Polarization-dependent dark-field scattering spectroscopy of differently shaped isolated gold nanostructures and their orientation mapping

Islam, Md Monirul
Major Professor
Andrew C Hillier
Committee Member
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Chemical and Biological Engineering

Gold nanostructures exhibit high light absorption and scattering properties due to the localized surface plasmon resonance. When light interacts with these nanoparticles, electric fields near them are greatly enhanced, and the scattering of light maximize at the resonant wavelength which can be tuned by changing the shape, size, and the surrounding medium of the nanoparticles. This thesis focuses on the shape and orientation-dependent scattering pattern of the gold nanostructures by polarization-dependent dark-field scattering spectroscopy along with the shape and orientation mapping of the isolated nanostructures from their scattering color.

Chapter 1 introduces the fundamentals of the localized surface plasmon resonance and single-particle spectroscopy. The dependence of the resonant wavelength on different factors is discussed briefly along with some mathematical expressions of light absorption, scattering and extinction cross-section. A brief description of different types of single-particle spectroscopy techniques based on nanoparticle scattering, absorption, and extinction are also presented here. The working principle of dark-field spectroscopy is discussed in detail with examples. This chapter concludes with a brief description of our modified dark-field spectroscopy setup.

In chapter 2, we present a systematic study of the effect of substrates, orientations of the nanoparticles, and the polarization direction of the incident light on the scattering pattern of different shaped individual gold nanostructures like nanorods, nanowires, nanotriangles, and nanospheres by dark-field microscopy. The color and the scattering spectra of these nanostructures are correlated to their shape, size, and orientations using high-resolution scanning electron microscopy. We have found that the scattering patterns of spherical and triangular gold nanoparticles are independent of their orientation and polarization direction of light, whereas the nanorods and nanowires are highly dependent on both the orientation and polarization.

In Chapter 3, we demonstrate a robust and straightforward method for mapping the shape and orientation of differently shaped gold nanoparticles adsorbed to a solid surface using polarized dark-field microscopy. The scattering signals from the isolated gold nanoparticles in the optical far-field is captured with the CCD detector of the microscope. By analyzing the RGB color of the scattering signal as a function of the azimuthal angle, the shapes and orientations of the underlying nanostructures are accurately determined. Using this method, we have accurately determined the shape and orientation of various nanostructures, including nanorods and nanowires, spherical nanoparticles, nanoparticle dimers, and nanotriangles. This technique will provide a rapid and inexpensive complement to the typical structural analysis of nanoparticles that is achieved by electron microscopy.

Chapter 4 briefly summarizes the work presented in this thesis, along with some potential applications of the method we developed. We anticipate that with the single-particle dark-field spectroscopy technique, we will be able to characterize the complex nanostructures fabricated by the metallization of DNA origami.

In summary, this thesis demonstrates a comprehensive study of the scattering pattern of different shaped isolated plasmonic nanostructures and reveals a straightforward method to map the shape and orientation of these nanostructures.