Piezoresponse Force Microscopy Measurements Research Articles

Evaluation of ferroelectricity in a distorted wurtzite-type structure of Sc-doped LiGaO2.

Since the discovery of ferroelectricity in a wurtzite-type structure, this structural type has gathered much attention as a next-generation ferroelectric material due to its high polarization value combined with its high breakdown strength. However, the main targets of wurtzite-type ferroelectrics have been limited thus far to simple nitride/oxide compounds. The investigation of new ferroelectric materials with wurtzite-type structures is important for understanding ferroelectricity in such structures. We therefore focus on β-LiGaO2 in this study. Although AlN and ZnO possess well-known wurtzite-type structures (P63mc), β-LiGaO2 has a distorted wurtzite-type structure (Pna21), and there are no reports of ferroelectricity in LiGaO2. In this study, we have revealed that LiGaO2 exhibits relatively high barrier height energy for polarization switching, however, Sc doping effectively reduces that energy. Then, we conducted thin film preparation and evaluation for Sc-doped LiGaO2 to observe its ferroelectric properties. We successfully observed ferroelectric behavior by using piezoresponse force microscopy measurements for LiGa0.8Sc0.2O2/SrRuO3/(111)SrTiO3.

Revisiting contrast mechanism of lateral piezoresponse force microscopy

Piezoresponse force microscopy (PFM) has been widely used for the nanoscale analysis of piezoelectric properties and ferroelectric domains. Although PFM is useful because of its simple and nondestructive features, PFM measurements can be obscured by non-piezoelectric effects that could affect the PFM signals or lead to ferroelectric-like behaviors in non-ferroelectric materials. Many research studies have addressed related technical issues, but they have primarily focused on vertical PFM. Here, we investigate significant discrepancies in lateral PFM signals between the trace and the retrace scans, which are proportional to the scan angle and the cantilever lateral tilting discrepancy. The discrepancies in PFM signals are analyzed based on intrinsic and extrinsic components, including out-of-plane piezoresponse, electrostatic force, and other factors. Our research will contribute to the accurate PFM measurements for the visualization of ferroelectric in-plane polarization distributions.

Varied domain structures in 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 single crystals

Single crystals of 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-30PT) with a composition close to the morphotropic phase boundary have demonstrated remarkable electromechanical properties, stimulating intensive research interest in the field of piezoelectrics. Domain structures have long been associated with their overall piezoelectric properties, thereby garnering particular research interest for the enhancement of piezoelectric properties. Here, we report on three distinct domain structures in this material. Through a combination of X-ray diffraction reciprocal space mapping and piezoresponse force microscopy measurements, an M A crystal structure has been confirmed. The three-dimensional polar vectors of the three domain structures have been reconstructed with two 4M A and one 2M A domain structures. Correlations between different domain structures and their local electrical switching properties have been revealed. Our study of the PMN-30PT single crystal examines diverse domain structures at the mesoscale in detail, which provides valuable insight into the relationship between structures and properties of the material.

High field dielectric response in κ-Ga2O3 films

κ-Ga2O3 has been predicted to be a potential ferroelectric material. In this work, undoped Ga2O3 films were grown by either plasma-enhanced atomic layer deposition (PEALD) or metal organic chemical vapor deposition (MOCVD) on platinized sapphire substrates. 50 nm thick PEALD films with a mixture of κ-Ga2O3 and β-Ga2O3 had a relative permittivity of ∼27, a loss tangent below 2%, and high electrical resistivity up to ∼1.5 MV/cm. 700 nm thick MOCVD films with predominantly the κ-Ga2O3 phase had relative permittivities of ∼18 and a loss tangent of 1% at 10 kHz. Neither film showed compelling evidence for ferroelectricity measured at fields up to 1.5 MV/cm, even after hundreds of cycles. Piezoresponse force microscopy measurements on bare κ-Ga2O3 showed a finite piezoelectric response that could not be reoriented for electric fields up to 1.33 MV/cm.

Thick Does the Trick: Genesis of Ferroelectricity in 2D GeTe-Rich (GeTe)m (Sb2 Te3 )n Lamellae.

The possibility to engineer (GeTe)m (Sb2 Te3 )n phase-change materials to co-host ferroelectricity is extremely attractive. The combination of these functionalities holds great technological impact, potentially enabling the design of novel multifunctional devices. Here an experimental and theoretical study of epitaxial (GeTe)m (Sb2 Te3 )n with GeTe-rich composition is presented. These layered films feature a tunable distribution of (GeTe)m (Sb2 Te3 )1 blocks of different sizes. Breakthrough evidence of ferroelectric displacement in thick (GeTe)m (Sb2 Te3 )1 lamellae is provided. The density functional theory calculations suggest the formation of a tilted (GeTe)m slab sandwiched in GeTe-rich blocks. That is, the net ferroelectric polarization is confined almost in-plane, representing an unprecedented case between 2D and bulk ferroelectric materials. The ferroelectric behavior is confirmed by piezoresponse force microscopy and electroresistive measurements. The resilience of the quasi van der Waals character of the films, regardless of their composition, is also demonstrated. Hence, the material developed hereby gathers in a unique 2D platform the phase-change and ferroelectric switching properties, paving the way for the conception of innovative devicearchitectures.

Bias-polarity-dependent asymmetric domain switching in fatigued epitaxial BiFeO3 (001) capacitors

We investigated domain switching dynamics in pristine and fatigued epitaxial BiFeO3 (001) capacitors using piezoresponse force microscopy and switching current measurements. In contrast to domain growth-dominant switching in the pristine capacitors, we observed two different bias-polarity-dependent switching dynamics in the fatigued capacitors. Under a positive bias on the top electrode of the fatigued capacitors, the domain walls were strongly pinned, indicating that nucleation played a critical role in domain switching. However, when a negative bias was applied, the domain wall velocity decreased owing to weak pinning of the domain walls, but domain growth remained the governing process of the domain switching behavior. The intriguing asymmetric domain switching and related domain wall pinning behavior in the fatigued BiFeO3 capacitors may be attributed to the formation of a defect layer near the interface between the ferroelectric film and electrode.

Piezo-Enhanced Photocatalytic Performance of Bismuth Ferrite-Based Thin Film for Organic Pollutants Degradation

This work addresses the global sustainable development concerns by investigating the enhancement of piezo-photocatalytic efficiency in bismuth ferrite-based thin films synthesized using reactive high-power impulse magnetron sputtering. The influence of substrate type and Cr addition on structural, optical and ferroelectric properties of bismuth ferrite (BFO) based thin films was investigated. X-ray diffraction measurements showed the formation of different phases depending on the substrate used for sample growth. Compared to the BFO film deposited on FTO (F-SnO2), the Cr-doped BFO (BFCO) sample on SrTiO3 (STO) exhibits higher photodegradation efficiency (52.3% vs. 27.8%). The enhanced photocatalytic activity of BFCO is associated with a lower energy band gap (1.62 eV vs. 1.77 eV). The application of ultrasonic-wave vibrations simultaneously with visible light improved 2.85 times and 1.86 times the photocatalytic degradation efficiencies of BFO/FTO and BFCO/STO catalysts, respectively. The piezoresponse force microscopy (PFM) measurements showed that both catalysts exhibit ferroelectric behavior, but a higher piezoelectric potential was evidenced in the case of the BFO/FTO thin film. The enhancement of piezo-photodegradation efficiency was mainly attributed to the piezoelectric-driven separation and transport of photo-generated carriers toward the surface of the photocatalyst.

Effect of film thickness on laser induced surface structures formation on optically transparent polymer films

Laser-induced periodic surface structures can be prepared on polymer thin films, despite the absence of light absorption at the wavelength of the irradiating laser, by using a bilayer approach. Nanostructuring of a poly(3-hexylthiophene-2,5-diyl) (P3HT) layer, by laser irradiation at 532 nm, covered by a thin layer of a non-absorbing polymer, in this case, the ferroelectric copolymer poly(vinylidene fluoride - trifluoro ethylene),P(VDF-TrFE), allows the development of a grating-like structure on the surface. The range of thicknesses of the upper layer, which leads to the formation of LIPSS, has been determined. It is verified that for a thickness larger than 40 nm, no periodic structures are formed, neither in the upper layer nor in the bottom P3HT one. This can be explained considering a mechanical restriction which prevents P3HT rearrangement. The upper nanostructured surface layer exhibits ferroelectric properties as proven by piezoresponse force microscopy measurements.

Mechanism of polarization “Wake-Up” in ferroelectric Hafnia-Zirconia thin films

As an emerging non-volatile memory technology, HfO2-based ferroelectrics exhibit excellent compatibility with silicon CMOS process flows; however, during repeated programming and erase of the polarization state of HfO2-based ferroelectric capacitors, the magnitude of the measured polarization increases, a phenomenon known as “wake-up”. Here-in, we study the cause of wake-up effect in Hf0.5Zr0.5O2 (HZO) capacitors using non-destructive methods that probe statistically significant sample volumes. Synchrotron X-ray diffraction reveals a concerted shift in HZO Bragg peak position as a function of polarization switching cycle number in films prepared under conditions such that they exhibit extremely large (∼3000%) wake-up. Piezoresponse force microscopy measurements are utilized to visualize domain switching with wake-up. The combination of these observations clearly demonstrates that wake-up is caused by a field driven phase transformation of the tetragonal phase to the metastable ferroelectric orthorhombic phase during polarization switching of HZO capacitors.

Magnetoelectric multiferroic properties of BaTiO3-CoFe2O4-BaTiO3 tri-layer thin films fabricated by pulsed laser deposition

A magnetoelectric multiferroic bi-phase system with robust ferroelectric and ferro/ferri-magnetism response at room temperature would be ideally suitable for microelectronics, memory devices, and for spintronic applications. BaTiO3/CoFe2O4/BaTiO3(BTO/CFO/BTO) heterostructured films were pulsed laser deposition grown on Pt(111)/TiO2/SiO2/Si substrates. High quality polycrystalline films were grown by thermal annealing at 750 °C, at an oxygen partial pressure of 100 mTorr for 1 h. Crystal quality and phase formation information was monitored using X-ray diffraction (XRD). XRD confirm the growth of polycrystalline heterostructures and the coexistence of both perovskite BTO and spinel CFO phases in heterostructures. In order to obtain robust magnetoelectric coupling, we studied the tri-layer structure as a representative ferroelectric/ferrimagnetic/ferroelectric system. We report the ferroelectric, leakage current behavior, ferrimagnetic, and frequency dependent ME properties of the films. Ferroelectric BTO and ferrimagnetic inverse spinel CFO nature is also confirmed using the piezoresponse force microscopy and magnetic force microscopy measurements. These nanostructures exhibit high saturation polarization (Ps ∼ 99.86 µC/cm2), saturation magnetization (Ms ∼ 51.48 emu/cm3), and a strong ME coupling coefficient of ∼ 274 mV/cm-Oe with a bias magnetic field of + 90 Oe, anda frequency of 1 kHz revealing them as prospective candidates for multifunctional applications.

Design and Piezoelectric Energy Harvesting Properties of a Ferroelectric Cyclophosphazene Salt.

Cyclophosphazenes offer a robust and easily modifiable platform for a diverse range of functional systems that have found applications in a wide variety of areas. Herein, for the first time, it reports an organophosphazene-based supramolecular ferroelectric [(PhCH2 NH)6 P3 N3 Me]I, [PMe]I. The compound crystallizes in the polar space group Pc and its thin-film sample exhibits remnant polarization of 5µCcm-2 . Vector piezoresponse force microscopy (PFM) measurements indicated the presence of multiaxial polarization. Subsequently, flexible composites of [PMe]I are fabricated for piezoelectric energy harvesting applications using thermoplastic polyurethane (TPU) as the matrix. The highest open-circuit voltages of 13.7V and the maximum power density of 34.60µWcm-2 are recorded for the poled 20wt.% [PMe]I/TPU device. To understand the molecular origins of the high performance of [PMe]I-based mechanical energy harvesting devices, piezoelectric charge tensor values are obtained from DFT calculations of the single crystal structure. These indicate that the mechanical stress-induced distortions in the [PMe]I crystals are facilitated by the high flexibility of the layered supramolecular assembly.

Halide Tunablility Leads to Enhanced Biomechanical Energy Harvesting in Lead-Free Cs2SnX6-PVDF Composites.

The main challenges impeding the widespread use of organic-inorganic lead halide perovskites in modern-day technological devices are their long-term instability and lead contamination. Among other environmentally convivial and sustainable alternatives, Cs2SnX6 (X = Cl, Br, and I) compounds have shown promise as ambient-stable, lead-free materials for energy harvesting, and optoelectronic applications. Additionally, they have demonstrated tremendous potential for the fabrication of self-powered nanogenerators in conjunction with piezoelectric polymers like polyvinylidene-fluoride (PVDF). We report on the fabrication of composites constituting solvothermally synthesized Cs2SnX6 nanostructures and PVDF. The electroactive phases in PVDF were boosted by the incorporation of Cs2SnX6, leading to enhanced piezoelectricity in the composites. First-principles density functional theory (DFT) studies were carried out to understand the interfacial interaction between the Cs2SnX6 and PVDF, which unravels the mechanism of physisorption between the perovskite and PVDF, leading to enhanced piezoresponse. The halide ions in the inorganic Cs2SnX6 perovskites were varied systematically, and the piezoelectric behaviors of the respective piezoelectric nanogenerators (PENGs) were investigated. Further, the dielectric properties of these halide perovskite-based hybrids are quantified, and their piezoresponse amplitude, piezoelectric output signals, and charging capacity are also evaluated. Out of the several films fabricated, the optimized Cs2SnI6_PVDF film shows a piezoelectric coefficient (d33) value of ∼200 pm V-1 and a remanent polarization of ∼0.74 μC cm-2 estimated from piezoresponse force microscopy and polarization hysteresis loop measurement, respectively. The optimized Cs2SnI6_PVDF-based device produced an instantaneous output voltage of ∼167 V, a current of ∼5.0 μA, and a power of ∼835 μW across a 5 MΩ resistor when subjected to periodic vertical compression. The output voltage of this device is used to charge a capacitor with a 10 μF capacitance up to 2.2 V, which is then used to power some commercial LEDs. In addition to being used as a pressure sensor, the device is employed to monitor human physiological activities. The device demonstrates excellent operational durability over a span of several months in an ambient environment vouching for its exceptional potential in application to mechanical energy harvesting and pressure sensing applications.

Field‐Induced Ferroelectric Phase Evolution During Polarization “Wake‐Up” in Hf0.5Zr0.5O2 Thin Film Capacitors

AbstractAs an emerging nonvolatile memory technology, HfO2‐based ferroelectrics exhibit excellent compatibility with silicon CMOS process flows; however, the reliability of polarization switching in these materials remains a major challenge. During repeated field programming and erase of the polarization state of initially pristine HfO2‐based ferroelectric capacitors, the magnitude of the measured polarization increases, a phenomenon known as “wake‐up”. In this study, the authors attempt to understand what causes the wake‐up effect in Hf0.5Zr0.5O2 (HZO) capacitors using nondestructive methods that probe statistically significant sample volumes. Synchrotron X‐ray diffraction reveals a concerted shift in HZO Bragg peak position as a function of polarization switching cycle number in films prepared under conditions such that they exhibit extremely large (≈3000%) wake‐up. In contrast, a control sample with insignificant wake‐up shows no such peak shift. Capacitance – voltage measurements show evolution in the capacitance loop with switching cycle number for the wake‐up sample and no change for the control sample. Piezoresponse force microscopy measurements are utilized to visualize the domain switching with wake‐up. The combination of these observations clearly demonstrates that wake‐up is caused by a field‐driven phase transformation of the tetragonal phase to the metastable ferroelectric orthorhombic phase during polarization switching of HZO capacitors.

Ferroelectricity in Niobium Oxide Dihalides NbOX2 (X = Cl, I): A Macroscopic- to Microscopic-Scale Study.

2D materials with ferroelectric and piezoelectric properties are of interest for energy harvesting, memory storage and electromechanical systems. Here, we present a systematic study of the ferroelectric properties in NbOX2 (X = Cl, I) across different spatial scales. The in-plane ferroelectricity in NbOX2 was investigated using transport and piezoresponse force microscopy (PFM) measurements, where it was observed that NbOCl2 has a stronger ferroelectric order than NbOI2. A high local field, exerted by both PFM and scanning tunneling microscopy (STM) tips, was found to induce 1D collinear ferroelectric strips in NbOCl2. STM imaging reveals the unreconstructed atomic structures of NbOX2 surfaces, and scanning tunneling spectroscopy was used to probe the electronic states induced at defect (vacancy) sites.

Room-temperature electric field-induced out-of-plane ferroelectric polarization in strain-free freestanding 2D SrTiO3 membranes

SrTiO3 (STO), a room-temperature paraelectric material in bulk form, has been a rich playground for emergent phenomena for decades. As an emerging material, great attention has been paid to freestanding 2D STO thin films. Recently, the room-temperature ferroelectricity has been unveiled in strained STO thin films; however, it remains an open question whether the strain-free freestanding 2D STO thin film is room-temperature ferroelectric or not. Here, we report the electric field-induced out-of-plane ferroelectric polarization in large-scale, freestanding, and strain-free 2D STO membranes at room temperature. High-resolution piezoresponse force microscopy measurements show that polarization in freestanding strain-free STO membranes can be switched by electric field and persist for an hour. The first-principles calculations suggest that the intrinsic defects such as oxygen vacancies could be linked to the observed spontaneous out-of-plane polarization in 2D STO membranes, which could be further enhanced by external electric field due to the induced symmetry breaking. Our work reports the unprecedented room-temperature ferroelectric polarization in strain-free freestanding 2D STO membranes, unlocking the great potential of freestanding 2D STO for the applications in novel electronic and logic-in-memory devices.

Comparative study of thermal annealing effect on undoped and Mn-doped (K,Na)NbO3 nanofibers

The (K,Na)NbO3 (KNN)-based ceramics have been deemed one of the most promising lead-free piezoelectric materials replacing lead-containing ones. In our study, we have prepared both undoped and Mn-doped KNN nanofibers via the electrospinning method and investigated how the thermal annealing process affects their structural, crystallographic, and piezoelectric properties. X-ray diffraction measurements suggest that the crystallization occurs around 450°C in these fibers, and with increasing annealing temperature, both undoped and Mn-doped nanofibers become granular with small grains forming along the fiber, accompanying the crystallization. Both exhibit increasing piezoelectric properties with annealing temperature based on the piezoresponse force microscopy measurements with Mn-doping, leading to a higher piezoelectric response.

The influence of the cooling rate onto efficiency of the poling by field cooling of BaTiO3 ceramics

The influence of the cooling rate during poling by cooling in a constant field of the barium titanate ceramics was studied by ex situ piezoresponse force microscopy measurements after macroscopic poling. The rearrangement of the domain structure and the value of the averaged local piezoelectric coefficient were shown to be largely determined by the cooling rate. It was revealed that the high cooling rate stimulated increasing the domain sizes, which in turn increased the average piezoelectric response due to a better alignment of the polarization.

Biferroelectricity of a homochiral organic molecule in both solid crystal and liquid crystal phases

Ferroelectricity, existing in either solid crystals or liquid crystals, gained widespread attention from science and industry for over a century. However, ferroelectricity has never been observed in both solid and liquid crystal phases of a material simultaneously. Inorganic ferroelectrics that dominate the market do not have liquid crystal phases because of their completely rigid structure caused by intrinsic chemical bonds. We report a ferroelectric homochiral cholesterol derivative, β-sitosteryl 4-iodocinnamate, where both solid and liquid crystal phases can exhibit the behavior of polarization switching as determined by polarization–voltage hysteresis loops and piezoresponse force microscopy measurements. The unique long molecular chain, sterol structure, and homochirality of β-sitosteryl 4-iodocinnamate molecules enable the formation of polar crystal structures with point group 2 in solid crystal phases, and promote the layered and helical structure in the liquid crystal phase with vertical polarization. Our findings demonstrate a compound that can show the biferroelectricity in both solid and liquid crystal phases, which would inspire further exploration of the interplay between solid and liquid crystal ferroelectric phases.

Revealing photovoltaic behavior in 2D hybrid perovskite ferroelectric single-crystalline microwire arrays for self-powered photodetectors

Hybrid perovskite ferroelectrics produce a revival of interest in ferroelectric photovoltaics because of the robust ferroelectricity coexisting with superior semiconducting properties. An electric field via perovskite ferroelectrics can affect their photovoltaic behavior; however, the fundamental understanding of the electric-field-induced effects remains comparatively elusive. Herein, (EA)2(MA)2Pb3Br10 single-crystalline microwire arrays (MWs) are synthesized for the fabrication of self-powered photodetectors and characterized with evident in-plane multiaxial ferroelectricity by piezoresponse force microscopy (PFM) measurements. Upon systematic investigations via a dynamic poling process, including electrical-poling-dependent photocurrent, Kelvin probe force microscopy (KPFM) mapping and ferroelectric polarization switching, we reveal that the coupling of ion migration and ferroelectric photovoltaic effect dominate the photovoltaic behavior within (EA)2(MA)2Pb3Br10 MWs. Such electric-field-induced effects are responsible for the self-powered ability in (EA)2(MA)2Pb3Br10 MWs-based photodetectors with accelerated response time, switchable photoelectric responses and large short-circuit current density. Our findings provide fundamental insight into the photovoltaic behavior under an electric field in perovskite ferroelectrics, and the electrical-poling-manipulated dynamics pave the way for innovative self-powered optoelectronic devices.

Enhancement of ferromagnetism in a multiferroic La–Co co-doped BiFeO3 thin films

BiFeO3-based materials have attracted considerable attention owing to their room-temperature multiferroic properties and ultrahigh ferroelectric polarization. In this study, La and Co co-doped BiFeO3 (BLFCO) thin films were fabricated on Pt (111)/Ti/SiO2/Si substrate buffered by Nb-doped (0.7 wt.%) SrTiO3 via pulsed laser deposition. Experiments revealed that the film deposited at 0.2 Pa comprised the predominant orientation of BiFeO3 (BFO) (111), and its surface root-mean-square roughness was 0.96 nm. The saturation magnetization of the film reached 25.3 emu cm3, which was an order of magnitude higher than that of the pure BFO film, as well as the remanent magnetization reached 1.8 emu cm−3. The result was attributed to the predominant orientation and small grain size of BLFCO films. The piezo-response force microscopy measurements revealed the co-doped film possesses well repeatable performance of polarization reversal, and the presence of ferroelectric orders with an asymmetric ‘butterfly’ structure. These results are helpful for further improving the performance of BFO multifunctional devices.