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  1. Free, publicly-accessible full text available May 10, 2024
  2. Free, publicly-accessible full text available April 26, 2024
  3. Abstract We present results from a stochastic cellular automata (CA) model developed and employed for examining the oxidation kinetics of NiAl and NiAl+Hf alloys. The rules of the CA model are grounded in diffusion probabilities and basic principles of alloy oxidation. Using this approach, we can model the oxide scale thickness and morphology, specific mass change and oxidation kinetics as well as an approximate estimate of the stress and strains in the oxide scale. Furthermore, we also incorporate Hf in the grain boundaries and observe the “reactive element effect”, where doping with Hf results in a drastic reduction in the oxidation kinetics concomitant with the formation of thin, planar oxide scales. Interestingly, although we find that grain boundaries result in rapid oxidation of the undoped NiAl, they result in a slower-growing oxide and a planar oxide/metal interface when doped with Hf. 
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  5. Constructing functional molecular systems for solar energy conversion and quantum information science requires a fundamental understanding of electron transfer in donor–bridge–acceptor (D–B–A) systems as well as competitive reaction pathways in acceptor–donor–acceptor (A–D–A) and acceptor–donor–acceptor′ (A–D–A′) systems. Herein we present a supramolecular complex comprising a tetracationic cyclophane having both phenyl-extended viologen (ExV 2+ ) and dipyridylthiazolothiazole (TTz 2+ ) electron acceptors doubly-linked by means of two p -xylylene linkers (TTzExVBox 4+ ), which readily incorporates a perylene (Per) guest in its cavity (Per ⊂ TTzExVBox 4+ ) to establish an A–D–A′ system, in which the ExV 2+ and TTz 2+ units serve as competing electron acceptors with different reduction potentials. Photoexcitation of the Per guest yields both TTz + ˙–Per + ˙–ExV 2+ and TTz 2+ –Per + ˙–ExV + ˙ in <1 ps, while back electron transfer in TTz 2+ –Per + ˙–ExV + ˙ proceeds via the unusual sequence TTz 2+ –Per + ˙–ExV + ˙ → TTz + ˙–Per + ˙–ExV 2+ → TTz 2+ –Per–ExV 2+ . In addition, selective chemical reduction of TTz 2+ gives Per ⊂ TTzExVBox 3+ ˙, turning the complex into a D–B–A system in which photoexcitation of TTz + ˙ results in the reaction sequence 2 *TTz + ˙–Per–ExV 2+ → TTz 2+ –Per–ExV + ˙ → TTz + ˙–Per–ExV 2+ . Both reactions TTz 2+ –Per + ˙–ExV + ˙ → TTz + ˙–Per + ˙–ExV 2+ and TTz 2+ –Per–ExV + ˙ → TTz + ˙–Per–ExV 2+ occur with a (16 ± 1 ps) −1 rate constant irrespective of whether the bridge molecule is Per + ˙ or Per. These results are explained using the superexchange mechanism in which the ionic states of the perylene guest serve as virtual states in each case and demonstrate a novel supramolecular platform for studying the effects of bridge energetics within D–B–A systems. 
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