Investigation of electrical, optical and mechanical control of ferroelectric domains using scanning probe microscopy

Abstract

This thesis compiles the applications and nanoscale understanding of electrical, optical and mechanical manipulation of ferroelectric domains in ((K0.5Na0.5)NbO3–2 mol% Ba(Ni0.5Nb0.5)O3−δ) (KNBNNO). KNBNNO is a novel bandgap (1.6 eV) engineered ferroelectric with an excellent piezo-response (100 pm/V). This made it interesting for light-dependent nanoscale investigations using scanning probe microscopy. The light is found to work analogous to an applied electric field and the charge carriers generated by the presence of light are found to be compensated by the movement of ferroelectric domains. The domain wall velocities achieved by the exposure to the light source are of the order of 0.01 nm/s. To make KNBNNO viable for practical applications, the domain wall velocities were further enhanced to 30,000 nm/s by the cumulative effect of light and low electric fields (< 4 kV/mm). The light illumination on this material is found to tune the material’s electrical conductivity by orders of magnitude and the charge injection due to illumination is governed by the ferroelectric photovoltaic effect. KNBNNO is also found to illustrate mechanical switching under atomic-force microscopy (AFM) tip pressure (> 0.4 μN). It is found that tip pressures higher than 3 μN can cause permanent deformation of the sample surface. The optical response of the materials is found to be influenced under the mechanical loading via an atomic force microscope tip. The presence of multiple domain switching mechanisms in KNBNNO makes it interesting for an ample spectrum of applications (neuromorphic computing and solid-state energy conversion) and to develop a fundamental understanding of more complex possible mechanisms. Based on the understanding gained, the phenomenon of electro-optic control of ferroelectric domains is utilized to modulate photo- and pyro-currents. Using this, a prototype monolithic light-effect transistor is presented. This could be a potential solution to the scaling limit of three-terminal transistors. In addition, two novel energy conversion cycles (Opto-electric and thermo-opto-electric) are proposed. Finally, the study is concluded with a hope to motivate the scientific community for utilizing the cumulative effect of light, electric field and mechanical force for novel devices and applications.