Effect of mechanical strain on the structure and properties of 2D materials and their heterostructures

Mohammadreza Daqiqshirazi

AutorIn

Mohammadreza Daqiqshirazi

Titel

Effect of mechanical strain on the structure and properties of 2D materials and their heterostructures

Zitierfähige Url:

https://nbn-resolving.org/urn:nbn:de:bsz:14-qucosa2-962908

Erstveröffentlichung

2025

Datum der Einreichung

01.11.2024

Datum der Verteidigung

04.03.2025

Abstract (EN)

Two-dimensional (2D) materials are a fascinating class of substances with diverse properties that make them suitable for various applications. They are particularly promising for optoelectronic and energy applications due to their thinness. They offer the potential to address the limitations of the silicon industry such as the increasing leakage currents (i.e., current which is non-functional and lost due to, e.g., spontaneous discharge of capacitors) with decreasing feature sizes and the heat management. However, using these materials in devices still requires overcoming several unresolved challenges. A major obstacle is to obtain precise control over the properties of these materials. Tuning these characteristics can be performed through various methods, such as adjusting the chemical composition of the materials or applying external fields. Strain is particularly attractive as it can be applied either during synthesis or dynamically during operation. Moreover, it can be applied in various forms, both uniform and non-uniform. Theoretical investigations have already provided detailed insight into the influence of uniform strain on optoelectronic properties of 2D materials. Yet, due to their low bending moduli, 2D materials are particularly susceptible to non-uniform strain fields, allowing them to bend easily in the out-of-plane direction. Although these strain fields are versatile for property modulation, non-uniform strain fields are rarely studied in theoretical works. This is mainly due to the computational complexity that arises after the breaking of symmetries by the strain field. In this thesis, I theoretically investigate the influence of strain on the properties of 2D materials within the framework of density functional theory (DFT). Initially, I discuss the role of wrinkling in 2D transition metal dichalcogenides (TMDCs, \ch{MX2}, where M = W, Mo,... and X = S, Se,...). Wrinkles convey a non-uniform, periodic strain field to the 2D structure and experimental observations have suggested the emergence of new optoelectronic phenomena, yet the impact of these fields have not been fully investigated theoretically. In my work, I investigate the role of wrinkles in nanoscale monolayers and bilayers of TMDCs, and I extend the analysis to heterobilayers. Furthermore, I highlight the critical role of spin-orbit coupling (SOC) in determining the electronic structure of these systems. Next, I address the effect of strain at the edges of 2D h--BN. When they are exfoliated from bulk and thinned under ion radiation, edges with different number of layers are formed. At these edges, localized (non-uniform) strain accumulates. This local strain can influence the charge density of the sample. Here, I particularly focus on connecting the DFT simulation results with experimental data obtained from aberration-corrected scanning transmission electron microscopy (TEM). I present how the correlation can be deciphered and linked to the experimental observations. Finally, I concentrate on atomic layer deposition (ALD)‌ synthesized \ch{Sb2Te3}-\ch{Sb2Se3} multilayered material. These materials experience another strain field, namely the interfacial strain, during synthesis. Moreover, defects are present in this synthesis. These defects introduce a local strain field that can influence the thermoelectricity in these materials. It is noteworthy that these layered samples are at the crossover to the 2D materials, since further reduction of the stack size will result in 2D structures. Here, I explain how strain affects the thermoelectric coefficients of \ch{Sb2Te3}-\ch{Sb2Se3} heterostructures and how this effect can be correlated with experimental data.

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