January 2022 Kentucky Lake Section Meeting

Quantum Chemical Simulations of Carbon Nanostructure Self-Assembly in Nonequilibrium

Featuring Dr. Stephan Irle,
Computational Chemistry and Nanomaterials Sciences Group,
Oak Ridge National Laboratory

Thursday, January 27th @ 7:00pm

Join Virtually via Zoom (Meeting ID TBA)

Abstract:  The density-functional tight-binding (DFTB) method [1] is an approximation to density functional theory (DFT) and allows a speedup of first principles electronic structure calculations by two to three orders of magnitude. In this talk, I will discuss DFTB-based simulations of nanoscale materials self-assembly in nonequilibrium on large length scales [2]. Fullerene, carbon nanotube, and graphene formation were simulated on the nanosecond time scale, considering experimental conditions as closely as possible. An approximate density functional method was employed to compute energies and gradients on-the-fly in direct MD simulations, while the simulated systems were continually pushed away from equilibrium via carbon concentration or temperature gradients. We find that carbon nanostructure formation from feedstock particles involves a phase transition of sp to sp2 carbon phases, which begins with the formation of Y-junctions, followed by a nucleus consisting of pentagons, hexagons, and heptagons. The dominance of hexagons in the synthesized products is explained via annealing processes that occur during the cooling of the grown carbon structure, accelerated by transition metal catalysts when present. The dimensional structures of the final synthesis products (0D spheres – fullerenes, 1D tubes – nanotubes, 2D sheets – graphenes) are induced by the shapes of the substrates/catalysts, and their interaction strength with carbon. Our work prompts a paradigm shift away from traditional anthropomorphic formation mechanisms solely based on thermodynamic stability. Instead, we conclude that nascent carbon nanostructures at high temperatures are dissipative structures described by nonequilibrium dynamics in the manner proposed by Prigogine, Whitesides, and others. As such, the fledgling carbon nanostructures consume energy while increasing the entropy of the environment, and only gradually anneal to achieve their familiar, final structure, maximizing hexagon formation wherever possible [2,3].

[1] a) Christensen, A. S.; Kubar, T.; Cui, Q.; Elstner, M. Semiempirical Quantum Mechanical Methods for Noncovalent Interactions for Chemical and Biochemical Applications, Chem. Rev. 2016, 116, 5301-5337; b) http://www.dftbplus.org
[2] Irle, S; Page, A. J.; Saha, B.; Wang, Y.; Chandrakumar, K. R. S.; Nishimoto, Y.; Qian, H.-J.; Morokuma, K. Atomistic mechanism of carbon nanostructure self-assembly as predicted by nonequilibrium QM/MD simulations, in: J. Leszczynski, M. K. Shukla, Eds. “Practical Aspects of Computational Chemistry II: An Overview of the Last Two Decades and Current Trends”, Springer-European Academy of Sciences, Chapter 5, pp. 105-172 (April 2, 2012). ISBN 978-94-007-0922-5. DOI: 10.1007/978-94-007-0923-2_5 Preprint: https://www.dropbox.com/s/n2o3sjnb0t1z6mr/5_Online%20PDF.pdf?dl=0
[3] Page, A. J.; Ding, F.; Irle, S.; Morokuma, K. Insights into carbon nanotube and graphene formation mechanisms from molecular simulations: a review, Rep. Prog. Phys. 2015, 78, 036501/1-38.

Bio: Dr. Stephan Irle is Group Leader of the Computational Chemistry and Nanomaterials Sciences Group at the Oak Ridge National Laboratory with more than 30 years of experience in computational chemistry and materials sciences in Germany, Austria, the United States, and Japan. He was a founding principal investigator at the Institute of Transformative Bio-Molecules (WPI-ITbM) at Nagoya University and member of the Japanese “post-K supercomputer” support project. His specialty is the quantum chemical study of complex systems on exascale and quantum computing platforms. Target areas are soft matter and biosimulations, excited states of large molecules, electrochemistry, catalysis and geosciences. Complementary studies of physicochemical properties, theoretical spectroscopy, and the development of methodologies including approximate quantum chemical methods accompany this research. Dr. Irle has more 300 publications in peer-reviewed journals, 45 book chapters and conference proceedings and authored 2 books. Dr. Irle received a B.S. and M.S. in Chemistry, both from the University of Siegen in Germany.  He received his Ph.D. in Chemistry from the University of Vienna in Austria.

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