Piezoresponse Force Microscopy Technique Research Articles

Influence of Nanoscale Dielectric Gap on Electrical Characterization in Scanning Probe Microscopy

Scanning Probe Microscopy (SPM) techniques, including Conductive-Atomic Force Microscopy (C-AFM), Kelvin Probe Force Microscopy (KPFM), and Piezoresponse Force Microscopy (PFM) or Electrochemical Strain Microscopy (ESM), are critical for the in situ, non-destructive analysis of material interfaces with high spatial resolution. Despite substantial advancements in SPM technologies, challenges related to their reliability and accuracy remain, primarily due to measurement artifacts and electrochemical reactions on the sample surface. These artifacts can significantly skew results, leading to misconceptions unless critically analyzed.This study explores the influence of the dielectric gap on electric characterization, particularly focusing on how variations in permittivity affect the results obtained from C-AFM and PFM techniques. By adjusting the permittivity of the dielectric medium (air, water with εr = 79, and silicone oil with εr = 2.1-2.8), we demonstrate that high permittivity media can significantly correct the discrepancies commonly observed in coercive field measurements, aligning them closer to values obtained from conventional capacitor-based polarization methods.Our findings illuminate the critical role of the dielectric gap in SPM, suggesting methodological adjustments for improved accuracy in the nanoscale electrical characterization of electrochemical materials. These insights offer significant implications for advancing the precision and reliability of SPM techniques in studying electronic materials.

SHG Assisted Mixed-Phases Anatomizing in a Molecular Ferroelectric.

Anatomizing mixed-phases, referring to analyzing the mixing profiles and quantifying the phases' proportions in a material, which is of great significance in the genuine applications. Here, by using second-harmonic generation (SHG)polarimetry and piezoresponse force microscopy (PFM) techniques, this work elucidates the contributions and distributions of two different symmetric phases mixed in an archetype monoaxial molecular ferroelectric, diisopropylammonium chloride (DIPACl). The two competing phases are preferred in thermodynamics or kinetic process respectively, and this work evidences the switching behavior between the two competing phases facilitated by an external electrical field as opposed to a heating process. This research contributes novel insights into phase engineering in the field of molecular ferroelectrics and is poised to serve as a potent analytical tool for subsequent applications.

Manipulation of domain structures in LiNbO3 thin films using focused ion beam

In this study, we utilized focused ion beam (FIB) technology to generate domains with a size of approximately 500 nm in LiNbO3 thin films. These films were synthesized using the rf sputtering technique. To investigate these domains, we employed scanning Kelvin probe microscopy (SKPM) and Piezoresponse force microscopy (PFM) techniques. PFM was specifically employed to examine the polarization characteristics of these domains. Notably, we observed distinct contrast in the phase data of neighboring domains. A phase contrast of 180° indicated that the polarization direction in the two neighboring polarized domains was antiparallel. Within the ferroelectric phase, the SKPM method measured a characteristic potential change of approximately 35 mV across neighboring ferroelectric domains with opposite polarization directions (180°-domains). Moreover, we observed that upon illumination, the average surface potential increased by approximately 80 mV. This increase has been attributed to the light-induced population of surface states.

New Insight into Nanoscale Identification of the Polar Axis Direction in Organic Ferroelectric Films.

Ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-co-TrFE)] thin films have been deposited by spin-coating onto the Bi0.5Na0.5TiO3(BNT)/LNO/SiO2/Si heterostructure. The copolymer microstructure investigated by using grazing-incidence wide-angle X-ray diffraction (GIWAXD) and deduced from the (200)/(110) reflections demonstrates that the b-axis in the P(VDF-co-TrFE) orthorhombic unit cell is either in the plane or out of the plane, depending on the face-on or on the two types of edge-on (called I and II) lamellar structures locally identified by atomic force microscopy (AFM). For edge-on I lamellae regions, the electroactivity (dzzeff ∼ -50.3 pm/V) is found to be twice as high as that measured for both edge-on II or face-on crystalline domains, as probed by piezoresponse force microscopy (PFM). This result is directly correlated to the direction of the ferroelectric polarization vector in the P(VDF-co-TrFE) orthorhombic cell: larger nanoscale piezoactivity is related to the b-axis which lies along the normal to the substrate plane in the case of the edge-on I domains. Here, the ability to thoroughly gain access to the as-grown polar axis direction within the edge-on crystal lamellae of the ferroelectric organic layers is evidenced by combining the nanometric resolution of the PFM technique with a statistical approach based on its spectroscopic tool. By the gathering of information at the nanoscale, two orientations for the polar b-axis are identified in edge-on lamellar structures. These findings contribute to a better understanding of the structure-property relationships in P(VDF-co-TrFE) films, which is a key issue for the design of future advanced organic electronic devices.

Improvement of the piezoelectric response of AlN thin films through the evaluation of the contact surface potential by piezoresponse force microscopy

Sputtered aluminum nitride (AlN) thin films were characterized by Piezoresponse Force Microscopy (PFM) technique using a methodology to decrease the contribution of the electrostatic forces to obtain a pure piezoelectric response. Our method is based on the sweeping of the DC voltage applied to the Atomic Force Microscope (AFM) tip under a fixed AC field to evaluate the contact surface potential difference (VCPD) between the tip and the sample used to measure the proper AlN piezoelectric coefficient (d33,eff), minimizing the electrostatic contribution. Kelvin probe Force Microscopy (KPFM) was employed as reference standard technique to measure the surface potential, confirming the reliability of the proposed experimental procedure on ceramic piezoelectric films, and simultaneously overcoming the disadvantages of the KPFM technique. The capability to tune surface potential of materials over a wide range of values opens new perspectives for the design of devices with changeable surface potential.

Analysis of multi-center topological domain states in BiFeO3 nanodot arrays

High-density ferroelectric BiFeO3 (BFO) nanodot arrays were developed through template-assisted tailoring of epitaxial thin films. By combining piezoresponse force microscopy (PFM) and Kelvin probe force microscopy (KPFM) imaging techniques, we found that oxygen vacancies in nanodot arrays can be transported in the presence of an electric field. Besides triple-center domains, quadruple-center domains with different vertical polarizations were also identified. This was confirmed by combining the measurements of the domain switching and polarization vector distribution. The competition between the accumulation of mobile charges, such as oxygen vacancies, on the interface and the geometric constraints of nanodots led to the formation of these topological domain states. These abnormal multi-center topological defect states pave the way for improving the storage density of ferroelectric memory devices.

Shift-Current Photovoltaics Based on a Non-Centrosymmetric Phase in In-Plane Ferroelectric SnS.

The shift-current photovoltaics of group-IV monochalcogenides has been predicted to be comparable to those of state-of-the-art Si-based solar cells. However, its exploration has been prevented from the centrosymmetric layer stacking in the thermodynamically stable bulk crystal. Herein, the non-centrosymmetric layer stacking of tin sulfide (SnS) is stabilized in the bottom regions of SnS crystals grown on a van der Waals substrate by physical vapor deposition and the shift current of SnS, by combining the polarization angle dependence and circular photogalvanic effect, is demonstrated. Furthermore, 180° ferroelectric domains in SnS are verified through both piezoresponse force microscopy and shift-current mapping techniques. Based on these results, an atomic model of the ferroelectric domain boundary is proposed. The direct observation of shift current and ferroelectric domains reported herein paves a new path for future studies on shift-current photovoltaics.

Exploring the effect of low concentration of stannum in lead-free BCT-BZT piezoelectric compositions for energy related applications

Piezoelectric, ferroelectric and electromechanical properties were studied at macroscopic and local scale level in stannum doped (Ba0.88Ca0.12)(SnxZr0.1-xTi0.9)O3 ceramics, with an objective to explore the effect of low content (0 ≤ x ≤ 0.5 at%) of the substituent (Sn) on the functional properties. The results exemplified that the substitution had an influence on the crystal lattice and microstructure that affected the dielectric, ferroelectric, and electromechanical properties. Enhancement in electrical/electromechanical properties were observed with stannum substitution. Optimal electrical properties were obtained in the composition with Sn = 0.3 at% that exhibited maximum piezoelectric constant d33 = 405 pC/N, planar electromechanical coupling factor kp ∼0.41, and saturation polarization Ps=12.1 μC/cm2. The same composition showed an electric-field strain response, Smax ∼0.08% and a converse piezoelectric coefficient, d33* of ∼525 pm/V. Local scale characterization via piezoresponse force microscopy technique revealed complex domain patterns comprising stripe-like macro-domains and featureless nano-sized domains. Energy harvesting and energy storage performance were evaluated for exploring their suitability in energy applications.

Electron beam-induced current imaging of ferroelectric domains and local polarization reversal in Hf0.5Zr0.5O2

Because of their full compatibility with CMOS technology, HfO2-based ferroelectrics, and especially Hf0.5Zr0.5O2 (HZO), attract a lot of attention. However, the overwhelming majority of measurement techniques provides only information about the cumulative electrical response of many domains of HZO, i.e., at the macroscopic level. So far, only piezoresponse force microscopy technique was applied to visualize distinct ferroelectric domains in HZO and to analyze the local switching behavior in the microscopic level. This work introduces the possibility of using electron beam-induced current (EBIC) technique in the scanning electron microscope to visualize the gradual polarization reversal of HZO and to obtain the local polarization dynamics. We show that although the local EBIC signal is affected by surrounding domains, studying the variations in the ferroelectric response of individual domains as well as the spread of the local stiffness and local imprint is possible by this method. Besides, we show the connection between the EBIC current and an electric field across passive non-ferroelectric layers at interfaces between HZO and metal electrodes, which opens up additional opportunities to use the EBIC technique for investigations of interface-dependent properties of HZO ferroelectrics in the future.

Investigation of the diameter-dependent piezoelectric response of semiconducting ZnO nanowires by Piezoresponse Force Microscopy and FEM simulations

Semiconducting piezoelectric nanowires (NWs) are promising candidates to develop highly efficient mechanical energy transducers made of biocompatible and non-critical materials. The increasing interest in mechanical energy harvesting makes the investigation of the competition between piezoelectricity, free carrier screening and depletion in semiconducting NWs essential. To date, this topic has been scarcely investigated because of the experimental challenges raised by the characterization of the direct piezoelectric effect in these nanostructures. Here we get rid of these limitations using the piezoresponse force microscopy technique in DataCube mode and measuring the effective piezoelectric coefficient through the converse piezoelectric effect. We demonstrate a sharp increase in the effective piezoelectric coefficient of vertically aligned ZnO NWs as their radius decreases. We also present a numerical model which quantitatively explains this behavior by taking into account both the dopants and the surface traps. These results have a strong impact on the characterization and optimization of mechanical energy transducers based on vertically aligned semiconducting NWs.

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.

Temperature dependent structural, dielectric, Raman, piezoresponse and photoluminescence investigations in sol-gel derived BCZT ceramics

0.5Ba(Ti0.8Zr0.2)O3-0.5(Ba0.7Ca0.3)TiO3 [50BZT-50BCT or BCZT] based compounds have been the focus of a lot of research, particularly motivated by their high piezoelectric effect. However, the literature lacks an elaborate investigation of the phase transition behavior in BCZT ceramics obtained by wet chemistry processing. Here, we present an in-depth study on the temperature dependence of x-ray diffraction (XRD), Raman scattering, dielectric properties, local piezoresponse and photoluminescence (PL) to investigate the sequence of phase transitions in the BCZT ceramic synthesized via a chemical route. Phase formation was determined by Rietveld analysis of XRD data, while compositional homogeneity and elemental quantification of the compound was validated using energy dispersive x-ray spectroscopy (EDX) and x-ray photoelectron spectroscopy (XPS) studies. Detailed fitting of XPS data indicated the existence of Ti3+ species (∼6%) in the prepared BCZT. Phase transitions were examined by analyzing the modifications in the XRD profile of Bragg reflection {200} and anomalies observed in the temperature variation of dielectric and Raman spectra studied over a wide temperature range starting from 10K to beyond Curie temperature. Crystallographic transformation temperatures obtained from dielectric measurement agreed well with those assessed from the temperature evolution of Raman spectra. In addition to other transitions, Raman scattering results revealed the existence of a transition from R3c to R3m phase near −175 °C, a transition that has not been interpreted in BCZT (and generally not observed in parent BaTiO3 compound). The luminescence response was studied by photoluminescence (PL) spectroscopy in the temperature range 15–300 K. The position of the PL peak was observed to shift with temperature and discontinuities in the wavelength shift were noted near phase transitions. Evolution of domain morphology with temperature was examined by piezoresponse force microscopy technique. Consolidated results assign the phase sequence in sol-gel derived BCZT as: R(R3c)→−175±10°CR(R3m)→−50±10°CO→40±10°CT→120±10°CC.

Formation of a quasi-equilibrium domain structure of crystals of the TGS group near TC

In the temperature range ΔT ≈ 321 K ÷ 322 K, the kinetics of the nonequilibrium domain structure of triglycine sulphate crystals, both pure and with specially introduced defects, has been studied by means of piezoresponse force microscopy technique. The temporal change in the domain structure as a set of regions with a scalar order parameter of P (r, t) = +1 and −1 for oppositely polarized domains was analysed by the behaviour of the space-time correlation function C(r,t) = ·Р(r,t)Р(0,t)Ò. At different distances from the Curie point Tc, the characteristic length Lc, as a scale measure of the average domain size, increases with time according to the power law Lc(t)~(t−t0)a. A decrease of the exponent a with distance from Tc can be a consequence of the transition of the domain structure of TGS crystals from a non-conservative state to aconservative one.

Frequency-dependent PFM signal induced by surface adsorbates

Piezoresponse force microscopy (PFM) is an indispensable tool for investigating local piezoelectric and ferroelectric properties by detecting the electromechanical strain response to an applied voltage on a surface. The PFM technique is highly sensitive to the surface state of a sample, such as surface adsorption and surface potential, which could lead to the misinterpretation of the quantitative value of PFM signal as well as the presence of ferroelectricity. In this study, we observed a frequency-dependent PFM signal generated by surface adsorbates. However, the surface adsorbates can be removed by sample cleaning albeit the cleaning induced a significant change in surface potential. Thus, correct evaluation of the effective piezoelectric coefficient was realized by cleaning the sample and ruling out the electrostatic contribution. This work demonstrates that one should be careful to consider the effect of adsorbates and the electrostatic effect to correctly evaluate the piezoelectric coefficient.

Humidity Effects on Domain Structure and Polarization Switching of Pb(Zn1/3Nb2/3)O3-x%PbTiO3 (PZN-x%PT) Single Crystals.

The effect of relative humidity on the domain structure imaging and polarization switching process of Pb(Zn1/3Nb2/3)O3-x%PbTiO3 (PZN-x%PT) ferroelectric single crystals has been investigated by means of the piezoresponse force microscopy (PFM) and piezoresponse force spectroscopy (PFS) techniques. It was found that the PFM amplitude increases with the relative humidity, and that the ferroelectric hysteresis loops at different relative humidity levels show that the coercive bias decreases with the increase in relative humidity. These observed phenomena are attributed to the existence of the water layer between the probe tip and the sample surface in a humid atmosphere, which could affect the effect of the electric field distribution and screening properties at the ferroelectric sample surface. These results provide a better understanding of the water adsorption phenomena at the nanoscale in regard to the fundamental understanding of ferroelectrics’ properties.

Nanoscale Ferroelectric Characterization with Heterodyne Megasonic Piezoresponse Force Microscopy.

Piezoresponse force microscopy (PFM), as a powerful nanoscale characterization technique, has been extensively utilized to elucidate diverse underlying physics of ferroelectricity. However, intensive studies of conventional PFM have revealed a growing number of concerns and limitations which are largely challenging its validity and applications. In this study, an advanced PFM technique is reported, namely heterodyne megasonic piezoresponse force microscopy (HM‐PFM), which uses 106 to 108 Hz high‐frequency excitation and heterodyne method to measure the piezoelectric strain at nanoscale. It is found that HM‐PFM can unambiguously provide standard ferroelectric domain and hysteresis loop measurements, and an effective domain characterization with excitation frequency up to ≈110 MHz is demonstrated. Most importantly, owing to the high‐frequency and heterodyne scheme, the contributions from both electrostatic force and electrochemical strain can be significantly minimized in HM‐PFM. Furthermore, a special measurement of difference‐frequency piezoresponse frequency spectrum (DFPFS) is developed on HM‐PFM and a distinct DFPFS characteristic is observed on the materials with piezoelectricity. By performing DFPFS measurement, a truly existed but very weak electromechanical coupling in CH3NH3PbI3 perovskite is revealed. It is believed that HM‐PFM can be an excellent candidate for the ferroelectric or piezoelectric studies where conventional PFM results are highly controversial.

Growth and Atomically Resolved Polarization Mapping of Ferroelectric Bi2WO6 Thin Films

Aurivillius ferroelectric $Bi_2WO_6$ (BWO) encompasses a broad range of functionalities, including robust fatigue-free ferroelectricity, high photocatalytic activity, and ionic conductivity. Despite these promising characteristics, an in-depth study on the growth of BWO thin films and ferroelectric characterization, especially at the atomic scale, is still lacking. Here, we report pulsed laser deposition (PLD) of BWO thin films on (001) $SrTiO_3$ substrates and characterization of ferroelectricity using the scanning transmission electron microscopy (STEM) and piezoresponse force microscopy (PFM) techniques. We show that the background oxygen gas pressure used during PLD growth mainly determines the phase stability of BWO films, whereas the influence of growth temperature is comparatively minor. Atomically resolved STEM study of a fully strained BWO film revealed collective in-plane polar off-centering displacement of W atoms. We estimated the spontaneous polarization value based on polar displacement mapping to be about 54 $\pm$ 4 ${\mu}C cm^{-2}$, which is in good agreement with the bulk polarization value. Furthermore, we found that pristine film is composed of type-I and type-II domains, with mutually orthogonal polar axes. Complementary PFM measurements further elucidated that the coexisting type-I and type-II domains formed a multidomain state that consisted of 90$\deg$ domain walls (DWs) alongside multiple head-to-head and tail-to-tail 180$\deg$ DWs. Application of an electrical bias led to in-plane 180$\deg$ polarization switching and 90$\deg$ polarization rotation, highlighting a unique aspect of domain switching, which is immune to substrate-induced strain.

Domain structure transition in compressively strained (100)/(001) epitaxial tetragonal PZT film

A 30-nm-thick epitaxial tetragonal (100)/(001) Pb(Zr0.22Ti0.78)O3 (PZT) thin film was grown at 600 °C on (001) KTaO3 (KTO) single-crystal substrate by metalorganic chemical vapor deposition. The a/c domain structure in the PZT film was analyzed in detail at room temperature by synchrotron x-ray diffraction (XRD) and piezoresponse force microscopy techniques. The temperature dependence of the crystal structure was investigated by XRD reciprocal space mapping and in-plane grazing incidence XRD. The PZT films were grown on KTO substrates under compressive strains from 700 °C to room temperature. This compressive strain gave rise to a perfect (001) orientation below a Curie temperature (TC) of ∼520 °C. An in-plane a-axis lattice parameter for the c domain kept the same value as that of the substrate lattice up to 350 °C despite the ferroelectric transition. Nucleation of the a domain started at around 350 °C. The formation of the a domain released the strain for the in-plane a-axis lattice parameter, as confirmed by in-plane analysis of the crystal structure. The results revealed that the in-plane average surface area of the PZT unit cell continued to match that of the KTaO3 substrate from 700 °C to room temperature, regardless of the domain structure evolution.

Observation of a stable fractionalized polar skyrmionlike texture with giant piezoelectric response enhancement

We report the creation of stable fractionalized polar skyrmionlike structures via flexoelectric effect in a prototypical multiferroic ${\mathrm{BiFeO}}_{3}$ thin film, which are directly imaged by angle-resolved piezoresponse force microscopy technique. We also demonstrate an approach to enhance piezoresponse at the nanoscale. Phase-field simulations reveal that a tip-driven flexoelectric effect allows the formation of an ordered polar skyrmionlike configuration that is topologically protected by the converse flexoelectric effect. Our findings open up avenues for designing and controlling spatially localized topological textures in rhombohedral ferroelectrics, and also could contribute to future developments for ferroelectric-based nanoelectronic devices.

Temperature-dependent Raman spectroscopy, domain morphology and photoluminescence studies in lead-free BCZT ceramic

Present work focuses on detailed temperature-dependent X-ray diffraction, Raman scattering, domain configuration, and photoluminescence (PL) studies in the (Ba0·85Ca0.15) (Zr0·10Ti0.90)O3 (BCZT) ceramics. The comprehensive Raman spectroscopy analysis in the present work not only validates the presence of the intermediate orthorhombic phase in BCZT, but also provides evidence of another transition: rhombohedral R3c phase to R3m at low temperature. Temperature behaviour of the lowest frequency transverse optical mode (soft E (TO) phonon) and hard modes was studied. Temperature dependence of peak positions, intensities, and linewidths of Raman phonon modes signalled the presence of phase transitions near −50 ± 5 °C, 0±5 °C, 35±5 °C and 110 ± 10 °C. Evolution of domain morphology occurring at phase transitions above room temperature was studied by piezoresponse force microscopy technique. Analysis of PL spectra revealed disorder/heterogeneity in the sample and indicated the existence of self-trapped excitons. PL spectra are composed of four distinct colour components (~2.55eV:blue, ~2.32eV:green, ~2.08eV:orange and ~1.78eV:red).