Research

Synthesis and Properties of Nanoparticles

The ability to make nanocrystals of high quality (uniform size, no defects except the ones we want, designed surface, etc.) is key to this area of science, and also interesting in its own right. We grow nanocrystals with well controlled sizes and shapes by injecting molecular precursors into hot liquids that also contain molecular species that will coordinate to the growing nanoparticle surfaces. Some important questions of solid state chemistry can be addressed in the synthesis of nanocrystals. How does nucleation of a solid occur? What governs the rate of growth of each facet of a crystal? What is the stress and strain at the interface between a core and a shell of different materials?

We can think of a simple nanocrystal as a type of artificial atom. In that case, the next step in nanocrystal synthesis is to learn how to make artificial molecules, in which nanocrystals (atoms) of differing composition, size, and shape, and interconnected with each other in a designed arrangement. To accomplish this, we develop modular approaches to the chemical transformation of nanocrystals. Three such transformations we have developed and exploited are: cation exchange, the nanoscale Kirkendall effect, and branching. With these we can make branched, segmented, hollow, and nested nanoparticles. We are working to better understand the mechanisms of each of these transformations, as well as searching for new and general nanoparticle chemical transformations.

In addition to fundamental studies of nanocrystal synthesis, we are interested in developing automated, self-correcting nanocrystal syntheses, surface derivitization, and methods for nanocrystal characterization and assembly. A feature of our current work is the development of new methods to observe the growth trajectories and formation mechanisms of individual colloidal nanocrystals in liquid solutions.

Development of Group III-V Semiconductor Nanocrystals

Our group is dedicated to researching III-V group semiconductor nanocrystals by exploring a variety of synthetic methods that enable precise control over composition, morphology, and size. In doing so, we can attain tunable optical and electronic properties that can be customized for diverse applications.

Compared to traditional semiconductor materials, nanocrystals offer superior abilities and performance for next-generation high-efficiency solar cells, light-emitting diodes, and sensors. Moreover, III-V nanocrystals are significantly less toxic than classical quantum dot systems, paving the way for broad integration into consumer and industrial electronics.

Quantum dots’ unique morphology makes them excellent candidates for optimization utilizing colloidal synthesis techniques. Core-shell structures can be fabricated, allowing for the layering of different semiconducting groups in the same nanocrystal. This technique enhances quantum yield and nanocrystal stability by passivating surface defects and reducing non-radiative recombination. Furthermore, alteration of the size and type of the surface ligands incorporated into the nanocrystal structure allows for the fine-tuning of optoelectronic properties such as charge transport and quantum yield; the nanocrystals can be synthesized with different ligands or the ligands can be exchanged post-synthesis. We additionally investigate how we can synthesize nanocrystals into different crystal structures via multi-step syntheses involving templating and cation/ anion exchanges, giving us access to crystal structures that are otherwise not possible from direct synthesis.

In order to observe nanocrystal properties such as size, shape, structure, and optoelectronic properties; we employ a variety of characterization techniques, including transmission electron microscopy (TEM), X-ray Diffraction (XRD), and photoluminescence spectroscopy (PL). The knowledge acquired from these investigations will lay the foundation for a new era of eco-friendly, high-performance, and energy-efficient devices, revolutionizing industries such as telecommunications, displays, renewable energy, and even art!

Spectroscopy and Structural Studies of Colloidal Nanocrystals.

Our experimental group is devoted to uncovering the complex relationships between the structure of nanocrystals, including their surfaces, and their resulting optical properties. Given the diversity of shapes, crystal structures, and morphologies within the nanoscale world, the optical properties of these nanocrystals often challenge conventional models. Our core endeavor is to map these intricate structure-function relationships, with an overarching goal to optimize their synthesis and effective application within optoelectronic devices.

We aim to create a comprehensive understanding of nanocrystals and their unique attributes, facilitating predictive models for their optical responses. This initiative underpins our efforts in achieving not only successful experimental results, but also advancing the broader scientific understanding of nanomaterials and their applications in optoelectronics.

Art with Nanoparticles

Craftsmen were arguably the first nanoscientists. Artifacts from as early as the Bronze Age reveal that artisans harnessed nanoscale phenomena to produce a variety of effects: lustre in early Islamic ceramics, enhanced strength in 9th century Syrian swords, and rich colors in ancient Roman, Egyptian, Chinese, and medieval European glass. Historians of science tell us that early craftsmen’s practices-gathering information through direct observation, measuring materials to promote reproducible results, and changing one variable at a time-did much to shape the methodology of science. Walk into a contemporary nanoscience lab, and you will find scientists using techniques that derive from ones developed by artists-for example, nanolithography and Lagmuir-Blodgett film-casting.

Thus, it is natural that an interdisciplinary lab such as ours would include an artist. Our artist-in-residence, Kate Nichols, was initially drawn to using nanomaterials because of her interest in structurally colored Morpho butteflies. (Morpho butterflies appear as brilliant blue despite their lack of blue pigmentation. Their blue color is structural rather than chemical, deriving from nanoscale features within their wings.) Once in our lab, Kate was inspired by medieval artisan’s use of plasmonic nanoparticles in stained glass windows. She synthesizes similar plasmonic nanoparticles and uses them to create macroscale art whose structural colors arise from surface plasmon resonance. In her painting, photography, and writing, Kate continues to explore natural photonic structures. Her work can be seen at The Leonardo, a museum of art and science in Salt Lake City, on her website, and at TED.

Energy Applications

The tunability of the optical, electronic, mechanical, and chemical properties of nanoparticles makes them well suited to tackle a number of energy conversion problems. Our research in this area focuses on optoelectronic devices like solar cells, as well as fuel and feedstock forming catalysis like water splitting and hydrocarbon conversion.

Our research in optoelectronic devices seeks to answer a number of fundamental questions: how do electrons move through nanoparticle solids? What is the fundamental trade-off between quantum confinement and electronic transport? What is the relationship between nanoparticle properties and device behavior? We are working to understand the organic-inorganic interface, as the oft-overlooked organic surface passivation of nanoparticles can have dramatic impacts on optoelectronic behavior. By tuning the morphology and composition of nanoparticles and nanoparticle films, we seek to control charge carrier mobility, dielectric constants, and nonradiative rates in order to create high-performing devices.

Our research also studies catalytic and photocatalytic fuel-forming reactions on nanoparticle surfaces. For example, the production of hydrogen from water using solar energy is a potentially clean and renewable source for hydrogen fuel, but there are still many materials-related obstacles to its widespread use. It is particularly difficult to find a stable semiconductor system with suitable band gap and electron affinity for visible light absorption and for driving the subsequent redox chemistry. Additional challenges facing the photocatalytic process include the quick recombination of photoinduced charge carriers, back reaction of intermediates on the catalyst surface, and the back reaction of the products. Our group is involved in the design of multicomponent nanoheterostructures which can drive photocatalytic hydrogen production. In particular, we are interested in understanding how spatial separation of absorber elements and catalysts at the nanoscale impacts photocatalytic hydrogen production. We have developed methods for studying photocatalytic hydrogen production in single nanoheterostructures, allowing us to advance our fundamental understanding of charge dynamics at the single particle level.

In addition, the high surface area of nanoparticles makes them attractive for use as heterogeneous catalysts. Our research aims to tune the structure of the catalyst by modifying its surface. A current feature of our research is the development of new nanoheterostructures for propane dehydrogenation and Fischer-Tropsch hydrocarbon synthesis. In particular, we are investigating composites of oxides and metals which may have improved anti-sintering behavior and synergistic catalytic properties. The catalyst we are designing leverage many recent synthetic advances, such as the nanoscale Kirkendall effect.