Simulating Raman spectroscopy of doped 2D materials

Doping 2D materials can tune their electronic, optical, magnetic, catalytic, and mechanical properties, but it is difficult to get conclusive evidence from experiments about where the dopants are located, particularly in multilayer or bulk materials. Raman spectroscopy can be a powerful tool for characterization, but help from simulations is needed to interpret the spectra in terms of dopant location in substitutional or intercalation sites. I will present our studies of Raman spectroscopy in Ni- and Re-doped MoS₂, with density-functional perturbation theory. In Ni-doped MoS₂, important for catalysis and lubrication, we find distinct fingerprints of the different doping sites, and analyze their origin in terms of activation of Raman-inactive modes, creation of new Ni-related modes, and shifts of existing modes [Guerrero et al., J. Phys. Chem. C 125, 13401 (2021)]. In Re-doped MoS₂, important for electronics, we again find distinct characteristics of the doping sites, mostly in terms of shifts of the pristine Raman peaks. We show however that this peak shifts have a more complex origin than the simple ideas of strengthening or weakening of bonds as commonly used. Re-doped MoS₂ is n-type and therefore has a metallic density of states, preventing usual approaches for Raman calculations based on the static dielectric constant. We overcome this problem by using atomic Raman tensors from the pristine material. Our benchmarks show this is a generally applicable method for Raman calculations of metallic doped materials [Guerrero and Strubbe, J. Phys. Chem. C https://doi.org/10.1021/acs.jpcc.2c03999 (2022)]. In an experimental collaboration, we used these results to identify the location of dopants in samples of Re-doped MoS₂ that showed an unusual increase in nanoscale friction with the number of layers [Acikgoz et al., Nanotechnology 34, 015706 (2023)]. Finally, I will show some results from an undergraduate/graduate class project in which students calculated Raman spectra of MoS_2xSe_2(1-x) monolayer alloys.