Optische und mechanische Manipulation plasmonischer Heißpunkte an Goldnanopartikelspitzen

In der vorliegenden Dissertation wurden auf alternativen Nanopartikelmorphologien basierende Hybridsysteme hinsichtlich ihrer optische Eigenschaften untersucht, um die bekannten Limitierungen und Unzulänglichkeiten der etablierten Nanopartikelsysteme und -formen, wie stäbchenförmige oder sphärische Nanopartikel, weitgehend zu beseitigen. Es werden sternförmige Goldnanopartikel und ihre besonderen plasmonischen Eigenschaften vorgestellt. Mit Methoden der Dunkelfeldspektroskopie, der Photoemissionelektronenmikroskopie und Rasterelektronenmikroskopie (SEM) werden die Nah- und Fernfeldeigenschaften einzelner Nanosterne mit ihrer Morphologie korreliert. So konnte der erstmalige experimentelle Nachweis der Lokalisierung von optisch angeregten Heißpunkten an den Spitzen der einzelnen Nanosterne geliefert werden. Durch geeignete Wahl der Polarisation und Wellenlänge werden Heißpunkte an mehreren oder ausschließlich an einzelnen Spitzen eines Nanosterns selektiv anregt und die detektierten Plasmonresonanzen nur aufgrund experimenteller Daten einzelnen Nanosternspitzen zugeordnet. Desweiteren werden Feldverstärkungsfaktoren im Bereich von 58-79 an den Nanosternspitzen zum ersten Mal direkt quantitativ bestimmt. Die hohen und für Moleküle leicht zugänglichen Heißpunkte an den Spitzen der Nanosterne werden verwendet, um die hervorragende Verstärkung der Ramanstreuung auf der Ebene einzelner Nanosterne zu demonstrieren. Die Ramanverstärkungsfaktoren von 10^7 für die untersuchten Nanosterne ohne Ausnutzung plasmonischer Kopplungseffekte, zeichnen die Nanosterne als wichtige Kandidaten für die Verwendung in komplexen dynamischen Umgebungen aus. Eine weitere Möglichkeit der Ausnutzung der an Spitzen lokalisierten Heißpunkte bieten Nanobipyramiden. Durch Manipulation mit einem Rasterkraftmikroskop lässt sich eine Positionierung der Heißpunkte bis zu 1 nm oberhalb der Oberfläche und an beliebigen Stellen auf einem Probensubstrat erreichen. Nanoresonatoren aus zwei Nanobipyramiden können in drei verschiedenen Adsorptionskonfigurationen hinsichtlich der zueinander weisenden Spitzen der zwei Nanopartikel mechanisch hergestellt werden. Durch die mechanische Änderung der Konfiguration des Nanoresonators kann die Position des im Partikelzwischenraum entstehenden Heißpunktes in der Höhe bewegt werden ohne wesentliche Änderung der spektralen Lage der gekoppelten Plasmonresonanz

Influence of nanoparticle dimensions on Rabi splitting strength

Strong coupling was detected between single gold nano-bipyramids and monolayer MoS 2. It was demonstrated that the coupling strength increases with nanoparticle size, even without increasing the number of excitons coupled into the system

Plasmon-assisted direction-and polarization-sensitive organic thin-film detector

Utilizing Bragg surface plasmon polaritons (SPPs) on metal nanostructures for the use in optical devices has been intensively investigated in recent years. Here, we demonstrate the integration of nanostructured metal electrodes into an ITO-free thin film bulk heterojunction organic solar cell, by direct fabrication on a nanoimprinted substrate. The nanostructured device shows interesting optical and electrical behavior, depending on angle and polarization of incidence and the side of excitation. Remarkably, for incidence through the top electrode, a dependency on linear polarization and angle of incidence can be observed. We show that these peculiar characteristics can be attributed to the excitation of dispersive and non-dispersive Bragg SPPs on the metal–dielectric interface on the top electrode and compare it with incidence through the bottom electrode. Furthermore, the optical and electrical response can be controlled by the organic photoactive material, the nanostructures, the materials used for the electrodes and the epoxy encapsulation. Our device can be used as a detector, which generates a direct electrical readout and therefore enables the measuring of the angle of incidence of up to 60\ub0 or the linear polarization state of light, in a spectral region, which is determined by the active material. Our results could furthermore lead to novel organic Bragg SPP-based sensor for a number of applications

Dye-doped spheres with plasmonic semi-shells: Lasing modes and scattering at realistic gain levels

We numerically simulate the compensation of absorption, the near-field enhancement as well as the differential far-field scattering cross section for dye-doped polystyrene spheres (radius 195 nm), which are half-covered by a silver layer of 10–40 nm thickness. Such silver capped spheres are interesting candidates for nanoplasmonic lasers, so-called spasers. We find that spasing requires gain levels less than 3.7 times higher than those in commercially available dye-doped spheres. However, commercially available concentrations are already apt to achieve negative absorption, and to narrow and enhance scattering by higher order modes. Narrowing of the plasmonic modes by gain also makes visible higher order modes, which are normally obscured by the broad spectral features of the lower order modes. We further show that the angular distribution of the far-field scattering of the spasing modes is by no means dipole-like and is very sensitive to the geometry of the structure

Metal-graphene nanostructures for SEIRA spectroscopy

Infrared spectroscopy is widely used technique for observing bioorganic materials, but sometimes scientist have to work with small amount of investigated materials and infrared light interacts poorly with nanometric size molecules. Taking into account unique electro-optical properties of graphene and metal we demonstrated possibility to use metal-graphene nanostructures for label-free detection of thymine. It was shown that IR spectra of thymine adsorbed on the composite nanostructures, such as Au "nanostars" with graphene, is more enhanced than whenthese nanoparticles are used without graphene. The enhancement in IR absorption for complex thymine/Au/graphene depends on size of Au nanoparticlesand thymine\u27s molecular group

Different Device Architectures for Bulk-Heterojunction Solar Cells

We report different solar cell designs which allow a simple electrical connection of subsequent devices deposited on the same substrate. By arranging so-called standard and inverted solar-cell architectures next to each other, a serial connection of the two devices can easily be realized by a single compound electrode. In this work, we tested different interfacial layer materials like polyethylenimine (PEI) and PEDOT:PSS, and silver as a non-transparent electrode material. We also built organic light emitting diodes applying the same device designs demonstrating the versatility of applied layer stacks. The proposed design should allow the preparation of organic bulk-heterojunction modules with minimized photovoltaically inactive regions at the interconnection of individual devices

Selective Excitation of Individual Plasmonic Hotspots at the Tips of Single Gold Nanostars

International audiencePlasmonic hotspots in single gold nanostars are located at the tips and can be excited selectively by laser light as evidenced by photoelectron emission microscopy. Selectivity is achieved through wavelength and polarization of the excitation light. Comparing photoelectron emission intensity and dark-field scattering spectra of the same individual nanostars reveals differences in terms of observable plasmon resonance wavelengths and field enhancements. Differences are explained with the underlying nearand far-field processes of the two techniques

Label-free biosensing based on single gold nanostars as plasmonic transducers

Gold nanostars provide high sensitivity for single nanoparticle label-free biosensing. The nanostars present multiple plasmon resonances of which the lower energy ones, corresponding to the nanostar tips and core-tip interactions, are the most sensitive to environmental changes. Streptavidin molecules are detected upon binding to individual, biotin-modified gold nanostars by spectral shifts in the plasmon resonances. Concentrations as low as 0.1 nM produce a shift of the tip related plasmon resonances of about 2.3 nm (5.3 meV).Fil: Dondapati, Srujan K.. Ludwig Maximilians Universitat; AlemaniaFil: Sau, Tapan K.. Ludwig Maximilians Universitat; AlemaniaFil: Hrelescu, Calin. Ludwig Maximilians Universitat; AlemaniaFil: Klar, Thomas A.. Technical University of Ilmenau; AlemaniaFil: Stefani, Fernando Daniel. Technical University of Ilmenau; Alemania. Ludwig Maximilians Universitat; Alemania. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Feldmann, Jochen. Ludwig Maximilians Universitat; Alemani

Optical Plasmons of Individual Gold Nanosponges

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