Elin Maria Kristina Larsson

Nanoplasmonic Probes of Catalytic Reactions

Plasmonic Probing of Catalysis An understanding of catalytic reactions on surfaces, such as those used in industrial processes, often requires some measure of reactant concentration on the surface. Often this is expressed as the surface coverage of metal particles that are dispersed on oxide supports. Although optical probes of surface coverage would be convenient, they usually lack sufficient sensitivity to detect the small number of molecules on the surface. Larsson et al. (p. 1091, published online 22 October) have used shifts in plasmon resonances to measure surface coverages. They grow oxide coatings, decorated with metal catalyst particles, on a nanoscale gold disk, and find that these model catalysts are within the region of plasmon sensitivity. Reactions such as CO oxidation on platinum can be followed for different ratios of reactant gases with a sensitivity of 0.1 monolayer of surface coverage. Reactant concentrations can be measured as plasmon frequency shifts for model catalysts grown on nanoscale gold disks. Optical probes of heterogeneous catalytic reactions can be valuable tools for optimization and process control because they can operate under realistic conditions, but often probes lack sensitivity. We have developed a plasmonic sensing method for such reactions based on arrays of nanofabricated gold disks, covered by a thin (~10 nanometer) coating (catalyst support) on which the catalyst nanoparticles are deposited. The sensing particles monitor changes in surface coverage of reactants during catalytic reaction through peak shifts in the optical extinction spectrum. Sensitivities to below 10−3 monolayers are estimated. The capacity of the method is demonstrated for three catalytic reactions, CO and H2 oxidation on Pt, and NOx conversion to N2 on Pt/BaO.

Indirect nanoplasmonic sensing: Ultrasensitive experimental platform for nanomaterials science and optical nanocalorimetry

Indirect nanoplasmonic sensing is a novel experimental platform for measurements of thermodynamics and kinetics in/on nanomaterials and thin films. It features simple experimental setup, high sensitivity, small sample amounts, high temporal resolution (<10(-3) s), operating conditions from UHV to high pressure, wide temperature range, and applicability to any nano- or thin film material. The method utilizes two-dimensional arrangements of nanoplasmonic Au sensor-nanoparticles coated with a thin dielectric spacer layer onto which the sample material is deposited. The measured signal is spectral shifts of the Au-sensor localized plasmons, induced by processes in/on the sample material. Here, the method is applied to three systems exhibiting nanosize effects, (i) the glass transition of confined polymers, (ii) catalytic light-off on Pd nanocatalysts, and (iii) thermodynamics and kinetics of hydrogen uptake/release in Pd nanoparticles <5 nm. In (i) and (iii), dielectric changes in the sample are detected, while (ii) demonstrates a novel optical nanocalorimetry method.

Ultrafast Vibrations of Gold Nanorings

We investigate the vibrational modes of gold nanorings on a silica substrate with an ultrafast optical technique. By comparison with numerical simulations, we identify several resonances in the gigahertz range associated with axially symmetric deformations of the nanoring and substrate. We elucidate the corresponding mode shapes and find that the substrate plays an important role in determining the mode damping. This study demonstrates the need for a plasmonic nano-optics approach to understand the optical excitation and detection mechanisms for the vibrations of plasmonic nanostructures.

Localized Surface Plasmons Shed Light on Nanoscale Metal Hydrides

Nanometer-sized hydrides are attractive for hydrogen storage. Nanofabricated model systems can be studied by a novel localized surface plasmon resonance (LSPR) based direct sensing approach and by quartz crystal microbalance. The role of nanoparticle size, shape and microstructure on the storage thermodynamics can be scrutinized.

A combined nanoplasmonic and electrodeless quartz crystal microbalance setup

We have developed an instrument combining localized surface plasmon resonance (LSPR) sensing with electrodeless quartz crystal microbalance with dissipation monitoring (QCM-D). The two techniques can be run simultaneously, on the same sensor surface, and with the same time resolution and sensitivity as for the individual techniques. The electrodeless QCM eliminates the need to fabricate electrodes on the quartz crystal and gives a large flexibility in choosing the surface structure and coating for both QCM-D and LSPR. The performance is demonstrated for liquid phase measurements of lipid bilayer formation and biorecognition events, and for gas phase measurements of hydrogen uptake/release by palladium nanoparticles. Advantages of using the combined equipment for biomolecular adsorption studies include synchronized information about structural transformations and extraction of molecular (dry) mass and degree of hydration of the adlayer, which cannot be obtained with the individual techniques. In hydrogen storage studies the combined equipment, allows for synchronized measurements of uptake/release kinetics and quantification of stored hydrogen amounts in nanoparticles and films at practically interesting hydrogen pressures and temperatures.

Nanoplasmonic sensing for nanomaterials science

Abstract Nanoplasmonic sensing has over the last two decades emerged as and diversified into a very promising experimental platform technology for studies of biomolecular interactions and for biomolecule detection (biosensors). Inspired by this success, in more recent years, nanoplasmonic sensing strategies have been adapted and tailored successfully for probing functional nanomaterials and catalysts in situ and in real time. An increasing number of these studies focus on using the localized surface plasmon resonance (LSPR) as an experimental tool to study a process of interest in a nanomaterial, with a materials science focus. The key assets of nanoplasmonic sensing in this area are its remote readout, non-invasive nature, single particle experiment capability, ease of use and, maybe most importantly, unmatched flexibility in terms of compatibility with all material types (particles and thin/thick layers, conductive or insulating) are identified. In a direct nanoplasmonic sensing experiment the plasmonic nanoparticles are active and simultaneously constitute the sensor and the studied nano-entity. In an indirect nanoplasmonic sensing experiment the plasmonic nanoparticles are inert and adjacent to the material of interest to probe a process occurring in/on this material. In this review we define and discuss these two generic experimental strategies and summarize the growing applications of nanoplasmonic sensors as experimental tools to address materials science-related questions.

Plasmon Hybridization in Stacked Double Gold Nanorings with Reduced Symmetry

Plasmon excitations in nanoparticles allow the manipulation and enhancement of optical processes at the nanoscale and are currently being investigated as components for numerous spectroscopic, photonic, and biomedical applications. Coupled plasmonic structures provide a route to enhanced optical tunability and performance. Such complex structures can be understood in terms of plasmon hybridization theory with the level of symmetry of the system playing a key role. We demonstrate a novel reduced-symmetry plasmonic nanostructure, combining concentric and non-concentric rings in a stack, prepared by a colloidal templating approach. Our experimental and theoretical analyses reveal hybridized dipolar and multipolar modes in the near infrared. Symmetry breaking in the system results in large electromagnetic-field enhancements (>240) and spectroscopic features originating from the hybridization of multipolar plasmons that would not be visible in the individual rings. These plasmonic nanostructures are highly tunable and suitable for surfaceenhanced spectroscopies and sensing applications. A range of metallic and metallodielectric plasmonic nanostructures has been generated by chemical synthesis or lithographic routes. In a number of nanostructure-based applications of plasmonics both the isolated response of the constituent nanostructures and their strong interparticle interactions and collective electrodynamic response have been exploited. Significant research focus has been placed on nanostructures combining multiple plasmonic motifs such as shells, rings, and stars. Plasmon hybridization theory provides a framework for classifying and designing such complex structures, giving a conceptually simple picture that draws on the rigorous analogy between interacting plasmons

Nanoplasmonic sensing for nanomaterials science, catalysis, and optical gas detection

In this chapter direct and indirect nanoplasmonic sensing approaches for applications in nanomaterials science and catalysis, as well as for gas sensing are discussed. It is illustrated how the typical features of nanoplasmonic sensors, e.g., high local and absolute sensitivity, high temporal resolution, remote readout, simple experimental arrangement, and generic robustness, together with a wide range of possible application conditions make the latter a potentially very powerful experimental tool to study processes on the surface and in the bulk of nanosized systems. The possibility to locally measure temperature at the nanoscale with nanoplasmonic optical calorimetry will also be discussed. Furthermore, numerous examples of nanoplasmonic sensors for gas-sensing applications will be reviewed and the role and potential of novel plasmonic metals will be addressed.