All-optical characterization of individual nanocrystal properties using calcite-assisted localization and kinetics (CLocK) microscopy

Plasmonic metal nanoparticles have unique optical properties that are governed by the particle’s size, shape, composition, and local environment, allowing materials to be tailor-designed and fine-tuned to meet the demands of various imaging, sensing, photocatalytic and photothermal applications. Bottom-up approaches are inexpensive and scalable means to efficiently produce plasmonic nanoparticles of specified size and shape, but colloidally prepared samples often suffer from both intraparticle and interparticle heterogeneity. When characterized in the ensemble, such heterogeneity inhibits our ability to properly relate nanoparticle structure and functionality, reducing our capacity to predict, control, and reproduce nanoparticle performance in application. Single-particle characterization techniques allow for a one-to-one correlation between nanoparticle structure and its resulting properties, but such studies are limited by the need for correlated electron microscopy to obtain single-particle structure. In this dissertation, we demonstrate an all-optical approach to characterize the size, shape, orientation, and chirality of individual nanocrystals using calcite-assisted localization and kinetics (CLocK) microscopy, a high-throughput, information-rich approach to screen both nanoparticle structure and its corresponding optical properties. Chapter 3 introduces CLocK microscopy, a scattering-based technique used to encode structural and orientational information into diffraction-limited images of single nanoparticles by use of a rotating birefringent calcite crystal. The basic principles of the calcite crystal and interpretation of the CLocK point-spread function (PSF) are discussed, as well as three examples showcasing the additional structural, spectral, and temporal information obtained from CLocK microscopy as compared to traditional dark-field scattering. In Chapter 4, color CLocK images are used to differentiate anisotropic gold nanorods of different size, where convolutional neural networks are trained to predict gold nanorod length, width, and aspect ratio directly from optical images within ~10% of the true value measured using electron microscopy. In Chapter 5, linearly polarized light is used to excite chiral and achiral nanoparticles, where the CLocK PSF is used to examine how each nanoparticle alters the ellipticity and polarization azimuth of scattered light. We show that left-handed, achiral, and right-handed nanoparticles can be differentiated in real-time by isolating the contributions of optical chirality and optical anisotropy to the chiroptical signal, where parameters representing circular/linear dichroism and birefringence are used to quantify chiroptical activity. Finally, Chapter 6 provides an outlook on future uses of CLocK microscopy, including monitoring dynamic processes in-situ, combining CLocK with techniques such as interference-based microscopy, and using finite-difference time domain (FDTD) methods to simulate CLocK images from known structures.