Simulating Raman spectra by combining first-principles and empirical potential approaches with application to defective MoS2

Zhennan Kou,Martti J Puska,Hannu-Pekka Komsa,Arsalan Hashemi,Arkady V Krasheninnikov

doi:10.1038/s41524-020-0320-y

Abstract

Successful application of two-dimensional transition metal dichalcogenides in optoelectronic, catalytic, or sensing devices heavily relies on the materials’ quality, that is, the thickness uniformity, presence of grain boundaries, and the types and concentrations of point defects. Raman spectroscopy is a powerful and nondestructive tool to probe these factors but the interpretation of the spectra, especially the separation of different contributions, is not straightforward. Comparison to simulated spectra is beneficial, but for defective systems first-principles simulations are often computationally too expensive due to the large sizes of the systems involved. Here, we present a combined first-principles and empirical potential method for simulating Raman spectra of defective materials and apply it to monolayer MoS2 with random distributions of Mo and S vacancies. We study to what extent the types of vacancies can be distinguished and provide insight into the origin of different evolutions of Raman spectra upon increasing defect concentration. We apply to our simulated spectra the phonon confinement model used in previous experiments to assess defect concentrations, and show that the simplest form of the model is insufficient to fully capture peak shapes, but a good match is obtained when the type of phonon confinement and the full phonon dispersion relation are accounted for.

Highlights

Structural defects can be either intentionally or unintentionally introduced into materials during their synthesis or processing, or they can naturally appear at finite temperatures due to the entropic contribution to the free energy of the system[1]
When calculating the Raman intensity of the supercell (SC) mode j, its Raman tensor RSC,j is obtained as a sum of the primitive unit cell (PUC) Raman tensors RPUC,i and the projections of the SC eigenmodes to the PUC eigenmodes at the Γ-point: 10 × 10 PUCs in the case of S2vac and Movac, and from 3 × 4 to 15 × 15 PUCs in the case of Svac randomly placed on the two sides of the layer
We have demonstrated a method for simulating Raman spectra of defective materials based on a combination of empirical potentials and first-principles calculations

Summary

Introduction

Structural defects can be either intentionally or unintentionally introduced into materials during their synthesis or processing, or they can naturally appear at finite temperatures due to the entropic contribution to the free energy of the system[1]. Doping by foreign impurities can be used to increase carrier concentration, but at the same time the mobility decreases due to enhanced scattering[2] They can enable new technologies, e.g., point defects can be used as single-photon emitters or qubit hosts[3,4] in the field of quantum information. With the discovery of new materials, defects continue to be an active and important research area for both engineers and scientists. This seems to be especially relevant with twodimensional (2D) materials, such as graphene and transition metal dichalcognides (TMDs). The concentration of defects can be much larger than that in bulk systems because of interactions with reactive species, but it is much easier to study and control defects in 2D systems than those buried in bulk materials (for an overview, see refs. 5–11)

Methods

Results

Conclusion