Optical Properties of Colloidal Silver Nanowires.

Silver nanowires are used in many applications, ranging from transparent conductive layers to Raman substrates and sensors. Their performance often relies on their unique optical properties that emerge from localized surface plasmon resonances in the ultraviolet. To tailor the nanowire geometry for a specific application, a correct understanding of the relationship between the wire's structure and its optical properties is therefore necessary. However, while the colloidal synthesis of silver nanowires typically leads to structures with pentagonally twinned geometries, their optical properties are often modeled assuming a cylindrical cross-section. Here we highlight the strengths and limitations of such an approximation by numerically calculating the optical and electrical response of pentagonally twinned silver nanowires and nanowire networks. We find that our accurate modeling is crucial to deduce structural information from experimentally measured extinction spectra of colloidally synthesized nanowire suspensions and to predict the performance of nanowire-based near-field sensors. On the contrary, the cylindrical approximation is fully capable of capturing the optical and electrical performance of nanowire networks used as transparent electrodes. Our results can help assess the quality of nanowire syntheses and guide in the design of optimized silver nanowire-based devices.

Super-Resolution Mapping of a Chemical Reaction Driven by Plasmonic Near-Fields.

Plasmonic nanoparticles have recently emerged as promising photocatalysts for light-driven chemical conversions. Their illumination results in the generation of highly energetic charge carriers, elevated surface temperatures, and enhanced electromagnetic fields. Distinguishing between these often-overlapping processes is of paramount importance for the rational design of future plasmonic photocatalysts. However, the study of plasmon-driven chemical reactions is typically performed at the ensemble level and, therefore, is limited by the intrinsic heterogeneity of the catalysts. Here, we report an in situ single-particle study of a fluorogenic chemical reaction driven solely by plasmonic near-fields. Using super-resolution fluorescence microscopy, we map the position of individual product molecules with an ∼30 nm spatial resolution and demonstrate a clear correlation between the electric field distribution around individual nanoparticles and their super-resolved catalytic activity maps. Our results can be extended to systems with more complex electric field distributions, thereby guiding the design of future advanced photocatalysts.

Simple and Fast High-Yield Synthesis of Silver Nanowires.

Silver nanowires (AgNWs) combine high electrical conductivity with low light extinction in the visible and are used in a wide range of applications, from transparent electrodes, to temperature and pressure sensors. The most common strategy for the production of AgNWs is the polyol synthesis, which always leads to the formation of silver nanoparticles as byproducts. These nanoparticles degrade the performance of AgNWs' based devices and have to be eliminated by several purification steps. Here, we report a simple and fast synthesis of AgNWs with minimal formation of byproducts, as confirmed by the spectral purity of the final solution. Our synthetic strategy relies on the use of freshly prepared AgCl and on the minimization of gas evolution inside the reaction vessel. The observed synthetic improvements can be of general validity for the polyol synthesis of metallic nanostructures of different shapes and compositions.

Charge carrier dynamics and photocatalytic activity of {111} and {100} faceted Ag3PO4 particles.

Silver orthophosphate is a highly promising visible light photocatalyst with high quantum yield for solar driven water oxidation. Recently, the performance of this material has been further enhanced using facet-controlled synthesis. The tetrahedral particles with {111} exposed facets demonstrate higher photocatalytic performance than the cubic particles with {100} exposed facets. However, the reason behind this large difference in photocatalytic performance is still not understood. In this work, we study the free charge carrier dynamics, such as mobility, lifetime, and diffusion lengths, for the {111}-faceted tetrahedral and the {100}-faceted cubic particles using time-resolved microwave conductivity measurements. An order of magnitude higher charge carrier mobility and diffusion length are found for the tetrahedral particles as compared to the cubic particles. The differences in crystal structure, surface composition, and optical properties are investigated in order to understand how these properties impact the charge carrier dynamics and the photocatalytic performance of differently faceted particles.

Super-resolution Mapping of Enhanced Emission by Collective Plasmonic Resonances.

Plasmonic particle arrays have remarkable optical properties originating from their collective behavior, which results in resonances with narrow line widths and enhanced electric fields extending far into the surrounding medium. Such resonances can be exploited for applications in strong light-matter coupling, sensing, light harvesting, nonlinear nanophotonics, lasing, and solid-state lighting. However, as the lattice constants associated with plasmonic particle arrays are on the order of their resonance wavelengths, mapping the interaction between point dipoles and plasmonic particle arrays cannot be done with diffraction-limited methods. Here, we map the enhanced emission of single fluorescent molecules coupled to a plasmonic particle array with ∼20 nm in-plane resolution by using stochastic super-resolution microscopy. We find that extended lattice resonances have minimal influence on the spontaneous decay rate of an emitter but instead can be exploited to enhance the outcoupling and directivity of the emission. Our results can guide the rational design of future optical devices based on plasmonic particle arrays.

Effect of ionic strength on intra-protein electron transfer reactions: The case study of charge recombination within the bacterial reaction center.

It is a common believe that intra-protein electron transfer (ET) involving reactants and products that are overall electroneutral are not influenced by the ions of the surrounding solution. The results presented here show an electrostatic coupling between the ionic atmosphere surrounding a membrane protein (the reaction center (RC) from the photosynthetic bacterium Rhodobacter sphaeroides) and two very different intra-protein ET processes taking place within it. Specifically we have studied the effect of salt concentration on: i) the kinetics of the charge recombination between the reduced primary quinone acceptor QA(-) and the primary photoxidized donor P(+); ii) the thermodynamic equilibrium (QA(-)↔QB(-)) for the ET between QA(-) and the secondary quinone acceptor QB. A distinctive point of this investigation is that reactants and products are overall electroneutral. The protein electrostatics has been described adopting the lowest level of complexity sufficient to grasp the experimental phenomenology and the impact of salt on the relative free energy level of reactants and products has been evaluated according to suitable thermodynamic cycles. The ionic strength effect was found to be independent on the ion nature for P(+)QA(-) charge recombination where the leading electrostatic term was the dipole moment. In the case of the QA(-)↔QB(-) equilibrium, the relative stability of QA(-) and QB(-) was found to depend on the salt concentration in a fashion that is different for chaotropic and kosmotropic ions. In such a case both dipole moment and quadrupole moments of the RC must be considered.