Broad Plasmonic Absorption Research Articles

Plasmonically enhanced two-photon absorption induced photoacoustic microscopy with laser-synthesized TiN nanoparticles

Combining photonic excitation and acoustic detection, photoacoustic imaging (PAI) presents one of the most promising noninvasive biomedical diagnostic modalities, but this technique still lacks efficient nano-sized contrast agents absorbing light in the region of relative tissue transparency (630–900 nm). Here, we explore the use of titanium nitride (TiN) nanoparticles (NPs) fabricated by methods of pulsed laser ablation in liquids as a contrast agent in PAI. When prepared in acetone, the NPs are spherical, have an average size of 25 nm, and exhibit a broad plasmonic absorption peak around 700 nm. We show that solutions of these NPs render possible a strong nonlinear photoacoustic response and the generation of photoacoustic images with 67 μm resolution within the biological transparency window. The observed effect is explained by a plasmonically enhanced two-photon absorption process in TiN NPs. Combined with earlier demonstrated capability of generating photothermal therapeutic effect, relative chemical purity, and excellent biocompatibility, laser-synthesized TiN NPs promise attractive applications in biomedical theranostics involving imaging modalities based on photoacoustics microscopy or tomography.

Synthesis of novel FeM (Co, Ni, Cu & Zn)/PDMS for magnetic actuators thin film fabrication by greener route

A novel PDMS-bimetallic magnetic nanoparticle thin films based magnetic actuators have been fabricated using an eco-friendly cost-effective facile method. The magnetic thin films have received greater attention for their variety of specialized applications, particularly in the medical field. Actuators find very broad applications in microfabrication, microelectronics, medicine, and lab-on-a-chip systems. The FeM(Co/Ni/Cu/Zn) bimetallic magnetic nanoparticles have been synthesized through greener Mimosa Pudica leaf extract. These synthesized nanoparticles are combined with PDMS polymer to form flexible thin films to fabricate magnetic actuators. Optical characterization of the FeM nanoparticles showed a broad plasmon absorption band around 1100–1600 nm, and their corresponding thin-film absorption band shifted negligibly. The magnetic investigations showed that the obtained nanoparticles and their thin films are ferromagnetic with a significant remanent magnetization. Particularly Fe-Cu magnetic NPs and their FeCu@PDMS film displayed 90.82, 55.79 emu/g Ms and 33.34, 11.68 emu/g Mr values at 300 K respectively. These results make the FeM@PDMS flexible films are promising for advanced magnetics memory devices and magnetic induced actuators fabrication.

Multi-spiked silver stars for ultrasensitive and multiplexed SERS detection of analytes

We report a facile synthesis procedure of multi-spiked star-shaped silver structures with sharp morphological features and broad plasmonic absorption for the fabrication of surface-enhanced Raman scattering (SERS) substrates. The multi-branched morphology of silver stars (AgSs) provides copious hot spots on SERS-active substrates to detect ultra-low concentrations of probe molecules. Significant Raman intensity enhancement factors of the order of ∼109 were obtained for the different analytes. The AgSs-based SERS-substrates show ultra-sensitive SERS performance for the detection of probe molecules rhodamine B, methylene blue, thiram, and phosmet with detectabilities 2.6 × 10−16, 1.97 × 10−17, 1.78 × 10−18, and 4.08 × 10−14 M concentration, respectively. The improved SERS sensitivity originated from the additional contribution of chemical enhancement, corroborated by the appearance of new Raman bands in the SERS spectrum. For practical applications, SERS based molecule detection requires high sensitivity, specificity, uniformity, and reproducibility of the SERS signal. We demonstrate the simultaneous detection of multiple analytes, with high specificity, from the complex mixture of molecules with different compositions. The fabricated SERS substrates provide excellent signal uniformity and reproducibility with ∼10% relative standard deviation for the multiple substrates. The real-life application of AgSs-based SERS substrates with the spiked apple juice and ITO-coated PET films without pre-processing is also demonstrated.

Solar-Thermal Conversion Characteristics of Surfactant-Free Gold Nanoparticles Produced By Plasma-Based Synthesis Method

Gold nanoparticles (Au NPs), with their unique plasmonic absorption in the visible spectrum, are promising candidates for the use as nanofluids for enhancing the solar-thermal conversion (STC) process in direct-absorption solar collectors (DASCs) [1-4]. Their absorption performance mainly depends on their size and morphology [5] controlled by the synthesis technique used [4], such as the widely-used wet chemistry-based methods. These chemical methods rely on the use of surfactants for controlling the nanoparticles size and shape, as well as achieving stability within the hosting fluid. However, utilizing chemical additives results in undesired by-products, which require washing, purification and recycling processes and also they eliminate the possibility of further functionalization of the produced Au NPs [6, 7]. Besides, surfactants increase the thermal resistance between the nanoparticle and the hosting fluid [8] and could also decompose and degrade over time, at operating temperatures as low as 70 °C [9].In the present work, the optical properties and STC characteristics of three types of Au NPs produced by different synthesis methods were experimentally investigated. The first type was produced using a plasma-induced non-equilibrium electrochemistry system, to form surfactant-free nanoparticles (sf-Au NPs). The second type was synthesized using Turkevich method to form PEGylated nanoparticles (p-Au NPs), while the third type is citrate-capped nanoparticles (c-Au NPs), supplied by Sigma. After washing and drying the Au NPs powder of each type, they were separately re-dispersed in ethylene glycol (EG) as a base fluid to give Au NPs concentration of 39.4 mg/L forming nanofluids of sf-Au NPs/EG, p-Au NPs/EG and c-Au NPs/EG.XPS results of the untreated and treated gold precursors, used for the synthesis process, showed that no gold ions were found in the treated sample, indicating the full reduction of the gold precursors. In addition, TEM images showed that all samples have the same average nanoparticle size of ~32 nm. However, the size and morphology of sf-Au NPs showed a broad range of sizes and shapes, while the other two samples have nanoparticles of fine spherical shapes and narrow size distribution. Furthermore, the optical properties results for a wavelength range of 280-900 nm, using UV-VIS spectroscopy, showed sf-Au NPs/EG nanofluid exhibited a broad plasmonic absorption peak over the visible spectrum, while the other two nanofluids have similar narrow peaks. Moreover, measurements from Zetasizer Nano ZS system showed that sf-Au NPs have a zeta potential of -25.8 mv, while the p-Au NPs and c-Au NPs have slightly higher values of -35.6 mV and -38.6 mV, respectively, which could be considered as good stabilization.A lab-scale DASC was built for investigating the STC performance of each nanofluid, under a uniform radiation intensity of ∼1000 W/m2 for 25 min, during which the nanofluids temperature change was recorded. Results revealed that sf-Au NPs/EG nanofluid achieved the highest solar-thermal conversion efficiency and specific absorption rate, compared to the other two surfactant-based Au NPs/EG nanofluids. The STC efficiency was enhanced to above 80%, compared to the efficiency of the base fluid, while 50-60% enhancement was reached by the surfactant-based nanofluids, highlighting the advantages of using plasm-liquid treatment method for the synthesis of sf-Au NPs for the use as nanofluids for solar energy harvesting in DASCs. References Zeiny, A., Jin, H., Bai, L., Lin, G. & Wen, D. 2018. A comparative study of direct absorption nanofluids for solar thermal applications. Solar Energy, 161, 74-82.Willets, K. A. & Van Duyne, R. P. 2007. Localized surface plasmon resonance spectroscopy and sensing. Rev. Phys. Chem., 58, 267-297.Lv, W. 2012. Plasmonic Nanoparticles and Their Suspensions for Solar Energy Conversion, Arizona State University.Xiong, Y., Long, R., Liu, D., Zhong, X., Wang, C., Li, Z.-Y. & Xie, Y. 2012. Solar energy conversion with tunable plasmonic nanostructures for thermoelectric devices. Nanoscale, 4, 4416-4420.Kamat, P. V., Flumiani, M. & Hartland, G. V. J. T. J. O. P. C. B. 1998. Picosecond dynamics of silver nanoclusters. Photoejection of electrons and fragmentation. 102, 3123-3128.Sakai, T., Enomoto, H., Torigoe, K., Sakai, H. & Abe, M. 2009. Surfactant-and reducer-free synthesis of gold nanoparticles in aqueous solutions. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 347, 18-26.Mariotti, D., Patel, J., Švrček, V. & Maguire, P. 2012. Plasma–liquid interactions at atmospheric pressure for nanomaterials synthesis and surface engineering. Plasma Processes and Polymers, 9, 1074-1085.Yu, W. & Xie, H. 2012. A review on nanofluids: preparation, stability mechanisms, and applications. Journal of nanomaterials, 2012, Wen, D. & Ding, Y. 2004. Effective thermal conductivity of aqueous suspensions of carbon nanotubes (carbon nanotube nanofluids). Journal of Thermophysics and Heat Transfer, 18, 481-485. Figure 1

Surface-ligand protected reduction on plasmonic tuning of one-dimensional MoO3−x nanobelts for solar steam generation

Sub-stoichiometric MoO3−x nanostructures with plasmonic absorption via creating oxygen vacancies have attracted extensive attentions for many intriguing applications. However, the synthesis of one-dimensional (1D) plasmonic MoO3−x nanostructures with widely tunable plasmonic absorption has remained a significant challenge because of their serious morphological destruction and phase change with increasing the concentration of oxygen vacancies. Here we demonstrate a surface-ligand protected reduction strategy for the synthesis of 1D MoO3−x nanobelts with tunable plasmonic absorption in a wide wavelength range from 200 to 2,500 nm. Polyethylene glycol (PEG-400) is used as both the reductant to produce oxygen vacancies and the surface protected ligands to maintain 1D morphology during the formation process of MoO3−x nanobelts, enabling the widely tunable plasmonic absorption. Owing to their broad plasmonic absorption and unique 1D nanostructure, we further demonstrate the application of 1D MoO3−x nanobelts as photothermal film for interfacial solar evaporator. The surface-ligand protected reduction strategy provides a new avenue for the developing plasmonic semiconductor oxides with maintained particle morphology and thus enriching their wide applications.

Photothermal Conversion Performance of Surfactant-Free Gold Nanoparticles Synthesised By Plasma-Induced Non-Equilibrium Electrochemistry and Compared with Surfactant-Based Samples

Most of the wet-chemistry synthesis routes in particular rely on the use of reducing, capping and stabilising agents to control the produced nanomaterials morphology and to provide steric stabilisation for their colloids. However, they introduce several undesired consequences, such as undergoing a multi-step process with a high level of control and time consumption for purifying and recycling their by-products, as incomplete purification may eliminate further functionalization. In addition, they contribute to the morphology and optical and thermal properties of the synthesised nanomaterials resulting in inaccurate analyses such as overestimation. Furthermore, the possibility of their decomposition and degradation at operating temperatures as low as 70 °C weakens the dependence on them in practical applications. Alternatively, plasma-induced non-equilibrium electrochemistry (PiNE) with its recent improvements in nanomaterial synthesis and surface functionalization offers a great opportunity to prepare nanomaterials in a single-step, safe, cost-effective and versatile method within a relatively short processing time. In this method, no reagents are needed as the plasma’s electrons initiate the reduction process and also provide electrostatic stabilisation for the nanomaterials formed. Therefore, in this work, we synthesised surfactant-free gold nanoparticles (sf-Au NPs) and re-dispersed them in ethylene glycol (EG) at a concentration of 39.4 mg/L in order to evaluate their optical properties and photothermal conversion (PTC) performance for direct absorption solar collectors (DASCs). The synthesis process was performed by generating an atmospheric-pressure microplasma on the surface of an aqueous solution of gold (III) chloride trihydrate (HAuCl4.3H2O) dissolved in distilled water at a concentration of 0.2 mM. The operating conditions used are a 25-sccm helium gas flow rate and a constant direct current of 5 mA applied to a carbon rod immersed in the solution. At a sample volume of 10 mL, a treatment time of only 15 min was sufficient to completely reduce the gold precursor into Au NPs, which was confirmed by fitting X-ray photoelectron spectroscopy results. For further evaluation, we compared the absorption and scattering characteristics of our sf-Au NPs with two additional surfactant-based Au NPs samples at same average particle size; a PEG-coated Au NPs sample synthesised by Turkevich-Frens method in a two-step process and a commercial citrate-capped sample. The morphology analysis, using transmission electron microscopy images, showed a wide range of sizes and several shapes of sf-Au NPs, whilst monodispersed and fine spherical shapes for both surfactant-based samples. Thus, after re-dispersion in EG at the same concentration, sf-Au NPs/EG nanofluid sample exhibited a broad plasmonic absorption peak over the visible spectrum, while p-Au NPs and c-Au NPs in EG had similar narrow peaks. As a result, the analysis of theoretical and experimental PTC results, under a solar irradiation of 1000 W/m2, revealed that sf-Au NPs/EG nanofluid achieved the highest PTC efficiency and specific absorption rate. The PTC process was enhanced by more than 80%, compared to EG as a base fluid, and 50-60 % was achieved by the surfactant-based samples. These results confirm the benefits of using PiNE for the synthesis of surfactant-free nanomaterials, particularly for direct volumetric absorption in solar thermal applications.

High Sensitive Biosensors Based on the Coupling Between Surface Plasmon Polaritons on Titanium Nitride and a Planar Waveguide Mode.

High sensitivity biosensors based on the coupling of surface plasmon polaritons on titanium nitride (TiN) and a planar waveguide mode were built; they were proved by sensing three different media: air, water and dried egg white; sensors described here could be useful for sensing materials with a refractive index between 1.0 and 1.6; in particular, materials of biological interest with a refractive index in the range 1.3–1.6, like those containing biotin and/or streptavidin. They were built by depositing Nb2O5/SiO2/TiN multilayer structures on the flat surface of D-shaped sapphire prisms by using the dc magnetron sputtering technique. Attenuated total reflection (ATR) experiments in the Kretschmann configuration were accomplished for the air/TiN/Prism and S/Nb2O5/SiO2/TiN/Prism structures, S being the sample or sensing medium. ATR spectra for plasmons at the TiN/air interface showed a broad absorption band for angles of incidence between 36 and 85°, with full width at half maximum (FWHM) of approximately 40°. For the S/Nb2O5/SiO2/TiN/Prism structures, ATR spectra showed a sharp reflectivity peak, within the broad plasmonic absorption band, which was associated with Fano resonances. The angular position and FWHM of the Fano resonances strongly depend on the refractive index of the sensing medium. ATR spectra were fitted by using the transfer-matrix method. Additionally, we found that angular sensitivity and figure of merit increase with increasing the refractive index of the sensing medium.

Spectrally broad plasmonic absorption in Ga and In nanoparticle hybrids

In this work, we use Joule-effect thermal evaporation to produce hybrid structures made of Ga and In nanoparticles (NPs) on Si (100) substrates. Taking advantage of the protective oxide shell, In NPs can be used as a template for a second deposition step without structural changes, enabling the hybridization of NPs of materials. These complex structures of mixed NPs present a spectrally broad plasmonic absorption that can be optically tuned with a wide range of photon energies from UV to IR regions with a full width at half maximum range of ∼400 to 800 nm. The results suggest that the localized surface plasmon resonance (LSPR) of the hybrid NPs is mainly due to the plasmonic coupling of the in-plane modes. Furthermore, different scenarios studied by discrete dipole approximation simulations show that the interconnection between NPs is extremely sensitive to the size and the local arrangement of the nanostructures. This kind of broadening and tunable LSPR may have interest for energy transfer applications, biosensing platforms and solar cells.

Silver nanoparticles by atomic vapour deposition on an alcohol micro-jet.

We achieved sputter deposition of silver atoms onto liquid alcohols by injection of solvents into vacuum via a liquid microjet. Mixing silver atoms into ethanol by this method produced metallic silver nanoparticles. These had a broad, log-normal size distribution, with median size between 3.3 ± 1.4 nm and 2.0 ± 0.7 nm, depending on experiment geometry; and a broad plasmon absorption band centred around 450 nm. We also deposited silver atoms into a solution of colloidal silica nanoparticles, generating silver-decorated silica particles with consistent decoration of almost one silver particle to each silica sphere. The silver–silica mixture showed increased colloidal stability and yield of silver, along with a narrowed size distribution and a narrower plasmon band blue-shifted to 410 nm. Significant methanol loss of 1.65 × 10−7 mol MeOH per g per s from the mature silver–silica solutions suggests we have reproduced known silica supported silver catalysts. The excellent distribution of silver on each silica sphere shows this technique has potential to improve the distribution of catalytically active particles in supported catalysts.

Semiconductor-Metal Nanofloret Hybrid Structures by Self-Processing Synthesis.

We present a synthetic strategy that takes advantage of the inherent asymmetry exhibited by semiconductor nanowires prepared by Au-catalyzed chemical vapor deposition (CVD). The metal-semiconductor junction is used for activating etch, deposition, and modification steps localized to the tip area using a wet-chemistry approach. The hybrid nanostructures obtained for the coinage metals Cu, Ag, and Au resemble the morphology of grass flowers, termed here Nanofloret hybrid nanostructures consisting of a high aspect ratio SiGe nanowire (NW) with a metallic nanoshell cap. The synthetic method is used to prepare hybrid nanostructures in one step by triggering a programmable cascade of events that is autonomously executed, termed self-processing synthesis. The synthesis progression was monitored by ex situ transmission electron microscopy (TEM), in situ scanning transmission electron microscopy (STEM) and inductively coupled plasma mass spectrometry (ICP-MS) analyses to study the mechanistic reaction details of the various processes taking place during the synthesis. Our results indicate that the synthesis involves distinct processing steps including localized oxide etch, metal deposition, and process termination. Control over the deposition and etching processes is demonstrated by several parameters: (i) etchant concentration (water), (ii) SiGe alloy composition, (iii) reducing agent, (iv) metal redox potential, and (v) addition of surfactants for controlling the deposited metal grain size. The NF structures exhibit broad plasmonic absorption that is utilized for demonstrating surface-enhanced Raman scattering (SERS) of thiophenol monolayer. The new type of nanostructures feature a metallic nanoshell directly coupled to the crystalline semiconductor NW showing broad plasmonic absorption.

Plasmonic ‘rainbow’ photocatalyst with broadband solar light response for environmental applications

We propose the concept of a ‘rainbow’ photocatalyst that consists of TiO2 modified with gold-silver alloy nanoparticles of various sizes and compositions, resulting in a broad plasmon absorption band that covers the entire UV–vis range of the solar spectrum. It is demonstrated that this plasmonic ‘rainbow’ photocatalyst is 16% more effective than TiO2 P25 under both simulated and real solar light for pollutant degradation at the solid-gas interface. With this we provide a promising strategy to maximize the spectral response for solar to chemical energy conversion.

Metamaterial Absorbers for Infrared Detection of Molecular Self-Assembled Monolayers.

The emerging field of plasmonic metamaterials has introduced new degree of freedom to manipulate optical field from nano to macroscopic scale, offering an attractive platform for sensing applications. So far, metamaterial sensor concepts, however, have focused on hot-spot engineering to improve the near-field enhancement, rather than fully exploiting tailored material properties. Here, we present a novel spectroscopic technique based on the metamaterial infrared (IR) absorber allowing for a low-background detection scheme as well as significant plasmonic enhancement. Specifically, we experimentally demonstrate the resonant coupling of plasmonic modes of a metamaterial absorber and IR vibrational modes of a molecular self-assembled monolayer. The metamaterial consisting of an array of Au/MgF2/Au structures exhibits an anomalous absorption at ~3000 cm−1, which spectrally overlaps with C-H stretching vibrational modes. Symmetric/asymmetric C-H stretching modes of a 16-Mercaptohexadecanoic acid monolayer are clearly observed as Fano-like anti-resonance peaks within a broad plasmonic absorption of the metamaterial. Spectral analysis using Fano line-shape fitting reveals the underlying resonant interference in plasmon-molecular coupled systems. Our metamaterial approach achieves the attomole sensitivity with a large signal-to-noise ratio in the far-field measurement, thus may open up new avenues for realizing ultrasensitive IR inspection technologies.

Sol–gel prepared Cu2O microspheres: linear and nonlinear optical properties

We report the simple sol–gel based synthesis of size-tunable monodispersed crystalline Cu2O microspheres (CMS) and the measurement of their linear and nonlinear optical properties. Optical absorption spectra show a broad plasmonic absorption band extending from 350 to 1100 nm in general agreement with calculations based on Mie theory, despite the relatively large micron size of the particles. Z-scan measurements (532 nm, 5 ns pulses) reveal that Cu2O microspheres are efficient optical limiters, comparable in strength to nanocarbons and metal/semiconductor nanoparticles. The effective two-photon absorption coefficients are numerically calculated from measured data. These can be tuned systematically by varying the particle size.

Radiolytic Preparation of Ultrafine Colloidal Gold Particles in Aqueous Solution: Optical Spectrum, Controlled Growth, and Some Chemical Reactions

Orange− or yellow−orange-colored aqueous dispersions of ultrafine gold particles (∼2 nm) result from the γ-irradiation of deaerated solutions containing hydrolyzed AuCl4- and poly(vinyl alcohol) or poly(vinylpyrrolidone), respectively. The particles have a weak and very broad plasmon absorption band with maximum in the 490 to 500 nm range, i.e., at substantially shorter wavelengths than expected from classical Mie theory using bulk dielectric data; moreover, the specific UV absorption increases as the particles become smaller. The particles are used as seeds in the radiolytic reduction of added Au(CN)2- to yield larger particles of any desired size and improved monodispersity. The particles partially react with oxygen. Chemisorption of 3-mercapto propionic acid strongly affects the electronics of the particles, as indicated by the changes in optical absorption. The early stages of AuCl4- reduction are also investigated. The reduction occurs essentially in two steps: (1) formation, and (2) reduction of Au+. A 203-nm absorption band is tentatively attributed to Au+. Without further irradiation, Au+ disappears thermally within hours to yield larger particles (20−70 nm).

Optical absorption of matrix isolated Li, Na, and Ag clusters and microcrystals

Optical absorption spectra are reported for Li, Na, and Ag in various stages of aggregation ranging from atom to microcrystal. The samples were prepared by the simultaneous condensation of rare gas and metal vapor onto a liquid helium temperature substrate. The dependence of the strength of the individual absorption lines on metal concentration and on selective light induced diffusion allowed the tentative assignment of spectral features to one, two, three, and four atom clusters. Comparison of the observed transition energies with an empirical, extended Hückel calculation seems to indicate that the trimers are triangular and the four atom clusters have tetrahedral shape. For metal concentrations higher than 10% the sharp molecular lines abruptly disappear and a single broad plasmon absorption, characteristic of microcrystals, appears. The shape of this absorption band can be explained using a continuum theory of dielectric response if microcrystal size, shape, and mutual interaction are properly taken into account.

Defect characterization in [formula omitted] at elevated temperatures. II: Neutron diffraction

We propose the concept of a ‘rainbow’ photocatalyst that consists of TiO2 modified with gold-silver alloy nanoparticles of various sizes and compositions, resulting in a broad plasmon absorption band that covers the entire UV–vis range of the solar spectrum. It is demonstrated that this plasmonic ‘rainbow’ photocatalyst is 16% more effective than TiO2 P25 under both simulated and real solar light for pollutant degradation at the solid-gas interface. With this we provide a promising strategy to maximize the spectral response for solar to chemical energy conversion.