Understanding the evolution of the structure and properties in metals from molecule-like to bulk-like has been a long sought fundamental question in science, since Faraday's 1857 work. We report the discovery of a Janus nanomolecule, Au 191 (SPh-tBu) 66 having both molecular and metallic characteristics, explored crystallographically and optically and modeled theoretically. Au 191 has an anisotropic, singly twinned structure with an Au 155 core protected by a ligand shell made of 24 monomeric [−S−Au−S−] and 6 dimeric [−S−Au−S−Au−S−] staples. The Au 155 core is composed of an 89-atom inner core and 66 surface atoms, arranged as [Au 3 @Au 23 @Au 63 ]@Au 66 concentric shells of atoms. The inner core has a monotwinned/stackingfaulted face-centered-cubic (fcc) structure. Structural evolution in metal nanoparticles has been known to progress from multiply twinned, icosahedral, structures in smaller molecular sizes to untwinned bulk-like fcc monocrystalline nanostructures in larger nanoparticles. The monotwinned inner core structure of the ligand capped Au 191 nanomolecule provides the critical missing link, and bridges the size-evolution gap between the molecular multipletwinning regime and the bulk-metal-like particles with untwinned fcc structure. The Janus nature of the nanoparticle is demonstrated by its optical and electronic properties, with metal-like electron−phonon relaxation and molecule-like long-lived excited states. Firstprinciples theoretical explorations of the electronic structure uncovered electronic stabilization through the opening of a shell-closing gap at the top of the occupied manifold of the delocalized electronic superatom spectrum of the inner core. The electronic stabilization together with the inner core geometric stability and the optimally stapled ligand-capping anchor and secure the stability of the entire nanomolecule.
The change in refractive index around plasmonic nanoparticles upon binding to biomolecules is routinely used in localized surface plasmon resonance (LSPR) based biosensors and in bio-sensing platforms. In this study, the plasmon sensitivity of hollow gold (Au) nanoshells is studied using theoretical modeling where the influence of shape, size, shell thickness and aspect ratio are addressed. Different shapes of hollow Au nanoshells are studied that include: sphere, disk, triangular prism, rod, ellipsoid, and rectangular block. Multi-layered Mie theory and discrete dipole approximation (DDA) were used to determine the LSPR peak position, and LSPR sensitivity as a function of size, shell thickness, shape, and aspect ratio. The change in LSPR peak wavelength per unit refractive index is defined as the sensitivity, and interesting results were obtained from the analysis. The rectangular block and rod-shaped Au nanoshells have shown maximum LSPR sensitivity when compared to other shaped Au nanoshells.In addition, increased sensitivity was observed for higher aspect ratio as well as for smaller shell thicknesses. The results are rationalized based on the inner and outer surface plasmonic coupling.
The influence of passivating ligand on electron-phonon relaxation dynamics of the smallest sized gold clusters was studied using ultrafast transient absorption spectroscopy and theoretical modeling. The electron dynamics in Au279, Au329, and Au329 passivated with 4-tert-butylbenzene thiol (TBBT), phenylethane thiol (SC2Ph) and hexane thiol (SC6), respectively, were investigated. These clusters were chosen as they are the smallest gold clusters reported till-date to show plasmonic behavior. Ultrafast transient absorption measurements were also carried out on Au~1400 (SC6) and Au~2000 (SC6) to understand the influence of the size on electron-phonon relaxation with the same passivating ligand. The study has revealed interesting aspects on the role of ligand on electron-phonon relaxation dynamics
Metal Enhanced Fluorescence (MEF) has promising applications in the field of optical displays, bio-sensing and photodynamic therapy. In this work, we exploit the plasmons of embedded silver nanoparticles (Ag NPs) fabricated by ion implantation to enhance the fluorescence of Coumarin515 dye (C515) via MEF. Ion Implantation of 70 keV Ag ions in quartz matrix at different fluences was carried out to synthesize Ag nanoparticles inside quartz matrix. The formation of Ag NPs is characterized by the optical absorption measurements and approximate sizes of Ag NPs was obtained from the fitting of the optical absorption spectra with Mie theory calculations. Rutherford Backscattering Spectrometry (RBS) measurement was used to obtain the depth profile and concentration Ag within the substrate. From the RBS results, it was determined that front edge of the layer containing Ag was formed at an average depth of 16 nm below the surface, which closely agreed with Stopping and Range of Ions in Matter (SRIM) calculations. Increase in the size of the NPs is observed as the fluence of the Ag within the substrate is increased. The MEF of drop casted C515 dye was studied using steady-state emission and excitation spectra measurements. Fluorescence enhancement factor ranging from 1.0 to 2.1 with a maximum enhancement for the largest size NP was obtained. The observed MEF was ascribed to a combination of plasmon enhancement with larger nanoparticles and to increased plasmonic hot spots.
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