Tip-based advanced characterizations of van der Waals materials for enhanced functionality

Haque, Md Farhadul

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https://hdl.handle.net/2142/115501 Copy

Description

Title

Tip-based advanced characterizations of van der Waals materials for enhanced functionality

Author(s)

Haque, Md Farhadul

Issue Date

2021-12-20

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

Nam, SungWoo

Doctoral Committee Chair(s)

Nam, SungWoo

Committee Member(s)

• Ferreira, Placid
• van der Zande, Arend
• Murphy, Catherine J

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)

• 2D materials
• AFM
• strain
• plasmon
• flexoelectricity
• heterostructure

Abstract

Two-dimensional (2D) van der Waals (vdW) materials have unique properties compared to conventional bulk materials. For example, transition metal dichalcogenides (TMDs) show bandgap energy transition from indirect to direct as their thickness is reduced to the monolayer limit. Reducing thickness also yields higher carrier mobility in 2D materials because of the confinement of a 2D surface along out-of-plane direction which results in reduced scattering of charge carriers (e.g., electron-phonon scattering). Enhanced electromechanical actuation has been demonstrated in nanoscale structures of three-dimensional materials compared to their bulk counterparts due to the large “free surface” effect. Probing such emerging properties at the nanoscale and investigating how they are correlated require advanced characterization techniques with atomic-scale precision. Often the spatial resolution of the characterization technique is limited by the resolution of the detection scheme. Therefore, I adopted advanced tip-based characterization technique to probe the unique plasmonic and electromechanical properties of 2D vdW materials at sub-30 nm spatial resolution. First, I investigated plasmonic resonance tunability of a crumpled graphene using photo-induced force microscopy (PiFM). PiFM is an effective technique for probing light-matter interaction at a sub-20 nm spatial resolution. By applying uniaxial compressive strain, we created crumpled structures of graphene with controlled topographic features (e.g., wavelength, aspect ratio) on a polymeric substrate. I observed a wide tunability of plasmon resonance wavelength (5.77-10.93 μm) which we attributed to both the variation in topography as well as efficient coupling with the incident photon. Our observed plasmon resonance followed a redshift trend with an increase in wavelength and aspect ratio of the crumples, which agrees well with our established numerical model. Finally, we demonstrated enhanced chemical imaging of a polymethyl methacrylate (PMMA) film aided by the effective plasmon resonance of the underlying crumpled graphene structure. The coupling between PMMA phonon and graphene plasmon showed a two-fold increase in the obtained near-field optical response. Our results suggest widely tunable, plasmonically-enhanced biosensing or spectroscopy in the infrared range. Next, I studied the electromechanical response driven by converse flexoelectric effect (CFE) of monolayer molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) using piezoresponse force microscopy (PFM). PFM is a popular technique for probing electromechanical response at a sub-30 nm spatial resolution. PFM measurements revealed strongly enhanced CFE of the atomically thin MoS2 and WSe2 than their bulk counterpart (~700% enhancement in MoS2, ~400% enhancement in WSe2). The bilayer MoS2 and WSe2 exhibited a 50% larger CFE than monolayer samples. Finite element analysis suggested that a puckering deformation during PFM measurement could suppress the CFE of the monolayer structure by ~70%. By inducing a built-in in-plane tension to reduce puckering, I observed a ~46% increase in the CFE of monolayer WSe2. Our findings can help understanding the fundamentals of flexoelectric behavior in 2D materials. Finally, I extended my study on electromechanical response on artificially stacked 2D vertical heterostructure. PFM measurement revealed almost five times enhancement in the electromechanical response on a MoS2/WSe2 vertical heterobilayer structure compared to the standalone MoS2 monolayer. I investigated the underlying mechanism behind the enhancement in light of charge transfer induced built-in dipole and the interlayer coupling strength. The charge transfer was qualitatively estimated by Kelvin probe force microscopy (KPFM). KPFM is a popular technique to probe surface potential at sub-20 nm spatial resolution. The interlayer coupling strength was estimated from the Raman and photoluminescence spectroscopy. The detailed analysis suggested that both charge transfer and interlayer coupling strength are responsible for the augmented electromechanical response of the heterostructure. The obtained knowledge is beneficial for next generation nanoscale actuator or energy harvesting devices.

Graduation Semester

2022-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Md Farhadul Haque

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