Thermoplasmonics of Ag Nanoparticles in a Variable-Temperature Bath

Abstract

Silver represents, by and large, the best plasmonic metal available, due to its very low optical losses in a broad photonenergy 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 localizedplasmon response, indicating Ag as a potential candidate for temperature-invariant thermoplasmonics applications.

Au–Ag Alloy Nanocorals with Optimal Broadband Absorption for Sunlight-Driven Thermoplasmonic Applications

“…Sunlight-driven thermoplasmonic applications demand for a set of requisites that are not easily satisfied by conventional plasmonic nanostructures. ,,, The solar spectrum extends from the near UV to the NIR, with 87.7% of energy comprised in the 350–1350 nm range and the 52.4% at wavelengths >700 nm. ,, Thus, panchromatic absorption in this wide range is a first important requisite, typically difficult to achieve in noble metal nanostructures without a simultaneous increase of size and scattering cross section. ,,, Photostability and chemical stability are other important features often limiting the exploitation of anisotropic metal nanoparticles obtained by chemical reduction with templating agents. ,, This is due to a generally high surface energy and the tendency to reshaping into compact spheroidal morphologies, either in the dark or at low illumination intensity. ,,, More inert plasmonic materials, like nitrides, have lower absorption cross section per unit volume compared to noble metals, and are seldom processable as a colloidal solution, as desirable for inclusion in matrixes and substrates. ,,, They also do not benefit of the easy surface chemistry of noble metals, which are functionalizable in one step with thiolated molecules. ,, The ability to conjugate metal nanoparticles with functional organic molecules is crucial for maintaining colloidal stability in complex liquid environments such as electrolyte solutions, biological fluids, or organic solvents. ,, Surface functionalization is key also for the addition of selectivity versus target chemical species and the formation of surface patterns or integration in specific matrixes. ,,, …”

“…However, the absorption cross section of Au NCs is not optimal for sunlight harvesting, due to the prevalence of gold interband transitions below 400 nm. Conversely, silver NPs are known to provide better plasmonic properties than gold, due to a negligible overlap with interband transitions, which is qualitatively evident from the fact that the plasmon absorption bands of Ag NPs are more intense than the interband transitions edge in optical absorption spectra. ,,, Quantitatively, in the visible range, this corresponds to extinction cross sections >3 times larger than Au NPs with the same geometry. , Besides, Au has a high cost, making gold nanostructures practically exploitable only for high-value added specific applications, such as in the biomedical field. ,, Silver is ca. 75 times less expensive than Au per unit gram, and ca.…”

“…75 times less expensive than Au per unit gram, and ca. 140 times less expensive per unit molar volume (equal for Ag and Au), which is the relevant quantity when comparing plasmonic properties, because it determines the free electron density. , Although Ag nanostructures have inferior chemical stability than Au, ,, it has been shown that alloying Ag with 10–20 at% of Au dramatically improves the resistance even in harsh chemical conditions, ,, thanks to surface Au segregation and passivation. , In fact, the alloying of metals provides several opportunities for tuning and optimizing materials properties along the desired applicative direction. ,,,− In the field of plasmonics, for instance, Au–Ag and Ag–Al nanoalloys were exploited for tunable surface enhanced Raman scattering substrates and the study of plasmon enhanced catalytic processes. This is also pushing to the continuous development and study of new alloys such as Ag–Cu or Au–Sn …”

Optical Response of CVD-Grown ML-WS2 Flakes on an Ultra-Dense Au NP Plasmonic Array

“…The red curve shows the absorption spectrum of the Au NP array, with the plasmonic peak at 2.125 ± 0.002 eV [48,58]. The dashed black curve is a Lorentzian fit (see Appendix B, Figure A2b for details).…”

“…The plasmonic substrate is composed of a regular array of Au NPs on lithium fluoride (LiF) realized by depositing 3 nm of Au film onto a uniaxial pre-nanopatterned LiF (110) surface by molecular beam epitaxy under ultra-high vacuum conditions (p base ∼10 −9 mbar), and annealing the sample to induce solid-state dewetting of the thin film [47,48]. Since the LSPR depends on many parameters, we optimized the fabrication conditions of the Au NP array in order to match its LSPR energy to that of the WS 2 A exciton.…”