Ph.D. Public Defense

Exploring Process-Induced Strain Engineering and Strain-Driven Effects in 2D Materials

Ahmad Azizimanesh

Supervised by Stephen Wu

Thursday, October 5, 2023
11 a.m.

703 Computer Studies Building
Meeting ID: 561 361 9460


The emergence of two-dimensional (2D) materials has revolutionized the field of semiconductors and holds immense importance across various disciplines. These atomically thin and layered materials, such as graphene, transition metal dichalcogenides (TMDs), and hexagonal boron nitride (hBN), possess exceptional physical and chemical properties distinct from 3D materials. Their atomic-scale thickness allows for extraordinary flexibility and mechanical strength in addition to remarkable electrical, thermal, and optical characteristics, making them highly desirable for next-generation electronic, optoelectronic, and energy storage devices.

Strain engineering also plays a crucial role in unlocking the potential of 2D materials. By applying mechanical strain to 2D materials, their electronic, optical, and mechanical properties can be finely tuned, enabling tailored functionalities for specific applications, and paving the way for innovative technological advancements.

In this thesis, we first introduce the strain engineering technique for 2D materials using evaporated stressed thin films. We show that using lithographically patterned evaporated thin films with intrinsic stress onto 2D MoS2, we can control tension/compression, magnitude, uniaxiality/biaxiality, and directionality of the resulting strain. We also explore the temperature and time stability of the proposed strain engineering method and study defects in the crystal of 2D MoS2 to enable using the proposed straining technique in various applications involving 2D materials.

In the second chapter, we explore the stacking order engineering in trilayer graphene (TLG). ABC stacked TLG displays distinctive electronic and optical properties, including superconductivity attributed to its enhanced density of states. Despite its unique features, it is thermodynamically less stable than ABA-TLG and less possible to appear in exfoliated TLG samples. We show that by applying a large enough uniaxial strain with patterned stressor layers, we can induce layer slippage in ABA-TLG and create stable ABC-TLG.

In the next chapter, we study in-plane strain transfer through the layers (c-axis) of different types of 2D materials using evaporated stressors. Due to the weak interlayer bonds between the layers of 2D materials, the layer-by-layer strain transfer from the evaporated stressors is incomplete. This property of 2D materials is called the strain transfer length scale and is a crucial parameter in engineering strain in different 2D materials and 2D heterostructures. We explore the strain transfer length scale in 2D hBN and graphene as two of the most utilized 2D materials using uniformly evaporated stressors through Raman spectroscopy.

In chapter 5, we use the patterned evaporated stressors to engineer strain in 1T’ MoTe2 and create a strain-induced semimetallic to semiconducting phase change in this material. We study the phase changes through Raman spectroscopy using optically transparent evaporated stressors. Additionally, using metallic contacts as the stressors and MoTe2 as the channel, we show a memristive switching behavior through a combination of strain- induced and electric-field-induced phase changes in MoTe2. The proposed MoTe2 phase change memristors show exceptionally low switching voltage and high on/off ratio due to the unique double-mechanism phase switching.