Strain-Engineered Ferroelastic Structures in PbTiO3 Films and Their Control by Electric Fields

Type-printable photodetector arrays for multichannel meta-infrared imaging

Multichannel meta-imaging, inspired by the parallel-processing capability of neuromorphic computing, offers considerable advancements in resolution enhancement and edge discrimination in imaging systems, extending even into the mid- to far-infrared spectrum. Currently typical multichannel infrared imaging systems consist of separating optical gratings or merging multi-cameras, which require complex circuit design and heavy power consumption, hindering the implementation of advanced human-eye-like imagers. Here, we present printable graphene plasmonic photodetector arrays driven by a ferroelectric superdomain for multichannel meta-infrared imaging with enhanced edge discrimination. The fabricated photodetectors exhibited multiple spectral responses with zero-bias operation by directly rescaling the ferroelectric superdomain instead of reconstructing the separated gratings. We also demonstrated enhanced and faster shape classification (98.1%) and edge detection (98.2%) using our multichannel infrared images compared with single-channel detectors. Our proof-of-concept photodetector arrays simplify multichannel infrared imaging systems and offer potential solutions in efficient edge detection in human-brain-type machine vision.

Strain Engineering of Domain Coexistence in Epitaxial Lead-Titanite Thin Films

Phase and domain structures in ferroelectric materials play a vital role in determining their dielectric and piezoelectric properties. Ferroelectric thin films with coexisting multiple domains or phases often have fascinating high sensitivity and ultrahigh physical properties. However, the control of the coexisting multiple domains is still challenging, thus necessitating the theoretical prediction. Here, we studied the phase coexistence and the domain morphology of PbTiO3 epitaxial films by using a Landau–Devonshire phenomenological model and canonic statistical method. Results show that PbTiO3 films can exist in multiple domain structures that can be diversified by the substrates with different misfit strains. Experimental results for PbTiO3 epitaxial films on different substrates are in good accordance with the theoretical prediction, which shows an alternative way for further manipulation of the ferroelectric domain structures.

Tunable Nanoscale Evolution and Topological Phase Transitions of a Polar Vortex Supercrystal

Understanding the phase transitions and domain evolutions of mesoscale topological structures in ferroic materials is critical to realizing their potential applications in next-generation high-performance storage devices. Here, the behaviors of a mesoscale supercrystal are studied with 3D nanoscale periodicity and rotational topology phases in a PbTiO₃ /SrTiO₃ (PTO/STO) superlattice under thermal and electrical stimuli using a combination of phase-field simulations and X-ray diffraction experiments. A phase diagram of temperature versus polar state is constructed, showing the formation of the supercrystal from a mixed vortex and a-twin state and a temperature-dependent erasing process of a supercrystal returning to a classical a-twin structure. Under an in-plane electric field bias at room temperature, the vortex topology of the supercrystal irreversibly transforms to a new type of stripe-like supercrystal. Under an out-of-plane electric field, the vortices inside the supercrystal undergo a topological phase transition to polar skyrmions. These results demonstrate the potential for the on-demand manipulation of polar topology and transformations in supercrystals using electric fields. The findings provide a theoretical understanding that may be utilized to guide the design and control of mesoscale polar structures and to explore novel polar structures in other systems and their topological nature.

Deconvolution of Phonon Scattering by Ferroelectric Domain Walls and Point Defects in a PbTiO3 Thin Film Deposited in a Composition-Spread Geometry

We present a detailed analysis of the temperature dependence of the thermal conductivity of a ferroelectric PbTiO₃ thin film deposited in a composition-spread geometry enabling a continuous range of compositions from ∼25% titanium deficient to ∼20% titanium rich to be studied. By fitting the experimental results to the Debye model we deconvolute and quantify the two main phonon-scattering sources in the system: ferroelectric domain walls (DWs) and point defects. Our results prove that ferroelectric DWs are the main agent limiting the thermal conductivity in this system, not only in the stoichiometric region of the thin film ([Pb]/[Ti] ≈ 1) but also when the concentration of the cation point defects is significant (up to ∼15%). Hence, DWs in ferroelectric materials are a source of phonon scattering at least as effective as point defects. Our results demonstrate the viability and effectiveness of using reconfigurable DWs to control the thermal conductivity in solid-state devices.

Relaxation Mechanisms and Strain-Controlled Oxygen Vacancies in Epitaxial SrMnO3 Films

SrMnO₃ has a rich epitaxial strain-dependent ferroic phase diagram, in which a variety of magnetic orderings, even ferroelectricity, and thus multiferroicity, are accessible by gradually modifying the strain. Different relaxation processes, though, including the presence of strain-induced oxygen vacancies, can severely curtail the possibility of stabilizing these ferroic phases. Here, we report on a thorough investigation of the strain relaxation mechanisms in SrMnO₃ films grown on several substrates imposing varying degrees of strain from slightly compressive (-0.39%) to largely tensile ≈+3.8%. First, we determine the strain dependency of the critical thickness (t _c) below which pseudomorphic growth is obtained. Second, the mechanisms of stress relaxation are elucidated, revealing that misfit dislocations and stacking faults accommodate the strain above t _c. Yet, even for films thicker than t _c, the atomic monolayers below t _c are proved to remain fully coherent. Therefore, multiferroicity may also emerge even in films that appear to be partially relaxed. Last, we demonstrate that fully coherent films with the same thickness present a lower oxygen content for increasing tensile mismatch with the substrate. This behavior proves the coupling between the formation of oxygen vacancies and epitaxial strain, in agreement with first-principles calculations, enabling the strain control of the Mn³⁺/Mn⁴⁺ ratio, which strongly affects the magnetic and electrical properties. However, the presence of oxygen vacancies/Mn³⁺ cations reduces the effective epitaxial strain in the SrMnO₃ films and, thus, the accessibility to the strain-induced multiferroic phase.