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Computational approaches to the dynamics at surfaces and applications to graphene-like materials

Speaker: 
Sergei Manzhos (Dept. Mech. Eng. NUS)
Date: 
Wed, 17/10/2012 - 10:30am to 11:30am
Location: 
S13-02-14 (Physics Resource Room, NUS)
Event Type: 
Seminars

Abstract

Ability to model reactions at interfaces is critical for the development of new catalysts and energy conversion and storage technologies. Specifically, graphene, doped graphene, and other carbon allotropes have shown potential as effective noble metal-free catalysts for oxygen reduction1. Calculations of processes at graphene-like materials have the potential to guide functional material design.

Most theoretical studies focus on static calculations of structures and energetics to describe or predict essentially dynamic outcomes (the volcano curve is an example). This is because the modeling of dynamics requires a potential energy surface (PES), and for the vast majority of molecule-surface systems, PES's do not exist. This makes theoretical predictions shaky. I will focus on methods to predict dynamics at the surfaces, including reaction dynamics and vibrational dynamics. I will describe a method to build potential energy surfaces, including multi-body effects and without imposing a pre-determined functional form, so that all chemical interactions can be accounted for 2. 

A catalyst can be characterized by vibrational signatures of reactants, intermediates, and products. If anharmonicity and coupling of modes are significant, the harmonic frequencies from DFT codes can be in error of dozens or hundreds cm-1 (on top of the error due of the electronic structure code), thwarting spectral assignment and thereby material design. Today there do not exist computational tools to compute routinely anharmonic spectra. I will present a method we have been developing to compute anharmonic vibrational spectra for molecules at interfaces3. 

I will also discuss strain and its effect on the phonon spectrum of graphene. Stress-strain relations including only Young modulus and third-order elastic moduli are still used4. Even a quick calculation shows higher-order expansions are needed. For planar graphene, a comprehensive theoretical analysis of stress-strain exists5. It still needs be computed for other graphene-like materials. We are also interested in out-of-plane distortions: a recent high-profile study reported changes in the phonon spectrum of graphene when under stress in non-planar configurations6. Our preliminary calculations show that the frequency shift is a non-linear function of strain and is also mode-dependent. This suggests an approach to characterize graphene-like materials.

1. S. Kattel, P. Atanassov, B. Kiefer, J Phys Chem C 116, 17378 (2012); P. Wu, P. DU, H. Zhang, C. Cai, J Phys Chem C 116, 20472 (2012); L. Qu, Y. Liu, J.-B. Baek, L. Dai, ACS Nano 4, 1321 (2010); Z. Yang, Z. Yao, G. Li, G. Fang, H. Nie, Z. Liu, X. Zhou, X. Chen, S. Huang, ACS Nano 6, 205 (2012)
2. S. Manzhos, K. Yamashita, Surf Sci 604, 555 (2010); S. Manzhos, K. Yamashita, T. Carrington, Comput Phys Commun 180, 2002 (2009)
3. S. Manzhos, T. Carrington, K.  Yamashita, Surf Sci 605, 616 (2011); M. Chan, K. Yamashita, T. Carrington, S. Manzhos, MRS Proceedings, in print.
4. C. Lee, X. Wei, J. W. Kysar, J. Hone, Science 321, 385 (2008)
5. X. Wei, B. Fragneaud, C. A. Marianetti, J. W. Kysar, Phys Rev B 80, 205407 (2009)
6. J.-U. Lee, D. Yoon, H. Cheong, Nano Lett 12, 4444 (2012)

 

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