91 research outputs found
Solid-state dewetting of thin Au films studied with real-time, in situ spectroscopic ellipsometry
No full textWe report the design and testing of a small, high vacuum chamber that allows real-time, in situ spectroscopic ellipsometry (SE) measurements; the chamber was designed to be easily inserted within the arms of a commercial ellipsometer. As a test application, we investigated the temperature-induced solid-state dewetting of thin (20 to 8 nm) Au layers on Si wafers. In situ SE measurements acquired in real time during the heating of the samples reveal features that can be related to the birth of a localized surface plasmon resonance (LSPR), and demonstrate the presence of a temperature threshold for the solid-state dewetting
Monitoring the solid-state dewetting of densely packed arrays of Au nanoparticles
No full textWe report a real time, in-situ spectroscopic ellipsometry study of the temperature-induced solid-state dewetting of Au nanowires into nanoparticles. Very large spectral variations are observed at different temperatures. Analysis of the key features in the acquired spectra reveals two different regimes: up to 300 °C the variation in the optical response is dominated by solid-state dewetting, while above that temperature, smaller variations not compatible with such mechanism are visible. Therefore our ellipsometry measurements allow us to determine in real time at which temperature the solid-state dewetting ceases and the morphology of our sample becomes stable. We point out that this observation is possible thanks to the higher sensitiviy of ellipsometry with respect to reflectance/transmittance measurements
Thermoplasmonics of Ag Nanoparticles in a Variable-Temperature Bath
No full textSilver represents, by and large, the best plasmonic metal available, due to its very low optical losses in a broad photon-energy range encompassing all the visible optical spectrum. Its performances are, more often than not, severely hampered by the presence of a few-nanometer thick surface-tarnish layer; thermal annealing under high-vacuum (HV) conditions may however lead to its decomposition, thereby allowing to attain the clean-metal response. Here, we report an experimental investigation of the temperature dependence of the plasmonic response of Ag nanoparticles, either clean or tarnished, by means of in situ optical spectroscopies under HV conditions. For tarnished nanoparticles, we observed the temperature dynamics of thermal decomposition of the contamination layer in real time and compared it with the corresponding behavior of spatially extended, flat surfaces. For clean Ag nanoparticles we witness instead a remarkable temperature invariance of the localized-plasmon response, indicating Ag as a potential candidate for temperature-invariant thermoplasmonics applications
Temperature-dependent permittivity of silver and implications for thermoplasmonics
No full textSilver is an extremely appealing metal for plasmonics due to its very low optical losses in the visible and near-ultraviolet range and its relatively low reactivity. Within the emerging field of thermoplasmonics, where light-metal interactions are exploited to generate heat on the nanometric scale, knowledge of temperature-dependent complex permittivities of plasmonic materials is indispensable. We extracted the temperature-dependent complex permittivity of silver Ag by spectroscopic ellipsometry under high-vacuum conditions. For rising T, we observed an increase of the free-electron contribution to the imaginary part of the permittivity Im[Ag] and a temperature-dependent absorption band splitting off the interband absorption edge in the 320-360-nm range. Around 340 nm the relative increase of Im[Ag] at 600 K with respect to its room-temperature value is around 500%. In order to understand the implications of this behavior on silver thermoplasmonics, we computed the temperature-dependent extinction efficiency of oblate Ag ellipsoids with localized plasmon resonance within the 320-360-nm range. We predict that dramatic damping of the plasmon resonance occurs for increasing temperature, possibly leading to intriguing self-limiting effects in Ag thermoplasmonics
Plasmonics of Au nanoparticles in a hot thermodynamic bath
No full textElectromagnetically-heated metal nanoparticles can be exploited as efficient heat sources at the nanoscale. The assessment of their temperature is, however, often performed indirectly by modelling their temperature-dependent dielectric response. Direct measurements of the optical properties of metallic nanoparticles in equilibrium with a thermodynamic bath provide a calibration of their thermo-optical response, to be exploited for refining current thermoplasmonic models or whenever direct temperature assessments are practically unfeasible. We investigated the plasmonic response of supported Au nanoparticles in a thermodynamic bath from room temperature to 350 °C. A model explicitly including the temperature-dependent dielectric function of the metal and finite-size corrections to the nanoparticles' permittivity correctly reproduced experimental data for temperatures up to 75 °C. The model accuracy gradually faded for higher temperatures. Introducing a temperature-dependent correction that effectively mimics a surface-scattering-like source of damping in the permittivity of the nanoparticles restored good agreement with the data. A finite-size thermodynamic effect such as surface premelting may be invoked to explain this effect
Detecting ultrathin ice on materials for optical coatings at cryogenic temperatures
No full textThe performance of optical cavities in gravitational wave detectors (GWD) is negatively affected by the growth of ice layers when operating at cryo temperatures. Loss of performance begins when the ice overlayer is only a few-nm thick. Careful planning is then required to minimize, monitor and take into account the presence of ultrathin ice on cryo-cooled optical surfaces. Here we employed spectroscopic ellipsometry (SE) to study icing on the surfaces of SiO2 and Ti:Ta2O5 thin films, two materials used in the high-reflective mirrors of current GWD. SE measurements were performed at 75 K. The data presented suggest that SE is a most convenient tool to monitor in operando the ice formation on the surfaces of GWD mirrors. Furthermore, ultrathin ice layers can affect the evaluation of the optical properties of materials at low temperatures, a valuable task for those next-generation GWD that will operate at cryogenic temperatures. The characterization of an ultrathin ice overlayer ( < 10 nm) allowed to determine for the first time the low-temperature optical properties of Ti:Ta2O5. The same approach could be applied to determine the low-temperature optical properties of other dielectric films, thus helping to screen new materials for cryo-operated GWD mirrors
A Tunable Polymer–Metal Based Anti‐Reflective Metasurface
No full textAnti-reflective surfaces are of great interest for optical devices, sensing, photovoltaics, and photocatalysis. However, most of the anti-reflective surfaces lack in situ tunability of the extinction with respect to wavelength. This communication demonstrates a tunable anti-reflective surface based on colloidal particles comprising a metal core with an electrochromic polymer shell. Random deposition of these particles on a reflective surface results in a decrease in the reflectance of up to 99.8% at the localized surface plasmon resonance frequency. This narrow band feature can be tuned by varying the pH or by application of an electric potential, resulting in wavelength shifts of up to 30 nm. Electrophoretic particle deposition is shown to be an efficient method for controlling the interparticle distance and thereby further optimizing the overall efficiency of the anti-reflective metasurface
Optical dielectric function of two-dimensional WS2 on epitaxial graphene
No full textHeterostacks composed of two-dimensional WS2 and graphene exhibit interesting opto-electronic properties, including ultrafast charge transfer and fast, efficient photodetection. The optical properties of WS2 are heavily influenced by the presence of graphene in the heterostack, yet, so far, their characterization in the whole visible range is missing. In this work we report the complex dielectric function of two-dimensional WS2 flakes on epitaxial graphene on silicon carbide, obtained by means of spectroscopic ellipsometry (SE). The so-called A, B and C excitonic features are precisely identified, and significant differences with respect to literature data of WS2 on fused silica are highlighted. Since this investigation is based on SE, the complex dielectric function of WS2 is retrieved without performing Kramers Kronig analysis, which is instead necessary when employing reflectance or transmittance spectroscopy. Furthermore, the approach described in this work can be used in principle to characterize any two-dimensional flakes, both in terms of complex dielectric function and percentage of surface coverage
Effects of Mixing and Annealing on the Optical Properties of TiO2:Ta2O5 Amorphous Oxide Coatings
No full textWe determine the optical properties of amorphous, mixed titania-tantala coatings as a function of the mixing ratio and thermal annealing. The Urbach energy is proposed as a good estimator of the quality of the coatings
Plasmonics of Au/Polymer Core/Shell Nanocomposites for Thermoresponsive Hybrid Metasurfaces
No full textWe investigated the temperature-dependent optical response of ordered lattices of noninteracting gold-core/poly(N-isopropylacrylamide)-shell nanoparticles (NPs), a system with proven photothermal and sensing capabilities. For the first time on this system, we exploited in situ spectroscopic ellipsometry (SE) to determine the complex, temperature-dependent optical properties of the lattice - a key piece of information which, however, is often overlooked. In doing so, we took full advantage of large-scale colloidal self-assembly, which makes NP lattices accessible to conventional SE. A quantitative interpretation of the SE data was obtained through an effective model based on the actual characteristics of the NPs and their dielectric environment. The model allowed to estimate temperature-dependent morphological parameters, such as the distance between the gold cores and the substrate, also yielding the complex permittivity of the plasmonic NP lattice. Thus, by combining the high sensitivity of SE with proper modeling, we provide a comprehensive characterization of thermoresponsive NP lattices. The approach proposed here is instrumental to the analysis and design of functional hybrid metasurfaces with plasmonic functionalities, including particle-to-film coupled systems
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