Near field of strongly coupled plasmons: uncovering dark modes.

Strongly coupled plasmons in a system of individual gold nanoparticles placed at subnanometer distance to a gold film (nanoparticle-on-plane, NPOP) are investigated using two complementary single particle spectroscopy techniques. Optical scattering spectroscopy exclusively detects plasmon modes that couple to the far field via their dipole moment (bright modes). By using photoemission electron microscopy (PEEM), we detect in the identical NPOPs near-field modes that do not couple to the scattered far field (dark modes) and are characterized by a strongly enhanced nonlinear electron emission process. To our knowledge, this is the first time that both far- and near-field spectroscopy are carried out for identical individual nanostructures interacting via a subnanometer gap. Strongly resonant electron emission occurs at excitation wavelengths far off-resonant in the scattering spectra.

Field emission of electrons generated by the near field of strongly coupled plasmons.

Field emission of electrons is generated solely by the ultrastrong near-field of strongly coupled plasmons without the help of a noticeable dc field. Strongly coupled plasmons are excited at Au nanoparticles in subnanometer distance to a Au film by femtosecond laser pulses. Field-emitted electrons from individual nanoparticles are detected by means of photoelectron emission microscopy and spectroscopy. The dependence of total electron yield and kinetic energy on the laser power proves that field emission is the underlying emission process. We derive a dynamic version of the Fowler-Nordheim equation that yields perfect agreement with the experiment.

Near field guided chemical nanopatterning.

This article demonstrates the possibility of creating well-defined and functional surface chemical nanopatterns using the optical near field of metal nanostructures and a photosensitive organic layer. The intensity distribution of the near field controlled the site and the extent of the photochemical reaction at the surface. The resulting pattern was used to guide the controlled assembly of colloids with a complementary surface functionality onto the substrate. Gold colloids of 20 nm diameter were covalently bound to the activated nanosites and proved the functionality of the suboptical wavelength structures and enabled direct visualization by means of electron microscopy. Our results prove, for the first time, the possibility of using optical near field to perform chemical reactions and assembly at the nanoscale.

Reusable localized surface plasmon sensors based on ultrastable nanostructures.

Nanoparticle arrays created by nanosphere lithography are widely used in sensing applications since their localized surface plasmon resonances are extremely sensitive to changes in the local dielectric environment. A major drawback for any biologically oriented sensing application of conventionally produced particle arrays is the lack of stability of the nanoparticles in aqueous media and buffer solutions. Here, a robust and reusable nanoscale sensing platform based on localized surface plasmon resonances of gold nanoparticles embedded in a silicon dioxide matrix is presented. The architecture exhibits extremely high stability in aqueous environments and can be regenerated several times by simple mechanical cleaning of the surface. The platforms surface is ultraflat by design, thus making it an ideal substrate for any bio-oriented sensing application.

Sensitivity of crescent-shaped metal nanoparticles to attachment of dielectric colloids.

The response of the plasmonic resonances of crescent-shaped nanoparticles to the attachment of a dielectric colloidal particle was investigated. The colloid serves as a model analyte which is easy to handle and allows for benchmarking of the sensing capabilities of plasmonic resonators. A clear red shift of the resonances is observed in agreement with the prediction from numerical simulations. From the response for different colloid positions, we obtain information on the nanoscale near field distribution. A field localization to length scales of 20 nm is proven directly. All resonators under study show a comparable response which is important for possible sensing application. We estimate that a further increase of the sensitivity by a factor of 8 would allow for label-free single biomolecule detection.

Parallel preparation of densely packed arrays of 150-nm gold-nanocrescent resonators in three dimensions.

Metallic nanostructures show interesting optical properties due to their plasmonic resonances, and when arranged in three-dimensional (3D) arrays hold promise for optical metamaterials with negative refractive index. Towards this goal a simple, cheap, and parallel method to fabricate large-area, ordered arrays of 150-nm gold nanocrescents supporting plasmonic resonances in the near-infrared spectral range is demonstrated. In this process hexagonally ordered monolayers of monodisperse colloids are prepared by a simple floating technique, and subsequently the individual particles are size-reduced in a plasma process and used as a shadow mask with the initial lattice spacing. The resulting two-dimensional array of plasmonic resonators is coated with a transparent silica layer, which serves as a support for a second layer prepared by the identical process. The mutual orientation of the nanostructures between the individual layers can be freely adjusted, which determines the polarization-dependent absorption of the array and opens the possibility to introduce chirality in this type of 3D metamaterial. The iteration of this simple and efficient methodology yields 3D arrays with optical features as sharp as those of the individual nanocrescents, and shows strong potential for large-scale production of high-quality optical metamaterials.

One-pot preparation of dendrimer-gold nanoparticle hybrids in a dipolar aprotic solvent.

We present a simple one-pot, one-step method to obtain stable and nearly monodisperse gold nanoparticles in dipolar aprotic solvents. Novel thiomethyl-functionalized polyphenylene dendrimers are used to control the growth and stabilize the nanoparticles in suspension. The dendrimer functionalized gold nanoparticles have an average size of roughly 10 nm and are stable in suspension for several weeks. The stability in dipolar aprotic solvents and the great functionalization flexibility offered by the dendrimers make these metal/dendrimer hybrid systems promising for applications such as nanophotonics, molecular electronics, and sensing.

Plasmon mediated confocal dark-field microscopy.

An efficient mode for scanning confocal dark-field microscopy through a thin gold film is established that takes advantage of the intermediate excitation of surface plasmons both in the excitation and in the emission process. This concept is verified by experimental investigation of the effective point-spread function, the intensity distribution of the scattered radiation and by comparison with a classical dark-field geometry. The wavelength-dependence of both the signal strength and the point-spread function are discussed.

Electrically conductive and optically transparent Sb-doped SnO2 STM-probe for local excitation of electroluminescence.

We demonstrate that an optically transparent and electrically conductive antimon-doped tin-oxide tip that is prepared in a sol-gel process can be used as a probe for scanning tunnelling microscopy (STM), yielding atomic vertical and nanometre lateral resolution. Emission of visible light from the tunnelling junction between gold particles and the tip is observed for bias voltages above 7 V. In contrast to the metallic tips generally used in STM, this tip does not significantly perturb the local optical response. Therefore, the tunnelling induced light can be used to map the optical near-field of surface structures with the tunnel gap acting as highly localised light source for the investigation of near-field enhancement in complex metal structures.

Plasmon hybridization and strong near-field enhancements in opposing nanocrescent dimers with tunable resonances.

A novel dimer nanostructure architecture featuring two symmetrically arranged crescents with opposing, nanometer-sized tips in close proximity is fabricated by colloidal lithography. This structure exhibits a strong and highly localized electrical near-field in the gap region between the tips. The close proximity of the tips in the nanocrescent dimers leads to a strong coupling process which generates new hybrid plasmon modes with different optical resonances. The optical properties of both single crescents and dimeric double crescent arrangements are investigated in detail, and correlations between resonance wavelengths and geometrical parameters are established. We apply plasmon hybridization theory to explain the spectral shifts between coupled and uncoupled crescent nanostructures based on simple geometric arguments for all polarization-dependent resonances. Computer simulations support the hybridization model and were further used to examine and compare the near-field enhancement of single and opposing double crescents. For close proximities of the two opposing crescents, a strong near-field with an enhancement factor of approximately 53 was detected. Compared to the near-field enhancement of approximately 20 for single crescents, the proximity of the second crescents further increases the near-field to more than seven times the initial value.

Fluorescence enhancement from individual plasmonic gap resonances.

We studied the fluorescence enhancement of a dye-loaded polyphenylene dendrimer in a gap of 2-3 nm between a silver film and single silver particles with an average diameter of 80 nm. This sphere-on-plane geometry provides a controllable plasmonic resonator with a defined dye position. A strong fluorescence signal was seen from all particles, which was at least 1000 times stronger than the signal from the plane dye-coated metal surface. The fluorescence emission profile varied between the particles and showed light emission at higher energies than the free dye, which we assigned to hot luminescence. The maximum fluorescence emission peak shifted along with the scattering maximum of the plasmonic resonance. Two classes of scattering resonators could be distinguished. Up to a significant line-broadening, the response of the "sphere-on-plane"-like cases resembled the theoretical prediction for a perfect sphere-on-plane geometry. Resonators which deviate strongly from this ideal scenario were also found. Electron microscopy did not show significant differences between these two classes, suggesting that the variations in the optical response are due to nanoscale variations of shape and roughness in the gap region. The strong modifications of the dye emission spectrum suggested the presence of physical mechanisms at very small metal/dye separations, which are beyond a simple wavelength-dependent enhancement factor.

Synthesis of direct white-light emitting carbogenic quantum dots.

A facile chemical method has been developed to synthesise highly efficient functionalized carbon dots; when illuminated with 407 nm light, both the solution and film emitted white-light directly.

Controlled polyelectrolyte coating of glass-supported metal nanostructures.

We investigated the coating of glass-supported gold nanostructures with polyelectrolyte multilayers. To achieve selective coating of either the metal or the glass, thiol and silane monolayers with different functionalities were used. Self-assembled monolayers of an oligo-(ethylene glycol)-terminated silane on glass allow for complete passivation and fully selective deposition of the polyelectrolytes on the metallic structures. This glass passivation forces more material on the gold than without use of this silane. Gold passivation was achieved on Au(111) but was not successful on the metallic nanostructures, giving rise to polyelectrolytes irregularly deposited on the nanoparticles in the form of blobs.

Bridging of gold nanoparticles by functional polyphenylene dendrimers.

We describe a protocol for the in-suspension bridging of gold particles with shape-persistent polyphenylene dendrimers, carrying multiple lipoic acid groups at their periphery. A solvent mixture is identified where both reactants are stable as individuals. Upon mixing, aggregates with stoichiometry-dictated size form rapidly. Initially, the spacing between individual gold colloids is large enough that their absorption spectrum is only slightly different from isolated particles. The aggregates rearrange within hours, leading to strong optical coupling between the gold particles and a corresponding spectral shift.

Simple, one-step synthesis of gold nanowires in aqueous solution.

A simple procedure to synthesize gold nanowires based on the reduction of hydrogen tetrachloroaurate by 2-mercaptosuccinic acid in aqueous solution is presented. This procedure requires no additional capping or reduction agent and produces wires with an apparent curly morphology several micrometers in length with diameters as thin as 15 nm. Some of the wires produced end in a ribbonlike structure, finally terminated by a flat triangular prism. Investigations by scanning electron microscopy, transmission electron microscopy (bright and dark field), scanning transmission electron microscopy, and atomic force microscopy as well as conductivity measurements indicate fully connected, polycrystalline gold objects.

Fluorescence intensities of chromophores in front of a thin metal film.

The fluorescence intensity from a planar multilayered system with a chromophore separated from a gold film by a dielectric spacer is measured quantitatively. The direction of excitation and the spacer thickness are varied and the angular distribution of the emission is recorded as well as its polarization. The experimental data are compared to the predictions obtained from classical electromagnetic theory, taking into account the refractive indices of the layer system as well as the nonradiative decay rate and the relative orientation of absorption and emission dipole moments of the dye. Excellent agreement is found for a spacer thickness above 15 nm if proper values for these parameters are used. Samples with thinner spacer layers show significant deviations from classical theory.

Reduced photobleaching of chromophores close to a metal surface.

The photobleaching of chromophores in front of a metal film is measured by recording the emitted fluorescence intensity from an ensemble of chromophores as a function of time. A strong dependence of the photostability on the distance from the metal surface is found. The experimental data are well described in a classical electromagnetic model with the additional assumption that photobleaching occurs at a constant rate from the excited state. The metal interface influences the photostability of the chromophores in two ways, first by altering the excitation rate by local enhancement of the electromagnetic field and second by altering the electromagnetic decay rate.