ICMCTF 2023 Session C3-2-FrM: Thin Films and Novel Surfaces for Energy II

Ferroelectric (FE) materials such as BaTiO₃, KNbO₃, and Pb(Zr,Ti)O₃ have dipole moments aligned to one direction. FEs have been already industrially applied as capacitor, piezoelectric actuator, filters, optical devices, and so on. However, antiferroelectric (AFE), such as PbZrO₃, AgNbO₃, and NaNbO₃, has two sublattices of the dipole moments, and each sublattice have dipole moments with opposite directions. As a whole, AFE does not have a net polarization and is structurally centrosymmetric. Under a smaller external electric field (E), AFE shows a linear response of electric displacement (D) against E like a paraelectric. Applying larger E causes flipping of dipole moments in one sublattice parallel to E. This constrained aligned dipole moments in one direction are comparable to a FE state that has a net polarization. However, once E is decreased, dipole moments in one sublattice flip to the original direction and consequently the structure returns to an original centrosymmetric and non-polar state. Utilizing such a characteristic of AFEs under E, a field-induced FE, they can be applied for the energy storage especially for the power devices, utilizing two step D-E responses of a linear one near the origin followed by a hysteresis loop at larger E.

Design of FE is easier compared to AFE because of the rich accumulated scientific knowledge of FE both in the structure and property during last 100 years. However, chemical design of AFE is still difficult due to the reason that the number of AFEs is limited compared to that of FEs. This study is going to try giving a comprehensive explanation on the phase stability of complex oxide materials including perovskites in the bulk and thin film including metastable state. Candidate compounds of AFE will be discussed for various phases in A₂O₃ and ABO₃.

The reactive sputtering process is widely used at the academic and industrial scales to deposit thin films of various nitrides, oxides, oxynitrides… For some materials, the crystallization of the film can occur even when the deposition temperature is fixed at room temperature (Cu₂O, ZnO, TiN, …). Nevertheless, other materials require to heat the substrates during the deposition to obtain a crystallized film (VO₂, LaCoO₃, Si₃N₄, …).

Zinc germanium nitride (ZnGeN₂) exhibit optoelectronics properties that makes this abundant material an interesting compound to replace the indium and gallium nitrides for LEDs applications. As previously mentioned by several groups, the deposition of crystalline ZnGeN₂ using sputtering processes can only be achieved by heating the substrate during the growth.

A new approach to describe the mechanism of growth induced by the temperature during reactive co-sputtered ZnGeN₂ deposition is reported. The complex crystallization of this system requires the deposition of films at high temperature involving difficulties in controlling the stoichiometry of the films due to the evaporation of zinc atoms relative to germanium at these temperatures. The range of temperature considered in this work is between 70°C to 280°C. The study of this material is done at different scales and a model is proposed to bring answers to the observations made. X-ray/electron diffraction, transmission electron microscopy combined with in-depth SIMS analyses reveal the origin of two different zones of composition and crystallization. At a temperature above 130°C, the first step of growth is described by an under-stoichiometry of Zn entailing an amorphous phase. The second zone appears after an increase of the zinc content makes it possible to obtain a stoichiometric crystallized material. Hypotheses are announced, supported by calculations of sputtered zinc atom energy and zinc vaporization temperature in the present conditions of growth. The model provides a good agreement with the observation of the difference of composition for films deposited at temperatures above 130°C in our sputtering process. In this paper, an original work on the phenomena taking place at different stages of the growth of ZnGeN₂ thin films is described and gives new elements of understanding to the previous studies in the literature.

Taking inspiration from the electrocytes in the electric eel, consisting of numerous “subnanometer-scale” rectified ion channels allowing unidirectional and amplified ion transport, assorted artificial solid-state ion channel membranes have been developed over these past few years. Nevertheless, the conventional membrane designs were only limited to either one structure homogenous or two structures heterogeneous membranes. Herein, we report a feasible yet versatile strategy to fabricate the breakthrough membrane design that has never been reported before. An engineered metal-organic framework (MOF)-based ionic diode membrane (termed as ZIF-8/PSS@BANM), composed of a continuous zeolitic imidazolate framework-8 (ZIF-8) membrane incorporated with negatively space-charged polystyrene sulfonate (PSS) and branch-type alumina nanochannel membrane (BANM), was successfully developed (Fig. 1). Results obtained confirm that the heterogeneous membrane is with high geometry gradient from sub-micro-scale to sub-nano-scale and high space charge property. We show that the engineered MOF-based heterogeneous sub-nano-channel membrane can vividly rectify ion transport even in LiCl-methanol solution with a ratio as high as ~13 (Fig. 2a), capable of amplifying ionic current uni-directionally at the subnanometer-scale confinement, due to the broken symmetries in channel sizes, charges, and wettabilities (Fig. 2b). We thereby probe the use of this subnanometer-scale MOF-based ionic-diode membrane in harvesting osmotic energy from organic solutions. Captivatingly, a record power output of up to ~9.58 W/m² at a 50-fold LiCl gradient in methanol can be harnessed (Fig. 3a), majorly because of its unique geometry gradient channel structure (Fig. 3b), outperforming the bandgap of the commercial benchmark value (5 W/m²) and the other conventional membranes structures. The continuous MOF-based heterogeneous membrane we developed here is with high novelty in membrane technology and the design strategy provides a promising approach for constructing new multifunctional biomimetic membranes towards advanced high-performance energy conversion devices.