Difference In Lattice Constants Research Articles

Atomic controlled shell thickness on Pt@Pt3Ti core-shell nanoparticles for efficient and durable oxygen reduction

The exploitation of durable and highly active Pt-based electrocatalysts for the oxygen reduction reaction (ORR) is essential for the commercialization of proton exchange membrane fuel cells (PEMFCs). Herein, we designed Pt@Pt3Ti core-shell nanoparticles with atomic-controllable shells through precise thermal diffusing Ti into Pt nanoparticles for effective and durable ORR. Combining theoretical and experiment analysis, we found that the lattice strain of Pt3Ti shells can be tailored by precisely controlling the thickness of Pt3Ti shell in atomic-scale on account of the lattice constant difference between Pt and Pt3Ti to optimize adsorption properties of Pt3Ti for ORR intermediates, thus enhancing its performance. The Pt@Pt3Ti catalyst with one-atomic Pt3Ti shell (Pt@1L-Pt3Ti/TiO2-C) demonstrates excellent performance with mass activity of 592 mA mgPt−1and durability nearly 19.5-fold that of commercial Pt/C with negligible decay (2%) after 30,000 potential cycles (0.6-1.0 V vs. RHE). Notably, at higher potential cycles (1.0 V-1.5 V vs. RHE), Pt@1L-Pt3Ti/TiO2-C also showed far superior durability than Pt/C (9.6% decayed while 54.8% for commercial Pt/C). This excellent stability is derived from the intrinsic stability of Pt3Ti alloy and the confinement effect of TiO2-C. The catalyst's enhancement was further confirmed in PEMFC configuration.

A comparative DFT study of MgFe2O4 and MnFe2O4 spinel ferrites at various pressures to investigate the structural, mechanical, electronic, magnetic and optical properties for multifunctional applications

The structural, electronic, mechanical, optical, and magnetic properties of the MgFe2O4 and MnFe2O4 ferrites were investigated under pressures ranging from 0 to 20 GPa using the DFT-based CASTEP Code. XRD analyses indicated differences in lattice constants, volumes and d-spacing between these ferrites, with decreasing trends under applied pressure. The MgFe2O4 ferrite has lower X-ray density, but higher electronic bandgap energy compared to the MnFe2O4 ferrite, both showing semiconductor behavior. Magnetic properties show a decrease in total magnetic moment and saturation magnetization under applied pressure for both ferrites, making them potential materials for transformer and inductor cores. Mechanical properties confirm their stability, elastic behavior, ductility and ionic bond nature, suitable for electrochemical devices. The MgFe2O4 ferrite exhibits dominant absorption and optical conductivity in the visible region compared to the MnFe2O4 ferrite, making it a favorable choice for optoelectronic and photocatalytic applications.

Cross-interface growth mechanism of nanotwins in extremely high stacking-fault energy ceramic layer

Twins have been proven to greatly enhance the hardness, toughness, thermal stability, and wear resistance of materials. However, the formation of twins in transition nitride ceramics is exceptionally difficult due to their prohibitively high stacking-fault energy. Consequently, reports on nanotwinned ceramic films have been limited in scope. In this study, we utilize the template effect of multilayer structures to induce the growth of nanotwins within the high stacking-fault energy TiN and AlN layers. Theoretical analysis reveals that the growth of nanotwins across the interface is facilitated by the template effect and large internal stress. The former fosters the epitaxial growth of nanotwinned two-sided structures into the TiN and AlN layers. Simultaneously, the pronounced internal stress engenders substantial lateral stress in the TiN and AlN layers immediately above the underlying nanotwin region, ultimately leading to the formation of nanotwins. In addition, due to the differences in lattice constants and crystal structures of TiN and AlN, the maximum thickness of TiN layer that can sustaining stable growth of nanotwins is about 10 nm, whereas for AlN layer is just about 1.5 nm. This study proposes a promising approach to introduce twins in ceramic films with exceedingly high stacking-fault energies and enable advancements in the toughening of ceramic films.

Experimental and numerical study of Si annealed by laser sources of the visible spectrum (red and blue wavelength)

Laser annealing (LA) is an increasingly important process in the semiconductor manufacturing industry. So far, lasers of nonvisible spectrum have been mostly used for semiconductor applications. The study using a visible wavelength laser is not nearly found though it could be a potential tool to expand applications of the LA. In this study, the silicon (Si) deposited on oxide was annealed using laser sources of visible spectrum. LA of Si was performed using a single laser source of either red or blue wavelengths or a dual-laser source using both wavelengths. The surface morphology and microstructure of the annealed Si were analyzed. A numerical simulation was also conducted to predict the temperature and melt pool characteristics of Si during the annealing process. Surface roughness increased with increasing heat input (or power density) of the laser beam, owing to the considerable melting of Si. The dual laser using a relatively high power density produced Si with high polycrystallinity and large grains. However, the Si lattice constant difference induced by different annealing conditions was negligible. Simulation results showed that temperature, melt pool length, and duration increased with the power density of the dual-laser beam. This contributed to the growth of large Si grains.

Molecular dynamics simulations of interface structure and deformation mechanisms in metal/ceramic composites under tension

The role of interfaces on plastic deformation of metal/ceramic composites is multifaceted, which is due to the fact that interfaces can emit, hinder, annihilate or store dislocations. In this work, molecular dynamics simulation was employed to investigate the interfacial structure and deformation mechanism of the coherent and semi-coherent Al/MgO(110) configurations under uniaxial tensile loading. In particular, rectangular mismatch dislocation networks were observed at the interface of a semi-coherent Al/MgO(110) configuration after relaxation, which were due to the difference in lattice constants between the constituent phases. Besides, the strain concentration at the dislocation nodes was found, being conducive to the emission of misfit dislocations into the Al matrix. Moreover, unlike the coherent configuration, the elastic zone in the stress-strain curve of the semi-coherent Al/MgO(110) configuration was relatively narrow and had two yield points. The first yield point was attributed to the nucleation of misfit dislocations from the interface dislocation nodes, whereas the second one was ascribed to the nucleation of lattice dislocations in the Al matrix. Finally, the failure stress of the semi-coherent configuration was less than that of the coherent system.

Alpha-phase gallium oxide-based UVC photodetector with high sensitivity and visible blindness

This paper describes the heteroepitaxial growth of high-quality alpha-phase gallium oxide (α-Ga2O3) on a sapphire substrate without a buffer layer via the hydride vapor phase epitaxy method. Here, compressive strain arising from the difference in lattice constants between the substrate and α-Ga2O3 is relieved near the interface, resulting in a high crystal quality of 32.72 arcsec (full width at half maximum value) in the high-resolution x-ray diffraction 2θ scan spectrum. Subsequently, the fabricated hetero α-Ga2O3-based photodetector with a metal–semiconductor–metal structure operating under ultraviolet radiation in the C-band (UVC) demonstrates a high UVC responsivity of 5 × 102 A W−1 and a high visible blindness of 8.14 × 104 at 235 nm. The photodetector utilizes photogenerated holes trapped near the interface of the metal electrode, inducing amplified electron current flow. The developed hetero α-Ga2O3-based UVC photodetector can be used to detect early signs of fire, flames, or corona discharge in visible light environments for social and industrial safety applications.

Rotational and dilational reconstruction in transition metal dichalcogenide moiré bilayers

Lattice reconstruction and corresponding strain accumulation plays a key role in defining the electronic structure of two-dimensional moiré superlattices, including those of transition metal dichalcogenides (TMDs). Imaging of TMD moirés has so far provided a qualitative understanding of this relaxation process in terms of interlayer stacking energy, while models of the underlying deformation mechanisms have relied on simulations. Here, we use interferometric four-dimensional scanning transmission electron microscopy to quantitatively map the mechanical deformations through which reconstruction occurs in small-angle twisted bilayer MoS2 and WSe2/MoS2 heterobilayers. We provide direct evidence that local rotations govern relaxation for twisted homobilayers, while local dilations are prominent in heterobilayers possessing a sufficiently large lattice mismatch. Encapsulation of the moiré layers in hBN further localizes and enhances these in-plane reconstruction pathways by suppressing out-of-plane corrugation. We also find that extrinsic uniaxial heterostrain, which introduces a lattice constant difference in twisted homobilayers, leads to accumulation and redistribution of reconstruction strain, demonstrating another route to modify the moiré potential.

Absence of near-ambient superconductivity in LuH2±xNy

A recent study demonstrated near-ambient superconductivity in nitrogen-doped lutetium hydride1. This stimulated a worldwide interest in exploring room-temperature superconductivity at low pressures. Here, by using a high-pressure and high-temperature synthesis technique, we have obtained nitrogen-doped lutetium hydride (LuH2±xNy), which has a dark-blue colour and a structure with the space group Fmbar{3}m as evidenced by X-ray diffraction. This structure is the same as that reported in ref. 1, with a slight difference in lattice constant. Raman spectroscopy of our samples also showed patterns similar to those observed in ref. 1. Energy-dispersive X-ray spectroscopy confirmed the presence of nitrogen in the samples. We observed a metallic behaviour from 350 K to 2 K at ambient pressure. On applying pressures from 2.1 GPa to 41 GPa, we observed a gradual colour change from dark blue to violet to pink-red. By measuring the resistance at pressures ranging from 0.4 GPa to 40.1 GPa, we observed a progressively improved metallic behaviour; however, superconductivity was not observed above 2 K. Temperature dependence of magnetization at high pressure shows a very weak positive signal between 100 K and 320 K, and the magnetization increases with an increase in magnetic field at 100 K. All of these are not expected for superconductivity above 100 K. Thus, we conclude the absence of near-ambient superconductivity in this nitrogen-doped lutetium hydride at pressures below 40.1 GPa.

Impenetrable Barrier at the Metal-Mott Insulator Junction in Polymorphic 1H and 1T NbSe2 Lateral Heterostructure.

When a metal makes contact with a band insulator, charge transfer occurs across the interface leading to band bending and a Schottky barrier with rectifying behavior. The nature of metal-Mott insulator junctions, however, is still debated due to challenges in experimental probes of such vertical heterojunctions with buried interfaces. Here, we grow lateral polymorphic heterostructures of single-layer metallic 1H and Mott insulating 1T NbSe2 by molecular beam epitaxy. We find a one-dimensional metallic channel along the interface due to the appearance of quasiparticle states with an intensity decay following 1/x2, indicating an impenetrable barrier. Near the interface, the Mott gap exhibits a strong spatial dependence arising from the difference in lattice constants between the two phases, consistent with our density functional theory calculations. These results provide clear experimental evidence for an impenetrable barrier at the metal-Mott insulator junction and the high tunability of a Mott insulator by strain.

Microstructure evolution and formation mechanism of CoCrCu1.2FeNi high entropy alloy during the whole process of semi‐solid billet preparation

• Coarsening kinetic equation describing solid grain growth was established. • Segregation in FCC 1 phase reduced and the lattice constant difference between the two phases decreased after isothermal heat treatment. • During heating up, FCC 1 and FCC 2 phase recrystallized successively, FCC 2 phase was spheroidized to a certain extent. • FCC 2 grains coalesced, grew up and spheroidized and the preferred orientations basically disappeared as heated to semi-solid range. Dual face-centered cubic (FCC) CoCrCu 1.2 FeNi semi-solid billets were prepared by semisolid isothermal treatment of wrought high entropy alloy (HEA) (SSITWH) method, and the microstructure evolution in the whole process of billets preparation was systematically investigated by optical microscopy, scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy. The hot deformed feedstock was mainly composed of deformation structure with preferred orientations and a small number of dynamically recrystallized grains of FCC 1 phase. In the semi-solid stage, the effect of temperature and soaking time on semi-solid microstructure was studied in the range of 1130–1250 °C and 5–120 min. The semi-solid microstructure was evaluated quantitatively. The average grain size and average shape factor increased with the increase of soaking time and isothermal temperature. After isothermal heat treatment, the segregation of Cu in FCC 1 phase reduced to a certain extent. Semi-solid coarsening kinetics analysis showed that the alloy had low coarsening coefficients. When the temperatures were 1130 °C, 1175 °C, 1200 °C, 1225 °C and 1250 °C, the coarsening coefficients were 1.08 μm 3 /s, 5.95 μm 3 /s, 6.17 μm 3 /s, 17.58 μm 3 /s, 38.67 μm 3 /s, respectively. A coarsening kinetic equation describing solid grain growth was established. During heating up, FCC 1 and FCC 2 phase recrystallized successively. At higher temperature, FCC 2 phase was spheroidized to a certain extent. When temperature was raised to semi-solid range, the grains of FCC 2 phase coalesced, grew up and spheroidized and the preferred orientations basically disappeared. The types of semi-solid melting characteristics of HEAs were summarized in this paper. The semi-solid melting behavior of alloys is essentially affected by phase structure, phase number, phase volume content and composition.

Recent Advances of Preparation and Application of Two-Dimension van der Waals Heterostructure

With paramount electrical, optical, catalytic, and other physical and chemical properties, van der Waals heterostructures (vdWHs) have captured increasing attention. vdWHs are two-dimension (2D) heterostructures formed via van der Waals (vdW) force, paving the way for fabricating, understanding, and applications of 2D materials. vdWHs materials of large lattice constant difference can be fabricated together, forming a series of unique 2D materials that cannot form heterostructures earlier. Additionally, vdWHs provide a new platform to study the interlayer interactions between materials, unraveling new physics in the system. Notably, vdWHs embody short-range bonds weaker than covalent and ionic bonds, almost only interactions between nearest particles are considered. Owing to a clear interface, vdW interaction between two different components, devices made by vdWHs can bring amazing physicochemical properties, such as unconventional superconductivity, super capacitance in intercalation 2D structure, etc. Recently, impressive progress has been achieved in the controlled preparation of vdWHs and various applications, which will be summarized in this review. The preparation methods comprise mechanical exfoliation, liquid phase stripping, physical vapor deposition, chemical vapor deposition, and metalorganic chemical vapor deposition. The applications sections will focus on photoelectric devices, logic devices, flexible devices, and piezotronics. Finally, some perspectives in the future on the controlled preparation of vdWHs with desired properties for advanced applications will be discussed.

Creation of unexplored tunnel junction by heterogeneous integration of InGaAs nanowires on germanium

Heteroepitaxy has inherent concerns regarding crystal defects originated from differences in lattice constant, thermal expansion coefficient, and crystal structure. The selection of III–V materials on group IV materials that can avoid these issues has however been limited for applications such as photonics, electronics, and photovoltaics. Here, we studied nanometer-scale direct integration of InGaAs nanowires (NWs) on Ge in terms of heterogenous integration and creation of functional materials with an as yet unexplored heterostructure. We revealed that changing the initial Ge into a (111)B-polar surce anabled vertical InGaAs NWs to be integrated for all In compositions examined. Moreover, the growth naturally formed a tunnel junction across the InGaAs/Ge interface that showed a rectification property with a huge current density of several kAcm−2 and negative differential resistance with a peak-to-valley current ratio of 2.8. The described approach expands the range of material combinations for high-performance transistors, tandem solar cells, and three-dimensional integrations.

Machine-learning-guided descriptor selection for predicting corrosion resistance in multi-principal element alloys

More than $270 billion is spent on combatting corrosion annually in the USA alone. As such, we present a machine-learning (ML) approach to down select corrosion-resistant alloys. Our focus is on a non-traditional class of alloys called multi-principal element alloys (MPEAs). Given the vast search space due to the variety of compositions and descriptors to be considered, and based upon existing corrosion data for MPEAs, we demonstrate descriptor optimization to predict corrosion resistance of any given MPEA. Our ML model with descriptor optimization predicts the corrosion resistance of a given MPEA in the presence of an aqueous environment by down selecting two environmental descriptors (pH of the medium and halide concentration), one chemical composition descriptor (atomic % of element with minimum reduction potential), and two atomic descriptors (difference in lattice constant (Delta {{{mathrm{a}}}}) and average reduction potential). Our findings show that, while it is possible to down select corrosion-resistant MPEAs by using ML from a large search space, a larger dataset and higher quality data are needed to accurately predict the corrosion rate of MPEAs. This study shows both the promise and the perils of ML when applied to a complex chemical phenomenon like corrosion of alloys.

Simulation study on the diversity and characteristics of twin structures in GaN

Gallium nitride (GaN) is a promising material for high-frequency and high-power electronics owing to its high thermal stability, wide bandgap, and high breakdown voltage. Monocrystalline GaN films are usually produced by metal organic chemical vapor phase deposition using Si, SiC, or Al 2 O 3 as the substrate. However, the difference in lattice constants and thermal expansion coefficients causes many defects in GaN epilayers . In particular, the formation of twins will strongly influence the electronic properties of GaN-based devices. In this study, the solidification process of GaN was simulated by molecular dynamics at a cooling rate of 10 12 K/s, and the twin structures in GaN were discussed. The results of this simulation showed two types of twins, including coherent twin and rotating twin, which were analyzed by visualization technology. The formation of twins was related to the coexistence of zinc blende and wurtzite phases. Furthermore, the types of twins were closely associated to the stacking ways of zinc blende and wurtzite structures. In general, it is essential to study the microstructure and the structural evolution of twin GaN, in order to give references to experiments about the growth and characteristics of twins. • Zinc blende and wurtzite nucleated randomly and formed lots of complex structures at the beginning of crystallization. • Coherent and rotating twins were formed according to different stacking methods of zinc blende and wurtzite structures. • The rules for the combination of twin structures in GaN crystals have been summarized and it can be generalized. • An error in the stacking sequence can cause the metastable phase c-GaN to start seeding the h-GaN phase.

Mn-Doped XAlO3 (X = Y, Tb) Single-Crystalline Films Grown onto YAlO3 Substrates: Raman Spectroscopy Study toward Visualization of Mechanical Stress

Single-crystalline films (SCFs) of perovskites TbAlO3 (TbAP) and YAlO3 (YAP) doped with Mn ions were grown using the liquid-phase epitaxy (LPE) method on YAlO3 single-crystal (SC) substrates and investigated using Raman spectroscopy. The cross-section of the above-mentioned epitaxial structures was probed to obtain Raman signals of TbAP:Mn, YAP:Mn SCFs, and the YAP SC substrate. The undisturbed Raman signals of TbAP:Mn and YAP:Mn were recorded. For the TbAP:Mn sample, due to the difference in lattice constants between the SCF and the substrate, significant changes in the Raman spectra were observed. Over the interface between the SCF and the substrate, called the transition layer, gradual changes in the Raman spectra were observed. The changes are connected with the replacement of Tb3+ ions with Y3+ cations and are manifested as shifts of the modes assigned to translational, librational, and internal vibrations. Angular-dependent measurements of polarized spectra allowed determination of the TbAP:Mn SCF orientation in the ac plane (010). High-resolution luminescence spectra confirmed electronic transition attributed to Tb3+ cations and Nd3+ trace impurities in the YAP substrate. The Raman map recorded for the region close to the transition layer showed a shift of the Raman band at 552 cm–1 (YAP) related to the changes in the lattice constants and mechanical stress.

Localization transitions and mobility edges in quasiperiodic ladder

We investigate localization properties of two-coupled uniform chains (ladder) with quasiperiodic modulation on interchain coupling strength. We demonstrate that this ladder is equivalent to two Aubry–André chains when two legs are symmetric. Analytical and numerical results indicate the appearance of mobility edges in asymmetric ladder systems. We propose an easy-to-engineer quasiperiodic Moiré superlattice ladder system comprising two-coupled uniform chains. An irrational lattice constant difference results in a quasiperiodic structure. Numerical simulations indicate that such a system supports the existence of mobility edges. Furthermore, we demonstrate that the mobility edges can be detected through a dynamical method, that is based on the measurement of survival probability in the presence of a single imaginary negative potential. The results provide insights into localization transitions and mobility edges in experiments.

Phase stability, mechanical properties and melting points of high-entropy quaternary metal carbides from first-principles

With a combination of first-principles calculations and thermodynamics formalism of configurational mixing entropy, we have constructed three-dimensional phase diagram in terms of thermodynamic and structural parameters including the configurational mixing entropy and enthalpy, the temperature of the melting point, and the lattice constant difference of the constitute carbides for fifteen equiatomic quaternary high-entropy metal carbide (HEMC) ceramics of group IVB and VB refractory metals (RM = Ti, Zr, Hf, V, Nb, and Ta). We further predicted nine new HEMCs and provided an explanation for the existence of six experimentally realized quaternary HEMCs. In addition, our calculations of the melting points and mechanical properties show that the HEMCs have the unique properties of high hardness, high fracture toughness, and ultrahigh melting points. The computational procedure involved in this work may be used to design new high-entropy ceramics for specific applications.

Determination of the Fermi velocity of graphene on MoS2 using dual mode scanning tunneling spectroscopy

We have measured the Fermi velocity of graphene on MoS2 using dual mode scanning tunneling spectroscopy. In our method, the tunnel current (I) and the derivate of the current to the gap width (dI/dz) are measured simultaneously as a function of sample bias (V) at a constant tip-substrate distance. Owing to the difference in lattice constants between graphene (2.461 Å) and MoS2 (3.161 Å), a moiré structure is found with a periodicity of 1.07 nm and a twist angle of 4°. Our experiments reveal that the electronic band structure of graphene remains intact in the bandgap region of MoS2. The Fermi velocity of graphene on MoS2 is determined to be 1.2±0.1×106 m/s.

Effects of Zn x Mn1−x S buffer layer on nonpolar AlN growth on Si (100) substrate

Thin film growth of Zn x Mn1−x S on a Si (100) substrate by sputtering was investigated for nonpolar AlN film growth on Si (100) substrate. The Zn x Mn1−x S buffer layer reduces the large differences in thermal expansion coefficient and lattice constants between AlN and Si. Although the solubility of ZnS in MnS is less than 5% at 800 °C in bulk form, the insertion of a room-temperature MnS layer between Zn x Mn1−x S and Si enabled (100)-oriented cubic-Zn x Mn1−x S film growth even at x = 9.5%, which is a metastable phase and a phase separation region in bulk form. On the (100)-oriented cubic Zn x Mn1−x S film, nonpolar AlN growth was achieved by sputtering. Furthermore, X-ray photoelectron spectroscopy measurements revealed that the Zn x Mn1−x S film improved the stability of the AlN/Zn x Mn1−x S interface. Zn x Mn1−x S has the potential to enable nonpolar AlN growth on large-diameter Si (100) substrates.

Interplay of Magnetic Properties and Doping in Epitaxial Films of h-REFeO3 Multiferroic Oxides.

Multiferroic materials demonstrating coexistence of magnetic and ferroelectric orders are promising candidates for magnetoelectric devices. While understanding the underlying mechanism of interplaying of ferroic properties is important, tailoring their properties to make them potential candidates for magnetoelectric devices is challenging. Here, the antiferromagnetic Neel ordering temperature above 200 K is realized in successfully stabilized epitaxial films of (Lu,Sc)FeO3 multiferroic oxide. The first-principles calculations show the shrinkage of in-plane lattice constants of the unit cells of the films on different substrates which corroborates well the enhancement of the Neel ordering temperature (TN ). The profound effect of lattice strain/stress at the interface due to differences of in-plane lattice constants on out of plane magnetic properties and on spin reorientation temperature in the antiferromagnetic region is further elucidated in the epitaxial films with and without buffer layer of Mn-doped LuFeO3 . Writing and reading ferroelectric domains reveal the ferroelectric response of the films at room temperature. Detailed electron microscopy shows the presence of lattice defects in atomic scale. First-principles calculations show that orbital rehybridization of rare-earth ions and oxygen is one of the main driving force of ferroelectricity along c-axis in thin films of hexagonal ferrites.