Browsing by Subject "Growth (materials)"
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Item Open Access Mesoscopic model of nucleation and Ostwald ripening/stepping: Application to the silica polymorph system(American Inst of Physics, Woodbury, NY, United States, 2000) Ozkan, G.; Ortoleva, P.Precipitation is modeled using a particle size distribution ~PSD! approach for the single or multiple polymorph system. A chemical kinetic-type model for the construction of the molecular clusters of each polymorph is formulated that accounts for adsorption at a heterogeneous site, nucleation, growth, and Ostwald ripening. When multiple polymorphs are accounted for, Ostwald stepping is also predicted. The challenge of simulating the 23 order of magnitude in cluster size ~monomer, dimer, . . . , 1023-mer! is met by a new formalism that accounts for the macroscopic behavior of large clusters as well as the structure of small ones. The theory is set forth for the surface kinetic controlled growth systems and it involves corrections to the Lifshitz–Slyozov, Wagner ~LSW! equation and preserves the monomer addition kinetics for small clusters. A time independent, scaled PSD behavior is achieved both analytically and numerically, and the average radius grows with Rave}t1/2 law for smooth particles. Applications are presented for the silica system that involves five polymorphs. Effects of the adsorption energetics and the smooth or fractal nature of clusters on the nucleation, ripening, and stepping behavior are analyzed. The Ostwald stepping scenario is found to be highly sensitive to adsorption energetics. Long time scaling behavior of the PSD reveals time exponents greater than those for the classical theory when particles are fractal. Exact scaling solutions for the PSD are compared with numerical results to assess the accuracy and convergence of our numerical technique. © 2000 American Institute of Physics. @S0021-9606~00!70123-1#Item Open Access Preparation of Au and Au-Pt nanoparticles within PMMA matrix using UV and X-ray irradiation(2009) Ozkaraoglu, E.; Tunc, I.; Süzer, ŞefikAu and Au-Pt alloy nanoparticles are prepared and patterned at room temperature within the PMMA polymer matrix by the action of 254 nm UV light or X-rays. The polymer matrix enables us to entangle the kinetics of the photochemical reduction from the nucleation and growth processes, when monitored by UV-vis spectroscopy. Accordingly, increase of the temperature to 50 °C of the reaction medium increases the nucleation and growth rates of the nanoparticle formation by more than one order of magnitude, due to enhanced diffusion and nucleation at the higher temperature, but has no effect on the photochemical reduction process. Presence of Pt ions also increases the same rate, but by a factor two only. Similar photochemical reduction and particle growth take also place within the PMMA matrix, when these metal ions are subjected to prolonged exposure to X-rays, as evidenced by XPS analysis. Both angle-resolved and charge-contrast measurements using XPS reveal that the resultant Au and Pt species are in close proximity to each other, indicating the Au-Pt alloy formation to be the most likely case.Item Open Access XPS studies of SiO2/Si system under external bias(American Chemical Society, 2003) Ulgut, B.; Süzer, ŞefikThermally grown SiO2 layers on Si (100) substrate have been subjected to different external voltage bias during XPS analysis to induce changes in the measured binding energy difference between Si4+ and Si0 in Si2p and SiKLL regions. The Si2pp binding energy difference increases from 3.2 to 4.8 for samples containing 1-7 nm oxide thickness, and furthermore, this difference can be influenced by application of an external bias to the sample. Application of negative d.c. bias increases the binding energy difference, whereas positive bias decreases it. The voltage dependence of the binding energy difference exhibits a sigmoid character with an abrupt change near 0 V. Both the binding energy difference and differential change between the positive and negative bias have similar functional dependence on the thickness. This is attributed to differential charging between the silicon oxide layer and silicon substrate, which is decreased when a positive bias is applied to the sample (and therefore attracting a larger proportion of the stray electrons from the vacuum chamber to partially neutralize the oxide). Similarly, when negative bias is applied, the stray electrons are repelled from the sample resulting in less neutralization and an increased differential charging. Through external biasing, it is determined that charging in the SiO2/Si system persists all of the way down to 1 nm. Application of a.c. (square-wave) bias is equivalent to simultaneous application of negative and positive bias together. However, the differential change in the binding energy difference in the positive and negative cycle is frequency dependent and approaches to the d.c. results at lower frequencies.