Introduction

Ferroelectric materials are part of the piezoelectric materials group, which are non-centrosymmetric dielectrics. All ferroelectrics can be considered as piezoelectric materials but not all piezoelectric materials are ferroelectrics as visualized in Figure1.

Piezoelectric materials can convert mechanical energy into electricity when the material is subjected to for example compression stretching or bending due to the rearrangement of the domains within the material.

This conversion of energy can occur the other way around, electrical energy via an external electrical field will be converted into mechanical energy. Piezoelectric materials are used in many applications such as capacitors, sensors, and microphones.^[1]

Figure1. Non-centrosymmetric dielectrics, piezoelectric group (Figure: Azad Karis).

Ferroelectric materials have this unique property, where the polarization of the dielectric material remains permanently within the material even when the external electric field is removed. This nonlinear polarization will continue in the same direction, and it can be reversed with an external electrical field.

Some examples of ferroelectric materials are polyvinylidene fluoride (PVDF), Lead-Zirconium-Titanium oxide (PZT) thin films^[1][2]and liquid crystals, more specifically chiral smectic liquid crystals such as cholesteryl-benzoate and DOBAMBC (p-decyloxybenzylidene p′-amino 2-methyl butyl cinnamate).^[3]

Measurement

The ferroelectric domain patterns can be imaged with Piezoresponse Force Microscopy (PFM). The PFM can be used to determine the topography, polarization direction, and physical displacement of the material.

This method is preferable compared to transmission electron microscope or scanning electron microscope for imaging the domains, because PFM is more versatile, easy-to-handle and the sample doesn’t require specific preparation.^[4]A schematic of the PFM is shown in Figure 2.

Figure 2. Schematic of the Piezoresponse Force Microscopy set-up. (Figure: Azad Karis)

The imaging of the domains is achieved by applying voltages, both AC and DC to the metalized tip and bringing the tip into contact with the surface of the material. The material is also being applied by a voltage (DC).^[5]

The PFM amplitude provides information on the magnitude of the local electromechanical coupling while the PFM phase image gives the local polarization orientation.

The orientation of polarization within the domains can be observed from the obtained data by using the vertical or lateral PFM imaging methods:

• Vertical (out-of-plane signal): the alignment of the electric field and polarization orientation will result in either expansion (positive deflection) or contraction (negative deflection) of the domain. These changes are detected by the photodiode via the tip.
• Lateral (in-plane signal): Electric field is applied orthogonal to the polarization which results in a torsional deformation of the tip which forces the laser spot to move horizontally. This movement is caused due to the shearing of the material.^[1] 

PFM example

Tao et al.^[6]fabricated a ferroelectric BZT-0.5BCT thin film by RF magnetron sputtering on Pt/Ti/SiO2/Si substrates using ceramic targets of Ba(Zr0.2Ti0.8)O3–0.5 Ba0.7Ca0.3TiO3. Different distances (d) of 6 cm, 6.5 cm, and 7 cm between the substrates and the target were used to synthesize the thin film to study the distance effect on the domains’ orientation.

The samples were kept at 500 °C during the synthesis and they were later annealed for 1 h at 700 °C. Film thickness was measured to be approximately 100 nm when d=6.5 cm.  Tao et al. performed PFM on the three samples to study both amplitude and phase of the thin film. The results are shown in Figure 3; amplitude (a) and phase (b) of the sample with d=6 cm, (c) and (d) for d=6.5 cm, (e) and (f) for d=7 cm, respectively. According to Tao et al., the distance between the substrate and the sputtering target of 6.5 cm gives the most uniform and ordered domain structure.

Figure 3. PFM results of amplitude (a) and phase (b) of the sample with d=6 cm, (c) and (d) for d=6.5 cm, (e) and (f) for d=7 cm, respectively. (License: CC BY-NC-ND 4.0)^[6]

References