Persistent photoconductivity in strained epitaxial BiFeO3 thin films

Brief Theoretical Overview of Bi-Fe-O Based Thin Films

This paper will provide a brief overview of the unique multiferroic material Bismuth ferrite (BFO). Considering that Bismuth ferrite is a unique material which possesses both ferroelectric and magnetic properties at room temperature, the uniqueness of Bismuth ferrite material will be discussed. Fundamental properties of the material including electrical and ferromagnetic properties also will be mentioned in this paper. Electrical properties include characterization of basic parameters considering the electrical resistivity and leakage current. Ferromagnetic properties involve the description of magnetic hysteresis characterization. Bismuth ferrite can be fabricated in a different form. The common forms will be mentioned and include powder, thin films and nanostructures. The most popular method of producing thin films based on BFO materials will be described and compared. Finally, the perspectives and potential applications of the material will be highlighted.

A light-heat synergism in the sub-bandgap photocatalytic response of pristine TiO2: a study of in situ diffusion reflectance and conductance

Pristine TiO₂ materials are mainly used as photocatalysts under super-bandgap light illumination. The sub-bandgap (SBG) photocatalytic response has seldom been investigated and the mechanism of action remains unclear. In the current research, we firstly study the SBG light electronic transition of pristine P25 TiO₂ by means of in situ diffusion reflectance and (photo)conductance measurements under finely controllable conditions. It is revealed that the SBG light can promote valence band (VB) electrons to the exponentially-distributed gap states of the TiO₂, which can then be thermally activated to the CB states. A hole in the VB and an electron in the CB can be generated by the synergism of a SBG photon and heat. It is also seen that the photoinduced electrons can transfer to O₂ through the CB states, and that the holes can be captured by isopropanol molecules. As a result, isopropanol dehydrogenation can occur over pristine TiO₂ under SBG light illumination. It is seen that the photocatalytic activity increases with temperature and the energy of the SBG photons, in agreement with the light-heat synergistic electric transition via the exponential gap states. The present research reveals a mechanism for the SBG light photocatalytic response of pristine TiO₂ materials, which is important in designing highly-active visible light active photocatalysts.

Oxide and Organic–Inorganic Halide Perovskites with Plasmonics for Optoelectronic and Energy Applications: A Contributive Review

The ascension of halide perovskites as outstanding materials for a wide variety of optoelectronic applications has been reported in recent years. They have shown significant potential for the next generation of photovoltaics in particular, with a power conversion efficiency of 25.6% already achieved. On the other hand, oxide perovskites have a longer history and are considered as key elements in many technological applications; they have been examined in depth and applied in various fields, owing to their exceptional variability in terms of compositions and structures, leading to a large set of unique physical and chemical properties. As of today, a sound correlation between these two important material families is still missing, and this contributive review aims to fill this gap. We report a detailed analysis of the main functions and properties of oxide and organic–inorganic halide perovskite, emphasizing existing relationships amongst the specific performance and the structures.

Deep-Level Traps Responsible for Persistent Photocurrent in Pulsed-Laser-Deposited β-Ga2O3 Thin Films

Gallium oxide (β-Ga2O3) is emerging as a promising wide-bandgap semiconductor for optoelectronic and high-power electronic devices. In this study, deep-level defects were investigated in pulsed-laser-deposited epitaxial films of β-Ga2O3. A deep ultraviolet photodetector (DUV) fabricated on β-Ga2O3 film showed a slow decay time of 1.58 s after switching off 250 nm wavelength illumination. Generally, β-Ga2O3 possesses various intentional and unintentional trap levels. Herein, these traps were investigated using the fractional emptying thermally stimulated current (TSC) method in the temperature range of 85 to 473 K. Broad peaks in the net TSC curve were observed and further resolved to identify the characteristic peak temperature of individual traps using the fractional emptying method. Several deep-level traps having activation energies in the range of 0.16 to 1.03 eV were identified. Among them, the trap with activation energy of 1.03 eV was found to be the most dominant trap level and it was possibly responsible for the persistent photocurrent in PLD-grown β-Ga2O3 thin films. The findings of this current work could pave the way for fabrication of high-performance DUV photodetectors.

Strongly enhanced and tunable photovoltaic effect in ferroelectric-paraelectric superlattices

Ever since the first observation of a photovoltaic effect in ferroelectric BaTiO₃, studies have been devoted to analyze this effect, but only a few attempted to engineer an enhancement. In conjunction, the steep progress in thin-film fabrication has opened up a plethora of previously unexplored avenues to tune and enhance material properties via growth in the form of superlattices. In this work, we present a strategy wherein sandwiching a ferroelectric BaTiO₃ in between paraelectric SrTiO₃ and CaTiO₃ in a superlattice form results in a strong and tunable enhancement in photocurrent. Comparison with BaTiO₃ of similar thickness shows the photocurrent in the superlattice is 10³ times higher, despite a nearly two-thirds reduction in the volume of BaTiO₃ The enhancement can be tuned by the periodicity of the superlattice, and persists under 1.5 AM irradiation. Systematic investigations highlight the critical role of large dielectric permittivity and lowered bandgap.

Approaching Charge Separation Efficiency to Unity without Charge Recombination

Improving the efficiency of charge separation (CS) and charge transport (CT) is essential for almost all optoelectronic applications, yet its maximization remains a big challenge. Here we propose a conceptual strategy to achieve CS efficiency close to unity and simultaneously avoid charge recombination (CR) during CT in a ferroelectric polar-discontinuity (PD) superlattice structure, as demonstrated in (BaTiO_{3})_{m}/(BiFeO_{3})_{n}, which is fundamentally different from the existing mechanisms. The competition of interfacial dipole and ferroelectric PD induces opposite band bending in BiFeO_{3} and BaTiO_{3} sublattices. Consequently, the photoexcited electrons (e) and holes (h) in individual sublattices move forward to the opposite interfaces forming electrically isolated e and h channels, leading to a CS efficiency close to unity. Importantly, the spatial isolation of conduction channels in (BaTiO_{3})_{m}/(BiFeO_{3})_{n} enable suppression of CR during CT, thus realizing a unique band diagram for spatially orthogonal CS and CT. Remarkably, (BaTiO_{3})_{m}/(BiFeO_{3})_{n} can maintain a high photocurrent and large band gap simultaneously. Our results provide a fascinating illumination for designing artificial heterostructures toward ideal CS and CT in optoelectronic applications.

Template-free fabrication of BiFeO3 nanorod arrays: multiferroic and photo-electrochemical performances

BiFeO₃ (BFO) has been widely investigated in many forms and morphologies because of its combined multiferroic and photovoltaic properties. However, direct growth of vertically aligned BFO nanorods on an underlying substrate has remained a challenge. In this work, we report template free growth of BiFeO₃ nanorod arrays on fluorine doped tin oxide coated glass substrate. This has been achieved by a two-step process, in which FeOOH nanorods are grown by chemical bath deposition and converted into BFO using bismuth (Bi) coating by physical vapour deposition (PVD). Both DC sputtering and thermal evaporation are attempted under PVD and the results suggest that Bi deposited by DC sputtering leads to well-defined BFO nanorods, which show superior performance in both multiferroic and photoelectrochemical studies. Piezoelectric force microscopy data shows the signature butterfly loop that confirms piezoelectric behaviour with a d ₃₃ value of 8 pmV^-1 in the BFO nanorods grown by DC sputtering. Further, the M-H hysteresis curve for the same samples reveals a remanent magnetization (M_r) value of 0.54 emu cc^-1 and antiferromagnetic nature at room temperature. Finally, a stable photocurrent density of 0.05 mA cm^-2 is achieved at 0.8 V vs Ag/AgCl under 1 Sun illumination. This work opens up new avenues for BFO in applications involving 1D nanostructures.

Progress in BiFeO3-based heterostructures: materials, properties and applications

BiFeO3-based heterostructures have attracted much attention for potential applications due to their room-temperature multiferroic properties, proper band gaps and ultrahigh ferroelectric polarization of BiFeO3, such as data storage, optical utilization in visible light regions and synapse-like function. Here, this work aims to offer a systematic review on the progress of BiFeO3-based heterostructures. In the first part, the optical, electric, magnetic, and valley properties and their interactions in BiFeO3-based heterostructures are briefly reviewed. In the second part, the morphologies of BiFeO3 and medium materials in the heterostructures are discussed. Particularly, in the third part, the physical properties and underlying mechanism in BiFeO3-based heterostructures are discussed thoroughly, such as the photovoltaic effect, electric field control of magnetism, resistance switching, and two-dimensional electron gas and valley characteristics. The fourth part illustrates the applications of BiFeO3-based heterostructures based on the materials and physical properties discussed in the second and third parts. This review also includes a future prospect, which can provide guidance for exploring novel physical properties and designing multifunctional devices.

Strain-gradient mediated local conduction in strained bismuth ferrite films

It has been recently shown that the strain gradient is able to separate the light-excited electron-hole pairs in semiconductors, but how it affects the photoelectric properties of the photo-active materials remains an open question. Here, we demonstrate the critical role of the strain gradient in mediating local photoelectric properties in the strained BiFeO₃ thin films by systematically characterizing the local conduction with nanometre lateral resolution in both dark and illuminated conditions. Due to the giant strain gradient manifested at the morphotropic phase boundaries, the associated flexo-photovoltaic effect induces on one side an enhanced photoconduction in the R-phase, and on the other side a negative photoconductivity in the morphotropic \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T\prime$$\end{document}T′-phase. This work offers insight and implication of the strain gradient on the electronic properties in both optoelectronic and photovoltaic devices.

In semiconductors strain gradients can separate light induced electron-hole pairs via the flexo-photovoltaic effect. Here the authors show that this effect can also account for the enhancement of the photoconduction in certain phase regions at the morphotropic phase boundary in Bismuth Ferrite films.

Band Gap Tuning of Solution-Processed Ferroelectric Perovskite BiFe1-x Co x O3 Thin Films

Ferroelectric perovskite oxides are emerging as a promising photoactive layer for photovoltaic applications because of their very high stability and their alternative ferroelectricity-related mechanism for solar energy conversion that could lead to extraordinarily high efficiencies. One of the biggest challenges so far is to reduce their band gap toward the visible region while simultaneously retaining ferroelectricity. To address these two issues, herein an elemental composition engineering of BiFeO₃ is performed by substituting Fe by Co cations, as a means to tune the characteristics of the transition metal-oxygen bond. We demonstrate by solution processing the formation of epitaxial, pure phase, and stable BiFe_1-x Co _x O₃ thin films for x ≤ 0.3 and film thickness up to 100 nm. Importantly, the band gap can be tuned from 2.7 to 2.3 eV upon cobalt substitution while simultaneously enhancing ferroelectricity. As a proof of concept, nonoptimized vertical devices have been fabricated and, reassuringly, the electrical photoresponse in the visible region of the Co-substituted phase is improved with respect to the unsubstituted oxide.

Controlling the ferroelectric and resistive switching properties of a BiFeO3 thin film prepared using sub-5 nm dimension nanoparticles

In recent years, BiFeO₃ has attracted significant attention as an interesting multiferroic material in the exploration of fundamental science and development of novel applications. Our previous study (Phys. Chem. Chem. Phys.18, 2016, 25409) highlighted the interesting physicochemical features of BiFeO₃ of sub-5 nm dimension. The study also accentuated the existence of weak ferroelectricity at sub-5 nm dimensions in BiFeO₃. Based on this feature, we have prepared thin films using sub-5 nm BiFeO₃ nanoparticles and explored various physicochemical properties of the thin film. We report that during the formation of the thin film, the nanoparticles aggregated; particularly, annihilation of their nanotwinning nature was observed. Qualitatively, the Gibbs free energy change ΔG governed the abovementioned processes. The thin film exhibited an R3c phase and enhanced Bi-O-Fe coordination as compared to the sub-5 nm nanoparticles. Raman spectroscopy under the influence of a magnetic field shows a magnetoelectric effect, spin phonon coupling, and magnetic anisotropy. We report room-temperature ferroelectric behavior in the thin film, which enhances with the application of a magnetic field; this confirms the multiferroic nature of the thin film. The thin film shows polarization switching ability at multiple voltages and read-write operation at low bias (±0.5 V). Furthermore, the thin film shows negative differential-complementary resistive switching behavior in the nano-microampere current range. We report nearly stable 1-bit operation for 10² cycles, 10⁵ voltage pulses, and 10⁵ s, demonstrating the paradigm device applications. The observed results thus show that the thin films prepared using sub-5 nm BiFeO₃ nanoparticles are a promising candidate for future spintronics and memory applications. The reported approach can also be pertinent to explore the physicochemical properties and develop potential applications of several other nanoparticles.

2D Hybrid Nanomaterials for Selective Detection of NO2 and SO2 Using "Light On and Off" Strategy

In order to distinguish NO₂ and SO₂ gas with one sensor, we designed a paper chip assembled with a 2D g-C₃N₄/rGO stacking hybrid fabricated via a layer-by-layer self-assembly approach. The g-C₃N₄/rGO hybrid exhibited a remarkable photoelectric property due to the construction of a van der Waals heterostructure. For the first time, we have been able to selectively detect NO₂ and SO₂ gas using a "light on and off" strategy. Under the "light off" condition, the g-C₃N₄/rGO sensor exhibited a p-type semiconducting behavior with a low detection limit of 100 ppb of NO₂, but with no response toward SO₂. In contrast, the sensor showed n-type semiconducting behavior which could detect SO₂ at concentration as low as 2 ppm under UV light irradiation. The effective electron transfer among the 2D structure of g-C₃N₄ and rGO nanosheets as well as highly porous structures could play an important role in gas sensing. The different sensing mechanisms at "light on and off" circumstances were also investigated in detail.

Controllable Photovoltaic Effect of Microarray Derived from Epitaxial Tetragonal BiFeO3 Films

Recently, the ferroelectric photovoltaic (FePV) effect has attracted great interest due to its potential in developing optoelectronic devices such as solar cell and electric-optical sensors. It is important for actual applications to realize a controllable photovoltaic process in ferroelectric-based materials. In this work, we prepared well-ordered microarrays based on epitaxially tetragonal BiFeO₃ (T-BFO) films by the pulsed laser deposition technique. The polarization-dependent photocurrent image was directly observed by a conductive atomic force microscope under ultraviolet illumination. By choosing a suitable buffer electrode layer and controlling the ferroelectric polarization in the T-BFO layer, we realized the manipulation of the photovoltaic process. Moreover, based on the analysis of the band structure, we revealed the mechanism of manipulating the photovoltaic process and attributed it to the competition between two key factors, i.e., the internal electric field caused by energy band alignments at interfaces and the depolarization field induced by the ferroelectric polarization in T-BFO. This work is very meaningful for deeply understanding the photovoltaic process of BiFeO₃-based devices at the microscale and provides us a feasible avenue for developing data storage or logic switching microdevices based on the FePV effect.

Photocarrier transport and dynamics in mixed-phase BiFeO3 films

We report a remarkable photoinduced relaxation process and its dependence of thickness and temperature in mixed-phase BiFeO₃ films grown on (001) LaAlO₃ substrates. When the films are illuminated by the light above the bandgap, their resistances are reduced with the increase of temperature. The photoinduced change of resistance reaches to the maximum of about 2.17 × 10^5% at 300 K. It is noted that the relaxation processes of the resistance are significantly different between T-like phase and T-R mixed phase due to structural strain, symmetry breaking and built-in electric field at the phase boundaries. These results provide more insights into intrinsic mechanisms of mixed-phase multiferroic materials and potential applications in all-oxide photoelectric devices.