Episode 10: Relaxor ferroelectric thin film characterized at the nanoscale

In this podcast episode, MRS Bulletin’s Sophia Chen interviews Lane Martin from Rice University about characterization of relaxor ferroelectrics, materials with noteworthy energy-conversion properties used in sensors and actuators. Martin’s research team investigated the material’s behavior at the nanoscale. The researchers found that the specific thin film they studied—the alloy lead magnesium niobate lead titanate—exhibited excellent properties down to 25–30 nm thick before they would start to shift. This work was published in a recent issue of Nature Nanotechnology.

Transcript

SOPHIA CHEN: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on hot topics in materials research. My name is Sophia Chen. Engineers have generally found it useful to make electronic devices smaller. Make the transistor smaller, and you can cram more of it on a chip and improve performance. Make sensors smaller, on the micron or nanoscale, and you can integrate more functionality in a small device.

LANE MARTIN: The smaller we can make them, the faster they can respond, the more different things we can do, the more we can stick them into your phone and all this kind of stuff.

SOPHIA CHEN: That’s Lane Martin, a materials researcher at Rice University. He’s studying a type of material known as relaxor ferroelectrics. Right now, researchers are interested in integrating relaxor ferroelectric materials into nanoscale devices.

LANE MARTIN: The problem has been, while we’re really good at making these micro- and nanoscale devices in general, and we take these materials and shrink them down to those sizes, they don’t work like they do in the when they’re big.

SOPHIA CHEN: Relaxor ferroelectrics are good for sensors because they are extremely responsive to many types of external stimuli. Specifically, its lattice structure changes drastically in response to pressure, temperature, and applied electric fields. This makes them useful, for example, for tiny accelerometers or actuators that could be put in medical devices such as ultrasounds and even microphones and sonar. But researchers found that as they shrunk relaxor ferroelectric samples down to about a micron, their properties would change.

LANE MARTIN: The problem was, as you shrink these things down, they typically get less good at doing their jobs.

SOPHIA CHEN: Researchers weren’t sure of the physics that caused the material to be less effective. So that’s what Martin and his colleagues wanted to understand.

LANE MARTIN: If we do shrink these things down, what are the things we're going to run into? What are the problems? What are the length scales that are really going to limit the fundamental function of these materials? That’s what we were trying to understand.

SOPHIA CHEN: To understand Martin’s recent study, let’s back up and explain what type of material relaxor ferroelectrics are. 

LANE MARTIN: If you take a relaxor and a ferroelectric and stick them together, you get a relaxor ferroelectric.

SOPHIA CHEN: For real. That’s what a relaxor ferroelectric is, a combination of a relaxor and a ferroelectric. So, what’s a ferroelectric? Martin explains it in analogy to a ferromagnet like the one that sticks to your refrigerator. If you zoom in on that fridge magnet, you’ll see little magnetic moments that are aligned with each other. Because the tiny domains are all aligned, they add up to an overall magnetic moment in the whole material. 

In the ferroelectric, instead of it being made of magnetic moments, they consist of electric charges that form tiny dipoles that all align with each other. 

LANE MARTIN: Here we have positive and negatively charged atoms that can slightly displace relative to each other, and they can create a little dipole in the system, right? So there’s a north end, south end, and you can have a little electric dipole at the unit cell level, the kind of basic Lego building block level of these materials.

SOPHIA CHEN: Those dipoles add up to create an intrinsic large-scale electric dipole moment in the material. So what’s a relaxor? A relaxor is a disordered cousin of the ferroelectric. Within it, some of its electric dipoles are aligned, but only on short length scales around 5 to 10 nanometers. They don’t line up en masse to create that large-scale dipole moment like in a ferroelectric. Without an applied field, the dipoles are not all aligned in the same direction. Also, the dipoles in a relaxor are transient and evolve with time and temperature. Now that we’ve covered the ferroelectric and the relaxor, let’s go back to the relaxor ferroelectric. When you combine the relaxor and the ferroelectric, the configuration of charges in each push and pull against each other. Martin explains. 

LANE MARTIN: The ferroelectric is trying to get all those polarizations to line up right. So it’s fighting against that disorder of the relaxor and saying, hey, let’s make a little ordered packet of stuff inside of here. And that gives you this kind of material that’s sitting right at the cusp of where it wants to be disordered and where it wants to be ordered. And then when you poke it, big changes can happen inside of this material.

SOPHIA CHEN: This culminates in tiny nanometer-scale domains of aligned dipoles. 

LANE MARTIN: They're kind of like long ellipsoidy shapes.

SOPHIA CHEN: The ellipsoids are around 5 to 10 nanometers along their short axis, and 20 to 30 nanometers along their long axis. The material they studied was an alloy called lead magnesium niobate lead titanate. 

LANE MARTIN: The way we would abbreviate it is we would say PMNPT.

SOPHIA CHEN: They created this PMNPT using a technique called pulsed-laser deposition, where they fired a UV laser at a target and the resulting material lands on a substrate to form a thin film. Then, they studied the material using several techniques including scanning transmission electron microscopy and x-ray diffraction. They studied samples of varying thicknesses, from 5 to 55 nanometers thick. They found that as the films became as thin as 25 to 30 nanometers, ellipsoid-shaped domains would start to become squeezed and rotate. The ellipsoids were about that length across along their long axis. 

LANE MARTIN: Right when the film thickness reaches that long axis thickness, or length of these materials, then it kind of rolls over and starts to go downhill in terms of its performance.

SOPHIA CHEN: Martin says that this work is significant because it deepens the field’s fundamental understanding of relaxor ferroelectrics.

LANE MARTIN: Now the implications of this are for what we can do with it in terms of future. How small of a device could we make from these materials and still have it function and behave the way we wanted to? Are there thickness regimes that we should focus on to get the best properties out of these materials?

SOPHIA CHEN: In future work, Martin plans to use the same fabrication techniques to build up heterostructures of this material along with others, to create thin films with desired properties. This work was published in a recent issue of Nature Nanotechnology. My name is Sophia Chen from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on X, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.