Electric-field, strain, and bandgap gradient in van der Waals semiconductors for advanced devices

Lee, Yeageun

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https://hdl.handle.net/2142/120563 Copy

Description

Title

Electric-field, strain, and bandgap gradient in van der Waals semiconductors for advanced devices

Author(s)

Lee, Yeageun

Issue Date

2023-04-28

Director of Research (if dissertation) or Advisor (if thesis)

Nam, SungWoo

Doctoral Committee Chair(s)

Nam, SungWoo

Committee Member(s)

• Saif, M. Taher A.
• Espinosa-Marzal, Rosa M.
• Cai, Lili

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Two-dimensional (2D) materials
• Transition metal dichalcogenides (TMDs)
• Electric-field gradient
• Strain gradient
• Bandgap gradient
• Strain engineering
• Three-dimensional (3D) structuring
• Flexoelectric effect
• Stretchable devices
• Nano-actuator
• Energy harvester
• Photodetector

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDs) such as monolayer molybdenum disulfide (MoS2) have been extensively studied due to their outstanding mechanical properties, as well as semiconducting nature with direct bandgap. In particular, high Young’s modulus, ultimate tensile strength, and breaking strain of 2D TMDs enable a wide range of applications including flexible and stretchable electronics. In addition, these superb mechanical properties allow to create high strain gradient throughout 2D TMDs by out-of-plane structuring such as bending, wrinkling, or crumpling. Out-of-plane deformation of 2D materials builds strain gradient in 2D materials, and strain gradient engineering of 2D TMDs is a powerful approach to enable new functions. Flexoelectric (or converse flexoelectric) effect is an electric polarization under non-zero strain gradient (or a mechanical response under non-zero electric field (E-field) gradient). Flexoelectric effect exists in all dielectrics, and the size of the effect increases quadratically as the material’s thickness decreases. Therefore, a strong strain gradient in 2D TMDs is highly advantageous for flexoelectric devices. A strain gradient also allows to modulate the bandgap of 2D TMDs, and the resulting bandgap gradient induces photo-excited excitons to drift to low bandgap area. 2D TMDs have been widely utilized for optoelectronic devices, and the bandgap modulation can provide additional functionalities on the devices. In the first part of this dissertation, I will discuss a localized out-of-plane deformation of 2D materials to form 2D-3D mixed-dimensional structure. The superb mechanical properties of 2D materials enable to demonstrate this unique structure. By applying O2 plasma on an elastomeric substrate, a stiff skin layer was formed on top of the substrate. Then, the skin layer was fractured by applying strains to create crack-assisted patterns. The applied strains were localized at surface openings between two crack-assisted patterns, and 2D materials located on the opening was deformed by releasing the strain. The localized strain at the crumpled region was estimated to be 1000 %, which is 3.3 times higher than the macroscale pre-strain. The resultant 2D-3D mixed-dimensional structure was fabricated into a strain sensor, and its gauge factor was 4 times higher than delocalized crumpled graphene, which originated from strain localization and amplification. The second part focuses on E-field gradient in 2D materials and flexo-actuators operated by converse-flexoelectric effect. A monolayer MoS2 was employed to actuate a 600 nm thick beam-type actuator with asymmetric electrode design to induce E-field gradient. In-plane strain and strain gradient generated by the MoS2 active layer induce ~42 nm of dynamic out-of-plane actuation under 40 V peak-to-peak AC voltage. The actuation of the device can be controlled in nanoscale range by modulating the applied voltage since the actuation is linearly proportional to the applied voltage. Furthermore, the device shows robust performance up to 1010 cycles and maintains ~70 % of its actuation under 10 K cryogenic condition in contrast to the commercial piezoelectric actuator maintaining only ~40% of its actuation under the same condition. In the third part, strain gradient in 2D materials and flexoelectric energy harvesters are investigated. 2D MoS2 was again utilized as an active material to generate high electrical signals from an applied strain gradient. On a pre-stretched very high bond (VHB) tape, 2D graphene, 2D MoS2, and 2D graphene were vertically stacked and released to have a crumpled structure. The fabricated flexoelectric energy devices showed 1 V of voltage and 10 nA of current generation under repetitive bending motions. However, the devices showed limited capability on harvesting energy from the translational motion. To overcome this challenge, the asymmetric device configuration was achieved by two-step stamping-crumpling methods and resulted in a periodic contact between one graphene electrode and the MoS2 active layer while maintaining a full contact between the other graphene electrode and the MoS2 active layer. As a result, the device with asymmetric device configuration showed 3 mV of voltage and 0.01 nA of current generation under repetitive translational motions. Furthermore, the crumpled structure allowed high deformation endurance, and the devices demonstrated robust energy harvesting ability up to 100 % and 50 % stretching strain for symmetric and asymmetric configurations, respectively. In the last part of this dissertation, MoS2 photodetectors having bandgap gradient created by strain engineering are discussed. Out-of-plane deformation of a graphene-MoS2-graphene heterostructure was achieved by utilizing biaxially pre-stretched VHB substrate. The photodetection capability of the device was then compared with a relatively flat graphene-MoS2-graphene photodetector. Due to the higher light absorbance, enhanced photoresponsivity under tensile stress, and the exciton drift by bandgap gradient of the crumpled structure, the crumpled device generated approximately an order of magnitude higher photocurrent (~ 0.3 nA) compared to that of the flat device (~ 0.03 nA). In conclusion, this study has successfully showcased effective strategies for generating E-field, strain, and bandgap gradients in 2D materials, and their potential applications in actuators, energy harvesters, and sensors. The unique properties of 2D materials have enabled the creation of extremely high gradients in the active layer, leading to exceptional device performance. Our approach provides valuable insights for the research field to develop simple and effective methods to utilize gradients of 2D materials in various device applications. We believe that these findings have important implications for the future development of advanced engineering systems.

Graduation Semester

2023-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2023 Yeageun Lee

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