Two-Component Dielectric Function of Gold Nanostars: Novel Concept for Theoretical Modeling and Its Experimental Verification

Abstract

Plasmon resonances of Au nanostars (AuNSTs) can be tuned across 600–2000 nm, which makes them an attractive platform for applications. Rational design of AuNST morphology requires adequate computational models. The common approach, based on electromagnetic simulations with a bulk dielectric function, is not applicable to sharp nanostar spikes, which are typical of plasmon resonances above 800 nm. We suggest a two-component dielectric function in which the nanostar core is treated as a bulk material, whereas the size-corrected dielectric function of the spikes is treated in terms of a modified Coronado–Schatz model. In contrast to common simulations with bulk Au constants and in agreement with experimental observations, the simulated nanostar spectra show a strong reduction in the Q factor of the plasmonic peak, whereas the polarization-averaged spectra with bulk Au constants are not broadened. For some nanostructures, an increase in the size-correction damping parameter of the electrons results in an unusual increase in the absorption plasmonic peak. Other atypical behaviour is a decrease in the scattering cross section with increasing spike length. The effect of NIR water absorption on the absorption and extinction cross sections is small, and the absorption peak demonstrates the expected small decrease in the absorbing media. Surprisingly, however, water absorption increases the scattering peak by 30%. For the common surfactant-free Vo-Dinh AuNSTs we report, for the first time, very intense SWIR plasmonic peaks around 1900 nm, in addition to the common strong peak in the UV–vis–NIR band (here, at 1100 nm). For bilayers of AuNSTs in air, we recorded two similarly intense peaks near 800 and 1500 nm. To simulate the experimental extinction spectra of colloids and bilayers on glass in air, we develop a statistical model that includes the major fraction of typical Vo-Dinh AuNSTs and two minor fractions of sea urchins and particles with protrusions. In contrast to general belief, we show that the common UV–vis–NIR plasmonic peak of surfactant-free AuNSTs is related to short spikes on a spherical core, whereas long spikes produce an intense SWIR plasmonic mode. Such a structural assignment of vis–NIR and SWIR peaks does not seem to have been reported previously for surfactant-free nanostars. With our model, we demonstrate good agreement between simulated and measured spectra of colloids and bilayers on glass in air.
This research was supported by the Russian Scientific Foundation (Project No. 18-14-00016). The work by VK on AuNST synthesis and characterization was supported by the Russian Scientific Foundation (Project No. 19-72-00120).
The work by SZ and YA on electromagnetic simulations was supported by the Russian Foundation for Basic Research (Project No. 19-07-00378)

Speaker

Nikolai Khlebtsov
Institute of Biochemistry and Physiology of Plants and Microorganisms Russian Academy of Sciences, Saratov State University
Russian Federation

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