational and experimental effort shows that these nanostars combine the unique advantages of nanostructures fabricated from the top-down and those synthesized from the bottom-up, showcasing a unique plasmonic response that remains largely unaltered on going from the single particle to the ensemble. Furthermore, they display multiple, well-separated, narrow resonances, the most intense of which extends in space much farther than that observed before for any plasmonic mode localized around a colloidal nanostructure. Importantly, the unique close correlation between morphology and plasmonic response leads the resonant modes of these particles to be tunable between 600 and 2000 nm, a unique feature that could find relevance in cutting edge technological applications

Molybdenum(VI) Oxosulfato Complexes in MoO₃–K₂S₂O₇–K₂SO₄ Molten Mixtures: Stoichiometry, Vibrational Properties, and Molecular Structures

The structural and vibrational properties of molybdenum­(VI) oxosulfato complexes formed in MoO₃–K₂S₂O₇ and MoO₃–K₂S₂O₇–K₂SO₄ molten mixtures under an O₂ atmosphere and static equilibrium conditions were studied by Raman spectroscopy at temperatures of 400–640 °C. The corresponding composition effects were explored in the <i>X</i>_MoO₃⁰ = 0–0.5 range. MoO₃ undergoes a dissolution reaction in molten K₂S₂O₇, and the Raman spectra point to the formation of molybdenum­(VI) oxosulfato complexes. The MoO stretching region of the Raman spectrum provides sound evidence for the occurrence of a dioxo Mo­(O)₂ configuration as a core. The stoichiometry of the dissolution reaction MoO₃ + <i>n</i>S₂O₇^2– → C^2<i>n</i>– was inferred by exploiting the Raman band intensities, and it was found that <i>n</i> = 1. Therefore, depending on the MoO₃ content, monomeric MoO₂(SO₄)₂^2– and/or associated [MoO₂(SO₄)₂]_<i>m</i>^2<i>m</i>– complexes are formed in the binary MoO₃–K₂S₂O₇ molten system, and pertinent structural models are proposed in full consistency with the Raman data. A 6-fold coordination around Mo is inferred. Adjacent MoO₂²⁺ cores are linked by bidentate bridging sulfates. With increasing temperature at concentrated melts (i.e., high <i>X</i>_MoO₃⁰), the observed spectral changes can be explained by partial dissociation of [MoO₂(SO₄)₂]_<i>m</i>^2<i>m</i>– by detachment of S₂O₇^2– and formation of a MoOMo bridge. Addition of K₂SO₄ in MoO₃–K₂S₂O₇ results in a “follow-up” reaction and formation of MoO₂(SO₄)₃^4– and/or associated [MoO₂(SO₄)₃]_<i>m</i>^4<i>m</i>– complexes in the ternary MoO₃–K₂S₂O₇–K₂SO₄ molten system. The 6-fold Mo coordination comprises two oxide ligands and four O atoms linking to coordinated sulfate groups in various environments of reduced symmetry. The most characteristic Raman bands for the molybdenum­(VI) oxosulfato complexes pertain to the Mo­(O)₂ stretching modes: (1) at 957 (polarized) and 918 (depolarized) cm^–1 for the ν_s and ν_as Mo­(O)₂ modes of MoO₂(SO₄)₂^2– and [MoO₂(SO₄)₂]_<i>m</i>^2<i>m</i>– and (2) at 935 (polarized) and 895 (depolarized) cm^–1 for the respective modes of MoO₂(SO₄)₃^4– and [MoO₂(SO₄)₃]_<i>m</i>^4<i>m</i>–. The results were tested and found to be in accordance with ab initio quantum chemical calculations carried out on [MoO₂(SO₄)₃]^4– and [{MoO₂}₂(SO₄)₄(μ-SO₄)₂]^8– ions, in assumed isolated gaseous free states, at the DFT/B3LYP (HF) level and with the 3-21G basis set. The calculations included determination of vibrational infrared and Raman spectra, by use of force constants in the Gaussian 03W program