Type of Document Dissertation Author Lee, Nicholas Jabari Ouma Author's Email Address email@example.com URN etd-11112005-173308 Title Parallel Molecular Dynamics Simulations of Pressure-Induced Structural Transformations in Cadmium Selenide Nanocrystals Degree Doctor of Philosophy (Ph.D.) Department Physics & Astronomy Advisory Committee
Advisor Name Title Rajiv Kalia Committee Chair Roger McNeil Committee Co-Chair Erno Sajo Committee Member James Matthews Committee Member Priya Vashishta Committee Member Randall W. Hall Dean's Representative Keywords
- quantum dots
- phase transformations
Date of Defense 2005-11-07 Availability unrestricted AbstractParallel molecular dynamics (MD) simulations are performed to investigate pressure-induced solid-to-solid structural phase transformations in cadmium selenide (CdSe) nanorods. The effects of the size and shape of nanorods on different aspects of structural phase transformations are studied. Simulations are based on interatomic potentials validated extensively by experiments. Simulations range from 105 to 106 atoms. These simulations are enabled by highly scalable algorithms executed on massively parallel Beowulf computing architectures.
Pressure-induced structural transformations are studied using a hydrostatic pressure medium simulated by atoms interacting via Lennard-Jones potential. Four single-crystal CdSe nanorods, each 44Å in diameter but varying in length, in the range between 44Å and 600 Å, are studied independently in two sets of simulations. The first simulation is the downstroke simulation, where each rod is embedded in the pressure medium and subjected to increasing pressure during which it undergoes a forward transformation from a 4-fold coordinated wurtzite (WZ) crystal structure to a 6-fold coordinated rocksalt (RS) crystal structure. In the second so-called upstroke simulation, the pressure on the rods is decreased and a reverse transformation from 6-fold RS to a 4-fold coordinated phase is observed.
The transformation pressure in the forward transformation depends on the nanorod size, with longer rods transforming at lower pressures close to the bulk transformation pressure. Spatially-resolved structural analyses, including pair-distributions, atomic-coordinations and bond-angle distributions, indicate nucleation begins at the surface of nanorods and spreads inward. The transformation results in a single RS domain, in agreement with experiments. The microscopic mechanism for transformation is observed to be the same as for bulk CdSe. A nanorod size dependency is also found in reverse structural transformations, with longer nanorods transforming more readily than smaller ones. Nucleation initiates at the center of the rod and grows outward.
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