Creating Polarization Gradient to Expand Ferroelectric Thin Film Application Range

For the first time, researchers have developed a polarization gradient in a thin film which will assist in largely expanding the functional temperature range for ferroelectrics, an important material employed in different everyday applications.

Researchers from the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have described their successful research in a paper published in the May 10th issue of the journal Nature Communications. The study opens the door for producing devices with the ability to support wireless communications in intense environments such as polar regions of Earth and inside nuclear reactors.

Ferroelectric materials are highly valued due to their spontaneous polarization, which can be reversed by applying an electric field and also due to the potential to minimize electric charges on application of physical pressure. Besides functioning as transducers, capacitors and oscillators, they can also be used in applications such as ultrasound imaging, transit cards and push-button ignition systems.

Scientists at the Berkeley Lab developed a strain and chemical gradient in a 150-nm barium strontium titanate thin film. Barium strontium titanate is an extensively used ferroelectric material. The researchers could make direct measurement of the tiny atomic displacements in the material by employing cutting-edge advanced microscopy at Berkeley Lab, thus discovering gradients in the polarization. The polarization ranged from 0 to 35 µC/cm2 throughout the thickness of the thin-film material.

Tossing out textbook predictions

Traditional physics and engineering textbooks wouldn’t have predicted this observation. Creating gradients in materials costs a lot of energy—Mother Nature doesn’t like them—and the material works to level out such imbalances in whatever way possible. In order for a large gradient like the one we have here to occur, we needed something else in the material to compensate for this unfavorable structure. In this case, the key is the material’s naturally occurring defects, such as charges and vacancies of atoms, that accommodate the imbalance and stabilize the gradient in polarization.

Lane Martin, Faculty Scientist, Berkeley Lab's Materials Science Division

Developing a polarization gradient proved advantageous as it expanded the temperature range for ideal performance of the ferroelectric material. The functioning of barium titanate is largely dependent on temperature, where the effects are comparatively small at room temperature and a sharp, large peak is observed at nearly 120 °C. Hence it is difficult to accomplish an optimally controlled, dependable function when the temperature varies more than a rather narrow gap. Therefore, to enable the material to be used for applications performed at and near ambient temperature, researchers fine-tune the chemical properties of the material, despite which the range of temperatures in which the material is functional remains comparatively narrow.

The new polarization profile we have created gives rise to a nearly temperature-insensitive dielectric response, which is not common in ferroelectric materials,” stated Martin. “By making a gradient in the polarization, the ferroelectric simultaneously operates like a range or continuum of materials, giving us high-performance results across a 500-degree Celsius window. In comparison, standard, off-the-shelf materials today would give the same responses across a much smaller 50-degree Celsius window.”

Exceeding the apparent expansions to colder and hotter environments, the scientists observed that the expanded temperature range can reduce the number of components required in electronic devices and can possibly minimize the power consumption of wireless phones.

The smartphone I’m holding in my hand right now has dielectric resonators, phase shifters, oscillators—more than 200 elements altogether—based on similar materials to what we studied in this paper. About 45 of those elements are needed to filter the signals coming to and from your cell phone to make sure you have a clear signal. That’s a huge amount of real estate to dedicate to one function.

Lane Martin, Faculty Scientist, Berkeley Lab's Materials Science Division

Since the resonance of the ferroelectric materials gets modified due to changes in temperature, constant adaptations are performed to match the materials with the wavelength of the signals transmitted from cellular towers. In order to tune the signal, power is required. The more out of tune the signal is, then more power is required by the phone to obtain a clear signal for the caller. A material that has a polarization gradient capable of operatingover large temperatures regimes can minimize the power required to tune the signal.

Faster detectors enable new imaging techniques

Obtaining an in-depth knowledge of the polarization gradient involved the use of epitaxial strain—a technique that involves growing a crystalline overlayer on a substrate, but with a mismatch in the lattice structure. This strain engineering method is generally used in the production of semiconductors and assists in regulating the structure and improving performance of materials.

Latest developments in electron microscopy have enabled scientists not only to acquire atomic-scale structural data of the strained barium strontium titanate but also to perform direct measurement of the strain, as well as the polarization gradient.

We have established a way to use nanobeam scanning diffraction to record diffraction patterns from each point, and afterwards analyze the datasets for strain and polarization data. This type of mapping, pioneered at Berkeley Lab, is both new and very powerful.

Andrew Minor, Director of the National Center for Electron Microscopy at Berkeley Lab's Molecular Foundry

According to Minor, one more important factor is the detector’s speed. In the case of the Nature Communications paper, data were acquired at a rate of 400 frames per second, which is faster than the rate of 30 frames per second achieved only a few years earlier. This method is now accessible for users at the Foundry.

We’re seeing a revolution in microscopy related to the use of direct electron detectors that is changing many fields of research,” stated Minor, who is also a UC Berkeley professor of materials science and engineering. “We’re able to both see and measure things at a scale that was hard to imagine until recently.”

Postdoctoral researcher Anoop Damodaran and graduate student Shishir Pandya from UC Berkeley’s Department of Materials Science and Engineering are the co-lead authors of the paper. Other study co-authors of the paper are researchers from the University of Pennsylvania, the Carnegie Institution for Science and Rutgers University.

The DOE’s Office of Science, the Army Research Office, the National Science Foundation, the Gordon and Betty Moore Foundation and the Carnegie Institution for Science supported this research.

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