Solventless synthesis, characterization, self-assembly of colloidal nanocrystals

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Sigman, Michael Barron

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New and general synthetic methods for materials confined to nanometer length scales are needed to provide both an experimental route for exploring material properties as a function of size and a viable means of production for commercial applications. A solventless synthesis technique was developed to produce metal sulfide and oxy-chloride nanocrystals including Cu2S, Bi2S3, and Pb3O2Cl2. A metal thiolate or metal chloride-octanoate serves as the molecular precursor for particle formation via thermolytic decomposition. Monodisperse Cu2S nanoplatelets were synthesized with the c-axis of the hexagonal high chalcocite crystal structure oriented across the width of the disks and the {100} facets oriented along the edges. Preferred adsorption and increased surface reactivity of dodecanethiol on the more energetic {100} crystal facets results in the hexagonal prism morphology. vii In comparison, Bi2S3 nanorods and nanowires with the orthorhombic bismuthinite crystal structure grow preferentially in the [001] direction. The aspect ratio depends on the choice of sulfur source. Nanowires were formed using dodecanethiol, while elemental sulfur results in shorter nanorods. Increased reaction temperature produced crossed networks of nanofabric with highly oriented growth resulting from the heterogeneous nucleation of wires 90o from the surface of existing wires. Pb3O2Cl2 nanobelts with the orthorhombic mendipite crystal structure were also produced. These belts are highly birefringent with a difference in the refractive index exceeding 0.48 with respect to the [010] and [100] crystallographic directions compared to the value of 0.07 for bulk mendipite. Self-assembly methods are also needed to arrange nanocrystals into films suitable for incorporation into devices. A fundamental study of three-dimensional nanocrystal superlattice formation was performed by depositing metallic nanocrystals from different solvents. Nanocrystals drop cast from chloroform produced smooth films, while hexane resulted in mounds. Interparticle attraction is 20% higher in hexane contributing to the observed increase in growth of the films in the [111] superlattice direction. The combination of nanocrystal synthesis techniques with self-assembly methods offers the ability to produce materials with size and shape dependent properties that may then be utilized to improve the functionality of devices such as LEDs, photovoltaics, or surface sensitive sensors in the future.




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