Combined Extinction and Absorption UV–Visible Spectroscopy as a Method for Revealing Shape Imperfections of Metallic Nanoparticles

Abstract

Metallic nanoparticle solutions are routinely characterized by measuring their extinction spectrum (with UV-vis spectroscopy). Theoretical predictions such as Mie theory for spheres can then be used to infer important properties, such as particle size and concentration. Here we highlight the benefits of measuring not only the extinction (the sum of absorption and scattering) but also the absorption spectrum (which excludes scattering) for routine characterization of metallic nanoparticles. We use an integrating sphere-based method to measure the combined extinction-absorption spectra of silver nanospheres and nanocubes. Using a suite of electromagnetic modeling tools (Mie theory, T-matrix, Surface Integral Equation methods), we show that the absorption spectrum, in contrast to extinction, is particularly sensitive to shape imperfections such as roughness, faceting, or edge rounding. We study in detail the canonical case of silver nanospheres, where small discrepancies between experimental and calculated extinction spectra are still common and often overlooked. We show 1 Page 1 of 32 ACS Paragon Plus Environment Analytical Chemistry that this mismatch between theory and experiment becomes much more important when considering the absorption spectrum, and can no longer be dismissed as experimental imperfections. We focus in particular on the quadrupolar localized plasmon resonance of silver nanospheres, which is predicted to be very prominent in the absorption spectrum but is not observed in our experiments. We consider and discuss a number of possible explanations to account for this discrepancy, including: changes in the dielectric function of Ag, size polydispersity, and shape imperfections such as elongation, faceting, and roughness. We are able to pinpoint faceting and roughness as the likely causes for the observed discrepancy. A similar analysis is carried out on silver nanocubes to demonstrate the generality of this conclusion. We conclude that the absorption spectrum is in general much more sensitive to the fine details of a nanoparticle geometry, compared to the extinction spectrum. The ratio of extinction to absorption also provides a sensitive indicator of size for many types of nanoparticles, much more reliably than any observed plasmon resonance shifts. Overall, this work demonstrates that combined absorption-extinction measurements provide a much richer characterization tool for metallic nanoparticles. The study of the optical properties of metallic nanoparticles (NPs) began as an attempt to understand fundamental interactions between light and nanomaterials. It has now become a fruitful source of practical applications for physicists, biologists, chemists and engineers, in fields as diverse as drug delivery, 1 nanocatalysis, 2 single-molecule detection, 3 and solar cells. 4 Following chemical or physical synthesis, NPs are usually thoroughly characterized to verify whether they will serve their intended purpose. UV-vis extinction spectroscopy is the most commonly used tool to measure their optical pr...

Mie theory and the dichroic effect for spherical gold nanoparticles: an experimental approach

“…Experimental analysis is possible but is much rarer due to its complexity. For example, techniques utilising photoacoustic imaging, 33 darkeld microscopy, 34,35 integrating sphere technology, [36][37][38] and others, 39,40 have been developed to measure absorption and scattering spectra of nanoparticle systems. Despite the signicant and valuable information obtained with these techniques, they have not yet achieved widespread use.…”

Full optical characterization of single nanoparticles using quantitative phase imaging

“…SM techniques have been used to measure absorption and scattering cross sections originating this time from the same signal [28,29]. Recently, the use of an integrating sphere implemented upon an optical microscope has been demonstrated to enable absorption and scattering measurements on single NPs [30,31]. The main drawbacks of all these techniques are that they require expensive equipment such as lasers, modulating elements, and lock-in amplifiers, and they are sensitive to optical alignments.…”

“…The analysis procedures are also complex and often based on a precise knowledge of the point spread function of the microscope [32]. These drawbacks make SM techniques impractical for routine characterization of NPs [31]. Very recently, Zilli et al introduced a technique aimed to measure all the optical cross sections of individual NPs at once, which is not based on the use of a focused laser and SM measurements [32].…”

Selective Thermoplasmonic Embedment Of Supported Silver Nanocrystals

“…While numerical modelling is an excellent tool to use to determine various properties of different nanomaterials using theoretical calculations, there can be discrepancies between experimental results and those calculated by computational programs. [159][160][161][162] Typically discrepancies between experimental and computation methods can be attributed to the fact that properties such as size distribution, geometric parameters and dielectric functions of the actual materials used may not be identical to values inputted or estimated by the softwares. [159][160][161][162] Another parameter than can differ considerably between computational and experimental systems is the type of light source used.…”

“…[159][160][161][162] Typically discrepancies between experimental and computation methods can be attributed to the fact that properties such as size distribution, geometric parameters and dielectric functions of the actual materials used may not be identical to values inputted or estimated by the softwares. [159][160][161][162] Another parameter than can differ considerably between computational and experimental systems is the type of light source used. 163 In these experiments the light source used to thermoplasmonically excite the AgNCs is a laser of unknown polarization.…”