Surface functionalization and self-assembly of ligand-stabilized silicon nanocrystals
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Silicon nanocrystals or quantum dots combine the abundance and nontoxicity of silicon with size-tunable energy band structure of quantum dots to form a new type of functional material that has applications in biomedical fluorescence imaging, photodynamic therapy, light-emitting devices, and solar cells. The surface is the major concern for using silicon nanocrystals in bio-related applications. Room temperature hydrosilylation is introduced to functionalize silicon nanocrystals in the dark to minimize temperature/photon-induced side reactions that can potentially damage the nanocrystal surface and capping ligands. As a proof of concept, silicon nanocrystals are passivated with styrene at room temperature, without showing styrene polymerization. Silicon nanocrystals are also conjugated to iron oxide nanocrystals through room temperature hydrosilylation to generate fluorescent/magnetic cell labeling probes. Thermally-induced thiolation is used to generate silicon nanocrystals passivated with silicon-sulfur bond that is metastable and can turn to silicon-carbon bond through a ligand exchange. The band gap and emission color of silicon nanocrystals depend on size. Monodisperse silicon nanocrystals and their self-assembly are of great importance for the applications in light-emitting devices and solar cells. Silicon nanocrystals are size-selected through a modified size-selective precipitation. Face-centered cubic superlattices are formed with monodisperse silicon nanocrystals, and characterized by using grazing incidence small angle X-ray scattering. The structure of silicon nanocrystal superlattice is stable at temperatures up to 375oC, due to the covalent Si-C bond on the nanocrystal surface. Silicon and gold nanocrystals are assembled to a simple hexagonal AlB2 binary superlattice that shows interesting thermal behavior. Finally, superlattices made with alkane thiol-capped sub-2 nm gold nanocrystals are used as model systems to study the superlattice phase transitions. Halide ions are found to be critical for order-to-order structural rearrangements in dodecanethiol-capped 1.9 nm gold nanocrystals superlattices at 190oC. Reversible amorphous-to-crystalline transition upon heating is discovered for octadecanethiol capped 1.66 nm gold nanocrystal superlattices, which is attributed to the ligand melting transition.