Surface modification of group 14 nanocrystals

• Author / Creator

  Kelly, Joel Alexander

• Group 14 semiconductor nanocrystals exhibit size-dependent optoelectronic properties through the influence of quantum size effects, notably intense photoluminescence (PL). A crucial step toward the realization of their technological potential is the development of effective methods to control the surface chemistry of freestanding nanocrystals. This thesis describes five investigations with this central goal in mind, based on silicon (Si) and silicon-germanium alloy (SixGe1-x) nanocrystals obtained from hydrogen silsesquioxane (HSQ). These composite materials were etched using hydrofluoric acid (HF), to yield freestanding hydride-terminated nanocrystals that undergo a hydrosilylation reaction with terminal olefins, with thermal or near-UV initiation. This reaction produced well-dispersed nanocrystals with increased stability against oxidation, which degrades their PL. The use of near-UV initiation was studied in detail, based on its compatibility with a wide range of olefins and the observation of unique reactivity unseen for hydrosilylation of molecular species. This reactivity of Si nanocrystals was size-dependent, consistent with an exciton-mediated mechanism previously proposed. This observation was explored through separation of a mixture of sizes based on their hydrosilylation reactivity. Exciton-mediated reactivity was also observed in the HF etching of oxide-embedded Si nanocrystal composites. The PL from these materials could be conveniently controlled by irradiating the etching solution, presenting a strategy for controlling the nanocrystal size and polydispersity; however, defect-containing nanocrystals were suggested to be inactive in this etching pathway. The origin of luminescence from Si nanocrystals functionalized using near-UV hydrosilylation was demonstrated to arise from quantum confinement effects using X-ray absorption spectroscopy. These results confirm that the alkyl grafting does not appreciably alter the emission pathway. Oxidation caused by ambient exposure reduced the ensemble PL quantum yield and shift the emission maximum. The morphology of nanocrystals obtained from the co-reaction of HSQ and GeI2 solubilized with trialkylphosphines was evaluated spectroscopically and microscopically to be a heterogeneous mixture of Si- and Ge-rich nanocrystals. The trialkylphosphine used to facilitate GeI2 co-precipitation influences nanocrystal nucleation and growth processes. Indirect evidence was presented for the involvement of Ge in the emission pathway.

• Subjects / Keywords

  • Hydrosilylation
  • Surface chemistry
  • X-ray absorption spectroscopy
  • Quantum confinement
  • Photoluminescence
  • Semiconductor nanocrystals

• Graduation date

  Spring 2012

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3WP9TF9B

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.