Measured Extinction Spectra Research Articles

Optimization of the Bulk Refractive Index Sensitivity of Silver NanoPrisms

AbstractThe sensitivity and optical properties of silver nanoprisms (triangular plates with round‐truncated corners) are investigated in this paper. Results of boundary element method simulations are compared with experimental results and literature data. Based on electron microscopy images of the synthesized nanoprisms, a single‐particle model is set up for simulations with three running parameters: edge length, thickness, and roundness (defined as the radius of the circumscribed circle divided by the edge length). These geometric parameters can be optimized during chemical synthesis to create sensors with improved sensitivity. The effect of biomolecular layers is also investigated. As a novel approach to improve the agreement between the simulated and experimentally measured extinction spectra, the single‐particle model is extended to consider the variation of the prisms' parameters in the form of distributions. The resulting extinction cross‐section spectra correspond well with the experimental data. The calculated bulk refractive index sensitivity is 670 nm/RIU (RIU stands for refractive index unit) for the single particle model (length = 150 nm, thickness = 10 nm, and roundness = 0.1), while (690 ± 5) nm/RIU for the extended model. The presented model and obtained relations between sensitivity and geometry can be effectively used to design and optimize the fabrication technologies for silver nanoprism‐based sensing applications.

Particle shapes and infrared extinction spectra of nitric acid dihydrate (NAD) crystals: optical constants of the β-NAD modification

Abstract. Satellite- and aircraft-based mid-infrared measurements of polar stratospheric clouds (PSCs) have provided spectroscopic evidence for the presence of β-NAT (nitric acid trihydrate) particles. Metastable nitric acid hydrate phases such as α-NAT and α-NAD (nitric acid dihydrate) have been frequently observed in laboratory experiments but not yet detected as a constituent of PSCs in atmospheric measurements. As for the β-NAD modification, its formation was first observed in X-ray diffraction measurements when the low-temperature α-NAD phase was warmed to a temperature above 210 K. Its infrared spectrum has been reported, but so far no optical constants have been derived that could be used as input for infrared retrievals of PSC composition. In this work, we show that β-NAD particles were efficiently formed in isothermal heterogeneous crystallisation experiments at 190 K from supercooled HNO3/H2O solution droplets containing an embedded mineral dust or meteoric smoke particle analogue. An inversion algorithm based on a T-matrix optical model was used to derive for the first time the mid-infrared complex refractive indices of the β-NAD modification from the measured extinction spectrum of the particles. In contrast to the heterogeneous crystallisation experiments, the α-NAD phase was formed when the HNO3/H2O solution droplets did not contain a solid nucleus and crystallised homogeneously. Using a light-scattering detector that recorded two-dimensional scattering patterns of the crystallised NAD particles, we were able to determine predominant shapes of the α- and β-NAD crystals. We found that α-NAD grew into elongated, needle-shaped crystals, while β-NAD particles were compact in shape. This agrees with previously reported images of α- and β-NAD particles grown on the cryo-stage of an environmental scanning electron microscope. While direct evidence for the existence of metastable NAD in the polar stratosphere is still lacking, our experiments add to the wealth of previous laboratory studies that have identified various conditions for the rapid growth of metastable compositions. In the atmosphere, these could be intermediate states that transform into thermodynamically stable NAT on longer timescales in aged PSCs.

Space-resolved chemical information from infrared extinction spectra

A new method is presented for the extraction of the complex index of refraction from the extinction efficiency, Q_{ext} {({tilde{nu }})}, of homogeneous and layered dielectric spheres that simultaneously removes scattering effects and corrects measured extinction spectra for systematic experimental errors such as baseline shifts, tilts, curvature, and scaling. No reference spectrum is required and fit functions may be used that automatically satisfy the Kramers–Kronig relations. Thus, the method yields the complex refractive index of a sample for unambiguous interpretation of the chemical information of the sample. In the case of homogeneous spheres, the method also determines the radius of the sphere. In the case of layered spheres, the method determines the substances within each layer. Only a single-element detector is required. Using numerically computed Q_{ext}({tilde{nu }}) data of polymethyl-methacrylate and polystyrene homogeneous and layered spheres, we show that the new reconstruction algorithm is accurate and reliable. Reconstructing the complex refractive index from a published, experimentally measured raw absorbance spectrum shows that the new method simultaneously corrects spectra for scattering effects and, given shape information, corrects raw spectra for systematic errors that result in spectral distortions such as baseline shifts, tilts, curvature, and scaling.

Optical Properties of Colloidal Silver Nanowires.

Silver nanowires are used in many applications, ranging from transparent conductive layers to Raman substrates and sensors. Their performance often relies on their unique optical properties that emerge from localized surface plasmon resonances in the ultraviolet. To tailor the nanowire geometry for a specific application, a correct understanding of the relationship between the wire’s structure and its optical properties is therefore necessary. However, while the colloidal synthesis of silver nanowires typically leads to structures with pentagonally twinned geometries, their optical properties are often modeled assuming a cylindrical cross-section. Here we highlight the strengths and limitations of such an approximation by numerically calculating the optical and electrical response of pentagonally twinned silver nanowires and nanowire networks. We find that our accurate modeling is crucial to deduce structural information from experimentally measured extinction spectra of colloidally synthesized nanowire suspensions and to predict the performance of nanowire-based near-field sensors. On the contrary, the cylindrical approximation is fully capable of capturing the optical and electrical performance of nanowire networks used as transparent electrodes. Our results can help assess the quality of nanowire syntheses and guide in the design of optimized silver nanowire-based devices.

Spectroscopic quantification of the nanoparticle production efficiency of copper wire explosion

Wire explosion is a technically straightforward green method for nanoparticle production; however, the determination of polydisperse and multimodal particle size distribution and thereby the assessment of the efficiency of the process pose serious challenges. Fitting extinction spectra derived from the Mie theory to the measured extinction spectra of colloidal solutions produced by the explosion of copper wires of 70 μm diameter and 20 mm length by discharging a capacitor of 435 nF charged to voltages ranging from 4 to 12 kV in an aqueous environment allows for determining the size distribution of the particles in the 10–300 nm diameter range and provides information on their oxidation state as well. In underwater wire explosion of copper, the vast majority of the nanoparticles transforms into copper oxides already during the process. The size distribution remains bimodal throughout the whole charging voltage range investigated. The nanoparticles follow a lognormal size distribution with a mode at 30 nm, while the fine particle population extending from 100 to 300 nm in diameter is dominated by rather normally distributed copper droplets with modes at 160–170 nm. The energy injected into the wire controls the individual concentration of both the fine and nanoparticles with the modes and ranges remaining hardly affected. The number concentration of the nanoparticles increases monotonically with the energy, spanning more than one order of magnitude. The throughput assessed by the total mass of the nanoparticles divided by the total mass of all species identified in the sol within the 0–300 nm diameter range increases monotonically from 1% at 4 kV up to 41% at 12 kV, revealing that wire explosion can be optimized for efficient nanoparticle production.

Signatures of Small Morphological Anisotropies in the Plasmonic and Vibrational Responses of Individual Nano-objects.

The plasmonic and vibrational properties of single gold nanodisks patterned on a sapphire substrate are investigated via spatial modulation and pump-probe optical spectroscopies. The features of the measured extinction spectra and time-resolved signals are highly sensitive to minute deviations of the nanodisk morphology from a perfectly cylindrical one. An elliptical nanodisk section, as compared to a circular one, lifts the degeneracy of the two nanodisk in-plane dipolar surface plasmon resonances, which can be selectively excited by controlling the polarization of the incident light. This splitting effect, whose amplitude increases with nanodisk ellipticity, correlates with the detection of additional vibrational modes in the context of time-resolved spectroscopy. Analysis of the measurements is performed through the combination of optical and acoustic numerical models. This allows us first to estimate the dimensions of the investigated nanodisks from their plasmonic response and then to compare the measured and computed frequencies of their detectable vibrational modes, which are found to be in excellent agreement. This study demonstrates that single-particle optical spectroscopies are able to provide access to fine morphological characteristics, representing in this case a valuable alternative to traditional techniques aimed at postfabrication inspection of subwavelength nanodevice morphology.

Controlled Gold Nanoparticle Placement into Patterned Polydimethylsiloxane Thin Films via Directed Self-Assembly

An economically scalable and reproducible method to assemble nanoparticles (NPs) into ordered arrays with high fidelity remains a fundamental roadblock. Methods like directed self-assembly have shown the highest promise resulting in >85% density of NP-filled prepatterned polymer cavities. This work refines directed self-assembly by controlling the evaporation rate, substrate velocity (deposition rate), and NP diameter resulting in reproducible fabrication of ordered arrays with areas >2 mm×2 mm and ~100% density of filled cavities. Measured optical spectra showed a blueshift in the localized surface plasmon resonance (LSPR) and surface lattice resonance (SLR) peaks with increasing NP density for both 100 and 150 nm gold (Au) NPs. Discrete dipole approximation (DDA), coupled dipole approximation (CDA), rapid semi-analytical CDA (rsa-CDA), and Mie theory simulations closely matched extinction per nanoparticle (extinction/NP) calculations for measured extinction spectra. An ordered array containing 150 nm AuNPs was used for comparison with rsa-CDA estimates using both polydimethylsiloxane (PDMS) and glass refractive indices (RI) resulting in peak location estimates within 1.7% and comparable relative increases in peak heights. Both the measured and simulated SLR peak heights were shown to significantly increase when the array was on glass as opposed to within PDMS.

Observation of Second Harmonic Generation in Lightning-Bolt-Like Shaped Nanostructured Metasurface

Second harmonic generation (SHG) was observed on irradiation of a symmetry-broken gold nanostructured surface with an ultrashort pulse of low input power. A metasurface consisting of lightning-bolt-like shaped gold (Au) nanostructures in a periodic arrangement was fabricated on a glass substrate and utilized to excite surface plasmons. The measured extinction spectrum had a dual band maximized at approximately 605 nm and 786 nm under illumination with linearly polarized light. The two-photon induced photoluminescence and SHG intensity were dependent on the polarization states and wavelength of the femtosecond laser pulse irradiation. Furthermore, the SHG intensity peak was linearly dependent on the input power of the incident laser beam, and the spectral property of the emitted second harmonic was correlated to the extinction spectrum of the nanostructured metasurface. The SHG peak over the pumping wavelengths was resonant with a transverse multipolar plasmon mode. The findings provide useful guidelines for efficient production of second harmonic luminescence.

Dual-blade-like shaped nanostructured metasurface inducing dual-band extinction for dye fluorescence enhancement

The author experimentally reports here strong fluorescence enhancement (FLE) induced by a gold nanostructured surface. The metasurface consisting of periodic arrays of dual-blade-like shaped nanostructures was fabricated on a glass substrate and manipulated to excite surface plasmons. The measured extinction spectrum has a dual band maximized at approximately 645 nm and 795 nm under a linearly polarized light illumination. The author employed the dye molecule IR125 as a fluorophore whose fluorescence was significantly boosted by near-field interactions between the plasmon and the molecule. The resulting FLE induced by the near-field enhancement is attributed to the plasmonic resonances and correlated to the extinction of the metal nanostructure. The calculated Purcell enhancement factor, which simulates a spontaneous emission enhancement of an emitting dipole placed in the vicinity of the nanostructure, is in good agreement with the measured FLE.

Self-Catalyzed Vapor-Liquid-Solid Growth of Lead Halide Nanowires and Conversion to Hybrid Perovskites.

Lead halide perovskites (LHPs) have shown remarkable promise for use in photovoltaics, photodetectors, light-emitting diodes, and lasers. Although solution-processed polycrystalline films are the most widely studied morphology, LHP nanowires (NWs) grown by vapor-phase processes offer the potential for precise control over crystallinity, phase, composition, and morphology. Here, we report the first demonstration of self-catalyzed vapor-liquid-solid (VLS) growth of lead halide (PbX2; X = Cl, Br, or I) NWs and conversion to LHP. We present a kinetic model of the PbX2 NW growth process in which a liquid Pb catalyst is supersaturated with halogen X through vapor-phase incorporation of both Pb and X, inducing growth of a NW. For PbI2, we show that the NWs are single-crystalline, oriented in the ⟨1̅21̅0⟩ direction, and composed of a stoichiometric PbI2 shaft with a spherical Pb tip. Low-temperature vapor-phase intercalation of methylammonium iodide converts the NWs to methylammonium lead iodide (MAPbI3) perovskite while maintaining the NW morphology. Single-NW experiments comparing measured extinction spectra with optical simulations show that the NWs exhibit a strong optical antenna effect, leading to substantially enhanced scattering efficiencies and to absorption efficiencies that can be more than twice that of thin films of the same thickness. Further development of the self-catalyzed VLS mechanism for lead halide and perovskite NWs should enable the rational design of nanostructures for various optoelectronic technologies, including potentially unique applications such as hot-carrier solar cells.

Global scale variability of the mineral dust long-wave refractive index: a new dataset of in situ measurements for climate modeling and remote sensing

Abstract. Modeling the interaction of dust with long-wave (LW) radiation is still a challenge because of the scarcity of information on the complex refractive index of dust from different source regions. In particular, little is known about the variability of the refractive index as a function of the dust mineralogical composition, which depends on the specific emission source, and its size distribution, which is modified during transport. As a consequence, to date, climate models and remote sensing retrievals generally use a spatially invariant and time-constant value for the dust LW refractive index. In this paper, the variability of the mineral dust LW refractive index as a function of its mineralogical composition and size distribution is explored by in situ measurements in a large smog chamber. Mineral dust aerosols were generated from 19 natural soils from 8 regions: northern Africa, the Sahel, eastern Africa and the Middle East, eastern Asia, North and South America, southern Africa, and Australia. Soil samples were selected from a total of 137 available samples in order to represent the diversity of sources from arid and semi-arid areas worldwide and to account for the heterogeneity of the soil composition at the global scale. Aerosol samples generated from soils were re-suspended in the chamber, where their LW extinction spectra (3–15 µm), size distribution, and mineralogical composition were measured. The generated aerosol exhibits a realistic size distribution and mineralogy, including both the sub- and super-micron fractions, and represents in typical atmospheric proportions the main LW-active minerals, such as clays, quartz, and calcite. The complex refractive index of the aerosol is obtained by an optical inversion based upon the measured extinction spectrum and size distribution. Results from the present study show that the imaginary LW refractive index (k) of dust varies greatly both in magnitude and spectral shape from sample to sample, reflecting the differences in particle composition. In the 3–15 µm spectral range, k is between ∼ 0.001 and 0.92. The strength of the dust absorption at ∼ 7 and 11.4 µm depends on the amount of calcite within the samples, while the absorption between 8 and 14 µm is determined by the relative abundance of quartz and clays. The imaginary part (k) is observed to vary both from region to region and for varying sources within the same region. Conversely, for the real part (n), which is in the range 0.84–1.94, values are observed to agree for all dust samples across most of the spectrum within the error bars. This implies that while a constant n can be probably assumed for dust from different sources, a varying k should be used both at the global and the regional scale. A linear relationship between the magnitude of the imaginary refractive index at 7.0, 9.2, and 11.4 µm and the mass concentration of calcite and quartz absorbing at these wavelengths was found. We suggest that this may lead to predictive rules to estimate the LW refractive index of dust in specific bands based on an assumed or predicted mineralogical composition, or conversely, to estimate the dust composition from measurements of the LW extinction at specific wavebands. Based on the results of the present study, we recommend that climate models and remote sensing instruments operating at infrared wavelengths, such as IASI (infrared atmospheric sounder interferometer), use regionally dependent refractive indices rather than generic values. Our observations also suggest that the refractive index of dust in the LW does not change as a result of the loss of coarse particles by gravitational settling, so that constant values of n and k could be assumed close to sources and following transport. The whole dataset of the dust complex refractive indices presented in this paper is made available to the scientific community in the Supplement.

Generation of Localized Surface Plasmon Resonance Using Hybrid Au-Ag Nanoparticle Arrays as a Sensor of Polychlorinated Biphenyls Detection.

In this study, the hybrid Au–Ag hexagonal lattice of triangular and square lattice of quadrate periodic nanoparticle arrays (PNAs) were designed to investigate their extinction spectra of the localized surface plasmon resonances (LSPRs). First, their simulating extinction spectra were calculated by discrete dipole approximation (DDA) numerical method by changing the media refractive index. Simulation results showed that as the media refractive index was changed from 1.0 to 1.2, the maximum peak intensity of LSPRs spectra had no apparent change and the wavelength to reveal the maximum peak intensity of LSPRs spectra was shifted lower value. Polystyrene (PS) nanospheres with two differently arranged structures were used as the templates to deposit the hybrid Au–Ag hexagonal lattice of triangular and square lattice of quadrate periodic PNAs by evaporation method. The hybrid Au–Ag hexagonal lattice of triangular and square lattice of quadrate PNAs were grown on single crystal silicon (c-Si) substrates, and their measured extinction spectra were compared with the calculated results. Finally, the fabricated hexagonal lattices of triangular PNAs were investigated as a sensor of polychlorinated biphenyl solution (PCB-77) by observing the wavelength to reveal the maximum extinction efficiency (λmax). We show that the adhesion of β-cyclodextrins (SH-β-CD) on the hybrid Au–Ag hexagonal lattice of triangular PNAs could be used to increase the variation of λmax. We also demonstrate that the adhesion of SH-β-CD increases the sensitivity and detection effect of PCB-77 in hexagonal lattice of triangular PNAs.

Particle characterization using THz spectroscopy

THz extinction spectroscopy extends UV-Vis and NIR-spectroscopy to characterize particles from fine powders and dust to sand, grains and granulated materials. We extract particle sizes from the spectral position of the first peak of the interference structure and size distributions from the visibility of the fine ripple structure in the measured extinction spectra. As such, we can demonstrate a route for a quick determination of these parameters from single measurements.

Tuning the Plasmonic Extinction Resonances of Hexagonal Arrays of Ag Nanoparticles

Plasmonically enhanced effects on a self-assembled, hexagonal array layer of ~4-nm silver nanoparticles are analyzed using three-dimensional finite-difference time-domain (3D FDTD) simulations and compared against experimentally measured extinction spectra. The effect of particle size, lattice spacing, and lack of monodispersity of the hexagonal array of silver nanoparticles on the extinction resonance was investigated to help determine optimal design specifications for efficient organic solar power harvesting.

Possibilities and limitations of non-destructive monitoring of the unicellular green microalgae (Chlorophyta) in the course of balanced growth

Timely information about basic growth parameters, such as dry weight, cell number, and pigment (chlorophyll and carotenoid) content is of importance for the evaluation of physiological conditions and growth kinetics of microalgal suspension cultures. The possibility of non-destructive determination of these parameters using the optical density (total light attenuation due to light scattering and absorption by the microalgal cells and the pigments contained therein) spectra of the cell suspension was shown for unicellular green algae Ettlia carotinosa Komarek and Haematococcus pluvialis Flotow (Chlorophyceae). Estimation of the pigment content of morphologically heterogeneous cultures (such as H. pluvialis) with a reasonable accuracy required the light-scattering compensation of the measured extinction spectra. On the other hand, the scattering signal is useful for dry weight and cell density estimation (in the cultures with insignificant morphological heterogeneity). The spectral regions sensitive to the variation in the target culture parameters was found, and the spectral indices linearly correlated with the latter were constructed. The possibilities and limitations of the optical methods suitable for non-destructive monitoring of the microalgal cultures in the course of their balanced growth and the potential of the developed approach for microalga biotechnology are discussed.

Extraction of absorption and scattering contribution of metallic nanoparticles toward rational synthesis and application.

Noble metal nanoparticles have unique localized surface plasmon resonance (LSPR), leading to their strong absorption and scattering in the visible light range. Up to date, the common practice in the selection of nanoparticles for a specific application is still based on the measured extinction spectra. This practice may be erroneous, because the extinction spectra contain both absorption and scattering contribution that may play different roles in different applications. It would be highly desirable to develop an efficient way to obtain the absorption and scattering spectra simultaneously. Herein, we develop a method to use the experimentally measured extinction and scattering signals to extract the absorption and scattering spectra that is in excellent agreement with that simulated by discrete dipole approximation (DDA). The heating curve measurement on the three types of gold nanorods, with almost the same extinction spectra but different absorption and scattering contribution, convincingly reveals an excellent correlation between the heating effect and the absorption strength rather than the extinction strength. The result demonstrates the importance to obtain the scattering and absorption spectra to predict the potential application for different types of nanoparticles, which in turn will screen efficiently nanoparticles for a specific application.

Formation and characterization of silver nanoparticles embedded in optical transparent materials for plasmonic sensor surfaces

• We analyzed particle forming behavior of thin Ag seed films before/after annealing. • We examined passivation layers produced via different deposition methods. • Ag particles embedded in Al 2 O 3 films were tested as LSPR-sensor-surface. • LSPR shifted 6.15 nm for a refractive index change of 0.07 using glucose solutions. Plasmonic nanostructures promise sensing capabilities with the potential for ultrasensitive and robust assays in life sciences and biomedicine. Silver island films represent an interesting and straightforward alternative for the implementation of substrate-attached plasmonic nanostructures. The temperature-induced particle-forming behavior of thin silver seed films deposited on glass substrates and on polycrystalline silicon films is represented. The measured extinction spectra reflect the different size distributions and shapes. The covering of the particles with different optical transparent film materials like ZnO, Al 2 O 3 , SiN x , and SiO x leads to a further shift in the resonance maximum due to their refractive index. The SiO x system shows an additional long term change in the extinction spectrum in contrast to ZnO, Al 2 O 3 , and SiN x . Thin silver films covered with Al 2 O 3 were used in order to proof the system as a sensor element for analyte detection (glucose solution).

Optical properties of plasmonic nanoparticles distributed in size determined from a modified Maxwell‐Garnett‐Mie theory

AbstractThe optical properties of spherical gold and silver nanoparticles distributed in size are investigated through a new effective medium theory. It is about the modified Maxwell‐Garnett‐Mie theory (MMGM) that takes into account the intrinsic confinement, dynamic effects and the nanoparticle radius distribution. In the case of monodispersed nanoparticles with a radius in the 1 nm‐25 nm range, MMGM is equivalent to the Mie theory. Compared to the conventional effective medium theory, we demonstrate that MGMM gives better description of the measured extinction spectra of colloidal gold and silver solutions. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Synthesis and characterization of silver nanoparticles in a nanoporous glass

We obtained a metal-dielectric composite by thermal restoration of silver atoms from an alcohol solution of a precursor in nanoporous glass with pores with a radius of 2 nm. The concentration, size, and asphericity degree of metal nanoparticles formed in the pores are characterized according to the measured extinction spectra of the material.

Desert dust observation from space – Application of measured mineral component infrared extinction spectra

Measured extinction spectra of six different components of desert dust have been used as input for a mineral dust retrieval algorithm for the Infrared Atmospheric Sounding Interferometer IASI based on Singular Vector Decomposition. The use of measured dust component extinction spectra rather than dust sample-based optical properties widely used in thermal infrared applications improves retrieval results significantly. Correlation between IASI derived dust optical depth (at 0.5 μm) and AERONET sun-photometer observations from six months over Northern Africa, the Arabian Peninsula and Asia improved from formerly 0.623 to now 0.757 while the root-mean-square difference decreased from 0.322 to now 0.174 and the bias decreased (in terms of its absolute value) from −0.181 to 0.003. The dynamically adjusted e−1 envelope of the deviation histogram between AERONET and IASI dust AOD at 0.5 μm is at 0.160 while 80% of all IASI observations are within AOD0.5 μm ± 0.2 of respective AERONET values.