I. NANOCRYSTALS: BUILDING BLOCKS FOR SOLID STATE CHEMISTRY AND MATERIALS DESIGN

Nanometer size inorganic crystals are playing an increasingly important role in solid state physics, chemistry, materials science, and even biology. Many fundamental properties of a crystal (e.g., ionization potential, melting point, band gap, saturation magnetization) depend upon the solid being periodic over a particular length scale, typically in the nm regime. By precisely controlling the size and surface of a nanocrystal, its properties can be tuned. Using techniques of molecular assembly, new nanocrystal based materials can in turn be created.

II. SCALING LAWS

As the number of atoms in a cluster increases, there is a critical size above which one particular bonding geometry; characteristic of an extended solid "locks in." As more atoms are added, the total volume and the number of surface atoms change, but the basic nature of the chemical bonds in the cluster does not. In this regime, the properties of nanocrystals vary smoothly, slowly extrapolating to bulk values, according to scaling laws. Many scaling laws have been hypothesized, a few are verified. For instance, the band gap of a semiconductor, such as Si, InAs, or CdSe, all increase with size, roughly as 1/r², and their melting temperatures all decrease with size, roughly as 1/r, and these observations can be described well theoretically. Other size dependent scaling laws are topics of current research: How long does it take for a crystal to isomerize between two stable bonding geometries? How do the selection rules for absorption and emission of light depend upon the crystal size (translational symmetry)? What is the largest crystal that can be made defect free? In our fundamental studies of nanocrystal physics, we employ a wide range of spectroscopic and structural experimental tools, as well as computer simulation.

III. SYNTHESIS

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 by injecting organometallic precursors into pure, hot surfactants. 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 a crystal? What is the stress and strain at the interface between a core and a shell of different materials? 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.

IV. MATERIALS DESIGN TARGETS

• Nanocrystal/polymer composites for light emitting diodes and photovoltaics
• Single nanocrystal-single electron transistor (with P. McEuen, Physics)
• Nanocrystal/antibody conjugates as biological tag molecules (with S. Weiss, LBNL)
• DNA directed assembly of nanocrystal patterns (with P. Schultz)
• Nanocrystal photo-catalysis
• Mechanical properties of nanocrystal composites

References

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