Piezoresponse Force Microscopy Studies Research Articles

Molecular beam epitaxy and characterization of ferroelectric quaternary alloy Sc0.2Al0.45Ga0.35N

In this study, we demonstrate ferroelectricity in high-quality monocrystalline quaternary alloy ScAlGaN. Sc0.2Al0.45Ga0.35N films are grown by plasma-assisted molecular beam epitaxy and exhibit a surface roughness of 0.5 nm, limited by the roughness of the underlying molybdenum template. Polarization-electric field and positive-up-negative-down measurements reveal unambiguous ferroelectric switching with a coercive field of ∼5.5 MV cm−1 at 10 kHz and high remanent polarization of ∼150 μC cm−2. Time-dependent measurements suggest that the polarization reversal behavior adheres to the Kolmogorov–Avrami–Ishibashi model and follows a scheme of domain nucleation and growth. Detailed piezoresponse force microscopy studies further elucidate the evolution of polarity reversal domains in wurtzite nitride ferroelectrics and support the notion that the growth of inversion domains occurs via an in-plane motion of the domain walls. The realization of functional ferroelectric quaternary alloys in the wurtzite nitride family extends beyond being a technical demonstration. The additional degree of bandgap, band alignment, lattice parameter, and piezoelectric constant tunability achievable through quaternary alloys unveils a vast dimension through which wurtzite nitride ferroelectrics can be optimally engineered for a broad variety of high-performance electronic, optoelectronic, and acoustic devices and systems.

Synthesis and Optical Properties of One Year Air-Stable Chiral Sb(III) Halide Semiconductors.

Chiral hybrid metal-halide semiconductors (MHS) pose as ideal candidates for spintronic applications owing to their strong spin-orbit coupling (SOC), and long spin relaxation times. Shedding light on the underlying structure-property relationships is of paramount importance for the targeted synthesis of materials with an optimum performance. Herein, we report the synthesis and optical properties of 1D chiral (R-/S-THBTD)SbBr5 (THBTD = 4,5,6,7-tetrahydro-benzothiazole-2,6-diamine) semiconductors using a multifunctional ligand as a countercation and a structure directing agent. (R-/S-THBTD)SbBr5 feature direct and indirect band gap characteristics, exhibiting photoluminescence (PL) light emission at RT that is accompanied by a lifetime of a few ns. Circular dichroism (CD), second harmonic generation (SHG), and piezoresponse force microscopy (PFM) studies validate the chiral nature of the synthesized materials. Density functional theory (DFT) calculations revealed a Rashba/Dresselhaus (R/D) spin splitting, supported by an energy splitting (ER) of 23 and 25 meV, and a Rashba parameter (αR) of 0.23 and 0.32 eV·Å for the R and S analogs, respectively. These values are comparable to those of the 3D and 2D perovskite materials. Notably, (S-THBTD)SbBr5 has been air-stable for a year, a record performance among chiral lead-free MHS. This work demonstrates that low-dimensional, lead-free, chiral semiconductors with exceptional air stability can be acquired, without compromising spin splitting and manipulation performance.

Interface-Tuning of Ferroelectricity and Quadruple-Well State in CuInP2S6 via Ferroelectric Oxide.

Ferroelectric van der Waals CuInP2S6 possesses intriguing quadruple-well states and negative piezoelectricity. Its technological implementation has been impeded by the relatively low Curie temperature (bulk TC ∼ 42 °C) and the lack of precise domain control. Here we show that CuInP2S6 can be immune to the finite size effect and exhibits enhanced ferroelectricity, piezoelectricity, and polar alignment in the ultrathin limit when it is interfaced with ferroelectric oxide PbZr0.2Ti0.8O3 films. Piezoresponse force microscopy studies reveal that the polar domains in thin CuInP2S6 fully conform to those of the underlying PbZr0.2Ti0.8O3, where the piezoelectric coefficient changes sign and increases sharply with reducing thickness. High temperature in situ domain imaging points to a significantly enhanced TC of >200 °C for 13 nm CuInP2S6 on PbZr0.2Ti0.8O3. Density functional theory modeling and Monte Carlo simulations show that the enhanced polar alignment and TC can be attributed to interface-mediated structure distortion in CuInP2S6. Our study provides an effective material strategy to engineer the polar properties of CuInP2S6 for flexible nanoelectronic, optoelectronic, and mechanical applications.

Flexible Piezoelectric Nanogenerators Based on One-Dimensional Neutral Coordination Network Composites

Metal–organic coordination polymers are modular systems whose structures can be modified in numerous ways to introduce and influence non-linear optical and electrical properties. However, their full potential as piezoelectric nanogenerators for self-powered electronics is yet to be uncovered. Here, we report a Zn(II)-based ferroelectric one-dimensional coordination network {[Zn(L1)(bpy)]·(H2O)1.5}∞ (1) derived from a flexible dicarboxylate ligand [PhPO(NH-(C6H4COOH))2] (L1H2) and 2,2′-bipyridine as a co-ligand. The origin of polarization in 1, despite its neutral structure, is due to the polyhedral distortions around the Zn(II) center as revealed by ab initio calculations. The presence of polarizable domains was visualized by piezoresponse force microscopy (PFM) experiments. Also, from the PFM studies, a sizable converse piezoelectric coefficient (d33) value of 19.4 pm/V was noticed for 1, which is unprecedentedly high for the class of neutral-network coordination polymers. Furthermore, flexible composite devices comprising a thermoplastic polyurethane (TPU) polymer with different weight percentages (wt %) of 1 were prepared and examined for application as piezoelectric nanogenerators. Notably, the champion device of this series (poled 5 wt % 1-TPU composite) exhibits a highest open-circuit voltage of 5.6 V and power density output of 14.6 μW/cm2.

Revisiting of δ−PVDF nanoparticles via phase separation with giant piezoelectric response for the realization of self-powered biomedical sensors

Piezo-, pyro-, ferro-electricity in carbon-fluoro polymers such as poly(vinylidene fluoride) (PVDF), its co-polymers and their nanostructures are gaining remarkable technological interest in self-powered flexible electronics. However, to achieve nanostructures of PVDF with preferred electroactive phase and enhanced piezoelectric coefficient is still remains a challenge. In this regard, we have introduced a nanoprecipitation technique to machinate PVDF nanoparticles with predominant piezoelectric delta (δ) phase (which is least studied till date) using the bi-solvent phase separation technique. It is noteworthy that δ-phase of PVDF possess excellent piezoelectric properties which is comparable to β-phase, nevertheless it has been rarely explored because of its ultra-high electric field (~MV/m) based processing conditions adopted so far. In this context, solvent based phase separation approach is most convenient and thus expected to be an industrially viable approach to scale up piezoelectric δ−PVDF nanoparticles which has enormous technological and commercial merits. The piezoresponse force microscopy (PFM) study is performed to confirm the nanoscale piezo- and ferro-electricity in δ−PVDF nanoparticles that manifests the giant piezoelectric coefficient (d33) of ~−43pm/V and optimum phase reversal (∆φ≈ 180°) behavior under very low applied bias (i.e., at ~ ± 5 V). As a proof of concept, a flexible piezoelectric nanogenerator (f-PNG) was fabricated comprising of δ−PVDF nanoparticles. It generates promising electrical output with power density of 2 µW/cm2. Moreover, its ability to track the physiological signal such as arterial pulse detection indicates the potential utility of δ−PVDF nanoparticles based self-power sensors and actuators.

Thickness-Dependent Polar Domain Evolution in Strained, Ultrathin PbTiO3 Films.

Ferroelectric ultrathin films have great potential in electronic devices and device miniaturization with the innovation of technology. In the process of product commercialization, understanding the domain evolution and topological properties of ferroelectrics is a prerequisite for high-density storage devices. In this work, a series of ultrathin PbTiO3 (PTO) films with varying thicknesses were deposited on cubic KTaO3 substrates by pulsed laser deposition and were researched by Cs-corrected scanning transmission electron microscopy (STEM), reciprocal space mapping (RSM), and piezoresponse force microscopy (PFM). RSM experiments indicate the existence of a/c domains and show that the lattice constant varies continuously, which is further confirmed by atomic-scale STEM imaging. Diffraction contrast analysis clarifies that with the decrease in PTO film thickness, the critical thickness for the formation of a/c domains could be missing. When the thickness of PTO films is less than 6 nm, the domain configurations in the ultrathin PTO films are the coexistence of a/c domains and bowl-like topological structures, where the latter ones were identified as convergent and divergent types of meron. In addition, abundant 90° charged domain walls in these ultrathin PTO films were identified. PFM studies reveal clear ferroelectric properties for these ultrathin PTO films. These results may shed light on further understanding the domain evolution and topological properties in ultrathin ferroelectric PTO films.

Estimation of gradient size of interfacial strain and its optimization for effective magnetoelectric coupling in (CoFe2O4) – (0.93Na0.5Bi0.5TiO3 – 0.07BaTiO3), 2-2 nano-composites

Strain-mediated coupling between the magnetic and electrically ordered phases plays a significant role in magnetoelectric (ME) nano-composites. This study explores a method to analyse and quantify interfacial strain using a grazing angle scan (α) in a ME composite optimised for a specific microstructure. The details of strain around the interface CoFe2O4 (CFO) – 0.93Na0.5Bi0.5TiO3 – 0.07BaTiO3 (NBT-BT) was determined by performing ‘α’ scan, in order to gather information at various depths of the NBT-BT layer around maximum intensity (110) reflection. The strain around the interface was observed to dominate over a spatial region of ∼20–30 nm away from the interface. The Piezoresponse force microscopy (PFM) studies performed near the interface reveal that the strain constrain experienced by the ferroelectric layer operates such that polarisation rotation and domain wall motion are constrained compared to the strain relaxed region of the film. For effective strain transfer, heterostructures grown with optimised thicknesses (∼20–30 nm) exhibited a superior inverse piezomagnetic effect.

Spontaneous and Induced Ferroelectricity in the BiFe1−xScxO3 Perovskite Ceramics

High‐pressure synthesis method allows obtaining single‐phase perovskite BiFe1−x Sc x O3 ceramics in the entire concentration range. As‐prepared compositions with x from 0.30 to 0.55 have the antipolar orthorhombic Pnma structure but can be irreversible converted into the polar rhombohedral R3c or the polar orthorhombic Ima2 phase via annealing at ambient pressure. Microstructure defects and large conductivity of the high‐pressure‐synthesized ceramics make it difficult to study and even verify their ferroelectric properties. These obstacles can be overcome using piezoresponse force microscopy (PFM) addressing ferroelectric behavior inside single grains. Herein, the PFM study of the BiFe1−x Sc x O3 ceramics (0.30 ≤ x ≤ 0.50) is reported. The annealed samples show a strong PFM contrast. Switching of domain polarity by an electric field confirms the ferroelectric nature of these samples. The as‐prepared BiFe0.5Sc0.5O3 ceramics demonstrate no piezoresponse in accordance with the antipolar character of the Pnma phase. However, application of a strong enough electric field induces irreversible transition to the ferroelectric state. The as‐prepared BiFe0.7Sc0.3O3 ceramics show coexistence of ferroelectric and antiferroelectric grains without poling. It is assumed that mechanical stress caused by the sample polishing can be also a driving force of phase transformation in these materials alongside temperature and external electric field.

Effect of correlated oxide electrodes on disorder pinning and thermal roughening of ferroelectric domain walls in epitaxial PbZr0.2Ti0.8O3 thin films

We report the competing effects of disorder pinning and thermal roughening on ferroelectric domain walls as a function of temperature in epitaxial $\mathrm{Pb}{\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_{3}$ thin films deposited on (001) $\mathrm{SrTi}{\mathrm{O}}_{3}$ substrates buffered by three types of correlated oxide electrodes, ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}\mathrm{Mn}{\mathrm{O}}_{3}$, $\mathrm{LaNi}{\mathrm{O}}_{3}$, and $\mathrm{SrIr}{\mathrm{O}}_{3}$. Piezoresponse force microscopy studies show that the 50-nm $\mathrm{Pb}{\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_{3}$ films are uniformly polarized in the as-grown states, with the patterned domain structures persisting above 700 \ifmmode^\circ\else\textdegree\fi{}C. For all three types of films, the domain wall roughness is dominated by two-dimensional (2D) random bond disorder at room temperature, and transitions to 1D thermal roughening upon heating. The roughness exponent $\ensuremath{\zeta}$ increases progressively from 0.3 to 0.5 within a temperature window that depends on the bottom conducting oxide type, from which we extracted the distribution of disorder pinning energy. We discuss the possible origins of the disorder pinning and the effect of the correlated oxide electrodes on the energy landscape of DW motion.

Ferroelectric domain evolution in a Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 piezoceramic studied using piezoresponse force microscopy

Lead-free Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BZT-0.5BCT) ceramics have drawn attention in recent years because of their outstanding dielectric and electromechanical properties, such as a piezoelectric coefficient d33 ∼ 620 pC N−1 and a large signal of d33* ∼ 1100 pm V−1 at 0.5 kV mm−1 at room temperature (RT). These particular properties are relevant to a range of applications. However, the structural origin of this high piezoelectric coefficient is still a subject of discussion. An in-depth understanding of the ferroelectric domain evolution of BZT-0.5BCT ceramics is crucial for probing the underlying mechanisms and for guiding practical applications. Using piezoresponse force microscopy (PFM), we have directly visualized the evolution of the BZT-0.5BCT domain structure using temperature and electric field stimulation on micrometer and nanometer scales. The PFM results unambiguously evidence the coexistence of wedge-shaped and lamellar domains with miniaturized nanodomain structures at RT. The temperature- and electric-field-dependent PFM study presented here highlights the critical role of wedge-shaped domains in domain evolution. Wedge-shaped domains turn into small domains with curved domain walls after the heating cycle and then become lamellar domains after the poling cycle at RT. Transitional domain structures with an increased density of nanodomains appear in both the thermal and poling cycles. More interestingly, the electric-field-dependent domain structure evolution at different temperatures shows better domain structure reversibility at high temperatures than at temperatures close to the phase boundary. This demonstrates that the BZT-0.5BCT ceramic has superior stability at medium temperatures (40 °C–60 °C), implying excellent stability for applications.

Assembly of Close-Packed Ferroelectric Polymer Nanowires via Interface-Epitaxy with ReS2.

The flexible, transparent, and low-weight nature of ferroelectric polymers makes them promising for wearable electronic and optical applications. To reach the full potential of the polarization-enabled device functionalities, large-scale fabrication of polymer thin films with well-controlled polar directions is called for, which remains a central challenge. The widely exploited Langmuir-Blodgett, spin-coating, and electrospinning methods only yield polymorphous or polycrystalline films, where the net polarization is compromised. Here, an easily scalable approach is reported to achieve poly(vinylidene fluoride-trifluoroethylene) P(VDF-TrFE) thin films composed of close-packed crystalline nanowires via interface-epitaxy with 1T'-ReS2 . Upon controlled thermal treatment, uniform P(VDF-TrFE) films restructure into about 10 and 35 nm-wide (010)-oriented nanowires that are crystallographically aligned with the underlying ReS2 , as revealed by high-resolution transmission electron microscopy. Piezoresponse force microscopy studies confirm the out-of-plane polar axis of the nanowire films and reveal coercive voltages as low as 0.1 V. Reversing the polarization can induce a conductance switching ratio of >108 in bilayer ReS2 , over six orders of magnitude higher than that achieved by an untreated polymer gate. This study points to a cost-effective route to large-scale processing of high-performance ferroelectric polymer thin films for flexible energy-efficient nanoelectronics.

Synthesis and Assembly of Zinc Oxide Microcrystals by a Low-Temperature Dissolution-Reprecipitation Process: Lessons Learned About Twin Formation in Heterogeneous Reactions.

Cobalt‐doped zinc oxide single crystals with the shape of hexagonal platelets were synthesized by thermohydrolysis of zinc acetate, cobalt acetate, and hexamethylenetetramine (HMTA) in mixtures of ethanol and water. The mineralization proceeds by a low‐temperature dissolution–reprecipitation process from the liquid phase by the formation of basic cobalt zinc salts as intermediates. The crystal shape as well as twin formation of the resulting oxide phase can be influenced by careful choice of the solvent mixture and the amount of doping. An understanding of the course of the reaction was achieved by comprehensive employment of analytical techniques (i.e., SEM, XRD, IR) including an in‐depth HRTEM study of precipitates from various reaction stages. In addition, EPR as well as UV/Vis spectroscopic measurements provide information about the insertion of the cobalt dopant into the zincite lattice. The Langmuir–Blodgett (LB) technique is shown to be suitable for depositing coatings of the platelets on glass substrates functionalized with polyelectrolyte multilayers and hence is applied for the formation of monolayers containing domains with ordered tessellation. No major differences are found between deposits on substrates with anionic or cationic surface modification. The adherence to the substrates is sufficient to determine the absolute orientation of the deposited polar single crystals by piezoresponse force microscopy (PFM) and Kelvin probe force microscopy (KPFM) studies.

Observation of Unconventional Dynamics of Domain Walls in Uniaxial Ferroelectric Lead Germanate

AbstractApplication of scanning probe microscopy techniques such as piezoresponse force microscopy (PFM) opens the possibility to re‐visit the ferroelectrics previously studied by the macroscopic electrical testing methods and establish a link between their local nanoscale characteristics and integral response. The nanoscale PFM studies and phase field modeling of the static and dynamic behavior of the domain structure in the well‐known ferroelectric material lead germanate, Pb5Ge3O11, are reported. Several unusual phenomena are revealed: 1) domain formation during the paraelectric‐to‐ferroelectric phase transition, which exhibits an atypical cooling rate dependence; 2) unexpected electrically induced formation of the oblate domains due to the preferential domain walls motion in the directions perpendicular to the polar axis, contrary to the typical domain growth behavior observed so far; 3) absence of the bound charges at the 180° head‐to‐head (H–H) and tail‐totail (T–T) domain walls, which typically exhibit a significant charge density in other ferroelectrics due to the polarization discontinuity. This strikingly different behavior is rationalized by the phase field modeling of the dynamics of uncharged H–H and T–T domain walls. The results provide a new insight into the emergent physics of the ferroelectric domain boundaries, revealing unusual properties not exhibited by conventional Ising‐type walls.

Atomic-Scale insight into the reversibility of polar order in ultrathin epitaxial Nb:SrTiO3/BaTiO3 heterostructure and its implication to resistive switching

Ferroelectric heterostructures with bi-stable state of polarization are appealing for data storage as well as tunable functionalities such as memristor behavior. While an increasing number of experimental and theoretical studies suggest that polarization persists in ultrathin epitaxial heterostructures approaching just a couple of unit cells, the switching of such polar order is much less well understood, and whether polarization can be reversed in ultrathin ferroelectric heterostructures remains to be answered. Here we fabricate high-quality 7-unit cell thick BaTiO3 (BTO) films on Nb-doped single crystalline SrTiO3 (NSTO) substrate, and demonstrate their apparent yet unambiguously false polarization reversal due to charge injection using comprehensive piezoresponse force microscopy (PFM) studies. The presence of weak polar order consistent with linear piezoelectricity is confirmed at the atomic scale by high resolution integrated differential phase contrast (IDPC) of scanning transmission electron microscopy (STEM) as well as macroscopic second harmonic generation (SHG), while the lack of polarization reversal under the voltage applied is supported by density functional theory calculation showing the persistence of dead layer on the surface. Nevertheless, poling-induced electric conduction differing by two orders of magnitude is observed, demonstrating resistive switching in ferroelectric heterostructure in the absence of polarization reversal, even with weak polar order. Our finding has technological implications on emerging memristor applications with potentially more accessible states than bi-stable polarization modulated mechanism, and raises technical challenges to unambiguously demonstrate polarization switching in ultrathin films at their critical size limit.

Enhanced photovoltaic properties of gradient calcium-doped BiFeO3 films

The photovoltaic properties of ferroelectric films have been extensively studied due to their potential applications in the fields of photodetection, energy conversion harvesting and storage. However, the effect of the gradient distribution of oxygen vacancies on the photovoltaic properties remains unclear. Herein, we prepared BiFeO3 (BFO) and two types of gradient calcium-doped BiFeO3 (BiFeO3/Bi0.95Ca0.05FeO2.975/Bi0.90Ca0.10FeO2.950/Bi0.85Ca0.15FeO2.925: BCFO-1 and Bi0.85Ca0.15FeO2.925/Bi0.90Ca0.10FeO2.950/Bi0.95Ca0.05FeO2.975/BiFeO3: BCFO-2) films deposited on fluorine-doped tin oxide glass substrates. Piezoresponse force microscopy studies indicate the upward self-polarization phenomenon in BFO and BCFO-1 films, while the downward self-polarization phenomenon in BCFO-2 films. The J–V characteristic curves show rectifier behaviors for these films due to the configuration of oxygen vacancies. For photovoltaic response, the open circuit voltage of BCFO-1 films is more than 2 times higher than that of BFO films, and the short-circuit photocurrent densities of BCFO-1 and BCFO-2 films are increased by nearly 32 and 6 times, respectively. The direction and magnitude of photovoltaic response are possibly associated with the energy band modulation governed by self-polarization and the gradient distribution of oxygen vacancies. The work demonstrates the merits of gradient doping with lower valence cation towards enhanced photovoltaic properties with high stability for ferroelectric oxides.

Optimization of electrical and photovoltaic properties of Au-BiFeO<sub>3</sub> nanocomposite films

Ferroelectric films, are an important class of photoelectric functional material, which possess the following characteristics: the breaking of their symmetry can lead to self-polarization and this polarization state can be regulated by external stimuli. The photovoltaic properties of ferroelectric films have been extensively investigated due to their potential applications in the field of photodetection, energy conversion harvesting and nonvolatile storage. In view of the small photocurrent density and the degradation of photovoltaic property caused by the depolarization effect in ferroelectric films, it is necessary to explore an approach to improving the self-polarization phenomenon and regulating the conduction mechanism to further optimize their photovoltaic properties. Here in this work, BiFeO<sub>3</sub> (BFO) films dispersed with Au nanoparticles are deposited on FTO glass substrates by the sol-gel method to obtain the Au-BFO nanocomposite films. Moreover, the relationships between Au content (0 mol%, 0.25 mol%, 0.5 mol%, 1 mol% and 3 mol%) and microstructure, electrical and photovoltaic properties of Au-BFO nanocomposite films are investigated to determine the optimal Au content. Piezoresponse force microscopy studies show that the Au-BFO nanocomposite film with 0.5 mol% Au has the strong self-polarization phenomenon. With the increase of Au content, the conduction mechanism of the Au-BFO nanocomposite films is described by the space-charge limited current theory but not the Schottky emission model any more. The photovoltaic properties of the Au-BFO nanocomposite films first increase and then decrease. When Au content is 0.5 mol%, the Au-BFO nanocomposite film has the best photovoltaic property. The open-circuit voltage and short-circuit photocurrent density of the Au-BFO nanocomposite film with 0.5 mol% Au increase nearly 3 and 5 times counterparts of the BFO film, respectively. The photovoltaic effects of Au-BFO nanocomposite films are improved mainly by regulating the self-polarization phenomenon and conduction mechanism. This study demonstrates the merits of BFO films dispersed with Au nanoparticles, specifically, the photovoltaic properties of Au-BFO nanocomposite films are further optimized. In this work, we propose a simple and effective method to regulate the electrical and photovoltaic properties of ferroelectric films, which provides a new perspective for further understanding the photovoltaic effects of ferroelectric films.

Nanoscale Probing of Ferroelectric Domain Switching Using Piezoresponse Force Microscopy

In ferroelectric materials, piezoresponse force microscopy (PFM) has been widely used to explore ferroelectric domain switching. In this article, we review the fundamentals of nanoscale probing of ferroelectric domain switching using PFM, including the basic principles of PFM and a variety of PFM studies on local domain switching. We also introduce advanced PFM techniques for exploring switching behavior. Finally, we discuss several issues and perspectives in nanoscale probing of ferroelectric domain switching using PFM. PFM has played an important role in exploring switching behavior in ferroelectric materials, and it could be further developed to probe more detailed switching information. Key words: Ferroelectric domain, Piezoresponse force microscopy, Switching behavior, Capacitor

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

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

Anisotropic strain: A critical role in domain evolution in (111)- Oriented ferroelectric films

Domain behavior of (111)- oriented perovskite ferroelectric films is significantly different from (001)-/(101)- oriented ones, resulting in enhancing property responses such as a superior susceptibility and a reduced coercive field. However, the domain structures and evolutions with the thickness of (111)-oriented ferroelectric films, which are crucial to further understand the distinctive properties, are still obscure. In this study, the ferroelectric domains of (111)- oriented PbTiO3 films are investigated by transmission electronic microscopy (TEM) and piezoresponse force microscopy (PFM). We identify the domain evolution with the film thicknesses under anisotropic strains imposed by the orthorhombic GdScO3 (101)O substrates. Contrast analysis and electron diffraction patterns reveal that only four ferroelectric variants evolve in PTO films: d2+, d2-, d3+ and d3-, with the polarization directions along [010], [01¯0], [001] and [001¯], respectively. Two kinds of domain walls are formed: “inclined” (011) domain walls for d2-/d3+ (d2+/d3-) domains, “normal” (011¯) domain walls for d2-/d3+ (d2+/d3-) domains. The width of periodically distributed d2+/d3+ (d2-/d3-) domains increases with film thickness following the square root rule. Aberration-corrected scanning transmission electronic microscopy demonstrates the lattice characteristics of domains in (111)- oriented PbTiO3 films are consistent with tetragonal ferroelectric domains. PFM studies reveal that both the out-of-plane and in-plane polarization components of d2-/d3+ (d2+/d3-) domains are non-identical, whereas the d2+/d3+ (d2-/d3-) domains possess uniform out-of-plane polarization component and non-uniform in-plane polarization component. This study discloses the domain structure in (111)- oriented tetragonal ferroelectric films under anisotropic strains and the association with the ferroelectric properties.

Atomic-Scale Insight into the Reversibility of Polar Order in Ultrathin Epitaxial Nb:SrTiO <sub>3</sub>/BaTiO <sub>3</sub> Heterostructure and Its Implication to Resistive Switching

Ferroelectric heterostructures with bi-stable state of polarization are appealing for data storage as well as tunable functionalities such as memristor behavior. While an increasing number of experimental and theoretical studies suggest that polarization persists in ultrathin epitaxial heterostructures approaching just a couple of unit cells, the switching of such polar order is much less well understood, and whether polarization can be reversed in ultrathin ferroelectric heterostructures remains to be answered. Here we fabricate high-quality 7-unit cell thick BaTiO3 (BTO) films on Nb-doped single crystalline SrTiO3 (NSTO) substrate, and demonstrate their apparent yet unambiguously false polarization reversal due to charge injection using comprehensive piezoresponse force microscopy (PFM) studies. The presence of weak polar order consistent with linear piezoelectricity is confirmed at the atomic scale by high resolution integrated differential phase contrast (IDPC) of transmission electron microscopy (TEM) as well as macroscopic second harmonic generation (SHG), while the lack of polarization reversal under the voltage applied is supported by density functional theory calculation showing the persistence of dead layer on the surface. Nevertheless, poling-induced electric conduction differing by two orders of magnitude is observed, demonstrating resistive switching in ferroelectric heterostructure in the absence of polarization reversal, even with weak polar order. Our finding has technological implications on emerging memristor applications with potentially more accessible states than bi-stable polarization modulated mechanism, and raises technical challenges to unambiguously demonstrate polarization switching in ultrathin films at their critical size limit.