A Quantitative Study of the Environmental Effects on the Optical Response of Gold Nanorods

Abstract

The effects of the dielectric environment on the optical extinction spectra of gold nanorods were quantitatively studied using individual bare and silica-coated nanorods. The dispersion and amplitude of their extinction cross-section, dominated by absorption for the investigated sizes, were measured using spatial modulation spectroscopy (SMS). The experimental results were compared to calculations from a numerical model that included environmental features present in the measurements and the morphology and size of the corresponding nanorods measured by transmission electron microscopy. The combination of these experimental and theoretical tools permits a detailed interpretation of the optical properties of the individual nanorods. The measured optical extinction spectra and the extinction cross-section amplitudes were well reproduced by the numerical model for silica-coated gold nanorods, for which the silica shell provides a controlled environment. In contrast, additional environmental factors had to be assumed in the model for bare nanorods, stressing the importance of controlling and characterizing the experimental conditions when measuring the optical response of bare surface-deposited single metal nanoparticles.

Linear and ultrafast nonlinear plasmonics of single nano-objects

“…Conversely, excellent agreement was restored by adjusting n sur value (e.g., n sur ≈ 1.4 for gold nanorod experiments) [62] or surrounding the nano-objects by a water droplet in the modeling [69]. This suggests the presence of residual surfactant molecules or solvent (and possibly other impurities) around deposited nanoparticles.…”

“…This is most usually done using either atomic force microscopy [18] or transmission or scanning electron microscopies (TEM or SEM). In the latter cases, adapted substrates have to be used, i.e., thin ones transparent to both photons and electrons (e.g., a 40 nm silica one) [41,[69][70][71] or conducting ones (e.g., ITO) [50,72], respectively. However, as electron exposure may affect the object properties, and in particular its optical spectra, care must be taken performing optical and electron microscopy imaging studies on the same nano-object.…”

“…The use of a refractive index of n sur =1 for the surrounding medium above the substrate (figure 3a) led to strong discrepancies between measured and simulated Ω R values [69].…”

Imaging nano-objects by linear and nonlinear optical absorption microscopies

“…1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 In particular, SMS has been used to generate precise data about electron-surface scattering in gold and silver nanoparticles, and how radiation damping affects the spectra of these materials. [2,3,12,13,30,78,92,100,114,115] A key point in these studies is that SMS allows the linewidths to be correlated to dimensions directly from the optical experiments. This is extremely useful for unraveling the effects of radiation damping from surface scattering, which scale differently with size.…”

“…However, SMS has an advantage in that it directly yields the extinction cross-section of the nano-object, which allows the size of the particle to be determined. [2, 16,25,30,77,92,114] In the following, the ways these different imaging techniques are implemented are briefly described. Examples of different experiments in our laboratory are then given, focusing on recent imaging measurements on single semiconductor and metal nano-objects with the idea that every picture tells a story.…”