Crystal Grain Boundaries Research Articles

Tunable Plasmon Resonance in Silver Nanodisk-on-Mirror Structures and Scattering Enhancement by Annealing.

In this study, we evaluated the surface plasmon characteristics of periodic silver nanodisk structures fabricated on a dielectric thin-film spacer layer on a Ag mirror substrate (NanoDisk on Mirror: NDoM) through finite difference time domain (FDTD) simulations and experiments involving actual sample fabrication. Through FDTD simulations, it was confirmed that the NDoM structure exhibits two sharp peaks in the visible range, and by adjusting the thickness of the spacer layer and the size of the nanodisk structure, sharp peaks can be obtained across the entire visible range. Additionally, we fabricated the NDoM structure using electron beam lithography (EBL) and experimentally confirmed that the obtained peaks matched the simulation results. Furthermore, we discovered that applying annealing at an appropriate temperature to the fabricated structure enables the adjustment of the resonance peak wavelength and enhances the scattering intensity by approximately five times. This enhancement is believed to result from changes in the shape and size of the nanodisk structure, as well as a reduction in grain boundaries in the metal crystal due to annealing. These results have the potential to contribute to technological advancements in various application fields, such as optical sensing and emission enhancement.

Small molecule induced interfacial defect healing to construct inverted perovskite solar cells with high fill factor and stability

Chemical defects at the surface and grain boundaries of perovskite crystals cause deterioration of conversion efficiency and stability of perovskite solar cells (PSCs). In this study, a multifunctional additive, 5-fluoro-2-pyrimidine carbonitrile (FPDCN) molecule, is added into the perovskite precursor solution in order to passivate the uncoordinated Pb2+ by the cyanogen (–CN) group and pyrimidine N in FPDCN. Interestingly, fluorine (F) atoms interact with FA+ to form hydrogen bonds, which could regulate the perovskite crystallization process for the formation of high-quality perovskite crystals. Besides, the F atoms in FPDCN increase the water contact angle of perovskite films. As a result, the carrier extraction and transport in the perovskite film are significantly enhanced, and the non-radiative recombination is suppressed. The corresponding devices achieve a champion photovoltaic conversion efficiency (PCE) of 20.7 % and a fill factor (FF) of over 83 %. The device based on FPDCN shows long-term stability under a high-humidity atmospheric environment by maintaining 85 % of the initial efficiency after aging of 700 h in the glove box. This study provides a simple and convenient method to prepare stable and efficient PSCs by optimizing the perovskite precursor solution.

The mechanical anisotropy of Mg-Zn-Y alloys with columnar crystals in specific orientation

The Mg99.2Zn0.2Y0.6 alloys were prepared by directional solidification, and the effect of loading modes on mechanical anisotropy and deformation mechanism of the alloys was studied. The microstructure of the alloy is columnar crystals whose growth orientations are mainly concentrated in <112(_)0>. When the tensile load is parallel to longitudinal grain boundaries of columnar crystals, the alloy has high yield strength (232 MPa), owing to basal <a> slip with hard orientation and {101(_)1} contraction twinning with high critical resolved shear stress (CRSS), but work hardening is not obvious. When the compressive load is parallel to longitudinal grain boundaries of columnar crystals, the alloy has high compressive strength (337 MPa) and good plasticity (maximum strain, 24 %), as well as distinct work hardening, which are related to (112(_)1) [112(_)6(_)] twins and good strain compatibility of grain boundaries. When the compressive load is perpendicular to longitudinal grain boundaries, the alloy shows high plasticity (maximum strain, 31 %) because the strip-like extension twins are parallel to the longitudinal grain boundaries during deformation.

Memristor Effect in Layered Film Structures

Equivalent electrical circuits of multilayer film structures with memristor switching of resistance at interlayer boundaries and at the boundaries of crystal grains in each layer are proposed. Numerical modeling of the current-voltage characteristics of such structures has shown that their loop-shaped form, typical of memristors, is transformed into a linear ohmic dependence of the total current on the magnitude of the applied external voltage as both the number of layers and the number of grains in each layer increase. A certain combination of the number of layers and grains in a layer has been established, at which the maximum total current flowing through the structure and the ratio of resistances in the “off” and “on” states reach the highest values.

Thermodynamically Driven Tilt Grain Boundaries of Monolayer Crystals Using Catalytic Liquid Alloys.

We report a method to precisely control the atomic defects at grain boundaries (GBs) of monolayer MoS2 by vapor-liquid-solid (VLS) growth using sodium molybdate liquid alloys, which serve as growth catalysts to guide the formations of the thermodynamically most stable GB structure. The Mo-rich chemical environment of the alloys results in Mo-polar 5|7 defects with a yield exceeding 95%. The photoluminescence (PL) intensity of VLS-grown polycrystalline MoS2 films markedly exceeds that of the films, exhibiting abundant S 5|7 defects, which are kinetically driven by vapor-solid-solid growths. Density functional theory calculations indicate that the enhanced PL intensity is due to the suppression of nonradiative recombination of charged excitons with donor-type defects of adsorbed Na elements on S 5|7 defects. Catalytic liquid alloys can aid in determining a type of atomic defect even in various polycrystalline 2D films, which accordingly provides a technical clue to engineer their properties.

Bulk Incorporation with 4‐Methylphenethylammonium Chloride for Efficient and Stable Methylammonium‐Free Perovskite and Perovskite‐Silicon Tandem Solar Cells

AbstractMethylammonium (MA)‐free perovskite solar cells have the potential for better thermal stability than their MA‐containing counterparts. However, the efficiency of MA‐free perovskite solar cells lags behind due to inferior bulk quality. In this work, 4‐methylphenethylammonium chloride (4M‐PEACl) is added into a MA‐free perovskite precursor, which results in greatly enhanced bulk quality. The perovskite crystal grains are significantly enlarged, and defects are suppressed by a factor of four upon the incorporation of an optimal concentration of 4M‐PEACl. Quasi‐2D perovskites are formed and passivate defects at the grain boundaries of the perovskite crystals. Furthermore, the perovskite surface chemistry is modified, resulting in surface energies more favorable for hole extraction. This facile approach leads to a steady state efficiency of 23.7% (24.2% in reverse scan, 23.0% in forward scan) for MA‐free perovskite solar cells. The devices also show excellent light stability, retaining more than 93% of the initial efficiency after 1000 h of constant illumination in a nitrogen environment. In addition, a four‐terminal mechanically stacked perovskite‐silicon tandem solar cell with champion efficiency of 30.3% is obtained using this MA‐free composition. The encapsulated tandem devices show excellent operational stability, retaining more than 98% of the initial performance after 42 day/night cycles in an ambient atmosphere.

Partially‐Saturated Brines Within Basal Ice or Sediments Can Explain the Bright Basal Reflections in the South Polar Layered Deposits

AbstractStrong radar reflections have been previously mapped at the base of the Martian South Polar Layered Deposits. Here, we analyze laboratory measurements of dry and briny samples to determine the cause of this radar return. We find that liquid vein networks consisting of brines at the grain boundaries of ice crystals can greatly enhance the electrical conductivity, thereby causing strong radar reflections. A brine concentration of 2.7–6.0 vol% in ice is sufficient to match the electrical properties of the basal reflection as observed by Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS). When brine is mixed with sediments, the brine‐ice mixture in the pores must be 2–5 times more concentrated in salt, increasing the brine concentration to 6.3–29 vol%. Our best fit of the median observed MARSIS value suggests a salt‐bulk sample concentration of ∼6 wt%. Thus, salt enhancement mechanisms on the order of a magnitude greater than the Phoenix landing site are needed. To form brine, the basal reflector must reach a temperature greater than the eutectic temperature of calcium perchlorate of 197.3 ± 0.2 K, which may be possible if more complex thermal modeling is assumed. Colder metastable brines are possible, but stability over millions of years remains unclear. Conversely, gray hematite with a concentration of 33.2–59.0 vol% possess electrical properties that could cause the observed radar returns, but require concentrations 2–3 times larger than anywhere currently detected. We also argue that brines mixed with high‐surface‐area sediments, or dry red hematite, jarosite, and ilmenite cannot create the observed radar returns at low temperatures.

Improved performance with boron-doped carbon quantum dots in perovskite solar cells

This study reports the using of boron-doped carbon quantum dots (B-CQDs), which can convert ultraviolet to blue light, as an additive in MAPbI3 based perovskite solar cells (PSCs) for improved photovoltaic performance. After a successful facile synthesis of B-CQDs, our work showed that the appropriate addition of B-CQDs (5% vol.) into MAPbI3 absorber layer can passivate the grain boundaries of perovskite crystals, improve the crystallinity and the grain sizes. Additionally, optical, and photovoltaic characteristics showed that the addition of B-CQDs can efficiently improve the charge transport properties and the hole extraction, while decreasing the recombination losses. In comparison to control device, power conversion efficiency (PCE) of champion device was increased from 10.4% to 12.7%. This approach of constructing inverted p-i-n PSCs with B-CQDs additives offers a simple way for the preparation of efficient and stable perovskite photovoltaics for future application.

Correlation of crystal orientation, crystal morphology and mechanical properties of directionally solidified Mg-xGd alloys

The Mg-xGd (x = 4–16, wt%) alloys with columnar crystals were prepared by directional solidification, and the effect of Gd content on the crystal orientation, microstructure, and tensile properties at room temperature were studied. The microstructure of the Mg-4Gd alloy is single α-Mg solid solution, and the microstructure of the Mg-6Gd alloy is composed of α-Mg and Mg24Gd distributed along the longitudinal grain boundaries of columnar crystals, while the microstructure of the Mg-(8−16)Gd alloys consists of α-Mg and lamellar eutectic phase. The Mg-4Gd alloy with growth orientation concentrated in< 112(_)0 > (hard orientation) has high yield strength (186 MPa) and moderate elongation (12 %), because {101(_)1} compression twinning are activated to coordinate strain. However, the Mg-6Gd alloy with growth orientation concentrated in<1(_)21(_)2 > (soft orientation) has excellent elongation (38 %), as a result of activated {101(_)2} tension twinning. When the Gd content exceeds 12 %, plenty of eutectic phases will cause low strength and poor plasticity of the Mg-Gd alloys.

Energy Transfer Assisted Fast X-ray Detection in Direct/Indirect Hybrid Perovskite Wafer.

Metal halide perovskite scintillators encounter unprecedented opportunities in indirect ionizing radiation detection due to their high quantum yields. However, the long scintillation lifetime of microseconds upon irradiation, known as the afterglow phenomenon, obviously limits their fast development. Here, a new type of hybrid X‐ray detector wafer combining direct methylamine lead iodide (MAPbI3) semiconductor and indirect zero‐dimensional cesium copper iodide (Cs3Cu2I5) scintillator through low‐cost fast tableting processes is reported. Due to the fast energy transfer from Cs3Cu2I5 to MAPbI3, the device response time to X‐rays is dramatically reduced by nearly 30 times to 36.6 ns, which enables fast X‐ray detection capability by a large area detector arrays within 1 s. Moreover, Cs3Cu2I5 exists at the grain boundaries of MAPbI3 crystals, and blocks the paths of mobile ions of perovskite, leading to the lowest detectable dose rate of hybrid X‐ray detector is thus reduced by 1.5 times compared with control MAPbI3 direct‐type semiconductor, and 10 times compared with the Cs3Cu2I5 indirect‐type scintillator. The direct/indirect hybrid wafer also exhibits improved operation stability at ambient conditions without any encapsulation. This new kind of hybrid X‐ray detectors provides strong competitiveness by combining the advantages of both direct perovskite semiconductors and indirect perovskite scintillators for next‐generation products.

Bernal Boron Nitride Crystals Identified by Deep-Ultraviolet Cryomicroscopy.

The presence of metastable Bernal stacking boron nitride is verified by combining second harmonic generation (SHG) and photoluminescence (PL) spectroscopy. The scanning confocal cryomicroscope, operating in the deep-ultraviolet range, shows a one-to-one correlation between inversion symmetry breaking probed by SHG and the detection of an intense PL line at ∼6.035 eV, the specific signature of the noncentrosymmetric Bernal stacking. The coherent character of the Bernal phase in boron nitride crystals is demonstrated by two-photon excitation spectroscopy. Direct and indirect excitons are simultaneously detected in the emission spectrum; they are quasi-degenerate, in agreement with theoretical predictions for Bernal boron nitride. The transition from AA' to AB stacking is characterized by an intense emission from stacking faults at the grain boundaries of hexagonal and Bernal boron nitride crystals.

Liberating photocarriers in mesoporous single-crystalline SrTaO2N for efficient solar water splitting

Porous single-crystalline SrTaO2N, combining structural homogeneity and high porosity, afford a great promise as an active photocatalyst. The lack of grain boundaries in SrTaO2N porous single crystals (PSCs) enables fast photocarrier migration from bulk to the surface whilst the high porosity offers adequate accessible surface to perform photocatalytic reactions. In this work, we demonstrate a facile synthesis of SrTaO2N PSCs via a topotactic conversion route. These SrTaO2N PSCs deliver exceptional activity and stability for photocatalytic O2 production from water with a record-breaking apparent quantum efficiency as high as 17.9% at 420 ± 20 nm. Overall water splitting with H2/O2 equals 2 has also been attained in a Z-scheme system employing SrTaO2N PSCs as the O2-evolution moiety. These results signify a paradigm to improve photocatalytic performance based on PSCs, which extends the toolbox to achieve efficient solar water splitting over conventional photocatalysts.

Perovskite Single-Crystal Thin Film Devices Using Lithography Assisted Epitaxy

Strain engineering is an emerging technique to physically manipulate and enhance semiconductor device performance. Chen et al. recently combined strain engineering with halide perovskites, demonstrating a novel approach toward improvement in optoelectronic properties.

Pile-up transmission and reflection of topological defects at grain boundaries in colloidal crystals

Crystalline solids typically contain large amounts of defects such as dislocations and interstitials. How they travel across grain boundaries (GBs) under external stress is crucial to understand the mechanical properties of polycrystalline materials. Here, we experimentally and theoretically investigate with single-particle resolution how the atomic structure of GBs affects the dynamics of interstitial defects driven across monolayer colloidal polycrystals. Owing to the complex inherent GB structure, we observe a rich dynamical behavior of defects near GBs. Below a critical driving force defects cannot cross GBs, resulting in their accumulation near these locations. Under certain conditions, defects are reflected at GBs, leading to their enrichment at specific regions within polycrystals. The channeling of defects within samples of specifically-designed GB structures opens up the possibility to design novel materials that are able to confine the spread of damage to certain regions.

Cryo-quenched Fe–Ni–Cr alloy decorative steel single crystals II: Alloy phases, structure, hardness, tensile, tribological, magnetic and electronic properties

Abstract We have previously reported (Boatner et al., J. Alloys Compd. 691 (2017) 666–671) on the discovery, formation, processing, and application of a new decorative ternary steel. alloy and on its use in a wide range of practical applications including: custom and commercial knives, art objects, jewelry, electronics, and furniture. This decorative property results when polished single crystals of the 70 wt%Fe-15 wt%Ni-15 wt% Cr austenitic alloy are cryo-quenched into the martensitic phase resulting in the formation of a three-dimensional raised pattern of mixed austenitic/martensitic laths. These laths propagate across large dimensions of the material due to the absence of grain boundaries in the single crystal. The macroscopic optically reflective 3-D decorative pattern reproduces the structural symmetry inherent in the single-crystal orientation. This pattern results solely from the metallurgical phase properties of the ternary alloy and is completely distinct from the properties of Damascus steels or more modern so-called pattern-welded steels. Here we use X-ray diffraction, nanoindentation, hardness measurements/scratch testing, tribological measurements, and alloy resistivity and magnetization methods to obtain a more fundamental and comprehensive study of the effects of the austenitic/martensitic phase transition on the structural and physical properties of the 70 wt%Fe-15 wt%Ni-15 wt% Cr alloy. These results reveal new insight into irreversible phenomena that are associated with the inhibited phase transition on heating of the two-phase, mixed austenitic/martensitic structure that is formed subsequent to cryo-quenching the alloy crystal to 77 K.

Novel approach for clean utilization of complex low-grade metal resources using silicon as metal getter

A method to utilize complex low-grade metal resources cleanly is necessary for the sustainable development of resources and the environment. Si is a widely consumed material in the semiconductor and photovoltaic industries; however, its purity must be upgraded for specific uses. In this study, inexpensive low-purity Si (∼99.32%) was employed as a metal getter to clean accumulated low-grade metal resources by extracting valuable elements from the low-grade resources and to make the manufacture of high-purity Si (∼99.99%) possible. Two common low-grade metal resources were employed in the study as examples: 1) a Ti-bearing blast furnace slag containing Ti and V and 2) a spent automotive catalyst containing Zr, rare-earth elements (Ce, Nd, and La) and platinum group metals (Pt, Pd, and Rh). First, the valuable elements were extracted from the low-grade metal resources using Si to prepare the Si–valuable-element alloy. The extraction process released no carbon gas, because Si acted as both a reductant and a metal getter. Then, the extracted valuable elements were observed to concentrate at the grain boundaries of Si crystals during the solidification process. Finally, the concentrated valuable elements were completely discharged into lixiviants with a minor loss of the Si matrix, i.e., the concentrated valuable elements could be completely separated from the Si matrix through acid leaching. The purity of the Si matrix was significantly enhanced after acid leaching, and high-purity Si was prepared. The high-purity Si, an important high-value byproduct, can be utilized in the semiconductor and photovoltaic industries after further purification or recycled again as a metal getter to treat low-grade metal resources repetitively. This new approach is a promising method for the large-scale treatment of other complex low-grade metal resources.

In situ fabrication of hierarchical iron oxide spheres@N-doped 3D porous graphene aerogel for superior lithium storage

To improve the electronic conductivity and dramatic volume change of iron oxide as anode for lithium-ion batteries, we finally fabricate a hierarchical aerogel composite of FeOx spheres encapsulated in porous N-doped three-dimensional graphene (FeOx@N-3DG) via a facile in situ electrostatic self-assembly strategy assisted with hydrothermal. In the hierarchical architecture, the distributed FeOx spheres that consist of Fe2O3 and Fe3O4 crystals are intimately wrapped by N-3DG acting as protective elastomer to buffer the volume change during cycling. Moreover, the integrally interconnected porous structure is beneficial for fast electron transfer and penetration of the electrolyte. More importantly, the abundant grain boundaries of Fe2O3 and Fe3O4 crystals inside FeOx spheres provide smooth channels for fast lithium-ion transport and increase lithium storage sites. Therefore, FeOx@N-3DG anode material exhibits high specific capacity (1100 mAh g−1 after 200 cycles at 0.5 A g−1), excellent rate performance (550 mAh g−1 at 5 A g−1) and outstanding long cycling stability (81.6% of capacity retention after 1000 cycles at 1 A g−1), which is attributed to the superior surface capacitance behavior in the hierarchical architecture during charging and discharging. Thus, the hierarchical FeOx@N-3DG aerogel has potential to be an advanced anode material for lithium-ion batteries.

Sculpted grain boundaries in soft crystals.

Engineering the grain boundaries of crystalline materials represents an enduring challenge, particularly in the case of soft materials. Grain boundaries, however, can provide preferential sites for chemical reactions, adsorption processes, nucleation of phase transitions, and mechanical transformations. In this work, "soft heteroepitaxy" is used to exert precise control over the lattice orientation of three-dimensional liquid crystalline soft crystals, thereby granting the ability to sculpt the grain boundaries between them. Since these soft crystals are liquid-like in nature, the heteroepitaxy approach introduced here provides a clear strategy to accurately mold liquid-liquid interfaces in structured liquids with a hitherto unavailable level of precision.

On the study of microstructure of annealed C+and H2+ Co_implanted 6H-SiC

The microstructure of annealed single crystal 6H-SiC implanted with C+ and H2+ ions were characterized by glancing incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). After annealing, cracks were observed to form in the surface of C+ ions implanted sample. However, blisters as circular clusters were observed in the surface of the H2+ ions implanted and annealed sample. Compared with the samples implanted with only single type of ions, both cracks and blisters with different morphologies were found to appear in the surface of C+ and H2+ ions co-implanted sample. The morphology and distribution of blisters and exfoliations in the C+ and H2+ co-implanted samples were more irregular and inhomogeneous than those in the H2+ ions implanted sample, which can be attributed to the randomly distributed columnar crystals formed during the annealing process. The grain boundaries of these columnar crystals could be used as migration channels for hydrogen, which influence the size evolution of blisters and exfoliations in the sample surface during annealing process.

Atomically Sharp Dual Grain Boundaries in 2D WS2 Bilayers.

It is shown that tilt grain boundaries (GBs) in bilayer 2D crystals of the transition metal dichalcogenide WS2 can be atomically sharp, where top and bottom layer GBs are located within sub-nanometer distances of each other. This expands the current knowledge of GBs in 2D bilayer crystals, beyond the established large overlapping GB types typically formed in chemical vapor deposition growth, to now include atomically sharp dual bilayer GBs. By using atomic-resolution annular dark-field scanning transmission electron microscopy (ADF-STEM) imaging, different atomic structures in the dual GBs are distinguished considering bilayers with a 3R (AB stacking)/2H (AA' stacking) interface as well as bilayers with 2H/2H boundaries. An in situ heating holder is used in ADF-STEM and the GBs are stable to at least 800 °C, with negligible thermally induced reconstructions observed. Normal dislocation cores are seen in one WS2 layer, but the second WS2 layer has different dislocation structures not seen in freestanding monolayers, which have metal-rich clusters to accommodate the stacking mismatch of the 2H:3R interface. These results reveal the competition between maintaining van der Waals bilayer stacking uniformity and dislocation cores required to stitch tilted bilayer GBs together.