Aspects of colloidal nanocrystals: patterning, catalysis and doping

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Stowell, Cynthia Ann

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Colloidal nanocrystals have many advantages over those synthesized by other means due to the flexibility not only in synthesis conditions but in post-synthesis assembly. Three aspects of colloidal nanocrystals that demonstrate this versatility were studied: the self-assembled patterning of nanocrystals into arrays through the use of fluid dynamics, the catalytic properties of nanocrystals as a function of ligand type and reaction cycle, and the doping of III-V semiconductor nanocrystals with magnetic atoms during colloidal synthesis. While much work has been done on the thermodynamically-driven formation of monodisperse or bi-modal nanocrystal superlattices, another option exists for nanocrystal self-assembly: formations driven by fluid dynamics. Hexagonal networks of gold nanocrystals were observed after drop casting gold nanocrystals in chloroform on different substrates. The honeycomb-shaped structures were calculated to be created by surface tension driven (Marangoni) convection. The honeycomb networks have a lattice parameter of 4.3 µm and their formation is highly dependant not only particle size and size distribution but concentration of particles within solution. A new synthesis for iridium nanocrystals was developed. The iridium particles could be synthesized with any of five stabilizing molecules. Taking advantage of this fact, the effect of capping ligands on the catalysis of 1-decene hydrogenation was studied. Ligands that stabilized the iridium well prevented hydrogenation while “weak” capping ligands allowed the iridium nanocrystals to reach turnover rates as high as 270 s-1. Recycling the catalytic particles also affected the activity, as the turnover frequency increased with each cycle until the particle began to agglomerate and fall out of solution. MnxIn1-xAs and MnxIn1-xP nanocrystals ranging from 2 to 10 nm in diameter were synthesized with up to xMn=0.025 for InMnAs and xMn=0.11 for InMnP. Surface exchange and magnetic measurements confirmed that much of the dopant resides in the nanocrystal core and modifies the magnetic properties of the host material through antiferromagnetic superexchange interactions. The effective Bohr magnetons of Mn in the synthesized InMnAs nanocrystals ranged from 2.2 to 3.7 µ B Mn atom, and from 3.4 to 5.1 µ B Mn atom for InMnP, values below the theoretical value of 5.9 µ B Mn atom. This result is attributed to antisite defects and interstitial doping.