Nanoscale ferroelectrics offer the potential for improved devices such as a digital memory medium. They also prompt a number of fundamental physical questions based on the influence of size on the properties of ferroelectric materials. The work within the Functional Materials group is focused on the characterisation of materials at the nanoscale as well as understanding the metrology employed to investigate ferroelectric materials at this scale.

Standardisation of nanoscale techniques

Piezoresponse Force Microscopy (PFM) is already widely used as an imaging technique, mapping piezoresponse of samples. However in most cases, the piezoresponse is represented with an arbitrary unit scale. Static measurements (non-scanning) of the piezoelectric properties, typically in the form of hysteresis loops, can also be made with this technique. This work aims to investigate many of the challenges facing the quantification of PFM measurements by conducting an inter-laboratory comparison. It is intended that this work will lead to future work that will provide a standard method for the quantification of the piezoresponse with a view to accurately determining the electromechanical coupling of the sample. This will allow for results from different sources to be reliably compared and could also provide a reference measurement for PFM imaging..

More details:

• Performance Related Properties for Electroceramics (VAMAS)
• Correlation of electron backscatter diffraction and piezoresponse force microscopy for the nanoscale characterization of ferroelectric domains in polycrystalline lead zirconate titanate (NPL publication)

Epitaxial Lead Zirconate Titanate (PZT) thin film showing Piezoresponse Force Microscopy (PFM) image showing topography (left), vertical PFM amplitude (middle), and vertical PFM phase (right). PFM images show square regions that have been poled with the AFM probe.

Measurements on nanoscale ferroelectric materials

Our research in this area is concerned with pushing the resolution and exploring quantification of microscopy measurements of ferroelectric materials with nanoscale domain structure. We are exploring the effect of such external influences as size, temperature and stress on the configuration of the domain structure. Our high speed electrical pulse measurements are designed to explore the temporal response of thin film ferroelectric materials to electrical pulses with few picosecond rise times.

The measurements of nanoscale ferroelectrics we are currently using include:

• Microscopy based analysis of nanoscale domains in ferroelectric materials
  • Scanning probe microscopy
  • Scanning electron microscopy including electron backscatter diffraction, backscatter and secondary electron imaging
• High speed electrical pulse measurements on ferroelectric thin films

Modelling of nanoscale ferroelectric materials

Modelling areas include:

• Analytical descriptions of fundamental ferroelectric performance
• Atomistic modelling of defects in ferroelectric materials: in particular, the effect of oxygen vacancies on the polarisation and domain configuration
• Atomistic modelling of nanoscale ferroelectrics including:
  • The investigation of size and shape effects on polarisation
  • The influence of edges
  • The influence of defects on domain configuration in nanoscale ferroelectrics

Diagram of the embedded cluster model used at NPL for the simulation of nanoscale ferroelectrics. Includes a quantum cluster embedded in a classical superstructure to enable quantum mechanical description of appropriately sized nanoscale structures.

People

Anna Kimmel (Modelling)

Publications

• Correlation of electron backscatter diffraction and piezoresponse force microscopy for the nanoscale characterization of ferroelectric domains in polycrystalline lead zirconate titanate
  T. L Burnett, P. M. Weaver, J. F. Blackburn, M. Stewart, M. G. Cain
  J. App. Phys., 108 (4), 042001 (2010)
  doi: 10.1063/1.3474940