Activation Energy For Migration Research Articles

Asymmetric Bipolar Resistive Switching of Halide Perovskite Film in Contact with TiO2 Layer.

Halide perovskite materials such as methylammonium lead iodide (CH3NH3PbI3) have attracted considerable interest for the resistive random-access memory applications, which exploit a dramatic change in the resistance by an external electric bias. In many semiconductor films, the drift, accumulation, and chain formation of defects explain the change in the resistance by an external bias. This study demonstrates that the interface of CH3NH3PbI3 with TiO2 has a significant impact on the formation and rupture of defect chains and causes the asymmetric bipolar resistive switching in the Au/CH3NH3PbI3/TiO2/FTO device (FTO = fluorine-doped tin oxide). When a negative bias is applied to the Au electrode, iodine interstitials with the lowest migration activation energy move toward TiO2 in the CH3NH3PbI3 layer and pile up at the CH3NH3PbI3-TiO2 interface. Under the same condition, oxygen vacancies in the TiO2 layer also travel to the CH3NH3PbI3-TiO2 interface and strongly attract iodine interstitials. As a result, a Schottky barrier appears at the CH3NH3PbI3-TiO2 interface, and the resistance of Au/CH3NH3PbI3/TiO2/FTO becomes much larger than that of Au/CH3NH3PbI3/FTO in the high resistance state. The frequency dependence of the capacitance confirms the asymmetric appearance of a large space charge polarization at the CH3NH3PbI3-TiO2 interface, which causes the unique bipolar resistive switching behavior with the on/off ratio (103) and retention time (>104 seconds) at -0.85 V in Au/CH3NH3PbI3/TiO2/FTO film.

(Invited) Fast Ion Dynamics in Solids As Probed By NMR: Disorder, Dimensionality and Correlation Effects

NMR offers plenty of tools to analyze ionic jump processes in crystalline and amorphous solids at atomic scale. Both high-resolution and time-domain 1H(2H), 6,7Li, 19F, 23Na NMR helps shedding light on the origins of rapid self-diffusion in materials being relevant for energy storage.It is well accepted that in materials with strong site preferences, such as LiAlO2, the Li+ ions are subjected to extremely slow exchange processes. The loss of this site preference, as it is the case in glasses and some nanocrystalline phases, may, however, lead to rapid cation diffusion. Examples that benefit from this effect include cation-mixed, high-entropy fluorides ((Ba,Ca)F2) and LiTi2(PS4)3. In general, in non-equilibrium phases site disorder, polyhedra distortions, strain and the various types of local defects will affect both the migration activation energy and the corresponding Arrhenius pre-factor. Whereas in (Me,Ca)F2 (Me = Ba, Pb) cation mixing influences F anion dynamics, in Li6PS5X (X = Br, Cl, I) the potential landscape can be manipulated by anion site disorder and the introduction of extrinsic defects. In some cases, this leads to rather flat landscapes enabling the ions to have access to concerted displacement processes, as is seen in LLZO-type garnets. On the other hand, in the mixed conductor Li4+x Ti5O12 cation-cation repulsions immediately lead to a boost in Li+ diffusivity at the early stages of chemical lithiation. For Na-bearing closo-borates, the influence of rotational anion dynamics is usually considered to explain long-range cation transport. Recent examples of our group also include Li- and Na-conducting MOFs, for which, as in the case of the closo-borates and the famous Na-β''-alumina, transitions from correlated to uncorrelated motion are observed at sufficiently high temperature.Finally, rapid diffusion is also expected for materials that are able to guide the ions along (macroscopic) pathways with confined dimensions, as we have recently observed for layer-structured fluorides such as RbSn2F5 or MeSnF4 with their buried interfaces.

Defect and dopant properties in CaMnO3

CaMnO3-based ceramics have been the subject of considerable research due to their potential application in solid oxide fuel cells, thermoelectric generators, and catalysis. The computational modeling technique based on the classical pair-wise potentials has allowed atomic-scale insights into the defect chemistry, diffusion of Ca2+ and O2− ions, and solution of various dopants in this material. The Ca/Mn anti-site was found to be the most favorable intrinsic defect suggesting disorder, which would be sensitive to synthesis conditions. The second most favorable disorder in CaMnO3 involves loss of CaO, resulting in calcium and oxygen vacancies, which in turn can promote vacancy mediated self-diffusion. The activation energy for oxygen migration (1.25 eV) is much lower than that for calcium (4.42 eV). Favorable isovalent dopants on the Ca and Mn sites were found to be Fe2+ and Ge4+, respectively. The formation of O vacancies can be facilitated by doping of single dopants Fe2+ and Al3+ on the Mn site. Dual dopants Ni–Fe and Al–Ga on the Mn site can also facilitate the introduction of oxygen vacancies required for the vacancy assisted oxygen diffusion.

Decoupling the effects of defects on efficiency and stability through phosphonates in stable halide perovskite solar cells

Summary Understanding defects is of paramount importance for the development of stable halide perovskite solar cells (PSCs). However, isolating their distinctive effects on device efficiency and stability is currently a challenge. We report that adding the organic molecule 3-phosphonopropionic acid (H3pp) to the halide perovskite results in unchanged overall optoelectronic performance while having a tremendous effect on device stability. We obtained PSCs with ∼21% efficiency that retain ∼100% of the initial efficiency after 1,000 h at the maximum power point under simulated AM1.5G illumination. The strong interaction between the perovskite and the H3pp molecule through two types of hydrogen bonds (H…I and O…H) leads to shallow point defect passivation that has a significant effect on device stability but not on the non-radiative recombination and device efficiency. We expect that our work will have important implications for the current understanding and advancement of operational PSCs.

Self-assembling crystalline peptide microrod for neuromorphic function implementation

Self-assembling crystalline peptide microrod for neuromorphic function implementation

Preparation of anti‐migration transition layer and its application in cast‐in‐case solid rocket motors

AbstractThe polyester‐based transition layers were first synthesized with poly (diethylene glycol adipate), glycerin, and toluene diisocyanate. Then they were coated between the propellant and the liner to improve the bonding performance of the charge and inhibit the migration of the energetic plasticizer in the propellant. The effects of R value (the ratio of the quantity of isocyanate functions NCO with respect to the quantity of hydroxyl functions OH) on migration resistance and bonding performance were also studied. The overall performance was optimized at R = 1.5: after the transition layer was applied, the binding strength was greatly increased from 0.14 to 0.64 MPa, which had good application prospects. Meanwhile, due to the strong interactions between the polar ester groups and N‐butyl‐N‐nitratoethyl nitramine (NENA) in the transition layers, the portion of NENA migrating into liners were reduced from 7.6% to 6.0% (reduced by 21%), indicating that the introduction of polyester transition layers could significantly improve the anti‐migration effect. The activation energies of migration for NENA in the transition layers were about 66.3 ~ 94.3 kJ/mol, which were equivalent to the value of strong hydrogen bonding.

Grain Size Influences Activation Energy and Migration Pathways in MAPbBr3 Perovskite Solar Cells.

Ion migration in perovskite layers can significantly reduce the long-term stability of the devices. While perovskite composition engineering has proven an interesting tool to mitigate ion migration, many optoelectronic devices require a specific bandgap and thus require a specific perovskite composition. Here, we look at the effect of grain size to mitigate ion migration. We find that in MAPbBr3 solar cells prepared with grain sizes varying from 2 to 11 μm the activation energy for bromide ion migration increases from 0.17 to 0.28 eV. Moreover, we observe the appearance of a second bromide ion migration pathway for the devices with largest grain size, which we attribute to ion migration mediated by the bulk of the perovskite, as opposed to ion migration mediated by the grain boundaries. Together, these results suggest the beneficial nature of grain engineering for reduction of ion migration in perovskite solar cells.

Defects, diffusion, dopants and encapsulation of Na in NaZr2(PO4)3

NaZr2(PO4)3 is an attractive ceramic material in the application of Na-ion batteries due to its high ionic conductivity and low thermal expansion. Here we employ classical simulation techniques to examine the defect processes, Na-ion migration and solution of various dopants in NaZr2(PO4)3. We find that the Na Frenkel (0.70 eV) is the most favourable defect process ensuring the formation of Na vacancies required for the vacancy assisted Na-ion migration. Long-range sodium vacancy migration was considered and it is calculated that Na-ions diffuse fast with a low activation energy of migration of 0.26 eV. The most favourable isovalent dopants on the Na and Zr sites are the K and Ge, respectively. The formation of Na interstitials and oxygen vacancies can be favoured by doping of Yb, Y, Ga and In on the Zr site. Encapsulation of Na was considered using density functional theory simulations and it is found that the incorporation is exoergic. Furthermore, the formation of Na+ ions and electrons simultaneously constitutes this material metallic together with high concentration of Na+ ions. The final encapsulated composite can be suitable for the preparation of electrodes in rechargeable Na-ion batteries.

<i>Ab Initio</i> Study of Helium in Tantalum: Interaction, Migration, and Clustering with Helium and Vacancies

Ab initio calculations based on the Density Function Theory (DFT) have been performed to study the interaction between helium and helium, helium and vacancy, migration of helium, and the stability of small helium-vacancy clusters in tantalum. The following results are found: (I) The tetrahedral interstitial helium atoms have weak interactions in tantalum, suggesting that no stable covalent bond is formed between this two helium atoms; (II) The stability of small helium-vacancy clusters is investigated. The interstitial helium atom and vacancy to the clusters are found to be positive in almost all case, i.e., all interactions are attractive; (III) The activation energies for a substitutional helium atom migration by the dissociation or vacancy mechanisms are estimated under the irradiation condition.

Recovery and recrystallization in molybdenum studied by positron annihilation and X-ray methods

The study investigated the recrystallization and induced changes in positron annihilation characteristics and the crystallographic texture of rolled molybdenum sheets as a function of annealing temperature. After deformation, the positron lifetime spectrum revealed dislocations and divacancies, however, above 200 °C where the recovery process of the deformed material began clusters with five vacancies were detected. Above 500 °C the step-like dependence of the mean positron lifetime on temperature was registered which is associated with recrystallization as was proofed by XRD measurements. This type of dependency can be well described using the positron diffusion model, which allows determining the activation energy of the grain boundary migration. This energy was determined as equal to 1.41 ± 0.23 eV. Variation of positron lifetime with temperature can be linked with changes in the texture. The rotated cube {100} 〈110〉 component dominates in the rolled material texture. The {111} <110> − {111} 〈112〉 fiber, typical for the recrystallization texture were registered for the samples annealed at 800 °C and 1050 °C.

Effect of vanadium doping on the electrical charge transport and dielectric relaxation properties of Sodium Bismuth Titanate perovskite

In this work, we report the effect of vanadium doping on the conductivity and relaxation dynamics of the charge carriers in Sodium Bismuth Titanate perovskite material (Na0.5Bi0.5TiO3) prepared by a unique technique which is a combination of chemical and mechanical methods. The parent material was prepared using the citrate auto-ignition method and the mechanical alloying technique was used to incorporate V2O5 in the NBT structure. The X-ray diffraction analysis confirmed the formation of the rhombohedral phase of NBT perovskite with space group R3c in all the compositions. The microstructural analysis was done using Field Emission Scanning Electron Microscopy, Energy-dispersive X-ray spectra, and the elemental mapping of the samples. The conductivity mechanism was analyzed using the complex impedance method. The sample with 15% doping showed the highest conductivity with minimum activation energy. The activation energies for conduction, relaxation, and migration of charge carriers were found to be comparable with each other. The frequency-dependent dielectric data was analyzed using Havriliak–Negami (HN) formalism and suggested non-Debye type relaxation behaviour. The 15% dopped composition showed the highest dielectric strength. The scaling behaviour of impedance and modulus spectra confirmed the temperature-independent conduction and relaxation mechanisms.

Zero-Dimensional Lead-Free FA3Bi2I9 Single Crystals for High-Performance X-ray Detection.

Direct X-ray detectors based on metal halide perovskites and their derivatives exhibit high sensitivity and low limit of detection (LoD). Compared with three-dimensional (3D) hybrid lead halide perovskites, low-dimensional A3Bi2I9 perovskite derivatives (A = Cs, Rb, NH4, CH3NH3(MA)) present better stability, greater environmental friendliness, and comparable X-ray detection performance. Here, we report FA3Bi2I9 (FA= CH(NH2)2) single crystals (SCs) as a new member of the A3Bi2I9 series for X-ray detection, which were prepared by the nucleation-controlled secondary solution constant temperature evaporation (SSCE) method. Centimeter-sized FA3Bi2I9 SCs show a full width at half-maximum (fwhm) of 0.0096°, which is superior to that of recently reported Cs3Bi2I9 (0.058°) and MA3Bi2I9 SCs (0.024°) obtained by inverse temperature crystallization (ITC). The as-grown FA3Bi2I9 SC shows a large resistivity of 7.8 × 1010 Ω cm and a high ion migration activation energy (Ea) of 0.56 eV, which can guarantee a low noise level and good operational stability under a large external bias. The FA3Bi2I9 SC detector exhibits a LoD of 0.2 μGyair s-1, a sensitivity of 598.1 μC Gyair -1 cm -2, and an X-ray detection efficiency of 33.5%, which are much better than those of the commercialized amorphous selenium detector. Results presented here will provide a new lead-free perovskite-type material to achieve green, sensitive, and stable X-ray detectors.

Defect energetics in the SrTiO3-LaCrO3 system

Oxide interfaces have attracted a huge amount of interest in recent years due to their novel functionalities that are not possible in bulk. Here we employ atomistic simulation techniques to study the defect properties and oxygen ion migration energy in bulk SrTiO3, LaCrO3 and SrTiO3–LaCrO3 interface. The SrO and La2O3 “Schottky-like” defects are found to be the most favourable intrinsic defects in their respective bulk structures and also at interface layers. While defect energies calculated at interface layers are not significantly changed compared to that calculated in the bulk LaCrO3, significant changes are noted in Schottky defects and oxygen Frenkel between bulk SrTiO3 and the interface. The activation energies for the oxygen ion diffusion in bulk SrTiO3 and LaCrO3 are calculated to be 0.64 eV and 0.53 eV, respectively, suggesting that both materials will present favourable diffusion pathways. In general, there is a reduction in the activation energies of oxygen ion migration at the interface compared to bulk structures. In particular, in the Cr layer, activation energy is calculated to be 0.36 eV implying that interface plays an important role in the oxygen ion migration and therefore such an interface is of interest for use in advanced electrochemical devices.

Self-diffusion in garnet-type Li7La3Zr2O12 solid electrolytes

Tetragonal garnet-type Li7La3Zr2O12 is an important candidate solid electrolyte for all-solid-state lithium ion batteries because of its high ionic conductivity and large electrochemical potential window. Here we employ atomistic simulation methods to show that the most favourable disorder process in Li7La3Zr2O12 involves loss of Li2O resulting in lithium and oxygen vacancies, which promote vacancy mediated self-diffusion. The activation energy for lithium migration (0.45 eV) is much lower than that for oxygen (1.65 eV). Furthermore, the oxygen migration activation energy reveals that the oxygen diffusion in this material can be facilitated at higher temperatures once oxygen vacancies form.

Accommodation and diffusion of Nd in uranium silicide - U3Si2

Uranium silicide, U3Si2, is considered as an advanced nuclear fuel for commercial light water reactors with improved accident tolerance as well as competitive economics. Nd is employed as a local burnup indicator for conventional oxide fuels due, among other reasons, to its low mobility in the UO2 fuel matrix and its high fission product yield. As part of the studies necessary to determine whether Nd can be considered as a candidate burnup indicator in the U3Si2 concept fuel, we investigate the mobility of Nd in U3Si2. In this work, density functional theory (DFT) calculations are performed to predict the most stable accommodation sites of Nd in U3Si2, found to be within the uranium sublattice. Based on DFT calculations of binding energies and migration activation energies, we investigate Nd diffusion by computing the transport coefficients within the framework of the self-consistent mean-field method. Our calculations predict that the diffusion ratio of Nd to U is smaller in U3Si2 than in UO2. Moreover, at the individual maximum centerline temperature of the fuel, the diffusion of Nd in U3Si2 is much slower than in UO2. From this perspective, Nd represents a good candidate burnup indicator, in similarity to that in UO2.

First-principles study of formation and performance of diamond (111)/Al interface

The simple and convenient metallic mask method is a significant method of preparing diamond nanostructures. The metallic mask method has poor repeatability and can not give the ideal results, because it is supported by no theory about formation of surface mental nanoparticles and its technological parameters are optimized by no experimental techniques that are expensive either. Aiming at the formation and performance of the diamond/Al interface, this paper adopts the first-principles to study the adsorption and migration behavior of Al atoms on the H-terminated diamond surface and the structure of the diamond/Al interface. The results show that the highest adsorption energy is at the T4 position, which is only 0.181 eV, through comparing the adsorption energies of Al atoms at the highly symmetrical positions (Top, Br, H3 and T4) on the surface of the H-terminated diamond (111). The adsorption energies at these different positions are similar and the maximum difference is only 0.019 eV. There is formed no chemical bond, although Al has partial charge transfer on the H-terminated surface through the analysis of differential charge density and worse layout distribution. This phenomenon can be considered as electrostatic adsorption. That is to say, the adsorption of Al atoms are physical adsorption. The smooth potential energy surface also makes it easier for Al atoms to migrate on the diamond surface. The calculation results reveal that the migration activation energies of the two possible migration paths (from T4 position to Br position and from T4 to Top position) are 0.011 eV and 0.026 eV respectively. The above results imply that the metal Al and diamond are mainly connected by weak force, so the adhesion work of the three diamond/Al interface structures is compared based on the geometric stacking structure. The results show that the adhesion work of the three interfaces is around 0. These results indicate that the stability of the diamond/Al interface is not high and the stable structure of the interface is easily destroyed when the external environment changes. This speculation can be confirmed in molecular dynamics. When the simulated temperature is 300 ℃, the liquefied metal Al obviously accumulates into spheres. According to the above research results, we deduce that the metallic mask method does not require high requirements for the relationship between the metal and the substrate material, which depends mainly on the surface topography of the base material. This research provides an important theoretical reference for understanding the formation mechanism of metal nanomasks.

Effect of Elemental Substitution in (Ba,Sr)6RE2Co4O15−BaCe0.5Pr0.3Y0.2O3-δ for Cathodes of Proton-Conducting Ceramic Fuel Cells

Solid oxide fuel cells are promising energy conversion devices because of high efficiency and high fuel flexibility, but their high-operating temperature (700-900 ºC) causes problems such as fast degradation of components and long start-up/shut-down time. To overcome these problems, low- temperature operation is preferable. Proton-conducting ceramic fuel cells (PCFCs) are promising energy conversion devices for intermediate-temperature operation, especially below 600 ºC. This is because the activation energy of proton migration in proton-conducting oxides, such as rare earth-doped barium cerate, is lower than that of the oxide ion. At the current state, however, the performance of PCFCs is mainly limited by the cathode materials.Recently, we have reported that the composite of Ba4Sr2Sm2Co4O15 (BSSC4224)–BaCe0.5Pr0.3Y0.2O3 (BCPY) showed higher activity below 600 ºC in 3% humidified synthetic air, as compared with high-performance cathodes reported1). In this composite, BSSC4224 mainly served as an active site for the dissociative absorption of oxygen. In this study, then, the effect of the elemental substitution, especially the Ba/Sr ratio and rare-earth elements, in (Ba,Sr)6 RE 2Co4O15, on the ASR was studied. Symmetric cells consisting of the BaCe0.8Y0.2O3 (BCY) disk electrolyte and Ba5SrRE 2Co4O15 (RE =Nd, La, Eu, Sm, Pr, and Gd), Ba4Sr2Gd2Co4O15 or Ba5SrGd2Co4O15 (BSGC5124)–BCPY electrodes were fabricated. The electrode performance was measured in 3% humidified synthetic air (21% O2–79% N2) at 450–700 ºC by impedance spectroscopy.In impedance analysis, BSGC5124 had the lowest ASR below 550 ºC among Ba5SrRE 2Co4O15 electrodes. Then, the composite of BSGC5124–BCPY was prepared and its performance was compared with that of BSSC4224–BCPY composite. We found that BSGC5124–BCPY has comparable area-specific resistance and activation energy to BSSC4224–BCPY although BSGC5124 itself had much lower area-specific resistance and activation energy than BSSC4224. Therefore, we concluded that the dissociative adsorption of oxygen on the BSGC5124 phase is not the rate limiting step for the oxygen reduction reaction in this composite system. [1] T. Matsui, K. Manriki, K. Miyazaki, H. Muroyama, K. Eguchi, J. Mat er. Chem. A, 6, 14188 (2018).

Eliminating the electric field response in a perovskite heterojunction solar cell to improve operational stability

Intrinsic and extrinsic ion migration is a very large threat to the operational stability of perovskite solar cells and is difficult to completely eliminate due to the low activation energy of ion migration and the existence of internal electric field. We propose a heterojunction route to help suppress ion migration, thus improving the operational stability of the cell from the perspective of eliminating the electric field response in the perovskite absorber. A heavily doped p-type (p+) thin layer semiconductor is introduced between the electron transporting layer (ETL) and perovskite absorber. The heterojunction charge depletion and electric field are limited to the ETL and p+ layers, while the perovskite absorber and hole transporting layer remain neutral. The p+ layer has a variety of candidate materials and is tolerant of defect density and carrier mobility, which makes this heterojunction route highly feasible and promising for use in practical applications.

2D Perovskite Single Crystals with Suppressed Ion Migration for High‐Performance Planar‐Type Photodetectors

2D Dion-Jacobson (DJ) perovskites have recently drawn much attention owing to their superior charge transport and high environmental stability. Unfortunately, their intrinsic optoelectronic properties are largely unexplored due to the lack of high-quality single crystals available for accurate measurements. Herein, a reactive, low-temperature-gradient crystallization process to grow high-quality 2D perovskite single crystal (BDAPbI4 ) using symmetric straight chain 1,4-butanediammonium as a short-chain insulating spacer is developed. It is found that the BDAPbI4 single crystal exhibits a direct bandgap with effective charge collection (μτ = 1.45 × 10-3 cm2 V-1 ). In particular, the BDAPbI4 single crystal shows a high ion migration activation energy (0.88 eV) that is desired for stability. Moreover, the planar-type photodetectors made of BDAPbI4 single crystals demonstrate excellent photoresponse performance, including large dynamic linear range (150 dB), high responsivity (927 mA W-1 ), fast response speed, and exceptional stability. Theoretical calculations reveal that the enhanced carrier transport properties are mainly attributed to the interlayer-induced coupling of the inorganic layers. Furthermore, the rigid structure of BDAPbI4 , resulting from the strong hydrogen bonds between the organic cations and the inorganic framework, will result in both low defect density and low ion migration, leading to superior carrier transport properties and detection performance.

Shallow Iodine Defects Accelerate the Degradation of α-Phase Formamidinium Perovskite

Summary Shallow defects are mostly benign in covalent semiconductors, such as silicon, given that they do not constitute non-radiative recombination sites. In contrast, the existence of shallow defects in ionic perovskite crystals might have significant repercussions on the long-term stability of perovskite solar cells (PSCs) because of the metastability of the ubiquitous formamidinium lead triiodide (FAPbI3) perovskite and the migration of charged point defects. Here, we show that shallow iodine interstitial defects ( I i ) can be generated unintentionally during commonly used post-fabrication treatments, which can lower the cubic-to-hexagonal transformation barrier of FAPbI3-based perovskites to accelerate its phase degradation. We demonstrate that concurrently avoiding the generation of I i and the more effective passivation of iodine vacancies ( V I ) can improve the thermodynamic stability of the films and operational stability of the PSCs. Our most stable PSC retained 92.1 % of its initial performance after nearly 1,000 h of continuous illumination operational stability testing.