Liquid–Liquid Phase Transition in Nanoconfined Silicon Carbide

Helical insertion of polyphenylene chains into confined cylindrical slits composed of two carbon nanotubes

The helical insertion behavior of poly(para-phenylene) (PP) chains into confined cylindrical slits constructed by two carbon nanotubes (CNTs) with different diameters is studied by molecular dynamics simulations. The contribution of system energy and each energy component to helical self-assembly is discussed to further explain the conditions, driving force and mechanism. The width and length of the slit, the diameter of the outer tube and the temperature have a great impact on the helical insertion of PP chains. Two equations are proposed to confirm the diameter and the distances between the PP helix and the inner and outer walls of the given CNTs. The helical self-assembly of PP with different numbers of chains inserted into the slits is further studied. This study has a great benefit in understanding the conformational behavior of polymers, even biological macromolecules in confinements.

Liquid-Liquid Phase Transition in Metallic Droplets

We report theoretical evidence of the substrate-induced liquid-liquid phase transition (LLPT) behaviors in a single Al droplet and Ti-Al droplets. The Al droplet can produce an LLPT induced by substrates in part, forming a special three-layer structure. However, the introduction of a Ti droplet can promote the LLPT in an Al droplet. Al and Ti droplets do not coalesce into a homogeneously mixed droplet but produce the ordered liquid films. The substrate-induced LLPT in the Al droplet is characterized by the transition from the disordered to ordered structure. Results indicate that the substrate and the Ti droplet are the driving forces to promote the LLPT. The LLPT of the Ti-Al droplets in the wedge-shaped substrate is also observed, indicating that the confined Ti-Al droplets are more likely to undergo an LLPT.

Crystallization behavior of a confined CuZr metallic liquid film with a sandwich-like structure

Despite the fact that its crystal state is thermodynamically stable, Cu₆₄Zr₃₆ alloy is prone to form metastable glass at a high cooling rate. However, the confinement can induce nano-crystallization with a novel sandwich-like hierarchical structure consisting of pure Cu layers, pure Zr layers and mixed layers by conducting molecular dynamics simulations. The liquid-to-crystal transition temperature and interatomic repulsion softness display abnormal oscillations, instead of monotonous variation, as the wall-wall separation increases. When the confinement size is 10 Å and 12 Å, the transition temperature reaches a maximum, resulting from the pending new sandwich layer. The atomic movement and dynamical heterogeneity are demonstrated to play a vital role in the abnormal oscillation behavior of physical properties of the nano confined metallic glass. The sandwich-like structure can alter the Cu-Zr bond fraction, which eventually influences the liquid-to-crystal transition temperature and interatomic repulsion softness. Our findings provide a deep insight into the hierarchical nanostructures and its liquid-to-crystal transition characteristics under confinement at the atomic level.

Nature of Hexagonal Silicon Forming via High-Pressure Synthesis: Nanostructured Hexagonal 4H Polytype

Hexagonal Si allotropes are expected to enhance light absorption in the visible range as compared to common cubic Si with diamond structure. Therefore, synthesis of these materials is crucial for the development of Si-based optoelectronics. In this work, we combine in situ high-pressure high-temperature synthesis and vacuum heating to obtain hexagonal Si. High pressure is one of the most promising routes to stabilize these allotropes. It allows one to obtain large-volume nanostructured ingots by a sequence of direct solid-solid transformations, ensuring high-purity samples for detailed characterization. Thanks to our synthesis approach, we provide the first evidence of a polycrystalline bulk sample of hexagonal Si. Exhaustive structural analysis, combining fine-powder X-ray and electron diffraction, afforded resolution of the crystal structure. We demonstrate that hexagonal Si obtained by high-pressure synthesis correspond to Si-4H polytype (ABCB stacking) in contrast with Si-2H (AB stacking) proposed previously. This result agrees with prior calculations that predicted a higher stability of the 4H form over 2H form. Further physical characterization, combining experimental data and ab initio calculations, have shown a good agreement with the established structure. Strong photoluminescence emission was observed in the visible region for which we foresee optimistic perspectives for the use of this material in Si-based photovoltaics.

Density dependent structural phase transition for confined copper: origin of the layering

Confinement presents the opportunity for novel structural transition scenarios not observed in three-dimensional systems. Here, we report a comprehensive molecular dynamic (MD) study of the structural phase transition induced by density for an ordinary metal copper (Cu) confined between two parallel panel walls. At 4.19 g cm-3 < ρ < 4.66 g cm-3, a notable structural phase transition occurs between the triangle unit cell structure and quasi-square unit cell structure upon densification. Both the bond order parameter (BOP) and angular distribution function (ADF) can provide evidence for the transition. We highlight the fact that when the sole decrease of the atom distance cannot adapt to the further densification, the system starts to adjust the neighboring bond angle and promote the layering transition, thus inducing the structural phase transition. At the metastable coexistence zone, the viscosity exhibits a remarkable drop and the diffusion coefficient shows a notable increase, both facilitating the accomplishment of the structural transition. Our results will trigger more interest on the phase transition under confinement in a metallic system.

Abnormal separation of the silicon–oxygen bond in the liquid layering transition of silicon dioxide in a nanoslit

Layering transition and separation of silicon and oxygen in liquid SiO₂ become obvious due to the strengthening of the nanoconfined effect.

Thermoresponsive Polyoxometalate/Ionic Liquid Supramolecular Gel Electrolytes for Supercapacitors: Fabrication, Structure, and Heteropolyanion Structure Effect

We report the fabrication, structure, and heteropolyanion structure effect of polyoxometalate (POM)/ionic liquid (IL) supramolecular gels. These supramolecular gels exhibit ordered structures, as a result of their excellent reversible self-assembly, and they show various physicochemical properties, determined by the heteropolyanion structure effect of POM anions. Specifically, the formation of POM/IL supramolecular gels results in a highly ordered layer-shape structure, which has been calculated using X-ray powder diffraction patterns and proven by transmission electron microscopy images for the first time. When these POM/IL supramolecular gels are heated, they become viscous liquid sols, with melting isotropic drops and even flowerlike structures on microscopic scales, while it undergoes a reversible gel-sol phase transformation from gel to sol. The heteropolyanion structure effect in these two IL gels, [TBTP]₅PW₁₀V₂O₄₀ and [TBTP]₈P₂W₁₆V₂O₆₂, on their physicochemical properties is demonstrated. The POM structures have a strong structure effect on the physicochemical properties. As the size of heteropolyanions increases, there is a significant improvement in the conductivity, thermal performance, and oxidizability, with a lower phase inversion temperature, which means that the Dawson-type compound, [TBTP]₈P₂W₁₆V₂O₆₂, has a higher conductivity, lower melting point, stronger oxidizability, and better thermal performance than the Keggin-type compound, [TBTP]₅PW₁₀V₂O₄₀, under the same conditions.

The role of liquid–liquid transition in glass formation of CuZr alloys

The structure evolution during LLTs is beneficial to the glass forming ability (GFA) of Cu–Zr systems.

Facile fabrication of thermal-control ionic liquid compound based on undecatungstophosphoindic polyoxometalate with fast ionic conductivity

Thermal-depended conductive properties of the ternary-POM-based layer-structure ionic liquid [TOAMe]₄[In(H₂O)PW₁₁O₃₉] was investigated. It shows fast ionic conductivity as high as 2.2 × 10⁻³ S cm⁻¹ at 90 °C with the conductive activation energy of 30.19 kJ mol⁻¹, which is related to its state, obviously increasing during the reversible phase transformation from a quasi-solid-state gel phase to an isotropic sol phase.