Browsing by Author "Sayle, T.X.T."

Now showing 1 - 7 of 7
  • Thumbnail Image
    ItemOpen Access
    Atomistic Models and Molecular Dynamics
    (2007-04-01T00:00:00Z) Sayle, D.C.; Sayle, T.X.T.; José, A. Rodriguez; Marcos, Fernández-García
    Here we show how atomistic computer simulation can help experiment unravel the rich structuralcomplexity of oxide nanomaterials and, ultimately, aid the fabrication of nanomaterials withimproved, tuneable or indeed new properties. We first explore the simulation methodologies:energy minimisation, monte-carlo, genetic algorithms and molecular dynamics together with thepotential models used to describe the interactions between metal and oxide ions. These tools can beused to generate realistic structures that include all the essential microstructural features observedexperimentally, such as surface structure (morphology, surface energy, faceting, surface steps,corners and edges), grain-boundaries and dislocations, intrinsic and extrinsic point defects andepitaxy. We show how the theoretician is able to capture all these (experimentally observed)structural details by attempting to simulate crystallisation. Equipped with realistic models,important properties can be calculated, including: electronic, chemical (catalytic activity, ionicdiffusion and conductivity) and mechanical (hardness, elastic constants). This is illustrated bycalculating the ease of oxygen extraction from the surface of a CeO2 nanocrystal compared with thebulk parent material with implications for oxidative catalysis. Throughout this chapter weemphasise the importance of molecular graphics - a much maligned and underrated tool - butwithout which, the generation of much of the simulation and experimental data would not havebeen possible.
  • Thumbnail Image
    ItemOpen Access
    Converting Ceria Polyhedral Nanoparticles into Single-Crystal Nanospheres
    (2008-05-27T14:14:25Z) Xiangdong, F.; Sayle, D.C.; Zhong, L.W.; Paras, M.S.; Santora, B.; Sutorik, A.C.; Sayle, T.X.T.; Yi, Y.; Yong, D.; Xudong, W.; Yie-Shein, Her
    Ceria (CeO2) nanoparticles are one of the key abrasive materials for chemical-mechanical planarization (CMP) of advanced integrated circuits. However, CeO2 nanoparticles synthesized by all existing techniques are faceted with irregular faceted-shapes, and they scratch the silicon wafers with increased defect concentrations. Here, we show for the first time an innovative approach for large-scale synthesis of spherical, single-crystal, CeO2 nanoparticles. Our synthetic strategy involves doping the CeO2 system with titanium, using flame temperatures that facilitate crystallization of the CeO2, yet retains the TiO2 in a molten state. In conjunction with Molecular Dynamics simulation, we show that under these conditions, the inner CeO2 core evolves in a single-crystal spherical-shape without faceting, because, throughout the crystallization, it is completely encapsulated by a molten 1-2 nm shell of TiO2, which, in liquid state, minimizes the surface energy. The single-crystal nanospheres reduce CMP defects by 80% and increase the silica removal rate by 50%, which will facilitate precise and reliable mass-manufacturing of chips for nanoelectronics at a precision of sub-nanometers. The principle demonstrated here could be applied to other oxide systems.
  • Thumbnail Image
    ItemOpen Access
    Elastic Deformation in Ceria Nanorods via a Fluorite-to-Rutile Phase Transition
    (American Chemical Society, 2010-02-28T00:00:00Z) Sayle, T.X.T.; Sayle, D.C.
    Atomistic simulations reveal that ceria nanorods, under uniaxial tension, can accommodate over 6% elastic deformation. Moreover, a reversible fluorite-to- rutile phase change occurs above 6% strain for a ceria nanorod that extends along [110]. We also observe that during unloading the stress increases with decreasing strain as the rutile reverts back to fluorite. Ceria nanorods may find possible application as vehicles for elastic energy storage.
  • Thumbnail Image
    ItemOpen Access
    Generating structural distributions of atomistic models of Li2O nanoparticles using simulated crystallisation
    (Royal Society of Chemistry, 2010-12-31T00:00:00Z) Sayle, T.X.T.; Ngoepe, P.E.; Sayle, D.C.
    Simulated crystallisation has been used to predict that Li2O nanoparticles comprise octahedral morphologies bounded by {111} and truncated by {100} with inverse fluorite crystal structure. We observe that by changing the temperature of the (simulated) crystallisation, changes in the microstructure can be realised, such a strategy facilitates the generation of full atomistic models with microstructural distributions similar to the structural diversity observed synthetically.
  • Thumbnail Image
    ItemOpen Access
    High-pressure crystallisation of TiO2 nanoparticles
    (American Scientific Publishers, 2007-03-31T00:00:00Z) Sayle, D.C.; Sayle, T.X.T.
    The full atomistic structure of a TiO2 nanocrystal, about 7 nm in diameter and comprising 16,000 atoms, has been generated using simulated melting and crystallisation, performed under high-pressure. Specifically, the nanoparticle was heated to 6000 K after which the molten nanoparticle was crystallised at 2000 K under 20 GPa pressure. The resulting nanocrystal comprises rutile- and alpha-PbO2-structured domains (alpha-PbO2 has been identified experimentally as a high-pressure phase of TiO2) expresses (111), (010), (001), and (110) surfaces facilitating a polyhedral morphology and includes grain-boundaries and grain- junction. Molecular graphics images of the various microstructural features are presented together with snapshots of the crystallisation.
  • Thumbnail Image
    ItemOpen Access
    Nanopolycrystalline materials; A general atomistic model for simulation
    (Royal Society of Chemistry, 2010-03-31T00:00:00Z) Sayle, D.C.; Mangili, B.C.; Price, D.W.; Sayle, T.X.T.
    We present a general strategy for generating full atomistic models of nanopolycrystalline materials including bulk and thin film. In particular, models for oxidenanoparticles were constructed using simulated amorphisation and crystallisation and used to populate a library of oxidenanoparticles (amorphous and crystalline) with different radii. Nanoparticles were then taken from this library and positioned, within a specific volume, using Monte Carlo techniques, to facilitate a tight-packed structure. The grain-size distribution of the polycrystalline material was controlled by selecting particular sized nanoparticles from the library. The (randomly oriented) grains facilitated a polycrystalline oxide, which comprised a network of general grain-boundaries. To help validate the model, gas diffusion through the (polycrystalline) oxide material was then simulated and the activation energy calculated directly. Specifically, we explored Hetransport in UO2, which is an important material with respect to both civilian and military applications. We found that Hetransport proceeds much faster through the grain-boundary and grain-junction network compared with intracrystalline UO2 regions, in accordance with experiment.
  • No Thumbnail Available
    ItemOpen Access
    Simulating Mn02 Nanoparticles using Simulated Amorphisation and Recrystallisation
    (American Chemical Society , 2011-03-18) Sayle, T.X.T.; Catlow, C.R.A.; Maphanga, R.R.; Ngoepe, P.E.; Sayle, D.C.