The search for novel plasmonic nanostructures, which can act simultaneously as optical detectors and stimulators, is crucial for many applications in the fields of biosensing, electro- and photocatalysis, electrochemistry, and biofuel generation. In most of these areas, a large surface-to-volume ratio, as well as high density of active surface sites, is desirable. We investigate sponge-like, that is, fully porous, nanoparticles, called nanosponges, where both the gold and the air phase are fully percolated in three dimensions. We correlate, on a single nanoparticle basis, their optical scattering spectra (using dark field microscopy) with their individual morphology (using electron microscopy). We find that the scattering spectra of nanosponges depend only weakly on their size and outer shape, but are greatly influenced by their unique percolation, in qualitative agreement with numerical simulations.

KEYWORDS giant shell quantum dots, successive ion layer adsorption and reaction, random lasing, exciton-exciton interactions, plasmonics ABSTRACT While over the last years the syntheses of colloidal quantum dots (CQDs) with core/shell structures were continuously improved to obtain highly efficient emission, it has remained a challenge to use them as active materials in laser devices. Here, we report on a successful demonstration of random lasing at room temperature in films of CdSe/CdS CQDs with different core/shell band alignments and extra thick shells. Even though the lasing process is based on random scattering, we find systematic dependencies of the laser thresholds on film morphology and excitation spot size. This systematics suggests that random lasing experiments are a valuable tool for testing nanocrystal materials, providing a direct and simple feedback for the further development of colloidal gain materials towards lasing in continuous wave operation.

Bulk gold shows photoluminescence (PL) with a negligible quantum yield of ∼10–10, which can be increased by orders of magnitude in the case of gold nanoparticles. This bears huge potential to use noble metal nanoparticles as fluorescent and unbleachable stains in bioimaging or for optical data storage. Commonly, the enhancement of the PL yield is attributed to nanoparticle plasmons, specifically to the enhancements of scattering or absorption cross sections. Tuning the shape or geometry of gold nanostructures (e.g., via reducing the distance between two nanoparticles) allows for redshifting both the scattering and the PL spectra. However, while the scattering cross section increases with a plasmonic redshift, the PL yield decreases, indicating that the common simple picture of a plasmonically boosted gold luminescence needs more detailed consideration. In particular, precise experiments as well as numerical simulations are required. Hence, we systematically varied the distance between the tips of two gold bipyramids on the nanometer scale using AFM manipulation and recorded the PL and the scattering spectra for each separation. We find that the PL intensity decreases as the interparticle coupling increases. This anticorrelation is explained by a theoretical model where both the gold-intrinsic d-band hole recombination probabilities as well as the field strength inside the nanostructure are considered. The scattering cross section or the field strength in the hot-spot between the tips of the bipyramids are not relevant for the PL intensity. Besides, we not only observe PL supported by dipolar plasmon resonances, but also measure and simulate PL supported by higher order plasmonic modes.

Huge spectral coverage of random lasing throughout the visible up to the infrared range is achieved with star-shaped gold nanoparticles (“nanostars”). As intrinsically broadband scattering centers, the nanostars are suspended in solutions of various laser dyes, forming randomly arranged resonators which support coherent laser modes. The narrow emission line widths of 0.13 nm or below suggest that gold nanostars provide an efficient coherent feedback for random lasers over an extensive range of wavelengths, all together spanning almost a full optical octave from yellow to infrared.

We demonstrate random lasing with star-shaped gold nanoparticles ("nanostars") as scattering centers embedded in a dye-doped gain medium. It is experimentally shown that star-shaped gold nanoparticles outperform those of conventional shapes, such as spherical or prolate nanoparticles. The nanoparticles are randomly distributed within a thin film of gain medium, forming resonators which support coherent laser modes. Driven by single-pulsed excitation, the random lasers exhibit coherent lasing thresholds in the order of 0.9 mJ/cm(2) and spectrally narrow emission peaks with linewidths less than 0.2 nm. The distinguished random laser comprising nanostars is likely to take advantage of the high plasmonic field enhancements, localized at the spiky tips of the nanostars, which improves the feedback mechanism for lasing and increases the emission intensity of the random laser.

An electromagnetic wave impinging on a gold nanosponge coherently excites many electromagnetic hot-spots inside the nanosponge, yielding a polarization-dependent scattering spectrum. In contrast, a hole, recombining with an electron, can locally excite plasmonic hot-spots only within a horizon given by the lifetime of localized plasmons and the speed carrying the information that a plasmon has been created. This horizon is about 57 nm, decreasing with increasing size of the nanosponge. Consequently, photoluminescence from large gold nanosponges appears unpolarized.

Localized-plasmon voltammetry (LPV) bears great potential for electrochemical sensing applications beyond conventional cyclic voltammetry. In order to determine the limitations of this method, it is of utmost necessity to investigate the response toward chemical instability of the plasmonic electrode. We therefore investigated electrooxidation of a gold nanowire array with LPV in acidic electrolytes with different pH values. LPV shows excellent agreement with simultaneously recorded cyclic voltammograms up to the onset of oxygen evolution. Beyond that point, LPV still appears to provide meaningful signals. Further, with LPV the pH dependent reduction potentials of electrochemically grown gold oxides were determined and show a linear characteristic over the investigated pH range according to Nernst’s equation.

We show that multiphoton thiol–ene polymerized structures comprise unreacted thiol moieties, which can be used for postpolymerization gold metallization. Some of the thiol groups located at the surface are not involved in thiol–ene reactions and, therefore, can serve as nucleation seeds for the synthesis of ∼50 nm sized gold nanoislands. Additionally, we show that the nanoislands can be used for immobilization of fluorescent molecules. We observed a significant enhancement of the fluorescence signal on the nanoisland-functionalized polymer structures when compared to the structured polymers without gold. To show a possible application, we bound peroxidase to the gold nanoislands. Peroxidase activity has been verified by chemiluminescence of the converted luminol substrate.