A library of ab initio Raman spectra for automated identification of 2D materials

Alireza Taghizadeh, Ulrik Leffers, Thomas G. Pedersen & Kristian S. Thygesen
Nature Communications volume 11, Article number: 3011 (2020) Cite this article

Raman spectroscopy is frequently used to identify composition, structure and layer thickness of 2D materials. Here, we describe an efficient first-principles workflow for calculating resonant first-order Raman spectra of solids within third-order perturbation theory employing a localized atomic orbital basis set. The method is used to obtain the Raman spectra of 733 different monolayers selected from the Computational 2D Materials Database (C2DB). We benchmark the computational scheme against available experimental data for 15 known monolayers. Furthermore, we propose an automatic procedure for identifying a material based on an input experimental Raman spectrum and apply it to the cases of MoS2 (H-phase) and WTe2 (T{}^{\prime}-phase). The Raman spectra of all materials at different excitation frequencies and polarization configurations are freely available from the C2DB. Our comprehensive and easily accessible library of ab initio Raman spectra should be valuable for both theoreticians and experimentalists in the field of 2D materials.

Following the discovery of graphene in 20041, the field of two-dimensional (2D) materials has grown tremendously during the last decade. Today, more than 50 different monolayer compounds including metals2,3, semiconductors4,5,6, insulators7, ferromagnets8, and superconductors9,10, have been chemically grown or mechanically exfoliated from layered bulk crystals11. The enormous interest in 2D materials has mainly been driven by their unique and easily tunable properties (as compared to 3D bulk crystals), which make them attractive for both fundamental research and technological applications in areas such as energy conversion/storage, (opto)-electronics, and photonics6,12,13. Among the various experimental techniques used for characterizing 2D materials, Raman spectroscopy plays a pivotal role14 thanks to its simplicity, non-destructive nature, and high sensitivity towards key materials properties such as chemical composition, layer thickness (number of layers), inter-layer coupling, strain, crystal symmetries and sample quality15,16,17.

Raman spectroscopy is a versatile technique for probing the vibrational modes of molecules and crystals from inelastically scattered light, and is widely used for identifying materials through their unique vibrational fingerprints18. There are various types of Raman spectroscopies that differ in the number of photons or phonons involved in the scattering process18. Here we focus on the first-order Raman processes in which only a single phonon is involved. Typically, this is the dominant scattering process in defect-free samples (which are considered here). Note that Raman processes involving defect states or several phonons may also play important roles in some 2D crystals such as graphene19. As shown schematically in Fig. 1(a), the light scattered from a crystal appears in three distinct frequency bands: A strong resonance at the incident frequency ωin due to Rayleigh (elastic) scattering, and weaker resonances due to Raman (inelastic) scattering at ωin???ων and ωin + ων forming Stokes and anti-Stokes bands, respectively. Here, ων is the frequency of a (Raman active) vibrational mode of the crystal, i.e. a phonon. Depending on the symmetry of the phonon modes and polarization of the electromagnetic fields, a phonon mode may be active or inactive in the Raman spectrum.

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