Daniel E. Gómez

Contributions from radiation damping and surface scattering to the linewidth of the longitudinal plasmon band of gold nanorods: a single particle study

The scattering spectra of single gold nanorods with aspect ratios between 2 and 4 have been examined by dark field microscopy. The results show that the longitudinal plasmon resonance (electron oscillation along the long axis of the rod) broadens as the width of the rods decreases from 14 to 8 nm. This is attributed to electron surface scattering. Analysis of the data using gamma = gamma(bulk) + Anu(F)/L(eff), where L(eff) is the effective path length of the electrons and nu(F) is the Fermi velocity, allows us to determine a value for the surface scattering parameter of A = 0.3. Larger rods with widths of 19 and 30 nm were also examined. These samples also show spectral broadening, which is attributed to radiation damping. The relative strengths of the surface scattering and radiation damping effects are in excellent agreement with recent work on spherical gold nanoparticles by Sönnichsen et al., Phys. Rev. Lett., 2002, 88, 077402; and by Berciaud et al., Nano Lett., 2005, 5, 515.

A new method to position and functionalize metal-organic framework crystals

With controlled nanometre-sized pores and surface areas of thousands of square metres per gram, metal-organic frameworks (MOFs) may have an integral role in future catalysis, filtration and sensing applications. In general, for MOF-based device fabrication, well-organized or patterned MOF growth is required, and thus conventional synthetic routes are not suitable. Moreover, to expand their applicability, the introduction of additional functionality into MOFs is desirable. Here, we explore the use of nanostructured poly-hydrate zinc phosphate (α-hopeite) microparticles as nucleation seeds for MOFs that simultaneously address all these issues. Affording spatial control of nucleation and significantly accelerating MOF growth, these α-hopeite microparticles are found to act as nucleation agents both in solution and on solid surfaces. In addition, the introduction of functional nanoparticles (metallic, semiconducting, polymeric) into these nucleating seeds translates directly to the fabrication of functional MOFs suitable for molecular size-selective applications.

Surface Plasmon Mediated Strong Exciton−Photon Coupling in Semiconductor Nanocrystals

We present an experimental demonstration of strong coupling between a surface plasmon propagating on a planar silver thin film and the lowest excited state of CdSe nanocrystals. Attenuated total reflection measurements demonstrate the formation of plasmon-exciton mixed states, characterized by a Rabi splitting of approximately 112 meV at room temperature. Such a coherent interaction has the potential for the development of nonlinear plasmonic devices, and furthermore, this system is akin to those studied in cavity quantum electrodynamics, thus offering the possibility to study the regime of strong light-matter coupling in semiconductor nanocrystals under easily accessible experimental conditions.We present an experimental demonstration of strong coupling between a surface plasmon propagating on a planar silver substrate, and the lowest excited state of CdSe nanocrystals. Variable-angle spectroscopic ellipsometry measurements demonstrated the formation of plasmon-exciton mixed states, characterized by a Rabi splitting of ∼ 82 meV at room temperature. Such a coherent interaction has the potential for the development of plasmonic non-linear devices, and furthermore, this system is akin to those studied in cavity quantum electrodynamics, thus offering the possibility to study the regime of strong light-matter coupling in semiconductor nanocrystals at easily accessible experimental conditions.

Surface Plasmon Resonances in Strongly Coupled Gold Nanosphere Chains from Monomer to Hexamer

We present experimental data on the light scattering properties of linear chains of gold nanoparticles with up to six nanoparticles and an interparticle spacing of 1 nm. A red shift of the surface plasmon resonance with increasing chain length is observed. An exponential model applied to the experimental data allows determination of an asymptotic maximum resonance at a chain length of 10-12 particles. The optical data are compared with analytical and numerical calculation methods (EEM and BEM).

Distance and wavelength dependent quenching of molecular fluorescence by Au@SiO2 core-shell nanoparticles.

Gold nanoparticles and nearby fluorophores interact via electromagnetic coupling upon light excitation. We determine the distance and wavelength dependence of this coupling theoretically and experimentally via steady-state and time-resolved fluorescence spectroscopy. For the first time, the fluorescence quenching of four different dye molecules, which absorb light at different wavelengths across the visible spectrum and into the near-infrared, is studied using a rigid silica shell as a spacer. A comprehensive experimental determination of the distance dependence from complete quenching to no coupling is carried out by a systematic variation of the silica shell thickness. Electrodynamic theory predicts the observed quenching quantitatively in terms of energy transfer from the molecular emitter to the gold nanoparticle. The plasmonic field enhancement in the vicinity of the 13 nm gold nanoparticles is calculated as a function of distance and excitation wavelength and is included in all calculations. Relative radiative and energy transfer rates are determined experimentally and are in good agreement with calculated rates. We demonstrate and quantify the severe effect of dye-dye interactions on the fluorescence properties of dyes attached to the surface of a silica nanoparticle in control experiments. This allows us to determine the experimental conditions, under which dye-dye interactions do not affect the experimental results.

Influence of Particle-Substrate Interaction on Localized Plasmon Resonances

We present a theory for determining the localized surface plasmon resonance shifts of arbitrarily shaped metal nanoparticles on a substrate. Using a pseudoparticle concept, an expression for the particle-substrate interaction is derived, providing both physical insight and formulas to estimate the shifted plasmon resonance. The theory is verified against measured scattering spectra of nanorods on substrates. Simple formulas are provided to calculate the resonance of nanorods, spheres, and ellipsoids on dielectric substrate.

Review of the Synthetic Chemistry Involved in the Production of Core/Shell Semiconductor Nanocrystals

Passivation of CdSe semiconductor nanocrystals can be achieved by overcoating the particles with a homogeneous shell of a second semiconductor. Shell layers are grown in monolayer steps to ensure homogeneous growth of the shell. The relative band edges of the two materials determine the photoreactiveity of the resultant core-shell nanocrystals. The critical role of ligands in minimizing nucleation of the shell material during the growth of the passivating layer is emphasized. The delocalization of charge carriers into the shell layers can be followed spectroscopically during the growth processes. The relative spectral shifts are directly correlated to the relative energies of the band edges.

Optical properties of single semiconductor nanocrystals

We present an overview of the current progress in the understanding of the (steady state) optical properties of individual II-VI semiconductor nanocrystals. We begin with a presentation of the conceptual development of the theory required to model the electronic structure of these systems. This is followed by an overview of the current experimental results obtained from the spectroscopy of individual semiconductor nanocrystals, and in particular, we focus on the study of photoluminescence intermittency (blinking) and spectral diffusion. Where possible, we link the experimental observations to the predictions of current theories. We conclude that the surface of small semiconductor crystals plays an important role in determining their optical properties.

The Dark Side of Plasmonics

Plasmonic dark modes are pure near-field modes that can arise from the plasmon hybridization in a set of interacting nanoparticles. When compared to bright modes, dark modes have longer lifetimes due to their lack of a net dipole moment, making them attractive for a number of applications. We demonstrate the excitation and optical detection of a collective dark plasmonic mode from individual plasmonic trimers. The trimers consist of triangular arrangements of gold nanorods, and due to this symmetry, the lowest-energy dark plasmonic mode can interact with radially polarized light. The experimental data presented confirm the excitation of this mode, and its assignment is supported with an electrostatic approximation wherein these dark modes are described in terms of plasmon hybridization. The strong confinement of energy in these modes and their associated near fields hold great promise for achieving strong coupling to single photon emitters.

Optical properties of metal nanoparticle coated silica spheres: a simple effective medium approach

The preparation of gold particle coated silica microspheres is described and the optical absorption spectra for various surface coverages of gold particles are presented. Onion-shell particles with the structure silica-gold-silica are also described. An effective medium model is used to quantify the surface plasmon shifts observed in such particles, which predicts the shifts in surface plasmon band for both solvent refractive index and surface coverage. The resultant microspheres exhibit absorption spectra resembling those of the isolated nanocrystals. However they possess extinction cross-sections orders of magnitude higher than the nanocrystals and can be centrifuged and redispersed more easily than the parent nanocrystals. The same effect can be obtained by coating silica spheres with gold nanorods, so that the plasmon absorption band can be tuned through the visible and NIR wavelength range. It is proposed that such micron-sized “nanoparticles” provide a practical means to amplify the unique optical properties of nanocrystals and make them more amenable to colloid processing.