Exploration of Novel Functional Properties in Ferroelectrics via Scanning Probe Microscopy

Scanning Probe Microscopy (SPM) has emerged as an essential instrument to probe and characterise materials, correlate local responses spatially, and manipulate and write features directly. With the development of SPM techniques, ferroelectrics at the nanoscale have attracted great interests in the past decade to exploit their novel properties and functionality for a wide range of applications based on photovoltaic, nanoelectronics and micromechanical systems. Of various characterisation tools, SPM techniques are effectively used to probe mechanical and electrical properties in a variety of ferroelectrics. Initially, the mechanical force of AFM tips is used to study and control phase transition behaviour of a multiferroic material, BiFeO3 (BFO). Force-Distance (F-D) spectroscopy is also employed to measure mechanical properties and softening behaviour during phase transformation between T- and R- phase in highly strained BFO thin films grown on LaAlO3 (LAO) substrate. Extraordinary softening behaviour, compared to bulk BFO, is observed in this BFO film. This work is followed in the latter chapter by extending the force based techniques to investigate elastic properties of doped BFO samples. Force-volume mapping technique is used along with single-point F-D curve measurements to reveal statistics of elasticity between undoped, Ca-doped and La-doped BFO thin films. These results reveal tuneable elasticity of BFO thin films as a result of chemical doping. Structural and electronic properties of a ferroelectric BaTiO3-δ are also studied by several techniques of SPM. By an after-growth oxygen cooling process, oxygen deficient barium titanate are characterised by PFM and c-AFM, showing strong correlation between oxygen vacancy and electronic conduction and resistive switching properties. Pulsed-mode current-voltage (I-V) spectroscopy measurements are shown to evaluate fatigue behaviour of resistive switching. These results highlight the use of robust PFM and c-AFM techniques with pulse mode I-V measurements to fully characterise electronic properties and the functionality of oxygen deficient ferroelectric thin films for promising storage memory applications.Extensive studies of electronic properties and conduction at domain walls in tensile strained BFO grown on GdScO3 (GSO) are carried by SPM along with X-ray diffraction (XRD) and Scanning Transmission Electron Microscopy (STEM) analysis. Enhanced domain wall conduction is observed from c-AFM images in comparison to PFM images. Moreover, temperature dependent c-AFM images conclude that the electric transport in the material is thermally activated for phase boundaries between R- and O- phase. STEM observations reveal structurally wide boundaries as a pivotal point for the enhanced conduction at the boundaries. These results open new pathways of using phase boundaries in this system for nanoelectronic applications.Overall, these studies suggest that SPM can provide highly detailed information and a deep insight into novel properties in ferroelectrics, experimentally evaluate theoretical modelling and simulations and their applicability in nanoscale regimes and develop newly emerging paradigms that consider these materials as active elements for promising nanoscale devices