Structure Design and Properties Modification Based on Wrinkling of Two-Dimensional Nanomaterials
thesisposted on 08.02.2018, 00:00 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.