Seminarium Fizyki Ciała Stałego

The purpose of this work is to study the vibrational properties of crystal lattices (phonons) in a choice of two-dimensional materials and their heterostructures on the grounds of quantum-mechanical simulations based on the density functional theory (DFT), with particular emphasis on the impact of the lattice temperature. The following two-dimensional structures (based on the transition metal dichalcogenides) will be considered: semiconducting single-layered 1H-MoS2 and 1H-WS2, multi-layered 2H-WS2 [1], 1H-MoS2/1H-WS2 heterostructures with different relative stacking between the layers, 1H-MoS2/graphene heterostructure, and highly anharmonic semi-metallic 1T-TiS2 material [2]. All the listed-above systems are significant from the point of view of their potential applications, enabling us to supplement physical properties of the famous graphene – from the point of view of its electrical, thermal, optical, mechanical, and chemical properties.The undertaken studies, based on rigorous theoretical grounds described in the literature of the 1960s, include the calculation of key effects induced by the anharmonicity of interatomic interactions, such as the thermal expansion of the structure and anharmonic phonon-phonon interactions (in particular three- and four-phonon processes) – affecting the temperature dependence of the effective phonon frequencies and their lifetimes. Each of the above-listed anharmonic effects has been studied separately, enabling us to better understand their nature in each studied two-dimensional material and their dependence on the structure’s geometry. The obtained theoretical temperature-dependent propagation parameters of main phonon modes agree very well with available spectroscopic measurement data, indicating the DFT calculations’ ability to well reproduce this physical property. Therefore, the proposed methodology might be successfully extended to more advanced problems such as complex multi-layered structures and other phonon-limited physical properties (e.g., thermal conductivity).Acknowledgments: Research was funded by the Warsaw University of Technology within the Excellence Initiative: Research University (IDUB) programme. We also acknowledge the usage of computer cluster DWARF at Warsaw University of Technology supported by the Polish National Science Center (NCN) under Contracts No. UMO-2017/26/E/ST3/00428 and UMO-2017/27/B/ST2/02792.References[1] K. Wilczyński, A.P. Gertych, K. Czerniak-Łosiewicz, J. Sitek, M. Zdrojek, “Phonon anharmonicity in multi-layered WS2 explored by first-principles and Raman studies”, Acta Materialia 240, 118299, (2022).[2] K. Wilczyński, A.P. Gertych, M. Zdrojek, “Explaining Mysterious “Shoulder” Raman Band in TiS2 by Temperature-dependent Anharmonicity and Defects”. J. Phys. Chem. C 127, 20870–20880 (2023).