Experimental demonstration of the suppression of optical phonon splitting in 2D materials by Raman spectroscopy

TL;DR

This study experimentally confirms that optical phonon splitting is suppressed in 2D materials due to dipole interactions, challenging the application of non-analytical corrections in Raman spectral modeling.

Contribution

It provides experimental evidence that non-analytical corrections should not be used for modeling Raman spectra of few-layered 2D materials, supporting recent theoretical predictions.

Findings

Optical phonon splitting is absent in 2D materials due to dipole interactions.

Non-analytical corrections are not applicable for modeling Raman spectra of certain 2D materials.

Layer parity and stacking influence the Raman spectral features.

Abstract

Raman spectroscopy is one of the most extended experimental techniques to investigate thin-layered 2D materials. For a complete understanding and modeling of the Raman spectrum of a novel 2D material, it is often necessary to combine the experimental investigation to density-functional-theory calculations. We provide the experimental proof of the fundamentally different behavior of polar 2D vs 3D systems regarding the effect of the dipole-dipole interactions, which in 2D systems ultimately lead to the absence of optical phonons splitting, otherwise present in 3D materials. We demonstrate that non-analytical corrections (NACs) should not be applied to properly model the Raman spectra of few-layered 2D materials, such as WSe$_{2}$ and h-BN, corroborating recent theoretical predictions [Nano Lett. 2017, 17 (6), 3758-3763]. Our findings are supported by measurements performed on tilted samples that allow increasing the component of photon momenta in the plane of the flake, thus unambiguously setting the direction of an eventual NAC. We also investigate the influence of the parity of the number of layers and of the type of layer-by-layer stacking on the effect of NACs on the Raman spectra.