Nanoscale Ferroelectric Control of Novel Electronic States in Layered Two-Dimensional Materials (Final Report)

In this DOE Early Career project “Nanoscale Ferroelectric Control of Novel Electronic States in Layered Two-Dimensional Materials,” the PI’s group has combined ferroelectric field effect with nanoscale domain imaging and writing to design the electronic and optical properties of two-dimensional (2D) van der Waals materials, including graphene and transition metal dichalcogenides MoS2 and ReS2. The van der Waals materials have been prepared into field effect transistor (FET) devices with ferroelectric gates. Through domain patterning in a ferroelectric polymer PVDF-TrEF top-gate via conductive atomic force microscopy, the team has created programmable Schottky junctions in monolayer MoS2, where both barrier height and I-V rectifying polarity can be reconfigured. The transport anisotropy of monolayer to few-layer ReS2 has been mapped out by defining the entire channel into an insulating state and creating nanoscale conducting paths along different directions through domain writing in the ferroelectric top-gate. The result shows that the conductivity along and perpendicular to the Re-chain can differ by >5.5x104. Theoretical modeling points to the band origin of the transport anomaly and reveals the emergence of a flat band in few-layer ReS2. The interfacial epitaxial relation between ReS2 and PVDF-TrFE further promotes the formation of close-packed, highly ordered PVDF-TrFE nanowires with width of 35 nm and 10 nm. Nonvolatile modulation of quantum Hall effect has been achieved in graphene FETs with a ferroelectric oxide Ba0.4Sr0.6TiO3 back-gate. Scattering from the remote surface optical phonon in Ba0.4Sr0.6TiO3 limits the room temperature mobility of graphene to be about 3x104 cm2/Vs. Steep-slope switching has been achieved in MoS2 FETs back-gated by polycrystalline Pb(Zr,Ti)O3, which signals a static-state negative capacitance mode without involving an additional dielectric layer. Piezoresponse force microscopy studies show that the sub-threshold swing can be well correlated with the domain wall density in Pb(Zr,Ti)O3. The team also observes an unconventional filtering effect of the second harmonic generation response at the MoS2/Pb(Zr,Ti)O3 heterointerface, which can be accounted for by the alignment between one of the polar axes of MoS2 and the chiral dipole rotation at the surface of domain wall in Pb(Zr,Ti)O3. The research supported by this DOE grant has significantly advanced the fundamental understanding and functional design of ferroelectric/2D van der Waals heterostructures for their implementation towards energy applications.