Molecular Adsorption and Resonance Coupling at the Colloidal Gold Nanoparticle Interface

Second harmonic generation is used to investigate the adsorption properties of malachite green, brilliant green, and methyl green to the surface of 80 nm colloidal gold nanoparticles capped with mercaptosuccinic acid in water. The experimental results are fit using the modified Langmuir model to obtain the free energies of adsorption and the adsorbate site densities for each cationic triphenylmethane molecular dye. Malachite green is observed to bind more strongly than brilliant green or methyl green to the nanoparticle surface but has a lower adsorbate site density, indicating differences in image-charge effects, adsorbate–adsorbate repulsions, and adsorption tilt angles. Complementary measurements from extinction spectroscopy show plasmonic and molecular resonance coupling leading to the formation of new polaritonic states and Fano-type resonances with corresponding plasmon and molecular spectral depletions as the adsorbate concentration is increased. The changes in the resonance coupling spectra are compared to the second harmonic generation molecular adsorption results and demonstrate the sensitivity of plasmonic–molecular interactions

Strongly Coupled Electron–Phonon Dynamics in Few-Layer TiSe₂ Exfoliates

Ultrafast electron diffraction is used to probe the time-resolved dynamics in a few-layer TiSe₂ sample. At normal incidence, the suppression of the Bragg diffraction peak intensities following photoexcitation displays strongly biexponential behavior. For tilted samples, changes in the diffraction peak positions reveal coherent acoustic vibrations that are dependent on the sample thickness and that further permit a calculation of the Young’s modulus. The complex room temperature lattice dynamics observed are attributed to strong electron–phonon coupling and electron–lattice equilibration processes, which support a Jahn–Teller origin of the charge density wave behavior in TiSe₂. Additionally, the significant role that the related Kohn anomalies may play in the electron transport dynamics and transition mechanism of this material is emphasized. These results demonstrate the importance of strongly coupled electron–phonon dynamics in the relaxation of electronically excited room temperature TiSe₂, which is expected to impact its applicability in optoelectronics

Enhanced Photothermal Effects and Excited-State Dynamics of Plasmonic Size-Controlled Gold–Silver–Gold Core–Shell–Shell Nanoparticles

The synthesis, characterization, and excited-state dynamics of colloidal gold–silver–gold core–shell–shell nanoparticles are reported. These plasmonic nanoparticles are spherical in shape with uniform shells. The plasmonic extinction peak wavelengths can be controlled over the visible and near-infrared regions by varying the thicknesses of the gold and silver shells. These unique spectroscopic properties make these nanoparticles potential candidates for biologically relevant applications including photothermal cancer therapy and biosensing. The ratio of the gold shell thickness to the overall particle size shows a linear dependence with the position of the plasmon extinction peak wavelength. Temperature measurements after laser irradiation show that the colloidal core–shell–shell nanoparticles have a higher photothermal effect compared to spherical gold nanoparticles and gold nanorods. Transient absorption measurements determine that the phonon–phonon scattering lifetime is considerably faster in the core–shell–shell nanoparticles than in the gold nanospheres and gold nanorods, which contributes to the higher photothermal efficiencies. In addition, the synthesis of extended core–shell architectures with controllable core and shell dimensions of alternating gold/silver shells is reported for advanced plasmonic engineering

Anomalous Size-Dependent Excited-State Relaxation Dynamics of NanoGUMBOS

The synthesis, characterization, and ultrafast spectroscopy of size-selected nanospheres of ruthenium bipyridine–bis­(pentafluoroethylsulfonyl)­imide ([Ru­(bipy)₃]­[BETI]₂) in water are reported. These studies represent the first experimental evidence of phonons with nanosecond lifetimes in organometallic nanomaterials. Thermally stable, crystalline nanoparticles of [Ru­(bipy)₃]­[BETI]₂ are derived from a group of uniform materials based on organic salts (GUMBOS). Excited-state relaxation dynamics are studied using pump–probe time-resolved transient absorption spectroscopy, and the results are compared to corresponding measurements of aqueous Ru­(bipy)₃Cl₂. The nanoGUMBOS show spectral shifts and size-dependent relaxation dynamics for nanoparticle diameters varying from 20 to 100 nm, characterized by excited-state decay dynamics similar to those of the precursor dye at higher pump pulse energies with an additional pathway attributed to intermolecular energy transfer, where all lifetimes increase with increasing nanoparticle size. Long-lived acoustic phonon oscillations with size-dependent frequencies are also observed, where the phonon frequency increases as the nanoparticle size increases, suggesting a very low coupling between electronic and phonon degrees of freedom and a strong hydrophobic interaction with the aqueous solvent. These studies provide new insights into the photodynamics of these novel nanoGUMBOS for potential advances in dye-sensitized solar cells and other optoelectronic devices, including hot-carrier extraction photovoltaics

Silica–Conjugated Polymer Hybrid Fluorescent Nanoparticles: Preparation by Surface-Initiated Polymerization and Spectroscopic Studies

Organic/inorganic hybrid nanoscale materials possess fascinating optical, electronic, magnetic, and catalytic properties that are promising for a variety of practical applications. Such properties can be dramatically affected by the hierarchical structure and molecular organization in the nanomaterials. Herein, we employed surface-initiated Kumada catalyst-transfer polymerization to prepare hybrid materials consisting of shells of conjugated polymers (CPs)polythiophene or poly­(<i>p</i>-phenylene)and their block copolymers covalently attached to the surface of silica nanoparticles. Because of the controlled chain-growth mechanism of surface-initiated polymerization, we obtained structurally well-defined CP blocks in the diblock copolymer shells, which produced distinct spectroscopic properties related to the intraparticle excitation energy transfer between the nanoscale polymer shell components, as well as the formation of interfacial exciplex states. The spectroscopic phenomena were further understood via time-resolved transient absorption spectroscopy studies. Overall, the surface-initiated polymerization provided an efficient tool to prepare structurally defined and highly stable organic polymer shell–inorganic core nanoparticles with tunable spectroscopic characteristics not achievable from corresponding single-component systems