Interfacial Properties of Graphene and 2D Materials Heterostructures Investigated by Scanning Probe Microscopy

Abstract

<p>2D materials, e.g., graphene, and heterostructures have extraordinary properties compared to their 3D counterparts, and have great potential for a broad range of applications, including flexible electronic devices, nanocomposites, and transistors. However, in most of these applications the 2D materials need to interface with other materials such as substrates or other 2D heteroststructures for not only device functionality but also mechanical stability. The interfacial properties of 2D materials and heterostructures greatly affect the performance of these 2D materials-based devices and thus call for further investigation. </p><p>In this dissertation, advanced scanning probe microscopy (SPM) techniques, including contact resonance atomic force microscopy (CR-AFM) and piezoresponse force microscopy (PFM), are applied to study the interfacial mechanical and piezoelectrical properties of graphene and 2D materials heterostructures. For the first time, CR-AFM is demonstrated with the sensitivity to local stiffness changes that arise from a single atomic layer of a van-der-Waals-adhered material. To this end, a new approach, combining CR-AFM with first-principles calculations and continuum mechanics modeling, is introduced, which can yield a quantitative subsurface atomic structure fingerprint for 2D materials and heterostructures, as demonstrated on an ideal model system – epitaxial graphene on SiC (0001). This model system is further investigated with PFM, which revealed a new source of piezoelectricity in graphene layers that arises from the presence of interfacial dipole moments induced by the polarization in the substrate. The last part of the dissertation discusses the interfacial mechanical properties of graphene deposited onto self-assembled-monolayers (SAMs). CR-AFM experiments and molecular dynamics (MD) simulations show that the surface energy of the SAM strongly affects the amount of water molecules present at the graphene-SAM interface, which in turn influences the elastic modulus of these graphene-SAM heterostructures. The SPM methods used in this dissertation can provide rich structure-property information about interfaces and surfaces, and can be used to understand other interfacial problems of fundamental and practical interest in 2D materials and heterostructures, such as nanoconfined water and 2D layered hybrid organic-inorganic perovskites.</p>