Recent advances in artificial neuromorphic applications based on perovskite composites

High-performance perovskite materials with excellent physical, electronic, and optical properties play a significant role in artificial neuromorphic devices. However, the development of perovskites in microelectronics is inevitably hindered by their intrinsic non-ideal properties, such as high defect density, environmental sensitivity, and toxicity. By leveraging materials engineering, integrating various materials with perovskites to leverage their mutual strengths presents great potential to enhance ion migration, energy level alignment, photoresponsivity, and surface passivation, thereby advancing optoelectronic and neuromorphic device development. This review initially provides an overview of perovskite materials across different dimensions, highlighting their physical properties and detailing their applications and metrics in two- and three-terminal devices. Subsequently, we comprehensively summarize the application of perovskites in combination with other materials, including organics, nanomaterials, oxides, ferroelectrics, and crystalline porous materials (CPMs), to develop advanced devices such as memristors, transistors, photodetectors, sensors, light-emitting diodes (LEDs), and artificial neuromorphic systems. Lastly, we outline the challenges and future research directions in synthesizing perovskite composites for neuromorphic devices. Through the review and analysis, we aim to broaden the utilization of perovskites and their composites in neuromorphic research, offering new insights and approaches for grasping the intricate physical working mechanisms and functionalities of perovskites.

Review of Systematic Tendencies in (001), (011) and (111) Surfaces Using B3PW as Well as B3LYP Computations of BaTiO3, CaTiO3, PbTiO3, SrTiO3, BaZrO3, CaZrO3, PbZrO3 and SrZrO3 Perovskites

We performed B3PW and B3LYP computations for BaTiO3 (BTO), CaTiO3 (CTO), PbTiO3 (PTO), SrTiO3 (STO), BaZrO3 (BZO), CaZrO3 (CZO), PbZrO3 (PZO) and SrZrO3 (SZO) perovskite neutral (001) along with polar (011) as well as (111) surfaces. For the neutral AO- as well as BO2-terminated (001) surfaces, in most cases, all upper-layer atoms relax inwards, although the second-layer atoms shift outwards. On the (001) BO2-terminated surface, the second-layer metal atoms, as a rule, exhibit larger atomic relaxations than the second-layer O atoms. For most ABO3 perovskites, the (001) surface rumpling s is bigger for the AO- than BO2-terminated surfaces. In contrast, the surface energies, for both (001) terminations, are practically identical. Conversely, different (011) surface terminations exhibit quite different surface energies for the O-terminated, A-terminated and BO-terminated surfaces. Our computed ABO3 perovskite (111) surface energies are always significantly larger than the neutral (001) as well as polar (011) surface energies. Our computed ABO3 perovskite bulk B-O chemical bond covalency increases near their neutral (001) and especially polar (011) surfaces.

DFT computations combined with semiempirical modeling of variations with temperature of spectroscopic and magnetic properties of Gd3+-doped PbTiO3

The rare-earth or 3d transition metal dopants in perovskites have potential to induce interesting features, thus opening opportunities for investigations and applications. Hence, understanding some features, i.e., defect structure, site of incorporation, valence state, and mechanism of charge compensation, in a wide range of temperature is crucial for their technological applications. A comprehensive understanding of the mechanism of structural changes in PbTiO₃ doped with trivalent rare-earths is significant for their potential applications in photonics. To unravel the structural changes, we utilize the density functional theory (DFT) to optimize structural data, which then serve as input for the semiempirical superposition model (SPM) analysis of spectroscopic and magnetic properties of Gd³⁺-doped PbTiO₃. We compute the formation energies of the doped compounds with and without O-vacancy to determine the stable composition. Analysis of the Bader electron charges computed using DFT plus quantum theory of atoms in molecules enables elucidating the effects of the Gd dopant and O-vacancy on the ionic and covalent bonds and, thereby, chemical stability of the compositions. To explain and corroborate the zero-field splitting parameters (ZFSPs) measured by EMR and the lattice parameter changes obtained from XRD, we employ SPM. The optimized structures obtained from ab initio computations for various structural models of Gd³⁺ doped PbTiO₃ are utilized as input data for SPM calculations of ZFPs. This enables theoretical analysis of variations of ZFSPs from 5 to 780 K. The results were fine-tuned by matching with available experimental EMR data for Gd³⁺ probes in PbTiO₃ nanoparticles. Modeling has been carried out considering several possible structural models and the role of an O-vacancy around Gd³⁺ centers. The results show that the two-fold modeling approach, combining DFT and SPM, provides a reliable description of experimental data. Comparative analysis indicates that the Ti-site is less favorable for being replaced by Gd³⁺ with/without O-vacancy. This analysis confirms the plausibility of the Pb²⁺ site for Gd³⁺ dopants and sheds light on the changes of crystal structure during the phase transitions occurring in PbTiO₃ with decreasing temperature.

Theoretical study on ferroelectric nitrides with super-wurtzite structures for solar energy conversion applications

Polarized structured nitride semiconductors are attractive due to their unique and environment-friendly electronic properties. The stability, ferroelectricity and photocatalytic and photovoltaic properties of super-wurtzite Mg₂XN₃ (X = Bi, Mo, Nb, Sb, Ta, Tc and W) were determined based on first principles calculations in this study. The calculated results indicate that Mg₂XN₃ (X = Sb, Ta, Bi and Nb) are stable polar nitrides by phonon frequencies, elastic coefficients and ferroelectric analysis. Mg₂XN₃ (X = Sb, Ta and Nb) with large ferroelectric polarization strength could absorb ultraviolet light to promote photocatalytic water splitting for hydrogen production. Mg₂BiN₃ is a new excellent photovoltaic candidate due to its ideal energy band, high electron mobility, high absorption coefficient and large ferroelectric polarization strength.

Magnetic performance and defect characterization studies of core-shell architectured MgFe2O4@BaTiO3 multiferroic nanostructures

Multiferroics that permit manipulation of the magnetization vector exclusively by electric fields have spawned extensive interest for memory and logic device applications. In line with this understanding, we herein report the encapsulation of non-ferroelectric magnesium ferrite (MgFe2O4) nanoparticles in a ferroelectric shell of BaTiO3 to produce a system with engineered dielectric, magnetic, magneto-electric and ferroelectric properties. The interface effect on the strain transfer was observed to strongly influence the magneto-electric coupling and the electric and magnetic properties of the system. The model polyhedral image of MgFe2O4@BaTiO3 has helped to get an insight into the core-shell structure. The multiferroicity induced by the excellent coupling between the ferroelectric and magnetostrictive phases at the core-shell interface unlocks wide prospects for device downscaling and information storage applications. The influence of magnetostrictive stress on the magneto-electric coupling effects and domain dynamics was further studied using transmission electron microscopy (TEM) and atomic force microscopy images. Interestingly, the realization of a superparamagnetic multiferroic system has been a breakthrough and facilitates ultra high density magnetic data storage technologies. Evidence for spontaneous polarization and the ferroelectric trait exhibited by the multiferroic samples was revealed from the P-E hysteresis loop. The investigation of defect evolution in the system was carried out using positron annihilation lifetime spectroscopy (PALS) and coincidence Doppler broadening spectroscopy (CDBS) of annihilation radiation and the studies revealed thermal diffusion of positrons into the interfacial regions within the core-shell structure and the "formation and pick-off annihilation of orthopositronium atoms". It is concluded that interface engineering is a strong means for manipulation of the magnetic, dielectric and magneto-electric properties in multiferroic heterostructures for high density electrical energy and magnetic data storage.

Studies of Multiferroic Palladium Perovskites

We have studied the atomic force microscopy (AFM), X-ray Bragg reflections, X-ray absorption spectra (XAS) of the Pd L-edge, Scanning electron microscopey (SEM) and Raman spectra, and direct magnetoelectric tensor of Pd-substituted lead titanate and lead zirconate-titanate. A primary aim is to determine the percentage of Pd⁺⁴ and Pd⁺² substitutional at the Ti-sites (we find that it is almost fully substitutional). The atomic force microscopy data uniquely reveal a surprise: both threefold vertical (polarized out-of-plane) and fourfold in-plane domain vertices. This is discussed in terms of the general rules for Voronoi patterns (Dirichlet tessellations) in two and three dimensions. At high pressures Raman soft modes are observed, as in pure lead titanate, and X-ray diffraction (XRD) indicates a nearly second-order displacive phase transition. However, two or three transitions are involved: First, there are anomalies in c/a ratio and Raman spectra at low pressures (P = 1 − 2 GPa); and second, the c/a ratio reaches unity at ca. P = 10 GPa, where a monoclinic (M_c) but metrically cubic transition occurs from the ambient tetragonal P4 mm structure in pure PbTiO₃; whereas the Raman lines (forbidden in the cubic phase) remain until ca. 17 GPa, where a monoclinic-cubic transition is known in lead titanate.

Chemical-bond effect on epitaxial strain in perovskite sodium niobate

Epitaxial films and heterostructures of perovskite oxides attract tremendous scientific interest because of the unique phenomena therein. Especially important is the epitaxial growth of films subjected to substrate-induced misfit strain. We show here that in contrast to conventional misfit-controlled epitaxy, chemical bonds determine the crystal stability and strain in epitaxial films of sodium niobate on different cubic substrates. Strain relaxation in sodium niobate is independent of misfit magnitude and proceeds through perovskite-specific tilting of oxygen octahedra in addition to common defect formation. The observed structural relaxation evidences a major role of a large internal strain that originates from chemical bonds in the perovskite cell. The effect of chemical bonds on film strain is anticipated to also control the epitaxy of other perovskite oxides and related compounds.

Defect dipole-induced domain reorientation of NdFeO3–PbTiO3 thin films

The increasing temperature induces the realignment of the defect dipoles in NdFeO₃–PbTiO₃ thin films, which further reorients the domains.