Observation of long-range orientational order in monolayers of polydisperse colloids.

Polydisperse amorphous-silicon colloidal particles ranging from approximately 10 to 140 nm in diameter were evaporated onto carbon substrates. The particles formed close-packed monolayers in which each particle had 6-fold nearest-neighbor coordination characteristic of a hexagonal lattice yet completely lacked positional order. Orientational correlation functions were calculated for the particles and found to be constant throughout the aggregate, indicating the occurrence of long-range orientational order. Computer simulations revealed that the structural organization in this system resulted from capillary immersion forces that lead to a size separation as the particles deposit from the evaporating solvent onto the substrate.

Synthesis of amorphous silicon colloids by trisilane thermolysis in high temperature supercritical solvents.

Colloidal submicrometer-diameter amorphous silicon (a-Si) particles are synthesized with >90% yield by thermal decomposition of trisilane (Si3H8) in supercritical hexane at temperatures ranging from 400 to 500 degrees C and pressures up to 345 bar. A range of synthetic conditions was explored to optimize the quality of the product. Under the appropriate synthetic conditions, the colloids are spherical and unagglomerated. The colloids can be produced with average diameters ranging from 50 to 500 nm by manipulating the precursor concentration, temperature, and pressure. Relatively narrow particle size distributions, as measured by transmission electron microscopy (TEM), with standard deviations about the mean as low as approximately +/-10% could be obtained in some cases. We explored the thermal annealing of the amorphous silicon particles after isolation from the reactor and found that crystallization to diamond structure silicon occurred at temperatures as low as 650 degrees C. The amorphous and crystalline materials were characterized by X-ray diffraction and high resolution scanning and transmission electron microscopy.

Electrochemistry and electrogenerated chemiluminescence from silicon nanocrystal quantum dots.

Reversible electrochemical injection of discrete numbers of electrons into sterically stabilized silicon nanocrystals (NCs) (approximately 2 to 4 nanometers in diameter) was observed by differential pulse voltammetry (DPV) in N,N'-dimethylformamide and acetonitrile. The electrochemical gap between the onset of electron injection and hole injection-related to the highest occupied and lowest unoccupied molecular orbitals-grew with decreasing nanocrystal size, and the DPV peak potentials above the onset for electron injection roughly correspond to expected Coulomb blockade or quantized double-layer charging energies. Electron transfer reactions between positively and negatively charged nanocrystals (or between charged nanocrystals and molecular redox-active coreactants) occurred that led to electron and hole annihilation, producing visible light. The electrogenerated chemiluminescence spectra exhibited a peak maximum at 640 nanometers, a significant red shift from the photoluminescence maximum (420 nanometers) of the same silicon NC solution. These results demonstrate that the chemical stability of silicon NCs could enable their use as redox-active macromolecular species with the combined optical and charging properties of semiconductor quantum dots.