Grain Boundaries In Films Research Articles

Defect Nanostructure and its Impact on Magnetism of α-Cr2 O3 Thin Films.

Thin films of the magnetoelectric insulator α-Cr2 O3 are technologically relevant for energy-efficient magnetic memory devices controlled by electric fields. In contrast to single crystals, the quality of thin Cr2 O3 films is usually compromised by the presence of point defects and their agglomerations at grain boundaries, putting into question their application potential. Here, the impact of the defect nanostructure, including sparse small-volume defects and their complexes is studied on the magnetic properties of Cr2 O3 thin films. By tuning the deposition temperature, the type, size, and relative concentration of defects is tailored, which is analyzed using the positron annihilation spectroscopy complemented with electron microscopy studies. The structural characterization is correlated with magnetotransport measurements and nitrogen-vacancy microscopy of antiferromagnetic domain patterns. Defects pin antiferromagnetic domain walls and stabilize complex multidomain states with a domain size in the sub-micrometer range. Despite their influence on the domain configuration, neither small open-volume defects nor grain boundaries in Cr2 O3 thin films affect the Néel temperature in a broad range of deposition parameters. The results pave the way toward the realization of spin-orbitronic devices where magnetic domain patterns can be tailored based on defect nanostructures without affecting their operation temperature.

Enhanced Performance of Carbon‐Based, Fully Printed Mesoscopic Perovskite Solar Cells through Defects Passivation

AbstractCarbon‐based, fully printed mesoscopic perovskite solar cells (PMPSCs) possess outstanding environmental stability, which can be fabricated by screen‐printing techniques with low costs in large area. The performance of PMPSCs is severely hindered by strong carrier recombination from ionic defects at the grain boundaries in perovskite films. In this work, ethylene carbonate (EC) with electron‐rich CO functional groups are successfully introduced in to (5‐AVA)xMA1−xPbI3 PMPSCs as an additive to effectively suppress carrier recombination by passivating the defects at grain boundaries. The density functional theory calculations and film characterization reveal that EC effectively reduces charge trapping density by passivation of defects. With the introduction of EC, the power conversion efficiency (PCE) of PMPSCs with EC is increased up to 15.28% from 12.99% of PMPSCs without EC. In addition, PMPSCs with EC show excellent stability and better moisture resistance under air conditions. The PMPSCs with EC keep above 90% of the original PCE after 55 days stored in air with a humidity of 60 ± 10%. The work offers a strategy to passivate defects in high‐performance PMPSCs with excellent stability.

Interfacial Modification Engineering with Cs3Cu2I5 Nanocrystals for Efficient and Stable Perovskite Solar Cells

Ion defects at surface and grain boundaries (GBs) of perovskite films are paramount factors that influence the power conversion efficiency (PCE) of organic–inorganic hybrid perovskite solar cells (PSCs). Various methods have been proposed to solve the problems, but it still remains a great challenge because of the sophisticated and multiplicity of these defects. Herein, a modification method is developed that all‐inorganic low‐dimensional cesium copper halide nanocrystals (Cs3Cu2I5 NCs) are introduced to reduce the ionic defects of perovskite films at the surface and GBs. The champion device modified by Cs3Cu2I5 NCs exhibits remarkable promotion of PCE from 19.88% to 22.03% and fill factor from 74.09% to 82.21%. The results show that the solid‐state interdiffusion process occurs between the perovskite films and Cs3Cu2I5 NCs, which can improve the crystallinity, reduce interfacial recombination loss, and enhance the interfacial carrier transport in PSCs. Especially, the element distribution of Cs3Cu2I5 NCs in perovskite films affects the effectiveness of ionic defects passivation in the perovskite lattice. The PSCs device retains 91.6% of its initial PCE after 504 h under high moisture conditions. This work demonstrates an interfacial engineering strategy using the characteristic perovskite NCs, which provides a passivation method to achieve the high‐performance PSCs.

Passivation of iodine vacancies of perovskite films by reducing iodine to triiodide anions for high-performance photovoltaics

Inverted perovskite solar cells (PSCs) exhibit great potential in large-scale fabrication due to the low-temperature manufacturing process and low cost compared to normal PSCs. However, defects at the surface and grain boundaries (GBs) of perovskite films, such as iodine vacancies, lead to low efficiency and poor stability. Herein, we report a strategy to passivate the defects in situ with tetrabutylammonium chloride (TABCl). Both the surface defects and GB defects are passivated after perovskite film growth. Moreover, TABCl modifies iodine vacancies by reducing I2 to iodide ions, leading to a decrease in charge recombination in the films and enhanced device performance. The power conversion efficiency (PCE) of devices increases from 18.52% to 20.36 % by TBACl modification, and TBACl also reduces the Voc loss in the PSCs. Meanwhile, TBACl enhances the UV light stability of devices tested by continuous UV light irradiation due to the decreased defects by TBACl. The PSCs could maintain over 90% PCE under continuous UV light irradiation for 450 min. This work not only presents a method to effectively passivate the defects in situ including the surface defects and GB defects but also demonstrates a novel strategy to modify the iodine vacancies by the reducibility of TBACl. In addition, the reducibility of TBACl suppresses the degradation of perovskite films, leading to improved stability.

Passivating defects via 4-cyanobenzenaminium iodide enables 22.44% efficiency perovskite solar cells

Defects can easily form at the surfaces and grain boundaries of solution-processed organic-inorganic halide perovskites films, leading to undesired non-radiative recombination and ion migration in the device. Consequently, this limits the performance of perovskite solar cells (PSCs). Herein, a new organic ammonium halide salt with cyano (-CN) functional group, 4-cyanobenzenaminium iodide (CNBAI), was developed to passivate the defects in the bulk of perovskite films. It was found that the CNBAI can improve the film morphology of the perovskite layer and passivate the various types of defects in the perovskite. On the one hand, the -CN group could bind with the under-coordinated Pb2+ and form intermolecular halogen bonds with the iodide atom in perovskite, as illustrated by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy measurements. On the other hand, CNBAI could fill the iodide vacancy with its own iodide ions. For the first time, we demonstrate experimentally that the -CN group can simultaneously interact with Pb2+ and form CN…I halogen bond in the perovskite. In addition, the introduced CNBAI improved energy alignment between the perovskite and adjacent charge transport layers. Finally, the PSCs fabricated with CNBAI-based perovskite films achieved the best PCE of 22.4% with negligible hysteresis and displayed good long-term stability.

Synergistic effect of surface active agent in defect passivation by for ambient air-synthesized halide perovskite solar cells

The unavoidable defect states located on the surface or grain boundaries (GBs) of the perovskite film can seriously deteriorate the performance and stability of perovskite solar cells (PSCs). In this work, an efficient and low-cost surface-active agent, sodium laurylsulfonate (SLS) has been incorporated into perovskite precursors in order to achieve effective defect passivation of humid air-fabricated PSCs. The zwitterionic SLS agent can not only improve crystallinity of perovskite film but also play the role of defect passivation originated from both negative- and positive-charged defect states. Density Functional Theory (DFT) calculation proves that the sulfonic acid group S=O and Na+ cations in SLS passivator can bond with the uncoordinated lead and halogen ions, leading to the suppression of carrier non-radiative recombination. As a result, the performance of champion device with SLS additives improves from 16.68 to 19.93%. Moreover, SLS passivation agent can also enhance the long-term stability of unencapsulated devices under ambient condition, which maintains 91% of its initial efficiency after 21 days. This study probes into the zwitterionic surface active agent with π-conjugated system for long term defect passivation of perovskite optoelectronic devices.

Influence of Preparation Conditions on Long-Term Stability of Magnetoresistive Properties of Nanostructured La–Sr–Mn–Co–O Films Grown by PI MOCVD

The results of preparation and post-treatment conditions on the long-term stability of Co-substituted nanostructured manganite La <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.81</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.19</sub> Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.09</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.06</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> films are presented. The films were grown on polycrystalline Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> substrates by pulsed-injection metal-organic chemical vapor deposition technique at 600 °C keeping different growth rates (9, 13, and 27 nm/min), thus inducing different levels of disorder in the films. For comparison, the reference epitaxial films were deposited on the LaAlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> substrate. The changes of resistivity and magnetoresistance (MR; up to 20 T) during the long-term storage of the films and accelerated aging by annealing the films in O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> or Ar atmosphere at ~100 °C were studied. It was found that the annealing atmosphere and long-term storage have a significant influence on the film resistivity and metal–insulator transition temperature, while MR magnitude is affected insignificantly. The aging effects have the highest influence on the nanostructured film grown at the highest growth rate, while for epitaxial films, the changes are negligible. The observed changes were analyzed using Mott’s variable-range hopping model and explained by oxygen depletion and diffusion in grain boundaries of nanostructured films. The ways to stabilize the main parameters of the manganite–cobaltite films for the development of high magnetic field sensors are discussed.

Defect Chemistry of Individual Grains with and without Grain Boundaries of Al-Doped Ceria Determined Using Well-Defined Microelectrodes

The deconvolution of grain boundary and grain interior contributions to the total impedance of a polycrystalline material and their influence on its conductivity can be challenging, depending on the specific material. In this study, we present a microelectrode approach which enables the direct characterization of the electrochemical transport processes of individual grains with and without a grain boundary of microcrystalline oxide thin films by using electrochemical impedance spectroscopy. A combination of photolithography and electron-beam lithography was used to prepare well-defined microelectrodes onto a ceria thin film sintered at 1500 °C for 8 h to contact individual grains with and without a grain boundary. Impedance data was collected in the temperature range between 613 and 773 K and in an oxygen partial pressure range between 0.2 and 10–5 bar. The conductivity of single grains increases with increasing oxygen partial pressure accompanied by a decrease of the activation energy from 1.32 to 0.93 eV. In contrast, the conductivity of grains with a grain boundary remains constant in the oxygen partial pressure range investigated. These differences are analyzed quantitatively in terms of the defect chemistry of the grain interior and the near grain boundary region, which differs due to the segregation of impurities at the grain boundaries. The results demonstrate that locally resolved measurements of the impedance of microcrystalline samples are a powerful tool to determine the transport characteristics of individual grains and grain boundaries as well as the equilibrium constants of the underlying defect chemical reactions.

Self-aligned CH3NH3PbBr3 perovskite nanowires via dielectrophoresis for gas sensing applications

Metal halide perovskites (MHPs) with environment-dependent optoelectronic properties can provide a new platform for highly sensitive and active gas sensor materials. While MHPs are a crystalline semiconductor with high carrier mobility and long carrier diffusion length, the formation of efficient channels for charge carriers can be hindered by the intrinsic point defects and impurities as well as grain boundaries in the solution-cast films with an uncontrolled morphology. Furthermore, a precise control of nanostructured MHP materials and their distribution with respect to device electrodes still remains challenging, which requires tedious efforts for additional fabrication. Herein, we demonstrate that the dielectrophoresis process can allow for self-alignment of single-crystalline MHP nanowires (NWs) with a uniform spatial distribution and orientation on interdigitated electrodes. We found that MAPbBr3 NWs array sensors exhibited the highest sensitivity (∼120% and τ = 40 s at 100 ppm) for H2S gas molecules among oxidizing (NO2) and reducing gases (NH2 and H2S), whereas no significant response was observed for MAPbCl3 NWs. The MAPbBr3 NW array-based sensor showed a long time stability exceeding one month with less than 20% derivation during first two weeks. The characteristic response of MHP NWs array significantly depends on the interaction of oxygen molecules adsorbed at their surface with the environmental target species, in which their surface conductivity can be modulated by the variation of trap states related to surface defects. Our work demonstrates a simple and facile route to synthesis and self-alignment of MHP NWs for chemiresistive gas sensors, broadening the range of technological applications of MHPs.

Trifluoromethyl‐Group Bearing, Hydrophobic Bulky Cations as Defect Passivators for Highly Efficient, Stable Perovskite Solar Cells

Solution‐processed perovskite films are rich in surface defects and grain boundaries, which limits their performance and stability in photovoltaic application. Surface passivation using bulky organic cations can effectively reduce the surface defects of a perovskite film without affecting its fundamental properties. Herein, the use of hydrophobic bulky aromatic molecules, namely 4‐trifluoromethyl‐benzylammonium iodide/bromide (CF3BZA‐I/Br), as defect‐passivators to heal the surface defects and grain boundaries of perovskite films is introduced. Owing to the presence of the trifluoromethyl (CF3) moieties, CF3BZA‐I/Br‐passivated perovskite films exhibit a hydrophobic surface with significantly fewer grain boundaries. By suppressing the surface and interfacial imperfections, CF3BZA‐Br‐treated perovskite solar cells achieve an outstanding power conversion efficiency (PCE) of 20.75%. The PCE improvement originates mainly from the reduction of trap states and nonradiative carrier recombination. The ultrathin hydrophobic barrier layer formed after passivation also shields the perovskite film surface from moisture ingress and environmental degradation, leading to improved stability of the devices. By optimizing the passivation conditions, the bulky CF3BZA‐I/Br molecules could be the ideal defect passivators, with versatile applications in a wide variety of perovskite optoelectronics.

Dispersed SnO2 colloids using sodium dodecyl benzene sulfonate for high-performance planar perovskite solar cells

Recently, SnO2-based perovskite solar cells (PSCs) have become more promising than traditional-material-based PSCs because of their simple low-temperature solution approach and low cost; meanwhile, the high-temperature process of mesoporous TiO2-based PSCs still offers a higher power conversion efficiency (PCE). In planar-based PSCs, nonradiative recombination of electron-hole pairs at the SnO2/perovskite interface leads to a loss of potential and a reduced PCE. Here, the SnO2-SDBS electron transport layer (ETL) is employed in PSCs to depress the defects of the SnO2 layer and the grain boundaries of the perovskite film. The surfactant SDBS was used as a wetting agent for SnO2 nanoparticles, to improve dispersibility. In addition, the benzene sulfonic acid group of SDBS attached to the SnO2-SDBS film can control the process of perovskite crystallization, producing a vertically aligned growth and large-grain perovskite layer. Finally, the open-circuit voltage was improved from 1.038 to 1.077 V and the PCE increased from 18.01% to 19.25% in the devices using the SnO2-SDBS ETL. The incorporation of SnO2-SDBS as an ETL paves the way to further performance enhancements of SnO2-based planar PSCs.

Non-equilibrium methods for synthesis and modification of gallium oxide

Synthesis and modification of gallium oxide as a wide-bandgap semiconductor is a topical task in the fields of power electronics, UV detectors, gas sensors, telecommunication. In the present work, the Ga2O3 films deposited on sapphire substrates by magnetron sputtering have been studied. The influence of deposition parameters and subsequent annealing on the structure and optical properties of the synthesized films is analyzed. Ion doping of magnetron-deposited films with silicon is carried out by the ion implantation method. It is shown by the Raman scattering and optical transmission spectroscopy that ion irradiation leads to the disordering of the crystal structure, but subsequent annealing results in a partial recovery of the structure. Hall-effect measurements for irradiated and then annealed films do not reveal the formation of a conducting layer. Apparently, this is due to the fact that the main contribution to the resistance is made by grain boundaries in the magnetron-deposited films.

In-Situ polymerization of PEDOT in perovskite Thin films for efficient and stable photovoltaics

Solution-processed polycrystalline perovskite films have detrimental defects in the bulk and surfaces/grain boundaries, especially the uncoordinated lead ions (Pb2+) with low formation energy, limiting both the performance and stability of the photovoltaic devices. In this work, a conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) is incorporated into a bulk perovskite film by a facile in-situ polymerization method, where 5-bromo-2,3-dihydro-thieno[3,4-b][1,4]dioxine (BEDOT) as monomer is directly added to a perovskite precursor solution and then in-situ polymerized within the perovskite film during the film formation. The PEDOT is chemically anchored at the surface and grain boundaries of the perovskite film via interaction of sulfur and oxygen atoms in PEDOT with uncoordinated lead ions in the perovskite, passivating the surface and bulk defects and also improving the charge extraction efficiency in the solar cell. Moreover, robust and hydrophobic properties of PEDOT can enhance the thermal and moisture resistance of the perovskite film. As a result, a champion power conversion efficiency (PCE) of 22.58% is achieved, along with significantly enhanced environmental, thermal, and light-soaking stability.

Effects of Sm implantation on the structure and magnetic properties of polar ZnO films

The present study reports a large ferromagnetism obtained by implanting Sm ions in Zn-polar and O-polar ZnO films prepared by molecular beam epitaxy (MBE) on sapphire substrates. X-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and superconducting quantum interference device (SQUID) measurements were carried out to characterize the structure, surface morphology, elemental chemical states and magnetic properties of the Sm-doped polar ZnO films. XRD and AFM show that the doped ions in Zn-polar films are located at the grain boundary, while the doping ions in the O-polar films are incorporated into the grains. The magnetic moment for the as-implanted O-polar film is higher than that of the Zn-polar film. After annealing at 600 ℃ for 30 mins., there is a clear reduction in the saturation magnetization for the O-polar film, while the magnetic moment of the Zn-polar film shows little change. Raman and XPS results show that a large number of grain boundaries in the O-polar films are repaired and O vacancy (Vo) defects are reduced. We concluded that the magnetization of polar ZnO film after annealing is attributed to the combined effects of the Vo defects introduced by implantation and the local moments associated with the Sm ions.

Short-Range Migration of A-Site Cations Inhibit Photoinduced Phase Segregation in FAxMAyCs1–x–yPbI3–zBrz Single Crystals

Mixed halide perovskites, particularly for mixed iodine (I) and bromine (Br), are promising semiconductor materials for multi-junction solar cells due to their band gap modulations; however numerous research studies have shown that halides undergo ion migration under light excitation, resulting in undesired phase segregation, which severely affects the device long-term stability. In high-Br-content (40%) mixed halide perovskites, I-rich and Br-rich regions simultaneously come into being under illuminations, whereas in a low Br content (<10%), the photoinduced phase segregation process is still unclear until now. On the other hand, the severity of phase segregation relies on defect density, especially prone to occur at grain boundaries in thin films. Here, we investigated the regulation of the phase segregation process by A-site cations in FAxMAyCs1–x–yPbI3–zBrz single crystals (SCs) under illuminations that evade disturbance of grain boundaries. It reveals that the introduction of MA, Cs, and Br into FAxMAyCs1–x–yPbI3–zBrz SCs leads to crystal lattice distortion, which confins the long-range migration of ions, and only the short-range migration at a local range occurs under illuminations. This short-range migration further affects the directional polarization of MA and brings lattice relaxation, which is helpful for eliminating local stress imbalance and shallow level traps, ultimately enhancing the stability under illumination.

Introduction of 4-hydroxybenzaldehyde as interface modifier with multidimensional defects passivation effect for high-performance perovskite solar cells

The various types of surfaced point defects and grain boundaries (GBs) of perovskite films have become an obstacle to the development of the overall power conversion efficiency (PCE) and long-term stability of perovskite solar cells (PSCs). Recently, the interfacial modification engineering has been demonstrated as an efficient strategy to reduce defects density and suppress non-radiative recombination. Herein, we report a novel interfacial modification material, 4-hydroxybenzaldehyde (HBA) with multifunctional groups (–CHO and –OH) to modify the interface between perovskite film and hole transport layer (HTL). It is found that the HBA interfacial modifier cannot only passivate one-dimensional point defects, including uncoordinated lead ion (Pb2+) and FA+/MA+ vacancies by coordination interaction or hydrogen bond, but also passivate two-dimensional extended defects via promoting secondary growth of grains to reduce the number of GBs. Consequently, the PCE is significantly improved from 18.84% for the control device to 20.89% for HBA-modified device. Meanwhile, the device with HBA interfacial modifier exhibits negligible hysteresis and excellent environmental stability against humidity and thermal. This study provides important insight to reasonably design passivation molecular structure for obtaining highly efficient and stable PSCs.

High-sensitivity and versatile plasmonic biosensor based on grain boundaries in polycrystalline 1L WS2 films

Structural defects play an important role in exploitation of two-dimensional layered materials (2DLMs) for advanced biosensors with the increasingly high sensitivity and low detection limit. Grain boundaries (GBs), as an important type of structural defect in polycrystalline 2DLM films, potentially provide sufficient active defect sites for the immobilization of bioreceptor units via chemical functionalization. In this work, we report the selective functionalization of high-density GBs with complementary DNA receptors, via gold nanoparticle (AuNP) linkers, in wafer-scale polycrystalline monolayer (1L) W(Mo)S2 films as versatile plasmonic biosensing platforms. The large surface area and GB-rich nature of the polycrystalline 1L WS2 film enabled the immobilization of bioreceptors in high surface density with spatial uniformity, while the AuNPs perform not only as bioreceptor linkers, but also promote detection sensitivity through surface plasmon resonance enhancement effect. Therefore, the presented biosensor demonstrated highly sensitive and selective sub-femto-molar detection of representative RNA sequences from the novel coronavirus (RdRp, ORF1ad and E). This work demonstrates the immense potential of AuNP-decorated GB-rich 2DLMs in the design of ultra-sensitive biosensing platforms for the detection of biological targets beyond RNA, bringing new opportunities for novel healthcare technologies.

Thermoelasticity of Water in Silicate Melts: Implications for Melt Buoyancy in Earth's Mantle

AbstractA comprehensive analysis of experimental data and theoretical simulations on the partial molar volume of water in silicate melt indicates that finite strain theory successfully describes the compression of the H2O component dissolved in silicate melt at high pressures and temperatures. However, because of the high compressibility of the water component, a fourth order equation of state fit is required to accurately simulate experimental results on water's volume in silicate melts at a deep upper mantle, transition zone, and lower mantle pressures. Data from previous shock compression experiments on hydrous minerals in which melting occurs along the Hugoniot are used to provide an experimental constraint on the partial molar volume of water in silicate melt at deep mantle temperatures and pressures. The equation of state of the water component indicates that, depending on elastic averaging technique, the amount of water that could be present in neutrally or negatively buoyant mafic/ultramafic melts above the 410 km seismic discontinuity is upper‐bounded at 5.6 wt%: smaller than previously inferred, and consistent with melt being confined to a narrow depth range above the 410 km discontinuity. If melt is predominantly distributed along grain boundaries in low aspect ratio films, extents of melting as low as 2% could produce observed seismic velocity reductions. The ability of the lowermost mantle to contain negatively buoyant hydrous liquids hinges on the trade‐off between iron content and hydration: at these depths, substantially higher degrees of hydration could be present within partial melts.

Activation energy and its fluctuations at grain boundaries of Er3+:BaTiO3 perovskite thin films: Effect of doping concentration and annealing temperature

An improved understanding of the grain growth kinetics in Erbium-doped Barium Titanate (BTE), has attracted great interest in its applications in photonic and electronic devices. To study the effects of doping and thermal treatment on the physical properties of films, the chemical surface, structural phase, grain size and fluctuations of the grain boundary of BTE films were analyzed and then their effects on the activation energy of the grain growth of the BTE films were revealed. BTE thin films were deposited on Si(100) substrates by electron beam physical vapor deposition (EB-PVD). XRD pattern analysis of the BTE films revealed a tetragonal crystal structure. Williamson–Hall analysis was used to confirm the tensile strain of the BTE thin films. The experimental results indicate that the Er doping in BaTiO3 show strong response by increasing the activation energy of the grain growth and increasing the grain size of the BTE films, while fluctuations of the surface roughness of the BTE films from 0.47 nm to 24.80 nm were also observed. Thin films: BTE2, BTE3, and BTE6 exhibit a grain growth exponent (n) of 5.71, 6.50, and 8.22 that respectively suggest a surface diffusion-controlled grain growth in these films. The activation energies for the grain growth came about 6.63, 6.95 and 19.7 kJ/mol for BTE2, BTE3 and BTE6, respectively, which were close to the values reported for the activation energy in BaTiO3. This was consistent with the high grain boundary density of the BTE films, which was responsible for the mechanism of surface diffusion and point defects. This behavior has been observed in case of BaTiO3, but for Erbium-doped barium titanate it has never been reported.

Untangling the effects of octahedral rotation and ionic displacements on the electronic structure of BiFeO3

The electronic structure and related properties of perovskites ${\mathrm{ABO}}_{3}$ are strongly affected by even small modifications in their crystalline structure. In the case of ${\mathrm{BiFeO}}_{3}$, variations in the octahedral rotations and ionic displacements lead to significant changes in the band gap. This effect can possibly explain the wide range of values (2.5--3.1 eV) reported in the literature, obtained from samples of varied structural qualities, including polycrystalline films, epitaxial films grown by pulsed-laser deposition and molecular beam epitaxy, nanowires, nanotubes, and bulk single crystals. Using hybrid density-functional calculations, we investigate the dependence of the electronic structure on the crystal lattice distortions of the ferroelectric-antiferromagnetic ${\mathrm{BiFeO}}_{3}$, disentangling the effects of the ferroelectric ionic displacements and the antiferrodistortive octahedral rotations on the band gap and the band-edge positions. The band gap is shown to vary from 3.39 eV for the rhombohedral ground-state (R3c) structure down to 1.58 eV for the perfect cubic (Pm$\overline{3}$m) structure, with changes in the conduction band being much more prominent than in the valence band. The gap varies linearly with the ferroelectric ionic displacements, but nonlinearly with the octahedral rotations around the pseudocubic [111${\mathrm{]}}_{c}$ axis, and this is explained in terms of the different interactions between Bi $6s, 6p$, Fe $3d$, and O $2p$ bands. We argue that such large variation of the band gap with structural changes may well explain the large scattering of the reported values, especially if significant deviations from the equilibrium crystal structure are found near domain boundaries, extended defects, or grain boundaries in polycrystalline films.