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.
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.
In 1998, working with Shimon Weiss, the Alivisatos group showed the first use of colloidal quantum dots for biological imaging. Since then, the group has continued to research and expand the intersection of nanotechnology and biology. Biological work in the group can be generally divided into two categories: the coupling of nanoparticles and biomolecules to create novel materials and investigate fundamental properties of nanoparties; the demonstration of unique nanoparticle properties suitable for bioimaging and sensing.
Work in the first category includes the development of methods to isolate gold nanoparticle-DNA conjugates and the assembly of pyramidal and chiral nanoparticles with DNA. Those projects laid a foundation for the controlled synthesis of plasmonically coupled nanoparticles bound by highly programmable biomolecular constructs. Such constructs allow for experimental observation of distance and size-dependent plasmonic phenomenon that are predicted to occur between gold and silver particles.
Furthermore, distance-dependent plasmon coupling has been used by the Alivisatos group in the development of a plasmon ruler for biological applications. This tool allows for the investigation of dynamics and activity of biological processes. Building a plasmon ruler with the ability to resolve nanoscale conformational changes in three dimensions is an exciting continuation of the plasmon ruler work.
Semiconductor nanoparticle applications to biology are also a very promising field of research. Quantum dots have several advantages over traditional fluorescent dyes used in biology. Dots do not bleach rapidly, have tunable fluorescence through the visible range and into the near IR, have narrow emission bands, and a large range of excitation wavelengths. The Alivisatos group also uses properties unique to higher level nanoparticle structures for sensing otherwise difficult to resolve biological signals. For example cellular traction forces can be quantified through the force-dependent fluorescence wavelength of tetrapod quantum dots.
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.