Aaron E. Saunders

Colloidal Hybrid Nanostructures: A New Type of Functional Materials

One key goal of nanocrystal research is the development of experimental methods to selectively control the composition and shape of nanocrystals over a wide range of material combinations. The ability to selectively arrange nanosized domains of metallic, semiconducting, and magnetic materials into a single hybrid nanoparticle offers an intriguing route to engineer nanomaterials with multiple functionalities or the enhanced properties of one domain. In this Review, we focus on recent strategies used to create semiconductor-metal hybrid nanoparticles, present the emergent properties of these multicomponent materials, and discuss their potential applicability in different technologies.

Visible Light-Induced Charge Retention and Photocatalysis with Hybrid CdSe−Au Nanodumbbells

Visible light photocatalysis is a promising route for harnessing of solar energy to perform useful chemical reactions and to convert light to chemical energy. Nanoscale photocatalytic systems used to date were based mostly on oxide semiconductors aided by metal deposition and were operational only under UV illumination. Additionally, the degree of control over particle size and shape was limited. We report visible light photocatalysis using highly controlled hybrid gold-tipped CdSe nanorods (nanodumbbells). Under visible light irradiation, charge separation takes place between the semiconductor and metal parts of the hybrid particles. The charge-separated state was then utilized for direct photoreduction of a model acceptor molecule, methylene blue, or alternatively, retained for later use to perform the reduction reaction in the dark.

Inverse Opal Nanocrystal Superlattice Films

We describe the single-step self-organization of nanocrystal superlattice films infused with spatially ordered arrays of micrometer-size pores. In a humid atmosphere, water droplets condense on the surface of evaporating thin-film solutions of nanocrystals. Nanocrystals coated with the appropriate ligands stabilize the water droplets, allowing them to grow to uniform size and ultimately pack into very ordered arrays. The droplets provide a temporary template that casts an ordered macroporous nanocrystal film. This process could serve as a reliable bottom-up self-assembly approach for fabricating two-dimensional waveguides with tunable optical properties for single-chip integration of photonic and electronic technologies.

Challenges in quantum dot-neuron active interfacing

Semiconductor nanocrystal quantum dots (qdots) are now being explored in applications requiring active cellular interfaces, such as biosensing and therapeutics in which information is passed from the qdot to the biological system, or vice versa, to perform a function. These applications may require surface coating chemistry that is different from what is commonly employed for passive interface applications like labeling (i.e., thick polymer coatings such as poly(ethylene glycol) (PEG)), in which the only concern is nonspecific sticking to cells and biocompatibility. The thick insulating coatings that are generally needed for labeling are generally not suitable for active qdot-cell interface applications. There is currently little data regarding the interactions between viable cells and qdots under physiological conditions. Our initial investigations using mercaptoacetic acid-coated CdS and CdTe qdots as a simple model to interface with neuron cell surface receptors under physiological conditions uncovered two significant technological hurdles: nonspecific binding and endocytosis. Nonspecific binding can be extensive and in general there appears to be greater nonspecific binding for larger particle sizes, but this also depends sensitively on the particle surface characteristics and the type of neuron, possibly indicating a detailed relationship between particle-cell affinity and cell membrane chemistry. More importantly, qdot endocytosis occurs rapidly at physiological temperature for the different nerve cell types studied, within the first five minutes of exposure to both CdS and CdTe qdots, regardless of whether the molecular coatings specifically recognize cell surface receptors or not. As a consequence, new strategies for tagging cell surface recognition groups for long-term active interfacing with cells under physiological conditions are needed, which requires more sophisticated ligands than MAA but also the absence of thick insulating coatings.

Seed-mediated growth of shape-controlled wurtzite CdSe nanocrystals: platelets, cubes, and rods.

Prior investigations into the synthesis of colloidal CdSe nanocrystals with a wurtzite crystal structure (wz-CdSe) have given rise to well-developed methods for producing particles with anisotropic shapes such as rods, tetrapods, and wires; however, the synthesis of other shapes has proved challenging. Here we present a seed-mediated approach for the growth of colloidal, shape-controlled wz-CdSe nanoparticles with previously unobserved morphologies. The synthesis, which makes use of small (2-3 nm) wz-CdSe nanocrystals as nucleation sites for subsequent growth, can be tuned to selectively yield colloidal wz-CdSe nanocubes and hexagonal nanoplatelets in addition to nanorod and bullet-shaped particles. We thoroughly characterize the morphology and crystal structures of these new shapes, as well as discuss possible growth mechanisms in the context of control over surface chemistry and the nucleation stage.

Structural Characterization and Temperature-Dependent Photoluminescence of Linear CdTe/CdSe/CdTe Heterostructure Nanorods

Linear CdTe|CdSe|CdTe heterostructure nanorods are synthesized by using a colloidal sequential reactant injection technique [Shieh et al., J. Phys. Chem. B 2005, 109, 8538-8542]. The composition profiles of the individual nanorods are verified by using nanobeam elemental mapping by energy dispersive X-ray spectroscopy (EDS) and the photoluminescence emission spectra of the linear CdTe|CdSe|CdTe heterostructure nanorods are measured as a function of the temperature (down to 5 K). Photoluminescence is observed to occur from electron-hole recombination in both the CdSe core and across the heterojunction. Thermally activated trapping is found to influence both luminescence processes, thereby being more significant for the type II recombination across the CdSe|CdTe interface.