UC Berkeley

A spherical nanocrystal, also known as a quantum dot, is often referred to as an artificial atom due to its remarkable discrete and size-tunable physical properties. Just as bonding between natural atoms forms molecules, coupling between discrete groupings of artificial atoms creates artificial nanocrystal molecules. There are two primary mechanisms by which this coupling can occur: plasmonic coupling through organic linkers, and electronic coupling through inorganic interconnections. Analogous to natural molecules, artificial nanocrystal molecules with a myriad of physiochemical properties may be obtained from a discrete set of atomic building blocks. Enormous potential exists for exciting and useful behaviors to arise in these new materials. Thus, the design and characterization of nanocrystal molecules is a significant endeavor.

This dissertation explores the new physical properties that can arise in a nanocrystal molecule, and demonstrates the use of artificial molecules as probes in biological and materials systems. Chapter 1 provides an introduction to nanocrystal molecules, highlighting their synthetic preparation and physical properties.

Biomolecules can serve as the organic linker bonds between plasmonic nanocrystals, and additionally provide interesting systems to probe. Chapter 2 describes the use of gold nanocrystal molecules to elucidate a force-generating ultrastructure present in prokaryotic systems.

Inorganic interconnections enable nanocrystal molecules of different shapes and behaviors. Chapter 3 investigates the strain-dependent photoluminescence of CdSe/CdS nanocrystal molecules with three different topologies: the sphere, rod, and tetrapod. The effects of shape on strain-dependence are discussed. Importantly, tetrapod nanocrystals are observed to exhibit differential photoluminescence behavior under hydrostatic and non-hydrostatic environments, suggesting that the tetrapod may be useful as a gauge of local stress.

Chapter 4 presents the design and implementation of a luminescent nanocrystal stress gauge. The tetrapod stress gauge is calibrated in a simple uniaxial tensile geometry. Tetrapod luminescence is further used to reveal the local microscale stress profile of a polymer fiber. The microscale measurements support previous observations of local mobility and predictions of local stress behavior.

In addition to shape, the chemical composition of artificial atom components alters the optoelectronic behavior of the resultant nanocrystal molecule. Typically, an organic or nanocrystal fluorophore exhibits radiative emission from a single transition. Chapter 5 discusses the observation of multiple radiative emission peaks in fluorescence spectroscopy studies of single CdSe/CdS tetrapods. The dependence of multiple emission on external factors including CdS arm length, incident power, and excitation energy suggest that this photophysical phenomenon is due to spatially direct and indirect transitions within the nanocrystal molecule. Unique nanocrystal molecule designs will likely allow further control over the lifetime, oscillator strength, and energy of the indirect transition.

A significant advantage of colloidal nanocrystals is their ease of incorporation into many different systems. Chapter 6 highlights the tetrapod quantum dot as a dynamic nanocrystal probe that fluorescently reports cellular forces with spatial and temporal resolution. Rat-derived cardiomyocytes are cultured on a luminescent tetrapod array and induce clear time-dependent spectral shifts in the array emission. The small size and colloidal state of the tetrapod suggest that it may be further developed as a tool to measure cellular forces in vivo and with nanoscale spatial resolution. The tetrapod can additionally be utilized to convert biochemical inputs into optical signals, which may be useful for applications including synthetic biology designs.

Concluding remarks are presented in Chapter 7.