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Heavy oil, with high viscosity and complex compositions, often faces a series of challenges in the process of its exploitation and utilization. There are huge amounts of compositions with different...

Metal intercalation under graphene has attracted extensive experimental and theoretical research because of its capability to manipulate the electronic structure and properties of graphene. However...

We performed molecular dynamics simulations to study the crystallization of the P3Sn4 phase from P2Sn5 liquid using a machine learning (ML) interatomic potential with desirable efficiency and accur...

Amorphous diamond structures are generated by quenching high-density high-temperature liquid carbon using tight-binding molecular-dynamics simulations. We show that the generated amorphous diamond structures are predominated by strong tetrahedral bonds with the sp3 bonding fraction as high as 97%, thus exhibit an ultra-high incompressibility and a wide band gap close to those of crystalline diamond. A small amount of sp2 bonding defects in the amorphous sample contributes to localized electronic states in the band gap while large local strain gives rise to localization of vibrational modes at both high and low frequency regimes.

Metal doping is a common strategy for the regulation of the structure of carbon nitride materials at the molecular level. A wide range of intriguing applications of metal-doped carbon nitride (M–CN) materials have recently been investigated from the mainstream of photocatalysis to other emerging fields, including electrocatalysis, organic synthesis, biosensors, and nanozymes. From a fundamental point of view, M–CN is an essential branch of carbon nitride materials and opens a new avenue for the preparation of single-atom catalysts and even endows the materials with new exciting properties. In particular, M–Nx structures as active sites in numerous M–CN materials have been widely demonstrated using both experiments and theoretical calculations. This review summarizes the state-of-the-art preparation methods, characterization techniques, and diverse applications of M–CN. Finally, future perspectives on the fabrication and application prospects of M–CN materials are discussed.

We systematically study the effects of Gd adsorption and intercalation on the electronic band structure of double-layer epitaxial graphene on Si-terminated SiC(0001) by first-principles calculations. We show that Gd adsorption and intercalation exhibit strong effects on the coupling between the graphene layers and between the buffer layer and substrate. Different adsorption/intercalation geometries can result in very different electron band structures. The number of Dirac cones and the positions of the Dirac cones relative to the Fermi level can be effectively manipulated through controlling the Gd adsorption/intercalation geometries. Our calculations provide useful insights to guide the experimental design of graphene-based materials with desirable functionalities for applications.

As a class of metal-free two-dimensional (2D) semiconductor materials, polymeric carbon nitrides have attracted wide attention recently due to its facile regulation of the molecular and electronic structures, availability in abundance and high stability. According to the different ratios of C and N atoms in the framework, a series of CxNy materials have been successfully synthesized by virtue of various precursors, which further triggers extensive investigations of broad applications ranging from sustainable photocatalytic reactions and highly sensitive optoelectronic biosensing. In view of topological structures on their electronic structures and material properties, the as-reported CxNy could be generally classified into two main categories with three- or six-bond-extending frameworks. Owing to the effective n→π* transition in most CxNy materials, the relative energy level of the lone-pair electrons on N atoms is high, which thus endows the materials with the capability of visible light absorption. Meanwhile, the different repeating units, bridging groups and defect sites of these two kinds of CxNy allow them to effectively drive a diverse of promising applications that require specific electronic, interfacial and geometric properties. This review paper aims to summarize the recent progress in topological structure design and the relevant electronic band structures and striking properties of CxNy materials. In the final part, we also discuss the existing challenges of CxNy and outlook the prospect possibilities.

Z-scheme photocatalytic systems are an ideal band alignment structure for photocatalysis because of the high separation efficiency of photo-induced carriers while simultaneously preserving the strong reduction activity of electrons and oxidation activity of holes. However, the design and construction of Z-scheme photocatalysts is challenging because of the need for appropriate energy band alignment and built-in electric field. Here, we propose a novel approach to a Z-scheme photocatalytic system using density functional theory calculations with the HSE06 hybrid functional. The undesirable type-I g-C3 N4 /MoSe2 heterojunction is transformed into a direct Z-scheme system through boron doping of g-C3 N4 (B-doped C3 N4 /MoSe2 ). Detailed analysis of the total and partial density of states, work functions and differential charge density distribution of the B-doped C3 N4 /MoSe2 heterojunction shows the proper band alignment and existence of a built-in electric field at the interface, with the direction from g-C3 N4 to MoSe2 , demonstrating a direct Z-scheme heterojunction. Further investigation on the absorption spectra reveals a large enhancement of the light absorption efficiency after boron doping. The results consistently confirm that electronic structures and photocatalytic performance can be effectively manipulated by a facile boron doping. Modulating the band alignment of heterojunctions in this way provides valuable insights for the rational design of highly efficient heterojunction-based photocatalytic systems.

The demand for disorder-tolerant quantum logic and spin electronics can be met by generating and controlling dissipationless spin currents protected by topology. Dirac fermions with helical spin-locking surface transport offer a way of achieving such a goal. Yet, surface-bulk coupling can lead to strong Dirac electron scattering with bulk carriers and phonons as well as impurities, assisted by such dissipative channel, which results in “topological breakdown”. Here, we demonstrate that coherent lattice vibrations periodically driven by a single-cycle terahertz (THz) pulse can significantly suppress such dissipative channel in topological insulators. This is achieved by reducing the phase space in the bulk available for Dirac fermion scattering into during coherent lattice oscillations in Bi2Se3. This light-induced suppression manifests as a remarkable transition exclusively in surface transport, absent for bulk, above the THz electric fields for driving coherent phonons, which prolongs the surface transport lifetime. These results, together with simulations, identify the critical role of spin–orbit coupling for the “phase space contraction” mechanism that suppresses the surface-bulk coupling. Imposing vibrational quantum coherence into topological states of matter may become a universal light control principle for reinforcing the symmetry-protected helical transport.

Interactions between defects in graphene and the lattice distortion and electronic charge localization induced by the defect interactions are studied by tight-binding (TB) calculations using the recently developed three-center TB potential model. The interaction between two 5–7 Stone–Wales defects gliding along the zig-zag (ZZ) direction of graphene, which has been observed by experiment, is studied at first to validate the TB calculations. Reconstructed divacancy defect pairs and di-adatom defect pairs separated along the glide ZZ and armchair (AC) directions in graphene, respectively, are then studied. We show that the characteristics (i.e., attractive or repulsive) and the strength of interactions between these defects are dependent on the type of defects and on the direction and distance of the defect separation on graphene. Although elastic interaction due to graphene lattice distortion induced by the defect has significant contribution to the total interaction energy, redistribution of electron char...

Due to the high cost and insufficient resources of lithium, alternative sodium-ion batteries have been widely investigated for large-scale applications. NaFePO4 has the highest theoretical capacity of 154 mA h g-1 among the iron-based phosphates, which makes it an attractive cathode material for Na-ion batteries. Experimentally, LiFePO4 has been highly successful as a cathode material in Li-ion batteries because its olivine crystal structure provides a stable framework during battery cycling. In NaFePO4, maricite replaces olivine as the most stable phase. However, the maricite phase is experimentally found to be electrochemically inactive under normal battery operating voltages (0-4.5 V). We found that partial substitutions of Na with Li stabilize the olivine structure and may be a way to improve the performance of NaFePO4 cathodes. Using the previously developed structural LiFePO4 database, we examined the low-energy crystal structures in the system when we replace Li with Na. The known maricite and olivine NaFePO4 phases are reconfirmed and an unreported phase with energy between them is identified by our calculations. Besides, the Li-doped olivine type compound LixNa1-xFePO4 with mixed alkali ions retains better energetic stability compared with the other two types of structures of the same composition, as long as the proportion of Li exceeds 0.25. The thermodynamic stability of o-type LixNa1-xFePO4 can be further improved at finite temperatures. The primary limitation of the calculations is that we mainly focus on the zero-temperature condition; however, the relative stability of the structures may vary depending on the ambient temperature.

Phase change memory is an excellent candidate for next-generation memory technologies with a high operation speed, but the memory capacity is not very satisfactory, due to which engineers have to add the 3D stacking technology (3D XPoint) for new products. Alternatively, multi-level storage is an easy approach to enable large data density and probably future neuromorphic computing. Lately, K2Sb8Se13 has attracted considerable attention as a multi-level phase change material because it exhibits an interesting amorphous-to-amorphous (polyamorphic) transformation before crystallization, and these two polyamorphic states as well as the crystalline phase show distinct resistances, adding a new data state to the existing “0” and “1”. Understanding and stabilizing this new amorphous state is the key to the application of this material; here, we have investigated these two amorphous states through ab initio simulations. We found that these two states showed obvious differences in the local structures, and the void concentration revealed by the low-electron-density areas indicated stronger interactions between the atomic clusters in the denser phase. The density of states and electron localization function were analyzed and we confirmed that adding electronic holes were largely responsible for the decrease in resistance. In this work, we have discovered the origin of multi-level resistance states in K2Sb8Se13, paving the way for the design of new phase change memory devices based on this material.

Phase-change materials such as Ge–Sb–Te compounds have attracted much attention due to their potential value in electrical data storage. In contrast to the amorphous and crystalline phases, supercooled liquids are far from being deeply understood despite their inevitable role in both amorphization and crystallization processes. To this end, we have studied the dynamics properties and structural characteristics of liquid and supercooled liquid Ge3Sb2Te6 during the fast cooling process. As the temperature decreases, chemical bonds become more homogeneous, but coordination numbers of Ge, Sb and Te atoms change very little. Meanwhile, the structural order of short-range configuration is obviously enhanced. Further studies suggest that Ge-centered, Sb-centered and Te-centered configurations change to the more ordered defective octahedrons mainly by adjusting the bond-angle relationship and bond length, rather than just by changing the coordination environment. It is the more ordered octahedrons that promote the formation of medium-range order. Our findings provide a deep insight into the origin of local structural order in supercooled liquid Ge3Sb2Te6, which is of great importance for the comprehensive understanding of amorphization and crystallization processes.

The adsorption and wetting on quartz surfaces by aqueous solutions of xylyl-substituted biquaternary ammonium salt gemini surfactants with different spacer groups (C3 and C6), have been investigated. The interfacial properties of surfactant solutions such as contact angle, adhesional tension (γLV cos θ), quartz–water interfacial tension (γSL) as well as adhesion work (WA) have been estimated. The obtained results show that C3 and C6 have similar adsorption behavior on quartz surfaces. Before critical micelle concentration (cmc) is reached, the contact angles of gemini surfactants slowly increase with the increasing concentration, and the adsorption amount at the water–air interface is almost the same as those at a quartz–water interface. After reaching cmc, the gemini surfactant Cn molecules form a more compact adsorption film through bending the flexible spacer chain, instead of forming a bi-layer. As a result, a further increase in quartz–liquid interfacial tension (γSL) and a consequent increase in contact angle have been observed after cmc. Gemini C6 shows a stronger ability towards hydrophobic modification at a quartz surface than C3, demonstrating the contribution of the longer methylene spacer to the hydrophobic modification of the quartz surface.

New magnetic materials for energy and information-processing applications are of paramount importance in view of significant global challenges in environmental and information security. The discovery and design of materials requires efficient computational and experimental approaches for high throughput and efficiency. When increasingly powerful computational techniques are combined with special non-equilibrium fabrication methods, the search can uncover metastable compounds with desired magnetic properties. Here we review recent results on novel Fe-, Co- and Mn-rich magnetic compounds with high magnetocrystalline anisotropy, saturation magnetization, and Curie temperature created by combining experiments, adaptive genetic algorithm searches, and advanced electronic-structure computational methods. We discuss structural and magnetic properties of such materials including Co– and/or Fe–X compounds (X = N, Si, Sn, Zr, Hf, Y, C, S, Ti, or Mn), and their prospects for practical applications.

The complex interplay between energetic and kinetic factors that governs the phase and morphology selections can originate at the earliest stage of crystallization in the amorphous parent phases. Because of the extreme difficulties in capturing the microscopic nucleation process, a detailed picture of how initial disordered structures affect the transformation pathway remains unclear. Here, we report the experimental observation of widely varying phase selection and grain size evolution during the devitrification of a homogeneous melt-spun glassy ribbon. Two different crystalline phases $\ensuremath{\theta}\text{\ensuremath{-}}{\mathrm{Al}}_{5}\mathrm{Sm}$ and $\ensuremath{\varepsilon}\text{\ensuremath{-}}{\mathrm{Al}}_{60}{\mathrm{Sm}}_{11}$ are found to form in the different regions of the same metallic glass (MG) ribbon during the devitrification. The grain size of the $\ensuremath{\varepsilon}\text{\ensuremath{-}}{\mathrm{Al}}_{60}{\mathrm{Sm}}_{11}$ phase shows a strong spatial heterogeneity. The coarse-grained $\ensuremath{\varepsilon}\text{\ensuremath{-}}{\mathrm{Al}}_{60}{\mathrm{Sm}}_{11}$ phase coupled with the small volume fraction of the $\ensuremath{\theta}\text{\ensuremath{-}}{\mathrm{Al}}_{5}\mathrm{Sm}$ phase is preferably formed close to the wheel side of the melt-spun ribbon. Combining experimental characterization and computational simulations, we show that phase selection and microstructure evolution can be traced back to different types and populations of atomic clusters that serve as precursors for the nucleation of different crystalline phases. Inhomogeneous cooling rates cause different structure orders across the glass sample during the quenching process. Our findings provide direct insight into the effect of structural order on the crystallization pathways during the devitrification of MG. It also opens an avenue to study the detailed nucleation process at the atomic level using the MG as a platform and suggests the opportunity of microstructure and property design via controlling the cooling process.

Van der Waals (vdW) heterostructures, which are produced by the precise assemblies of varieties of two-dimensional (2D) materials, have demonstrated many novel properties and functionalities. Here we report a nano-plasmonic study of vdW heterostructures that were produced by depositing ordered molecular layers of pentacene on top of graphene. We find through nano-infrared (IR) imaging that surface plasmons formed due to the collective oscillations of Dirac fermions in graphene are highly sensitive to the adjacent pentacene layers. In particular, the plasmon wavelength declines systematically but nonlinearly with increasing pentacene thickness. Further analysis and density functional theory (DFT) calculations indicate that the observed peculiar thickness dependence is mainly due to the tunneling-type electron transfer from pentacene to graphene. Our work unveils a new method for tailoring graphene plasmons and deepens our understanding of the intriguing nano-optical phenomena due to interlayer couplings in novel vdW heterostructures., Comment: 21 pages

Unremitting efforts have been made to develop high-performance photoelectrochemical (PEC) water-splitting system to produce clean hydrogen fuel using sunlight. In this work, a novel way, combining highly-ordered nanowires (NWs) structure and piezotronic effect of p-type ZnO has been demonstrated to dramatically enhance PEC hydrogen evolution performance. Systematic characterizations indicate that the Sb atoms uniformly dope into ZnO NWs and substitute Zn sites with the introduction of two zinc vacancies to form the shallow acceptor SbZn–2VZn complex. Detailed synchrotron-based X-ray absorption near-edge structure (XANES) experiments in O K-edge and Zn L-edge further confirm the formation of the complex, and theoretical calculation verifies the Sb5+ state dominating the complex. The optimal photocurrent density of the 0.2Sb/ZnO-anneal NWs can reach −0.85 mA/cm2 (0 VRHE) which is 17.2 times larger than that of the n-ZnO NWs under sunlight illumination (100 mW/cm2). Furthermore, the piezotronic effect can be introduced to regulate the charge separation and transfer in the ZnO NWs through modulating the band structure near the interface. The photocurrent density can further increase to −1.08 mA/cm2 (0 VRHE) under a 0.6% tensile strain, which is 27.4% enhancement with respect to the ZnO sample without strain. These results provide an efficient way to design and develop high-performance photoelectrodes toward PEC hydrogen evolution.

In recent years, some arguments about the existence of medium-range order (MRO) in the Zr-Rh system have been put forward. However, research on the structural features of the Zr-Rh binary alloy at the atomic level is still lacking. This study uses ab initio molecular dynamics simulations to systematically study the local structures of Zr77Rh23 from the liquid to the glassy states. Pair correlation function (PCF), coordination number (CN), Honeycutt–Anderson(HA) index, bond-angle distribution functions, and the Voronoi tessellation method are used to reveal a clear icosahedral-like configuration in the amorphous Zr77Rh23 alloy. It is noteworthy that the splitting in the second peak of the partial PDF implies the existence of a medium range order (MRO) in the Zr77Rh23 system. We obtain the local order in three-dimensional atomic density distributions by using a new atomistic cluster alignment (ACA) method. Interestingly, a Bergman-type MRO is observed in the glassy Zr77Rh23. Furthermore, the spatial distribution and connections of icosahedral-like clusters are shown to further demonstrate the MRO network. Our findings shed light on the nature of atomic local structures of amorphous Zr77Rh23 alloy and have important implications to understanding the formation of various MROs in metallic glasses.

For lithium-sulfur batteries (LSBs), the dissolution of lithium polysulfide and the consequent "shuttle effect" remain major obstacles for their practical applications. In this study, we designed a new cathode material comprising MoSe2/graphene to selectively adsorb polysulfides on the selenium edges and thus to mitigate their dissolution. More specifically, few-layered MoSe2 was first grown on nitrogen-doped reduced graphene oxide (N-rGO) using the chemical vapor deposition method and then infiltrated with sulfur as the cathode for LSBs. An initial capacity of 1028 mA h g-1 was achieved for S/MoSe2/N-rGO at 0.2 C, higher than 981 and 405.1 mA h g-1 for pure graphene and sulfur, respectively, along with enhanced cycling durability and rate capability. Moreover, the density functional theory simulation, in addition to the experimental adsorption test, X-ray photoelectron spectroscopy analysis, and transmission electron microscopy technique, reveals the dual roles that MoSe2 plays in improving the performance of LSBs by functioning as the binding sites for lithium polysulfides and as the platform that enables fast Li-ion diffusion by reducing its diffusion barrier. The reported finding suggests that the transition-metal selenides could be an efficient alternative material as the cathode for LSBs.

The formation possibility of a new (Zr0.25Nb0.25Ti0.25V0.25)C high-entropy ceramics (ZHC-1) was first analyzed by the first-principles calculations and thermodynamical analysis and then it was successfully fabricated by hot pressing sintering technique. The first-principles calculation results showed that the mixing enthalpy of ZHC-1 was 5.526 kJ/mol and the mixing entropy of ZHC-1 was in the range of 0.693R–1.040R. The thermodynamical analysis results showed that ZHC-1 was thermodynamically stable above 959 K owing to its negative mixing Gibbs free energy. The experimental results showed that the as-prepared ZHC-1 (95.1% relative density) possessed a single rock-salt crystal structure, some interesting nanoplate-like structures, and high compositional uniformity from nanoscale to microscale. By taking advantage of these unique features, compared with the initial metal carbides (ZrC, NbC, TiC and VC), it showed a relatively low thermal conductivity of 15.3 ± 0.3 W/(m⋅K) at room temperature, which was due to the presence of solid solution effects, nanoplates and porosity. Meanwhile, it exhibited the relatively high nanohardness of 30.3 ± 0.7 GPa and elastic modulus of 460.4 ± 19.2 GPa and the higher fracture toughness of 4.7 ± 0.5 MPa m1/2, which were attributed to the solid solution strengthening mechanism and nanoplate pullout and microcrack deflection toughening mechanism.

AZ31 magnesium alloy was produced via variable-plane rolling (VPR) under different temperatures. The effects of rolling temperature on the microstructure, texture and mechanical properties were investigated. A double-peak basal texture was formed during VPR treatment. Dynamic recrystallization (DRX) was observed during the VPR treatment, and it can reduce the texture intensity owing to the coalescence of sub-grains. Four types of twins were observed in the VPR treatment: {10-12} extension twins, {10-13}, {10-11} contraction twins and {10-11}-{10-12} double twins. The suitable temperature of AZ31 magnesium alloy for VPR treatment is determined as 623 K through experiments with tensile yield strength of 240 MPa, compressive yield strength of 162 MPa and elongation of 13%. Owing to the important impact of temperature on mechanical properties, the alloy was further VPRed with a decrease in temperature pass-by-pass. The high temperature of 623 K at the initial passes randomized the texture, which led to notably high ductility.

Chalcogenide alloys in pseudobinary line between Sb2Te3 and GeTe are extensively utilied in phase change memories for optical and electronic contrast between crystalline and amorphous phases. Different from the crystalline structure, the understanding of amorphous structures is still insufficient due to disorder and distortion. By employing first-principle molecular dynamics simulations and atomistic cluster alignment analysis, the short-range orders and the associated distortions of amorphous Sb2Te3, Ge1Sb2Te4, Ge2Sb2Te5, Ge3Sb2Te6 and GeTe are investigated to explore the origin of nature difference. The results reveal that Ge Ge and Sb Sb bonds present a notable competitive mechanism with GeTe content. The GeTe content has a great influence on Ge- and Te-centered short-range orders but little influence on the structures of Sb-centered clusters, especially for the octahedral sites and unidentified structures. The tetrahedrons in Ge-centered clusters of each alloy show a close proportion, but the fraction of tetrahedrons in total clusters increases with the increasing GeTe content due to the increase in the ratio of Ge atom. As for the distortions of clusters, the distorted tetrahedrons in Ge2Sb2Te5 are closest to the standard tetrahedron, the Peierls distortion of Ge-centered 6-fold octahedron reduces with an increase in GeTe content whereas that of Sb-centered 6-fold octahedron shows a little change. Additionally, it is found that GeTe content can inhibit the formation of nanocavity but contribute to the formation of the odd rings. Our findings deepens the understanding of amorphous structures, which can promote the design and application of phase-change materials.

A driblet of magnetic fluids (MFs) falls on an annular magnet, forming a closed liquid ring. The magnetized MFs can produce liquid support due to magnetostatic force. The air cushion enclosed by the MFs sealing ring may generate gas support as the magnet bottom combines with a substrate. The supporting capacity supplied by the liquid-gas contributes to friction reduction. Research shows such supporting is affected by the surface magnetic field and field distribution. Tribological results confirm that low friction can be obtained since the tribo-pairs are separated by the supporting force and the friction originates from the fluid viscosity. Such design would be significant for solving the “cold welding” as well as the “stick-slip” phenomenon, especially in precise sliding machine.

Phase change memory is an emerging nonvolatile memory technology, recently becoming the center of attention to bridge the speed gap between fast dynamic random access memory and slow flash-based solid-state drives. Lately, CrGeTe3 has been investigated as a special phase change material with an inverse resistance and density change. This material has excellent properties such as good thermal stability, ultralow-energy glass formation process and almost zero mass-density change upon crystallization. Here, we analyzed the amorphous structure of this abnormal material in detail through ab initio simulations and discovered that the metallic-like tight atomic packing is the origin of the high carrier concentration and high mass density in the amorphous phase. Furthermore, the bonding analysis confirms that it is the short Cr–Cr bonds that lead to high packing efficiency in the amorphous local order. Our results discovered the material gene of the amorphous CrGeTe3, paving the way for the design of high-performance memory devices based on this material.

The adsorption of xylyl-substituted biquaternary ammonium salt Gemini surfactants with different spacer (C3 and C6) at polytetrafluoroethylene (PTFE) and polymethylmethacrylate (PMMA) surfaces has been investigated and the different adsorption parameters such as surface tension, contact angle, adhesional tension, solid-water interfacial tension and work of adhesion have been estimated. The results show that C3 and C6 have similar adsorption behaviors at PTFE and PMMA surfaces. C3 and C6 adsorb gradually at a PFTE-water interface via hydrophobic interactions and the adsorption amounts at the water-air interface are almost three times higher than those at the PTFE-water interface due to the steric hindrance effect. However, the contact angle keeps constant throughout the experimental concentration range because the decrease in surface tension just counterbalances the decrease in PFTE-water interfacial tension. On the other hand, C3 and C6 adsorb at the PMMA surface via polar interactions between xylyl and functional groups of PMMA before CMC. Similar to PTFE, the increase in PMMA-water interfacial tension compensates the decrease in surface tension and the contact angle also shows a stationary value before the CMC. A bi-layer structure of C3 and C6 will be formed at the PMMA-water interface via hydrophobic interaction and PMMA-water interfacial tension decreases consequently after the CMC, which results in the decrease in contact angle.

The crystal structures of boron hydrides in a pressure range of 50-400 GPa were studied using the genetic algorithm (GA) method combined with first-principles density functional theory calculations. BH4 and BH5 are predicted to be thermodynamically unstable. Two new BH2 structures with Cmcm and C2/c space group symmetries, respectively, were predicted, in which the B atoms tend to form two-dimensional sheets. The calculated band structures showed that in the pressure range of 50-150 GPa, the Cmcm-BH2 phase has very small gaps, while the C2/c-BH2 phase at 200-400 GPa is metallic. The superconductivity of the C2/c-BH2 structure was also investigated, and electron-phonon coupling calculations revealed that the estimated Tc values of C2/c-BH2 are about 28.18-37.31 K at 250 GPa.

Phase-change material such as Ge 2 Sb 2 Te 5 is usually utilized to store data due to the pronounced contrast in optical and electrical properties between crystalline and amorphous phases. As the density differs in the two phases, it is necessary to explore the influence of pressure on the structures of Ge 2 Sb 2 Te 5 , especially for the supercooled liquid which is an inevitable state in the formation of the two phases. The short-range structures in supercooled liquid Ge 2 Sb 2 Te 5 under compression have been investigated by using ab initio molecular dynamics simulation. The supercooled liquid eventually changes to a solid with an increase in pressure. During the process, tetrahedrons decrease slightly, revealing that tetrahedral structures are insensitive to the pressure. Octahedrons increase as the pressure is less than 3.7 GPa and then decrease, suggesting that a moderate pressure can promote the formation of octahedrons. The body-centered-cubic, face-centered-cubic and hexagonal closed-packed structures are observed at 9.9 GPa and then increase gradually. Additionally, it is noticed that Sb- and Te-centered clusters prefer Ge-centered clusters to forming the high-coordinated short-range structures during the compression process. Our research make us aware of the effects of pressure on short-range structures in supercooled liquid, which is of great importance in the application of phase-change materials.

Transition metal oxides have attracted a lot of interest for the high energy density in asymmetric supercapacitors, but the fast capacity fading, low electrical conductivity and hard clean industrialization still limit the practical applications. In order to overcome the challenge, a novel type of polyoxometalate-assisted hydrothermal strategy for the synthesis of manganese dioxide as an example for asymmetric supercapacitors is reported. The samples are prepared in one step fashion, avoiding complicated equipment and acid corrosion process simultaneously, which are fully researched the morphology, chemical components and the surface area thereby finding its impact on electrochemical performance. Results in electrochemical test demonstrate that the α-MnO2 have good capacitance (235 F g–1) in neutral electrolytes. The asymmetric capacitors assembled by manganese dioxide and activated carbon (AC) as electrode materials possessed large specific energy (27.6 Wh kg–1 at 200 W kg–1 and 19.4 Wh kg–1 at 5000 W kg...

Tailored design/fabrication of electroanalytical materials with highly-active exposed crystal planes is of great importance for the development of electrochemical sensing. In this work, combining experimental and theoretical efforts, we reported a facile strategy to fabricate TiO2 nanocrystals with tunable electrochemical performance for heavy metal detection. Density functional theory (DFT) calculations indicated that TiO2 (001) facet showed relative larger adsorption energy and lower diffusion energy barrier toward heavy metal ions, which is favorable for obtaining better electrochemical stripping behaviors. Based on this prediction, a series of TiO2 nanocrystals with different ratios of exposed (001) and (101) facets were synthesized. Electrochemical stripping experiments further demonstrated that with the increase of the percentage of exposed (001) facet, the sensitivity toward Pb(II) and Cd(II) was increased accordingly. When the percentage of exposed (001) facet was increased from 7% to 80%, the sensitivity increased by 190% and 93% for Pb(II) and Cd(II), respectively. Our work provides an effective route to construct advanced electroanalytical materials for sensing.

We studied the electronic structure of ${\mathrm{PtPb}}_{4}$ using laser angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations. This material is closely related to ${\mathrm{PtSn}}_{4}$, which exhibits exotic topological properties such as Dirac node arcs. The Fermi surface (FS) of ${\mathrm{PtPb}}_{4}$ consists of two electron pockets at the center of the Brillouin zone (BZ) and several hole pockets around the zone boundaries. Our ARPES data reveal significant Rashba splitting at the $\mathrm{\ensuremath{\Gamma}}$ point, in agreement with DFT calculations. The presence of Rashba splitting may render this material of potential interest for spintronic applications.

Manipulating the electronic properties of graphene has been a subject of great interest since it can aid material design to extend the applications of graphene to many different areas. In this paper, we systematically investigate the effect of lead (Pb) intercalation on the structural and electronic properties of epitaxial graphene on the SiC(0001) substrate. We show that the band structure of Pb-intercalated few-layer graphene can be effectively tuned through changing intercalation conditions, such as coverage, location of Pb, and the initial number of graphene layers. Lead intercalation at the interface between the buffer layer (BL) and the SiC substrate decouples the BL from the substrate and transforms the BL into a $p$-doped graphene layer. We also show that Pb atoms tend to donate electrons to neighboring layers, leading to an $n$-doping graphene layer and a small gap in the Dirac cone under a sufficiently high Pb coverage. This paper provides useful guidance for manipulating the electronic properties of graphene layers on the SiC substrate.

Sb-Based phase-change materials have exhibited tremendous advantages in both data storage and reconfigurable photonic devices. Despite the intensive studies on their structures and properties in the crystalline state, the widely used amorphous phase remains elusive. Here, we investigate amorphous Sb2Te3, Sb2Se3, and Sb2S3 through ab initio calculations to link their unique properties to the local structure and bonding nature. We discover that Sb forms shorter and stronger bonds with Se and S than Te, and the average bonding angles of Se (92.0°) and S (94.1°) show larger distortion than that of Te (91.5°). This leads to larger Peierls-like distortion in Sb2Se3 and Sb2S3. On the other hand, more charge transfer and void fraction are presented, opening band gaps and leading to different electronic and optical properties. In contrast, Sb2Te3, due to its semiconducting behavior and low thermal stability, enables its application in phase-change memory. Our results reveal the physics of vastly different electronic and optical properties induced by S, Se, and Te alloying, providing an effective strategy for materials design.

The joint activity of multiple engineered nanoparticles (ENPs) has attracted much attention in recent years. Many previous studies have focused on the combined toxicity of different ENPs with nanostructures of the same dimension. However, the mixture toxicity of multiple ENPs with different dimensions is much less understood. Herein, we investigated the toxicity of the binary mixture of TiO2 nanospherical particles (NPs) and TiO2 nanotubes (NTs) to two freshwater algae with different morphology, namely, Scenedesmus obliquus and Chlorella pyrenoidosa. The physicochemical properties, dispersion stability, and the generation of reactive oxygen species (ROS) were determined in the single and binary systems. Classical approaches to assessing mixture toxicity were applied to evaluate and predict the toxicity of the binary mixtures. The results show that the combined toxicity of TiO2 NPs and NTs to S. obliquus was between the single toxicity of TiO2 NTs and NPs, while the combined toxicity to C. pyrenoidosa was higher than their single toxicity. Moreover, the toxicity of the binary mixtures to C. pyrenoidosa was higher than that to S. obliquus. A toxic unit assessment showed that the effects of TiO2 NPs and NTs were additive to the algae. The combined toxicity to S. obliquus and C. pyrenoidosa can be effectively predicted by the concentration addition model and the independent action model, respectively. The mechanism of the toxicity caused by the binary mixtures of TiO2 NPs and NTs may be associated with the dispersion stability of the nanoparticles in aquatic media and the ROS-induced oxidative stress effects. Our results may offer a new insight into evaluating and predicting the combined toxicological effects of ENPs with different dimensions and of probing the mechanisms involved in their joint toxicity.

Though photoluminescence (PL) of Si quantum dots (QDs) has been known for decades and both theoretical and experimental studies have been extensive, their luminescence mechanism has not been elaborated. Several models have been proposed to explain the mechanism. A deep insight into the origin of light emissions in Si QDs is necessary. This work calculated the ground- and excited state properties of hydrogenated Si QDs with various diameters, including full hydrogen passivation, single Si[double bond, length as m-dash]O ligands, single epoxide and coexisting Si[double bond, length as m-dash]O and epoxide structures in order to investigate the dominant contribution states for luminescence. The results revealed that even a single oxygen atom in hydrogenated Si QDs can dramatically change their electronic and optical properties. Intriguingly, we found that a size-independent emission, the strongest among all possible emissions, was induced by the single Si[double bond, length as m-dash]O passivated Si-QDs. In non-oxidized Si-QDs exhibiting a core-related size-tunable emission, the luminescence properties can be modulated by the ligands of Si QDs, and a very small number of oxygen ligands can strongly influence the luminescence of nanocrystalline silicon. Our findings deepen the understanding of the PL mechanism of Si QDs and can further promote the development of silicon-based optoelectronic devices.

We analyze several methods of obtaining the accurate relativistic total energy (TE) variations using traditional perturbation theories (PTs) and proposed coupling constant integration (CCI) methods. For this purpose, we perform benchmark calculations within the density functional theory taking the spin-orbit coupling (SOC) and its derivative as a perturbation. The TE change due to SOC addition obtained from both PTs and CCI is shown to reach the accuracy of fully self-consistent TE calculations. Similar accuracy is also obtained even for the magnetocrystalline anisotropy energy (MAE). The real advantage of the proposed methods is to use PTs and CCI methods in those electronic structure methods where accurate total energies currently cannot be obtained with required accuracy. Correspondingly, we demonstrate the applicability of suggested methods for calculations of MAE in different magnetic materials using a dynamic mean-field method. All suggested PTs and CCI methods also provide convenient site, orbital, and spin decompositions of the TE variation, creating a powerful way to analyze microscopic physics in strongly correlated materials.

An open-access database specialized for magnetic compounds, as well as for magnetic clusters, is developed with a focus on magnets free from rare earths. Data-intensive methods are used to facilitate the theoretical and experimental design and discovery of new magnetic materials. The utility of the datasets for computational screening, machine-learning modeling, and experimental fabrication is discussed.

We propose a scheme of decomposition of the total relativistic energy in solids to intra- and interatomic contributions. The method is based on a site variation of such fundamental constant as the speed of light. As a practical illustration of the method, we tested such decomposition in the case of a spin-orbit interaction variation for the decomposition of the magnetic anisotropy energy (MAE) in CoPt. We further studied the ${\ensuremath{\alpha}}^{\ensuremath{''}}\text{\ensuremath{-}}{\mathrm{Fe}}_{16}{\mathrm{N}}_{2}$ magnet doped by Bi, Sb, Co, and Pt atoms. It was found that the addition of Pt atoms can enhance the MAE by as much as five times while Bi and Sb substitutions double the total MAE. Using the proposed technique, we demonstrate the spatial distribution of these enhancements. Our studies also suggest that Sb, Pt, and Co substitutions could be synthesized by experiments.

An interatomic potential for the Al–Tb alloy around the composition of Al90Tb10 is developed using the deep neural network (DNN) learning method. The atomic configurations and the corresponding total potential energies and forces on each atom obtained from ab initio molecular dynamics (AIMD) simulations are collected to train a DNN model to construct the interatomic potential for the Al–Tb alloy. We show that the obtained DNN model can well reproduce the energies and forces calculated by AIMD simulations. Molecular dynamics (MD) simulations using the DNN interatomic potential also accurately describe the structural properties of the Al90Tb10 liquid, such as partial pair correlation functions (PPCFs) and bond angle distributions, in comparison with the results from AIMD simulations. Furthermore, the developed DNN interatomic potential predicts the formation energies of the crystalline phases of the Al–Tb system with an accuracy comparable to ab initio calculations. The structure factors of the Al90Tb10 metallic liquid and glass obtained by MD simulations using the developed DNN interatomic potential are also in good agreement with the experimental X-ray diffraction data. The development of short-range order (SRO) in the Al90Tb10 liquid and the undercooled liquid is also analyzed and three dominant SROs, i.e., Al-centered distorted icosahedron (DISICO) and Tb-centered ‘3661’ and ‘15551’ clusters, respectively, are identified.

An appropriate treatment of electronic correlation effects plays an important role in accurate descriptions of physical and chemical properties of real materials. The recently proposed Correlation Matrix Renormalization theory with Sum Rule correction (CMR) for studying correlated electron materails has shown good performance in molecular systems and a periodic Hydrogen chain in comparison with various quantum chemistry and quantum Monte Carlo calculations. This work gives a detailed formulation and computational code implementation of CMR in multi-band periodic lattice systems. This lattice CMR ab initio theory is highly efficient, has no material specific adjustable parameters, and has no double counting issues faced by the hybrid approaches like LDA+U, DFT+DMFT and DFT+GA type theories. Benchmark studies on materials with s and p orbitals in this study show that CMR in its current implementation consistently performs well for these systems as the electron correlation increases from the bonding region to the bond breaking region.

The correlation between structure and glass formability in glassy systems is a long-standing puzzle. To solve this puzzle, many descriptors based on the short-range order (SRO) have been proposed. Here we show that the SRO, however, offers little help in explaining the glass formability and stability; instead it is the formation of medium-range order that stabilizes the glass against crystallization by suppressing the atomic rearrangement and compositional change. Our results provide a perspective for understanding the correlation between structure and stability in glasses.

We introduce Gutzwiller conjugate gradient minimization (GCGM) theory, an ab initio quantum many-body theory for computing the ground-state properties of infinite systems. GCGM uses the Gutzwiller wave function but does not use the commonly adopted Gutzwiller approximation (GA), which is a major source of inaccuracy. Instead, the theory uses an approximation that is based on the occupation probability of the on-site configurations, rather than approximations that decouple the site-site correlations as used in the GA. We test the theory in the one-dimensional and two-dimensional Hubbard models at various electron densities and find that GCGM reproduces energies and double occupancies in reasonable agreement with benchmark data at a very small computational cost.

The development of long-range ferromagnetic (FM) order in dilute magnetic topological insulators can induce dissipationless electronic surface transport via the quantum anomalous Hall effect. We measure the magnetic excitations in a prototypical magnetic topological crystalline insulator, ${\mathrm{Sn}}_{0.95}{\mathrm{Mn}}_{0.05}\mathrm{Te}$, using inelastic neutron scattering. Neutron diffraction and magnetization data indicate that our ${\mathrm{Sn}}_{0.95}{\mathrm{Mn}}_{0.05}\mathrm{Te}$ sample has no FM long-range order above a temperature of 2 K. However, we observe slow, collective FM fluctuations $(l70\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{eV})$, indicating proximity to FM order. We also find a series of sharp peaks originating from local excitations of antiferromagnetically (AF) coupled and isolated Mn-Mn dimers with ${J}_{\mathrm{AF}}=460\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{eV}$. The simultaneous presence of collective and localized components in the magnetic spectra highlight different roles for substituted Mn ions, with competition between FM order and the formation of AF-coupled Mn-Mn dimers.

Nanocellulose is a functional material derived from natural carbon-based polymers. These nanomaterials are biodegradable and renewable in nature and hence are seen as environmentally-friendly materials in many applications. The use of such innovative materials is accelerating and inescapable there is a need to test these presumed environmentally-friendly materials with regard to their ecotoxicity. Here, the acute toxicity and the oxidative stress of nanocelluloses as induced to three aquatic organisms of different trophic levels, namely Scenedesmus obliquus, Daphnia magna, and Danio rerio, were studied in relation to the composition and morphology of the celluloses. Wood-based cellulose nanocrystals (CNCs), cotton-based CNCs, and cotton-based cellulose nanofibrils were selected as model compounds. The results clearly demonstrated a lack of impact of the different nanocellulose materials on apical endpoints like growth inhibition and mortality after short-term exposure. The nanocellulose materials did activate oxidative stress as evoked by reactive oxygen species in the three aquatic organisms. Key factors ascertained to induce the oxidative stress were the composition and morphology. The nanocellulose induced oxidative stress was observed for all the species at concentrations higher than 0.01 mg/L. This finding suggests a more general revelation of oxidative stress being a characteristic mechanism for nanocellulose toxicity to aquatic organisms.

In glassy materials, the Johari-Goldstein secondary (β) relaxation is crucial to many properties as it is directly related to local atomic motions. However, a long-standing puzzle remains elusive: why some glasses exhibit β relaxations as pronounced peaks while others present as unobvious excess wings? Using microsecond atomistic simulation of two model metallic glasses (MGs), we demonstrate that such a difference is associated with the number of string-like collective atomic jumps. Relative to that of excess wings, we find that MGs having pronounced β relaxations contain larger numbers of such jumps. Structurally, they are promoted by the higher tendency of cage-breaking events of their neighbors. Our results provide atomistic insights for different signatures of the β relaxation that could be helpful for understanding the low-temperature dynamics and properties of MGs.

A metastable e -Al60Sm11 phase appears during the initial devitrification of as-quenched Al-10.2 at.% Sm glasses. The e phase is nonstoichiometric in nature since Al occupation is observed on the 16f Sm lattice sites. Scanning transmission electron microscopic images reveal profound spatial correlation of Sm content on these sites, which cannot be explained by the “average crystal” description from Rietveld analysis of diffraction data. Thermodynamically favorable configurations, established by Monte Carlo (MC) simulations based on a cluster-expansion model, also give qualitatively different correlation functions from experimental observations. On the other hand, molecular dynamics simulations of the growth of e -Al60Sm11 in undercooled liquid show that when the diffusion range of Sm is limited to ∼4 A, the correlation function of the as-grown crystal structure agrees well with that of the scanning transmission electronic microscopy (STEM) images. Our results show that kinetic effects, especially the limited diffusivity of Sm atoms plays the fundamental role in determining the nonstoichiometric site occupancies of the e -Al60Sm11 phase during the crystallization process.

Nitric oxide (NO) donors are valuable tools to probe the profound implications of NO in health and disease. The elusive nature of NO bio-relevance has largely limited the use of spontaneous NO donors and promoted the development of next generation NO donors, whose NO release is not only stimulated by a trigger, but also readily monitored via a judiciously built-in self-calibration mechanism. Light is without a doubt the most sensitive, versatile and biocompatible method of choice for both triggering and monitoring, for applications in complex biological matrices. Herein, we designed and synthesized an N-nitroso rhodamine derivative (NOD560) as a photo-triggered and photo-calibrated NO donor to address this need. NOD560 is essentially non-fluorescent. Upon irradiation by green light (532 nm), it efficiently release NO and a rhodamine dye, the dramatic fluorescence turn-on from which could be harnessed to conveniently monitor the localization, flux, and dose of NO release. The potentials of NOD560 for in vitro biological applications were also exemplified in in vitro biological models, i.e. mesenchymal stem cell (MSC) migration suppression. NOD560 is expected to complement the existing NO donors and find widespread applications in chemical biological studies.

In order to further improve the electroanalytical performance toward heavy metal ions, core-ring structured NiCo2O4 nanoplatelets were used to modify glass carbon electrode (GCE) for the determination of heavy metal ions in water. Owing to the high surface area of NiCo2O4 nanoplatelets, the Pb(II) sensitivity increased by a factor of 1.70, and the detection limit decreased by a factor of 2.64 as compared to solid NiCo2O4 nanoparticles modified GCE. Interestingly, NiCo2O4 nanoplatelets showed different sensitivities toward heavy metal ions with the same valence states, following the order Pb(II) > Cd(II) > Hg(II) > Cu(II). To better and scientifically understand the difference in sensitivity, adsorption and desorption abilities were integrated into account. Density functional theory calculations verified that the adsorption capability of NiCo2O4 toward Pb(II) was strongest among all heavy metal ions, thereby resulting in the largest sensitivity. Further desorption current measurements indicated the large desorption barrier of Cu(II) was another important factor leading to its lowest sensitivity. Finally, the applicability of the proposed method was demonstrated by the detection of heavy metal ions in real seawater.

The effect of B sites on the catalytic activities of oxygen evolution reaction (OER) for perovskite oxides La0.6Sr0.4CoxFe1-xO3-δ (x = 0, 0.2, 0.4, 0.6, 0.8, 1, denoted as LSF, LSCF-28, LSCF-46, LSCF-64, LSCF-82 and LSC, respectively) prepared by a convenient and simple method of electrospinning technique is reported. The prepared La0.6Sr0.4CoxFe1-xO3-δ catalysts possess almost same crystal structures, similar morphologies (except for the LSC catalyst) and slightly different BET surface areas. Upon the optimization of the Co/Fe atomic ratio, the optimal LSCF-82 catalyst exhibits the OER performance with a low onset potential of 1.541 V, a small Tafel slope of 80.56 mV dec-1, a high charge-transfer rate and a large electrochemical surface area in 0.1 M KOH solution. LSCF-82 catalyst exhibits the long-term stability under the catalytic operation condition for 12 h. Such catalytic activity may mainly cause by the synergy of higher catalytic activity Co and lower catalytic activity Fe. Thus, the reasonable optimization of the transition metal composition in B sites for the perovskite oxides is in favor of the improvement of OER performance. Keywords: B sites, La0.6Sr0.4CoxFe1-xO3-δ, Perovskite oxides, Oxygen evolution reaction

Interatomic potentials based on neural-network machine learning (ML) approach to address the long-standing challenge of accuracy versus efficiency in molecular-dynamics simulations have recently attracted a great deal of interest. Here, utilizing Pd-Si system as a prototype, we extend the development of neural-network ML potentials to compounds exhibiting various types of bonding characteristics. The ML potential is trained by fitting to the energies and forces of both liquid and crystal structures first-principles calculations based on density-functional theory (DFT). We show that the generated ML potential captures the structural features and motifs in $\mathrm{P}{\mathrm{d}}_{82}\mathrm{S}{\mathrm{i}}_{18}$ and $\mathrm{P}{\mathrm{d}}_{75}\mathrm{S}{\mathrm{i}}_{25}$ liquids more accurately than the existing interatomic potential based on embedded-atom method (EAM). The ML potential also describes the solid-liquid interface of these systems very well. Moreover, while the existing EAM potential fails to describe the relative energies of various crystalline structures and predict wrong ground-state structures at $\mathrm{P}{\mathrm{d}}_{3}\mathrm{Si}$ and $\mathrm{P}{\mathrm{d}}_{9}\mathrm{S}{\mathrm{i}}_{2}$ composition, the developed ML potential predicts correctly the ground-state structures from genetic algorithm search. The efficient ML potential with DFT accuracy from our study will provide a promising scheme for accurate atomistic simulations of structures and dynamics of complex Pd-Si system.