Ferroelectric materials, because of their robust spontaneous electrical polarization, are widely used in various applications. Recent advances in modelling, synthesis and characterization techniques are spurring unprecedented advances in the study of these materials. In this Review, we focus on thin-film ferroelectric materials and, in particular, on the possibility of controlling their properties through the application of strain engineering in conventional and unconventional ways. We explore how the study of ferroelectric materials has expanded our understanding of fundamental effects, enabled the discovery of novel phases and physics, and allowed unprecedented control of materials properties. We discuss several exciting possibilities for the development of new devices, including those in electronic, thermal and photovoltaic applications, and transduction sensors and actuators. We conclude with a brief survey of the different directions that the field may expand to over the coming years. Strain engineering can be used to control the properties of thin-film ferroelectric materials, which are promising for electronic, thermal, photovoltaic and transduction applications. This Review addresses fundamental aspects, novel ways to control materials properties and the development of new ferroelectric-based devices.

https://escholarship.org/content/qt51w3s3s1/qt51w3s3s1.pdf?t=p8f5p7

Thin-film ferroelectric materials and their applications

The past several decades have witnessed significant advances in the synthesis and applications of PIEZOelectric semiconductors, an important class of materials, including piezoelectric, pyroelectric, and ferroelectric semiconductors. The intriguing combination of physical and chemical phenomena in PIEZOelectric semiconductors has triggered much interest in PIEZOcatalysis, that is, catalysis enabled by PIEZOpotential (i.e., piezopotential, pyropotential, and ferropotential)-induced built-in electric fields, which is the focus of this Minireview. First, the PIEZOelectric materials are briefly introduced. Second, recent developments in PIEZOcatalysis are highlighted, including the introduction of representative PIEZOelectric semiconductors, their possible catalytic mechanisms, novel techniques to produce their PIEZOelectric effects during the catalytic process, and several examples of PIEZOcatalysis. Finally, the challenges in the field and exciting opportunities to further improve the PIEZOcatalytic efficiency are discussed.

Enabling PIEZOpotential in PIEZOelectric Semiconductors for Enhanced Catalytic Activities

Photocatalytic hydrogen evolution, as an environmental-friendly, energy-saving and simple process, has been widely reported. However, there are some important factors that prohibit practical application of photocatalytic hydrogen production, including the low efficiency of solar energy utilization, and the lack of response in the dark. Temperature fluctuation is a very common phenomenon in our daily life, which is rarely reported for use for hydrogen production. Here we demonstrate direct hydrogen evolution through the harvesting of cold–hot 298–323 K (25–50 °C) alternation energy by using pyroelectric nanomaterial Ba0.7Sr0.3TiO3 (BST). The yield of hydrogen achieved was 46.89 μmol per gram of catalyst after 36 thermal cycles and the rate of hydrogen generation was up to 1.30 μmol g−1 per thermal cycle. The minimum of the conduction band of BST is more negative than the conduction band level of H+/H2, enabling the reduction of water to form hydrogen. This efficient and eco-friendly pyro-catalytic method provides a promising future for hydrogen evolution utilizing natural cold–hot temperature fluctuation energy.

Pyro-catalytic hydrogen evolution by Ba0.7Sr0.3TiO3 nanoparticles: harvesting cold–hot alternation energy near room-temperature

Ferroelectrics, with their spontaneous switchable electric polarization and strong coupling between their electrical, mechanical, thermal, and optical responses, provide functionalities crucial for a diverse range of applications. Over the past decade, there has been significant progress in epitaxial strain engineering of oxide ferroelectric thin films to control and enhance the nature of ferroelectric order, alter ferroelectric susceptibilities, and to create new modes of response which can be harnessed for various applications. This review aims to cover some of the most important discoveries in strain engineering over the past decade and highlight some of the new and emerging approaches for strain control of ferroelectrics. We discuss how these new approaches to strain engineering provide promising routes to control and decouple ferroelectric susceptibilities and create new modes of response not possible in the confines of conventional strain engineering. To conclude, we will provide an overview and prospectus of these new and interesting modalities of strain engineering helping to accelerate their widespread development and implementation in future functional devices.

/pdf/new-modalities-of-strain-control-of-ferroelectric-thin-films-28aswamq6f.pdf

New modalities of strain-control of ferroelectric thin films

We summarize the recipes and describe the role of sputtering parameters in producing highly c-axis Aluminum Nitride (AlN) films for piezoelectric applications. The information is collated from the analysis of around 80 journal articles that sputtered this film on variety of substrate materials, processes and equipment. This review will be a good starting point to catch up with the state-of-the-arts research on the reactive sputtering of AlN (002) thin film, as well as its evolving list of piezoelectric applications such as energy harvesters.

Reactive Sputtering of Aluminum Nitride (002) Thin Films for Piezoelectric Applications: A Review.

Electric and pyroelectric properties of AlN thin films deposited by reactive magnetron sputtering on Si substrate

Intrinsic pyroelectric properties of thick, coarse grained Ba1−xSrxTiO3 ceramics

Epitaxial Pb(Zr0.2Ti0.8)O3 layers of good structural quality were grown on single crystal SrTiO3 substrates. The pyroelectric coefficient was estimated from the signal generated by the ferroelectric film working as a pyroelectric detector in the voltage mode, without pre-poling procedure. The obtained value is as high as 1.9 × 10−3 C/m2 K. The large value is attributed to the presence of 90° ferroelectric domains and to the compressive misfit strain, leading to an enhanced ferroelectric polarization.

Giant pyroelectric coefficient determined from the frequency dependence of the pyroelectric signal generated by epitaxial Pb(Zr0.2Ti0.8)O3 layers grown on single crystal SrTiO3 substrates

Effect of Li doping on the electric and pyroelectric properties of ZnO thin films

The role of Ag addition on the structural, dielectric, and mechanical harvesting response of 20%(xAg − (1 − x)BaTiO3) − 80%PVDF (x = 0, 2, 5, 7 and 27 vol.%) flexible composites is investigated. The inorganic fillers were realized by precipitating fine (~3 nm) silver nanoparticles onto BaTiO3 nanoparticles (~60 nm average size). The hybrid admixtures with a total filling factor of 20 vol.% were embedded into the PVDF matrix. The presence of filler enhances the amount of β-PVDF polar phase and the BaTiO3 filler induces an increase of the permittivity from 11 to 18 (1 kHz) in the flexible composites. The addition of increasing amounts of Ag is further beneficial for permittivity increase; with the maximum amount (x = 27 vol.%), permittivity is three times larger than in pure PVDF (εr ~ 33 at 1 kHz) with a similar level of tangent losses. This result is due to the local field enhancement in the regions close to the filler-PVDF interfaces which are additionally intensified by the presence of silver nanoparticles. The metallic addition is also beneficial for the mechanical harvesting ability of such composites: the amplitude of the maximum piezoelectric-triboelectric combined output collected in open circuit conditions increases from 0.2 V/cm2 (PVDF) to 30 V/cm2 for x = 27 vol.% Ag in a capacitive configuration. The role of ferroelectric and metallic nanoparticles on the increasing mechanical-electric conversion response is also been explained.

/pdf/increasing-permittivity-and-mechanical-harvesting-response-3aycme20.pdf

Mihaela Botea

Papers

Electric and pyroelectric properties of AlN thin films deposited by reactive magnetron sputtering on Si substrate

Intrinsic pyroelectric properties of thick, coarse grained Ba1−xSrxTiO3 ceramics

Giant pyroelectric coefficient determined from the frequency dependence of the pyroelectric signal generated by epitaxial Pb(Zr0.2Ti0.8)O3 layers grown on single crystal SrTiO3 substrates

Effect of Li doping on the electric and pyroelectric properties of ZnO thin films

Increasing Permittivity and Mechanical Harvesting Response of PVDF-Based Flexible Composites by Using Ag Nanoparticles onto BaTiO3 Nanofillers