High-performance ferroelectric non-volatile memory based on La-doped BiFeO3 thin films

An ultrathin (6.2 nm) ferroelectric La_0.1Bi_0.9FeO₃ (LBFO) film was epitaxially grown on a 0.7 wt% Nb-doped SrTiO₃ (001) single-crystal substrate by carrying out pulsed laser deposition to form a Pt/La_0.1Bi_0.9FeO₃/Nb-doped SrTiO₃ heterostructure. The LBFO film exhibited strong ferroelectricity and a low coercive field. By optimizing the thickness of the LBFO film, a resistance OFF/ON ratio of the Pt/LBFO (∼6.2 nm)/NSTO heterostructure of as large as 2.8 × 10⁵ was achieved. The heterostructure displayed multi-level storage and excellent retention characteristics, and showed stable bipolar resistance switching behavior, which can be well applied to ferroelectric memristors. The resistance switching behavior was shown to be due to the modulating effect of the ferroelectric polarization reversal on the width of the depletion region and the height of the potential barrier of the LaBiFeO₃/Nb-doped SrTiO₃ interface.

An ultrathin (6.2 nm) ferroelectric La_0.1Bi_0.9FeO₃ (LBFO) film was epitaxially grown on a 0.7 wt% Nb-doped SrTiO₃ (001) single-crystal substrate by carrying out pulsed laser deposition to form a Pt/La_0.1Bi_0.9FeO₃/Nb-doped SrTiO₃ heterostructure.     

Effective doping control in Sm-doped BiFeO3 thin films via deposition temperature

Sm-doped BiFeO₃ (Bi_0.85Sm_0.15FeO₃, or BSFO) thin films were fabricated on (001) SrTiO₃(STO) substrates by pulsed laser deposition (PLD) over a range of deposition temperatures (600 °C, 640 °C and 670 °C). Detailed analysis of their microstructure via X-ray diffraction (XRD) and transmission electron microscopy (TEM) shows the deposition temperature dependence of ferroelectric (FE) and antiferroelectric (AFE) phase formation in BSFO. The Sm dopants are clearly detected by high-resolution scanning transmission electron microscopy (HR-STEM) and prove effective in controlling the ferroelectric properties of BSFO. The BSFO (T_dep = 670 °C) presents larger remnant polarization (Pr) than the other two BSFO (T_dep = 600 °C, 640 °C) and pure BiFeO₃ (BFO) thin films. This study paves a simple way for enhancing the ferroelectric properties of BSFO via deposition temperature and further promoting BFO practical applications.

Sm-doped BiFeO₃ (Bi₀.₈₅Sm_0.15FeO₃) thin films were fabricated on (001) SrTiO₃ substrates by PLD over a range of deposition temperatures. The Sm dopants are clearly detected by high-resolution scanning transmission electron microscopy.

As a promising alternative lead-free piezoelectric material, BiFeO₃ (BFO) thin films have attracted enormous research interest due to their remarkable multiferroic properties and piezoelectric response.^1–4 However, the high leakage current and large coercive field are factors limiting the extensive use of BFO.⁵ Since the polarization is mainly induced by the Bi³⁺ ions, various rare-earth elements (e.g., La, Nd, Sm, and Gd) have been doped into the Bi-site to improve the overall ferroelectric response.^6–8 According to the site-engineering concept, the doping of foreign elements causes chemical pressure and controls the volatility of Bi atoms in BFO systems.^9,10 The structural information of rare-earth-doped BFO (Re-BFO) systems is therefore upgraded and a preliminary phase diagram is proposed.^11–14Sm-doped BFO (Bi_1−xSm_xFeO₃, BSFO) thin film has attracted research attention, due to the narrow concentration range at room temperature.¹⁵ It shows ferroelectric (FE) rhombohedral to antiferroelectric (AFE) orthorhombic phase transitions as the Sm doping amount increases. Morphotropic phase boundary (MPB) appears at around x = 0.13–0.15, and high values of out-of-plane piezoelectric coefficient (d₃₃ ∼ 110 pm V⁻¹) and enhanced dielectric constant at x = 0.14 are reported in such systems.^16,17 Structure studies show that three main phase, FE R3c phase, AFE PbZrO₃-like phase, and paraelectric Pnma phase, coexist at the MPB composition.¹⁸ In this regard, many efforts have been made on the investigation of phase transitions under external stimuli.^19,20 However, there are not much work on the structure analysis of BSFO at the atomic-scale level. It will be interesting to investigate the microstructure with high-resolution transmission electron microscopy (TEM) and high-resolution scanning transmission electron microscopy (HR-STEM). The detailed microstructure information will reveal how the Sm dopants distribute in the overall BSFO lattice. In addition, BSFO films in previous studies are mostly prepared by solid phase synthesis and sol–gel method^15,20,21 with very few reports using pulsed laser deposition (PLD).^17,22 In those prior reports, Sm-doping amounts in films were controlled by changing the composition of the deposition targets. Deposition temperature has been proven as an effective parameter for PLD in controlling the doping amount, thin film microstructure and the related properties. In the Ag-doped ZnO (SZO) system, deposition temperature was directly used to control the density of stacking faults and consequently affect the electrical transport properties.²³ Table S1^† lists several reported deposition temperatures for Re-BFO thin films, which is in the range of 520 °C to 850 °C. Here, we used three different substrate temperatures, 600 °C, 640 °C and 670 °C, for the BSFO film fabrication via PLD. Pure BFO film was also grown as a reference sample for comparison. Besides the detailed microstructure, the corresponding ferroelectric property measurements were conducted on the BSFO thin films to investigate how the Sm dopants affect the ferroelectric behavior of BSFO.In the present work, Bi_0.85Sm_0.15FeO₃ (BSFO) was selected as the model system. The BSFO target was synthesized by a conventional solid-state sintering method using high-purity Bi₂O₃ (99.99%), Fe₂O₃ (99.95%) and Sm₂O₃ (99.90%) powders. Thin films were grown on (001) single-crystal SrTiO₃ (STO) substrates epitaxially via PLD. KrF excimer laser with a wavelength of 248 nm was used as the laser source. Three different substrate temperatures, 600 °C, 640 °C and 670 °C, were applied in the deposition. For all depositions, oxygen partial pressure was kept at 200 mTorr and the deposition rate was 5 Hz. The films were cooled down to room temperature at a cooling rate of 10 °C min⁻¹ in 200 torr oxygen atmospheres. The growth condition and parameters of BFO film is same with our previous report.²⁴ Au top contacts with 100 nm thickness and 0.1 mm² were deposited by a custom-built magnetron sputtering system. The Au sputter target (99.99% pure) is made by Williams Advanced Materials.X-ray diffraction (XRD) spectra were collected by a PANalytical Empyrean system using Cu K_α radiation. The Raman spectra were measured by Renishaw''s inVia Raman microscope. The microstructure analysis was performed on FEI TALOS F200X TEM/STEM operated at 200 kV. The energy-dispersive X-ray spectroscopy (EDS) chemical mapping was acquired by the SuperX EDS system with four silicon drift detectors. Ferroelectric characterization was conducted by Precision LC II Ferroelectric Tester (Radiant Technologies, Inc.). Fig. 1(a) shows the θ–2θ XRD spectra of the as-prepared thin film samples deposited at 600 °C, 640 °C and 670 °C. All the films display BSFO (00l) diffraction peaks, indicating highly textured BSFO along c-axis. It is noted that BSFO (003) peak shifts from 71.62°, to 70.82° and to 70.70° with the deposition temperature increasing from 600 °C to 640 °C, and 670 °C. The corresponding out-of-plane lattice parameters are calculated to be 3.951 Å, 3.987 Å and 3.993 Å, respectively and are summarized in Fig. 1(b). Compared with the out-of-plane lattice parameter of pure BFO film (∼4.000 Å) on STO substrate,²⁴ three BSFO samples show smaller lattice parameters than that of pure BFO. This is because partial Bi³⁺ (radius = 1.030 Å) ions have been substituted by Sm³⁺ ions with smaller radius (radius = 0.958 Å).²⁵ The BSFO peak at around 32.155° (denoted as “*”) in Fig. 1(a) comes from rhombohedral (110) peak and the peak disappears when the deposition temperature increases to 670 °C. The above results indicate that the deposition temperature influences the Sm-doping amount and the BSFO crystal structure. Raman analysis has been conducted on all BSFO and pure BFO films. The Raman spectra were fitted with Lorentzian curves, as shown in Fig. S1.^† The reported data of bulk polycrystalline Bi_1−xSm_xFeO₃ was taken as reference.²⁶ The overall shape of peaks is the same, which implies that the main structure of BSFO sample is the same as the rhombohedral R3c structure of BFO.Open in a separate windowFig. 1(a) θ–2θ XRD spectra of BSFO film deposited at 600 °C, 640 °C and 670 °C. (b) The summary of out-of-plane lattice parameter. (c–e) Reciprocal space map (RSM) results of BSFO (103) peaks.To further analyze the detailed phase information, asymmetric reciprocal space mapping (RSM) measurements were performed around the (103) diffraction peak for all the three BSFO samples and the results are shown in Fig. 1(c)–(e). The RSM pattern of sample (T_dep = 600 °C) exhibits four domains, which are shown by red triangles. It indicates the existence of rhombohedral-like phase which has antipolar nature.^27–29 For the other two samples (T_dep = 670 °C and T_dep = 640 °C), peak split along Q_x direction is not apparent and the narrower width along Q_x direction is observed, indicating less structure distortion in BSFO films deposited at higher temperature. These results provide direct evidence that the deposition temperature significantly affects the domain structure of the BSFO film.In order to analyze the microstructure structure, TEM analysis has been applied on two samples (T_dep = 670 °C and T_dep = 600 °C). Fig. 2(a) and (c) show the overall films stacks of BSFO on STO substrates. The corresponding selected area electron diffraction (SAED) patterns of BSFO thin films only demonstrate the rhombohedral-like phases. It is interesting to note that the TEM image of high deposition temperature sample (T_dep = 670 °C) exhibits few dark lines. And the dark line density in the high deposition temperature sample is obviously higher than that in the lower deposition temperature one. The image contrast is proportional to ∼Z² (Z, atomic number) in TEM bright-field mode. The dark line is therefore proposed be related with Sm, owing to Z_Sm is larger than Z_Bi and Z_Fe. It also suggests that the higher deposition temperature introduce more Sm dopants in BSFO films than the lower one. The Fig. 2(b) and (d) are high resolution TEM (HR-TEM) images from local areas, which were analyzed by Fast Fourier Transform (FFT). The spots (marked by red arrows) in the FFT image of the sample (T_dep = 670 °C) correspond to the incommensurate phase, which is caused by the competition between ferroelectric (FE) and antiferroelectric (AFE) phases. Different phase emerges in the sample (T_dep = 600 °C), and the corresponding spots are marked by red circle. It has been proved as the antipolar orthorhombic AFE phase, linked to the macroscopic AFE behavior. The phase information of Bi_0.85Sm_0.15FeO₃ thin film in this study is different from previous reported BSFO with only AFE phases.¹⁶ The above microstructure analysis shows the effect of deposition temperature on the phase formation in BSFO system even with 15 atomic percent Sm.Open in a separate windowFig. 2(a and c) Cross-sectional TEM images of BSFO thin films (670 °C and 600 °C) on STO substrates. (b and d) High resolution TEM images. The insets in (a and c) show the corresponding (SAED) pattern of BSFO thin films. The insets in (b and d) show the fast-Fourier transformed (FFT) images from the blue squared region.STEM analysis was then performed to resolve the composition information. Fig. 3(a) shows the high-resolution STEM (HR-STEM) image of the 670 °C sample. The high angle annular dark field mode (HADDF) image intensity is proportional to the atomic number Z. Thus, the white line areas in STEM image correspond to the dark lines in TEM images. We further examined the composition distribution across this Sm layer using the intensity line profile (Fig. 3(b)), which provides a direct interpretation of composition information in the HAADF imaging mode. It is obvious that the position near white line area has higher intensity than other areas. This result proves that Sm³⁺ ion has been doped into BFO system effectively. Energy dispersive spectroscopy (EDS) measurements have been further applied on the same area. The EDS mapping of Bi and Sm elements are shown in Fig. S2(b) and (c).^† The EDS line-scan analysis of Sm is overlaid on the HADDF image and displayed in Fig. S2(a).^† The line profile reflects high content of Sm corresponding to the white line area. Therefore, both the TEM and STEM results show that the higher deposition temperature BSFO sample (T_dep = 670 °C) has higher Sm-doping amount.Open in a separate windowFig. 3(a) HR-STEM image of BSFO film deposited at 670 °C. (b) Enlarged view of the yellow dashed square area from (a). The intensity line profile is inserted along the marked blue line.The ferroelectric behaviors were characterized by the polarization–electric field (P–E) hysteresis loops. Fig. 4(a)–(c) show the P–E loops of BSFO samples with three different deposition temperatures, while Fig. 4(d) shows the loop of pure BFO as a comparison. The polarization measurement as a function of electric field measurement was carried out at room temperature for several times to ensure the reproducibility of the measurements. The BSFO sample (T_dep = 670 °C) with higher Sm-doping amount exhibits obvious enhanced polarization. The remnant polarization (Pr) for the film is determined to be 17 μC cm⁻², much larger than other two BSFO samples and the BFO sample. It was proposed that the incorporation of Sm could break the short-range dipolar regions, surmount the local barrier and transform it to the long-range polar structure.³⁰ The BFO film with four-variant domains exhibits a lower electric filed and remnant polarization Pr.³¹ It was also found that the formation of bridging phase could enhance piezoelectric and dielectric properties of BSFO.¹⁶ In this work, the BSFO sample (T_dep = 600 °C) shows four structural domain structure and the lowest Pr. With the increase amount of Sm-doping amount, incommensurate phase appears in the 670 °C sample. We conclude that the higher deposition temperature introduces the higher Sm-doping amount, which further assists the incommensurate phase formation and suppresses the AFE phase. In addition, the unsaturated P–E loop of BFO film indicates it suffers from high leakage current, which is generally due to the appearance of Bi deficiencies and oxygen vacancies.³² The Sm–O bond enthalpy (565 ± 13 kJ mol⁻¹) is stronger than the Bi–O bond enthalpy (337 ± 12.6 kJ mol⁻¹).³³ Therefore, the higher polarization exhibited by BSFO samples (T_dep = 670 °C) than the pure BFO proves that the Sm dopant could compensate for the Bi loss and suppress the formation of oxygen vacancies. The P–E loop results were compared with prior reports on Re-BFO films. As shown in Table S1,^† the remnant polarization is quite different for Re-BFO systems. It is due to the different ionic radii of the rare earth elements, which result in different structural distortion of BFO and diverse critical doping ratio for phase transitions. Besides the phase variants, the polarization is closely related with the orientation of crystalline structures.Open in a separate windowFig. 4(a–c) Polarization hysteresis measurements for BSFO film deposited at 670 °C, 640 °C and 600 °C. (d) Polarization hysteresis measurements for BFO film.This study demonstrates that Sm-doping amount in BSFO thin films can be effectively tuned via deposition temperature. The Sm dopants influence phase formation of BSFO and further control the macroscopic ferroelectric properties. The local incommensurate phase presented by Bi_0.85Sm_0.15FeO₃ with higher Sm-doping amount than the reported ones (Bi_0.86Sm_0.14FeO₃), which is extremely helpful in constructing phase diagram of BSFO. More interestingly, the existence and location of Sm dopants in BSFO thin film have been directly demonstrated by the HR-STEM and corresponding EDS analysis. This work is also beneficial for the exploration of other Re-BFO films with deposition temperatures and detailed structure analysis, which is an important step toward the practical applications of Re-BFO in electronic devices.     

Preparation and photoelectrochemical properties of BiFeO3/BiOI composites

BiFeO₃ thin films were spin coated onto FTO. BiFeO₃/BiOI composites have been successfully synthesized by an electrochemical deposition method. The morphology, structure and optical absorption properties of the as-synthesized samples were characterized via XRD, SEM, and UV-Vis DRS. The effect of the BiOI electrodeposition cycles on the photoelectrochemical properties of the BiFeO₃/BiOI composites were investigated. The results showed that the photoelectrochemical properties were enhanced under simulated solar light. The composite could achieve an optimum photocurrent density of 16.03 μA cm⁻² at 0 V (vs. Ag/AgCl), which is more than twice that of pure BiFeO₃ thin films (6.3 μA cm⁻²). In addition, the Mott–Schottky curves indicate an improvement in the carrier density of the composite. The enhanced photoelectrochemical properties of the composites can be attributed to the formation of a heterojunction at the interface and the band bending of the ferroelectric material BiFeO₃.

We report the preparation, optical absorption and enhanced photoelectrochemical properties of novel BiOI-decorated BFO thin films heterostructures.     

Static and dynamic electro-optical properties of liquid crystals mediated by ferroelectric polymer films

This paper reports the electro-optical properties of high resistivity nematic liquid crystals sandwiched between ferroelectric polymer films. Interactions between liquid crystals and the film result in a series of interesting optical and electro-optical features. For example, the visualization of ferroelectric domains by means of liquid crystals has been known for decades. However, here we demonstrate that liquid crystals can also reveal the fractal dimension of multi—domain poly(vinylidene fluoride)-based films. Unidirectionally rubbed films made of poly(vinylidene fluoride)-based (PVDF) materials align liquid crystals (LC) homogeneously, with the pretilt angle on the order of 1–2 degrees. This property was implemented in the design of hybrid cells composed of liquid crystals sandwiched between PVDF-based films. The designed PVDF|LC|PVDF cells exhibit tunable electro-optical performance originating from the presence of the PVDF-based films. More specifically, (i) the threshold voltage characterizing the transition of liquid crystals from a planar to a homeotropic state can be tuned by varying the film thickness, and (ii) total fall time (turn-off time) can be controlled by varying the frequency and amplitude of the driving voltage. This frequency dependence of the fall time is strongly pronounced at a relatively high voltage applied across the cell. In the low frequency regime, an increase in the turn-off time can be approximated as a linear function of the applied electric field. An electric-field induced polarization of the PVDF-based films is considered a major reason leading to the afore-mentioned amplitude and frequency dependence of the switching time.

Liquid crystals (LC) can reveal the fractal dimension of multi-domain ferroelectric films (FF) while these films can control the switching time of FF–LC hybrids.     

Aqueous synthesis of tin- and indium-doped WO3 films via evaporation-driven deposition and their electrochromic properties

M-doped WO₃ (M = Sn or In) films were prepared from aqueous coating solutions via evaporation-driven deposition during low-speed dip coating. Sn- and In-doping were easily achieved by controlling the chemical composition of simple coating solutions containing only metal salts and water. The crystallinity of the WO₃, Sn-doped WO₃, and In-doped WO₃ films varied with heating temperature, where amorphous and crystalline films were obtained by heating at 200 and 500 °C, respectively. All the amorphous and crystalline films showed an electrochromic response, but good photoelectrochemical stability was observed only for the crystalline samples heated at 500 °C. The crystalline In–WO₃ films exhibited a faster electrochromic color change than the WO₃ or Sn–WO₃ films, and good cycle stability for the electrochromic response in the visible wavelength region.

WO₃ and M-doped WO₃ (M = Sn or In) electrochromic films were obtained from aqueous solutions via evaporation-driven deposition. The In–WO₃ films showed a faster electrochromic response than WO₃ and Sn–WO₃ films, and a good cycle stability.     

Tuning the catalytic properties of La–Mn perovskite catalyst via variation of A- and B-sites: effect of Ce and Cu substitution on selective catalytic reduction of NO with NH3

Perovskites with flexible structures and excellent redox properties have attracted considerable attention in industry, and their denitration activities can be further improved with metal substitution. In order to investigate the effect of Ce and Cu substitution on the physicochemical properties of perovskite in NH₃-SCR system, a series of La_1−xCe_xMn_1−yCu_yO₃ (x = 0, 0.1, y = 0, 0.05, 0.1, 0.2, 0.4) catalysts were prepared by citrate sol-gel method and employed for NO removal in the simulated flue gas, and the physical and chemical properties of the catalysts were studied using XRD, SEM, BET, XPS, DRIFT characterizations. The Ce substitution on A-site cation of LaMnO₃ can improve the denitration activity of the perovskite catalyst, and La_0.9Ce_0.1MnO₃ displays NO conversion of 86.7% at 350 °C. The characterization results indicate that the high denitration activity of La_0.9Ce_0.1MnO₃ is mainly attributed to the larger surface area, which contributes to the adsorption of NH₃ and NO. Besides, the appropriate Cu substitution on B-site cation of La_0.9Ce_0.1MnO₃ can further improve the denitration activity of perovskite catalyst, and La_0.9Ce_0.1Mn_0.8Cu_0.2O₃ displays the NO conversion of 91.8% at 350 °C. Although the specific surface area of La_0.9Ce_0.1Mn_0.8Cu_0.2O₃ is lower than La_0.9Ce_0.1MnO₃, the Cu active sites and the Ce³⁺ contents are more developed, making many reaction units formed on the catalyst surface and redox properties of catalyst improved. In addition, strong metal interaction (Ce⁴⁺ + Mn²⁺ + Cu²⁺ ↔ Ce³⁺ + Mn³⁺/Mn⁴⁺ + Cu⁺) and high concentrations of chemical adsorbed oxygen and lattice oxygen both strengthen the redox reaction on catalyst surface, thus contributing to the better denitration activity of La_0.9Ce_0.1Mn_0.8Cu_0.2O₃. Therefore, appropriate cerium and copper substitution will markedly improve the denitration activity of La–Mn perovskite catalyst. We also reasonably conclude a multiple reaction mechanism during NH₃-SCR denitration process basing on DRIFT results, which includes the Eley–Rideal mechanism and Langmuir–Hinshelwood mechanism.

Perovskites with flexible structures and excellent redox properties have attracted considerable attention in industry, and their denitration activities can be further improved with metal substitution.     

Effect of Mn2+ substitution on the structure,properties and HER activity of cadmium phosphochlorides

Cadmium phosphochlorides, Cd₄P₂Cl₃ and Cd₇P₄Cl₆, possess cadmium atoms differently bonded to chlorine and phosphorus ligands. A combined experimental and theoretical study has been carried out to examine the effect of manganese substitution in place of cadmium in these compounds. Experimentally it is found that manganese prefers the Cd₇P₄Cl₆ phase over Cd₄P₂Cl₃. First-principles calculations reveal, stabilization of Cd₇P₄Cl₆ upon Mn-substitution with a significant reduction in the formation energy when Mn²⁺ is substituted at Cd-sites coordinated octahedrally by Cl-ligands. Substitution of Mn²⁺ at two different Cd-sites in these compounds not only alters their formation energy differently but also causes a notable change in the electronic structures. In contrast to n-type conductivity in pristine Cd₇P₄Cl₆, Mn²⁺ substituted Cd_7−yMn_yP₄Cl₆ analogues exhibit p-type conductivity with a remarkable enhancement in the photochemical HER activity and stability of the system. Photochemical properties of pristine and substituted compounds are explained by studying the nature of charge carriers and their dynamics.

Mn²⁺ prefers the Cd-sites having larger number of tightly bounded Cl-ligands. Pure Cd₇P₄Cl₆ exhibits n-type conductivity whereas Cd_5.8Mn_1.2P₄Cl₆ exhibits p-type conductivity. The HER activity of Cd_7−yMn_yP₄Cl₆ is superior to that of pristine Cd₇P₄Cl₆.     

Strain coupling of ferroelastic domains and misfit dislocations in [101]-oriented ferroelectric PbTiO3 films

High-index perovskite ferroelectric thin films possess excellent dielectric permittivity, piezoelectric coefficient, and exotic ferroelectric switching properties. They also exhibit complications in the ferroelastic domains, misfit dislocations, and strain-relaxation behaviors. Exploring the relationship of the ferroelastic domains and misfit dislocations may be of benefit for promoting the high-quality growth of these thin films. Here, the strain field of the ferroelastic domains and misfit dislocations in [101]-oriented PbTiO₃/(La, Sr)(Al, Ta)O₃ epitaxial thin films were investigated by advanced aberration-corrected (scanning) transmission electron microscopy (TEM) combined with geometry phase analysis (GPA). Two types of misfit dislocations with projected Burgers vectors of a[001] or a[100] on the (010) plane were elucidated, whose strain fields included in-plane strain and lattice rotation coupled with the c domains above them. Besides, it was demonstrated that the coupling was kept inside the PbTiO₃ films when the film thickness was increased. Furthermore, the polarization rotation was observed in both narrow c domains and around the misfit dislocation cores, which may be attributed to the flexoelectric effect. These results are expected to provide useful information for understanding the nucleation and propagation mechanism of ferroelastic domains and for further modifying the growth of high-index ferroelectric thin films.

The strain coupling of misfit dislocations and ferroelastic domains is revealed in [101]-oriented PbTiO₃/(La, Sr)(Al, Ta)O₃ films and flexoelectric-induced polarization rotation is observed around the misfit dislocation cores.     

Exploring the optical limiting,photocatalytic and antibacterial properties of the BiFeO3–NaNbO3 nanocomposite system

Thin films of BiFeO₃–NaNbO₃ composites were fabricated in a PMMA matrix. XRD and HRTEM were used for structural investigations. The grain size and surface morphology of samples were analysed through HRTEM images. The self-cleaning property of any material accelerates its industrial applications. Hence, along with the optical limiting performance, the photocatalytic and antibacterial activity of BiFeO₃–NaNbO₃ composite samples were also studied. BiFeO₃–NaNbO₃ films fabricated in the PMMA matrix exhibit strong optical nonlinearity when excited by 5 ns laser pulses at 532 nm. The origin and magnitude of the observed optical nonlinearity were explained on the basis of the weak absorption saturation and strong excited state absorption. The photocatalytic performance of samples was analysed by dye degradation method using Methyl Orange dye. The dye degradation rate in the presence of the catalyst is heeded in a particular time interval, which exhibits the photocatalytic performance of the samples. The destruction of microbial organisms that are in contact with the material was contemplated, which could prove its antibacterial activity. The effect of the particle size on the photocatalytic activity was also investigated.

Thin films of BiFeO₃–NaNbO₃ composites were fabricated in a PMMA matrix.     

Insights into the relationship between ferroelectric and photovoltaic properties in CsGeI3 for solar energy conversion

Materials such as oxide and halide perovskites that simultaneously exhibit spontaneous polarization and absorption of visible light are called photoferroelectrics. They hold great promise for the development of applications in optoelectronics, information storage, and energy conversion. Devices based on ferroelectric photovoltaic materials yield an open-circuit voltage that is much higher than the band gap of the corresponding active material owing to a strong internal electric field. Their efficiency has been proposed to exceed the Shockley–Queisser limit for ideal solar cells. In this paper, we present theoretical calculations of the photovoltaic properties of the ferroelectric phase of the inorganic germanium halide perovskite (CsGeI₃). Firstly, the electronic, optical and ferroelectric properties were calculated using the FP-LAPW method based on density functional theory, and the modern theory of polarization based on the Berry phase approach, respectively. The photovoltaic performance was evaluated using the Spectroscopic Limited Maximum Efficiency (SLME) model based on the results of first-principles calculations, in which the power conversion efficiency and the photocurrent density–voltage (J–V) characteristics were estimated. The calculated results show that the valence band maximum (VBM) of CsGeI₃ is mainly contributed by the I-5p and Ge-4s orbitals, whereas the conduction band is predominantly derived from Ge-4p orbitals. It can be seen that CsGeI₃ exhibits a direct bandgap semiconductor at the symmetric point of Z with a value of 1.53 eV, which is in good agreement with previous experimental results. The ferroelectric properties were therefore investigated. With a switching energy barrier of 19.83 meV per atom, CsGeI₃ has a higher theoretical ferroelectric polarization strength of 15.82 μC cm⁻². The SLME calculation also shows that CsGeI₃ has a high photoelectric conversion efficiency of over 28%. In addition to confirming their established favorable band gap and strong absorption, we demonstrate that CsGeI₃ exhibits a large shift current bulk photovoltaic effect of up to 40 μA V⁻² in the visible region. Thus, this material is a potential ferroelectric photovoltaic absorbed layer with high efficiency.

In addition to its favorable band gap and strong absorption, CsGeI₃ exhibits a large shift current bulk photovoltaic effect of up to 40 μA V⁻² in the visible region.     

Simultaneous tuning of optical and electrical properties in a multifunctional LiNbO3 matrix upon doping with Eu3+ ions

Combined photoluminescence (PL) and dielectric studies have been carried out on both undoped and Eu³⁺ doped LiNbO₃ compounds for their potential application in optical–electrical integration for the first time. Special focus has been given to simultaneously tuning both these physical properties. A PL study reveals that the blank compound is a blue emitting material, while upon doping with Eu³⁺ ions, the emitting color can be tuned from blue to red upon changing the excitation wavelength. Interestingly, the electrical property measurement of this ferroelectric compound showed that upon doping with Eu³⁺ ions, the remnant polarization was increased significantly. Density Functional Theory (DFT) based calculations were carried out to explain both the optical and electrical properties. It has been found that different defect centers are responsible for the bluish host emission while Eu³⁺ ions are energetically preferred to occupy the Nb site and gives rise to red emission. The DFT based results also showed that Eu³⁺ ions induced more distortion into the nearby Nb-site, which is responsible for enhancement of the remnant polarization. Stark-splitting patterns in the PL study also showed that the point symmetry of LiNbO₃ upon Eu³⁺ doping changes from C_6v to D₃, which indicates that the structure becomes less symmetric. Overall, the study presents a novel approach to designing multifunctional materials for optical–electrical integration application and to tuning their physical properties simultaneously in the desired range.

PL and dielectric studies have been carried out on LiNbO₃ and Eu³⁺:LiNbO₃ compounds with a special focus on simultaneous tuning of optical and electrical properties for their potential application in optical–electrical integration.     

Effects of Ga substitution on electronic and thermoelectric properties of gapless semiconductor V3Al

Thermoelectric properties of the antiferromagnetic (AF) gapless semiconductor (GS) V₃Al were optimized by substituting Al with the isoelectric element Ga in the D0₃ structure. Structural and mechanical stability, electronic structure and transport properties of V₃Al_1−xGa_x (x = 0.25, 0.5, 0.75 and 1) compounds have been studied based on first-principles calculations with the combination of the semi-classical Boltzmann theory and deformation potential theory. All the compounds are structurally and mechanically stable gapless semiconductors. The Ga substitution for Al leads to an appreciably decreased thermal conductivity and an undesirable decrease in power factor, but contributes more to the decreased thermal conductivity. Consequently, the figure of merit (zT) is effectively improved in V₃Al_0.75Ga_0.25 and V₃Ga compounds with respect to V₃Al.

Reduced thermal conductivity and enhanced thermoelectric efficiency was demonstrated by Ga substitution for Al at room temperature in p-type V₃Al_1−xGa_x.     

Superlattice-like structure and enhanced ferroelectric properties of intergrowth Aurivillius oxides

Aurivillius oxides with an intergrowth structures have been receiving increasing interest because of their special structures and potential outstanding ferroelectric properties. In this work, Bi₃LaTiNbFeO₁₂–Bi₅Ti₃FeO₁₅ and Bi₃TiNbO₉–Bi₃LaTiNbFeO₁₂ compounds were successfully synthetised using a simple solid-state reaction method. X-Ray diffraction patterns and scanning transmission electron microscopy high angle annular dark field (STEM-HAADF) images confirm the 2–3 and the 3–4 intergrowth structures in Bi₃TiNbO₉–Bi₃LaTiNbFeO₁₂ and Bi₃LaTiNbFeO₁₂–Bi₅Ti₃FeO₁₅ compounds, respectively. A superlattice-like distortion in these oxides was proposed resulting from the combination of sub-lattices with different a and b parameters, which was validated by XRD refinements and Raman spectra. Polarization-electric field tests and pulsed polarization positive-up negative-down measurements demonstrate that such superlattice-like structures can effectively enhance the intrinsic ferroelectric polarization and coercive field of these oxides, especially when compared with their component oxides Bi₃TiNbO₉, Bi₃LaTiNbFeO₁₂ and Bi₅Ti₃FeO₁₅. Simultaneously, ferroelectric Curie temperatures of Bi₃TiNbO₉–Bi₃LaTiNbFeO₁₂ and Bi₃LaTiNbFeO₁₂–Bi₅Ti₃FeO₁₅ oxides are lowered because of the internal stress in the superlattice-like structure. Nevertheless, the paramagnetism of the samples is hardly influenced by their structure, while mainly related to their iron content, in which iron has a similar effective magnetic moment around 3.4–3.9.

Aurivillius oxides with an intergrowth structures have been receiving increasing interest because of their special structures and potential outstanding ferroelectric properties.     

Studies of La- and Pr-driven reverse distortion of FeO6 octahedral structure,magnetic properties and hyperfine interaction of BiFeO3 powder

The Bi_1−x−yLa_xPr_yFeO₃ (x = 0 and 0.05; y = 0, 0.10, 0.15 and 0.20) (BLPFO) powders were prepared using a hydrothermal method. The lattice structure of the samples was characterized by X-ray diffraction, which revealed an increase in the lattice constant of the doped samples evidencing the substitution of Bi by La and Pr ions. Raman spectroscopy was used to further analyse the structural distortion in the samples. Scanning electron microscopy was used to characterize the morphology of the samples. The atomic concentrations (%) of La and Pr elements in the samples were detected by Energy Dispersive X-ray spectroscopy. The ferromagnetism of the samples increased with the increase in La and Pr co-doping concentration as observed by vibrating sample magnetometry at room temperature. The evidence of reverse distortion of FeO₆ octahedral structure in the La and Pr co-doped samples was revealed by the Mössbauer spectra parameters: Is, Qs, H, Γ, χ² and area ratio (A₁/A₂) of two sextets.

Mössbauer spectra of the Bi_1−x−yLa_xPr_yFeO₃ (x = 0 and 0.05; y = 0, 0.10, 0.15 and 0.20) (BLPFO) powders.     

Structural transition and magnetic properties of Mn doped Bi0.88Sm0.12FeO3 ceramics

We investigated the effects of Mn doping on the crystal structure, phonon vibration, and magnetic properties of Bi_0.88Sm_0.12FeO₃ ceramics. Mn doping effectively modified the rhombohedral symmetry and induced a structural transition from an R3c rhombohedral to Pnam orthorhombic structure. Magnetic measurements revealed a weak ferromagnetic behavior, which was related to the canted antiferromagnetic order of the Pnam structure. The cycloidal spin structure of the R3c phase could not be suppressed by substitution of Mn at the Fe site. Studies on the self-phase transition and electric field-induced structural transition revealed many changes in coercivity and remanent magnetization, which are believed to originate from the R3c/Pnam phase switching along with spin frustration. Observations of the field step-dependent hysteresis loop and the ferromagnetic-like hysteresis loop after poling in an electric field provided direct evidence of phase boundary (PB) ferromagnetism and magnetic coupling at the PB.

We investigated the effects of Mn doping on the crystal structure, phonon vibration, and magnetic properties of Bi_0.88Sm_0.12FeO₃ ceramics.     

Improved energy storage performance of PbZrO3 antiferroelectric thin films crystallized by microwave radiation

Energy storage dielectric capacitors based on a physical charge-displacement mechanism have attracted much attention due to their high power density and fast charge–discharge characteristics. How to improve the energy storage capacity of dielectric materials has become an important emerging research topic. Here, antiferroelectric PbZrO₃ films were prepared by chemical solution deposition on Pt/Ti/SiO₂/Si substrates and crystallized by microwave radiation. The effects of microwave radiation on the antiferroelectric properties and energy storage performance were investigated. In contrast to ordinary heating, microwave radiation can crystallize the amorphous PbZrO₃ films into the perovskite phase at 750 °C in only 180 seconds. The PbZrO₃ films have a highly (100)-preferred orientation and dense microstructure, which is beneficial to enhance the stability of antiferroelectric phase and the electric breakdown strength. The PbZrO₃ films show a recoverable energy storage density of 14.8 J cm⁻³ at 740 kV cm⁻¹, which is approximately 40% higher than that of the PbZrO₃ films crystallized by ordinary heating. The results reveal that microwave radiation is an effective method to improve energy storage performance of antiferroelectric films.

We prepared amorphous PZO films by chemical solution deposition and then crystallized the films by microwave radiation. Using microwave radiation in the crystallization of AFE thin films is an effective method to improve their energy storage performance.     

Improved thermal and mechanical properties of bismaleimide nanocomposites via incorporation of a new allylated siloxane graphene oxide

A thermosetting resin system based on bismaleimide (BMI) has been developed via copolymerization with 4,4′-diaminodiphenylsulfone in the presence of a newly synthesized graphene oxide, modified using allylated siloxane (AS-GO). The curing behavior of the AS-GO-containing resin system was evaluated using curing kinetics. The dispersibility of AS-GO in the resin was observed through polarizing optical microscopy (POM), which indicates that AS-GO has good dispersibility in the resin due to GO modified with allylated siloxane which has a good phase compatibility with BMI. The effect of AS-GO on the thermomechanical and mechanical properties of the cured modified resin was also studied. Results of thermogravimetric analysis indicated that the cured sample systems display a high char yield at lower concentrations of AS-GO (≤0.5 wt%) with an improved thermal stability. Using dynamic mechanical analysis, a marked increase in glass transition temperature (T_g) with increasing AS-GO content was observed. Mechanical property analyses revealed a possible effect of AS-GO as a toughener, and the results showed that an addition of 0.3% AS-GO maximized the toughness of the modified resin systems, which was confirmed by analysis of fracture surfaces.

A thermosetting resin system based on bismaleimide has been developed via copolymerization of a new allylated siloxane graphene oxide.     

Control of graphene aerogel self-assembly in strongly acidic solution via solution polarity tuning

In view of their advantages (plasticity, low density, adjustable pore size, high porosity of >99.9%), three-dimensional graphene aerogels (GAs) are widely used for energy storage and adsorption separation, which has inspired the development and optimization of the corresponding synthetic techniques. In particular, self-assembly in the liquid phase features the benefits of tunability and sustainability and is viewed as a promising strategy of GA synthesis. During hydrothermal GA preparation, hydrophilic graphene oxide (GO) gradually turns lipophilic upon reduction, and the resulting phase transition separation and polarity change induce self-assembly into an aerogel. However, the effect of solution polarity on the structure or state of dispersed GO nanosheets, which affects the final property-determining process of automatic assembly, is still unclear. Herein, we prepared a series of GAs by hydrothermal reduction of unwashed GO with vitamin C in liquid-phase systems of different polarity and investigated the effects of polarity on the self-assembly process and aerogel properties using a range of instrumental techniques. The results showed that GO reduction is slowed down in weakly polar systems and further demonstrated that the shape of partially reduced graphene oxide (rGO) flakes depends on solution polarity. Flaky, layered, and stacked rGO particles obtained in strongly polar media self-assembled into anisotropic gully aerogels that were brittle and almost completely inelastic. Conversely, in weakly polar media, the prepared rGO sheets were twisted, which increased the number of contact points and modes between sheets and resulted in self-assembly into uniform-pore-structure honeycomb aerogels that showed good elasticity and could be repeatedly compressed.

Possible self-assembly mechanism and deformability of graphene aerogels prepared in water (a) and ethanol solution (b).     

Effect of ZnO on (ferroelectric) fatigue retention and thermal stability of ferroelectric properties in lead free (K0.5Na0.5)(Nb0.7Ta0.3)O3 ceramics

This work investigates and reports the effect of ZnO addition on the ferroelectric properties of (K_0.5Na_0.5)(Nb_0.7Ta_0.3)O₃ (KNNT) ceramics prepared by a solid state reaction method. Though literature is abundant on the study of the effect of ZnO on the sinterability, microstructure and electrical properties of KNN based materials, the effect of ZnO on their ferroelectric properties has seldom been studied in detail, especially in KNNT. In the current study, 2, 4 and 6 wt% of ZnO was added to KNNT ceramics. The XRD results revealed ZnO addition has no effect on the crystal symmetry of KNNT. However, a ZnO secondary phase was found in KNNT ceramics with 4 and 6 wt% ZnO doping. An increase in grain size was observed with increases in the concentration of ZnO, indicating a direct dependence of grain size on the concentration of ZnO in the KNNT matrix. From ferroelectric studies it was observed that a lower electric field was sufficient to get maximum polarization for ZnO doped KNNT samples compared to that of pure KNNT ceramics. A high remnant polarization (P_r = 14.0 μC cm⁻²) and lower coercive field (E_c = 5.6 kV cm⁻¹) was obtained for 2 wt% ZnO doped KNNT. These samples showed the least fatigue (0.8%) after 10⁹ cycles in comparison to pure (5%), 4 wt% ZnO doped (24.9%) and 6 wt% ZnO doped (30%) KNNT ceramics. The diminution in P_s, P_r, and E_c was only 26.0%, 26.2% and 18.5%, respectively, with an increase in measurement temperature, which indicates improved thermal stability in 2 wt% ZnO doped KNNT. From the present study the optimum concentration of ZnO in KNNT is identify to be 2.0 wt% and their improved properties in comparison to the pure KNNT ceramics are discussed in detail.

This work investigates and reports the effect of ZnO addition on the ferroelectric properties of (K_0.5Na_0.5)(Nb_0.7Ta_0.3)O₃ (KNNT) ceramics prepared by a solid state reaction method.