Currently, the research activities of our group are focused on non-linear dielectric thin films. We study nanometer-size ferroelectric single-crystals from various viewpoints: microstructure of nanometer-size ferroelectric domains, effect of crystal size on ferroelectric behavior, nucleation and growth inside nanometer diameter alumina pores, thermodynamic stability, phase transitions, dielectric behavior, polarization mechanisms, piezoelectric response, and pyroelectric currents.
In our lab we grow nanometer-size single-crystals inside a highly-dense array of alumina nano-pores. The single-crystals are grown towards a preferred crystallographic orientation under a tight control of temperature, composition, and applied electric field. The crystals nucleate from supersaturated liquid solutions at the bottom of the pores. The surface morphology and microstructure of the nanometer single-crystals are characterized using FIB and HRTEM techniques on vertical cross section samples. The crystallographic phase and orientation of the crystals are determined using XRD analysis. The composition and nature of chemical bonds are identified by AES, SIMS and FTIR. The dielectric properties are measured using LCR impedance analyzer and Radient instruments. The piezoelectric response and coefficient are determined by measuring the change of the sample radius of curvature as a function of applied electric field in-vertical to the film plane. The pyroelectric currents are measured by exposing the sample to a variable temperature cycle.
The achievements of two of our recent projects are detailed below.
Project 1: Ferroelectric nano-domains in ultra-thin films.
We have discovered multi-nanodomains in ultra-thin BaTiO3. The ferroelectric nano-domains have a width as small as one-unit cell. This finding, which was published in the Journal of Applied Physics, 94 (10), 6774-6778 (2003), violates two well established (theoretically and experimentally) scientific concepts. It violates the concept of a critical size below only single-domains exist. It also violates the concept that below critical size ferroelectricity disappears. The discovery of nano-domains in ultra-thin films opens the route for future ferroelectric nanometer-sized devices.
Project 2: High-density array of nano-ferroelectric domains having uniform polarization orientation.
We have fabricated and characterized a high-density array (1011 cm-2) of nano-ferroelectric single-crystals (20-60 nm in diameter) grown inside alumina nano-pores. The crystals have nano-domains oriented along the longitudinal axis of the pores and consequently uniform polarization direction in-vertical to the film surface. This nano-composite structure has enhanced polarization values and thermal stability relative to bulk-size crystals. The results of this study, which were published in Phys. Rev. B, 71, 184112 (2005), can be implemented in future high-resolution and high-sensitive ferroelectric, pyroelectric and piezoelectric devices.