Scanning Probe Microscopy and Electrical Transport Studies of Ferroelectric Thin Films and 2D van der Waals Materials

Abstract

In this dissertation, I present the scanning microscopy and electrical transport studies of ferroelectric thin films and ferroic/2D van der Waals heterostructures. Based on the conducting probe atomic force microscopy and piezo-response force microscopy (PFM) studies of the static and dynamic behavior of ferroelectric domain walls (DW), we found that the ferroelectric polymer poly(vinylidene-fluoride-trifluorethylene) P(VDF-TrFE) is composed of two-dimensional (2D) ferroelectric monolayers (MLs) that are weakly coupled to each other. We also observed polarization asymmetry in epitaxial thin films of ferroelectric Pb(Zr,Ti)O₃, which is attributed to the screening properties of the underlying conducting oxide. PFM studies also reveal ferroelectric relaxor-type behavior in ultrathin Sr(Zr,Ti)O₃ films epitaxially deposited on Ge.

We exploited scanning-probe-controlled domain patterning in a P(VDF-TrFE) top layer to induce nonvolatile modulation of the conduction characteristic of ML molybdenum disulfide (MoS₂) between a transistor and a junction state. In the presence of a DW, MoS₂ exhibits rectified I_ds-V_ds (IV) characteristics that are well described by the thermionic emission model. This approach can be applied to a wide range of van der Waals materials to design various functional homojunctions and nanostructures.

We also studied the interfacial charge transfer effect between graphene and magnetoelectric Cr₂O₃ via electrostatic force microscopy and Kelvin probe force microscopy, which reveal p-type doping with up to 150 meV shift of the Fermi level. The graphene/Cr₂O₃ heterostructure is promising for developing magnetoelectric graphene transistors for spintronic applications.