Ferroelectric Polarization Properties Research Articles

Lowering the coercive field of van der Waals ferroelectric NbOI2 with photoexcitation

Polarization switching in van der Waals ferroelectric materials driven by an electric field remains robust even at the atomic layer limit, paving the way for advances in the miniaturization and integration of ferroelectric devices. Thus, understanding the modulation of ferroelectric properties in two-dimensional ferroelectric materials is essential for their efficient nanoscale applications. NbOI2, a recently confirmed van der Waals ferroelectric, offers an ideal platform for investigating in-plane spontaneous polarization at the nanoscale. We explored the influence of laser excitation on the ferroelectric polarization properties of NbOI2. In multilayer NbOI2 devices with in-plane configurations, no significant current signals were detected along the c-axis or b-axis (the ferroelectric polarization axis) in the absence of illumination. However, under laser excitation, the material exhibited clear hysteresis loop behavior along both the c-axis and b-axis, indicating that laser excitation effectively reduces the coercive voltage. Furthermore, at the same excitation wavelength, the current peak along the c-axis was larger, with more pronounced hysteresis loops. Our experimental findings demonstrate that laser excitation can lower the coercive field in multilayer NbOI2 and induce different electrical hysteresis behavior along different crystallographic orientations, providing valuable insights for the development of NbOI2-based optoelectronic devices.

Robust Ferroelectricity in Nonstoichiometric 2D AgCr1-xS2 via Chemical Vapor Deposition.

Ferroelectricity in two-dimensional (2D) materials at room temperature has attracted significant interest due to their substantial potential for applications in non-volatile memory, nanoelectronics, and optoelectronics. The intrinsic tendency of 2D materials toward nonstoichiometry results in atomic configurations that differ from those of their stoichiometric counterparts, thereby giving rise to potential ferroelectric polarization properties. However, reports on the emergence of room temperature ferroelectric effects in nonstoichiometric 2D materials remain limited. This study reports the observation of room temperature ferroelectricity in nonstoichiometric AgCr1-xS2 ternary 2D transition metal dichalcogenides synthesized via chemical vapor deposition. The noncentrosymmetric crystal structure and switchable ferroelectric polarization are confirmed through second harmonic generation (SHG) and piezoresponse force microscopy (PFM) measurements. It is determined that the primary cause of ferroelectric polarization is the interlayer movement of ordered asymmetric Ag atoms under the influence of numerous chromium (Cr) vacancies along with interlayer atom displacement. Furthermore, two types of electrical devices based on in-plane (IP) and out-of-plane (OOP) polarization are demonstrated. This work offers a new perspective for fabricating ternary ultrathin 2D transition metal dichalcogenides ferroelectric materials and presents a potential pathway for creating exceptional multifunctional materials.

Atomic and electronic structure of monolayer ferroelectric GeS on Cu(111)

Two-dimensional (2D) ferroelectric materials are important materials for both fundamental properties and potential applications. Especially, group Ⅳ monochalcogenide possesses highest thermoelectric performance and intrinsic ferroelectric polarization properties and can sever as a model to explore ferroelectric polarization properties. However, due to the relatively large exfoliation energy, the creation of high-quality and large-size monolayer group Ⅳ monochalcogenide is not so easy, which seriously hinders the integration of these materials into the fast-developing field of 2D materials and their heterostructures. Herein, monolayer GeS is successfully fabricated on Cu(111) substrate by molecular beam epitaxy method, and the lattice structure and the electronic band structure of monolayer GeS are systematically characterized by high-resolution scanning tunneling microscopy, low-energy electron diffraction, <i>in-situ</i> X-ray photoelectron spectroscopy, Raman spectra, and angle-resolved photoelectron spectroscopy, and density functional theory calculations. All atomically resolved STM images reveal that the obtained monolayer GeS has an orthogonal lattice structure, which consists with theoretical prediction. Meanwhile, the distinct moiré pattern formed between monolayer GeS and Cu(111) substrate also confirms the orthogonal lattice structure. In order to examine the chemical composition and valence state of as-prepared monolayer GeS, <i>in-situ</i> XPS is utilized without being exposed to air. The measured spectra of XPS core levels suggest that the valence states of Ge and S elements are identified to be +2 and –2, respectively and the atomic ratio of Ge/S is 1∶1.5, which is extremely close to the stoichiometric ratio of 1∶1 for GeS. To further corroborate the quality and lattice structure of the monolayer GeS film, <i>ex-situ</i> Raman measurements are also performed for monolayer GeS on highly oriented pyrolytic graphene (HOPG) and multilayer graphene substrate. Three well-defined typical characteristic Raman peaks of GeS are observed. Finally, <i>in-situ</i> ARPES measurement are conducted to determine the electronic band structure of monolayer GeS on Cu(111). The results demonstrate that the monolayer GeS has a nearly flat band electronic band structure, consistent with our density functional theory calculation. The realization and investigation of the monolayer GeS extend the scope of 2D ferroelectric materials and make it possible to prepare high quality and large size monolayer group Ⅳ monochalcogenides, which is beneficial to the application of this main group material to the rapidly developing 2D ferroelectric materials and heterojunction research.

Effect of Local Structural Distortions on Antiferroelectric-Ferroelectric Phase Transition in Dilute Solid Solutions of KxNa1-xNbO3.

The fundamental principles that govern antiferroelectric (AFE)-ferroelectric (FE) transitions are not well understood for many solid solutions of perovskite compounds. For example, crystal chemical considerations based on the average Goldschmidt tolerance factor or ionic polarizability do not precisely predict the boundary between the AFE and FE phases in dilute solid solutions of alkali niobates, such as KxNa1-xNbO3 (x ≤ 0.02). Here, based on detailed structural analysis from neutron total scattering experiments, we provide insights about how the relative local distortions around the A- and B-sites of the ABO3 perovskite structure affect the AFE/FE order of the average crystallographic phases in KxNa1-xNbO3. We show that a higher (lower) ratio of B-site-centered distortions over A-site-centered distortions drives transition toward a long-range FE (AFE) phase, which is based on a competition between the long-range polarizing field of the Nb-O dipoles and the disordering effect of local distortions around the A-site. Our study provides a predictive tool for designing complex solid-solution perovskites with tunable (anti)ferroelectric polarization properties, which can be of interest for various energy-related applications such as high-density energy storage and solid-state cooling.

GnPs/PVDF decorated thermoplastic veils to boost the triboelectric nanogenerator output performance toward highly efficient energy harvesting

Triboelectric nanogenerators (TENGs) are state-of-the-art devices that generate electricity through the coupling effect of triboelectrification and electrostatic induction by harvesting mechanical energy. This study aims to boost the electrical performance of a TENG that harvests biomechanical energy to power wearable and portable electronic devices by employing a simple and inexpensive method based on GnPs. For this purpose, I have designed and produced a vertical-contact-separation mode TENG suitable for biomechanical energy harvesting using tribonegative materials Polyvinylidene difluoride (PVDF)/ Polyetheretherketone (PEEK) veil and tribopositive materials Polyethylene oxide (PEO)/ Glass and Nylon 6.6 nanofibers. The surface of the PEEK veil is coated with a PVDF solution with excellent tribonegativity and ferroelectric polarization properties using the doctor blade technique. The electrical performance of the TENG is assessed by connecting a capacitor or various load resistors to the circuit. 3 wt% GnPs@PVDF/PEEK & Nylon 6.6 nanofibers and 3 wt% GnPs@PVDF/PEEK & PEO/Glass veil TENGs deliver output voltages of 470 V and 303 V, respectively at 75.8 MΩ load resistance. The power densities of 3 wt% GnPs@PVDF/PEEK & Nylon 6.6 nanofibers and 3 wt% GnPs@PVDF/PEEK & PEO/Glass veil TENGs at 1.1 MΩ are 7.36 W/m2 and 4.04 W/m2, respectively. Root mean square (RRMS), peak to peak height (Rpp), and arithmetic mean height (Ra) of the surface roughness of 3 wt% GnPs@PVDF/PEEK are measured to be 17.59 µm, 151.08 µm, and 13.16 µm, respectively. Parallel to increased surface roughness, the output power, and output voltage of 3 wt% GnPs@PVDF/PEEK & PEO/Glass veil TENG increase by 4 and 2.23-fold, respectively. Further GnPs embedding reduces both surface roughness and electrical performance of the TENG. I believe that GnPs@PVDF/PEEK veil material has the potential to be used to make high-performance TENGs that can power portable, wearable, and self-powered electronics.

Enhancement of room-temperature magnetization in GaFeO3-type single crystals by Al and Sc doping

GaFeO3-type oxides are promising multiferroic materials due to the coexistence of spontaneous magnetization and ferroelectric polarization properties at room temperature. As these ferroic properties feature a large anisotropy, single crystals are required. However, the magnetization of GaFeO3-type single crystals remains low at room temperature. In this study, we largely enhanced the magnetization at room temperature of GaFeO3-type single crystals by increasing the Fe content and co-doping Sc3+ and Al3+. Single crystals of AlxSc0.1−xGa0.6Fe1.3O3 (x = 0.01–0.04) were prepared using the optical floating-zone method. The single crystals were rod-shaped, with a diameter and length of ∼6 mm and 7 cm, respectively. X-ray diffraction measurements confirmed the ferroelectric polarization of the crystals. In addition, they exhibited room-temperature ferrimagnetism, with Curie temperature in the range of 326–338 K; the crystals exhibit magnetic anisotropy along the a-axis. The magnetization of the single crystal at 300 K and 0.3 kOe was 13 emu g−1, which is over ten times larger than those of previously reported single crystals with GaFeO3-type crystal structure.

Flexible room-temperature multiferroic thin film with multifield tunable coupling properties

Flexible multiferroic materials that exhibit unique properties are considered as a promising solution for electronic applications in wearable products and implantable systems. In this work, we present a high quality and large-area multiferroic (1-x)BiTi(1-y)/2FeyMg(1-y)/2O3-(x)CaTiO3(BTFM-CTO) thin films grown on mica substrate by employing the sol-gel method, while the respective ferroelectric and magnetic effects are characterized at room temperature. Compared with the flat state, the in-plane ferroelectric domains of the BTFM-CTO thin films were switched under the application of a bending radius of 9 mm. This result indicates that the applied bending strain can switch the in-plane ferroelectric domain patterns by the manifestation of the respective flexoelectricity effect, whereas the external in-plane magnetic field can also switch the ferroelectric domain patterns. Thus, it is demonstrated that the flexible BTFM-CTO thin film can achieve multifield control ferroelectric polarization properties. Our work opens new paths for the developing novel flexible multiferroic devices via mechanical bending for various applications.

Electronic structures and ferroelectric properties of Ba-doped ZnO

Wurtzite ZnO has long been considered to be a promising candidate material for photovoltaic application due to its high power conversion efficiency. More interestingly and very recently, some research results suggested that the ferroelectric property of the photovoltaic material introduced by chemical elements doping can promote its power conversion efficiency significantly. Therefore, in order to understand the effect of Ba doping on the electronic structure and the ferroelectric properties of ZnO and to reveal the potentially optoelectronic properties of Zn1-xBaxO, the energy band structure, the density of states, and the polarizability and the relative dielectric constant of the bulk Ba-doped ZnO supercell system, in which the Zn atoms are partly and uniformly substituted by the Ba atoms, are investigated by using the first-principles method based on the density functional theory and other physical theory. The norm-conserving pseudopotentials and the plane-wave basis set with a cut-off energy of 600 eV are used in the calculation. The generalized gradient approximation refined by Perdew and Zunger (GGA-PBE), the local density approximation (LDA) and the local density approximation added Hubbard energy (LDA+U) are employed for determining the exchange-correlation energy respectively. Brillouin zone is set to be within 4×4×5K point mesh generated by the Monkhorst-Pack scheme. The self-consistent convergence of total energy is at 2.0×10-6 eV/atom. Additionally, in order to obtain a stable and accurate calculation result, the cell structure is optimized prior to calculation. The calculated results suggest that the bulk Ba-doped ZnO semiconductor system is still a semiconductor with a direct wide band gap. The band gap of Zn1-xBaxO increases gradually with Ba atom doping percentage increasing from 12.5% to 87.5%. Consequently, the ferroelectric polarization properties and the dielectric properties of the bulk Ba-doped wurtzite ZnO materials are tailored by doping Ba atoms. It indicates that the polarizability of Zn1-xBaxO system increases with Ba doping atomic percentage increasing, especially, the polarizability reaches to a maximum when the atomic percentage of doping is 75%. Meanwhile, the relative dielectric constant inversely decreases with Ba atomic percentage increasing. This is attributed to the effective contribution of Ba atoms to the density of state at the bottom of the valence band. The diagonalized components of polarizability imply that there are possible micro-domains in the supercell while applying externally electric field to it. And the supercell presents a nearly isotropic polarizability macroscopically due to the strong interaction among the electric dipole moments existing in the different domains.

Temperature dependences of ferroelectricity and resistive switching behavior of epitaxial BiFeO3 thin films**Project supported by the National Natural Science Foundation of China (Grant Nos. 51272078 and 51332007), the State Key Program for Basic Research of China (Grant No 2015CB921202), the Guangdong Provincial Universities and Colleges Pearl River Scholar Funded Scheme, China (2014), the International Science & Technology Cooperation Platform Program

We investigate the resistive switching and ferroelectric polarization properties of high-quality epitaxial BiFeO3 thin films in various temperature ranges. The room temperature current–voltage (I–V) curve exhibits a well-established polarization-modulated memristor behavior. At low temperatures (< 253 K), the I–V curve shows an open circuit voltage (OCV), which possibly originates from the dielectric relaxation effects, accompanied with a current hump due to the polarization reversal displacement current. While at relative higher temperatures (> 253 K), the I–V behaviors are governed by both space-charge-limited conduction (SCLC) and Ohmic behavior. The polarization reversal is able to trigger the conduction switching from Ohmic to SCLC behavior, leading to the observed ferroelectric resistive switching. At a temperature of > 298 K, there occurs a new resistive switching hysteresis at high bias voltages, which may be related to defect-mediated effects.

Study on polarization characteristics of BiFeO3thin films prepared by sol-gel spin-coating technology

The ferroelectric polarization properties of bismuth ferrite (BFO) thin films deposited on Pt(111)/TiO2/SiO2/Si(100) substrates by sol-gel spin-coating technology affected by the processes and dopants have been studied and analyzed tentatively. The results indicate BFO thin film should be introduced to a rapid thermal annealing (RTA) process at N2 atmosphere. The enhanced ferroelectric polarization properties were observed in Mn and La doped BFO thin films, because ion substitution may improve the inherent volatility of Bi atoms, valence fluctuation of Fe ions and magnetic spin structures, reducing the formation of oxygen vacancies. The increased remnant polarization observed in the BFO thin film with a lead zirconate titanate (PZT) seeding layer has also been analyzed as the probable results of small changes of lattice parameters caused by constraint stress, reduced defects and decreased leakage current density, which are contributed by the PZT seeding layer possibly. Moreover, the remnant polarization approximately 79 μC/cm2 is observed in (Bi0.95La0.05)FeO3 (BLFO) thin film with a PZT seeding layer, which is twice and treble the values of BFO thin films with and without a PZT seeding layer, indicating PZT seeding layer is much more effective for BLFO thin film which has a substitution at A site.

Cation-vacancy-induced low coercive field in La-modified SrBi2Ta2O9

We studied the effects of La modification on the crystal structure and the ferroelectric polarization properties of SrBi2Ta2O9 (SBT). The Rietveld refinement of neutron powder diffraction demonstrated that La3+ substitution, preferentially at the Sr2+ site (perovskite A site), resulted in A-site vacancies for charge compensation. Polarization measurements revealed that La modification led to a much lower coercive field and a larger remanent polarization. The tilt angle of the TaO6 octahedra from the c axis remained constant, but La modification decreased the tilt angle along the polarization direction (the component along the a axis). The results for Ca-, Ba-, and Pb-substituted SBT, as well as La-modified SBT, indicated that the TaO6 octahedral tilt along the a axis is closely related to coercive field for the SBT system.

Orientation Behavior and Ferro- and Piezoelectric Properties of Bi4-xPrxTi3O12 Polycrystalline Films

The significance of orientation control on ferroelectric polarization and electromechanical displacement properties was studied by comparing the difference in growth behavior among Bi4-xPrxTi3O12 films (BPT, x=0.3, 0.5, and 0.7, nominal) deposited on both Pt and Ir sputter-grown bottom electrode layers on Si substrates. BPT films were directly crystallized from precursor solutions and exhibited (111) and (110) preferential orientations on Ir layers but (117) orientation on Pt layers. Because of the tilt angle difference between the major spontaneous polarization Ps vector and the above-mentioned axes, BPT films deposited on Ir layers showed a remanent polarization Pr that is 10–20% larger than those of the films deposited on Pt layers. Furthermore, Pr and Curie temperature Tc monotonically decreased with increasing x. This corresponds to the fact that substitution of Pr3+ for Bi3+ may not improve the ferroelectric polarization properties. The values of longitudinal electromechanical displacement in BPT films deposited on Ir layers were measured by atomic force microscopy (AFM). The uniform response within the film and the top electrode was a direct consequence of the uniform growth of the films. A maximum displacement was measured at x=0.3 and this qualitatively agrees with the finding that the product εrPs can attain a maximum value with a positive value of x by considering the observed behavior that both Psat and Tc monotonically decrease with increasing x.