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Integrated ferroelectric oxides for energy conversion devices

Periodic Reporting for period 2 - FERROENERGY (Integrated ferroelectric oxides for energy conversion devices)

Reporting period: 2021-02-08 to 2022-02-07

A common strategy used by materials scientists to explore and manipulate properties of crystalline systems consists on synthesizing nanoscale single crystal films, which allow for complete control over structure-properties relationships. This is especially relevant in systems showing polar order (e.g. ferroelectrics) where the electrical polarization is directly connected to structural distortions. So far, this has been a very successful strategy to enhance materials’ susceptibilities, produce devices with novel functionalities and discover emergent phenomena, however this strategy imposes several restrictions:

- Strain tunability: The accessible lattice deformations that one can induce in the lattice of ferroelectric films is limited by the available single crystal oxide substrates for coherent growth.

- Integrability: The properties and devices fabricated on the growth substrates are in general not possible to implement on other substrates more convenient for technological applications (such as silicon or flexible polymers)

- Mechanical clamping: The strong bonding between the substrate and the film negatively affects the intrinsic functionality of the latter. In ferroelectrics, this increases the power and time needed to switch polarization, and decreases their piezoelectric response, limiting the performance in memory devices and nanoelectromechanical systems.

- Thermal connection with the substrate: The much larger thermal mass of the substrate causes the ultrafast thermalization of the films, preventing the direct measurement of temperature changes caused by external stimuli, and therefore thermal energy conversion capabilities can only be explored via simulations or indirect measurements.

The objective of FERROENERGY is to exploit a new methodology to produce freestanding single crystal nanomembranes of complex oxides that allow overcoming these limitations and expand the opportunities of ferroelectric oxides as next-generation materials for electronic devices and for energy conversion at the nanoscale.
During the initial period of the action, several oxide perovskites were synthesized as thin films and membranes integrated in silicon and flexible polymers. Five lines of work were devised from here:

1 - Strain engineering of ferroelectric membranes: Demonstration of the manipulation of structure and ferroelectric properties of the membranes by using the interlayer stress. This led to ferroelectric capacitor devices integrated on silicon and flexible polymer platforms with strain-engineered properties (Fig.1) showing ultrafast, low-voltage switching operation on silicon, and large dielectric tunability under low stress application on polymers. This work led to two high-impact publications in Advanced Materials and Nature Communications journals, and was presented in a talk by the Experienced Researcher (ER) at the ISAF-ECAPD conference 2021.

2- Strain gradient engineering of ferroelectric membranes: Studies of temperature dependence of flexoelectric coefficient were performed and partially presented in a PhD dissertation thesis from Stanford University, and synthesis of membranes with corrugated (wrinkled or buckled) structures was developed (Fig. 2). The effect of large local curvatures on the ferroelectric domain configuration is currently under exploration, in collaboration with Oak Ridge National Lab (ORNL), and work related to this research will be disseminated in future conferences during 2022.

3- Exploring the microscopic ordering physics of ferroelectrics with complex microscopic order: Nanometric membranes of archetypical ferroelectrics (BaTiO3), ferroelectric relaxors (PMN-PT) and antiferroelectrics (PbZrO3) were transferred to grids for examination in Transmission Electron Microscopy (TEM). These have allowed performing imaging with atomic resolution in the membranes and study the intricate microstructural arrangements of these materials produced by the spontaneous deformations of the membranes (Fig.3).

4- Direct measurement of electrocaloric effects on ferroelectric membranes. Fabrication of suspended capacitor structures failed due to unavoidable mechanical/electrical failure of the devices. As a contingency plan, membranes were transferred to nanocalorimetry chips fabricated by the Group of Thermal Properties of Nanoscale Materials, at ICN2 (Fig.4) allowing for heat capacity measurements on BaTiO3 membranes, which is a milestone result for the further application of this technique on electrocaloric measurement of ferroelectric capacitors.

5– Optomechanical effects on ferroelectric membranes: As an alternative to electrical actuation on suspended membranes, photoexcitation was used. A large mechanical deformation in response to near-UV laser excitation was found, by using an interferometric microscope at ICN2. The physical origin of this enhanced response is currently under investigation.

Further dissemination actions include the publication of two topical review articles on the recent developments of ferro-/pyro- electrics in synthesis, processing, and computational modeling. A third invited review article on recent advances on complex oxide membranes is expected to be published during 2022. The ER also participated in numerous seminars (at University of California Berkeley, Lawrence Berkeley National Lab and at ICN2) and attended exhibitions (e.g. Cal Day) and virtual workshops (e.g. Quorom, INTERSECT2021, III CANN 2021), discussing the topics related to the project with fellow researchers, technological partners and broader audiences.
The ER has developed beyond-the-state-of-the-art tools to investigate and manipulate complex materials in single-crystalline form at the nanoscale. The results obtained and published during the MSCA have demonstrated:

1 - A high-yield fabrication method to produce high-quality single-crystal membranes of ferroelectric oxides, of high interest to the researchers working in the oxide perovskites community.

2 - The excellent performance of CMOS-integrated ferroelectric capacitor devices. In particular, ultrafast operation and low power consumption, making these prototypical devices highly appealing for next generation non-volatile storage devices.

3 - Highly-efficient stress manipulation of dielectric properties in ferroelectric devices integrated on flexible polymer substrates, a promising path for applications on nanosensors.

The research of single crystal oxide nanomembranes is still under intensive development, and projects such as FERROENERGY are contributing to push forward the research in this emerging field. FERROENERGY has demonstrated that research can progress in many directions in this field, with both application-oriented studies, that could in the future develop into spin-off projects with stronger connections with industrial partners, and fundamental research projects that will contribute to expanding the knowledge in the physicochemical mechanisms governing the properties of complex oxide materials.

FERROENERGY has also built an international collaborative network involving groups in Spain (ICN2, ICMAB, IMA, IMB-CNM), United States (University of California Berkeley, Stanford University, ORNL), France (CEMES Toulouse, Synchrotron SOLEIL) and Switzerland (EPFL Lausanne), that, coordinated from ICN2 and with the support of several National and EU-funded projects will continue to push the frontiers of knowledge in the area of nanoscale complex oxide systems.
Strain-engineering of dielectric properties in integrated single-crystal ferroelectric membranes
Topography of a BaTiO3 membrane on Silicon showing corrugation (buckling) patterns
BaTiO3 membrane transferred to a Pt heater/sensor pad used for nanocalorimetry measurements
BaTiO3 flake transferred to a SiNx holey membrane support and top-view TEM image