Program Highlights

Shared MRSEC Facility Strengthening Collaboration

Axel Enders, Sumit Beniwal, Haidong Lu, Peter Dowben,
Alexei Gruverman, and Chang-Beom Eom  
 Nebraska MRSEC

The MRSEC Thin Film Growth and Characterization Facility combines state-of-the-art tools for the fabrication and in-situ characterization of thin films, nano- and hybrid structures under ultrahigh vacuum conditions. The facility stimulates collaboration between MRSEC investigators to explore new materials and to discover novel phenomena at the nanoscale, by providing a unique suite of synthesis tools for a broad range of materials classes, and a comprehensive set of ultra-sensitive, high-resolution equipment to investigate their properties.

Recently, the Nebraska MRSEC researchers used the Facility to investigate the stability of the ferroelectric polarization of barium titanate films as thin as only a few nanometers. Shown below are piezoresponse force microscopy phase images, taken on a 48-unit-cell-thick (about 19 nanometers) ferroelectric film of barium titanate on a strontium titanate substrate in ultrahigh vacuum. The films have been polarized perpendicular to the surface in square-in-a-square geometry, and the polarization has then been probed as a function of time. The right image shows that the polarization pointing into the plane (the inner square) is unstable and disappears over the time of 5 minutes, due to a loss of stabilizing surface charges in vacuum. Dipolar adsorbates, such as quinonoid zwitterion molecules, can be added to the surface to study how such adsorbates would affect long-term stability and surface polarization of the ferroelectric layers. Experiments like these aide to study the role of surface charges in ferroelectric thin films, and the surface pyroelectric effect. The shared facility is key in enabling this collaboration, by bringing together PIs with expertise in oxide film growth, local probe microscopy, and interface physics to jointly work on this pressing scientific question of stability and scalability of ferroelectrics.

These programs are supported by the National Science Foundation, Division of Materials Research, Materials Research Science and Engineering Program, Grant 1420645.

In-situ piezoresponse force microscopy phase images of a polarization pattern electrically written in a thin BaTiO₃ film on the SrTiO₃ substrate. The image on the right was taken minutes after the image on the left was written and shows degradation of the remanent polarization of the inner square where the sample is polarized into the film plane, whereas outward polarization in the outer square appears more stable.