Research Interests: back to home
Research areas: interfacial phenomena; synthesis, self-assembly, and integration of nanocrystals for studies of catalysis, bioimaging, and collective optical, electronic, magnetic properties; nanoporous materials for catalysis, sensors, sorption, and separations.
1. Nanocrystal synthesis, self-assembly, and integration - Nanometer-sized crystallites (NCs) of metals, semiconductors, and oxides exhibit new optical, electronic and chemical properties often different from molecules or macroscopic solids. The ability to adjust properties through control of size, shape, composition, crystallinity, and structure has led to a wide range of potential applications for NCs in areas like optics, electronics, catalysis, magnetic storage, and biological labeling. Furthermore, NC assembly into 2- and 3-D arrays is of interest for development of ‘artificial solids’ with collective optical and electronic properties that can be further tuned by the NC spacing and arrangement. Current monosized NCs are often passivated with a organic monolayer (alkanethiols, trioctylphosphine oxide, etc.) These NCs are hydrophobic and only soluble in nonpolar organic solvents. This is problematic for biological imaging and more generally for uniform incorporation of nanocrystals in hydrophilic matrices like silica or titania needed for the fabrication of robust, functional lasers. Second, the organic monolayer causes the NC arrays to be mechanically weak and often thermally and chemically unstable. These combined factors ultimately limit routine integration of nanocrystals into devices.
Our research focuses on the direct synthesis of water-soluble NC-micelles through surfactant encapsulation. Subsequent self-assembly of NC-micelles with hydrophilic precursors (metal oxides, water-solublw polymers etc.) leads to robust, ordered 3D NC arrays in bulk and thin film forms (see below). Our method allows precise positioning/spacing of NCs within the solid matrices, achieves high loadings while avoiding aggregation, and imparts needed mechanical and chemical robustness. In addition the matrix can be varied from insulating, to semiconducting, to conducting and/or its refractive index tuned by changing its composition. These ordered NC solids are useful for catalysts, photonics devices such as lasers, provide model platforms for fundamental studies of transport and collective phenomena. Integration of 3D gold NC/silica arrays into micro-capacitor shows linear I-V behavior at room temperature and collective coulomb blockade behavior at low temperature (78k). Other devices currently being explored include light emitting from gold/semiconductor NCs/rare earth/silica etc. We have demonstrated that the 3D gold/silica exhibit high performance for CO oxidation at room temperature. Switching from normal surfactants to phospholipids, the resulting lipid NC-micelles has been explored for biolabeling and sensing. In addition to the use of these biofunctionalized NC-micelles for biolabeling and biosensing, we are interested in development of new nano/bio materials at the interface between biomedical and biofunctionalized NCs.
Scheme 1. Processing diagrams for the synthesis of water-soluble and biocompatible nanocrystal-micelles and periodically ordered nanocrystal array thin films and solids.
2. Nanoporous materials - Molecular templating/imprinting techniques are used to synthesize nanostructured organic/inorganic hybrid functional materials. Amphiphilic molecules (such as surfactants, block copolymers, peptides, etc.) form micelles and liquid crystals in water media. Preferential interactions of hydrophilic precursors (metal oxides, polymers, etc.) with micelles and liquid crystals result in periodically ordered nanostructures that impart function from both organic and inorganic parts. Removal of surfactants leads to nanoporous materials. These materials are useful for catalysis, separation, low-k dielectrics, sensor, and gas storage. Currently we are interested in synthesis of nanoporous materials for hydrogen/methane storage and sensing.
