We report on nanoscale strain gradients in ferroelectric HoMnO3 epitaxial thin films, resulting in a giant flexoelectric effect. Using grazing-incidence in-plane x-ray diffraction, we measured strain gradients in the films, which were 6 or 7 orders of magnitude larger than typical values reported for bulk oxides. The combination of transmission electron microscopy, electrical measurements, and electrostatic calculations showed that flexoelectricity provides a means of tuning the physical properties of ferroelectric epitaxial thin films, such as domain configurations and hysteresis curves.

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Giant Flexoelectric Effect in Ferroelectric Epitaxial Thin Films

A previous study of the growth conditions has shown that single-phase BiFeO3 thin films can only be obtained in a narrow pressure-temperature window and that these films display a weak magnetic moment. Here we study in more detail the structure and the magnetism of single-phase BiFeO3 films by means of reciprocal space mapping, Raman spectroscopy and neutron diffraction. X-ray and Raman data suggest that the BiFeO3 structure is tetragonal for 70 nm-thick films and changes to monoclinic for 240 nm-thick films, thus remaining different from that of the bulk (rhombohedral) structure. In the 240 nm monoclinically distorted film neutron diffraction experiments allow the observation of a G-type antiferromagnetic order as in bulk single crystals. However, the satellite peaks associated with the long-wavelength cycloid present in bulk BiFeO3 are not observed. The relevance of these findings for the exploitation of the magnetoelectric properties of BiFeO3 is discussed.

Structural distortion and magnetism of BiFeO3 epitaxial thin films: a Raman spectroscopy and neutron diffraction study

The optical properties of monolayer and bilayer transition metal dichalcogenide semiconductors are governed by excitons in different spin and valley configurations, providing versatile aspects for van der Waals heterostructures and devices. Here, we present experimental and theoretical studies of exciton energy splittings in external magnetic field in neutral and charged WSe2 monolayer and bilayer crystals embedded in a field effect device for active doping control. We develop theoretical methods to calculate the exciton g-factors from first principles for all possible spin-valley configurations of excitons in monolayer and bilayer WSe2 including valley-indirect excitons. Our theoretical and experimental findings shed light on some of the characteristic photoluminescence peaks observed for monolayer and bilayer WSe2. In more general terms, the theoretical aspects of our work provide additional means for the characterization of single and few-layer transition metal dichalcogenides, as well as their heterostructures, in the presence of external magnetic fields. Excitons in various spin and valley configurations control the optical properties of ultrathin transition metal dichalcogenides. Here, the authors develop theoretical and experimental methods to determine the exciton g-factors for all possible spin-valley configurations of excitons in monolayer and bilayer WSe2, including valley-indirect excitons.

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Exciton g-factors in monolayer and bilayer WSe2 from experiment and theory.

Neuromorphic computing has the potential to accelerate high performance parallel and low power in-memory computation, artificial intelligence, and adaptive learning. Despite emulating the basic functions of biological synapses well, the existing artificial electronic synaptic devices have yet to match the softness, robustness, and ultralow power consumption of the brain. Here, we demonstrate an all-inorganic flexible artificial synapse enabled by a ferroelectric field effect transistor based on mica. The device not only exhibits excellent electrical pulse modulated conductance updating for synaptic functions but also shows remarkable mechanical flexibility and high temperature reliability, making robust neuromorphic computation possible under external disturbances such as stress and heating. Based on its linear, repeatable, and stable long-term plasticity, we simulate an artificial neural network for the Modified National Institute of Standards and Technology handwritten digit recognition with an accuracy of 94.4%. This work provides a promising way to enable flexible, low-power, robust, and highly efficient neuromorphic computation that mimics the brain.

Flexible electronic synapse enabled by ferroelectric field effect transistor for robust neuromorphic computing

Polarization-enhanced photocatalytic activity in non-centrosymmetric materials based photocatalysis: A review

Switchable ferroelectric diode and photovoltaic effects in polycrystalline BiFeO3 thin films grown on transparent substrates

Exploring and manipulating domain configurations in ferroelectric thin films are of critical importance for the design and fabrication of ferroelectric heterostructures with a novel functional performance. In this study, BiFeO3 (BFO) ultrathin films with various Bi/Fe ratios from excess Bi to deficient Bi have been grown on (La0.7Sr0.3)MnO3 (LSMO)-covered SrTiO3 substrates by a laser molecular beam epitaxy system. Atomic force microscopy and piezoresponse force microscopy measurements show that both the surface morphology and ferroelectric polarization of the films are relevant to Bi nonstoichiometry. More significantly, a Bi-excess thin film shows an upward (from substrate to film surface) uniform ferroelectric polarization, whereas a Bi-deficient thin film exhibits a downward uniform polarization, which means the as-grown polarization of BFO thin films can be controlled by changing the Bi contents. Atomic-scale structural and chemical characterizations and second-harmonic generation measurements reveal that two different kinds of structural distortions and interface atomic configurations in the BFO/LSMO heterostructures can be induced by the change of Bi nonstoichiometry, leading to the two opposite as-grown ferroelectric polarizations. It has also been revealed that the band gap of BFO thin films can be modulated via Bi nonstoichiometry. These results demonstrate that Bi nonstoichiometry plays a key role on the ferroelectric domain states and physical properties of BFO thin films and also open a new avenue to manipulate the structure and ferroelectric domain states in BFO thin films.

Manipulating the Ferroelectric Domain States and Structural Distortion in Epitaxial BiFeO3 Ultrathin Films via Bi Nonstoichiometry.

We report an effective internal electric field induced by Nb doping in BiFeO3 (BFO) thin films and the consequent effects on polarization backswitching and conduction behaviors. Bi(Fe1–xNbx)O3 (x =...

Internal Electric Field and Polarization Backswitching Induced by Nb Doping in BiFeO3 Thin Films

BiFeO 3 (BFO) thin films with BaTiO 3 (BTO)
or SrTiO 3 (STO) as buffer layer were epitaxially grown
on SrRuO 3 -covered SrTiO 3 substrates. X-ray diffraction
measurements show that the BTO buffer causes tensile strain in the
BFO films, whereas the STO buffer causes compressive strain. Different
ferroelectric domain structures caused by these two strain statuses
are revealed by piezoelectric force microscopy. Electrical and magnetical
measurements show that the tensile-strained BFO/BTO samples have reduced
leakage current and large ferroelectric polarization and magnetization,
compared with compressively strained BFO/STO. These results demonstrate
that the electrical and magnetical properties of BFO thin films can
be artificially modified by using a buffer layer.

Effects of BaTiO 3 and SrTiO 3 as the buffer layers of epitaxial BiFeO 3 thin films

The valley of transition metal dichalcogenides provides an additional platform to manipulate spin due to its unique selection rule. Normally, intralayer optical transitions in the magnetic field show a Zeeman splitting with a g factor of about −4. Here, we report a remarkable valley Zeeman effect exhibited by the splitting of excitonic emission in a bilayer WS2, with a value of g factor as large as −16.5. The observed large g factor results from the interlayer recombination, as the conduction band and the valence band are modified in opposite directions by the magnetic field in different layers. The interlayer recombination is due to the defect induced inversion symmetry breaking, which is theoretically not accessible in ideal bilayer WS2 with inversion symmetry. The large g factor of interlayer emission offers potential benefits for future optical spin control and detection.

Shilu Tian

Papers

Switchable ferroelectric diode and photovoltaic effects in polycrystalline BiFeO3 thin films grown on transparent substrates

Manipulating the Ferroelectric Domain States and Structural Distortion in Epitaxial BiFeO3 Ultrathin Films via Bi Nonstoichiometry.

Internal Electric Field and Polarization Backswitching Induced by Nb Doping in BiFeO3 Thin Films

Effects of BaTiO 3 and SrTiO 3 as the buffer layers of epitaxial BiFeO 3 thin films

Large g factor in bilayer WS2 flakes