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Structure Design and Properties Modification Based on Wrinkling of Two-Dimensional Nanomaterials

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thesis
posted on 08.02.2018 by Shikai Deng
Structure and strain engineering is the process of tuning a material's properties by altering its mechanical or structural attributes. Atomically thin two-dimensional nanomaterials (2DNMs), such as graphene, boron nitride, and transition metal dichalcogenides (MoS2, WS2, etc.), which have been extensively studied in recent years, are particularly well-suited for strain engineering because they can withstand large strains. Wrinkling has shown its great advantages to introduce well-controlled local structure and strain in 2DNMs. However, the studies on understanding of the wrinkles formation, wrinkling processes for nanoscale confined and directed wrinkles, and wrinkle-effect and application in 2DNMs are still in its infancy. This thesis first shows that parallel and self-similar hierarchical wrinkles pattern can be formed on ultrathin cobalt/chromium film atop a contracting silicone oil meniscus. Interesting, these wrinkle attributes do not follow the standard von-Kármán wrinkling scaling near the edge, attributed to the added surface energy (L/λ ∝ (A/t)0.31). An energy model is developed and shows a linear relation between the amplitude and the length of wrinkles at all observed hierarchic levels (L ∝ A). Additionally, wrinkles (wavelength = 10 nm ∼ 10 μm) can be found in mechanical exfoliated MoS2 flakes on silicon-based substrates (SiO2 and Si3N4). A mechanical energy model is proposed that equates the adhesion energy of MoS2 on SiO2 and Si3N4 to the attributes of a single wrinkle in a MoS2 flake. The adhesion energy values of 0.170 ± 0.033 J m–2 for MoS2 on SiO2 and 0.252 ± 0.041 J m–2 for MoS2 on Si3N4 are determined. Further, we show that selective desiccation of a bacterium under impermeable and flexible graphene via a flap-valve operation produces axially aligned graphene wrinkles of wavelength 32.4–34.3 nm, consistent with modified Föppl–von Kármán mechanics (confinement ∼0.7 × 4 μm2). An electrophoretically oriented bacterial device with confined wrinkles aligned with van der Pauw electrodes is fabricated and exhibited an anisotropic barrier (ΔE = 1.69 meV). Finally, we show that wrinkles can induce uniaxial strain, spatially reconfigured doping distribution, phonon softening (2 cm-1/% deformation for Raman E12g mode), and reduction of the optical bandgap (40∼60 meV/% deformation) in multilayer MoS2 flakes. A larger barrier (ΔEA= 106.6 meV) and a higher carrier mobility are exhibited in the MoS2 devices with wrinkles in the field-effect transistor studies. Further, a 1000-fold improvement in the on/off ratio and a 10-fold photocurrent enhancement over flat MoS2 devices are also observed in optoelectronic studies. This phenomenon is attributed to the exciton funneling and the built-in potential induced by bandgap-reduction and doping-variation in wrinkled devices.

History

Advisor

Berry, Vikas

Chair

Berry, Vikas

Department

Chemical Engineering

Degree Grantor

University of Illinois at Chicago

Degree Level

Doctoral

Committee Member

Liu, Ying Chaplin, Brian Yang, Zheng Huang, Jiaxing

Submitted date

December 2017

Issue date

15/11/2017

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