Insights of the Multimode Orientation-Dependent Initial Oxidation of Aluminum by First-Principles Theory Calculations.

A fundamental understanding of the oxidation mechanisms of aluminum (Al) alloys is of great importance for its applications in corrosion, catalysis, sensors, etc. In this work, we systematically investigated the first-stage oxidation behaviors of three low-index Al facets with O coverage up to two monolayers (ML) by using density-functional theory (DFT). The large negative adsorption energies indicated favorable oxidation on all three facets. However, distinctive structural and electronic changes induced by the adsorption of oxygen have led to different oxidation modes. More specifically, the oxidation process proceeded by "intercalating" into the subsurface region along the (111) plane out of the (110) facet with spontaneous O ingress into (110) far below one ML, as revealed by the electron density distribution, whereas the oxide ad-layer grew in a "layer-by-layer" mode on Al(111) and (001) facets. Moreover, various Al-O complexes with different atomic coordination numbers (CN), configurations, and sizes may be indicators of the tendency of an Al surface to be oxidized. Besides, the oxide phases formed on (111)/(001) and (110) assembled the Al-O bond distribution within α-Al2O3 and γ-Al2O3, respectively.

Self-Sacrificed Additive in Preparing PbI2 Film Enables the Oriented Growth of Perovskite Crystals for Improved Solar Cell Performance.

Due to the limitation of the diffusion kinetics of organic amine salts on the PbI2 layer in the two-step method, prepared perovskite particles are small in size, have many defects, and are randomly oriented, and the cell efficiency and stability are difficult to guarantee due to PbI2 residues. Here, we added a volatile additive, N,N,N',N'-tetramethylethylenediamine (TMEDA), to the PbI2 precursor solution and formed preaggregated atomic clusters with PbI2 through TMEDA, which reduced the Gibbs free energy of nucleation to obtain a porous PbI2 layer, and finally obtained a perovskite film with large particles, few defects, ideal crystal plane orientation, and no additive residues. The results show that the photoelectric conversion efficiency of the optimized device is increased by 1.68% (from 21.68% to 23.36%), and the unpackaged optimized device still maintains the maximum efficiency of 77% after being placed in the air for 1200 h. This study provides an effective way to fabricate efficient and stable perovskite solar cells by promoting the nucleation-induced crystallization orientation by volatile additives.

Determination of five-parameter grain boundary characteristics in nanocrystalline Ni-W by scanning precession electron diffraction tomography.

Determining the full five-parameter grain boundary characteristics from experiments is essential for understanding grain boundaries impact on material properties, improving related models, and designing advanced alloys. However, achieving this is generally challenging, in particular at nanoscale, due to their 3D nature. In our study, we successfully determined the grain boundary characteristics of an annealed nickel-tungsten alloy (NiW) nanocrystalline needle-shaped specimen (tip) containing twins using Scanning Precession Electron Diffraction (SPED) Tomography. The presence of annealing twins in this face-centered cubic (fcc) material gives rise to common reflections in the SPED diffraction patterns, which challenges the reconstruction of orientation-specific virtual dark field (VDF) images required for tomographic reconstruction of the 3D grain shapes. To address this, an automated post-processing step identifies and deselects these shared reflections prior to the reconstruction of the VDF images. Combined with appropriate intensity normalization and projection alignment procedures, this approach enables high-fidelity 3D reconstruction of the individual grains contained in the needle-shaped sample volume. To probe the accuracy of the resulting boundary characteristics, the twin boundary surface normal directions were extracted from the 3D voxelated grain boundary map using a 3D Hough transform. For the sub-set of coherent Σ3 boundaries, the expected {111} grain boundary plane normals were obtained with an angular error of <3° for boundary sizes down to 400 nm². This work advances our ability to precisely characterize and understand the complex grain boundaries that govern material properties.

Atomic Structure and 3D Shape of a Multibranched Plasmonic Nanostar from a Single Spatially Resolved Electron Diffraction Map.

Despite the interest in improving the sensitivity of optical sensors using plasmonic nanoparticles (NPs) (rods, wires, and stars), the full structural characterization of complex shape nanostructures is challenging. Here, we derive from a single scanning transmission electron microscope diffraction map (4D-STEM) a detailed determination of both the 3D shape and atomic arrangement of an individual 6-branched AuAg nanostar (NS) with high-aspect-ratio legs. The NS core displays an icosahedral structure, and legs are decahedral rods attached along the 5-fold axes at the core apexes. The NS legs show an anomalous anisotropic spatial distribution (all close to a plane) due to an interplay between the icosahedral symmetry and the unzipping of the surfactant layer on the core. The results significantly improve our understanding of the star growth mechanism. This low dose diffraction mapping is promising for the atomic structure study of individual multidomain, multibranched, or multiphase NPs, even when constituted of beam-sensitive materials.

Molecular Dynamics Simulation Study of Aluminum-Copper Alloys' Anisotropy under Different Loading Conditions and Different Crystal Orientations.

The commonly used aluminum-copper alloys in industry are mainly rolled plates and extruded or drawn bars. The aluminum-copper alloys' anisotropy generated in the manufacturing process is unfavorable for subsequent applications. Its underlying mechanism shall be interpreted from a microscopic perspective. This paper conducted the loading simulation on Al-4%Cu alloy crystals at the microscopic scale with molecular dynamics technology. Uniaxial tension and compression loading were carried out along three orientations: X-<1¯12>, Y-<11¯1>, and Z-<110>. It analyzes the micro-mechanisms that affect the performance changes of aluminum-copper alloys through the combination of stress-strain curves and different organizational analysis approaches. As shown by the results, the elastic modulus and yield strength are the highest under tension along the <11¯1> direction. Such is the case for the reasons below: The close-packed plane of atoms ensures large atomic binding forces. In addition, the Stair-rod dislocation forms a Lomer-Cottrell dislocation lock, which has a strengthening effect on the material. The elastic modulus and yield strength are the smallest under tension along the <110> direction, and the periodic arrangement of HCP atom stacking faults serves as the main deformation mechanism. This is because the atomic arrangement on the <110> plane is relatively loose, which tends to cause atomic misalignment. When compressed in different directions, the plastic deformation mechanism is mainly dominated by dislocations and stacking faults. When compressed along the <110> direction, it has a relatively high dislocation density and the maximum yield strength. That should be attributed to the facts below. As the atomic arrangement of the <110> plane itself was not dense originally, compression loading would cause an increasingly tighter arrangement. In such a case, the stress could only be released through dislocations. This research aims to provide a reference for optimizing the processing technology and preparation methods of aluminum-copper alloy materials.

Halogen-Functionalized Hole Transport Materials with Strong Passivation Effects for Stable and Highly Efficient Quasi-2D Perovskite Solar Cells.

The performance of quasi-two-dimensional (Q-2D) perovskite solar cells (PSCs) strongly depends on the interface characteristics between the hole transport material (HTM) and the perovskite layer. In this work, we designed and synthesized a series of HTMs with triphenylamine-carbazole as the core structure and modified end groups with chlorine and bromine atoms. These HTMs show deeper highest occupied molecular orbital energy levels than commercial HTMs. This reduced energy band mismatch between the HTM and perovskite layer facilitates efficient charge extraction at the interface. Moreover, these HTMs containing halogen atoms on the end groups could form halogen bonding with the Pb2+ ions at the buried interface of the perovskite layer, effectively passivating defects to suppress nonradiative recombination. Additionally, halogen bonding also contributes to the formation of vertically oriented perovskite crystals with a high quality. By incorporation of chlorohexane-substituted HTMs, the resultant Q-2D PSCs exhibited the highest power conversion efficiency of 21.07%. Furthermore, the devices show improved stability, retaining 97.2% of their initial efficiency after 1100 h of continuous illumination.

Interfacial Engineering for Controlled Crystal Mosaicity in Single-Crystalline Perovskite Films.

Organic-inorganic hybrid perovskites have attracted significant attention for optoelectronic applications due to their efficient photoconversion properties. However, grain boundaries and irregular crystal orientations in polycrystalline films remain issues. This study presents a method for producing crystalline-orientation-controlled perovskite single-crystal films using retarded solvent evaporation. It is shown that single-crystal films, grown via inverse temperature crystallization within a confined space, exhibit enhanced optoelectronic property. Using interfacial polymer layer, this method produces high-quality perovskite single-crystalline films with varying crystal orientations. Density functional theory calculations confirm favorable adsorption energies for (110) surfaces with methylammonium iodide and PbI2 terminations on poly(3-hexylthiophene), and stronger adsorption for (224) surfaces with I and methylammonium terminations on polystyrene, influenced by repulsive forces between the thiophene group and the perovskite surface. The correlation between charge transport characteristics and perovskite single-crystalline properties highlights potential advancements in perovskite optoelectronics, improving device performance and reliability.

Blade-Coating (100)-Oriented α-FAPbI3 Perovskite Films via Crystal Surface Energy Regulation for Efficient and Stable Inverted Perovskite Photovoltaics.

Photoactive black-phase formamidinium lead triiodide (α-FAPbI3) perovskite has dominated the prevailing high-performance perovskite solar cells (PSCs), normally for those spin-coated, conventional n-i-p structured devices. Unfortunately, α-FAPbI3 has not been made full use of its advantages in inverted p-i-n structured PSCs fabricated via blade-coating techniques owing to uncontrollable crystallization kinetics and complicated phase evolution of FAPbI3 perovskites during film formation. Herein, a customized crystal surface energy regulation strategy has been innovatively developed by incorporating 0.5 mol % of N-aminoethylpiperazine hydroiodide (NAPI) additive into α-FAPbI3 crystal-derived perovskite ink, which enabled the formation of highly-oriented α-FAPbI3 films. We deciphered the phase transformation mechanisms and crystallization kinetics of blade-coated α-FAPbI3 perovskite films via combining a series of in-situ characterizations and theoretical calculations. Interestingly, the strong chemical interactions between the NAPI and inorganic Pb-I framework help to reduce the surface energy of (100) crystal plane by 42 %, retard the crystallization rate and lower the formation energy of α-FAPbI3. Benefited from multifaceted advantages of promoted charge extraction and suppressed non-radiative recombination, the resultant blade-coated inverted PSCs based on (100)-oriented α-FAPbI3 perovskite films realized promising efficiencies up to 24.16 % (~26.5 % higher than that of the randomly-oriented counterparts), accompanied by improved operational stability. This result represented one of the best performances reported to date for FAPbI3-based inverted PSCs fabricated via scalable deposition methods.

Preparation, Deformation Behavior and Irradiation Damage of Refractory Metal Single Crystals for Nuclear Applications: A Review.

Refractory metal single crystals have been applied in key high-temperature structural components of advanced nuclear reactor power systems, due to their excellent high-temperature properties and outstanding compatibility with nuclear fuels. Although electron beam floating zone melting and plasma arc melting techniques can prepare large-size oriented refractory metals and their alloy single crystals, both have difficulty producing perfect defect-free single crystals because of the high-temperature gradient. The mechanical properties of refractory metal single crystals under different loads all exhibit strong temperature and crystal orientation dependence. Slip and twinning are the two basic deformation mechanisms of refractory metal single crystals, in which low temperatures or high strain rates are more likely to induce twinning. Recrystallization is always induced by the combined action of deformation and annealing, exhibiting a strong crystal orientation dependence. The irradiation hardening and neutron embrittlement appear after exposure to irradiation damage and degrade the material properties, attributed to vacancies, dislocation loops, precipitates, and other irradiation defects, hindering dislocation motion. This paper reviews the research progress of refractory metal single crystals from three aspects, preparation technology, deformation behavior, and irradiation damage, and highlights key directions for future research. Finally, future research directions are prospected to provide a reference for the design and development of refractory metal single crystals for nuclear applications.

Effects of Crystal Orientation and External Stress on the Static Recrystallization Behavior of an Ni-Based Single-Crystal Superalloy.

The deformation mechanism and static recrystallization (SRX) behavior of an Ni-based single-crystal superalloy are investigated. Indentation tests were performed to investigate the effects of crystal orientation and external stress on SRX behavior. Following solution heat treatment, the depth of the SRX layer below the indentation increases with a deviation angle (ß) from the [001] orientation. The slip analysis indicates that an increased deviation angle leads to an increase in the resolved shear stress on the slip plane and a decrease in the number of active slip systems. In addition, the variation pattern of the SRX layer depth with the deviation angle is consistent for different external stresses. The depth of the SRX layer also increases with external stress. The coarse γ' phases and residual γ/γ' eutectics obviously enhance the pinning effects on the expansion of recrystallized grain boundaries, resulting in slower growth rates of the recrystallized grains in interdendritic regions than those in dendrite core regions.

An Improved Ordinary State-Based Peridynamic Model for Granular Fractures in Cubic Crystals and the Effects of Crystal Orientations on Crack Propagation.

Material anisotropy caused by crystal orientation is an essential factor affecting the mechanical and fracture properties of crystal materials. This paper proposes an improved ordinary state-based peridynamic (OSB-PD) model to study the effect of arbitrary crystal orientation on the granular fracture in cubic crystals. Based on the periodicity of the equivalent elastic modulus of a cubic crystal, a periodic function regarding the crystal orientation is introduced into peridynamic material parameters, and a complete derivation process and expressions of correction factors are given. In addition, the derived parameters do not require additional coordinate transformation, simplifying the simulation process. Through convergence analysis, a regulating strategy to obtain the converged and accurate results of crack propagation paths is proposed. The effects of crystal orientations on crack initiation and propagation paths of single-crystal materials with different notch shapes (square, equilateral triangle, semi-circle) and sizes were studied. The results show that variations in crystal orientation can change the bifurcation, the number, and the propagation path direction of cracks. Under biaxial tensile loading, single crystals with semi-circular notches have the slowest crack initiation time and average propagation speed in most cases and are more resistant to fracture. Finally, the effects of grain anisotropy on dynamic fractures in polycrystalline materials under different grain boundary coefficients were studied. The decrease in grain anisotropy degree can reduce the microcracks in intergranular fracture and the crack propagation speed in transgranular fracture, respectively.

Heat-Resistant Carbon-Coated Potassium Magnesium Hexacyanoferrate Nanoplates for High-Performance Potassium-Ion Batteries.

Metal hexacyanoferrates (HCFs) are regarded as promising cathode materials for potassium-ion batteries (PIBs) on account of their low cost and high energy density. However, the difficult-to-remove [Fe(CN)6] vacancies and crystal water lead to structural instability and capacity deterioration as well as the stereotype of poor thermostability of conventional HCFs. Herein, we report (100) face-oriented potassium magnesium hexacyanoferrate (KMgHCF) nanoplates with low [Fe(CN)6] vacancies and high crystallinity, enabling thermostability up to 550 °C, high-temperature carbon coating and crystal water elimination. The as-obtained KMgHCF/C nanoplates exhibit superior potassium storage properties, including a large reversible capacity of 84.6 mAh g-1, a high voltage plateau of 3.87 V, excellent long-term cycling performance over 15000 cycles and high rate capability at 5 A g-1. The unprecedented cycling stability of KMgHCF/C is attributed to the synergistic effect of a highly reversible two-phase reaction, low [Fe(CN)6] vacancies and no crystal water, a specially exposed steady (100) surface, and a protective carbon coating. This work provides a new material selection and modification strategy for the practical application of HCFs in PIBs.

Controlled Nucleation and Oriented Crystallization of Methylammonium-Free Perovskites via In Situ Generated 2D Perovskite Phases.

Enhancing stability while maintaining high efficiency is among the primary challenges in the commercialization of perovskite solar cells (PSCs). Here, a crystal growth technique assisted by in situ generated 2D perovskite phases has been developed to construct high-quality 2D/3D perovskite films. The in situ generated 2D perovskite serve as templates for regulating the nucleation and oriented crystal growth in the α-FAPbI3-rich film. This led to a high film quality with much reduced trap density and an ultralong carrier lifetime. The obtained perovskite film shows excellent stability under extreme environment conditions (T = 200 °C, RH = 75 ± 5%). The corresponding PSC achieved an efficiency of 26.16% (certified 25.84%), along with excellent operational stability (T93 > 1300 h, T ≅ 50 °C) as well as outstanding high and low temperature cycle stability.

Improvement of Photovoltaic Performance of Perovskite Solar Cells by Synergistic Modulation of SnO2 and Perovskite via Interfacial Modification.

In the past decade, perovskite solar cell (PSC) photoelectric conversion efficiency has advanced significantly, and tin dioxide (SnO2) has been extensively used as the electron transport layer (ETL). Due to its high electron mobility, strong chemical stability, energy level matching with perovskite, and easy low-temperature fabrication, SnO2 is one of the most effective ETL materials. However, the SnO2 material as an ETL has its limitations. For example, SnO2 films prepared by low-temperature spin-coating contain a large number of oxygen vacancies, resulting in energy loss and high open-circuit voltage (VOC) loss. In addition, the crystal quality of perovskites is closely related to the substrate, and the disordered crystal orientation will lead to ion migration, resulting in a large number of uncoordinated Pb2+ defects. Therefore, interface optimization is essential to improve the efficiency and stability of the PSC. In this work, 2-(5-chloro-2-benzotriazolyl)-6-tert-butyl-p-cresol (CBTBC) was introduced for ETL modification. On the one hand, the hydroxyl group of CBTBC forms a Lewis mixture with the Sn atom, which reduces the oxygen vacancy defect and prevents nonradiative recombination. On the other hand, the SnO2/CBTBC interface can effectively improve the crystal orientation of perovskite by influencing the crystallization kinetics of perovskite, and the nitrogen element in CBTBC can effectively passivate the uncoordinated Pb2+ defects at the SnO2/perovskite interface. Finally, the prevailing PCE of PSC (1.68 eV) modified by CBTBC was 20.34% (VOC = 1.214 V, JSC = 20.49 mA/cm2, FF = 82.49%).

Precise Crystal Orientation Identification and Twist-Induced Giant Modulation of Optical Anisotropy in 1T'-ReS2.

The ability to precisely identify crystal orientation as well as to nondestructively modulate optical anisotropy in atomically thin rhenium dichalcogenides is critical for the future development of polarization programmable optoelectronic devices, which remains challenging. Here, we report a modified polarized optical imaging (POI) method capable of simultaneously identifying in-plane (Re chain) and out-of-plane (c-axis) crystal orientations of the monolayer to few-layer ReS2, meanwhile, propose a nondestructive approach to modulate the optical anisotropy in ReS2 via twist stacking. The results show that parallel and near-cross POI are effective to independently identify the in-plane and out-of-plane crystal orientations, respectively, while regulating the twist angle allows for giant modulation of in-plane optical anisotropy from highly intrinsic anisotropy to complete optical isotropy in the stacked ReS2 bilayer (with either the same or opposite c-axes), as well modeled by linear electromagnetic theory. Overall, this study not only develops a simple optical method for precise crystal orientation identification but also offers an efficient light polarization control strategy, which is a big step toward the practical application of anisotropic van der Waals materials in the design of nanophotonic and optoelectronic devices.

Manipulating Crystal Growth and Secondary Phase PbI2 to Enable Efficient and Stable Perovskite Solar Cells with Natural Additives.

In perovskite solar cells (PSCs), the inherent defects of perovskite film and the random distribution of excess lead iodide (PbI2) prevent the improvement of efficiency and stability. Herein, natural cellulose is used as the raw material to design a series of cellulose derivatives for perovskite crystallization engineering. The cationic cellulose derivative C-Im-CN with cyano-imidazolium (Im-CN) cation and chloride anion prominently promotes the crystallization process, grain growth, and directional orientation of perovskite. Meanwhile, excess PbI2 is transferred to the surface of perovskite grains or formed plate-like crystallites in local domains. These effects result in suppressing defect formation, decreasing grain boundaries, enhancing carrier extraction, inhibiting non-radiative recombination, and dramatically prolonging carrier lifetimes. Thus, the PSCs exhibit a high power conversion efficiency of 24.71%. Moreover, C-Im-CN has multiple interaction sites and polymer skeleton, so the unencapsulated PSCs maintain above 91.3% of their initial efficiencies after 3000 h of continuous operation in a conventional air atmosphere and have good stability under high humidity conditions. The utilization of biopolymers with excellent structure-designability to manage the perovskite opens a state-of-the-art avenue for manufacturing and improving PSCs.

Effect of Initial Position and Crystallographic Orientation on Grain Selection Procedure in Z-Form Selector for Ni-Based Single-Crystal Superalloy.

The grain selection process in a Z-form selector for Ni-based single-crystal superalloy was simulated using a macro-scale ProCAST software (2013 version) coupled CAFE module combined with an experiment to investigate the grain selection procedure and mechanism with different grain positions and crystal orientation relationships. A non-stationary solidification process was found in the Z-form selector, and the liquid-solid (L-S) interface was tilted in the same direction as the selector channel during directional solidification. Given that the grain boundary was parallel to the Z-form selector, the overgrowth rate of the bi-crystal in the selector channel was very low. The initial position of the bi-crystal in the selector channel has a greater effect on the overgrowth rate than the effects of primary and secondary orientations. The grain selection was a result of the coupling of the competitive grain growth effect and geometrical restriction effect. Finally, the selection grain mechanism within the Z-form selector was discussed, coalescing the temperature field and the grain competition growth mechanism.

Structure-Piezoelectric Property Relationships of Thin Films Composed of Electrospun Aligned Poly(vinylidene fluoride) Nanofibers.

In the past two decades, many studies on piezoelectric nanofibers (NFs) prepared from poly(vinylidene fluoride) (PVDF) and its copolymers, including single NFs, randomly oriented nonwoven mats, and aligned NFs, have been reported. However, studies on the relationships between the PVDF NF diameter, the orientation of the ß-phase crystals inside NFs, and the piezoelectric properties of the NFs are still limited. In this study, the effect of the fiber diameter on the internal molecular packing/orientation and piezoelectric properties of aligned PVDF NF thin films was investigated. Herein, piezoelectric thin films composed of densely packed, uniaxially aligned, PVDF NFs with diameters ranging from 228 to 1315 nm were prepared by means of electrospinning with a rotating collector and successive hot-pressing and poling. The effect of the diameters of PVDF NFs on their internal structures, as well as the piezoelectric properties of the thin films, was investigated. All prepared NFs mainly contained ß-phase crystals with a similar total crystallinity. The orientation of the ß-phase crystals inside the NFs increased with an increase in the fiber diameter, resulting in an improved transverse piezoelectric coefficient (d31) for the thin films. The output voltage of the prepared thin films reached a maximum of 2.7 V at 104 Hz.

High precision orientation mapping from 4D-STEM precession electron diffraction data through quantitative analysis of diffracted intensities.

The association of scanning transmission electron microscopy (STEM) and detection of a diffraction pattern at each probe position (so-called 4D-STEM) represents one of the most promising approaches to analyze structural properties of materials with nanometric resolution and low irradiation levels. This is widely used for texture analysis of materials using automated crystal orientation mapping (ACOM). Herein, we perform orientation mapping in InP nanowires exploiting precession electron diffraction (PED) patterns acquired by an axial CMOS camera. Crystal orientation is determined at each probe position by the quantitative analysis of diffracted intensities minimizing a residue comparing experiments and simulations in analogy to x-ray structural refinement. Our simulations are based on the two-beam dynamical diffraction approximation and yield a high angular precision (∼0.03°), much lower than the traditional ACOM based on pattern matching algorithms (∼1°). We anticipate that simultaneous exploration of both spot positions and high precision crystal misorientation will allow the exploration of the whole potentiality provided by PED-based 4D-STEM for the characterization of deformation fields in nanomaterials.

Regulating Crystal Orientation in VO2 for Aqueous Zinc Batteries with Enhanced Pseudocapacitance.

Although aqueous zinc batteries have attracted extensive interest, they are limited by relatively low rate capabilities and poor cyclic stability of cathodes. The crystal orientation of the cathode is one important factor influencing electrochemical properties. However, it has rarely been investigated. Herein, VO2 cathodes with different crystal orientations are developed via tuning the number of hydroxyl groups in polyol, such as using glycerol, erythritol, xylitol, or mannitol. The polyols serve as a reductant as well as a structure-directing agent through a hydrothermal reaction. Xylitol-derived VO2 shows a (110)-orientated crystalline structure and ultrathin nanosheet morphology. Such features greatly enhance the pseudocapacitance to 76.1% at a scan rate of 1.0 mV s-1, which is significantly larger than that (61.6%) of the (001)-oriented VO2 derived from glycerol. The corresponding aqueous zinc batteries exhibit a high energy storage performance with a reversible specific capacity of 317 mAh g-1 at 0.5 A g-1, rate ability of 220 mAh g-1 at 10 A g-1, and capacity retention of 81.0% at 10 A g-1 over 2000 cycles. This work demonstrates a facile method for tailoring VO2 crystal orientations, offers an understanding of the Zn2+ storage mechanism upon different VO2 facets, and provides a novel method to develop cathode materials toward advanced aqueous zinc batteries.