Stable Superlattices Research Articles

Regulating Interfacial Compositions to Build a Stable Superlattice Structure of Layered Oxide Cathode Materials for Sodium-Ion Batteries

Regulating Interfacial Compositions to Build a Stable Superlattice Structure of Layered Oxide Cathode Materials for Sodium-Ion Batteries

Hydrogen storage and capacity degradation mechanism of superlattice La-Y-Ni-based hydrogen storage alloy

New La-Y-Ni-based hydrogen storage alloys exhibit key technological advantages for energy storage; however, several challenges arise because of their insufficient cycle life. In this study, we design a non-stoichiometric structure and present a scheme for obtaining a stable superlattice structure by optimizing multi-component elements and understanding the structural evolution and anti-degradation mechanism for increasing the cycle life. The rationality of the scheme is verified by designing AB3.67-type La1.4Ce0.5Y4.1-xSmxNi20Mn1.2Al0.8 (x = 0, 0.2, 0.3, and 0.4) alloys. Substituting Y with an appropriate amount of Sm reduces capacity degradationin both in electrochemistry and solid/H2 reactions because it effectively inhibits the transition of the stable phase and this regulation reduces the mismatch of subunits in the Gd2Co7-type phase, especially the control [A2B4] subunit instead of the [AB5] subunit. Further, a capacity degradation mechanism is illustrated and the entire process is divided into three stages. The corrosion resistance of synergistic elements in the first stage (<100 cycles) of the electrochemical reaction is a decisive factor controlling the long-term cyclic stability of alloys. Although the amorphous phase in the solid-H2 reaction is the main factor responsible for decay, the La1.4Ce0.5Y3.8Sm0.3Ni20Mn1.2Al0.8 alloy shows better electrochemical properties, with a maximum discharge capacity, H2-absorption capacity, and capacity retention rate S100 of 377.7 mAh g−1, 1.64 wt%, and 90.9 %, respectively. After 500 cycles, the capacity retention rate is enhanced from 42 % to 60.9 %. The proposed scheme is expected to prom.

Bi2W2O9: A potentially antiferroelectric Aurivillius phase

Ferroelectric tungsten-based Aurivillius oxides are naturally stable superlattice structures, in which A-site deficient perovskite blocks [WnO3n+1]-2 (n = 1, 2, 3, ...) interleave with fluorite-like bismuth oxide layers [Bi2O2]+2 along the c -axis. In the n = 2 Bi2W2O9 phase, an in-plane antipolar distortion dominates but there has been controversy as to the ground state symmetry. Here we show, using a combination of first-principles density functional theory calculations and experiments, that the ground state is a non-polar phase of Pnab symmetry. We explore the energetics of metastable phases and the potential for antiferroelectricity in this n=2 Aurivillius phase.

On the Role of Superlattice Structures with Ni3X-type Composition in Selected Ni-Based Superalloys with Commercial Grades

Close-packed superlattice derivatives of the face-centered cubic (fcc) structure with Ni3X-type composition in some commercial Ni-based superalloys have been characterized, and their effect on tensile strength has been determined. Both the L12 and DO22 superlattices can form directly from the fcc structure by minor atoms rearrangement. However, the two superlattices can act as transient metastable phases which transform into more stable superlattices (L12 → DO24 and DO22 → D0a) with extended thermal exposure at elevated temperatures. It is shown that the stability and morphology of each superlattice are dependent upon the alloy composition. Although some superlattices produce adverse effects on tensile strength, others are found to have beneficial effects. The exact effect is correlated with the particular morphology and deformation mode. It is concluded that the DO22 superlattice can form the basis for designing alloys deformable by twinning and combine high strength and high ductility.

Irradiation-induced amorphization in the zirconium suboxide on Zr-0.5Nb alloys

We report for the first time the observation of irradiation-induced amorphization of the zirconium suboxide formed during aqueous corrosion of Zr-0.5Nb alloys. High-resolution transmission electron microscopy results reveal amorphization of the hexagonal-ZrO suboxide under heavy ion irradiation at cryogenic temperatures. This irradiation-induced amorphization behaviour is discussed in relation to the arrangement of oxygen interstitials and the formation of stable superlattices. The sensitivity of the suboxide to irradiation damage can lead to phase changes and the accumulation of defects near the oxide/metal interface, which needs to be taken into account in the development of mechanistic models addressing radiation-assisted acceleration of corrosion rates in zirconium alloys.

Monolayer atomic crystal molecular superlattices.

Artificial superlattices, based on van der Waals heterostructures of two-dimensional atomic crystals such as graphene or molybdenum disulfide, offer technological opportunities beyond the reach of existing materials. Typical strategies for creating such artificial superlattices rely on arduous layer-by-layer exfoliation and restacking, with limited yield and reproducibility. The bottom-up approach of using chemical-vapour deposition produces high-quality heterostructures but becomes increasingly difficult for high-order superlattices. The intercalation of selected two-dimensional atomic crystals with alkali metal ions offers an alternative way to superlattice structures, but these usually have poor stability and seriously altered electronic properties. Here we report an electrochemical molecular intercalation approach to a new class of stable superlattices in which monolayer atomic crystals alternate with molecular layers. Using black phosphorus as a model system, we show that intercalation with cetyl-trimethylammonium bromide produces monolayer phosphorene molecular superlattices in which the interlayer distance is more than double that in black phosphorus, effectively isolating the phosphorene monolayers. Electrical transport studies of transistors fabricated from the monolayer phosphorene molecular superlattice show an on/off current ratio exceeding 107, along with excellent mobility and superior stability. We further show that several different two-dimensional atomic crystals, such as molybdenum disulfide and tungsten diselenide, can be intercalated with quaternary ammonium molecules of varying sizes and symmetries to produce a broad class of superlattices with tailored molecular structures, interlayer distances, phase compositions, electronic and optical properties. These studies define a versatile material platform for fundamental studies and potential technological applications.

Free-Standing Metal Oxide Nanoparticle Superlattices Constructed with Engineered Protein Containers Show in Crystallo Catalytic Activity.

The construction of defined nanostructured catalysts is challenging. In previous work, we established a strategy to assemble binary nanoparticle superlattices with oppositely charged protein containers as building blocks. Here, we show that these free-standing nanoparticle superlattices are catalytically active. The metal oxide nanoparticles inside the protein scaffold are accessible for a range of substrates and show oxidase-like and peroxidase-like activity. The stable superlattices can be reused for several reaction cycles. In contrast to bulk nanoparticle-based catalysts, which are prone to aggregation and difficult to characterize, nanoparticle superlattices based on engineered protein containers provide an innovative synthetic route to structurally defined heterogeneous catalysts with control over nanoparticle size and composition.

Ordering and Dynamics for the Formation of Two-Dimensional Molecular Crystals on Black Phosphorene

Phosphorene, a monolayer of elemental phosphorus, has emerged as the most important 2D atomic crystal post graphene. As a consequence of its suitable direct band gap, it has high carrier mobility, thereby making it suitable for potential applications in nanoelectronic devices. In this article, we have performed molecular dynamics simulations for the first time to unveil the potential of black phosphorene (BP) as a platform to fabricate two-dimensional crystals through van der Waals epitaxy of organic molecules. Eight molecules (benzene, 1,3,5-trimethylbenzene, 1,3,5-trifluorobenzene, 1,3,5-trihydroxybenzene, 7,7′,8,8′-tetracyanoquinodimethane (TCNQ), pentacene, coronene, and fullerene) of various size, shape, and polarity are shown to form highly periodic 2D monolayers on BP. We show that self-assembly proceeds through a hierarchy of steps, beginning with adsorption on the surface followed by interplay between the kinetic energy and intermolecular interactions. Crystalline 2D layers are obtained through the minimization of van der Waals interactions for nonpolar and electrostatic interactions for polar molecules. Long-range attractive electrostatic interactions contribute significantly toward the stabilities of the superlattices, thereby making the melting points of polar crystals much higher compared to their nonpolar analogs (Tm (polar) > Tm (nonpolar)). Interestingly, although nonpolar fullerene is shown to form highly stable superlattices due to extensive intermolecular van der Waals interactions, the underlying BP monolayer helps to orient the molecules and, hence, indirectly assists molecular ordering. It also contributes to the overall stability of the molecular crystals through interfacial adsorption. A comparison of self-assembly patterns over BP vis-a-vis graphene and hexagonal boron nitride shows that the nonaromatic nature and undulation of black phosphorene do not affect the intrinsic stability of the assemblies. These findings offer new avenues for generating new molecular doped crystalline superlattices for semiconductor industry.

Dynamic tuning of DNA-nanoparticle superlattices by molecular intercalation of double helix.

Nanoparticle (NP) assembly using DNA recognition has emerged as a powerful tool for the fabrication of 3D superlattices. In addition to the vast structural diversity, this approach provides an avenue for dynamic 3D NP assembly, which is promising for the modulation of interparticle distances and, hence, for example, for in situ tuning of optical properties. While several approaches have been explored for changing NP separations in the lattices using responsiveness of single-stranded DNA (ss-DNA), far less work has been done for the manipulation of most abundant double-stranded DNA (ds-DNA) motifs. Here, we present a novel strategy for modulation of interparticle distances in DNA linked 3D self-assembled NP lattices by molecular intercalator. We utilize ethidium bromide (EtBr) as a model intercalator to demonstrate selective and isotropic lattice expansion for three superlattice types (bcc, fcc, and AlB2) due to the intercalation of ds-DNA linking NPs. We further show the reversibility of the lattice parameter using n-butanol as a retrieving agent as well as an increased lattice thermal stability by 12-14 °C due to the inclusion of EtBr. The proposed intercalator-based strategy permits the creation of reconfigurable and thermally stable superlattices, which could lead to tunable and functionally responsive materials.

InGaAs/GaAsP superlattice solar cells with reduced carbon impurity grown by low-temperature metal-organic vapor phase epitaxy using triethylgallium

In this paper, we investigated the effects of carbon incorporation on photovoltaic performance of InGaAs/GaAsP superlattice (SL) solar cells grown by low-temperature MOVPE (LT-MOVPE), which is required for stable SL growth on vicinal substrates. Using trimethylgallium (TMGa) as the gallium precursor, methyl radicals formed by its pyrolysis tend to be absorbed on the surface at low temperature, causing severe carbon incorporation and p-type background doping. High background carrier concentration flattens the band-lineup of the intrinsic region and blocks the carrier transport across the SLs, and resulted in serious degradation of photocurrent. Intentional sulfur doping to cancel out the background doping and hence to recover the built-in field greatly improved the cell performance, but was found to require very precise control of doping level to achieve an exact compensation doping condition. Use of triethylgallium (TEGa) instead of TMGa much reduced the carbon incorporation at low temperature and significantly enhanced the photocurrent extraction without sulfur doping treatment. By thinning GaAsP barriers to 3 nm to facilitate efficient tunneling transport, a 50-period SL cell with bandgap of 1.22 eV grown on 6°-miscut substrates achieved 1.13 times higher efficiency with 31% current enhancement as middle cell performance than a GaAs reference cell.

Mechanical properties of ground state structures in substitutional ordered alloys: High strength, high ductility and high thermal stability

We have studied the effect of atom arrangements in the ground state structures of substitutional ordered alloys on their mechanical properties using nickel–molybdenum-based alloys as model systems. Three alloys with nominal compositions of Ni–19.43at% Mo, Ni–18.53at% Mo–15.21at% Cr and Ni–18.72at% Mo–6.14at% Nb are included in the study. In agreement with theoretical predictions, the closely related Pt2Mo-type, DO22 and D1a superlattices with similar energies are identified by electron diffraction of ground state structures, which can directly be derived from the parent disordered fcc structure by minor atom rearrangements on {420}fcc planes. The three superlattices are observed to coexist during the disorder–order transformation at 700°C with the most stable superlattice being determined by the exact chemical composition. Although most of the slip systems in the parent disordered fcc structure are suppressed, many of the twinning systems remain operative in the superlattices favoring deformation by twinning, which leads to considerable strengthening while maintaining high ductility levels. Both the Pt2Mo-type and DO22 superlattices are distinguished by high strength and high ductility due to their nanoscale microstructures, which have high thermal stability. However, the D1a superlattice is found to exhibit poor thermal stability leading to considerable loss of ductility, which has been correlated with self-induced recrystallization by migration of grain boundaries.

Electronic Properties of Graphene/h-BN Bilayer Superlattices

We study the electronic properties of superlattices composed of a graphene and hexagonal boron nitride ( h -BN) bilayer in the framework of density functional theory. Depending on the stacking sequences of superlattices, various interesting electronic properties are observed. The stable superlattice with the shortest stacking period is found to be a narrow-gap semiconductor with a very small effective mass. On the other hand, almost linear dispersion relations should appear around the K point in a stable superlattice with a longer stacking period. These almost linear bands are found to split owing to weak but finite inter-graphene interactions mediated by h -BN bilayers.

Self-organization of carbide superlattice and nucleation of carbon nanotubes.

In metal-carbon systems with known stable compounds, carbide nanocrystals self-organize epitaxially on metal surfaces to form two-dimensional arrays during carbon deposition. The process is energetically driven by the competition between the strain and surface energies, and it appears to play an important role in the nucleation of single-walled carbon nanotubes. Interplay between energetics and kinetics controls carbon precipitation from the superlattice, such that the length scale of the carbide and superlattice appears to control the size and morphology of the precipitates. Furthermore, carbon precipitates appear to be "seedlings" of carbon nanotubes grown on top of the carbide nanocrystals. These findings reveal that the nucleation of carbon nanotubes is a nonequilibrium process and that a stable carbide superlattice can be used as an ordered template of carbon saturated "roots" for nucleating nanotube bundles with controlled diameter, spacing, and perhaps chirality.

The Synthesis of Metastable Skutterudites and Crystalline Superlattices

ABSTRACTWe have used controlled crystallization of elementally modulated reactants to prepare a series of kinetically stable, crystalline skutterudites (M'xM4Sb12 where M' = vacancy, RE, Hf,…; M = Ni, Fe, Co) and crystalline superlattices composed of promising thermoelectric materials. For the bulk synthesis of skutterudites, low angle diffraction data demonstrates that the elemental layers interdiffuse at temperatures below 150°C. Nucleation of the skutterudite structure occurs with at large exotherm on annealing at temperatures below 200°C regardless of the ternary metal. All of the metastable ternary compounds and the new metastable binary compounds were found to decompose exothermically on higher temperature annealing. The decomposition temperature ranged from 250°C for the binary compound NiSb3 to above 550°C for the rare earth containing cobalt compounds. The occupation of the ternary site was found to depend on the composition of the initial reactant and was varied from 0 to 1. Full occupancy typically required an excess of the filling cation. The lattice parameters of the compounds prepared at low temperatures are distinctly smaller than those prepared using traditional synthetic approaches. High temperature annealing converts the lattice parameters of the low temperature compounds to those prepared at higher temperatures using traditional synthetic approaches. Diffraction patterns of crystalline superlattices containing skutterudites prepared using elementally modulated reactants show splitting of high angle diffraction maxima as well as the presence of the expected low angle diffraction pattern from a supperlattice. The skutterudite superlattices are stable with respect to low temperature annealing.

DETERMINATION OF ORDERED STRUCTURES IN FACE-CENTRED CUBE INTERSTITIAL SOLUTIONS

The probability wave theory of atomic configuration is employed to study the most stable superlattices formed in fcc interstitial solutions. Eight basic ordered structures are deduced for octahedral interstices and seven basic ordered structures are deduced for tetragonal interstices.

Ground-state structures in ternary III-V semiconductor alloys

The probability wave of the atomic configuration is used to study the ordered structures of ternary III-V semiconductor alloys. The atomic configuration of the most stable superlattices formed in the ternary III-V semiconductor alloys is discussed. Some features of the experiments are explained.

Probability wave theory of the atomic configuration for multicomponent crystal structures and its application to the ordered structure of complex perovskite materials

The probability wave of the atomic configuration is used to study the occupation probabilities of different kinds of atoms at lattice sites in multicomponent solid solutions and predict the crystal structures of multicomponent solid solutions. The theory of the probability wave of atomic configuration is applied to study the atomic configuration of the most stable superlattices formed in the complex perovskite structure. Seven basic ordered structures are deduced theoretically; these have been compared with the experiment results.

Structural ability and electronic density of states in (001)- and (111)-oriented (GaP)1/(InP)1 strained-layer superlattices

The stability of (001)- and (111)-oriented (GaP)1/(InP)1 strained-layer superlattices (SLSS) is studied by the Keating model. The obtained deviations of the In-P and Ga-P bond lengths of the (001) stable superlattice structure from the constituent bulk lengths are -1.0% and 0.8%, respectively. A recursion method is used to calculate the bulk and surface electronic structures for both (001) and (111) (GaP)1/(InP)1 SLSS. It is found that the band gaps of the (001) and (111) structures, respectively, are smaller by 0.28 eV and 0.31 eV than the average of those of bulk InP and GaP. From the total-structural-energy and Fermi level calculation, the authors conclude that the strained-monolayer superlattice growth along the (001) direction is more stable than that along the (111) direction. A qualitative trend is proposed to elucidate the influence of strain on the electronic occupation in a strained-layer superlattice fabricated from III-V semiconductors, with the aid of two auxiliary systems. The localized states of a Si impurity in these systems are calculated.

Effect of strain on the electronic structures of ultrathin layer GaP/InP (111) superlattice: Bulk and surface

We have used the recursion method to calculate bulk and surface electronic structures of a (GaP) 1/(InP) 1 (111) strained-layer superlattice. The obtained energy gap for the stable superlattice is 1.88eV, which is smaller by 0.31eV than the average of the gaps of bulk GaP (2.91eV) and InP (1.48eV). We show how the fundamental gap and the electron occupation can be varied by changing the bond strain in these systems.

Interface and growth studies of α-Sn/CdTe(110) superlattices

Angular-resolved synchrotron radiation photoemission spectroscopy and reflection high-energy electron diffraction (RHEED) are used to study the molecular-beam epitaxy of CdTe/ α-Sn(110) interfaces and superlattices. Core level photoemission spectra indicate that both sides of the interface are stable, nonreactive, and abrupt for growth temperatures up to 100 °C. At the α-Sn/CdTe interface, the valence band maximum at Γ is at EV=1.1 eV below the Fermi level. This gives a valence band offset of ΔEV=1.1 eV, assuming zero band gap for the Sn. Stable superlattices of α-Sn/CdTe(110) have been grown at 100 °C. The surface quality of the superstructure degrades after the growth of several α-Sn/CdTe periods. After the growth of ten periods each 50-Å thick, the RHEED pattern shows mainly three-dimensional bulk diffraction, indicating increased surface, and interface roughness.