Jonathan E Spanier

Realization of tunable materials that are multifunctional and maintain high performance in dynamically changing environments is a fundamental goal of science and engineering. Tunable dielectrics form the basis of a wide variety of communication and sensing devices and require breakthrough performance improvement to enable next-generation technologies. Using phenomenological modeling, film growth, and characterization, we show that devices consisting of domain-wall-rich BaSrTiOfilms close to a polar-domain-variant phase boundary exhibit colossal dielectric tunability of 100:1 (99%) at a voltage (electric field) of ~15 V (750 kV/cm), resulting in a tunability-quality factor product figure of merit that rises to nearly 10, two orders of magnitude higher than the best previous reported values. Remarkably, varying the amplitude of alternating-current bias enables modulation of this tunability by 50%, owing to domain-wall motion. These results suggest that domain engineering is a powerful approach for achieving excellent modulation of functional properties in ferroelectric films.

Nanodielectrics based upon nanoscale Ba(Ti, M V )O 3 , where M = Nb or Ta, were prepared and electrically characterized for their potential use as a high permittivity dielectric layer. Nanocrystals of Ba(Ti, Nb)O 3 (BTNO) and Ba(Ti, Ta)O 3 (BTTO) of average size 20 nm (range 10-50 nm) with a non-centrosymmetric (polarizable) crystal structure were synthesized, dispersed in alcohol solvents and blended with three polymers of known but differing dielectric and electromechanical behavior: Polyvinylpyrrolidone (PVP), Polyfurfuryl alcohol (PFA) and Polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE). 0-3 nanoparticle-polymer pressed pellets, films and metal-insulator-metal devices were prepared for electrical characterization. Analysis of the Ba(Ti, M V )O 3 -PVP and Ba(Ti, M V )O 3 -PFA composites showed a high effective permittivity, low loss, low leakage and voltage tolerance, demonstrating the capability for high energy density capacitance. Effective permittivity, of 52 (BTNO-PFA) and 42 (BTTO-PFA) for pellet nanocomposites and 32 (BTNO-PVP) and 20 (BTNO-PVP) film nanocomposites were observed at 1 MHz respectively. Voltage breakdown strengths of 2133 V/mm (BTNO) and 833 V/mm (BTTO) were demonstrated respectively (threshold 0.1 μA). Linear and non-linear dielectric behavior was studied by polarization-electric field (P-E) hysteresis measurements. Nanocomposites of BTNO-PVDF-TrFE were prepared to assess the viability of making ferroelectric nanocomposites over a range of polymer-nanoparticle volume fractions.

Enhanced susceptibilities in ferroelectrics often arise near phase boundaries between competing ground states. While chemically-induced phase boundaries have enabled ultrahigh electrical and electromechanical responses in lead-based ferroelectrics, precise chemical tuning in lead-free alternatives, such as (K,Na)NbO
3
thin films, remains challenging due to the high volatility of alkali metals. Here, we demonstrate strain-induced morphotropic phase boundary-like polymorphic nanodomain structures in chemically simple, lead-free, epitaxial NaNbO
3
thin films. Combining ab initio simulations, thin-film epitaxy, scanning probe microscopy, synchrotron X-ray diffraction, and electron ptychography, we reveal a labyrinthine structure comprising coexisting monoclinic and bridging triclinic phases near a strain-induced phase boundary. The coexistence of energetically competing phases facilitates field-driven polarization rotation and phase transitions, giving rise to a multi-state polarization switching pathway and large enhancements in dielectric susceptibility and tunability across a broad frequency range. Our results open new possibilities for engineering lead-free thin films with enhanced functionalities for next-generation applications.
The authors demonstrate strain-induced morphotropic phase boundary-like nanodomains in lead-free NaNbO
3
thin films, enabling multi-state switching and large enhancements in dielectric susceptibility and tunability over a broad frequency range.

MXenes represent one‐of‐a‐kind materials to devise radically novel technologies and achieve breakthroughs in optoelectronics. To exploit their full potential, precise control over the influence of stoichiometry on optical and thermal properties, as well as device performance, must be achieved. Here, the characteristics of optoelectronic devices based on Ti 3 C 2 T x and Ti 2 CT x thin films are uncovered, highlighting the striking difference in their photothermal responses to laser irradiation under different experimental conditions. Even though their absorption coefficients at 450 nm are comparable, the thermal excitation and relaxation phenomena display markedly different kinetics: Ti 2 CT x devices show a strong asymmetry during the heating‐cooling cycle, with the heat dissipation kinetics being three orders of magnitude slower than Ti 3 C 2 T x and strongly influenced by environmental conditions. The findings are expected to stimulate fundamental investigations into the photothermal response of MXenes and open exciting prospects for their use in printed and wearable optoelectronics, including memory devices and neuromorphic computing.

Highly responsive, voltage-tunable dielectrics are essential for microwave-telecommunication electronics. Ferroelectric/relaxor materials have been leading candidates for such functionality and have exhibited agile dielectric responses. Here, it is demonstrated that relaxor materials developed from antiferroelectrics can achieve both ultrahigh dielectric response and tunability. The system, based on alloying the archetypal antiferroelectric PbZrO3 with the dielectric BaZrO3, exhibits a more complex phase evolution than that in traditional relaxors and is characterized by an unconventional multi-phase competition between antiferroelectric, ferroelectric, and paraelectric order. This interplay of phases can greatly enhance the local heterogeneities and results in relaxor characteristics while preserving considerable polarizability. Upon studying Pb1-xBaxZrO3 for x = 0-0.45, Pb0.65Ba0.35ZrO3 is found to provide for exceptional dielectric tunability under low bias fields (approximate to 81% at 200 kV cm(-1) and approximate to 91% at 500 kV cm(-1)) at 10 kHz, outcompeting most traditional relaxor ferroelectric films. This high tunability is sustained in the radio-frequency range, resulting in a high commutation quality factor (>2000 at 1 GHz). This work highlights the phase evolution from antiferroelectrics (with lower, "positive" dielectric tunability) to relaxors (with higher, "negative" tunability), underscoring a promising approach to develop relaxors with enhanced functional capabilities and new possibilities.

Ferroelectric nitrides attract immense attention due to their excellent electrical, mechanical, and thermal properties as well as for their compatibility with scalable semiconductor technology. The availability of high‐quality nitride films possessing tailorable coercive voltage and field, however, remains challenging, and is a key for deeper exploration of switching dynamics and practical applications in low‐power devices. 2D growth of epitaxial thin (≲20 nm) c ‐axis‐oriented Sc 0.3 Al 0.7 N films is reported on Al 2 O 3 (0001) and on electrically conductive 4 H ‐SiC (0001), obtained by reflection high‐energy electron diffraction‐monitored layer‐by‐layer physical vapor deposition growth. Films exhibit high quality, as evidenced by rocking curve full‐width at half‐maximum (FWHM) as narrow as ≈0.02°, and an atomically abrupt film‐substrate interface with low dislocation density. The coercive field of Sc 0.3 Al 0.7 N/4 H ‐SiC (0001) heterostructures is as low as 2.75 MV cm −1 . Moreover, a high endurance of >10 9 cycles at saturation polarization is achieved. Density functional theory calculations of a model system reveal that an improved crystal quality, including atomically abrupt ferroelectric nitride‐metal interface, facilitates the reduction in the switching barriers, and leads to reduced coercivity. These findings demonstrate the feasibility of obtaining high‐quality epitaxial ferroelectric nitride films on highly scalable and radiation‐resistant substrates, and their potential for energy‐efficient electronic devices.

The pursuit of smaller, energy-efficient devices drives the exploration of electromechanically active thin films (<1 µm) to enable micro- and nano-electromechanical systems. While the electromechanical response of such films is limited by substrate-induced mechanical clamping, large electromechanical responses in antiferroelectric and multilayer thin-film heterostructures have garnered interest. Here, multilayer thin-film heterostructures based on antiferroelectric PbHfO
and ferroelectric PbHf
Ti
O
overcome substrate clamping to produce electromechanical strains >4.5%. By varying the chemistry of the PbHf
Ti
O
layer (x = 0.3-0.6) it is possible to alter the threshold field for the antiferroelectric-to-ferroelectric phase transition, reducing the field required to induce the onset of large electromechanical response. Furthermore, varying the interface density (from 0.008 to 3.1 nm
) enhances the electrical-breakdown field by >450%. Attaining the electromechanical strains does not necessitate creating a new material with unprecedented piezoelectric coefficients, but developing heterostructures capable of withstanding large fields, thus addressing traditional limitations of thin-film piezoelectrics.

Switchable order parameters in ferroic materials are essential for functional electronic devices, yet disruptions of the ordering can take the form of planar boundaries or defects that exhibit distinct properties from the bulk, such as electrical (polar) or magnetic (spin) response. Characterizing the structure of these boundaries is challenging due to their confined size and three-dimensional (3D) nature. Here, a chemical antiphase boundary in the highly ordered double perovskite Pb
MgWO
is investigated using multislice electron ptychography. The boundary is revealed to be inclined along the electron beam direction with a finite width of chemical intermixing. Additionally, regions at and near the boundary exhibit antiferroelectric-like displacements, contrasting with the predominantly paraelectric matrix. Spatial statistics and density functional theory (DFT) calculations further indicate that despite their higher energy, chemical antiphase boundaries (APBs) form due to kinetic constraints during growth, with extended antiferroelectric-like distortions induced by the chemically frustrated environment in the proximity of the boundary. The three-dimensional imaging reveals the interplay between local chemistry and the polar environment, elucidating the role of antiphase boundaries and their associated confined structural distortions and offering opportunities for engineering ferroic thin films.

Calculation of Raman scattering from molecular dynamics (MD) simulations requires accurate modeling of the evolution of the electronic polarizability of the system along its MD trajectory. For large systems, this necessitates the use of atomistic models to represent the dependence of electronic polarizability on atomic coordinates. The bond polarizability model (BPM) is the simplest such model and has been used for modeling the Raman spectra of molecular systems but has not been applied to solid-state systems. Here, we systematically investigate the accuracy and limitations of the BPM parameterized from the density functional theory results for a series of simple molecules, such as CO2, SO2, H2S, H2O, NH3, and CH4; the more complex CH2O, CH3OH, CH3CH2OH, and thiophene molecules; and the BaTiO3 and CsPbBr3 perovskite solids. We find that BPM can reliably reproduce the overall features of the Raman spectra, such as shifts of peak positions. However, with the exception of highly symmetric systems, the assumption of non-interacting bonds limits the quantitative accuracy of the BPM; this assumption also leads to qualitatively inaccurate polarizability evolution and Raman spectra for systems where large deviations from the ground state structure are present.