Thursday, February 18, 2016

Theory of Graphene Raman Scattering

Eric J. Heller, Yuan Yang, Lucas Kocia, Wei Chen, Shiang Fang, Mario Borunda, and Efthimios Kaxiras

ACS Nano Article ASAP January 2016 doi:10.1021/acsnano.5b07676

Contributed by Alán Aspuru-Guzik

The recent paper by Eric Heller and collaborators is a tour de force that provides the definitive and correct theory for the unusual features of the Raman spectrum of  perhaps the most popular material of the decade, graphene. Heller and collaborators place in serious doubt literally more than 3000 papers that employ a fourth order perturbative ("double resonance") unphysical model. Instead, the authors employ the Kramers-Heisenberg Dirac model that "has been used ever since to explain more than half a million Raman spectra in a very wide range of systems".

 This paper is also a case study of negative inertia effect in science, where once a dominant theory is established as a de facto explanation, thousands of researchers in a subfield do not examine it further and hence it becomes a dogma. It also shows that unfortunately, cross-pollination of theoretical models and ideas between different communities of scientists can take many years to happen.

Several key elements came into play for this paper. The non-negotiable inclusion of coordinate-dependent transition moments is necessary to reproduce the spectra. In addition, Heller and co-workers introduce a new mechanism, named "transition sliding" that is responsible for the unusual high intensity overtones observed in the spectrum. This transition sliding relies on the linear dispersion and Dirac cones of graphene. The linear dispersion leads to a coherent addition of amplitude in the excited state that reinforces a particular state of graphene (2D). The authors argue that due to the lack of linear dispersion in most if not all other molecules, this effect is mostly absent in regular conjugated polymers due to the lack of constructive interference in such cases.

The article contains a comprehensive discussion of all the other spectral features of the material in a very lucid presentation style.

In summary, the authors make a convincing and irrefutable case for the fact that "[t]here is no reason that removing hydrogens from carbon materials should cause KHD to catastrophically fail and require replacement by a theory based on different physics." This paper should be a must-read for any scholar of two-dimensional materials and a case study for every graduate student of theoretical chemistry.

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