Plasmonenresonanzen von sichelförmigen metallischen Nanoobjekten

Metallische Objekte in der Größenordnung der optischen Wellenlänge zeigen Resonanzen im optischen Spektralbereich. Mit einer Kombination aus Kolloidlithographie, Metallfilmbedampfung und reaktivem Ionenstrahl¨atzen wurden Nanosicheln aus Gold bzw. Silber mit identischer Form und Orientierung in Sichelform mit einer Größe von 60nm bis 400nm hergestellt. Der Öffnungswinkel der Nanosicheln lässt sich kontinuierlich einstellen. Durch die einheitliche Orientierung lassen sich Messungen am Ensemble direkt auf das Verhalten des Einzelobjektes übertragen, wie ein Vergleich der Extinktionsspektren einer Ensemblemessung am UV/Vis/NIR-Spektrometer mit einer Einzelpartikelmessung in einem konfokalen Mikroskop zeigt. Die optische Antwort der Nanosicheln wurde als zwei-dimensionales Modell mit einer Finite Elemente Methode berechnet. Das Ergebnis sind mehrere polarisationsabhängige Resonanzen im optischen Spektrum. Diese lassen sich durch Variation des Öffnungswinkels und der Gr¨oße der Nanosichel verschieben. Durch Beleuchten lassen sich plasmonische Schwingungen anregen, die ein stark lokalisiertes Nahfeld an den Spitzen und in der Öffnung der Nanosicheln erzeugen. Das Nahfeld der Partikelresonanz wurde mit einer Fotolackmethode nachgewiesen. Die Untersuchungen am UV/Vis/NIR-Spektrometer zeigen mehrere polarisationsabhängige Resonanzen im Spektralbereich von 300 nm bis 3200 nm. Die Resonanzen der Nanosicheln lassen sich durch den Öffnungswinkel und den Durchmesser in der Größenordnung der Halbwertbreite im optischen Spektrum verschieben. In der Anwendung als Chemo- bzw. Biosensor zeigen Gold-Nanosicheln eine ähnliche Empfindlichkeit wie vergleichbare Sensoren auf der Basis von dünnen Metallstrukturen. Das Nahfeld zeichnet sich durch eine starke Lokalisierung aus und dringt, je nach Multipolordnung, zwischen 14 nm und 70 nm in die Umgebung ein. Quantenpunkte wurden an das Nahfeld der Nanosicheln gekoppelt. Die Emission der Quantenpunkte bei einer Wellenlänge von 860nm wird durch die Resonanz der Nanosicheln verstärkt. Die Nanosicheln wurden als optische Pinzette eingesetzt. Bei einer Anregung mit einem Laser bei einer Wellenlänge von 1064 nm wurden Polystyrolkolloide mit einem Durchmesser von 40 nm von den resonanten Nanosicheln eingefangen. Die Nanosicheln zeigen außergewöhnliche optische Eigenschaften, die mithilfe der Geometrieparameter über einen großen Bereich verändert werden können. Die ersten Anwendungen haben Anknüpfungspunkte zur Verwendung in der Sensorik, Fluoreszenzspektroskopie und als optische Pinzette aufgezeigt. -------------------------------------------------------------------------------- Inhaltszusammenfassung in einer weiteren Sprache (englisch) Metal nanoobjects sized in the order of the optical wavelength exhibit characteristic resonances in the optical regime. Crescent-shaped gold and silver nanoobjects, sized from 60 nm to 400 nm, were fabricated by a combination of colloidlithography, metal film evaporation and reactive ion-beam-etching. This combination results in nano-crescents on a substrate with uniform shape and orientation. The nano-crescents can be fabricated with different gap-angles. Due to the uniformity of these structured substrates the measurements of ensembles can be directly transfered to those of a the single particle. This was varified by the comparison of spectra of an ensemble with a single particle measurement. The spectra for the ensemble were measured with a UV/Vis/NIR-Spectrometer were as the spectra for a single particle measurement were taken from a confocal microscope. The optical response was calculated by implementing a two dimensional model with a Finite-Element-Method. As a result, multiple polarization dependent resonances occured in the optical spectra. The resonances can be tuned over a wide range using different gap angles. These excited plasmon resonances exhibit a strongly localized near-field at the tips and in the gap of the nano-crescents. A photo resist was used to image the near-field of the particle resonance at a wavelength of 532 nm. The measurements indicated a field enhancement in the gap of that resonance. The measurements with the UV/Vis/NIR-Spectrometer revealed multiple polarization dependent resonances in the optical regime from 300 nm to 3200 nm. These resonances can be tuned using the gap-angle and the diameter. Initial studies to apply gold-nano-crescents as a chemo- and biosensor have shown similar sensitivity to optical sensors based on thin metal films. As a result, the near-field penetrates the environment by 14 nm to 70 nm according to multipolarity of the excited plasmon-oscillations. Investigations of the coupling of semiconductor quantum dots to the near-field of the nano-crescents were performed. The emission of the quantum dots at the wavelength 860 nm has been enhanced by a resonance of the nano-crescents. The application of an optical nano-tweezer was investigated by illuminating the nanocrescents with a laser beam at 1064 nm. Polystyrene colloids with a diameter of 40nm were captured at certain positions. The nano-crescents have shown unusual optical properties, that can be manipulated over a wide range by varying the parameters of geometry. Initial applications indicate a potential in using the nano-crescents in sensing, fluorescence spectroscopy and as an optical nano-tweezer

High-latitude biomes and rock weathering mediate climate-carbon cycle feedbacks on eccentricity timescales (vol 11, 5013, 2020)

Correction to: Nature Communications https://doi.org/10.1038/s41467-020-18733-w, published online 6 October 2020

Photoemission Electron Microscopy as a tool for the investigation of optical near fields

Photoemission electron microscopy was used to image the electrons photoemitted from specially tailored Ag nanoparticles deposited on a Si substrate (with its native oxide SiO$_{x}$). Photoemission was induced by illumination with a Hg UV-lamp (photon energy cutoff $\hbar\omega_{UV}=5.0$ eV, wavelength $\lambda_{UV}=250$ nm) and with a Ti:Sapphire femtosecond laser ($\hbar\omega_{l}=3.1$ eV, $\lambda_{l}=400$ nm, pulse width below 200 fs), respectively. While homogeneous photoelectron emission from the metal is observed upon illumination at energies above the silver plasmon frequency, at lower photon energies the emission is localized at tips of the structure. This is interpreted as a signature of the local electrical field therefore providing a tool to map the optical near field with the resolution of emission electron microscopy.Comment: 10 pages, 4 figures; submitted to Physical Review Letter

Profile Prediction and Fabrication of Wet-Etched Gold Nanostructures for Localized Surface Plasmon Resonance

Dispersed nanosphere lithography can be employed to fabricate gold nanostructures for localized surface plasmon resonance, in which the gold film evaporated on the nanospheres is anisotropically dry etched to obtain gold nanostructures. This paper reports that by wet etching of the gold film, various kinds of gold nanostructures can be fabricated in a cost-effective way. The shape of the nanostructures is predicted by profile simulation, and the localized surface plasmon resonance spectrum is observed to be shifting its extinction peak with the etching time

Tuning resonances on crescent-shaped noble-metal nanoparticles

The geometry of crescent-shaped noble-metal nanoparticles is systematically varied in terms of shape and size. The resulting changes in the plasmonic resonances of these structures are investigated by extinction spectroscopy revealing a rich polarization-dependent response in the near- infrared region of the electromagnetic spectrum. A first approach towards the understanding of this behaviour, in analogy to previous models on confined modes in nanometric metal slabs, is presented and discussed. Variations in several geometrical parameters lead to changes in the optical response that can be understood within this model. Qualitative changes in the response are seen at the transition of the structures from an open ‘crescent’ to a fully connected ring, pointing to a high field localization between the two tips of the structure

High-latitude biomes and rock weathering mediate climate–carbon cycle feedbacks on eccentricity timescales

The International Ocean Discovery Programme (IODP) and its predecessors generated a treasure trove of Cenozoic climate and carbon cycle dynamics. Yet, it remains unclear how climate and carbon cycle interacted under changing geologic boundary conditions. Here, we present the carbon isotope (δ13C) megasplice, documenting deep-ocean δ13C evolution since 35 million years ago (Ma). We juxtapose the δ13C megasplice with its δ18O counterpart and determine their phase-difference on ~100-kyr eccentricity timescales. This analysis reveals that 2.4-Myr eccentricity cycles modulate the δ13C-δ18O phase relationship throughout the Oligo-Miocene (34-6 Ma), potentially through changes in continental weathering. At 6 Ma, a striking switch from in-phase to anti-phase behaviour occurs, signalling a reorganization of the climate-carbon cycle system. We hypothesize that this transition is consistent with Arctic cooling: Prior to 6 Ma, low-latitude continental carbon reservoirs expanded during astronomically-forced cool spells. After 6 Ma, however, continental carbon reservoirs contract rather than expand during cold periods due to competing effects between Arctic biomes (ice, tundra, taiga). We conclude that, on geologic timescales, System Earth experienced state-dependent modes of climate–carbon cycle interaction