Decoding Apparent Ferroelectricity in Perovskite Nanofibers - ACS

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Decoding apparent ferroelectricity in perovskite nanofibers Rajasekaran Ganeshkumar, Suhas Somnath, Chin Wei Cheah, Stephen Jesse, Sergei V. Kalinin, and Rong Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14257 • Publication Date (Web): 13 Nov 2017 Downloaded from http://pubs.acs.org on November 15, 2017

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ACS Applied Materials & Interfaces

Decoding apparent ferroelectricity in perovskite nanofibers Rajasekaran Ganeshkumar#, Suhas Somnath+ $, Chin Wei Cheah#, Stephen Jesse+ $, Sergei V Kalinin+ $ and Rong Zhao* # #

Engineering Product Development, Singapore University of Technology and Design, Republic

of Singapore 487372. +

The Institute of Functional Imaging of Materials, Oak Ridge National Laboratory, Oak Ridge,

Tennessee 37831, USA. $

The Centre for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge,

Tennessee 37831, USA.

KEYWORDS Nanofibers; Ferroelectricity; Potassium niobate; PFM; Band excitation PFM; cKPFM; Polarization switching; Electrospinning;

ACS Paragon Plus Environment

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ABSTRACT

Ferroelectric perovskites are an important group of materials underpinning a wide variety of devices ranging from sensors and transducers to non-volatile memories and photovoltaic cells. Despite the progress in material synthesis, ferroelectric characterization of nanoscale perovskites is still a challenge. Piezoresponse force microscopy (PFM) is one of the most popular tools for probing and manipulating nanostructures to study the ferroelectric properties. However, the interpretation of hysteresis data and alternate signal origins are critical. Here, we use a family of scanning probe microscopy (SPM) techniques to systematically investigate the ferroelectric behavior of electrospun potassium niobate (KNbO3) nanofibers. Band excitation (BE) SPM scans reveal that PFM signals are dominated by changes in resonant frequency due to rough nanofiber surfaces, rather than the actual local piezoelectric strength. We investigate bias-induced charge injection properties and electrostatic interactions on the PFM response of the nanofiber using contact mode Kelvin probe force microscopy (cKPFM). Furthermore, impact of relative humidity (RH) on the KNbO3 nanofiber’s piezoresponse, switching behavior and tip-induced charges are explored. The resultant data from BE scans were utilized to estimate the piezoelectric constants of the KNO nanofiber. These observations will provide clarity in studying newly developed ferroelectric nanostructures and unambiguously interpreting the PFM data.


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1. INTRODUCTION Nanoscale perovskite ferroelectrics remain an area of intense interest due to unique functional properties that enable large number of applications ranging from sensors, actuators, optical transducers to energy conversion devices.1–3 The additional level of novel phenomena emerges on transition to the nanoscale. Lately, synthesis of perovskite nanostructures such as BaTiO3 (BTO),4 BiFeO3 (BFO),5 KNbO3 (KNO)6 and Pb(Zr, Ti)O3 was widely reported and their properties have been investigated.7 Among fundamental studies of these materials, imaging ferroelectric domains and probing polarization dynamics at the nanoscale level utilizing piezoresponse force microscopy (PFM) is actively explored.8,9 PFM, a versatile scanning probe microscopy-based technique detects the electromechanical response induced by the converse piezoelectric effect when an ac bias is applied to the sample surface through a conductive tip.10,11 In most of the existing reports on perovskite nanostructures, presence of ferroelectricity is established via the detection of the PFM hysteresis loops, which serve as a preponderant evidence towards switchable ferroelectric polarization. In certain cases, these studies are complemented by observations of polarization domains.12,13 Beyond the qualitative studies through piezoresponse images, however, the quantitative analysis of the PFM data for these materials are inaccurate and contradict each other.14–18 For example, the piezoelectric coefficient (d33) values reported for KNO nanostructures vary from 30 pm V-1 to 105 pm V-1.14,15 In another case, Li et al. carried out PFM on BFO nanowire and determined the d33 to be around 31 pm V-1, while investigations of Xie et al. showed extremely small piezoresponse of 8.7 pm V-1 for a similar sized nanowire.17,18 Furthermore, the latter report questioned about the possibility of electrostatic interactions in the PFM data rather than pure piezoelectric signals. While synthesis


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routes are understood to an extent, piezoelectric characterization of nanoscale perovskites are still incomplete. On the other hand, the existence of non-ferroelectric signal contribution such as electrostatic interaction between tip and the surface11, electrochemical strain19 and hysteretic surface charging20 to Scanning Probe Microscopy (SPM)-type experiments were explored. For instance, the electrochemical strain can induce ferroelectric-like hysteresis loop due to complex set of ionic motions.21 Kim et al. observed the influence of electrostatic interaction on thin film of PZT, resulting in ferroelectric-like hysteresis loop despite the absence of polarization switching.22 Balke et al. reported PFM-type measurement schemes to differentiate ferroelectric and nonferroelectric signals through detailed investigations on PZT and HfO2 thin films.23 However, use of these new techniques to explore the signal origins in ferroelectric nanowires/nanofibers are barely discussed or considered. Here, we systematically investigate the ferroelectric properties of electrospun KNO nanofibers collected on SiO2/Si substrate. To complement the traditional single-frequency PFM (SFPFM),10 we use Band-Excitation PFM (BE-PFM),24 Band Excitation Piezoresponse Spectroscopy (BEPS),25 and contact-Kelvin Probe Force Microscopy (cKPFM)23 techniques to identify and decouple the piezoelectric behavior of the nanofiber from other phenomena including contact mechanics and electrostatics arising from the Atomic Force Microscopy (AFM) tip-sample interactions. We chose KNO as our material system since it is a promising, and environmentally friendly alternative for developing piezoelectric transducers and energy conversion devices compared to other perovskites.26,27 This work demonstrates robust experimental methodologies for characterizing newly synthesized ferroelectric nanostructures. Moreover, insights from this work will substantially advance the understanding of the


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ferroelectric properties of KNO at the nanoscale, thus offering a pathway required for potential applications.

2. MATERIALS AND METHODS 2.1 Synthesis and Characterization of KNO Nanofibers A sol-gel based far-field electrospinning process was used to synthesize long KNO nanofibers as reported earlier.28 The sol-gel was prepared by dissolving 0.2 g of Niobium Chloride (NbCl5; Sigma-Aldrich, 98%) and 0.667 g of Potassium Sorbate (C6H7KO2; Sigma-Aldrich, 99%) in 3 ml of methanol. The mixture was stirred for 1 hour and a clear solution was obtained by removing the chloride precipitates via centrifugation. 0.4 g of PVP polymer and 3 ml of 2-methoxyethanol were added to the existing solution and the mixture was magnetically stirred for few hours to obtain a homogenous sol-gel KNO precursor. During the electrospinning process, the precursor sol-gel was ejected from the syringe at a constant feeding rate of 0.4 ml h-1 and the fibers were collected on to a SiO2/Si substrate when a high electric field was applied between the syringe and the collector substrate. After few minutes of fiber collection, the samples were dried at 60 °C for 1 hour, followed by an annealing at 550 °C for 5 hours. All the experiments in this work were carried out on the as-annealed nanofibers. The surface morphology and geometry of the as-annealed KNO nanofibers were inspected using a field-induced scanning electron microscope (FE-SEM, JEOL JSM7600F). The structural properties of the nanofibers were investigated through a high-resolution transmission electron


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microscope (HR-TEM, FEI Technai-F20). KNO nanofiber was cut using FIB lift-out process for cross-sectional TEM imaging to clearly observe the morphology and domains.

2.2 PFM Experimental Setup PFM measurements were conducted to study the local piezoelectric response of the KNO nanofibers in both ambient and humid environments using an Asylum Research Cypher-ES AFM.29 We used Budget-sensors Multi75E-G AFM cantilevers having a nominal resonance frequency of 75 kHz in air, stiffness of about 3 Nm-1, and a Cr/Pt coated conductive tip. BE-scan, BEPS, and cKPFM experiments were performed using custom instrumentation software that controlled a National Instruments PXI-6115 data acquisition card. We measured local hysteresis loops using BEPS and cKPFM on an evenly spaced spatial grid (10 x 10) of points. The sample was electrically excited via the AFM tip with a 15 V bipolar triangular waveform (Fig. 4a) during both BEPS and cKPFM measurements to cycle through the polarization state of the material. The BE spectra in all datasets were fitted to a simple harmonic oscillator (SHO) model to yield 2D (x, y spatial positions, in the case of BE-scan), 4D (x, y position, DC switching voltage, switching cycle, in the case of BEPS) and 5D (x, y positions, DC switching voltage, switching cycle, DC measurement voltage, in the case of cKPFM) maps of local piezoresponse amplitude, phase, resonance frequency, and quality factor. This functional fitting facilitated analysis of local hysteresis loops and correlation of these BE measurements with surface topography. A more detailed description of the data acquisition and processing are reported elsewhere.20,25


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3. RESULTS AND DISCUSSION KNO nanofibers were synthesized using sol-gel based far-field electrospinning process.28,30 The nanofibers were collected on to a SiO2 covered silicon substrate and subjected to thermal annealing. The as-spun fibers were loaded into a horizontal tube furnace, which was heated to 550 °C, at a heating rate of 5 °C/min under atmospheric conditions for 5 hours. SEM micrographs in Fig. 1a shows that the nano-grains are stacked in one dimension resulting as continuous nanofibers. As observed in Fig. 1b, the KNO nanofibers are nano-polycrystalline in nature. The crystal phase and crystallinity of the as-annealed KNO nanofibers were verified by the X-ray diffraction (XRD) and Raman measurements.28 The XRD peak and Raman Spectra indicated the formation of nanofibers with pure perovksite phase – orthorhombic structure. The chemical composition of the KNO nanofibers was characterized using SEM-EDX (refer Fig. S1) which showed precise stoichiometry without any significant impurities or secondary phases. In addition, the High-Resolution Transmission Electron Microscopy (HR-TEM) images in Fig. 1c validated that the KNO nanofiber are orthorhombic in phase. The corresponding selected area electron diffraction (SAED) pattern of the nanofiber confirms that the crystal structure is perovskite. The lattice spacing along the growth and lateral direction are 0.397 nm and 0.409 nm, respectively, while the spacing in bulk KNO were reported to be 0.3984 nm and 0.4035 nm, respectively.31 The average crystallite size in the nanofiber sample was around 20 ± 5 nm, and was estimated by direct measurement of the size distribution using enhanced microscopy images as shown in Fig. S2. In PFM, a SPM probe with a conductive tip is brought into contact with the sample surface and an electrical voltage is applied between the tip and the sample. The sample surface deforms due to the inverse piezoelectric effect, which results in cantilever deflection. In typical PFM, the


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probe is electrically excited at a frequency near the resonance mode of the tip-surface system and the vibrations of the probe are measured at the same frequency using a lock-in amplifier. The amplitude and phase components of the cantilever deflection can be extracted which contains information about the local piezoelectric strength and polarization direction of the ferroelectric domains, respectively.29 Details regarding the PFM experimental setup are described in the Experimental section. Fig. 2 shows the results from conventional SF-PFM measurements on a single KNO fiber. Fig. 2a-b show the topography and deflection, while Fig. 2c-d show the corresponding piezoelectric amplitude and phase response maps, respectively. In SF-PFM, the observed piezoresponse arises from the convolution of the material response with the tip-sample dynamics due to the nonlinear nature of the tip-surface interaction. In other words, changes in the tip-sample contact due to variations in the sample topography, affect the tip-sample resonance frequency, which in turn affect the piezoresponse measured at the excitation frequency if the latter is close to the resonance. Thus, it is unclear whether changes in the measured piezoelectric amplitude arise from variations in the local piezoresponse or the topography. The strong spatial correlation of the piezoelectric amplitude and phase with the height in Fig. 2 clearly illustrates this weakness of SF-PFM. In order to overcome this ambiguity in SF-PFM imaging, we performed BE imaging on the same area of the KNO nanofiber. Unlike SF-PFM wherein the piezoresponse is measured at a single frequency, BE excites the tip at a band of frequencies cantered at the tip-sample resonance frequency and captures the piezoresponse at these frequencies. Thus, BE accommodates for variations in the resonance frequency and consistently provides accurate and unbiased information about the piezoresponse.25


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Fig. 3 shows the spatial maps of the quality factor of the resonance peak, resonance frequency, piezoresponse amplitude and phase at this resonance frequency obtained from BE imaging. The KNO fibers exhibit larger piezoresponse compared to the bare silica substrate surrounding the fibers which is evident from Fig. 3a. The comparison between the deflection map in Fig. 2b and the BE resonance frequency map in Fig. 3b illustrates that the rough nano-grain structures on the nanofiber result in large variations in the resonance frequency. Note the strong correlation between the bright areas in SF-PFM amplitude map (Fig. 2c) with the blue region in the BE frequency map (Fig. 3b). The changes in frequencies are in the order of 10 kHz, significantly larger than the width of the resonance peak itself. Furthermore, the amplitude and phase vary within the fiber indicating different polarity in different domains in the nanofiber and the granular interaction between multiple nano grains across the fiber surface. Note that change in polarity in Fig. 3c is not clearly evident because the contribution of electrostatic forces lowers the phase difference between domains which is particularly pronounced for materials with small response such as KNO nanofibers.32 Notable are also the large variations (90 - 210) in the quality factor (Q), as shown in Fig. 3d. Consequently, the SF-PFM piezoresponse measurement was dominated by changes in resonant frequency, rather than piezoresponse amplitude and relying on SF-PFM can result in inaccurate estimations of piezoelectric coefficients and other quantitative PFM characterization of the material. On the contrary, BE measurements decouple the cross-talk between the topography and the actual piezoresponse. To explore the potential ferroelectricity in these materials, we used BE-based voltage spectroscopy techniques such as BEPS & cKPFM for probing the polarization switching on the rough KNO nanofiber surface. At each spatial location, BEPS uses a bipolar triangular waveform to cycle through the polarization state of the material. At each voltage step (write voltage) in the


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triangular waveform, the local polarization state is measured with the switching bias turned off (read voltage set to 0 V), using a standard BE measurement in order to construct hysteresis loops. Additional details regarding BEPS are presented elsewhere.24 Fig. 4 shows the results of BEPS measurements on the KNO fiber and the surrounding silica substrate. The amplitude and phase spectrograms in Fig. 4a-c show two polarization switching cycles as a function of varying dc-bias. These spectrograms clearly show switching where the amplitude drops to the noise floor and the phase changes by 180 degrees. Similar to the BE-scan, the BE spectra for each voltage point was fitted to a simple harmonic oscillator model to obtain the quality factor, resonance frequency, amplitude and phase at the resonance frequency. The fitting parameters are then used to construct the ferroelectric hysteresis loops. In an effort to simplify the visualization and interpretation of data, we carefully separated the measurements over the nanofiber and the bare substrate and averaged the measurements to build the hysteresis loops a shown in Fig. 4d-e. A BEPS response map for evenly spaced 10 x 10 grid on the sample area and an average of 4 points from different areas on the nanofiber was used to construct the hysteresis loop as show in Fig. S3. The response map shows the amplitude variations over the fiber but either the loop shape or coercive field had no significant changes. The butterfly shaped amplitude loops acquired on the KNO fiber are characteristic switching in piezoelectric materials while those from silica mainly indicate electrostatic charging of the material. Relevant material properties such as the coercive biases (-3.2 V and +4.7 V), imprint bias, and remanence can be extracted from the hysteresis loops on the KNO fiber. We varied the dc bias window from 10 V to 30 V to understand the effect of bias on the hysteresis loop. As shown in Fig. S4, the loop shape remains largely unchanged and the variations in relevant parameters, such as the coercive biases, are relatively small. However, the piezoresponse did not show appreciable saturation


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even at high driving voltages which may be ascribed to two reasons: a) electrostatic charge injection in the nanofiber; b) gradual polarization switching in individual grains that spans the nanofiber, instead of the complete reversal that is seen in bulk/thin films of KNO. Charge injection from the tip is considered to be an important phenomenon and to identify whether they contribute to the PFM spectroscopy measurements above, we performed cKPFM measurements on the same area of the KNO nanofiber. cKPFM uses a sequence of bipolar triangular waveforms as in BEPS with the main difference being that the read voltage is varied among the cycles of the triangular waveforms allowing us to investigate non-ferroelectric contributions, such as electrostatics, to the measured piezoresponse. Typically, the read voltage is varied from a voltage just over the negative coercive bias to just under the positive coercive bias to study the effect of charge injection on the shape of the hysteresis loop. In other words, setting the read voltage to 0 V in cKPFM would result in a standard BEPS measurement while setting both the read and write voltages in cKPFM to 0 V results in a standard BE-imaging measurement. Additional details on cKPFM are presented elsewhere.20 Fig. 5 shows the piezoresponse as a function of the read voltage for KNO nanofiber and SiO2/Si substrate. The color of the lines signifies the write voltage, which was varied from -12 to +12 V. These measurements represent the average response from multiple spatial locations on the nanofiber and the silica substrate. In both cases, the curves are linear with negative slope and shifts along the x-axis after dc voltage pulses. Note that for an ideal ferroelectric material, the cKPFM curves must be nonlinear and follow the shape of the hysteresis loop.23 However, we observe that the curves obtained for both ferroelectric KNO nanofiber and non-ferroelectric SiO2 substrate are similar except for the width of the band, which confirms the strong electrostatic contribution to the hysteresis measurements of the KNO nanofiber. When read voltage is 0 V,


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the standard BEPS hysteresis loop can be obtained from cKPFM as presented in Fig 5b. To extract the surface potential (SP), a linear approximation was carried out along the x-intercept which plotted against the applied voltage as shown in Fig 5c. Note that the SP loops are identical to the BEPS loop formed due to the electromechanical (piezo) response of the KNO nanofiber which explains the presence of bias-induced phenomenon. This cKPFM band formation due to injection of charges into the sample surface leads to changes in the surface potential of the nanofiber, consequently a measurable change in PFM signals. Thus, we argue that results from BE imaging and spectroscopy confirm that the KNO nanofiber is piezoelectric in nature, but the observations from cKPFM experiments demonstrate that the polarization switching intrinsic to the fiber may be attributed to electret-like or relaxor-like ferroelectric behavior. To explore these studies further and potentially decouple surface electrochemistry and bulk ionic injection, we perform the studies as a function of humidity, controlling the water layers on the sample surface.33 Since alkali niobates such as BaNbO3, NaNbO3 and LiNbO3 are highly sensitive to relative humidity (RH), we explored the influence of the RH on the piezoelectric properties of the KNO nanofibers.34–36 The RH was maintained by introducing precisely metered flow rates of dry and humid air into a sealed AFM sample holder. For each RH level, the measurements were performed after 30 minutes delay to establish a stable environment. Fig. 6 shows the piezoresponse of the sample for various RH. The piezoresponse amplitude of the nanofiber was the highest when subjected to dry RH (Fig. 6a) and decreased gradually with increasing RH (Fig. 6b). Fig. 6c shows that the piezoresponse hysteresis loops on the KNO nanofiber, obtained via BEPS, shrank considerably with increasing RH. The hysteresis loop on silica shrank slightly compared to the nanofiber and the vertical shifts in the loop could be attributed to the measurements being acquired close to the noise level. The decrease in the


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piezoresponse and the shrinking of the hysteresis loops on the KNO nanofiber can be ascribed to the physisorbed water layers on to the surface of the nanofibers at higher RH strongly influencing the tip-surface adhesion forces,33 electric field and electrostatic interactions.28,37 Fig. 7 shows the humidity dependence of cKPFM on the sample for different RH environments. The cKPFM band of curves on the KNO nanofiber was wide for low RH levels and progressively contracted with increasing RH. The width of the cKPFM band is the measure of charge injection properties of the surface.23 Therefore, we argue that the injection of the charges into the sample surface at higher RH levels was reduced due to the wetted water layers on top of the nanofiber leading to contraction in the width of the cKPFM band. This phenomenon can be attributed to the increase of the lateral ionic transport which becomes fast compared to the time scale of cKPFM. The hysteresis loops from Fig. 6d and the cKPFM band of curves from Fig. 7b on silica illustrate a response like that of a lossy capacitor response. This can be ascribed to the high surface potential stability at high RH in post-annealed SiO2 films.38,39 In both cases, relative humidity had minimal influence on the measurements on the silica compared to those on KNO nanofiber. Thus, the qualitative analysis using the BE and cKPFM measurements in both ambient and humid environment provides a reliable and deeper understanding on the piezoresponse and switching behavior of perovskite KNO nanofibers. In PFM, the dynamic cantilever displacements are caused by the volume expansion due to converse piezoelectric effect which is proportional to the piezoelectric constant of the material. Therefore, the resultant data from three BE datasets (which includes room condition and two extreme environments) were utilized to accurately quantify the piezo constant of the KNO nanofiber using a methodology demonstrated to quantify PFM surface displacements by Balke et al.40 This methodology takes into account of


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BE piezoresponse amplitude (which is already converted into volts) and Q-factor information of each pixel along with the cantilever sensitivity (optical level sensitivity) and stiffness extracted from the AFM force-distance calibration curves. Fig. 8 presents the quantified piezoresponse at each point of the sample and the histogram shows the distribution of piezo constants for samples scanned at different environmental conditions. The peak value of the piezoelectric constant of the KNO nanofiber at ambient conditions is 1.325 pm V-1. Table 1 shows the sample details and their corresponding peak, mean and standard deviation piezo constant values estimated. At lower RH, the peak value increased to 3.689 pm V-1 indicates that the KNO nanofibers are weak piezoelectric material when compared to standard poled PPLN (7.5 pm V-1) and PZT thin films (~30 pm V-1).40,41 However, a direct comparison with earlier reports of KNO bulk/thin films may not be appropriate as we observe significant changes in the quantification due to the extreme environmental conditions during PFM scans. Thus, a quantitative analysis of piezoelectric activity in KNO nanofibers was demonstrated which will help in error-free estimation of relevant properties and interpretation of material physics when employed in potential applications.

4. CONCLUSION Here, we present the first systematic investigation of the piezoresponse and ferroelectric properties of potassium niobate (KNO) nanofibers at the nanoscale. We fabricated the nanofibers using a sol-gel based far-field electrospinning process and collected the nanofibers on a SiO2/Si substrate. We show that BE-PFM can clearly identify and decouple the piezoelectric behavior of the nanofiber from other phenomena including contact mechanics and electrostatics arising from the AFM tip-sample interactions. Conversely, the conventional single frequency PFM methods


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used in literature so far leads to gross misinterpretation of results. cKPFM measurements on the nanofibers unambiguously proved that the hysteresis loops measured using BEPS method arises from electrostatic contributions. Thus, proving that KNO nanofibers does not exhibit classical polarization switching and can be attributed to electret-like or relaxor-like behavior. We also observed decreasing piezoresponse and shrinking hysteresis loops with increasing relative humidity which could be attributed to changes in the electric field and electrostatics due to increased wetting of the sample surface and their influence in quantifying piezoelectric constants of the nanofiber. PFM characterization methodologies from this work can be used as guidelines for reliable and unbiased methods for studying the ferroelectric properties of nanofibers. Insights from this study can lead to the development and testing of the next generation of nanofibers for applications including piezoelectric transducers and energy conversion devices.

ASSOCIATED CONTENT Supporting Information. SEM-EDX and HR-TEM images of KNO nanofiber. BEPS response maps, piezoresponse and amplitude loops for varying tip bias windows obtained from BEPS measurements on the KNO nanofiber. The PFM data was analyzed using pycroscopy, which is an open-source python package for storing, analyzing, and visualizing microscopy data, available at https://github.com/pycroscopy/pycroscopy The following files are available free of charge. Supplementary Info (PDF)


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AUTHOR INFORMATION Corresponding Author Dr. Zhao Rong, Associate Professor, Singapore University of Technology and Design, Singapore [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT SPM experiments were conducted at the Center for Nanophase Materials Sciences (CNMS), Oak Ridge National Laboratory which is a DOE Office of Science User Facility managed by UTBattelle, LLC under contract no. DE-AC0500OR22725. We thank Rama K. Vasudevan and Nina Balke of CNMS for thoughtful discussions on bias-induced phenomenon and quantification of surface displacements, respectively.

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3

Nanofibers with

Dramatically Enhanced Sensitivity. Nanoscale 2012, 4, 408–413.


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Figure 1. (a) SEM micrograph of as-annealed KNO nanofibers; (b) TEM and (c) HR-TEM image of a single KNO nanofiber (inset: SAED pattern)

Figure 2. (a) Surface topography; (b) deflection, (c) piezoresponse amplitude and (d) phase maps of KNO nanofiber obtained from Single frequency Piezoresponse Force Microscopy


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Figure 3. (a) Amplitude; (b) frequency; (c) phase and (d) quality factor response from Band Excitation PFM scan of the KNO nanofiber

Figure 4. (a) Bipolar triangular waveform applied to the AFM tip; (b-c) Amplitude and phase spectra for two consecutive cycles of the ferroelectric switching on the surface of a KNO nanofibers in ambient; (d) piezoresponse – voltage hysteresis and (e) amplitude – voltage butterfly loops for a bias window of 15 V.


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Figure 5. cKPFM as a function of Vread for (a) KNO nanofiber. (b) Correlation of surface potentnial as extracted from panel a, and (c) off-field PFM hysteresis loop.(d) cKPFM curves measured for SiO2/Si substrate in ambient atmosphere.


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Figure 6. Band excitation PFM on KNO nanofiber at varying RH environments. Piezoresponse amplitude maps at (a) 2% RH and (b) 92% RH. The hysteresis loops obtained at seven different RH levels for (c) KNO nanofiber and (d) SiO2/Si substrate.

Figure 7. cKFPM band as a function of Vread at varying RH environments (dry to humid) for (a) KNO nanofiber and (b) SiO2/Si substrate.

Sample

Piezoresponse

Peak (pm/V)

Mean response (pm/V)

Standard Deviation (pm/V)

1.325

1.259

0.913

KNO-2 @ RH 2%

3.689

1.058

1.704

KNO-2 @ RH 92%

2.067

0.617

1.041

KNO-1 condition

@

ambient


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Table 1. KNO nanofiber samples scanned using BE technique at different environmental conditions and their corresponding peak, mean and standard deviation piezoelectric constants are estimated.

Figure 8. Histograms showing PFM (displacement per voltage) for KNO nanofiber samples at (a) room conditions, (b) RH 2% and (c) 92% RH. (d) Comparison and Gaussian fitting of quantified piezoresponse obtained from KNO sample data.


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Decoding Apparent Ferroelectricity in Perovskite Nanofibers - ACS

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