Nanoscale Probing of Interlayer Interactions in 2D Materials and vdW Heterostructures

2D layered materials;vdW heterostructures;Nano-scale characterization;Interlayer van der Waals interaction;Interlayer electrostatic interaction;Molecular dynamics simulations;Mechanical Engineering;Engineering;Mechanical Engineering

Rokni Damavandi Taher, HosseinLiang, Xiaogan ;

University of Michigan

关键词: 2D layered materials;    vdW heterostructures;    Nano-scale characterization;    Interlayer van der Waals interaction;    Interlayer electrostatic interaction;    Molecular dynamics simulations;    Mechanical Engineering;    Engineering;    Mechanical Engineering;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/149883/rokni_1.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Two-dimensional layered materials (2DLMs), such as ultrathin graphite (G), hexagonal boron nitride (hBN), transition metal dichalcogenides (e.g., molybdenum disulfide MoS2), and their newly-emerged van der Waals (vdW) heterostructures with strong in-plane covalent bonding but weak interlayer vdW bonding exhibit a unique combination of high elasticity, extreme mechanical flexibility, visual transparency, and superior (opto)electronic performance, making them ideally suited to modern (bio/opto)electronic devices. Since intrinsic ultrahigh surface to volume ratio in 2DLMs dictates an extremely strong and dominant vdW force in many processes related to not only synthesis, transfer and manipulation of 2DLMs but also fabrication, integration and performance of 2DLMs-based devices, precise nanoscale quantification of their weak interlayer vdW bonding is of fundamental, theoretical and technological importance for the large-scale production of such promising materials and continued development of flexible and transparent electronics incorporating these materials. Although physical and chemical properties of similar and dissimilar 2DLMs associated with in-plane covalent bonds are well studied both experimentally and theoretically, an extensive atomic and nanoscale characterization of their interlayer vdW-dependent properties (e.g., interfacial charge transfer/distribution, interfacial adhesion energy, etc.) is still a great challenge due to the highly anisotropic nature of weak interlayer vdW interactions, the lack of precise experimental techniques to quantify such complex interlayer behavior and difficult mechanical manipulation of ultrathin and highly transparent 2DLMs. This dissertation aims to address these inherent challenges by developing a novel atomic force microscopy (AFM)-assisted manipulation setup with ultrahigh force-displacement resolution combined with multiscale modeling at atomistic and continuum levels to advance our fundamental understanding of nanoscale interlayer electrostatic and vdW behavior of 2DLMs and their heterostructures. I first develop highly‒efficient and clean plasma‒assisted exfoliation and nanoimprint‒assisted shear exfoliation techniques to produce large‒scale, ordered graphene and MoS2 device arrays at micro and nanoscale. Then, in order to gain a deeper insight into the interlayer vdW interactions of 2DLMs during the exfoliation process, the mechanical response of interlayer vdW interactions to external shear or normal forces is qualitatively studied by gently moving an in situ flattened, conductive AFM tip with an attached 2D crystal nanomesa away from the substrate in a direction parallel or normal to the basal plane of 2D crystals.To investigate the atomistic details underlying my AFM-assisted shear/normal electrostatic exfoliation mechanisms, the electrostatic response of interlayer vdW interactions of few-layer graphene (FLG) to the external electric field is first studied using DC electrostatic force microscopy (DC-EFM). Then, I exploit the layered nature of FLG to develop a novel spatial discrete model that successfully accounts for both electrostatic screening and fringe field effects on the charge distribution of FLG systems. Next I implement, for the first time, 3D spatial charge distribution of FLG (obtained from the proposed spatial discrete model) into molecular dynamics (MD) simulations to further gain an atomistic insight into the electrostatic shear/normal exfoliation mechanisms. As the last piece in the puzzle of understanding the interlayer behavior of 2DLMs, the first direct quantitative characterization of interfacial adhesion behavior of both fresh and aged vdW homo/heterointerfaces is reported under different temperatures and humidity conditions through very well-defined interactions between AFM tip-attached 2D crystal nanomesas and different 2D crystal and SiOx substrates. The temperature of nanocontact interfaces in the range of -15⎼300°C is precisely control by microheaters on the top and a cooling stage underneath the substrate.