Polycrystalline Bismuth Research Articles

Strain engineering of the electro-optic effect in polycrystalline BiFeO3 films [Invited

Electro-optic thin film materials, which change their refractive index upon the application of an electric field, are crucial for the fabrication of optical modulators in integrated photonic circuits. Therefore, it is key to develop strategies to tune the linear electro-optic effect. Strain engineering has arisen as a powerful tool to optimize the electro-optic coefficients in ferroelectric thin films. In this report, the electro-optical properties of polycrystalline bismuth ferrite (BiFeO3) thin films are studied. The electro-optic coefficients (reff) of low-cost solution-processed BiFeO3 films under different substrate-induced thermal stress are characterized using a modified Teng-Man technique in transmission geometry. The influence of poling state and substrate stress on the electro-optical properties are discussed. The films show a notable piezo-electro-optic effect: the effective electro-optic coefficient increases both under compressive and tensile in-plane stress, with compressive stress having a much more profound impact. Electro-optic coefficients of 2.2 pm/V are obtained in films under a biaxial compressive stress of 0.54 GPa.

Optimization of large magnetoresistance of polycrystalline Bi film

The oriented growth and morphology of polycrystalline bismuth (Bi) films deposited on insulating substrates is investigated based on the molecular beam epitaxy method. The film thickness effect on the resistance-temperature characteristics and magnetoresistance properties is analyzed. The Bi film resistivity with thickness range [25, 65] nm is investigated using the four-probe method at 2–300 K. The magnetic field contributes to opening the energy gap of Bi thin films and a more obvious opening of the energy gap is observed in the thinner films. The Bi film has the weak antilocalization (WAL) effect and the thickness increase affects the WAL effect. The large magnetic field is beneficial to the observation of the WAL effect. The temperature-dependent resistivity of Bi thin films is described based on the Ziman-Faber theory. The competition between the metal surface state and the bulk state explains the semimetal-semiconductor transition and the magnetoresistance effect in the thickness range [25, 65] nm. In addition, the thickness effect on the magnetoresistance property is researched, which provides a new approach to specify the Bi thin film magnetoresistance.

Stress-tuning the bulk photovoltaic response in polycrystalline bismuth ferrite films

Bulk or anomalous photovoltaic effect in ferroelectrics has recently sparked interest due to the generation of switchable photovoltages that are not limited by the bandgap of the material. The development of strategies to tune its magnitude is a key for the development of light-driven devices, e.g., photostrictive actuators, photostrictive sensors, or reconfigurable waveguides. In this paper, the bulk photovoltaic effect of polycrystalline solution-deposited bismuth ferrite thin films is studied under different stress conditions induced by different substrates and quantified using a direct strain assessment via x-ray diffraction. An increase in the short-circuit bulk photovoltaic current of 150% is observed with a change from a compressive stress of 0.54 GPa to a tensile stress of 0.93 GPa. This change is attributed to intrinsic piezophotovoltaic effect, demonstrating the potential to strain engineer the bulk photovoltaic effect in thin films.

Spatially-Resolved Study of the Electronic Transport and Resistive Switching in Polycrystalline Bismuth Ferrite

Ferroelectric materials attract much attention for applications in resistive memory devices due to the large current difference between insulating and conductive states and the ability of carefully controlling electronic transport via the polarization set-up. Bismuth ferrite films are of special interest due to the combination of high spontaneous polarization and antiferromagnetism, implying the possibility to provide multiple physical mechanisms for data storage and operations. Macroscopic conductivity measurements are often hampered to unambiguously characterize the electric transport, because of the strong influence of the diverse material microstructure. Here, we studied the electronic transport and resistive switching phenomena in polycrystalline bismuth ferrite using advanced conductive atomic force microscopy (CAFM) at different temperatures and electric fields. The new approach to the CAFM spectroscopy and corresponding data analysis are proposed, which allow deep insight into the material band structure at high lateral resolution. Contrary to many studies via macroscopic methods, postulating electromigration of the oxygen vacancies, we demonstrate resistive switching in bismuth ferrite to be caused by the pure electronic processes of trapping/releasing electrons and injection of the electrons by the scanning probe microscopy tip. The electronic transport was shown to be comprehensively described by the combination of the space charge limited current model, while a Schottky barrier at the interface is less important due to the presence of the built-in subsurface charge.

Polycrystalline bismuth nanowire networks for flexible longitudinal and transverse thermoelectrics.

This paper reports on the preparation and the characterization of structural, electrical and thermoelectric properties of nanocomposite films formed from three-dimensional networks of polycrystalline bismuth (Bi) nanowires (NWs). The samples were fabricated by electrodeposition within polycarbonate (PC) templates with crossed cylindrical nanopores, yielding self-supported networks of Bi crossed nanowires (CNWs) with mean diameter values ranging from 23 nm to 230 nm. Temperature changes in electrical resistance and thermopower were studied by considering electric and thermal currents flowing in the plane of the films. While the values of the Seebeck coefficient are close to those of polycrystalline Bi for diameters greater than 100 nm, a progressive decrease in thermopower appears at smaller diameters, due to an increasing contribution of surface charge carriers as the diameter decreases. Transverse thermoelectricity based on the Nernst effect was also demonstrated on a network of Bi CNWs 230 nm in diameter. Such hierarchical architectures based on Bi CNWs are extremely robust, offering a reliable solution for the next generation of flexible thermoelectric devices.

Influence of excess bismuth oxide on the structural and dielectric characteristics of textured Bi4Ti3O12 ceramics

Bismuth titanate (Bi4Ti3O12), an Aurivillius family of ferroelectric oxide, has been widely investigated due to its large remnant polarization, high Curie temperature, high dielectric constant and relatively low tangent for the application in information memory storage devices. In this work, polycrystalline bismuth titanate ceramics samples with different amounts of excess Bi2O3 (3 and 5 wt%) were prepared using a low temperature, cost-effective molten salt synthesis method. The structural, microstructural and dielectric properties depend on the addition of excess Bi2O3 and the sintering duration of Bi4Ti3O12 ceramics. Moreover, the X-ray diffraction studies revealed the effect of excess bismuth oxide on the formation of preferential orientation along the c-axis. It is also observed that the orientation factor along the c-axis increases with an increase in the sintering duration for different content of bismuth oxide. The morphological studies revealed the oriented plate-like structure of bismuth titanate ceramics associated with the enhanced growth of grains. It is noticed from the EDAX analysis that the addition of excess bismuth oxide compensated the volatilization of bismuth ions in the crystal lattice of Bi4Ti3O12 ceramics. The dielectric property studies have exhibited a high value of dielectric constant (εr ~ 190), and low tangent loss factor (tanδ ~ 0.08) for the optimum bismuth oxide content of 5 wt% added Bi4Ti3O12 ceramics. Hence, we suggest that these materials could be utilized for the high energy storage capacity of non-volatile memory devices.

Electrical and optical properties of thin-film bismuth ferrite

The optical and electrical properties of bismuth ferrite thin films obtained by high-frequency magnetron sputtering in an atmosphere of argon and oxygen (80%+20%) has been studied. Investigations of the optical properties have shown that for polycrystalline bismuth ferrite films the optical band gap is ~2.3 eV, which is in the range of given in the literature values. The dependences of the specific electrical conductivity on the magnitude of the electric field has been studied for the synthesized films. It has been established that the electrical conductivity does not depend on the electric field strength up to the value of E=2.1·106 V/m. The experimental results are discussed in terms of the model of charge carrier injection from aluminum electrode into the conduction band of bismuth ferrite. Keywords: electrical conductivity, strong electric fields, optical absorption coefficient, memristor effect.

Электрические и оптические свойства тонкопленочного феррита висмута

The optical and electrical properties of bismuth ferrite thin films obtained by high-frequency magnetron sputtering in an atmosphere of argon and oxygen (80% + 20%) has been studied. Investigations of the optical properties have shown that for polycrystalline bismuth ferrite films the optical band gap is ~3 eV, which is in the range of given in the literature values. The dependences of the specific electrical conductivity on the magnitude of the electric field has been studied for the synthesized films. It has been established that the electrical conductivity does not depend on the electric field strength up to the value of E = 2.1106 V/m. The experimental results are discussed in terms of the model of charge carrier injection from aluminum electrode into the conduction band of bismuth ferrite.

Observing variations in carrier-scattering mechanisms of polycrystalline bismuth through its temperature-dependent properties

This study has investigated variations in the carrier scattering mechanisms of polycrystalline bismuth based on the temperature dependence of its transport coefficients across a range of temperatures from 10 to 300 K. A measurement of the continuous temperature dependence of the Hall coefficient revealed the characteristic extremes of its temperature-based parameters to be less than 50 K. The results demonstrated a clear variation in the component of the scattering mechanism pertaining to acoustic deformation. Furthermore, the temperature dependencies of carrier density, mobility, chemical potential, and band gap energy were also comprehensively investigated to determine their influence on carrier transport. The information obtained in this study is quite relevant for the realization of the carrier transport of Bi nanowire. The carrier transport of the wire exhibits a mean free path that is equivalent to the wire diameter and has been expected to introduce a quantum effect, thus enhancing the thermoelectric performance.

Phase Transformation, Photocatalytic and Photoluminescent Properties of BiPO4 Catalysts Prepared by Solid-State Reaction: Degradation of Rhodamine B

Polycrystalline bismuth phosphate BiPO4 was synthesized by solid-state reaction at different temperatures varying from 500 to 900 °C. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDS) and Raman spectroscopy. The low-temperature phase of BiPO4 has monoclinic structure with a space group P21/n, and was transformed into the monoclinic phase P21/m with a slight distortion of monoclinic lattice when it was heated above 500 °C. The effect of the transformation on the structure, morphology and photocatalytic properties was examined. The photocatalytic activity of each sample, in presence of Rhodamine B (RhB) in aqueous solution, was carried out and analyzed under UV light irradiation. Photoexperiments showed that the material prepared at 500 °C is the best catalyst with degradation efficiency of the order of 96% after 12 min of reaction time under UV light irradiation. This high photocatalytic efficiency could be due to their structural and morphological changes. The photocatalytic degradation mechanism of RhB in the presence of the best photocatalyst BiP-500 °C is proposed. The stability of the catalyst was also examined by carrying out four successive tests of the degradation in the presence of BiP-500 °C. Total organic carbon (TOC) was used to further estimate the rate of mineralization in the presence of BiP-500 °C (83% TOC removal). Photoluminescence experiments performed under UV-laser light irradiation revealed emissions in the green-orange range, with optimal intensities for the mix systems observed at 550 °C.

Casting synthesis of Bi12SiO20

Polycrystalline bismuth silicate Bi12SiO20 has been obtained by fusing a stoichiometric mixture of Bi2O3 and SiO2 followed by crystallization of the melt by casting with no seeding.

Anneal temperature dependence of resistive switching and photoelectric properties of Bismuth ferrite thin film prepared via sol–gel method

Enhancement on resistive switching characteristics of Bismuth ferrite (BFO) has attracted persistent interest in the past decades due to the inimitable interior structure. In the work, the amorphous and the polycrystalline Bismuth ferrite thin films have been successfully prepared via sol–gel method. We utilize X-ray diffraction, Transmission Electron Microscope and absorption spectrum to characterize the internal structure and calculate the band gap of Bismuth ferrite thin film samples. By comparing the current–voltage (I-V) characteristics between the amorphous and the polycrystalline sample, we investigate the amorphous sample has better resistive switching performance (switching ratio is 104) than that of polycrystalline (101) bismuth ferrite thin film devices. However, the polycrystalline Bismuth ferrite sample exhibits favorable switchable photovoltaic response and good stability. Additionally, we discuss the conduction mechanism via the double logarithm fitting of amorphous Bismuth ferrite sample. The article also indicates that resistive switching property is closely related with the defects such as oxygen vacancies. The article deepens the comprehension of resistive switching properties of amorphous and polycrystalline Bismuth ferrite thin film which would have further potential applications in advanced electronic devices such as new resistive memory.

Terahertz Photoconductivity Spectra of Electrodeposited Thin Bi Films.

Electron dynamics in the polycrystalline bismuth films were investigated by measuring emitted terahertz (THz) radiation pulses after their photoexcitation by tunable wavelength femtosecond duration optical pulses. Bi films were grown on metallic Au, Pt, and Ag substrates by the electrodeposition method with the Triton X-100 electrolyte additive, which allowed us to obtain more uniform films with consistent grain sizes on any substrate. It was shown that THz pulses are generated due to the spatial separation of photoexcited electrons and holes diffusing from the illuminated surface at different rates. The THz photoconductivity spectra analysis has led to a conclusion that the thermalization of more mobile carriers (electrons) is dominated by the carrier–carrier scattering rather than by their interaction with the lattice.

The scattering mechanism analysis of anisotropic (00l)-orientation bismuth films based on magnetoresistance measurement method

The magnetoresistance anisotropic effect of (00l)-oriented semimetal polycrystalline bismuth (Bi) films is proposed as a candidate for Ettingshausen device and magnetoresistance device. By tuning film samples, 29 nm thickness (00l)-oriented columnar bismuth films with different grain sizes were prepared. The electrical transport measurement results showed that due to the different grain sizes, the magnetoresistance anisotropy of the Bi films with the same thicknesses differed by 3 times at low temperatures (T = 2 K). The main collision mechanism controlling the Bi film resistance was discriminated by whether the surface scattering or grain boundary scattering, based upon whether the magnetoresistance depends on the thickness or the grain sizes. The analysis result indicated that the surface scattering of the Bi film directly affected the semimetal to semiconductor transition. The present study provides a pathway for the electronic device application operating at low temperature by controlling the grain sizes.

Dielectric studies of polycrystalline bismuth titanate Bi12TiO20 ceramic synthesized via economical chemical route

Bismuth titanate nanostructure, Bi12TiO20, (BTO) belonging to sillenite family was synthesized by chemical route at the sintering temperature 800 °C for a period of 6 h. The crystalline phase of the materials was investigated by the X-ray diffraction method. The morphology and particle size of the BTO sample were determined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis, respectively. The SEM images displayed the bimodal structure, and average grain size was found to be in the range of 0.6–3.7 µm. TEM observation confirmed the nanocrystalline nature of the particle having size of 50 ± 20 nm. The value of dielectric constant and dielectric loss for Bi12TiO20 ceramic was found to be 8.29 × 103 and 3.27, respectively, at the temperature 468 K and frequency 100 Hz.

Self-powered broadband photo-detection and persistent energy generation with junction-free strained Bi2Te3 thin films.

We experimentally demonstrate efficient broadband self-powered photo-detection and power generation in thin films of polycrystalline bismuth telluride (Bi2Te3) semiconductors under inhomogeneous strain. The developed simple, junction-free, lightweight, and flexible photo-detectors are composed of a thin active layer and Ohmic contacts on a flexible plastic substrate, and can operate at room temperature and without application of an external bias voltage. We attribute the observed phenomena to the generation of an electric field due to a spontaneous polarization produced by strain gradient, which can separate both photo-generated and thermally-generated charge carriers in bulk of the semiconductor material, without a semiconductor junction. We show that the developed photo-detectors can generate electric power during both the daytime and the nighttime, by either harnessing solar and thermal radiation or by emitting thermal radiation into the cold sky. To the best of our knowledge, this is the first demonstration of the power generation in a simple junction-free device under negative illumination, which exhibits higher voltage than the previously used expensive commercial HgCdTe photo-diode. Significant improvements in the photo-detector performance are expected if the low-charge-mobility polycrystalline active layer is replaced with high-quality single-crystal material. The technology is not limited to Bi2Te3 as the active material, and offers many potential applications in night vision, wearable sensors, long-range LIDAR, and daytime/nighttime energy generation technologies.

Mathematical modeling of thermal processes during "cassette" crystallization of chalcogenides

A new, relatively simple and highly efficient modification of the directional melt crystallization method in the form of a multi-cassette process has been considered. This study is based on Russian Patents and technological studies conducted at National Research and Technological University MISiS. As a result, mathematical models of a multi-cassette method have been developed for 3D radiation and conduction analysis of thermal processes in the entire volume of the heating unit and 2D analysis of convection and conduction heat exchange in a separate cassette. Parameters have been calculated on the basis of these mathematical models for clarifying the effect of heating unit component arrangement and dimensions on the formation of thermal fields in cassette units, the effect of vertical homogeneity of heat supply to the cassette unit and heating power reduction rate during crystallization on the shape of the crystallization front, as well as the effect of small asymmetry in cassette design and violation of cassette bottom cooling homogeneity on convection and asymmetrical heat transfer. Application of the conductive and radiative heat exchange model to the entire heating unit has allowed us to calculate process parameters on the basis of which we have analyzed the effect of heating unit components, their arrangement and temperature on the heat exchange conditions at the cassette unit boundaries. Application of the convective and conductive model to one growth cassette has shown that asymmetrical design and boundary thermal conditions as well as unstable vertical temperature gradient lead to the formation of convection vortices and substantial crystallization front deviation from planar shape. Calculations on the basis of the convective mass exchange model have shown that an increase in the crystallization rate by one order of magnitude greatly increases the tellurium flow into the crystal thus substantially altering the melt composition in the vicinity of the crystallization front and hence serving as a potential origin of dendrite growth. The authenticity of the calculation results has been verified in a number of tests aimed at analyzing the effect of heat and mass transport on crystallization front shape for cassette cooling rates that are typical of polycrystalline bismuth telluride growth processes.

Poling-induced inverse time-dependent microstrain mechanisms and post-poling relaxation in bismuth ferrite

Domain-wall dynamics under strong, super-coercive electric fields in polycrystalline bismuth ferrite (BiFeO3) are not well established due to the experimental difficulties in processing high phase purity perovskite with low electrical conductivity. Overcoming these difficulties, here we present x-ray diffraction measurements carried out in situ during electrical poling with a trapezoidal electric-field to investigate the domain wall dynamics and lattice strain in this material. It is observed that during field ramping, microscopic strains, i.e., non-180° domain texture and lattice strain, increase simultaneously. During DC field dwell, however, a lattice strain decrease occurs over time, accompanied by an increase in the non-180° domain texture. This inverse time-dependent trend of microscopic strain mechanisms is speculated to be due to mobile charged defects residing in domain wall regions. The configuration of these charged point defects may also play a role in the observed post-poling relaxations of non-180° domain texture and macroscopic piezoelectric coefficients on removal of the field. Since conducting domain walls have been recently identified in a number of ferroelectrics, these results should significantly impact the understanding of strain mechanisms not only in BiFeO3 but on a broader range of ferroelectric materials.

Study of CuO and V2O5 Effect on IR Spectra of Polycrystalline Bismuth Niobate

Bismuth niobate (BiNbO4) ceramics were fabricated by mixed oxide method and sintered by presureless sintering method. BiNbO4 ceramics doped with V2O5 additive in amount 0.125 wt%, 0.250 wt% and 1 wt% of was sintered at T = 910°C whereas BiNbO4 ceramics doped with 2 wt% of CuO additive was sintered at T = 890°C and T = 910°C. It was found that V2O5 additive improved morphology of the ceramic samples. However, the chemical composition of BiNbO4 ceramics in relation to bismuth oxide and niobium oxide manifested a tendency of lack of Bi2O3 component. Absorption bands for the BiNbO4 compound were identified. FTIR band positions associated with NbO6 octahedra suggested that the crystal structure changes after V2O5 incorporation.

Structural and Electrical Properties of Lead-Free Perovskite: Bi(Sr0.25Ti0.25Fe0.5)O3

The polycrystalline bismuth strontium ferric titanate (Bi(Sr0.25Ti0.25Fe0.5)O3), synthesized via a high-temperature solid-state reaction route, has been characterized by using different experimental procedures. Study of basic crystal structure with the help of X-ray diffraction data of the compound exhibits the rhombohedral crystal system. The quality of the sample was asserted by studying its microstructure and morphology. Detailed investigations of dielectric and electrical properties of Bi(Sr0.25Ti0.25Fe0.5)O3 in a broad frequency (1 kHz–1 MHz) and temperature (25–450 °C) ranges have provided various important information on conduction mechanism, structure-properties correlationship, etc. The high dielectric constant and relativly low tangent loss with multiple phase transitions have been observed in the material I the experimental frequency and temperature ranges.