Ferroelectric bubbles constitute a fairly new scientific area which is concerned with the conservation of electric bubbles for advanced microelectronic and energy storage devices. These bubbles are very fragile and require specific materials as well as conditions to create a film which can hold and preserve them inside. The electric bubbles develop in a three-layer extremely thin structure having alternating electrical properties – ferroelectric, dielectric, and ferroelectric again. They are made out of specially ordered dipoles forming twinned electric charges. The orientation of these dipoles depends on the local strain in the material and surface charges which make the dipoles enter their relative lowest energy level. Electric bubbles form only when specific conditions are met and can be disturbed by even small forces.
Now (2021), scientists at Argonne National Laboratory have discovered a method to conserve these bubbles by removing heterostructure thin films with electrical bubbles inside from a substrate without destroying the bubbles. Since bubble growth was the most difficult task in the experiment, despite the large variety of possible substrate materials, the research in a first stage was aimed at finding methods and materials to ensure efficient bubble growth. The scientists grew the bubbles in a very thin heterostructure film on a strontium titanate substrate. Then, they had to come up with an effective method of removing the heterostructure from the substrate without disturbing the bubbles. This was no mean feat as the bubble domains were only about 4 nanometers in radius and therefore difficult to see. To solve this problem, advanced scanning probe microscopy techniques with Fourier transform analysis were used which allowed the scientists to make the bubbles visible as well as analyse their properties in the freestanding films. They also measured their electronic and piezoelectric properties using scanning microwave impedance microscopy and piezoresponse force microscopy to monitor the state of the bubble domains. In case of bubble disintegration, an applied voltage would have made the capacitance change; however, it remained quite stable up to a fairly high voltage. These experiments validated the theoretical analyses the scientists had carried out which combined atomistic simulations with circuit theory. When removing the bubbles, the heterostructure film suddenly assumed a rippled appearance. Apprehensions that this change would impair the properties of the bubbles were found to be ungrounded; on the contrary, a change in the built-in voltage of the material seemed to actually protect the bubbles. Atomistic simulations suggested that the elastic energy at the free interfaces was the origin of the ripple formation.
Especially in the field of solar photovoltaics the use of ferroelectric bubbles for energy production has attracted a lot of attention. In 2016, scientists used the ferroelectric insulator barium titanate to demonstrate how photogeneration and collecting hot, non-equilibrium electrons through the bulk photovoltaic effect (BPVE) could produce greater-than-unity quantum efficiency. The absorbing effect being less than a tenth of the solar spectrum notwithstanding, the power conversion efficiency of the BPVE device under 1 sun illumination was greater than the maximum theoretical efficiency of a solar cell for a material of this bandgap. In this study, data for devices consisting of a single-tip electrode contact and an array with 24 tips was accumulated. The study found that the BPVE at the nanoscale was an excellent method for obtaining high-efficiency photovoltaic solar energy conversion.
In 2018, scientists designed a new organic-inorganic hybrid bilayered perovskite ferroelectric ((C4H9NH3)2(NH2CHNH2)Pb2Br7) with single crystals which had an above-room-temperature Curie temperature of ~322 K. It was conspicuous that the quantum-well structure of this perovskite showed two-photon absorption properties with a nonlinear optical absorption coefficient larger than those of traditional all-inorganic perovskite ferroelectrics. Ferroelectric and two-photon absorption were also researched. The aim of the study was to design new ferroelectrics with two-photon absorption and research their potentials in the photonic application.
The findings might have great impact on the energy sector as these bubbles have unusual electrical and mechanical properties whose potential has yet to be fully understood. The newly discovered nanoscale objects can possibly find application in many different areas as transformation of these bubbles results in an unusually high electromechanical response, which could be used in a wide range of devices in microelectronics and energy applications.
Ferroelectric bubbles could also reshape our whole approach to building supercomputers or energy harvesters. Until then, however, a lot more research will have to be carried out and prototypes designed to test their potential in practice.
Recently, inventors have been increasingly interested in perovskite ferroelectrics. For example, this can be seen in patent application US20210202689A1: Ferroelectric capacitor and method of patterning such
“Ferroelectric capacitor is formed by conformably depositing a non-conductive dielectric over the etched first and second electrodes, and forming..... In some embodiments, thin layer (e.g., approximately 10 nm) perovskite template conductors such as SrRuO3 coated on top of IrO2, RuO2, PdO2, PtO2, which have a non-perovskite structure but higher conductivity to provide a seed or template for the growth of pure perovskite ferroelectric at low temperatures, are used as conductive oxides for BE2 103 and TE2 106”.
Among other inventions, attention should be paid to the following: CN111416006A Two-dimensional layered perovskite ferroelectric multifunctional film and preparation process thereof, CN111370579A Preparation method of metal organic hybrid perovskite ferroelectric film, CN110498680A The perovskite ferroelectric ceramics and preparation method and application of twin crystal grain particle diameter distribution structure, US20210202510A1 Integration method of ferroelectric memory array.