Minimizing electrostatic interactions from piezoresponse force microscopy via capacitive excitation

Piezoresponse force microscopy(PFM)has emerged as one of the most powerful techniques to probe ferroelectric materials at the nanoscale,yet it has been increasingly recognized that piezoresponse measured by PFM is often influenced by electrostatic interactions.In this letter,we report a capacitive excitation PFM(ce-PFM)to minimize the electrostatic interactions.The effectiveness of ce-PFM in minimizing electrostatic interactions is demonstrated by comparing the piezoresponse and the effective piezoelectric coefficient measured by ce-PFM and conventional PFM.The effectiveness is further confirmed through the ferroelectric domain pattern imaged via ce-PFM and conventional PFM in vertical modes,with the corresponding domain contrast obtained by ce-PFM is sharper than conventional PFM.These results demonstrate ce-PFM as an effective tool to minimize the interference from electrostatic interactions and to image ferroelectric domain pattern,and it can be easily implemented in conventional atomic force microscope(AFM)setup to probe true piezoelectricity at the nanoscale.

Sol-Gel Based Soft Lithography and Piezoresponse Force Microscopy of Patterned Pb(Zr_(0.52)Ti _(0.48))O_3 Microstructures

Sol-gel based soft lithography technique has been developed to pattern a variety of ferroelectric Pb（Zr0.52Ti0.48）O3（PZT） microstructures,with feature size approaching 180 nm and good pattern transfer between the master mold and patterned films.X-ray diffraction and high-resolution transmission electron microscopy confirm the perovskite structure of the patterned PZT.Piezoresponse force microscopy（PFM） and switching spectroscopy piezoresponse force microscopy（SSPFM） confirm their piezoelectricity and ferroelectricity.Piezoresponse as high as 2.75 nm has been observed,comparable to typical PZT films.The patterned PZT microstructures are promising for a wide range of device applications.

Piezoresponse force microscopy and dielectric spectroscopy study of Ba_(0.6)Sr_(0.4)TiO_(3)thin films

Research on synthesis,characterization and determination of processing-structure-property relationships of commercially important ferroelectric thin films has been performed.The sol-gel type solution deposition technique was applied to produce good quality thin films of Ba_(0.6)Sr_(0.4)TiO_(3)(BST60/40)chemical composition on the stainless steel substrates.The thin films were characterized in terms of their microstructure,crystal structure,phase composition,piezoelectric and dielectric properties.It was found that the BST60/40 thin film adopted the cubic structure at room temperature with an elementary cell parameter a-3:971e8TÅ.Morphology of the thin film surface was studied with Atomic Force Microscopy(AFM).Average roughness of the thin films surface was found(Sa=0:055μm).Piezoresponse Force Microscopy(PFM)was applied for the thin film characterization.Active piezoelectric regions were found in BST60/40 thin film.Therefore,dielectric response measured at room temperature was studied in assumption of piezoelectric electric equivalent circuit.

POLAR STRUCTURES OF PbMg_(1/3)Nb_(2/3)O_(3)-PbTiO_(3) RELAXORS:PIEZORESPONSE FORCE MICROSCOPY APPROACH

Relaxor ferroelectrics are one of the mysterious objects of the solid-state physics studied over 50 years.The physical properties of relaxors were mainly assessed by scattering and dielectric methods and revealed the importance of polarization correlations with the short-range order,so-called polarization nanoscale regions.This paper review recent progress achieved by using piezoresponse force microscopy(PFM)to analyze the spatial distribution of polarization and its evolution with time,temperature,and electricfield for the most popular relaxor family:solid solutions between PbMg_(1/3)Nb_(2/3)O_(3) and PbTiO_(3).The PFM technique has proved to be a powerful tool for the investigation of local properties of relaxors where optical techniques obviously fail because of their lack of resolution.The PFM study of relaxors clearly helps to understand the mechanism of polarization distribution and illustrates the importance of mesoscopic polarization patterns that were until now overlooked by major theories of the relaxor state.

Nanoscale domain switching mechanism of Bi_(3.15)Eu_(0.85)Ti_3O_(12) thin film under the different mechanical forces

The switching process of ferroelectric thin films in electronic devices is one of the most important requirements for their application. Especially for the different external fields acting on the film surface, the mechanism of domain switching is more complicated. Here we observe the nanoscale domain switchings of Bi3.15Eu0.85Ti3O12 thin film under different mechanical forces at a fast scan rate. As the force increases from initial state to 247.5 n N, the original bright or grey contrasts within the selected grains are all changed into dark contrasts corresponding to the polarization vectors reversed from the up state to the down state, except for the clusters. As the mechanical force increases to 495 n N, the color contrasts in all of the selected grains further turn into grey contrasts and some are even changed into grey contrasts completely showing the typical 90° domain switching. When another stronger loading force 742.5 n N is applied, the phase image becomes unclear and it indicates that the piezoelectric signal can be suppressed under a sufficiently high force, which is coincident with previous experimental results. Furthermore, we adopt the domain switching criterion from the perspective of equilibrium state free energy of ferroelectric nanodomain to explain the mechanisms of force-generated domain switchings.

Ferroelectricity in hexagonal YFeO_3 film at room temperature

In this paper we report the leakage current, ferroelectric and piezoelectric properties of the YFe O3 film with hexagonal structure, which was fabricated on Si（111） substrate by a simple sol-gel method. The leakage current test shows good characteristics as the leakage current density is 5.4 × 10^-6A/cm^2 under 5 V. The dominant leakage mechanism is found to be an Ohmic behavior at low electric field and space-charge-limited conduction at high electric field region. The P–E measurements show ferroelectric hysteresis loops with small remnant polarization and coercive field at room temperature.The distinct and switchable domain structures on the nanometer scale are observed by piezoresponse force microscopy,which testifies to the ferroelectricity of the YFe O3 film further.

Imaging ferroelectric domains via charge gradient microscopy enhanced by principal component analysis

Local domain structures of ferroelectrics have been studied extensively using various modes of scanning probes at the nanoscale,including piezoresponse force microscopy(PFM)and Kelvin probe force microscopy(KPFM),though none of these techniques measure the polarization directly,and the fast formation kinetics of domains and screening charges cannot be captured by these quasi-static measurements.In this study,we used charge gradient microscopy(CGM)to image ferroelectric domains of lithium niobate based on current measured during fast scanning,and applied principal component analysis(PCA)to enhance the signal-to-noise ratio of noisy raw data.We found that the CGM signal increases linearly with the scan speed while decreases with the temperature under power-law,consistent with proposed imaging mechanisms of scraping and refilling of surface charges within domains,and polarization change across domain wall.We then,based on CGM mappings,estimated the spontaneous polarization and the density of surface charges with order of magnitude agreement with literature data.The study demonstrates that PCA is a powerful method in imaging analysis of scanning probe microscopy(SPM),with which quantitative analysis of noisy raw data becomes possible.

Creation and erasure of polar bubble domains in PbTiO_(3)films by mechanical stress and light illuminations

The controllable manipulation of polar topological structures(e.g.skyrmion bubble)in ferroelectric materials have been considered as a cornerstone for future programmable nano-electronics.Here,we present the effective creation and erasure of polar bubble states PbTiO_(3)(PTO)multilayers trigged by mechanical stress and light illumination,respectively.It was found that applying atomic force microscope(AFM)tip force can induced formation of nanoscale bubble domains from the initial monodomain state.Moreover,the created bubble domain can be eliminated by exposure to ultraviolet or infrared light illumination.The above results can be understood by modulation of depolarization screening charges and bias fields,as reflected by scanning Kelvin potential microscopic(SKPM)observations,whereby the flexoelectric effect from the tip force tends to remove the screening charges on top surface and modulate the bias field that favors the formation of bubble state while light illumination tends to recover the screen charges and favor the monodomain state.The results provide a good example for multi-field manipulation of polar topologies,which might create a new avenue towards the immerging new concept electronic devices.

Determination of polarization states in(K,Na)NbO3 lead-free piezoelectric crystal

Polarization switching in lead-free(K0.40Na0.60)NbO3(KNN)single crystals was studied by switching spectroscopy piezoresponse force microscopy(SS-PFM).Acquisition of multiple hysteresis loops on a closely spaced square grid enables polarization switching parameters to be mapped in real space.Piezoresponse amplitude and phase hysteresis loops show collective symmetric/asymmetric characteristics,affording information regarding the switching behavior of different domains.As such,the out-of-plane polarization states of the domains,including amplitudes and phases can be determined.Our results could contribute to a further understanding of the relationships between polarization switching and polarization vectors at the nanoscale,and provide a feasible method to correlate the polarization hysteresis loops in a domain under an electric field with the polarization vector states.

Ferroelectric domains and phase transition of sol-gel processed epitaxial Sm-doped BiFeO_(3)(001)thin films

BiFeO_(3),a room-temperature multiferroic material,has recently been increasingly applied as a potential lead-free piezoelectric material due to its large piezoelectricity achieved by doping.In this work,12%Smdoped BiFeO_(3)epitaxial thin films were fabricated on Nb-doped SrTiO_(3)(001)single crystal substrates via sol-gel method.The epitaxy was verified by reciprocal space mapping(RSM)and transmission electron microscope(TEM).The TEM results indicated the coexistence of R3c and Pbam phases in the film.The domains and piezoelectric properties from room temperature to 200℃were characterized by piezoresponse force microscopy(PFM).Domains became active from 110℃to 170℃,and domain configurations changed obviously.A partially fading piezoresponse indicated the emergence of antiferroelectric Pbam.The in-situ domain analysis suggested that the phase transition was accompanied by domain wall motion.Switching spectroscopy PFM(SS-PFM)was further conducted to investigate the piezoresponse during the phase transition.Anomalous responses were found in both ON and OFF states at 170℃,and the film exhibits typical antiferroelectric behavior at 200℃,implying that the completion of phase transition and structure turned to the Pbam phase.This work revealed the origin of the high piezoresponse of Sm-doped BiFeO_(3)thin films at the morphotropic phase boundary(MPB).

Synthesis, microstructures, and magnetoelectric couplings of electrospun multiferroic nanofibers

Multiferroic materials with two or more types of ferroic orders have attracted a great deal of atten- tion in the last decade for their magnetoelectric coupling, and new ideas and concepts have been ex- plored recently to develop multiferroic materials at nano-scale. Motivated by theoretical analysis, we synthesized single-phase BiFeO3 （BFO） nanofibers, Pb（Zr0.52Ti0.48）O3-CoFe2O4 （PZT-CFO） and Pb（Zro.52Tio.ns）Oa-NiFe204 （PZT-NFO） composite nanofibers, and CoFe2O4-Pb（Zr0.52Ti0.48）O3 （CFO-PZT） core-shell nanofibers using sol-gel based electrospinning. These nanofibers typically have diameters in the range of a few hundred nanometers and grain size in the range of 10s nanome- ters, and exhibits both ferroelectric and ferromagnetic properties. Piezoresponse force microscopy （PFM） based techniques have also been developed to examine the magnetoelectrie coupling of the nanofibers, which is estimated to be two orders of magnitude higher than that of thin films, con- sistent with our theoretical analysis. These nanofibers are promising for a variety of multiferroic applications.

Piezo-/ferroelectric phenomena in biomaterials: A brief review of recent progress and perspectives

There have been overwhelming observations of piezo-/ferroelectric phenomena in many biological tissues and macromolecules,boosting the development of bio-based smart devices and the applications using electromechanical coupling phenomena in biological systems.The electromechanical coupling is believed to be responsible for various biophysical behaviors and remarkable biomaterial properties.Despite the abundant phenomenal observations,the fundamental understanding of the piezo-/ferroelectric effect in biomaterials/systems and the rational design of biobased macroscopic materials with desired piezoelectric responses are still scarce.In this review,we firstly present remarkable historical events on the development of piezo-/ferroelectricity in biomaterials,followed by a brief overview of the fundamental physics of piezo-/ferroelectricity.The developments of biopiezo-/bioferroelectricity in protein-based biomaterials and their implications are highlighted subsequently.In experimental studies,to identify the intrinsic piezo-/ferroelectric properties from other effects or artifacts is usually elusive.This issue is also addressed and discussed in detail,especially using piezoelectric force microscopy(PFM)and spectroscopy techniques to investigate the local piezo-/ferroelectric phenomena in nanostructured materials are highlighted emphatically.