Alexey S. Konstantinov

Controlling ferroelectric phase symmetry through strain engineering is crucial for tailoring polarization topology and functional properties in oxide-based devices. Here, we show that a thin conductive buffer layer can over-ride substrate-induced strain in BaTiO3 (BTO) films on MgO. Although the lattice parameter of MgO(001) would be expected to induce tensile misfit strain, in 500 nm BaTiO3 films the thermal-expansion mismatch dominates,resulting in effective compressive stress upon cooling. Inserting a 50-nm La0.67Sr0.33MnO3 (LSMO) buffer layer reverses the effective strain state from compressive to tensile. Using temperature-dependent polarized Raman spectroscopy, X-ray diffraction, and piezoresponse force microscopy, we reveal an unconventional phase sequence upon cooling: paraelectric → orthorhombic (aa-phase, ∼407 K) → monoclinic (r -phase, ∼300–345 K), characterized in-plane and tilted polarization components. The LSMO layer not only serves as a bottom electrode but also modifies the strain state, while interfacial charge redistribution induces a built-in electric field, evidenced by self-polarization and asymmetric PFM hysteresis. These findings highlight the critical role of conductive buffer layers in controlling ferroelectric symmetry and enabling oxide heterostructures with tailored polarization textures for low-power spin- tronics, tunable microwave devices, and neuromorphic computing.

The widespread use of ferroelectric materials in tunable electronic devices is often limited by energy dissipation at domain walls in the ferroelectric state. Low dielectric losses and high tunability can be achieved by operating the material in the paraelectric phase under an external electric field. Under these conditions, a domain-free, field-stabilized ferroelectric-like state can be induced, particularly near phase boundaries or multicritical points. We investigate the electric-field control of dielectric and piezoelectric properties in paraelectric Pb _x Sr _{1− x} TiO ₃ (PST) thin films epitaxially constrained on cubic substrates. Using a Landau–Ginzburg–Devonshire thermodynamic potential that incorporates both polarization and octahedral tilting as coupled order parameters, we construct the room-temperature (300 K) phase diagram in the misfit strain–composition ( u _m – x ) plane. Within the paraelectric region, we analyze the system’s responseunder a perpendicular electric field ( E _z = 100 kV cm ⁻¹ ), which induces a uniaxial, domain-free ferroelectric-like state, often referred to as a ‘forced’ ferroelectric. We calculate the field-dependent permittivities ( ε 33 , ε 11 ) and induced piezoelectric coefficients ( d i j ,   e i j ) along a path within the paraelectric phase near the adjacent ferroelectric phase boundaries. Our results show a pronounced enhancement of dielectric tunability t ( ε ) and non-zero piezoelectric response in regions near the multicritical point ( u _m ≈ 0, x ≈ 0.138), where the paraelectric phase borders the c -phase (tetragonal, P 4 m m ), r -phase (rhombohedral-like monoclinic, C m ), and aa -phase (orthorhombic, A m m 2 ). The most pronounced effects occur near the boundary with the c -phase, where the out-of-plane polarization mode undergoes softening. These findings indicate that operating PST thin films in the paraelectric phase under an external bias enables low-loss, highly tunable behavior, making them promising candidates for reconfigurable microwave and electronic devices.

Vanadium dioxide with metal-to-insulator transition (MIT) that is triggered by heat, current or light is a promising material for modern active THz/mid-IR metasurfaces and all-optical big data processing systems. Multilayer VO2-based active metasurfaces are urgently needed however several important issues related to VO2 properties in VO2/TiO2/Al2O3 films should be thoroughly examined first. We study electrical, optical and structural properties of VO2 films as well as their composition and switching characteristics as function of the VO2 layer thickness in VO2/TiO2 composites. XRD analysis revealed an epitaxial growth of films with deformation of the monoclinic VO2 lattice to hexagonal symmetry. Reduced VO2 layer thickness from 170 nm to 20 nm results in increased phase transition temperature while the width of the resistance versus temperature hysteresis loop R(T) remains constant at ~6 °C for all VO2 thicknesses in the range of 20-170 nm. The resistance alteration ratio Rratio is reduced from 4,2×103 to 2.7×102 in thinner films. Raman spectra reveal a significant shift of VO2 lattice vibration modes for films thinner than 30 nm claiming a great structural strain whereas modes position for thicker VO2 layers are similar to those in bulk structure. Composition of VO2 films has revealed only a minor alteration of VO2/V2O5 phases ratio from 1.6 to 1.8 when the film thickness has been increased from 20 nm to 50 nm. Investigation of surface elemental composition and valence states of VO2 films revealed that VO2/V2O5 ratio remains practically unchanged with thickness reduction. The study of electrical MIT dynamics revealed the switching time of a 50 nm VO2 film to be as low as 800 ns.

The Ni‐PANI@GO composite electrode was fabricated via cost effective electrodeposition technique. According to the XRD, FTIR, Raman, SEM, and XPS analyses revealed that the nickel doped PANI@GO composite has been fabricated on the surface of the nickel foam. Addition of nickel significantly enhanced interaction between graphene with PANI leading to higher degree of polyaniline doping though imine groups. Electrochemical investigation revelated the significant performance of the Ni‐PANI@GO composite electrode, boosting an impressive capacitance of 4480 F/g at 40 A/g, surpassing previous Ni‐foam‐based binder‐free electrodes. Notably, Ni‐PANI@GO electrode displayed excellent catalytic activity in both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), generating a considerable volume of the gas bubbles at relatively modest overpotentials of 279 mV and 244 mV respectively. This event allows for the achievement of 20 mA cm⁻² current density. Furthermore, in the laboratory‐scale water electrolyzer, a low cell voltage of 1.72 V was achieved, facilitating a water‐splitting current density of 20 mA cm⁻². This study underscores the premising potential for the real‐world device's application of the versatile Ni‐PANI@GO composite electrode.

Decreasing the scale of vanadium dioxide (VO₂) structures is one of the ways to enhance the switching speed of the material. We study the properties of VO₂ films of altered thicknesses in the range of 20–170[ ]nm prepared on c-sapphire substrates with a TiO₂ sublayer by pulsed laser deposition (PLD) method. The synthesis regime to design a TiO₂ film was preliminarily optimized based on XRD data. XRD patterns reveal an epitaxial growth of the VO₂ films with distortion of the monoclinic cell to hexagonal symmetry. The positions of the lattice vibration modes in Raman spectra are similar to those in bulk VO₂ when the film thickness is greater than [ ][ ]nm. For VO₂ films thicker that [ ][ ]nm, a lattice strain results in the modes’ positions and intensity change. However, the electrically triggered transition in a [ ][ ]nm thick VO₂ film reveals forward and reverse switching times as short as 20[ ]ns and 400[ ]ns, correspondingly.

Phase change metasurfaces based on VO₂, which are pre-heated with electric current and optically addressed by projected structured light hologram, are considered to become a new paradigm in programmed THz/middle IR flat optics. Macroscopic quasi-homogeneous arrays of Au nanoparticles show large near IR absorption and a significant photothermal effect capable of boosting a light-triggered switching of VO₂ and are to be carefully examined. We propose a new approach to simultaneously probe the altered temperature and electric conductivity of a hybrid Au particle-VO₂ film composite by monitoring a phase shift and attenuating a surface acoustic wave in a YX128° cut LiNbO₃ substrate. The method shows a temperature resolution of 0.1 °C comparable with the best existing techniques for studying nanoobjects and surfaces. The laser-induced photothermal effects were characterized in a macroscopic array of Au nanostars (AuNSts) with different surface coverage. In a monolayer of 10 nm Au, coupled plasmonic nanoparticles were deposited on the LiNbO₃ substrate. An optically triggered insulator-metal transition assisted by photothermal effect in AuNSts/VO₂/TiO₂/LiNbO₃ composites was studied at varied light power. We believe that the proposed SAW-based method is of significant importance for the characterization and optimization of radiation absorbing or/and electrically heated elements of metasurfaces and other devices for lab-on-chip and optical communication/processor technology.