Piezoresponse Images Research Articles

Composition‐Dependent Ferroelectricity of LuFeO3 Orthoferrite Thin Films

AbstractThis work characterizes the structural, magnetic, and ferroelectric properties of epitaxial LuFeO3 orthoferrite thin films with different Lu/Fe ratios. LuFeO3 thin films are grown by pulsed laser deposition on SrTiO3 substrates with Lu/Fe ratio ranging from 0.6 to 1.5. LuFeO3 is antiferromagnetic with a weak canted moment perpendicular to the film plane. Piezoresponse force microscopy imaging and switching spectroscopy reveal room temperature ferroelectricity in Lu‐rich and Fe‐rich films, whereas the stoichiometric film shows little polarization. Ferroelectricity in Lu‐rich films is present for a range of deposition conditions and crystallographic orientations. Positive‐up‐negative‐down ferroelectric measurements on a Lu‐rich film yield ≈13 µC cm−2 of switchable polarization, although the film also shows electrical leakage. The ferroelectric response is attributed to antisite defects analogous to that of Y‐rich YFeO3, yielding multiferroicity via defect engineering in a rare earth orthoferrite.

Achieving Enhanced Energy‐Storage Performances in Bi0.5(K0.15Na0.85)0.5TiO3 Ceramics Modified by Sr(Ti0.85Zr0.15)O3

Dielectric Ceramics In article number 2200071, Zunping Xu and co-workers synthesize lead-free (1–x)Bi0.5(K0.15Na0.85)0.5 TiO3-xSr(Ti0.85Zr0.15)O3 ((1–x)BKNT-xSTZ, x = 0.1–0.4) ceramics by citrate combustion method. The hysteresis loops for 0.7BKNT-0.3STZ at different frequencies and different temperatures are shown in the two left graphics on the cover, while the bottom right image presents the piezoresponse force microscopy image of the 0.7BKNT-0.3STZ ceramic. The Wre and η are given in the top right image.

Electro-active properties of nanostructured films of cytosine and guanine nucleobases

The discovery of multifunctional properties related to electro-activity of organic systems of biomolecules is important for a variety of applications, especially for devices in the realm of biocompatible sensors and/or bioactuators. A further step towards such applications is to prepare thin films with the required properties. Here, the investigation is focused on the characterization of films of guanine and cytosine nucleobases, prepared by thermal evaporation—an industrial accessible deposition technique. The cytosine films have an orthorhombic non-centrosymmetric structure and grow in two interconnected nanostructured fractal patterns, of nearly equal proportion. Piezoresponse force microscopy images acquired at room temperature on the cytosine films display large zones with antiparallel alignment of the vertical components of the polarization vector. Guanine films have a dense nano-grained morphology. Our studies reveal electrical polarization switching effects which can be related to ferroelectricity in the films of guanine molecules. Characteristic ferroelectric polarization–electric-field hysteresis loops showing large electrical polarization are observed at low temperatures up to 200 K. Above this temperature, the guanine films have a preponderant paraelectric phase containing residual or locally induced nano-scopic ferroelectric domains, as observed by piezoresponse force microscopy at room temperature.

The surface piezoresponse of tetragonal tungsten bronze oxides with different ordering of cations in A-sublattice

The piezoresponse (PR) images of microsized surface areas of the hot-pressed ceramics samples K4Bi2Nb10O30 , K2Sr4NbwO3 0, and Na2Sr4Nbl0O 30, presenting different types of the inter-octahedral cations ordering, were obtained. The Fourier analysis of the PR data showed that the mesoscale regions form subsystems responsible for the electrical activity of the sample, the contribution of which manifests itself depending on the type of cation ordering.

Large domain-wall currents in epitaxial Au/BiFeO3/SrRuO3 thin-film capacitors with modulated oxygen vacancy and wall densities

Large domain wall (DW) conductivity in an insulating ferroelectric plays an important role in the future nanosensors and nonvolatile memories. However, the wall current was usually too small to drive high-speed memory circuits and other agile nanodevices requiring high output-powers. Here, a large domain-wall current of 67.8 μA in a high on/off ratio of ~4460 was observed in an epitaxial Au/BiFeO3/SrRuO3 thin-film capacitor with the minimized oxygen vacancy concentration. The studies from read current-write voltage hysteresis loops and piezo-response force microscope images consistently showed remaining of partially unswitched domains after application of an opposite poling voltage that increased domain wall density and wall current greatly. A theoretical model was proposed to explain the large wall current. According to this model, the domain reversal occurs with the appearance of head-to-head and tail-to-tail 180° domain walls (DWs), resulting in the formation of highly conductive wall paths. As the applied voltage increased, the domain-wall number increased to enhance the on-state current, in agreement with the measurements of current-voltage curves. This work paves a way to modulate DW currents within epitaxial Au/BiFeO3/SrRuO3 thin-film capacitors through the optimization of both oxygen vacancy and domain wall densities to achieve large output powers of modern domain-wall nanodevices.

Lowering of Tc in Van Der Waals Layered Materials Under In-Plane Strain.

The dependence of electromechanical behavior on strain in ferroelectric materials can be leveraged as parameter to tune ferroelectric properties such as the Curie temperature. For van der Waals materials, a unique opportunity arises because of wrinkling, bubbling, and Moiré phenomena accessible due to structural properties inherent to the van der Waals gap. Here, we use piezoresponse force microscopy and unsupervised machine learning methods to gain insight into the ferroelectric properties of layered CuInP2S6 where local areas are strained in-plane due to a partial delamination, resulting in a topographic bubble feature. We observe significant differences between strained and unstrained areas in piezoresponse images as well as voltage spectroscopy, during which strained areas show a sigmoid-shaped response usually associated with the response measured around the Curie temperature, indicating a lowering of the Curie temperature under tensile strain. These results suggest that strain engineering might be used to further increase the functionality of CuInP2S6 through locally modifying ferroelectric properties on the micro- and nanoscale.

Piezoresponse force microscopy imaging and its correlation with cantilever spring constant and frequency

Single-frequency piezoresponse force microscopy (PFM) images of a BiFeO3/DyScO3(110) thin film, obtained with long (kc = 0.82 N/m) and short (kc = 7.64 N/m) cantilevers, were analyzed as a function of the applied voltage frequency. For long cantilevers, the electrostatic and electrostrictive contributions were identified. These contributions were reduced with a frequency near the second mode of the contact resonance; while for short cantilevers, the first mode was necessary. A method for domain structure analyses, to discriminate a ferroelectric behavior from a non-ferroelectric, through the optimization of PFM images, is also described. The analysis can be extended to cantilevers with different spring constants and ferroelectric materials.

Electrical Tunability of Domain Wall Conductivity in LiNbO3 Thin Films.

Domain wall nanoelectronics is a rapidly evolving field, which explores the diverse electronic properties of the ferroelectric domain walls for application in low-dimensional electronic systems. One of the most prominent features of the ferroelectric domain walls is their electrical conductivity. Here, using a combination of scanning probe and scanning transmission electron microscopy, the mechanism of the tunable conducting behavior of the domain walls in the sub-micrometer thick films of the technologically important ferroelectric LiNbO3 is explored. It is found that the electric bias generates stable domains with strongly inclined domain boundaries with the inclination angle reaching 20° with respect to the polar axis. The head-to-head domain boundaries exhibit high conductance, which can be modulated by application of the sub-coercive voltage. Electron microscopy visualization of the electrically written domains and piezoresponse force microscopy imaging of the very same domains reveals that the gradual and reversible transition between the conducting and insulating states of the domain walls results from the electrically induced wall bending near the sample surface. The observed modulation of the wall conductance is corroborated by the phase-field modeling. The results open a possibility for exploiting the conducting domain walls as the electrically controllable functional elements in the multilevel logic nanoelectronics devices.

Microscopic study of polydopamine modified BaTiO3/poly(vinylidene fluoride-trifluoroethylene) nanocomposite films

Poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] has been intensively studied because of its good piezoelectric properties and at the same time being flexible and bio-compatible, making it a strong candidate for biomedical application. Doping with inorganic piezoceramic nanoparticles will enhance its ferro−/piezoelectricity. Further enhancements in ferro−/piezoelectricity of P(VDF-TrFE) are expected from surface modification of the nanoparticles through improving the bonding between the nanoparticles and polymer matrix. In this paper, a piezoresponse force microscopy (PFM) study on P(VDF-TrFE) doped with polydopamine modified BaTiO3 (BT-Pdop) nanoparticles is introduced. The microscopic study is complicated by the fine domain structures. As ferro−/piezoelectricity is directly related with domain size, instead of a domain-wise study, a statistical correlation (domain size) analysis was carried out for this composite. Since PFM measurement is based on the converse piezoelectric effect (the first harmonic response of the sample surface upon the applied electric stimulation), topographical effects should be taken into account in analyzing the related piezoresponse images. Hence, the correlation lengths of both topographical and piezoresponse images on the same area were studied. The doping dependence of the correlation length of BT-Pdop/P(VDF-TrFE) behaves differently for piezoresponse (polarization) and topography (grain) images. In particular, the correlation lengths of piezoresponse and topography images are in different ranges: 23–29 nm for the former and 55–66 nm for the latter; also, the composition dependences are different. Accordingly, the piezocorrelation reflects the material's real piezoresponse. It can also be deduced that the macroscopic polarization behavior in Pdop-BT/P(VDF-TrFE) results from extrinsic contributions.

Nanoscale studies of magnetoelectric coupling in multiferroic BTO–CFO composite

We report on the existence of interacting ferromagnetic and ferroelectric nanodomains in multiferroic 0.65BaTiO3–0.35CoFe2O4 (BTO–CFO) composite synthesized using a conventional solid-state route. X-ray diffraction study shows the coexistence of individual phases (BTO and CFO) of the composite material. Nanometric studies on the sample pellet of BTO–CFO composite are carried out using Scanning Probe Microscopy (SPM) to probe local ferroelectric and magnetic ordering. Importantly, the interactive nanodomains of individual phases are distinctly seen in the Atomic Force Acoustic Microscopy (AFAM) and Piezo-Force Microscopy (PFM) studies, which are done to investigate the magnetoelectric coupling in the BTO–CFO composite. Elasticity maps and piezoresponse images show local inhomogeneities with respect to the distribution of magnetoelectric coupling in the composite.

The local cationic configurations in the K2Sr4Nb10O30 lattice and peculiarities of its surface piezoresponse

ABSTRACTBasing on the X-ray diffraction studies, a possibility of six local cationic configurations was established for K2Sr4Nb10O30 (KSN) having a tetragonal tungsten bronze-type structure. The association of these locally charged configurations may lead to the formation of the electrically active nanoregions. The piezoresponse force microscopy studies revealed at the KSN surface the presence of a subsystem of nano- or meso-size regions, which may be electrically active. Distribution and reference size of these regions have been estimated using the Fourier filtering of the source piezoresponse image.

Magnetostriction-polarization coupling in multiferroic Mn2MnWO6

Double corundum-related polar magnets are promising materials for multiferroic and magnetoelectric applications in spintronics. However, their design and synthesis is a challenge, and magnetoelectric coupling has only been observed in Ni3TeO6 among the known double corundum compounds to date. Here we address the high-pressure synthesis of a new polar and antiferromagnetic corundum derivative Mn2MnWO6, which adopts the Ni3TeO6-type structure with low temperature first-order field-induced metamagnetic phase transitions (TN = 58 K) and high spontaneous polarization (~ 63.3 μC·cm−2). The magnetostriction-polarization coupling in Mn2MnWO6 is evidenced by second harmonic generation effect, and corroborated by magnetic-field-dependent pyroresponse behavior, which together with the magnetic-field-dependent polarization and dielectric measurements, qualitatively indicate magnetoelectric coupling. Piezoresponse force microscopy imaging and spectroscopy studies on Mn2MnWO6 show switchable polarization, which motivates further exploration on magnetoelectric effect in single crystal/thin film specimens.

Nanoscale investigation of ferroelectric and piezoelectric properties in (Pb,Ca)TiO3 thin films grown on LaNiO3/LaAlO3(1 0 0) and Pt/Si(1 1 1) using piezoresponse force microscopy

Single phase polycrystalline and strong textured Pb0.76Ca0.24TiO3 (PCT24) thin films have been grown on platinum coated silicon and LaAlO3(100) (LAO) substrates using LaNiO3 (LNO) as a buffer layer by chemical solution deposition. X-ray diffraction measurements showed that the PCT24 thin films crystallize in a highly a-oriented single phase on LNO/LAO(100). The effects of the LNO buffer layer, nature of the substrate, and film orientation on the ferroelectric and piezoelectric properties were investigated in the nanoscale range using piezoresponse force microscopy (PFM). Local piezoelectric hysteresis loops for PCT24/LNO/LAO(100) oriented films and PCT24/Pt/Si polycrystalline films were measured on selected grains. From these piezoloops, PCT24/LNO/LAO(100) oriented films show a higher response (maximum relative d33 value) than PCT24/Pt/Si polycrystalline films. Furthermore, the comparison of simultaneously acquired surface morphology and piezoresponse images of the PCT24/Pt/Si films revealed the presence of inactive grain regions. In contrast, highly a-oriented films showed a higher presence of active grains. Our observations suggest that the improvement of ferro/piezoelectric properties is greatly associated with the use of LNO buffer layers and the growth of highly a-oriented films.

Description of New Bioelectromechanical Properties in Alginate: An Insight with a Computer Simulation

Alginate biopolymer from Tropicalgin C302245 was studied by means of piezoresponse force microscopy imaging, scanning electron microscopy, powder X-rays, infrared spectroscopy and computer simulations. Local piezoresponse force microscopy images show possible ferroelectric domains detected in the out of plane mode and these results are confirmed by the second harmonic generation analysis. Alginate powder is composed by diatom frustules containing a cristobalite-like compound, amorphous silica and chitin. The experimental results are explained by MM+ and PM3 computer simulations that establish that the self-assembly of the alginate molecules enhance the polarization increasing the molecular collective dipole moment. Alginate molecular properties might open interesting possibilities for organic technological applications.

Single- and Few-Layers MoSe2 Nanoflowers: Synthesis, Characterization, and Their Piezoresponse

We demonstrate a hydrothermal process at temperature of 180oC to synthesize a single to few layers MoSe2 nanoflowers which exhibit a high surface-to-volume ratio and is stable under room temperature in ambient air. The crystal structure of the MoSe2 nanoflowers has been confirmed by X-ray diffraction pattern, Raman, and High-resolution transmission electron microscopy (HRTEM). On the basis of the scanning electron microscope (SEM) image, the size of the MoSe2 nanoflowers is in the range of the 300-500nm. HRTEM image reveals that the MoSe2 nanoflowers possess a great number of the single- and few-layers, which further confirms that our nanoflowers display the plentiful amount of active surface sites. In addition, the lattice fringes between each single layer is about 0.66 nm, which is slightly larger than the reported value of 0.62 nm for the MoSe2. The piezoresponse force microscopy image further shows that the MoSe2 nanoflowers exhibits significantly piezoelectric potential which is generated from the active surface sites of the petals in the MoSe2 nanoflowers. Reference[1-6] 1. Duerloo, K.-A.N., M.T. Ong, and E.J. Reed, Intrinsic piezoelectricity in two-dimensional materials. The Journal of Physical Chemistry Letters, 2012. 3(19): p. 2871-2876. 2. Kaasbjerg, K., K.S. Thygesen, and A.-P. Jauho, Acoustic phonon limited mobility in two-dimensional semiconductors: Deformation potential and piezoelectric scattering in monolayer MoS 2 from first principles. Physical Review B, 2013. 87(23): p. 235312. 3. Alyörük, M.M., et al., Promising Piezoelectric Performance of Single Layer Transition-Metal Dichalcogenides and Dioxides. The Journal of Physical Chemistry C, 2015. 119(40): p. 23231-23237. 4. Qi, J., et al., Piezoelectric effect in chemical vapour deposition-grown atomic-monolayer triangular molybdenum disulfide piezotronics. Nature communications, 2015. 6. 5. Wu, T. and H. Zhang, Piezoelectricity in Two ‐ Dimensional Materials. Angewandte Chemie International Edition, 2015. 54(15): p. 4432-4434. 6. Chhowalla, M., et al., The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets. Nature chemistry, 2013. 5(4): p. 263-275.

Microstructure and local electrical investigation of lead-free α-La2WO6 ferroelectric thin films by piezoresponse force microscopy

La2WO6 (LWO) thin films with the α high-temperature orthorhombic polymorph have been grown on (001)-LaAlO3 (LAO) substrates by pulsed laser deposition. The X-ray diffraction study evidences that the films are (00l)-oriented. This matching appears to be in good agreement with the compatibility between the α-LWO bulk/LAO substrate crystal lattices. Pole figure measurements lead to the following crystallographic relationships between the film and substrate: [100]α-LWO║ [110]LAO, [010]α-LWO ║ [11-0]LAO and [001]α-LWO ║ [001]LAO. Film cell parameters measured on a LaNiO3 buffer layer, setting as electrode for electric measurements, lead to a=16.217(5) Å, b=5.607(1) Å and c=8.918(8) Å. A Williamson–Hall plot carried out on a LWO film reveals that the microstrain is 0.16% along the growth direction while the coherent domain dimension is 16nm. Piezoresponse force microscopy imaging and piezoloops recording demonstrate that these films are piezoelectric/ferroelectric on nanoscale.

Positive Effect of an Internal Depolarization Field in Ultrathin Epitaxial Ferroelectric Films

The effect of intentionally introducing a large depolarization field in (001)‐oriented, epitaxial Pb(Zr0.2TiO0.8)O3 (PZT) ultrathin films grown on La0.67Sr0.33MnO3 (LSMO) buffered SrTiO3 (STO) substrates is investigated. Inserting between 3 and 10 unit cells of STO between two 3 nm thick PZT films significantly influences the out‐of‐plane (c) lattice constant as well as the virgin domain state. Piezoresponse force microscopy images reveal a nanoscale (180°) polydomain structure in these films. In comparison, a “reference” single layer PZT sample (6 nm thick without STO spacer) exhibits an elongated PZT c‐axis (0.416 nm) and is preferentially “down”‐polarized with large regions of monodomain contrast. It shows asymmetric switching loops (i.e., imprint) coupled with sluggish domain switching under external bias. It is shown that the insertion of STO drives a monodomain to 180° polydomain transition in the as‐grown state, which reduces the imprint by 80%. The insertion of the STO also profoundly improves dielectric leakage and hence the distribution of the applied electric field. Consequently, the critical pulse duration of the electric field required to initiate domain switching is reduced by two orders of magnitude relative to the reference sample. These results demonstrate the possibility of manipulating the depolarization field in such a way that it has positive effects on the ferroelectric behavior of ultrathin PZT films.

Low-temperature dynamics of ferroelectric domains in PbZr0.3Ti0.7O3 epitaxial thin films studied by piezoresponse force microscopy

Dynamics of domain boundaries is expected to change drastically at low absolute temperatures but direct experimental information for this temperature range is still lacking. To clarify the mechanism of low-temperature domain dynamics, we studied the growth of ferroelectric domains in the temperature range 4.2–295 K using the out-of-plane piezoresponse mode of a cryogenic atomic force microscope (AFM). Nanoscale 180° domains were created in epitaxial PbZr0.3Ti0.7O3 films by applying short voltage pulses between the conductive AFM tip brought into contact with the bare film surface and the bottom LaSr0.7Mn0.3O3 electrode. A quantitative analysis of acquired piezoresponse images enabled us to determine the in-plane domain size as a function of the writing voltage and pulse duration. It is found that at all studied temperatures the dependence of this size on the pulse duration can be fitted by a logarithmic function, which indicates that the domain-wall velocity exponentially depends on the driving electric field. The theoretical analysis of experimental data shows that the observed low-temperature domain dynamics is consistent with the creep of domain boundaries occurring in the presence of defects and structural nanoheterogeneities.

Nanoscale Ferroelectric Switchable Polarization and Leakage Current Behavior in (Ba0.50Sr0.50)(Ti0.80Sn0.20)O3 Thin Films Prepared Using Chemical Solution Deposition

Nanoscale switchable ferroelectric (Ba0.50Sr0.50)(Ti0.80Sn0.20)O3‐BSTS polycrystalline thin films with a perovskite structure were prepared on Pt/TiOx/SiO2/Si substrate by chemical solution deposition. X‐ray diffraction (XRD) spectra indicate that a cubic perovskite crystalline structure and Raman spectra revealed that a tetragonal perovskite crystalline structure is present in the thin films. Sr2+ and Sn4+ cosubstituted film exhibited the lowest leakage current density. Piezoresponse Force Microscopy (PFM) technique has been employed to acquire out‐of‐plane (OPP) piezoresponse images and local piezoelectric hysteresis loop in polycrystalline BSTS films. PFM phase and amplitude images reveal nanoscale ferroelectric switching behavior at room temperature. Square patterns with dark and bright contrasts were written by local poling and reversible nature of the piezoresponse behavior was established. Local piezoelectric butterfly amplitude and phase hysteresis loops display ferroelectric nature at nanoscale level. The significance of this paper is to present ferroelectric/piezoelectric nature in present BSTS films at nanoscale level and corroborating ferroelectric behavior by utilizing Raman spectroscopy. Thus, further optimizing physical and electrical properties, BSTS films might be useful for practical applications which include nonvolatile ferroelectric memories, data‐storage media, piezoelectric actuators, and electric energy storage capacitors.

Electrical transport and resistance switching characteristics of BiFeO3/Nb:SrTiO3/GaAs heterostructure fabricated by pulsed laser deposition

BiFeO3 thin films were epitaxially grown on (001) GaAs substrate by pulsed laser deposition with Nb doped SrTiO3 as a buffer layer. Piezoresponse force microscopy images exhibit effective ferroelectric switching of the heterostructure. The temperature-dependent current-voltage characteristics of the heterostructure reveal a resistance switching phenomenon and diode-like behavior with a rectifying ratio of 2 × 102 at the applied voltage of ±13.4 V. The electrical transport mechanism in the heterostructure has been illustrated by constructing the energy band structure. In addition, the resistance switching behavior in the heterostructure could be explained by the polarization modulation of the depletion region at the interface of the semiconductor and the ferroelectric layers.