Reconfigurable strain engineering of atomically-thin van der Waals materials

Kim, Jin Myung

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https://hdl.handle.net/2142/117523 Copy

Description

Title

Reconfigurable strain engineering of atomically-thin van der Waals materials

Author(s)

Kim, Jin Myung

Issue Date

2022-08-25

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

Nam, SungWoo

Doctoral Committee Chair(s)

Shim, Moonsub

Committee Member(s)

• Huang, Pinshane
• Zhang, Yingjie

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• 2D materials
• strain engineering
• photo-induced force microscopy

Language

eng

Abstract

Atomically-thin van der Waals (vdW) layered materials exhibit unique physical and chemical properties arising from quantum confinement in low dimensionality and lack of dangling bond. Their atomic structure consists of strong intralayer covalent bonding and weak interlayer vdW interaction, which offers extraordinary combination of high in-plane strength and out-of-plane flexibility. This opens up the new opportunities of strain engineering of two-dimensional (2D) materials in which emerging phenomena and functionalities are enabled and controlled by strain-induced modulation of electronic and phononic band structure. For instance, heterogeneous strain applied on transition metal dichalcogenides (TMDs) enables manipulation of exciton, electron-hole pair bound by Coulomb interaction, toward direction of lowering its potential energy, called exciton funneling. Unlike bulk semiconductor, the high mechanical strength and strain sensitivity allow for reconfigurable and reversible control of exciton drift. Furthermore, exciton drift in nanoscale strain gradient of TMDs leads to emission of antibunched photon (i.e., single photon emitters), which contributes to quantum communication and secured cryptography as an essential information unit. In this thesis, I aim to demonstrate advanced strain engineering in static and dynamic control of strain and strain gradient in 2D semiconductors, and analyze multi-dimensional optical properties of strain-modulated properties. First, I investigated strain-induced exciton drift in monolayer tungsten diselenide (WSe2) at room temperature via surface-instability driven surface wrinkling and pump-probe optical characterization. Multi-layered wrinkle architecture enabled high local strain more than 2.4% and optically-resolvable energy gradient (49 meV/μm) to WSe2. I observed strain gradient induced flux of high-energy excitons drifted to the nearest potential energy minima with high transport efficiency more than one order of magnitude improved than nanoscale strain engineering of exciton transport. The time-resolved characterization combined with theoretical calculation elucidated exciton dynamics in strain gradient and provided design parameters for potential straintronic exciton devices functional at GHz frequency. These results provide strong evidence for strain-driven manipulation of exciton funneling in two-dimensional semiconductors and open up future opportunities for quantum straintronics and excitonic phase transitions. Second, I studied dynamic modulation of strain-exciton coupling by surface acoustic wave (SAW). Theoretical model established on the experimental observation of strain-induced exciton funneling and critical role of bright/dark excitons in strain-exciton coupling. The calculation showed that the efficient modulation between dynamic strain and exciton drift is only made possible by resonant matching of SAW frequency and exciton motion parameters, including mobility and lifetime. I also demonstrated controlled exciton drift by tuning phase shift and pulsed excitation at specific time window. Furthermore, uni-directional exciton transport in 1-port propagating SAW device manifested potential to realize efficient and long-distance exciton drift. This work provides new insights for rational design of 2D vdW materials on SAW device for reconfigurable optoelectronic devices toward secured communications and exciton-based quantum phase applications. Third, I investigated nanoscale optical properties of 2D semiconductors using photo-induced force microscopy (PiFM). PiFM technique allows for near-field tip-sample dipole interactions induced by coherent laser excitation. I employed polarization-controlled visible wavelength laser source to investigate the in-plane excitonic response of 2D vdW semiconductors in visible-near IR wavelengths. This enables us to demonstrate strong resonance peaks of A and B excitons in various TMD materials with differing number of layers. Spatial resolution of the PiFM hyperspectral mapping was revealed less than 20 nm, indicating its capability of nanoscale optical characterization. Furthermore, we analyzed nanoscale strain modulation in wrinkled 2D semiconductors. Our findings pave the new ways for advanced nano-characterization for strain-tunable optical properties of 2D semiconductors beyond optical resolution limit. Lastly, I demonstrated structural characteristics and piezoresistance of laser-induced graphene (LIG) derived from filter paper. LIGs showed unique hierarchy in pore structure, enabled by pre-existing cellulous pore structure of filter paper and controlled carbonization of fire-retardant coated filter paper substrate. I observed linear piezoresistance in concave and convex bending, and complex modulation in twisting configuration. Particularly, LIG also exhibited high strain gauge behavior when strain level exceeds critical strain, attributed to dynamic reconstruction of LIG pore structure. The in situ characterization of pore structure under tensile strain revealed two different types of pore evolutions in linear regime and nonlinear regime, as well as flawless recovery of pore connectivity at unstrained state. Our results present new opportunities of facile production of low-cost, highly sensitive LIG strain/vibration sensors.

Graduation Semester

2022-12

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/117523

Copyright and License Information

Copyright 2022 Jin Myung Kim

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