Lateral Piezoresponse Force Microscopy Research Articles

Energy storage properties influenced by relaxor ferroelectric properties dependent on the growth direction of epitaxial Bi2SiO5 thin films

Ferroelectric Bi2SiO5 (BSO) thin films were deposited by pulsed laser deposition on Nb-doped (100), (110) and (111) SrTiO3 (Nb:STO) substrates, resulting in (001)-, (113)- and (204)-oriented epitaxial films. Due to the crystallinity of BSO, in which the Bi2O2 layers are formed perpendicular to the c-axis direction, the (001)-oriented epitaxial BSO thin film showed the lowest remanent polarization and the best leakage current characteristics. On the other hand, the (113)- and (204)-oriented films showed an increase in remanent polarization due to the improvement of a-oriented crystallinity. Through experiments using vertical and lateral piezoresponse force microscopy, it has been confirmed that the distribution of in-plane-oriented domains reducing remanent polarization decreases in the order of (001)-, (113)- and (204)-oriented epitaxial BSO thin films. The epitaxial BSO thin films that exhibit ferroelectric hysteresis loops similar to the relaxor ferroelectric thin films tended to have improved energy storage characteristics as a result of improved remanent polarization and saturation polarization. In particular, the (113)-oriented epitaxial BSO thin film showed a high recoverable energy density of about 41.6 J cm−3 and an energy storage efficiency of about 85.6%.

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.

Microscopic piezoelectric response of poly (3-hydroxybutyrate-co-3-hydroxyhexanote) fiber

Microscopic piezoelectric response of a poly (3-hydroxybutyrate-co-3-hydroxyhexanote) (PHBH) fiber, a polymer more biodegradable than poly (lactic acid) (PLA), are observed. The crystallization of PHBH fibers is enhanced by the annealing, and a strong piezoelectric response is observed in the particles. Different positive and negative piezoelectric responses are observed within a single PHBH fiber particle by lateral piezoresponse force microscopy. The piezoelectric response of PHBH fiber is suggested to be due to piezoelectricity, since the molecular chain of PHBH fiber has a helical structure similar to that of PLA and cellulose. In addition, the deformation is confirmed to be a displacement corresponding to particles of 100–300 nm in diameter.

Electromechanical Coupling in Collagen Measured under Increasing Relative Humidity.

The functional role of collagen piezoelectricity has been under debate since the discovery of piezoelectricity in bone in 1957. The possibility that piezoelectricity plays a role in bone remodeling has generated interest in the investigation of this effect in relevant physiological conditions; however, there are conflicting reports as to whether collagen is piezoelectric in a humid environment. In macroscale measurements, the piezoelectricity in hydrated tendon has been shown to be insignificant compared to dehydrated tendon, whereas, at the nanoscale, the piezoelectric effect has been observed in both dry and wet bone using piezoresponse force microscopy (PFM). In this work, the electromechanical properties of type I collagen from a rat tail tendon have been investigated at the nanoscale as a function of humidity using lateral PFM (LPFM) for the first time. The relative humidity (RH) was varied from 10% to 70%, allowing the piezoelectric behavior to be studied dry, humid, as well as in the hydrated range for collagen in physiological bone (12% moisture content, corresponding to 40-50% RH). The results show that collagen piezoresponse can be measured across the humidity range studied, suggesting that piezoelectricity remains a property of collagen at a biologically relevant humidity.

Giant Nonlinear Optical Response via Coherent Stacking ofIn-Plane Ferroelectric Layers.

Thin ferroelectric materials hold great promise for compact nonvolatile memory and nonlinear optical and optoelectronic devices. Herein, an ultrathin in-plane ferroelectric material that exhibits a giant nonlinear optical effect, group-IV monochalcogenide SnSe, is reported. Nanometer-scale ferroelectric domains with ≈90°/270° twin boundaries or ≈180° domain walls are revealed in physical-vapor-deposited SnSe by lateral piezoresponse force microscopy. Atomic structure characterization reveals both parallel and antiparallel stacking of neighboring van der Waals ferroelectric layers, leading to ferroelectric or antiferroelectric ordering. Ferroelectric domains exhibit giant nonlinear optical activity due to coherent enhancement of second-harmonic fields and the as-resulted second-harmonic generationwasobserved to be 100 times more intense than monolayer WS2 . This work demonstratesin-plane ferroelectric ordering and giant nonlinear optical activity in SnSe, which paves the way for applications in on-chip nonlinear optical components and nonvolatile memory devices.

What lies beneath? Investigations of atomic force microscopy-based nano-machining to reveal sub-surface ferroelectric domain configurations in ultrathin films

Multiferroic materials, encompassing simultaneous ferroelectric and ferromagnetic polarization states, are enticing multi-state materials for memory scaling beyond existing technologies. Aurivillius phase B6TFMO (Bi6TixFeyMnzO18) is a unique room temperature multiferroic material that could ideally be suited to future production of revolutionary memory devices. As miniaturization of electronic devices continues, it is crucial to characterize ferroelectric domain configurations at very small (sub-10 nm) thickness. Direct liquid injection chemical vapor deposition allows for frontier development of ultrathin films at fundamental (close to unit cell) dimensions. However, layer-by-layer growth of ultrathin complex oxides is subject to the formation of surface contaminants and 2D islands and pits, which can obscure visualization of domain patterns using piezoresponse force microscopy (PFM). Herein, we apply force from a sufficiently stiff diamond cantilever while scanning over ultrathin films to perform atomic force microscopy (AFM)-based nano-machining of the surface layers. Subsequent lateral PFM imaging of sub-surface layers uncovers 45° orientated striped twin domains, entirely distinct from the randomly configured piezoresponse observed for the pristine film surface. Furthermore, our investigations indicate that these sub-surface domain structures persist along the in-plane directions throughout the film depth down to thicknesses of less than half of an Aurivillius phase unit cell (˂ 2.5 nm). Thus, AFM-based nano-machining in conjunction with PFM allows demonstration of stable in-plane ferroelectric domains at thicknesses lower than previously determined for multiferroic B6TFMO. These findings demonstrate the technological potential of Aurivillius phase B6TFMO for future miniaturized memory storage devices. Next-generation devices based on ultrathin multiferroic tunnel junctions are projected.

Strong Piezoelectricity in 3R‐MoS2 Flakes

AbstractDistinct from conventional 2H‐MoS2, recently synthesized 3R‐MoS2 exhibits a noncentrosymmetric atomic structure of the trigonal “building blocks” and, thus, remarkable piezoelectric characteristics of ultrathin 3R‐MoS2 flakes are predicated theoretically. This paper reveals, for the first time, very high piezoelectricity in 3R‐MoS2 flakes experimentally. Through applying mechanical stress to a 48 nm 3R‐MoS2 flake, a high output power density of 65 mW m‐2 is obtained and is at least one order larger than those from the corresponding monolayer MoS2 flake. With out‐of‐plane lateral piezoresponse force microscopy technique, the two piezoelectric coefficients d33 and d13 are analyzed to be ≈0.9 and ≈1.6 pm V‐1, respectively. These piezoelectric coefficients are not apparently dependent on the flake thickness. The findings suggest that 3R‐MoS2 is of excellent piezoelectric properties and it can be an excellent material for novel piezoelectric devices.

Unraveling Strain Gradient Induced Electromechanical Coupling in Twisted Double Bilayer Graphene Moiré Superlattices.

Moiré superlattices of 2D materials with a small twist angle are thought to exhibit appreciable flexoelectric effect, though unambiguous confirmation of their flexoelectricity is challenging due to artifacts associated with commonly used piezoresponse force microscopy (PFM). For example, unexpectedly small phase contrast (≈8°) between opposite flexoelectric polarizations is reported in twisted bilayer graphene (tBG), though theoretically predicted value is 180°. Here a methodology is developed to extract intrinsic moiré flexoelectricity using twisted double bilayer graphene (tDBG) as a model system, probed by lateral PFM. For small twist angle samples, it is found that a vectorial decomposition is essential to recover the small intrinsic flexoelectric response at domain walls from a large background signal. The obtained threefold symmetry of commensurate domains with significant flexoelectric response at domain walls is fully consistent with the theoretical calculations. Incommensurate domains in tDBG with relatively large twist angles can also be observed by this technique. A general strategy is provided here for unraveling intrinsic flexoelectricity in van der Waals moiré superlattices while providing insights into engineered symmetry breaking in centrosymmetric materials.

Quantitative probe for in-plane piezoelectric coupling in 2D materials

Piezoelectric response in two-dimensional (2D) materials has evoked immense interest in using them for various applications involving electromechanical coupling. In most of the 2D materials, piezoelectricity is coupled along the in-plane direction. Here, we propose a technique to probe the in-plane piezoelectric coupling strength in layered nanomaterials quantitively. The method involves a novel approach for in-plane field excitation in lateral Piezoresponse force microscopy (PFM) for 2D materials. Operating near contact resonance has enabled the measurement of the piezoelectric coupling coefficients in the sub pm/V range. Detailed methodology for the signal calibration and the background subtraction when PFM is operated near the contact resonance of the cantilever is also provided. The technique is verified by estimating the in-plane piezoelectric coupling coefficients (d11) for freely suspended MoS2 of one to five atomic layers. For 2D-MoS2 with the odd number of atomic layers, which are non-centrosymmetric, finite d11 is measured. The measurements also indicate that the coupling strength decreases with an increase in the number of layers. The techniques presented would be an effective tool to study the in-plane piezoelectricity quantitatively in various materials along with emerging 2D-materials.

Imaging Graphene Moiré Superlattices via Scanning Kelvin Probe Microscopy.

Moiré superlattices in van der Waals heterostructures are gaining increasing attention because they offer new opportunities to tailor and explore unique electronic phenomena. Using a combination of lateral piezoresponse force microscopy (LPFM) and scanning Kelvin probe microscopy (SKPM), we directly correlate ABAB and ABCA stacked graphene with local surface potential. We find that the surface potential of the ABCA domains is ∼15 mV higher (smaller work function) than that of the ABAB domains. First-principles calculations show that the different work functions between ABCA and ABAB domains arise from the stacking-dependent electronic structure. Moreover, while the moiré superlattice visualized by LPFM can change with time, imaging the surface potential distribution via SKPM appears more stable, enabling the mapping of ABAB and ABCA domains without tip-sample contact-induced effects. Our results provide a new means to visualize and probe local domain stacking in moiré superlattices along with its impact on electronic properties.

Few-layer tin sulfide (SnS): Controlled synthesis, thickness dependent vibrational properties, and ferroelectricity

• Consistently ultrathin SnS was synthesized on SnS 2 van der Waals substrates. • Real-time microscopy establishes underlying growth processes. • Experiment and calculations identify thickness-dependent vibrational properties. • First high-quality PFM maps of SnS ferroelectric polarization and domains. • Non-equilibrium stacking provides net polarization independent of thickness. Group IV monochalcogenides, anisotropic van der Waals crystals (and black phosphorus analogues), are attracting increasing interest due to a number of exceptional properties including anisotropic optoelectronics and multiferroicity predicted and in part realized in the ultrathin limit. Due to their enhanced chemical reactivity, both exfoliation and synthesis of monolayer and few-layer crystals pose challenges not found in other 2D/layered materials, such as the transition metal dichalcogenides. Here, we show that SnS synthesis on SnS 2 van der Waals substrates can address these challenges and consistently produces few-layer flakes, a capability that is explained via analysis of real-time microscopy of the growth process. Raman spectroscopy combined with efficient computations of the Raman-active modes across an extended thickness range enables a comprehensive understanding of the evolution of the vibrational properties of SnS with number of layers. Lateral piezoresponse force microscopy provides unprecedented insight into the stacking-dependent polarization and ferroelectric domain structures in large few-layer SnS flakes. The combined results establish a basis for further fundamental studies and applications of SnS and other group IV monochalcogenides in the few-layer regime.

Chain substitution caused sub-fibril level differences in electromechanical structure and property of wild-type and oim/oim collagen fibers

Electromechanical changes in type I collagen caused by diseases are pivotal for monitoring bone health and offering informative message of constructing biobased smart devices. Here, we employ a mouse model of osteogenesis imperfecta (oim/oim), which is genetically modified through mutating the α-2 chain to α-1 chain in the collagen fibrils of the wild-type model (+/+ or heterotrimer), resulting in three α-1 chains in the collagen fibrils (i.e., homotrimer). Piezoresponse force microscopy (PFM) is used to directly visualize the sub-micrometer structures and piezoresponses of +/+ and oim/oim collagen fibers. Results show that the compact and highly ordered +/+ collagen fibers possess larger in-plane piezoresponses than the loosely packed and randomly distributed oim/oim collagen fibers. The mean values of the lateral PFM amplitude are 108.53 pm and 77.72 pm with interquartile ranges of 98.56–117.47 pm and 71.21–85.93 pm for +/+ and oim/oim collagen fibers, respectively. Molecular simulations demonstrate that the structural stability and electrically induced activity of heterotrimer are better than those of homotrimer, suggesting better biopiezoelectricity of comprising diverse polar residues (atomic charges) within the oriented heterotrimeric collagen molecular structure. Our study provides a new insight into the functional changes of human osteogenesis imperfecta.

Fibril size-dependent control of polar ordering in type I collagen membranes

The most abundant protein in the human body, collagen, is widely used in tissue culture and engineering applications, spanning from substrate functionalization to fibrillar architectures and three-dimensional constructs. Collagen piezoelectricity provides an opportunity to exploit electromechanical coupling in these applications, wherein an applied mechanical stress generates charge, which might influence ion screening, protein absorption, and cell response. In type I collagen, the polarization direction follows the fibril orientation. Thus, control of fibril orientation and size in a collagen film or membrane may provide control of the polarization, enabling the creation of regions of uniform polarization direction. Here, aligned substrate-supported type I collagen membranes having fibril sizes from ∼100–500 nm are deposited using different osmotic concentrations (90, 190, and 290 mOsm/kg, from low to high ionic strength) to investigate the correlation between fibril size and piezoelectric properties. Lateral piezoresponse force microscopy is used to show that regions of uniform polarization orientation, as determined through 2D correlation analysis, decrease with increasing fibril size.

Crystallographic polarity measurements in two-terminal GaN nanowire devices by lateral piezoresponse force microscopy∗∗Contribution of an agency of the U.S. government; not subject to copyright.

Lateral piezoresponse force microscopy (L-PFM) is demonstrated as a reliable method for determining the crystallographic polarity of individual, dispersed GaN nanowires that were functional components in electrical test structures. In contrast to PFM measurements of vertically oriented (as-grown) nanowires, where a biased probe tip couples to out-of-plane deformations through the d33 piezoelectic coefficient, the L-PFM measurements in this study were implemented on horizontally oriented nanowires that coupled to shear deformations through the d15 coefficient. L-PFM phase-polarity relationships were determined experimentally using a bulk m-plane GaN sample with a known [0001] direction and further indicated that the sign of the d15 piezoelectric coefficient was negative. L-PFM phase images successfully revealed the in-plane [0001] orientation of self-assembed GaN nanowires as part of a growth polarity study and results were validated against scanning transmission electron microscopy lattice images. Combined characterization of electrical properties and crystallographic polarity was also implemented for two-terminal GaN/Al0.1Ga0.9N/GaN nanowires devices, demonstrating L-PFM measurements as a viable tool for assessing correlations between device rectification and polarization-induced band bending.

Calibration of the in-plane PFM response by the lateral force curves

Calibration technique for lateral piezoresponse force microscopy (PFM) based on lateral force curves measurements is introduced and discussed in terms of relative error connected with experimental realization. Dependences of lateral calibration coefficient on the tip motion velocity, normal loading force, humidity environment, and position of laser spot on the cantilever beam are considered. The maximal error of the measurements does not exceed 10%, which is close to the error of typical vertical PFM signal calibration based on force-distance curves. Proposed calibration technique is easy and accurate, which is important for the future development of quantitative PFM method.

Probing the Microstructure of Methylammonium Lead Iodide Perovskite Solar Cells

The microstructure of absorber layers is pivotally important for all thin‐film solar technologies. Using electron backscattered diffraction (EBSD), the crystal orientation in methylammonium lead iodide thin films with submicrometer resolution is reported. For the vast majority of (110) oriented grains, the c‐axis of the perovskite unit cell is oriented in‐plane. Although some adjacent grains exhibit the same in‐plane horizontal orientation of the c‐axis, no universal horizontal orientation of the c‐axis within the sample plane exists. The (110) crystal orientation correlates with an in‐plane orientation of the ferroelectric polarization as investigated by vertical and lateral piezoresponse force microscopy (PFM). The individual grains with different crystal orientations that exhibit different ferroelectric patterns and surface potentials are identified. The strong correlation between crystal orientation and ferroelectric polarization allows conclusions to be drawn about the microstructure from PFM measurements and, likewise, the ferroelectric polarization to be derived from crystallographic observations by EBSD.

High-resolution angle-resolved lateral piezoresponse force microscopy: Visualization of in-plane piezoresponse vectors.

Piezoresponse force microscopy (PFM) is a widely used tool for ferroelectric domain imaging. Lateral PFM (LPFM) utilizes the torsional vibration mode of a probe cantilever; it can distinguish ferroelectric domains having different polarizations with respect to the axis perpendicular to the cantilever, but it is blind to the parallel axis innately. We introduce a high-resolution angle-resolved-LPFM technique that is capable of visualizing full two-dimensional in-plane piezoresponse vector fields. The LPFM signal is analyzed for each pixel with respect to the sample-probe orientation angle with the aid of an image registration technique, and the corresponding local in-plane piezoresponse vector is deduced from the amplitude and phase of the trigonometric curve fitting. This technique provides a pathway for the visualization of complicated ferroelectric and piezoelectric structures.

Ferroelectric properties and piezoresponse force micoroscopy study of Bi3TaTiO9 thin films

We investigate the ferroelectric properties and crystal structures of Bi3TaTiO9 (BTTO) thin films deposited on single-crystal Nb-doped (100) SrTiO3 substrates via pulsed laser deposition. The BTTO films exhibited either a (001)-epitaxial crystalline structure or a mixed a- and c-oriented polycrystalline structure depending on the substrate temperature. The ferroelectric polarization and piezoelectric coefficient of the mixed a- and c-oriented film were larger than those of the (001)-epitaxial film because its polar axis was perpendicular to the c-axis. Vertical and lateral piezoresponse force microscopy studies indicate that the ferroelectric domains of the (001)-epitaxial film were all parallel to the in-plane orientation, whereas the mixed a- and c-oriented film comprised both square grains with in-plane-oriented ferroelectric domains and longish grains with ferroelectric domains out of orientation with the plane.

Orientation of Ferroelectric Domains and Disappearance upon Heating Methylammonium Lead Triiodide Perovskite from Tetragonal to Cubic Phase

We study the spontaneous polarization of the archetypal semiconducting halide perovskite methylammonium lead triiodide (CH3NH3PbI3) that is currently being investigated for use in thin film solar cells and light-emitting diodes. Using both lateral and vertical piezoresponse force microscopy (PFM) to image polycrystalline thin films, we observed domains in the piezoresponse that reversibly appear and disappear below and above the tetragonal-to-cubic phase transition temperature. Importantly, we observe these domains to exhibit a piezoresponse that is predominantly in-plane for films with the (110) plane oriented parallel to the substrate, providing a measure of the polarization associated with specific crystal planes. We characterize the polarization and its temporal response using both local switching spectroscopy and time-dependent PFM spectra. These data show hysteresis loops with the polarization switching with bias but relaxing back on time scales of several minutes. Our results suggest the existence ...

Non-destructive determination of collagen fibril width in extruded collagen fibres by piezoresponse force microscopy

Extruded collagen fibres are a promising platform for tissue engineering applications. Ensuring that the functional properties of the engineered tissues possess similar structural properties as native tissues is important for biomedical applications. Advanced imaging tools including scanning electron microscopy (SEM) and atomic force microscopy (AFM) have revealed the structural features of collagen fibrils within such fibres; however, these techniques often require modification steps that can alter the sample in the process. Here, lateral piezoresponse force microscopy (LPFM), which is sensitive to the polar orientation of piezoelectric collagen fibrils, is demonstrated as a promising tool to assess the width of individual fibrils and moreover map their organisation and polar orientation without altering the sample. Within the fibres studied, the collagen fibrils showed a highly anisotropic arrangement with preferred alignment along the length of the fibre. Fibril widths of 74 ± 18 nm and 73 ± 19 nm in untreated and bleached fibres, respectively, were measured from LPFM amplitude images. These values agreed with values from SEM (70 ± 10 nm) and AFM (71 ± 19 nm) measurements that could only be obtained from bleached fibres.