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Energy-efficiency through new ultrathin ferroelectrics

Source: Cnm_argonne, CC BY-SA 2.0

 One of the greatest challenges nowadays is to develop energy-efficient electronics which can accommodate the requirements of modern electronic devices. As these devices have become ever smaller over the years the materials powering them have to become equally thinner without losing any of their efficiency. Ferroelectrics might present a solution to this problem. They are a class of materials in which some of the atoms are arranged off-center, which can cause a spontaneous internal electric charge or polarization. If exposed to an external voltage, the internal polarization of the material can be reversed. However, in conventional ferroelectric materials the internal polarisation is lost below around a few nanometers in thickness. Therefore, they are not compatible with current-day silicon technology.

Now (2022), a team of scientists at Argonne National Laboratory have created so far thinnest working ferroelectric managing to solve some of the aforementioned problems. The team of scientists found that an ultrathin layer of zirconium dioxide just half a nanometer thick displayed ferroelectricproperties, but only when it was grown particularly thin, approximately 1-2 nanometers in thickness. This marked a ground-breaking discovery as the material is not typically ferroelectric when in bulk form. Also, the scientists managed to switch the polarisation in the material back and forth when applying a small voltage.

In many other materials the ferroelectric behaviour is reduced if the material is only a few nanometres thick. In zirconium dioxide, however, a ferroelectric phase transition occurs even if it is thinner than two nanometres. This property might also apply to other fluorite-structured binary oxides. This approach which makes use of three-dimensional centrosymmetric materials reduced to the two-dimensional thickness limit, especially this model fluorite-structure system with its unusual ferroelectric size effects, offers substantial promise for electronics, demonstrated by proof-of-principle atomic-scale non-volatile ferroelectric memory on silicon.

To analyse and visualize the behaviour of the ultrathin ferroelectric device, the scientists used advanced photon x-ray diffraction, which gave them the insight into how this ferroelectricity emerged.

For many years, scientists have tried to improve the characteristics of ferroelectric materials. In 2017, scientists designed [(La0.9Sr0.1MnO3)n/(Pa0.9Ca0.1MnO3)n/(La0.9Sb0.1MnO3)n]m superlattices films and deposited them on Nb:SrTiO3 substrates using a laser molecular-beam epitaxy technology. Expected ferroelectricity occurred at low temperature. The superlattice was composed of transition metal manganite. Furthermore, the ferroelectric properties of the superlattices were improved by increasing the periodicity m, which may be attributed to the accumulation of the polarization induced by the frustration. The saturation magnetization and magnetic coercivity of films were found to have a strong periodic dependence. The research was able to verify previous theoretical studies of generating multiferroics experimentally and was a valuable contribution to designing or developing the novel magnetoelectric devices based on manganite ferromagnets.

Details about RHEED pattern and the sample structure. (a) RHEED pattern and intensity oscillations during deposition. Cross-sectional morphologies (b), macroscopic structure and FFT pattern (c) and surface (d) of the SL film

Source: Huanyu Pei, Shujin Guo, Lixia Ren, Changle Chen/ The Frustration-induced Ferroelectricity of a Manganite Tricolor Superlattice with Artificially Broken Symmetry/ Scientific Reports 7(1), December 2017/ DOI:10.1038/s41598-017-06640-y/ Open Source This is an Open Access article is distributed under the terms of the
Creative Commons Attribution 4.0 International (CC BY 4.0)

In 2020, scientists undertook the synthesis of a bis(diisobutyldithiophosphinato) lead(II) complex and its  application as a single source precursor for the nanostructured deposition of lead sulphide semiconductors. The synthesized complex was scrutinised using microelemental analysis, nuclear magnetic resonance spectroscopy, infrared spectroscopy and thermogravimetric analysis. This complex was then decomposed using the aerosol-assisted chemical vapour deposition technique at different temperatures in order to grow PbS nanostructures on glass substrates. These nanostructures were also analyzed by XRD, SEM, TEM and EDX methods. In a complex impedance plane plot, two relaxation processes were witnessed due to grains and grain boundaries contribution. A high value of dielectric constant was observed at low frequencies. These impedance spectroscopic results have corroborated the ferroelectric nature of the resultant PbS nanodeposition.

Complex impedance plane plot of PbS and Resistor-Capacitor circuit used for fitting

Source: Sadia Iram, Syeda Aqsa Batool, Azhar Mahmood, Effat Sitara/ Nanostructured Lead Sulphide Depositions by AACVD Technique Using Bis(Isobutyldithiophosphinato)Lead(II) Complex as Single Source Precursor and Its Impedance Study/ Nanomaterials 10(8):1438, July 2020/ DOI:10.3390/nano10081438/ Open Source This is an Open Access article is distributed under the terms of the
Creative Commons Attribution 4.0 International (CC BY 4.0)

The benefits of designing an ultra-thin ferroelectric component are evident: This work has a great technological impact, as it might spur the development of new two-dimensional materials. ​“Simply squeezing 3D materials to their 2D thickness limit offers a straightforward-yet-effective route to unlocking hidden phenomena in a wide variety of simple materials,” the scientists believe. ​This greatly expands the possibilities for materials design for next-generation electronics and can also include materials compatible with silicon technologies. Growing a few atomic layers of a 3D material can offer the potential for a new class of 2D materials surpassing the properties of conventional sheets of 2D materials like graphene.

The researchers are positive that this work will stipulate more research into two-dimensional 3D materials and reveal emergent electronic phenomena relevant for energy-efficient electronics. Considering the fact that conventional zirconium dioxide is already present in today’s state-of-the-art silicon chips this study offers substantial promise for energy-efficient electronics.